Design, Validation, and Optimization of a Rear Sub-Frame with Electric Powertrain Integration THESIS Presented in Partial Fulfil

Design, Validation, and Optimization of a Rear Sub-frame with Electric Powertrain

Integration

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

David Michael Walters, B.S.

Graduate Program in Mechanical Engineering

The Ohio State University

2015

Thesis Committee:

Dr. Giorgio Rizzoni, Advisor

Dr. Shawn Midlam-Mohler

David Michael Walters

2015

ABSTRACT

Government regulations and consumer desire continue to aggressively push automotive manufacturers to improve the fuel economy and emissions of new vehicle designs.

Vehicle weight reduction and the use of hybrid electric powertrains are becoming more commonly used methods for addressing a need for improved fuel economy and reduced vehicle emission. EcoCAR 2 is a three year collegiate design competition that involves

15 teams from universities across North America, competing to develop a vehicle with improved fuel economy and reduced emissions. Each team starts with a 2013 Chevrolet

Malibu and replaces the powertrain with the primary objective being the reduction of fuel consumption and emissions. The team from The Ohio State University incorporated a rear electric powertrain featuring an electric machine and single-speed transmission into their vehicle architecture. This resulted in the need for a customized rear cradle to support the addition of a rear electric powertrain. An initial custom cradle design was created by modifying an existing steel rear sub-frame to accommodate the addition of the rear electric powertrain. However, in order to reduce total vehicle weight and thus improve fuel economy and reduce emissions, a reduced mass, aluminum rear cradle was created for Ohio State's final vehicle design. This study covers the methodology surrounding the design, validation, and optimization of that final rear cradle design.

DEDICATION

To my loving wife, Laura, who helps me to be the best version of myself; to my parents,

Dick and Michele, who have supported me and shaped me into the man I am today, and

to my savior Jesus Christ who gives my life meaning.

iii

ACKNOWLEDGEMENTS

I would like to thank Dr. Rizzoni and Dr. Shawn Midlam-Mohler for their guidance and patience through this and other EcoCAR related endeavors during my time at The Ohio

State University. Their support and encouragement has helped me in addition to countless other students at OSU's Center for Automotive Research.

I would like to thank Josh Hendricks for conducting much of the work surrounding the design and fabrication of the OE modified cradle design as part of this project.

I would like to thank all contributors to the OSU EcoCAR 2 team for the camaraderie, support, and commitment to excellence in all things automotive.

Lastly, I would like to thank my friends and family for their patience and support during this stressful period of my life.

VITA

June 2005 ...... Circleville High School

May 2010 ...... B.S. Mechanical Engineering,

Ohio Northern University

January 2011 to Present ...... Graduate Research Associate, Department

of Mechanical Engineering,

The Ohio State University

FIELDS OF STUDY

Major Field: Mechanical Engineering

TABLE OF CONTENTS

ABSTRACT ...... II

DEDICATION...... III

ACKNOWLEDGEMENTS ...... IV

VITA...... V

TABLE OF CONTENTS ...... VI

LIST OF TABLES ...... XI

LIST OF FIGURES ...... XIII

CHAPTER 1: INTRODUCTION ...... 1

1.1 Introduction ...... 1

1.2 EcoCAR 2 Competition ...... 2

1.3 EcoCAR 2 Architecture ...... 5

vi 1.4 Motivation ...... 7

CHAPTER 2: LITERATURE REVIEW AND BACKGROUND...... 11

2.1 Introduction ...... 11

2.2 Design Considerations ...... 12

2.2.1 Material Selection ...... 12

2.2.2 System Interactions and Design Constraints...... 14

2.2.3 Environmental Exposure ...... 16

2.2.4 Manufacturability ...... 17

2.2.5 Crashworthiness and Structural Stiffness ...... 20

CHAPTER 3: TOOLS AND RESOURCES ...... 23

3.1 Design Modeling ...... 23

3.2 Finite Element Analysis ...... 24

3.2.1 FEA Process Introduction ...... 24

3.2.2 Pre-Analysis Processing...... 25

3.2.3 Mesh Generation ...... 54

3.2.4 Simulation Constraints and Loading Application ...... 63

3.2.5 Solving Simulation...... 69

3.2.6 Post-Processing Results ...... 71

3.3 Manufacturing Technologies ...... 81 vii CHAPTER 4: SIMULATION LOADING SCENARIOS ...... 83

4.1 Loading Conditions ...... 83

4.2 Two-Wheel Bump ...... 85

4.3 One-Wheel Bump ...... 85

4.4 Twist: Jounce and Rebound ...... 87

4.5 Forward Braking ...... 88

4.6 Reverse Braking ...... 89

4.7 Cornering ...... 90

4.8 Forward Acceleration ...... 91

4.9 Reverse Acceleration ...... 92

4.10 Max Torque ...... 93

4.11 Reverse Bump...... 95

4.12 Forward Impact ...... 95

CHAPTER 5: OE MODIFIED CRADLE DESIGN ...... 97

5.1 OE Modified Rear Sub-frame...... 97

viii 5.2 Component Integration ...... 98

5.3 Material Removal...... 100

5.4 Reinforcing ...... 101

5.5 Simulations ...... 103

5.6 Result Processing ...... 103

CHAPTER 6: REDUCED MASS ALUMINUM CRADLE...... 106

6.1 Approach ...... 106

6.2 Component Integration ...... 107

6.3 Model Development and Simulation ...... 107

6.4 Results and Comparison...... 111

6.5 Vehicle Testing ...... 117

CHAPTER 7: CONCLUSIONS AND FUTURE WORK ...... 120

7.1 Conclusions ...... 120

7.2 Future Work ...... 121

BIBLIOGRAPHY ...... 123

ix APPENDIX A: LIST OF SYMBOLS AND ABBREVIATIONS ...... 125

APPENDIX B: OE CRADLE AND OE MODIFIED CRADLE FEA FIGURES .. 127

APPENDIX C: REDUCED MASS ALUMINUM CRADLE FEA GIGURES ...... 138

LIST OF TABLES

Table 1: Vehicle Technical Specifications for the EcoCAR 2 Vehicle ...... 4

Table 2: Material Properties...... 13

Table 3: Sytem Interactions and Constraints ...... 15

Table 4: Two-Wheel Bump Loading Components ...... 85

Table 5: One-Wheel Bump Left-hand Side Loading Components ...... 86

Table 6: One-Wheel Bump Right-hand Side Loading Components ...... 86

Table 7: Twist - Left-hand Side Rebound, Right-hand Side Jounce Loading Components

...... 87

Table 8: Twist - Left-hand Side Jounce, Right-hand Side Rebound Loading Components

...... 88

Table 9: Forward Braking Loading Components ...... 89

Table 10: Reverse Braking Loading Components ...... 90

Table 11: Cornering - Left Turn Loading Components ...... 91 xi Table 12: Cornering - Right Turn Loading Components ...... 91

Table 13: Forward Acceleration Loading Components ...... 92

Table 14: Reverse Acceleration Loading Components ...... 93

Table 15: Maximum Torque in Forward Direction Loading Components ...... 94

Table 16: Maximum Torque in Reverse Direction Loading Components ...... 94

Table 17: Reverse Bump Loading Components ...... 95

Table 18: Forward Impact Loading Components ...... 96

Table 19: Established Maximum Stress ...... 112

Table 20: Stress Ratio and FOS of OE Modified and Aluminum Cradle Designs ...... 114

Table 21: OE Modified vs. Aluminum Weight Comparison ...... 116

Table 22: Maximum Displacement Summary ...... 117

xii

LIST OF FIGURES

Figure 1: OSU EcoCAR 2 Vehicle Architecture ...... 7

Figure 2: Creating Midsurfaces from Face Pairs ...... 27

Figure 3: Solid Body Selection ...... 28

Figure 4: Completed Surface Body Generation ...... 29

Figure 5: Show Hide Controls ...... 31

Figure 6: New FEM and Simulation from Existing Geometry ...... 32

Figure 7: New FEM Simulation Settings ...... 33

Figure 8: Promote Model Geometry to Finite Element Model (FEM) ...... 34

Figure 9: FEM Manipulation and Toolbar and Navigation ...... 35

Figure 10: Stitch Edge Icon ...... 36

Figure 11: Stitch Edge Selection Veiw ...... 37

Figure 12: Edges to be Stitched Together ...... 38

xiii Figure 13: Stitch Edge Selection...... 39

Figure 14: Stitch Edge View After Edges are Sticthed...... 40

Figure 15: Veiw After Stitching Completion...... 42

Figure 16: Create Point Icon ...... 43

Figure 17: Creating Point at Center of Arc ...... 44

Figure 18: Creating Point Between Two Points ...... 45

Figure 19: Creating Point in Center of Bolt Location...... 46

Figure 20: Created Point for Load Application ...... 47

Figure 21: Point Create in Center of Bushing Sleeve ...... 48

Figure 22: Points Generated at Center of All Loading and Constraining Geometries ..... 49

Figure 23: 1D Connection Icon...... 50

Figure 24: 1D Connection Settings Edge Selected for Weld Simulation ...... 51

Figure 25: 1D Connection Settings Face Selected for Weld Simulation ...... 52

Figure 26: 1D Connection - Point to Face for Bushing Simulation ...... 53

Figure 27: Mesh Collector Icon ...... 55

Figure 28: Mesh Collector Settings ...... 56 xiv Figure 29: PSHELL Properties ...... 57

Figure 30: Collector Setup for Aluminum Cradle ...... 59

Figure 31: 2D Mesh Icon ...... 60

Figure 32: 2D Mesh Settings ...... 60

Figure 33: 2D Mesh Generated with 1D Connections on Aluminum Cradle Design ...... 63

Figure 34: Overview of Loads and Constraints ...... 64

Figure 35: Constraint Icon ...... 65

Figure 36: Constraint Settings for Fixed Points Simulating Chassis Mounts ...... 66

Figure 37: Load Icon ...... 67

Figure 38: Settings for Force Loading ...... 68

Figure 39: Settings for Moment Loading...... 69

Figure 40: Solve Icon ...... 70

Figure 41: Solve Menu...... 70

Figure 42: Stress Results ...... 72

Figure 43: Displacement Results ...... 73

Figure 44: Results View Prior to Clean-up ...... 74 xv Figure 45: Results Post View Selection ...... 75

Figure 46: Post View Settings Display Tab ...... 75

Figure 47: Post View Settings Edges & Faces Tab ...... 76

Figure 48: Results Veiw Post Clean-up ...... 77

Figure 49: Cloned Solution with Altered Loading ...... 79

Figure 50: Post Processing View of Multiple Scenerio Results Stored ...... 80

Figure 51: Fixture to Maintaine Suspension and Chassis Mounting Locations ...... 81

Figure 52: Laser Cut Plates Tab and Slot Make the Assembly Self-Fixturing to a Degree

...... 82

Figure 53: SAE Vehicle Axis System [13] ...... 84

Figure 54: Rear View of Powertrain Interference on Stock Cradle ...... 98

Figure 55: Front View of Powertrain Interference on Stock Cradle ...... 99

Figure 56: OE Cradle with Highlighted Crossmembers for Removal ...... 100

Figure 57: OE Cradle with Cut Out Sections Removed ...... 101

Figure 58: OE Modified Cradle Final Design - Rear View ...... 102

Figure 59: OE Modified Cradle Final Design - Front View ...... 102

xvi Figure 60: Unrealistic Weld Stress Occurance ...... 104

Figure 61: Mounting Point Stress Concentration...... 105

Figure 62: Model Mounting Points and Constraints Multiple Viewing Angles ...... 107

Figure 63: New Design Comprised of Aluminum Plate and Tube - Upper Front View 109

Figure 64: New Design Comprised of Aluminum Plate and Tube - Lower Front View 110

Figure 65: New Design Comprised of Aluminum Plate and Tube - Upper Rear View 110

Figure 66: New Design Comprised of Aluminum Plate and Tube - Lower Rear View 111

Figure 67: Maximum Established Stress Comparison Plot ...... 113

Figure 68: Stress Ratio to Stock and Material FOS Plot ...... 115

Figure 69: The EcoCAR Vehicle Driving the Double Lane Change Test ...... 118

Figure 70: The EcoCAR 2 Vehicle Driving Over a the Large Bump Ride Profile ...... 118

Figure 71: Two-Wheel Bump Stress...... 127

Figure 72: Two-Wheel Bump Displacement ...... 127

Figure 73: One-Wheel Bump LHS Stress ...... 128

Figure 74: One-Wheel Bump LHS Displacement ...... 128

Figure 75: One-Wheel Bump RHS Stress ...... 128 xvii Figure 76: One-Wheel Bump RHS Displacement ...... 129

Figure 77: Twist - LHS Jounce, RHS Rebound Stress ...... 129

Figure 78: Twist - LHS Jounce, RHS Rebound Displacement ...... 129

Figure 79: Twist - LHS Rebound, RHS Jounce Stress ...... 130

Figure 80: Twist - LHS Rebound, RHS Jounce Displacement ...... 131

Figure 81: Forward Braking Stress ...... 131

Figure 82: Forward Braking Displacement...... 131

Figure 83: Reverse Braking Stress ...... 132

Figure 84: Reverse Braking Displacement ...... 132

Figure 85: Cornering - Left Turn Stress ...... 132

Figure 86: Cornering - Left Turn Displacement ...... 133

Figure 87: Cornering - Right Turn Stress ...... 133

Figure 88: Cornering - Right Turn Displacement ...... 133

Figure 89: Forward Acceleration Stress ...... 134

Figure 90: Forward Acceleration Displacement ...... 134

Figure 91: Reverse Acceleration Stress ...... 134 xviii Figure 92: Reverse Acceleration Displacement ...... 135

Figure 93: Max Torque - Forward Stress ...... 135

Figure 94: Max Torque - Forward Displacement ...... 135

Figure 95: Max Torque - Reverse Stress ...... 136

Figure 96: Max Torque - Reverse Displacement ...... 136

Figure 97: Reverse Bump Stress ...... 136

Figure 98: Reverse Bump Displacement ...... 137

Figure 99: Forward Impact Stress ...... 137

Figure 100: Forward Impact Displacement ...... 137

Figure 101: Two-Wheel Bump Max Stress ...... 138

Figure 102: Two-Wheel Bump Max Deflection ...... 138

Figure 103: One-Wheel Bump LHS Max Stress ...... 139

Figure 104: One-Wheel Bump LHS Max Deflection ...... 139

Figure 105: One-Wheel Bump RHS Max Stress ...... 140

Figure 106: One-Wheel Bump RHS Max Deflection ...... 140

Figure 107: Twist - LHS jounce, RHS rebound Max Stress...... 141 xix Figure 108: Twist - LHS jounce, RHS rebound Max Deflection ...... 141

Figure 109: Twist - LHS rebound, RHS jounce Max Stress...... 142

Figure 110: Twist - LHS rebound, RHS jounce Max Deflection ...... 142

Figure 111: Forward Braking Max Stress ...... 143

Figure 112: Forward Braking Max Deformation ...... 143

Figure 113: Reverse Braking Max Stress ...... 144

Figure 114: Reverse Braking Max Deformation ...... 144

Figure 115: Cornering - Left Turn Max Stress ...... 145

Figure 116: Cornering - Left Turn Max Deformation ...... 145

Figure 117: Cornering - Right Turn Max Stress ...... 146

Figure 118: Cornering - Right Turn Max Deformation ...... 146

Figure 119: Forward Acceleration Max Stress ...... 147

Figure 120: Forward Acceleration Max Deformation ...... 147

Figure 121: Reverse Acceleration Max Stress ...... 148

Figure 122: Reverse Acceleration Max Deformation ...... 148

Figure 123: Max Torque - Forward Max Stress ...... 149 xx Figure 124: Max Torque - Forward Max Deformation ...... 149

Figure 125: Max Torque - Reverse Max Stress ...... 150

Figure 126: Max Torque - Reverse Max Deformation ...... 150

Figure 127: Reverse Bump Max Stress ...... 151

Figure 128: Reverse Bump Max Deformation...... 151

Figure 129: Forward Impact Max Stress ...... 152

Figure 130: Forward Impact Max Deformation...... 152

xxi CHAPTER 1: INTRODUCTION

1.1 Introduction

This study examines the process of designing, validating, and prototyping a rear cradle, also known as a subframe, used by the Ohio State University EcoCAR 2 Team in the

EcoCAR 2 competition. Over the course of the 3 year competition, the rear cradle, which serves the dual function of structurally connecting the rear suspension components to the vehicle chassis and housing the rear electric powertrain, went through two distinct designs. Both designs went through multiple iterations in model space, with FEA simulations used to influence changes between each iteration, before being fabricated.

The first design was conducted with the primary objective being to modify an existing rear sub frame, designed for use with the existing suspension, in such a way as to enable the integration of a rear electric motor and single speed transmission, while maintaining the structural integrity provided by the original cradle. The second design was conducted with the goal of substantially reducing the total mass of the cradle. This second design involved creating an entirely new rear cradle, while maintaining the rear powertrain components and suspension mounting locations from the previous design. The new structure was optimized to minimize mass without reducing performance or structural integrity.

1 1.2 EcoCAR 2 Competition

The EcoCAR 2: Plugging In to the Future (EcoCAR 2) competition is a 3 year event empowering teams of university students to design and build advanced fuel efficient hybrid vehicles. The EcoCAR 2 competition is the most recently completed installment in over 25 years of DOE Advanced Vehicle Technology Competitions (AVTC). The

EcoCAR 2 competition is organized and sponsored primarily by the U.S. Department of

Energy (DOE) and General Motors (GM). The competition includes teams of students from 15 universities located across North America. Each team is given a 2013 Chevrolet

Malibu, and challenged to design and integrate a new powertrain that when compared to the production vehicle, demonstrates reduced fuel consumption, reduced well-to-wheel greenhouse gas emissions, and reduced tailpipe emissions. In addition to meeting all of these challenges with the new powertrain the teams are also challenged to maintain if not improve consumer acceptability in the areas of performance, utility, and safety.

The three-year competition is conducted in three phases, one phase per year, with each phase having a set of deliverables and presentations that are scored to determine a winner for each phase. The phases follow a Vehicle Development Process (VDP) designed to simulate a real-world engineering design process. The three phases of the VDP, modeled after the VDP used by GM, are design, build, and refine. The first year, teams design their vehicle architecture by creating and assembling the design virtually in 3D CAD model space. The second year, each team receives the 2013 Chevrolet Malibu and modifies it to match their CAD designs, creating a working prototype. The third year, the

2 designs are refined and optimized, taking the vehicle from a functional prototype to a showroom ready finished vehicle. [1]

In addition, to the general goals of the competition, teams are given measurable Vehicle

Technical Specifications (VTS) that serve as both requirements and targets for the vehicle to achieve in order to be competitive in the competition. The VTS for the EcoCAR 2 competitions are shown in Table 1, which has the values achieved by the production 2013

Malibu, which served as the starting platform for all the vehicles in the competition, the values that serve as requirements that must be met in order to be competitive, and the values achieved by the OSU EcoCAR 2 Team's design, classified as a Parallel-Series

PHEV.

3 Production Competition OSU Parallel- Specification 2013 Malibu Design Target Series PHEV ECOCAR COMPETITION REQUIREMENTS Acceleration 0-60 (s) 8.2 sec 11.5 sec 11.2 s Acceleration 50-70 (s) 8.0 sec 10 sec 5.4 s 43.7 m [143.4 Braking 60-0 54.8 m [180 ft] 43.7 m (143.4 ft) ft] Highway Gradeability @ 10+% @ 60 3.5% @ 60 mph 3.5% @ 60 mph 20 min mph

Cargo Capacity 16.3 ft3 7 ft3 7 ft3 Passenger Capacity 5 ≥ 4 5 Mass 1,589 kg ≤ 2078 kg 2,000 kg (4,410 lb) Starting Time ≤ 2 sec < 15 sec < 2 sec Ground Clearance 155 mm >127 mm >127 mm 736 km (457 Range ≥ 322 km [200 mi] 415 km (258 mi) mi) (CAFÉ) ECOCAR COMPETITION TARGETS Charge Depleting Range* N/A ** 72.0 km (44.7 mi) Charge Depleting Fuel N/A ** 0 lge/100km Consumption* 7.52 Charge Sustaining Fuel N/A ** (lge/100km(ge) Consumption* [670.0 Wh/km] 8.83 (lge/100 7.12 (lge/100 km) 2.61 (lge/100 km) UF-Weighted Fuel Energy km) Consumption* [787 Wh/km] [634 Wh/km] [232.2 Wh/km] UF-Weighted AC Electric N/A ** 192.7 (Wh/km) Energy Consumption* UF-Weighted Total 787 (Wh/km) 634 (Wh/km) 424.8 (Wh/km) Energy Consumption* UF-Weighted WTW 774 (Wh Petroleum Energy (PE) 624 (Wh PE/km) 79.9 (Wh PE/km) PE/km) Use* UF-Weighted WTW GHG 253 (g 204 (g GHG/km) 185.5 (g GHG/km) Emissions* GHG/km) Criteria Emissions Tier II Bin 5 Tier II Bin 5 < Tier II Bin 5

* Evaluated using EcoCAR 2 combined “4-cycle” weighting method, which weights US06, 505 and HWFET to reflect 43% city driving, 57% highway driving

** There is no competition design target, but EcoCAR teams are expected to report their predictions in these categories for their VTS.

Table 1: Vehicle Technical Specifications for the EcoCAR 2 Vehicle

1.3 EcoCAR 2 Architecture

The OSU EcoCAR 2 vehicle architecture is designed to be more efficient and have cleaner emissions than the 2013 Malibu base platform, while still meeting all the required

VTS as part of the competition. The vehicle design is classified as a series-parallel Plug- in Hybrid Electric Vehicle (PHEV). This design utilizes an 18.9 kW-hr high voltage battery pack to power two electric machines. An 80 kW electric machine is used to drive the rear axle via a single speed transmission. The front axle drive incorporates a 1.8L

Honda compressed natural gas engine, which has been converted to run on E85, and a second 80 kW electric machine. Both engine and electric machine can be used to apply power to the wheels using a 6-speed manual transmission that has been automated by the team to make it an automated manual transmission. The automation of the transmission utilizes the electric machine to speed match the two sides of the electrically, actuated clutch during shifting operations. This has the capability to produce smooth, efficient shifts more characteristic of an automatic transmission than a manual, which allows the vehicle to realize the efficiency increase benefits of a manual transmission, while maintaining the consumer acceptability of an automatic transmission.

The battery pack is charged by plugging the vehicle into a car charge port, utilizing the existing energy generation of the electric grid, or by using the front electric machine as a generator to convert mechanical energy from the engine into electric energy for storing in 5 the battery pack. This allows the vehicle to run in electric mode for short trips and urban driving where the electric drive efficiency is best realized relative to the E85 powered engine. The vehicle can also operate in parallel mode using the engine and rear electric machine for propulsion and the front electric machine to maintain charge in the battery.

This is useful for highway speeds and long trips because it utilizes the combined power of the engine and rear electric machine for achieving high speeds. This architecture also offers the advantage to maintain power using the E85 to provide extended range over greater distances than could be achieved by the electric capacity alone. Finally, the vehicle can operate in series mode by putting the front transmission into neutral and using the engine exclusively for electric generation through the front electric machine, while using the rear motor for vehicle propulsion. This allows the engine to run at peak efficiency for charging. Figure 1 shows the CAD model design layout of how the aforementioned powertrain components are integrated into the OSU EcoCAR 2 Vehicle

Architecture.

Figure 1: OSU EcoCAR 2 Vehicle Architecture

1.4 Motivation

The motivation behind the designs that serve as the subject of this study is twofold as it differs for each of the two designs. The motivation for the initial cradle design is fictional necessity. In order to incorporate the previously discussed vehicle architecture into the vehicle the rear electric machine and transmission need mounting provisions that do not negate any of the VTS requirements, such as minimum ride height, and allow the output shafts from that transmission to supply power to the wheels without interfering with the operation of the vehicle suspension.

7 The motivation for the second cradle design is to reduce the total mass of the cradle, while maintaining the functionality of the initial cradle design. Mass reduction of the rear cradle is driven by two reasons. The first reason for the reduction of the cradle mass is to meet the EcoCAR 2 competition requirements. The OSU vehicle design had to have non-critical components removed in year 2 of the competition in order to remain under the maximum weight limit set forth in the competition VTS requirements. None of the components removed affected the drivability of the vehicle, but the vehicle did have a reduced consumer acceptability and higher tailpipe emissions than it would have with all the designed components installed. Thus, reducing existing component mass in order to field the complete vehicle design without exceeding the maximum vehicle weight threshold is one reason for reducing mass in the initial rear cradle design.

The second motivating factor for reducing the total mass of the rear cradle design is to improve the overall vehicle efficiency. Vehicle mass is one of the most significant contributing factors to total vehicle efficiency. It has been shown that for every 100 kg reduction in vehicle mass the consumption of fuel will decrease by approximately 0.4 L per 100 km[2]. This improved efficiency also results in reduced CO2 emissions of roughly 8 g to 11 g per kilometer for every 100 kg reduction in mass[3].

The effects that reducing vehicle mass has on vehicle efficiency is clear when examining the resistive forces acting against the vehicle propulsion. These resistive forces are expressed mathematically in Equation 1, frequently referred to as the "road load" equation.

8 Equation 1

The variables for Equation 1 are defined in the following list:

A = Frontal area of vehicle

CD = Aerodynamic drag coefficient

fr = Rolling resistance coefficient

RRL = Road Load

V = Forward Velocity

W = Weight of vehicle

ρ = Density of air

θ = Pitch angle

This equation is the summation of three components, the rolling resistance, the aerodynamic forces, and the grade forces. Rolling resistance remains fairly constant and makes up the majority of road load in most driving cycles. Aerodynamic forces are negligible at load speeds and do not approach similar magnitudes as rolling resistance until highway speeds are reached in a typical passenger vehicle. Grade forces vary with terrain and can range from the most to least significant in magnitude depending on the road grade. Of these three components, vehicle weight is a driving variable in rolling

9 resistance and grade forces. This makes a strong argument for considering vehicle weight the most significant variable in determining the amount of force a vehicle propulsion system must overcome.

CHAPTER 2: LITERATURE REVIEW AND BACKGROUND

2.1 Introduction

An engine cradle is a key aspect of vehicle design. It serves a dual purpose of supporting the engine transmission and suspension of a vehicle, as well as isolating the cabin and occupants from the vibrations and shock generated by the powertrain and road. The first appearance of engine cradles was in the late 1960's. The initial occurrence was to provide a way to balance the riding comfort and handling for a luxury car. Typical engine cradles are fabricated by stamping heavy steel sheets into concave sections, which are welded together to form a tubular structure that is bolted to the vehicle frame or chassis. Another benefit is engine cradles provide a way to build the steering, engine, and transmission assemblies in a separate location from where this finished assembly is installed in the finished vehicle. This modular approach to assembly allows for improved flexibility and efficiency, which results in fast assembly time and reduced costs. Furthermore, engine cradles also serve the maintenance side of the automotive industry, allowing mechanics to remove broken parts more easily, thus reducing repair time and maintenance cost. As a result of the realization of advantages provided by engine cradles, a rear subframe with similar structure has been developed and received widespread adoption in the automotive industry. Rear subframes serve the purpose of carrying the rear suspension and occasionally drivetrain components for rear-wheel or all-wheel drive vehicles[4]. 11 The OSU EcoCAR 2 vehicle design, having nearly independent front and rear powertrains, takes the idea of a rear subframe a step further than traditional rear subframes. By integrating a full powertrain, including an electric motor, transmission, and suspension, this design is more similar to an engine cradle than a traditional rear subframe. Thus this design shall be thought of as a rear powertrain cradle, referred to as cradle through the majority of this study.

2.2 Design Considerations

Before designing and fabricating a cradle there are many aspects to consider. Most manufacturers in the automotive industry will take years designing, validating, optimizing, and testing before they even begin producing the cradle for installation in their vehicles. GM even released a publication describing most of the design considerations they considered in the design of an aluminum engine cradle for use in one of their vehicles. The list of considerations included in this publication include static stiffness, NVH performance, crashworthiness, strength and fatigue, weight, corrosion resistance, temperature and fluid exposure, and cradle clearances. [5] Many of these aspects in addition to a few others are considered in this design study as well.

2.2.1 Material Selection

For the initial cradle design, material compatibility with the existing cradle structure was the driving factor, since the added reinforcing structure had to be joined to existing framework of the cradle mounting points. Choosing a material that could be welded to the existing frame narrowed the material options to low carbon steel family of metals. 12 For the second cradle redesign, weight reduction was the driving factor; this led to a search of what materials have successfully been used for similar applications resulting in reduced weight compared to conventional steel cradle weldments. One study found that fabricating an engine cradle out of 6061-T4 aluminum substantially reduced the overall weight of the engine cradle without reducing the structural integrity. This study used

1/16" 4043 welding wire in a MIG welder for the aluminum welding of this cradle and hydro forming of aluminum tube to create the bends and shape of the cradle. [6]

The materials that make up the initial cradle design include a GM specification type of steel called GMW3032M-ST-S-HR340La, SAE 1010 steel, and A36 steel. The aluminum cradle is made entirely of 6061-T6 aluminum alloy. The properties of these materials are shown in Table 2.

Young's Yield Poisson's Modulus Strength Ratio Material [Mpa] [Mpa] GMW3032M 0.29 207,000 305 -ST-S-HR340La SAE 1010 0.29 207,000 340 A36 Steel 0.29 200,000 250 6061 Aluminum 0.33 68,900 242

Table 2: Material Properties

2.2.2 System Interactions and Design Constraints

In determining the constraints needed to define the design of the rear cradle, the first process is identifying all the systems interacting with the function and space of the rear cradle. Table 3 shows the systems that interact with the rear cradle design and the constraints they induce upon the cradle system.

14 System Interaction Constraint Four mounting points critical to cradle Four fixed points relative to each other function and structure and structural used to anchor design in space and Chassis space claim establishes upper limit of established upper limit of available available space for cradle structure. space for cradle structure Minimum ground (>127 mm) clearance Established lower limit of available Ground drives lower limit of available space for space for cradle structure cradle structure. Five mounting locations per side that Seven suspension components rely on must be structurally supported and in a the rear cradle for structural location and fixed location relative to chassis Suspension support relative to each other and mounting. And leave volume clear vehicle chassis. where links can occupy as suspension compresses and extends. Electric Supported entirely by rear cradle 16 transmission/electric machine Machine & structure for resisting weight and output mounting points located relative to the Single-Speed torque of drivetrain. chassis mounts. Transmission Leave volume clear where drive shafts Passes though area close to several Drive Shafts can occupy as suspension compresses suspension mounting points and extends. Leave path for exhaust pipe to pass with Must pass through region near enough clearance to avoid absorbing Exhaust transmission and suspension mounts detrimental amounts of radiant or conductive transfer. Partially defines forward limit of limit of Fuel tank Space claim near cradle mounting points available space for cradle structure DC-AC Partially defines forward limit of limit of Space claim near cradle mounting points Inverter available space for cradle structure Partially defines upper and rear limit of High Voltage Space claim near cradle mounting points limit of available space for cradle Battery Pack structure

Table 3: Sytem Interactions and Constraints

15 2.2.3 Environmental Exposure

The rear cradle being located on the under body of the vehicle and supporting the rear electric powertrain may be exposed to a variety of environmental factors including ambient temperatures ranging from approximately -80°C to 60°C, water, salt spray, automotive lubricants, automotive coolants, and road derris.

Of these factors, one of the most threatening to the successful function of the rear cradle designs is the water and salt spray, which can promote corrosion if mitigating measures are not taken. For steel components an enamel or epoxy based paint with a primer as recommended by paint manufacturer is an appropriate coating to protect the steel from the corrosive effects of water and salt spray. The 6061 aluminum alloy, used for the aluminum cradle design, has good corrosion resistance without any coating. However, where the aluminum interfaces with steel such as the bolted connections mounting the transmission to the cradle, galvanic corrosion can occur. The theory of galvanic corrosion is well established and good description are available [7], so details of the mechanics will not be discussed in this study. However, effective methods for galvanic corrosion prevention include electrically isolating the two metals using insulating washers, coatings or even tape, and protecting the bi-metal contacting surfaces from electrolytes that promote galvanic corrosion by coating with sealants or paints [8].

16 2.2.4 Manufacturability

Typically cradles are manufactured using multiple stampings of steel sheet metal welded together to form tubular structures, indeed this is the means by which the OE cradle modified for use on the OSU EcoCAR 2, year 2 vehicle design, was originally manufactured. Another method of manufacturing cradle designs is the use of hydroforming. Hydroforming is a metal forming process where structural members with closed sections are created from tubes. The process usually starts with a round tube being placed in a die, frequently with a mostly square or rectangular profile, then the tube is filled with fluid and hydraulic pressure is applied causing the walls of the tube to conform to the die creating the desired cross-section. Hydroforming is typically done with a constant wall thickness tube; however, in 1999, GM conducted a study investigating the use of variable wall thickness tubing to create a cradle with mass reduction without sacrificing structural strength. GM's study used a baseline steel cradle, an aluminum cradle made using constant wall thickness tube, and an aluminum cradle using variable wall thickness tubes. The findings of the study showed that after optimization both aluminum cradles had stress and displacement finite element results that were quite comparable to the base line steel cradle. However, the constant wall thickness aluminum cradle showed a 32.7% mass reduction and the variable wall thickness cradle showed a 36.8% mass reduction compared to the base line steel cradle.

[6] While hydroforming as a process proved too costly for use in fabricating the OSU

EcoCAR 2 rear cradle, the concept of varying material thicknesses to optimize the design was used in the development of this design. 17 Both stamping and hydroforming metal forming operations are useful for large scale production operation such as those implemented by automotive manufacturers; however, for the production of a single prototype, as is the use case for the OSU rear cradle design, either operation would prove fare to costly and require too much time to produce for the scope of the competition. As a result, metal working processes such as machining, cutting, grinding, bending, and welding were considered for potential manufacturing methods during the design of this cradle.

For both the initial modified OE steel cradle design and the reduced mass aluminum design, a combination of stock structural angle or tube and laser-cut plate are used. Laser tables are able to hold a profile tolerance of .13 mm, this allows complex profiles to be cut quickly and relatively inexpensively from the CAD model profile. By modeling notches or slots in one piece, and modeling a corresponding tab in another, the model can be created where the cut plate profiles help locate each plate relative to adjacent parts.

This reduces the complexity of the fixture needed to hold the parts in the same orientation as the geometry created in model space, for welding together.

For welding both iterations of rear cradle designs, metal inert gas (MIG) welding is used as the joining process. For welding the initial steel cradle, ER70S-3 filler wire is used with direct current arc generation. This is a common setup for welding mild steel and was selected because it would create welds with similar characteristics to the welds created during the initial fabrication of the OE cradle being modified to create the first design iteration of cradle for the OSU EcoCAR 2 design.

18 For welding the aluminum cradle design, multiple welding processes were considered.

Edison Welding Institute (EWI) wrote an article investigating the best welding methods for use on aluminum in automotive frame and sub-frame applications. The investigation focused on four types of arc welding suitable for welding aluminum. These welding methods included pulsed GMAW, variable polarity GMAW, twin-wire GMAW, and plasma arc welding. Pulse GMAW creates a high quality weld, generates low splatter, consistency and is frequently used in suspension components in industry. Variable polarity GMAW, also known as AC-GMAW, provides more control of weld penetration than other GMAW methods and is capable of welding lap fillet joints with gaps up to twice the material thickness. Twin-wire GMAW provides faster weld speeds than other

GMAW processes, which results in less material distortion due to heat. Lastly, plasma arc welding requires less surface prep, is capable of use on blind spot welding, can be used on materials up to 8 mm thick, and does not create degradation at weld bead start and finish like GMAW methods. In considering all of these aspects, variable polarity GMAW is used for the fabrication of the aluminum design because two areas of concern for the structural integrity of the design are the welding methods ability to fully penetrate the joints on the thicker 1/2" plates and fill gaps created due to tolerance stack-ups of multiple pieces being welded together. Both are issues variable polarity GMAW is identified as being the best at overcoming. [9]

19 2.2.5 Crashworthiness and Structural Stiffness

Crashworthiness is a critical consideration in vehicle engine cradle designs because the cradle takes up a large portion of the space and creates much of the structure between the occupants and any object that might be involved in a frontal collision. Though the rear cradle considered for this study is not the same as a front engine cradle, many of the same considerations apply; however, for this design a rear collision is the event of concern.

Crashworthiness for a cradle design requires a design to be optimized between conflicting requirements. One requirement for a cradle design is the need for the cradle to be stiff enough to support the powertrain weight and torque, in addition to the suspension loading. Counter to that requirement, is the need for the cradle to deform enough in a collision to allow the crush zones to collapse and absorb the energy. If the cradle is too stiff it can reinforce crush zones of the vehicle causing more collision energy than intended to reach the occupant portion of the vehicle. Ideally, a cradle will serve to adequately support the powertrain and suspension, and serve as an energy absorbing element in the event of a collision. Another design consideration of the cradle is the likely trajectory of the powertrain components during a collision. Most cradles are designed to divert the powertrain under the vehicle in the event of a collision so that the momentum transferred into the powertrain during the collision is aimed under instead of towards the vehicle.

Another consideration for the crashworthiness of the OSU EcoCAR 2 rear cradle design is the impact of using aluminum instead of steel for the final design and how the material

20 change may impact the crashworthiness of the vehicle. The question of if using aluminum in place of steel to lightweight a vehicle structure can be accomplished without decreasing the crashworthiness of the vehicle, was the driving point behind a study conducted by the Alcoa Technical Center, an aluminum manufacturer. This study focused on three main aspects surrounding this issue; the effects of size versus mass on crashworthiness, the potential to reduce vehicle mass by using aluminum body structures, and the energy absorption capability of aluminum structures. Alcoa found that the size of the structure geometry has a much greater impact on crashworthiness than the mass of the structure. Indeed, if the mass is reduced without decreasing the size of the structure, and thus the crush zones, the vehicle becomes safer for both the occupant of that vehicle and the occupants of other vehicles involved in a collision with that vehicle. Since aluminum alloys can have similar strength properties to steel with only one third of the density, it is an ideal material for reducing weight without compromising crashworthiness. Further experimentation by Alcoa also found that using identical thicknesses and geometry aluminum can absorb more energy than steel, making it not just a suitable substitute for steel, but a preferable one from a crashworthiness perspective [10].

The feasibility of using aluminum as a substitute for steel with the second cradle design iteration is further supported by a study conducted by the Ford Motor Company. Ford documented the process of designing a cradle using 6000 series aluminum alloy to absorb an average of 8,000 lbs of load while allowing adequate space for the frame rails to deform absorbing the impact. The design was created and optimized in modal space and validated using FEA. The study found that reducing cradle stiffness improved 21 crashworthiness, and increasing the number of structural members while reducing the wall thickness was a good method for reducing stiffness [11].

CHAPTER 3: TOOLS AND RESOURCES

3.1 Design Modeling

For this study, Siemens NX 8 and NX 9 are the two revisions of the software package used to generate the 3D CAD models that are used to drive the analysis and fabrication of the rear cradle designs. This software is used in part because the competition provided

CAD data, for both the Malibu base vehicle and the baseline rear cradle from the

LaCrosse, is native to this software. The use of this software makes manipulating the designs for the rear cradles easier and allows checks for potential vehicle interferences.

Developing the designs in model space helps streamline the design process because the model is easily converted into a simplified geometry for use in the Finite Element

Analysis (FEA), and plate profiles can be exported to be used as direct input files into a

CNC laser-cutting table, ensuring an accurate transition from design to fabricated assembly. The 3D CAD model of the designs is also used to validate that fabricated assembly matches the design validated using FEA. Critical measurements taken from the

3D model are compared to measurements taken from the fabricated cradle design to confirm that the design and fabrication match.

23 3.2 Finite Element Analysis

3.2.1 FEA Process Introduction

FEA was used to validate the design prior to creating a prototype. This tool allowed for multiple iterations of the design to be modeled and tested against the harshest potential loading conditions without expending the time and effort required to create and test a prototype structure for each iteration of the design. There are many types of FEA software available to perform the analysis for this design. Ultimately, the Advanced

Simulation application for NX9 was selected for validating this design. This software was chosen because the vehicle and component models where already loaded and assembled in NX and the Advanced Simulation application of uses NX NASTRAN for structural analysis which is a robust solver. This application has adequate capabilities of pre- processing to accurately simulate the design and sufficient post-processing to measure the needed results for the planned analysis. Also, since the Advanced Simulation application is integrated with the NX modeling software, using this application makes converting and manipulating the initial model through several iterations of design and analysis both efficient and accurate. This section will define the pre-processing and post-processing, as well as the solver method used to validate the cradle design against the specified loading scenarios.

The same method was used to perform FEA on both the modified OE cradle design and the light-weight aluminum design in order to assure the results were comparable. Both designs are almost entirely made up of material geometries with a constant wall 24 thicknesses. The only exception being two small thread bosses in the front cross-member of the OE cradle, which allow for the mounting of the LaCrosse AWD differential in the

OE configuration. Note these bosses were not used for mounting of the OSU EcoCAR2 vehicle powertrain. The constant wall thickness allows for the use of two-dimensional

(2D) elements in the FEA mesh instead of three-dimensional (3D) elements. This requires some extra effort to convert the cradle geometry from solid extrusions to midsurfaces, but saves a significant amount of time and processing power for both the mesh generation and the solver calculations.

3.2.2 Pre-Analysis Processing

There are several actions necessary to convert the solid model into a simplified geometry that can be used for generating a finite element mesh that is both accurate to the original solid model design and stable enough for both mesh generation and solving. This section will identify these actions in five sub-sections including: Converting Solid Bodies to

Surface Bodies, Creating New Simulation and Finite Element Model, Manipulating

Surfaces for Optimal Mesh Generation, Creating Points for Load and Constraint

Application, and Ridged Connections for Simulating Weld and Bushing Connections.

3.2.2.1 Converting Solid Bodies to Surface Bodies

After creating a solid model, the first step in being able to generate a 2D mesh that can be used to determine the stress and displacement exhibited in the structure when the loading

25 scenarios are applied is to convert the solid bodies into surfaces. In order to maintain accuracy to the structural properties of the design it is important that the surfaces be located at the mid-point between the two outer surfaces of the solid body. Later in the setup process a thickness will be assigned to each surface. The finite element solver calculations operate under the assumption that the surface is located at the mid plane of the body. This is why converting to mid-surfaces, not offset surfaces is critical.

In NX, with the solid model opened, the Advanced Simulation application is selected in order to access the needed toolbar. Once in Advanced Simulation the Midsurface by Face

Pairs button is selected in the Geometry Preparation section. This accesses the settings shown in Figure 2.

Figure 2: Creating Midsurfaces from Face Pairs

With the Select Solid Body button selected the bodies intended to be converted to midsurfaces are selected and change color as shown in Figure 3.

Figure 3: Solid Body Selection

Next, the strategy for generating the midsurface must be selected. For the majority of geometries in this design, Progressive was a sufficient strategy to generate the desired midsurface, though with some of the more complex geometry bodies it was necessary to undo the initial midsurface generation and regenerate using a Manual strategy. Using the

Manual strategy the inner and outer surfaces are manually selected for generating the midsurface.

After selecting the strategy, the Automatically Create Face Pairs button is selected. This generates the midsurfaces. Then selecting Apply will finalize the newly created midsurfaces and reset the Midsurface by Face Pairs setting to allow for additional

28 midsurface generation. Selecting OK will finalize the midsurfaces and close the setting window.

Once the midsurface is finalized it will appear as a solid line down the middle of the converted solid bodies as shown in Figure 4.

Figure 4: Completed Surface Body Generation

This process must be performed on all solid bodies in the model, however, it is prudent to take a methodical approach, converting sections of similar bodies at a time. For example, in Figure 3 all the vertical plates running from front to back are selected. These plates are all simple geometries, so using a progressive strategy will work for all of selected bodies.

By converting the model in sections and reviewing the generated midsurfaces after each

29 section is completed, it is easier to check that all midsurfaces were generated correctly than it would be if the entire geometry were selected and in one pass generating all midsurfaces at once.

Once all solid bodies are converted to midsurfaces, a good check to assure all bodies were converted to surfaces is to hide all the solid bodies, sketches, and datums, and see that the remaining surfaces, or sheet bodies as identified by NX, create the entire model geometry. To hide or show features in NX open the View tab and select the "Show and

Hide" button in the Visibility section. The menu shown in Figure 5 will appear and selecting the plus or minus symbols adjacent to a feature’s name will show or hide all occurrences of that feature, respectively.

Figure 5: Show Hide Controls

After confirming that all solid bodies have been converted to surfaces the model is ready to be converted into a Finite Element Model (FEM), which will be used to calculate the analysis results. It is important that before beginning the process to create an FEM from the existing model that all solid bodies be hidden, unless the body is intended to be used for a 3D mesh because converting to a surface is not feasible due to geometry complexity.

31 3.2.2.2 Creating New Simulation and Finite Element Model

In order to convert the existing surface model into a FEM, right click on the existing model in the Simulation Navigator and select the New FEM and Simulation as shown in

Figure 6.

Figure 6: New FEM and Simulation from Existing Geometry

Once New FEM and Simulation is selected the menus shown in Figure 7 will appear.

Figure 7 indicates the selections that were made to create the simulations used for the analysis conducted on the designs outlined in this study. Note that the Bodies to Use selection is set to All visible. This is the reason that only the surface bodies and needed solid bodies should be visible going into this process.

Figure 7: New FEM Simulation Settings

NX defaults to a static structural analysis, so very few settings had to be changed to create the desired simulation. Once through the menus in Figure 7, the next step is to promote the geometry for use in the FEM. This is accomplished by clicking on the

Promote button in the Start section of the Home tab. The pop-up window shown in Figure

8 will appear. Then the entire geometry must be selected. Each feature will change color

33 once selected, also shown in Figure 8. After confirming all the geometry is selected, selecting the OK button completes the promotion process.

Figure 8: Promote Model Geometry to Finite Element Model (FEM)

After the model is promoted, the FEM needs to be prepared for mesh generation. In order to gain access to the necessary toolbars for mesh preparation and generation, the FEM file name in the Simulation Navigation panel is double-clicked. This causes NX to switch from the simulation toolbars to the FEM toolbars. The FEM toolbars and how the FEM surface model appeared for the aluminum cradle design of this study are visible in Figure

9. 34

Figure 9: FEM Manipulation and Toolbar and Navigation

3.2.2.3 Manipulating Surfaces for Optimal Mesh Generation

Once access to the FEM toolbox is achieved the first step to having an FEM that is adequately prepared for mesh generation is stitching edges. Stitching edges is a tool used to merge any surfaces that may have been generated separately, for ease of modeling, but will be comprised of a single piece when the part is fabricated. An example of this is found on the back plate that extends from the left to the right side of the aluminum cradle design. This plate is fabricated of a single piece of 1/4" aluminum, but in order to apply the loading forces from the sway (or anti-roll) bar, the surface was split into sections representing the rectangular area to which the sway bar bushings mount. This split is

35 needed for loading, but it is important that the simulation treat these splits in the surface as if the plate were mechanically one piece. Thus the Stitch Edge command is used. This process is initiated by selecting the button shown in Figure 10, which is located in the

Polygon Geometry section on the Home tab of the tool bar.

Figure 10: Stitch Edge Icon

When the Stitch Edge button is selected, the settings window shown in Figure 11 appears and all edges are highlighted with a magenta coloring, which is also visible in Figure 11.

Figure 11: Stitch Edge Selection Veiw

The first setting to consider for stitching surfaces is the method options this defaults to

Automatic, which is suitable for the majority of geometry. The second setting is the type of geometry. For this example, the geometry consists of two adjacent edges, so edge to edge is used. Then, the Select Body button is selected allowing the desired bodies for stitching together to be identified. Figure 12 shows that the rear plate is simulating four separate plates before edge stitching is applied.

Figure 12: Edges to be Stitched Together

The four separate surface bodies change to orange as each is selected, and the Select

Body line in the settings indicates the total number bodies selected, as appears in Figure

13.

Figure 13: Stitch Edge Selection

The remaining settings for the stitch edge process allows the exclusion of some edges, in the event that not all connecting edges between the selected bodies require stitching, and tolerance parameters to control how much gap between edges the stitching process will allow when determining if two edges of selected surface bodies are adjacent.

Once all settings are complete, the stitch edge process is applied, and the surfaces that are stitched together will simulate a single pieces with homogeneous mechanical properties.

39 Selecting the Stitch Edge button again will make the open edges visible, once again denoted by magenta. For the example, Figure 14 shows the edges previously stitched no longer are highlighted with magenta, indicating the surface bodies joined at these edges simulate a single plate as oppose to the four plates simulated prior to edge stitching.

Figure 14: Stitch Edge View After Edges are Sticthed.

40 After completing the stitch edge process on all necessary surface bodies, an additional review of the entire geometry with the free edges highlighted is advisable. Figure 15 shows this view for the aluminum cradle design after completing all edge stitching processes on that design. Figure 15 also shows for the aluminum cradle design, edge stitching was required for the forward C-channel cross brace because this body was modeled as a mirrored symmetrical body for ease of modeling, but is fabricated out of a plate that extends the length of the cradle, as opposed to the two halves originally represented by the model.

Figure 15: Veiw After Stitching Completion

42 3.2.2.4 Creating Points for Load and Constraint Application

The loading scenarios described in the following chapter describe how the mounting points for the suspension components are loaded with three axial force components and three axial moment components. In order to accurately apply the loading moments, a point is generated at the center of the mounting geometry and then connected to the mounting interfaces via rigid elements, which will transfer the loads from the point to the geometry without absorbing any of the energy of the loading. This section describes the process used to generate these points that are used for applying the loads.

The first step in the process is the selection of the create point icon, which is shown in

Figure 16.

Figure 16: Create Point Icon

Once the create point icon is selected the point generation settings window appears on screen as shown in Figure 17.

Figure 17: Creating Point at Center of Arc

Figure 17 shows the camber link mounting geometry for the aluminum cradle design.

Because the mounting geometry for this is comprised of two slotted holes, the optimal load point is on the axis created by two points centered in each slot halfway between the two slotted surfaces. Creating this loading point requires several other points to be generated to properly position the load point.

The first point is created at the center of one of the four radii making up the two slotted holes. This is accomplished by using the Inferred Point setting in the Type section of the point generation setting, which generates the desired point the majority of the time. In

44 this case, the program correctly inferred that when the radius was selected, the center of the curve was the desired point. The program generates a preview of the point location in the form of an orange sphere. Selecting Apply in the point generation settings creates the point, which is represented by a blue plus (+) symbol, and resets the settings for generating additional points. In the example of the camber link mounting geometry, an additional point is placed at the center of the opposite radius. Then the Type in the point generation setting is changed from Inferred Point to Between Two Points, which allows the selection of two different existing points and locates a new point at the midpoint between the two existing points. Figure 18 shows the two selected existing points and the preview of the new point located between the other two points.

Figure 18: Creating Point Between Two Points

45 After creating the midpoint shown in Figure 18 the process is repeated on the other slotted hole shown in this example. Then the same point between two points method is used to create the point between the two generated midpoints creating the final loading point shown in Figure 19.

Figure 19: Creating Point in Center of Bolt Location

The completed loading point is visible as a generated point (+) in Figure 20. This final loading point represents the center of the bolt location where the camber link is attached to the rear cradle.

Figure 20: Created Point for Load Application

This point generation process is used to generate all the load points at the center of the suspension components mounting points. This process is also used to generate points at the center of the bushing sleeves that hold the bushings used to mount the cradle to the vehicle chassis. Figure 21 shows one of the points generated at the center of the bushing sleeve by generating points at the center of the upper and lower edge of the cylinder, and creating a point between those two points.

Figure 21: Point Create in Center of Bushing Sleeve

This process was completed at all constraining and loading geometries, creating points at the center of each cradle interface location as shown in Figure 22.

Figure 22: Points Generated at Center of All Loading and Constraining Geometries

3.2.2.5 Ridged Connections for Simulating Weld and Bushing Connections

In order to simulate the joining points between surface bodies, 1D connections are used.

The following describes the process used to create the recipes that guide the generation of the 1D connections when mesh generation is applied to the adjacent surface bodies. 1D connections are single line elements used to connect a point or edge to another point, edge, or surface. This analysis uses these 1D connections to connect bodies that are welded together, or have bolted or bushing connections. 1D connections are the simplest form of elements, so using these elements to connect the interfaces of the model greatly

49 reduces the complexity of the finite element equation set and improves the stability of the model, without significantly impacting the accuracy of results.

The first step in generating 1D connection is selecting the 1D Connection icon shown in

Figure 23. This icon is located in the Connections section on the Home tab of the toolbar.

Figure 23: 1D Connection Icon

The selection of the 1D Connection icon causes the 1D connection settings window to open. This window is visible in the top left corner of Figure 24. In order to describe the process of joining to surface bodies to simulate a welded joint the connection between the cradle bearing sleeve and the rear support tube is used as an example and is shown in

Figure 24.

Figure 24: 1D Connection Settings Edge Selected for Weld Simulation

Since this connection is between the outer surface of the bearing sleeve and the edge of the support tube, the Type setting for the 1D connection is set to Edge to Face. The source is then selected, in this example the edge of the tube highlighted in orange in

Figure 24. Then the Target is selected, the face of the bearing sleeve in this example, which is shown in Figure 25.

Figure 25: 1D Connection Settings Face Selected for Weld Simulation

After selecting the Target the settings are finalized by selecting OK or Apply, and the connection recipe is generated. The connections are represented by a set of light blue dots between the two features to be joined during mesh generation. This processes is repeated throughout the cradle design to create 1D connections at all locations that are joined by welding in the fabrication of the cradle.

Another type of connection in the cradle that 1D connections are used to simulate is the connections between the load and constraint points generated in the previous section and the interfacing surfaces of the cradle structure. One example of this kind of connection is 52 between the constraint point in the center of the bushing sleeve and the inner surface of the bushing sleeve. In this example, a Point to Face type of connection is used to connect the constraint point to the bushing sleeve, as is shown in Figure 26.

Figure 26: 1D Connection - Point to Face for Bushing Simulation

This 1D connection is simulating the function that is achieved by the bushing in the actual fabrication of the cradle design. Similarly, 1D connections are also created to

53 connect the load points generated in the previous section to the edges of the bolted joints of the suspension component mounting geometries.

3.2.3 Mesh Generation

After the proceeding FEM preparation is completed the FEM is ready for 2D mesh generation. This process consists of generating a series of quadrilateral surface elements across the entire FEM geometry. These elements create the links and nodes that define the equation set used to calculate the stress and displacement the loads create in the cradle geometry. The properties of these elements define the material properties and wall thickness of the material. This makes it critical that the correct properties are assigned to the correct surface bodies in order for the results to be accurate. For example, if the wrong properties, such as 1/2” thick high strength steel are assigned to a surface that is intended to be 1/4" thick aluminum in the fabricated design, the simulation will show significantly lower stress in that surface than would actually be experienced by that part of the cradle design in vehicle use. This could result in a potential unsafe geometry being used in the final design. For this reason, collectors are used to organize groups of surfaces with identical material properties together. This makes identifying which geometries are assigned which properties very clear, and makes reviewing or changing material properties between design iterations efficient.

54 3.2.3.1 Mesh Collector Setup

Before generating the 2D meshes, it is most efficient to set up the mesh collectors. This is accomplished by selecting the Mesh Collector icon shown in Figure 27.

Figure 27: Mesh Collector Icon

Selecting the Mesh Collector icon causes the mesh collector settings window to appear on screen. The mesh collector settings window is shown in Figure 28, and the Element

Topology settings are set to the correct values for the analysis conducted on the rear cradle designs. The only exception to this is the threaded boss on the OE and Modified designs that uses 3D Element Family and Solid Collector Type.

Figure 28: Mesh Collector Settings

3.2.3.2 Element Simulated Physical Properties

For the Physical Property section of the mesh collector settings, the PSHELL type is used for all collectors using 2D elements in this analysis. The specification of the PSHELL properties is set by selecting the wrench symbol in the Physical Property section. Once the wrench symbol is selected the PSHELL settings window shown in Figure 29 appears on screen.

Figure 29: PSHELL Properties

The name and label for the PSHELL are used for selecting a specific PSHELL property set in the collector setup. For this analysis, the name and properties were simply left in default state where each progressive PSHELL is numbered one higher than the previous

PSHELL. For the properties, Figure 29 shows the properties used to simulate the 1/4" thick 6061 Aluminum. The material and default thickness are changed to reflect the appropriate material and thickness of the geometry being assigned to each surface body.

57 As each mesh is generated, it is assigned to a mesh collector, so the PSHELL assigned to each mesh collector determines the simulated thickness and material of all the meshes stored within that mesh collector.

For the aluminum cradle design, the entire geometry is comprised of 6061 aluminum, but four different material thicknesses are used with the structure. The material thickness types are each assigned to a different 2D mess collector for the simulation. The collectors are named 1.5" Tube, 3mm collar, 1/2" plate, and 1/4" plate. These collectors specify the wall thickness for the tubes connecting the chassis bushing mounts to the plate structure, the collars the chassis bushing mounts press into, the 1/2" thick plate used in the main structure, and the 1/4" plate used in the main structure, respectively. These four different

2D mesh collectors are visible in the 2D collectors section of the Simulator Navigation panel in Figure 30.

Figure 30: Collector Setup for Aluminum Cradle

After all collectors are created and all materials and thickness intended for use in the design are loaded in to the proper PSHELL and assigned to the correct mesh collector, the actual meshes can be generated using the following procedure.

3.2.3.3 2D Mesh Generation

To generate a 2D mesh, the 2D Mesh icon, Figure 31, is selected. This opens the 2D mesh settings window shown in Figure 32.

Figure 31: 2D Mesh Icon

Figure 32: 2D Mesh Settings

60 The first portion of the mesh collector settings is selecting the object or objects to mesh.

All objects selected must be the same material and wall thickness, since each mesh generation can only be assigned to a single mesh collector. When using the FEA as an iteration validation check, as part of a multiple iteration design process, as is done for this study, it is prudent to only mesh very similar bodies, such as mirror bodies of a mostly symmetrical design, in the same mesh command. The reasoning behind this method is that some parts of the design will likely change from one design iteration to the next and any objects changes will need to have the corresponding mesh regenerated, so minimizing the number of objects sharing a single mesh generation will reduce the amount of meshes needing regenerated between design iteration.

Once the necessary objects are selected the element type is selected. For all the 2D analysis in this study the CQUAD4 element was used. This is a quadrilateral plate with membrane-bending or plain strain behavior[12]. This it is the most robust and versatile element for use on this kind of analysis, given the settings and geometry used.

The mesh parameters section is when the element size is specified. Smaller element sizes provide more accurate results, but increase the number of equations the solver needs to process, so if the elements are set too small the solver will take an excessive amount of time to solve the model or may run out of memory and not be able to produce results at all. For the analysis conducted in this study an element size of 2 mm was selected because it was small enough that two or more elements are needed to span the material

61 thickness of the all the pieces of the design, which is generally considered a good starting point for element sizing using the settings identified in this chapter.

After setting the mesh parameters the last section of the 2D mesh settings requires the selection of the destination mesh collector for the newly generated mesh. In this section one of the mesh collectors created in the previous section is selected based on the desired material and thickness properties the object is to be assigned. For the example, in Figure

32, the collector is chosen that will give any selected bodies the material properties of

6061 aluminum and a thickness of 1/2". After these settings are complete and the object or objects are selected. Clicking the Show Results button with generate a preview of the mesh and Apply or OK will create the finalized mesh. Figure 33 shows a portion of the cradle geometry after all meshes are generated. Each element is outlined in thin green lines and the dense green at the connections, between surfaces are the ridged connection that generate once both adjacent bodies are meshed, according to the recipe settings described in 3.2.2.5.

Figure 33: 2D Mesh Generated with 1D Connections on Aluminum Cradle Design

After creating all meshes the FEM is ready for simulation, additional settings are still needed. These settings accessed through the simulation toolbar, which is accessed by going to the window selection drop-down located above the toolbars and opening and activating the simulation file created with the FEM in 3.2.2.2, or opening the simulation file (.sim) through the File tab.

3.2.4 Simulation Constraints and Loading Application

The meshes are the framework used to create the equations, that when solved, maps the stress and displacement in the structure, but the loads and constraints are the inputs and boundaries that drive those equations. This section describes the process used to apply constraints and loads to the analysis. Figure 34, shows the four constraints located in the

63 center of the bushing sleeves used to mount the cradle to the vehicle chassis, and several of the loading forces and moments being applied to the center of the mounting points.

Figure 34: Overview of Loads and Constraints

3.2.4.1 Constraints

The four constraints are created by selecting the Constraint Type icon located in the

Loads and Constraints section on the Home tab of the simulation toolbar. The Constraint

Type icon appears as shown in Figure 35.

Figure 35: Constraint Icon

For this study, the User Defined option is used from the dropdown menu that appears when the Constraint Type icon is selected. This opens the User Defined Constraint settings shown in Figure 36.

Figure 36: Constraint Settings for Fixed Points Simulating Chassis Mounts

The constraint points created in 3.2.2.4 are selected as the objects to be constrained. Since the cradle is mounted to the vehicle chassis by rubber bushings, which allow some rotational movement, only the three translational Degrees of Freedom (DOF) are fixed while the three remaining rotational DOF are left free to simulate the rotation allowed by

66 the bushings. Once the constraints are created three blue lines with "x" at the end extend from the constraint point indicating the three DOF that are now fixed at that point.

3.2.4.2 Loads

The loads which are defined and discussed in detail in 0are created by first selecting the

Load Type icon, Figure 37, located next to the Constraint Type icon.

Figure 37: Load Icon

From the drop-down menu that appears when the Load Type icon is selected, the Force and Moment loads are used in this simulation. Selecting force causes the force setting window to appear as shown in Figure 38.

Figure 38: Settings for Force Loading

In this study, the Components type is used on all loading points for both force and moment loads. The load points created in 3.2.2.4 are each loaded with the component values defined in CHAPTER 4, by selecting the point and entering the defined components in the Components portion of the settings.

This process is then repeated on each loaded point selecting moment instead of force for the load type, which opens the moment settings shown in Figure 39.

Figure 39: Settings for Moment Loading

After all suspension mounting points have the correct force and moment components applied, the simulation is ready to be solved.

3.2.5 Solving Simulation

Solving the simulation is accomplished by selecting the Solve icon, Figure 40, located in the Solution section of the Home tab on the solution toolbar.

Figure 40: Solve Icon

Once the Solve icon is selected the solve menu will appear as shown in Figure 41.

Figure 41: Solve Menu

The model setup check is useful to have checked, as it will identify most issues that may cause the solver to fail prior to running the solver. This is useful because without this check some errors may cause the solver to process for a long period of time prior to determining an error and failing to produce results. Once the check completes without 70 any failures the solver will run even if some warnings exist. Due to the nature of the model setup check algorithm, some warnings are common and do not indicate any loss of accuracy in the results, just areas of note if issues arise.

3.2.6 Post-Processing Results

For scenarios in this study most runs take between 20 and 30 minutes to complete. After the solver finishes, the results are examined by opening the Post Processing Navigator, located as shown in Figure 42.

Figure 42: Stress Results

In order to examine the stress results, the Von-Mises highlighted in Figure 42 is selected.

In addition, the displacement results are viewable by selecting the Magnitude highlighted in Figure 43.

Figure 43: Displacement Results

Once selected, the results appear in a state that is not clear for understanding and interacting with the results map, as shown in Figure 44.

Figure 44: Results View Prior to Clean-up

The displacement map in Figure 44 appears difficult to interpret because the element outlines are masking the shading and the deformation is exaggerated. Both these issues are adjustable by selecting and editing the Post View, shown in Figure 45.

Figure 45: Results Post View Selection

In the Post View window that appears, Figure 46, un-checking the Deformation check box removes the exaggerated deformation, making the view clearer to interpret.

Figure 46: Post View Settings Display Tab

75 After un-checking the deformation setting, selecting the Edges & Faces tab provides access to the setting in Figure 47. Changing the Edges setting from opaque to none removes the grid lines that distort the results map.

Figure 47: Post View Settings Edges & Faces Tab

Also, the Legend tab settings can be used to change the maximum, minimum, and increments of the legend. This is useful when trying to identify areas that exceed a certain stress or displacement value. Setting the maximum legend value to the value not to be exceeded will cause all elements of the structure exceeding that value to turn red.

76 Figure 48 shows the same results and view as Figure 44 after the previously described clean-up actions are taken.

Figure 48: Results Veiw Post Clean-up

In this study, the same model is analyzed 15 different times with only the force and moment components changed between each one. An efficient way of accomplishing this is cloning and renaming the existing simulation and changing the load values before solving. By renaming the clone to a different solution, the results for the original solution 77 will be preserved and only the load values need changed before the new solution can be solved to provide the results of the new loading scenario. This process is repeated 14 times, naming each solution in accordance with the loading scenario it represents. After creating all 15 solutions the Simulator Navigation panel appears as shown in Figure 49.

Figure 49: Cloned Solution with Altered Loading

After all 15 solutions are run through the solver, corresponding results appear in the Post

Processing Navigator panel as shown in Figure 50.

Figure 50: Post Processing View of Multiple Scenerio Results Stored

This method is useful because after completing all 15 load cases, all results are still readily available for review. Selecting any result set and activating it allows access to drill down to the displacement or stress results for review or measurement.

80 3.3 Manufacturing Technologies

The fabrication method for both the OE modified cradle and aluminum cradle use the same approach. Using the base OE cradle from the LaCrosse as a template, a fixture is fabricated around the existing OE cradle with interfacing bolt connections and spacers holding the locations of all the suspension and chassis bushing mounting points. This fixture is anchored to a large steel plate that serves to simulate the minimum ground clearance plane for the design. Figure 51 shows this fixture as the aluminum cradle design is beginning to be assembled.

Figure 51: Fixture to Maintaine Suspension and Chassis Mounting Locations

81 Another method employed for ensuring the interfaces between laser-cut plates are located accurately, is modeling slots on one plate and interfacing tabs on the adjacent plate. This leverages the accuracy of the CNC laser-cutting table, which can cut with an accuracy of

±.005", to locate the panels and reduces the amount of time spent fabricating the fixture, since this method makes the parts self-fixturing for many of the part interfaces. An example of this slot and tab interface is visible by noticing the vertical rectangular slots located on the rear plate located in the upper right portion of Figure 51. These slots interface with tabs in the plates running front to rear shown in Figure 52.

Figure 52: Laser Cut Plates Tab and Slot Make the Assembly Self-Fixturing to a Degree

CHAPTER 4: SIMULATION LOADING SCENARIOS

4.1 Loading Conditions

In order to assure that the rear cradle design could withstand the wide variety of loading conditions capable of being generated during the majority of possible driving scenarios,

12 loading scenarios were provided by the EcoCAR 2 organizers to represent the loads the suspension components could transmit into the rear cradle design. Because neither of the rear cradles designed for this vehicle were symmetrical, it was necessary to simulate scenarios on both right and left sides. For example, the one-wheel bump scenario had to be simulated for both the right and left wheel. If the cradle design had been symmetrical, only one of the two simulations would have been needed to validate the design against a one-wheel bump. The rear electric powertrain components and vehicle geometry made it so a symmetrical design was not feasible, so the one-wheel bump, twist, and cornering scenarios all had to be simulated on both the right and left side. The tables in this section show the loading components for the various scenarios using F to denote force in

Newtons and T to denote moments, or torque, in Newton-meters. The sub-letters x, y, and z denote the axial direction of the force or moment, and the sub-letter m denotes the total magnitude of the cumulative force or moment components at each loading point. The

SAE Vehicle Axis System was used for this analysis and is defined relative to general vehicle structure in Figure 53. 83

Figure 53: SAE Vehicle Axis System [13]

In Table 4 through Table 18 the moment coordinates are denoted by the axis the moment revolves around, so the Roll (p), Pitch (q), and Yaw (r) components are denoted by Tx,

Ty, and Tz, respectively. This nomenclature is used to more closely align with the NX software's interface for moment component entry, which uses the nomenclature Mx, My, and Mz to denote the roll, pitch, and yaw moments, respectively.

84 4.2 Two-Wheel Bump

The Two-Wheel Bump loading scenario simulates both rear wheels hitting a bump simultaneously, similar to the loading seen when a vehicle travels over a speed bump at high speed. The loading forces and moments for this scenario are shown in Table 4.

(1) Two-Wheel Bump Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 456 -3254 1199 -15 -10 6 3497 19 Toe Link, Right 456 3254 1198 15 -10 -6 3498 19 Camber Link, Left -251 8743 -2711 -25 -33 5 9157 41 Camber Link, Right -251 -8744 -2709 24 -33 -5 9157 41 H-Arm, Rear, Left 51 -4927 -3281 -38 -11 18 5920 43 H-Arm, Rear, Right 52 4926 -3280 38 -11 -18 5919 43 H-Arm, Front, Left 1344 -4102 1583 -44 -13 13 4597 48 H-Arm, Front, Right 1344 4104 1581 44 -13 -13 4599 48 Sway Bar, Left -217 2404 250 0 0 0 2427 0 Sway Bar, Right -217 -2404 246 0 0 0 2426 0

Table 4: Two-Wheel Bump Loading Components

4.3 One-Wheel Bump

The One-Wheel Bump loading scenario simulates one rear wheel hitting a bump while the other rear wheel continues travel on flat terrain. The loading forces and moments for this scenario where the bump is acting on the left-hand side wheel and right-hand side wheel are shown in Table 5 and Table 6, respectively.

85 (2) One-Wheel Bump - LHS Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 738 -5522 1953 -15 -13 8 5904 21 Toe Link, Right 157 1019 364 13 -6 -3 1093 14 Camber Link, Left -415 14196 -4580 -25 -37 8 14923 46 Camber Link, Right -111 -3378 -777 20 -23 -1 3469 30 H-Arm, Rear, Left 299 -6527 -1567 -38 -13 24 6719 47 H-Arm, Rear, Right -169 2901 -4499 32 -6 -7 5356 33 H-Arm, Front, Left 2320 -7479 426 -44 -15 17 7842 50 H-Arm, Front, Right 529 1260 2323 36 -9 -6 2695 37 Sway Bar, Left -54 2104 1071 0 0 0 2362 0 Sway Bar, Right -321 -1901 -643 0 0 0 2032 0

Table 5: One-Wheel Bump Left-hand Side Loading Components

(3) One-Wheel Bump - RHS Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 157 -1019 364 -13 -6 3 1093 14 Toe Link, Right 739 5523 1952 15 -13 -8 5904 21 Camber Link, Left -111 3378 -777 -20 -23 1 3468 30 Camber Link, Right -415 -14196 -4578 25 -37 -8 14922 46 H-Arm, Rear, Left -169 -2901 -4500 -32 -6 7 5356 33 H-Arm, Rear, Right 299 6527 -1568 38 -13 -24 6719 47 H-Arm, Front, Left 529 -1259 2323 -36 -9 6 2695 37 H-Arm, Front, Right 2320 7481 424 44 -15 -17 7844 50 Sway Bar, Left -321 1900 -640 0 0 0 2030 0 Sway Bar, Right -54 -2103 1069 0 0 0 2359 0

Table 6: One-Wheel Bump Right-hand Side Loading Components

86 4.4 Twist: Jounce and Rebound

The Twist loading scenarios simulates one wheel being pushed upward toward the body of the vehicle, a motion referred to a jounce, and the other wheel is simultaneously allowed to travel downward extending the suspension, a motion referred to as a rebound.

This loading is similar to the loading seen when a vehicle travels over off road terrain with steep side slopes. The loading forces and moments for this scenario are shown for twisting in both directions in Table 7 and Table 8.

(5) Twist - LHS Rebound, RHS Jounce Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -11 37 -29 5 4 -1 49 7 Toe Link, Right 999 7802 2565 14 -16 -8 8273 22 Camber Link, Left 30 -368 -76 8 11 1 377 14 Camber Link, Right -583 -19603 -6165 24 -40 -9 20558 47 H-Arm, Rear, Left 0 105 -1181 13 5 -2 1185 14 H-Arm, Rear, Right 279 8993 -525 36 -14 -27 9012 47 H-Arm, Front, Left 9 135 393 15 6 -2 416 16 H-Arm, Front, Right 3063 10372 -731 42 -15 -19 10839 49 Sway Bar, Left 3 2518 -4762 0 0 0 5387 0 Sway Bar, Right 304 -3627 4985 0 0 0 6173 0

Table 7: Twist - Left-hand Side Rebound, Right-hand Side Jounce Loading Components

87 (4) Twist - LHS Jounce, RHS Rebound Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 1000 -7803 2566 -14 -16 8 8275 22 Toe Link, Right -11 -38 -29 -5 4 1 49 7 Camber Link, Left -583 19606 -6168 -24 -40 9 20561 47 Camber Link, Right 30 368 -77 -8 11 -1 377 14 H-Arm, Rear, Left 279 -8994 -524 -36 -14 27 9013 47 H-Arm, Rear, Right 0 -105 -1180 -13 5 2 1185 14 H-Arm, Front, Left 3063 -10373 -730 -42 -15 19 10840 49 H-Arm, Front, Right 9 -135 393 -15 6 2 416 16 Sway Bar, Left 305 3629 4986 0 0 0 6175 0 Sway Bar, Right 3 -2520 -4763 0 0 0 5388 0

Table 8: Twist - Left-hand Side Jounce, Right-hand Side Rebound Loading Components

4.5 Forward Braking

The Forward Braking loading scenario simulates heavy braking being applied to both rear wheels simultaneously, while the vehicle is traveling at high speed in the forward direction, similar to the loading seen when a vehicle has to brake suddenly when coming to an unexpected red light or traffic jam. The loading forces and moments for this scenario are shown in Table 9.

88 (6) Forward Braking Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 743 -4089 1296 -13 -14 19 4353 27 Toe Link, Right 743 4088 1296 13 -14 -19 4353 27 Camber Link, Left -214 5846 -1623 -22 -38 21 6071 49 Camber Link, Right -214 -5846 -1623 22 -38 -21 6070 49 H-Arm, Rear, Left 2350 7611 -1179 -33 -14 31 8053 48 H-Arm, Rear, Right 2350 -7612 -1179 33 -14 -31 8053 48 H-Arm, Front, Left 4339 -11471 -2570 -40 -15 21 12531 47 H-Arm, Front, Right 4340 11472 -2571 40 -15 -21 12532 47 Sway Bar, Left -166 1886 206 0 0 0 1905 0 Sway Bar, Right -166 -1886 205 0 0 0 1905 0

Table 9: Forward Braking Loading Components

4.6 Reverse Braking

The Reverse Braking loading scenario simulates heavy braking being applied to both rear wheels simultaneously, while the vehicle is traveling in the reverse direction, similar to the loading seen when a vehicle has to brake suddenly when backing up out of a driveway and an unexpected vehicle moves into the path. The loading forces and moments for this scenario are shown in Table 10.

89 (7) Reverse Braking Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -171 1119 -221 -13 -2 -12 1154 18 Toe Link, Right -171 -1119 -221 13 -2 12 1154 18 Camber Link, Left -125 2328 -467 -19 -15 -18 2377 30 Camber Link, Right -125 -2328 -467 19 -15 18 2377 30 H-Arm, Rear, Left -2021 -9921 -7020 -32 -1 -15 12320 36 H-Arm, Rear, Right -2021 9921 -7020 32 -1 15 12321 36 H-Arm, Front, Left -1672 4566 5751 -34 -6 -8 7531 36 H-Arm, Front, Right -1672 -4566 5751 34 -6 8 7531 36 Sway Bar, Left -200 1426 172 0 0 0 1450 0 Sway Bar, Right -200 -1426 172 0 0 0 1450 0

Table 10: Reverse Braking Loading Components

4.7 Cornering

The Cornering loading scenarios simulate traveling around tight turns at a velocity close to the maximum the vehicle can achieve while maintaining control though the entire turn.

This scenario is similar to those seen by a vehicle traveling through a round-about at speeds above the recommended limits. The loading forces and moments for this scenario turning both to the left and the right are shown in Table 11 and Table 12, respectively.

90 (8) Cornering - Left Turn Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 197 -1559 60 -1 0 2 1573 2 Toe Link, Right -765 -6545 -2040 16 -3 -7 6898 17 Camber Link, Left -138 4289 -191 -2 -1 3 4295 4 Camber Link, Right 260 13508 4181 23 -25 2 14142 33 H-Arm, Rear, Left -361 -3809 -2683 -3 1 6 4673 7 H-Arm, Rear, Right 1078 -11326 -8172 38 -6 8 14008 39 H-Arm, Front, Left 401 -1524 919 -3 0 4 1824 5 H-Arm, Front, Right -173 -3310 1805 42 -10 3 3774 43 Sway Bar, Left -135 1136 -3064 0 0 0 3271 0 Sway Bar, Right 204 -1875 3235 0 0 0 3744 0

Table 11: Cornering - Left Turn Loading Components

(9) Cornering - Right Turn Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -765 6546 -2041 -16 -3 7 6899 17 Toe Link, Right 197 1559 60 1 0 -2 1573 2 Camber Link, Left 260 -13508 4182 -23 -25 -2 14143 33 Camber Link, Right -138 -4289 -191 2 -1 -3 4295 4 H-Arm, Rear, Left 1078 11326 -8173 -38 -6 -8 14009 39 H-Arm, Rear, Right -361 3809 -2684 3 1 -6 4673 7 H-Arm, Front, Left -173 3310 1805 -42 -10 -3 3774 43 H-Arm, Front, Right 401 1524 919 3 0 -4 1824 5 Sway Bar, Left 204 1876 3236 0 0 0 3746 0 Sway Bar, Right -135 -1136 -3065 0 0 0 3272 0

Table 12: Cornering - Right Turn Loading Components

4.8 Forward Acceleration

The Forward Acceleration loading scenario simulates the loads applied to the cradle when the vehicle accelerates quickly in the forward direction, similar to when a driver 91 punches the gas when a light turns green trying to get up to speed limit quickly. The loading forces and moments for this scenario are shown in Table 13.

(10) Forward Acceleration Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -174 762 -141 -12 -13 -16 794 24 Toe Link, Right -174 -765 -143 13 -13 16 798 24 Camber Link, Left -149 2745 -638 -21 -34 -26 2822 47 Camber Link, Right -149 -2742 -641 21 -34 26 2820 47 H-Arm, Rear, Left -1535 -9886 -2824 -34 -9 -15 10395 38 H-Arm, Rear, Right -1536 9887 -2822 34 -9 15 10397 39 H-Arm, Front, Left -1983 4196 688 -38 -11 -8 4692 40 H-Arm, Front, Right -1987 -4207 692 38 -11 8 4704 41 Sway Bar, Left -234 1952 194 0 0 0 1975 0 Sway Bar, Right -228 -1957 232 0 0 0 1983 0

Table 13: Forward Acceleration Loading Components

4.9 Reverse Acceleration

The Reverse Acceleration loading scenario simulates the loads applied to the cradle when the vehicle accelerates quickly in the reverse direction, such as backing quickly out of a parking spot. The loading forces and moments for this scenario are shown in Table 14.

92 (11) Reverse Acceleration Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 513 -2559 725 -12 -1 16 2708 20 Toe Link, Right 515 2565 713 12 -1 -16 2712 20 Camber Link, Left -142 4321 -937 -18 -15 19 4424 30 Camber Link, Right -143 -4320 -921 18 -14 -20 4419 30 H-Arm, Rear, Left 699 2172 -5903 -28 -4 22 6329 35 H-Arm, Rear, Right 703 -2174 -5901 27 -4 -22 6328 35 H-Arm, Front, Left 2487 -5647 3460 -32 -8 15 7074 36 H-Arm, Front, Right 2494 5676 3431 31 -7 -15 7086 35 Sway Bar, Left -116 984 184 0 0 0 1008 0 Sway Bar, Right -122 -971 79 0 0 0 982 0

Table 14: Reverse Acceleration Loading Components

4.10 Max Torque

The Maximum Torque loading scenario simulates the loads transmitted into the cradle when the powertrain generates and transmits maximum torque to the wheels. The loading forces and moments for this scenario in the forward and reverse direction are shown in

Table 15 and Table 16, respectively.

93 (12) Max Torque - Forward Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -321 2379 -477 -9 -13 -24 2447 29 Toe Link, Right -321 -2381 -481 10 -13 24 2450 29 Camber Link, Left -216 761 -72 -16 -30 -36 794 50 Camber Link, Right -216 -762 -72 16 -30 36 795 50 H-Arm, Rear, Left -2260 -13512 -1960 -28 -7 -23 13840 37 H-Arm, Rear, Right -2260 13510 -1950 28 -7 23 13836 37 H-Arm, Front, Left -3827 8638 -186 -30 -9 -13 9450 34 H-Arm, Front, Right -3829 -8642 -190 31 -9 13 9454 35 Sway Bar, Left -211 1270 146 0 0 0 1295 0 Sway Bar, Right -207 -1274 179 0 0 0 1303 0

Table 15: Maximum Torque in Forward Direction Loading Components

(13) Max Torque - Reverse Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 870 -3999 1089 -12 2 24 4235 27 Toe Link, Right 871 4006 1068 12 2 -24 4236 27 Camber Link, Left -220 5325 -1170 -18 -10 33 5456 39 Camber Link, Right -221 -5324 -1147 18 -10 -33 5451 38 H-Arm, Rear, Left 1575 7479 -7228 -27 -3 29 10519 40 H-Arm, Rear, Right 1581 -7485 -7213 27 -3 -29 10514 40 H-Arm, Front, Left 4610 -10464 4714 -31 -7 19 12368 37 H-Arm, Front, Right 4621 10504 4665 31 -7 -19 12388 37 Sway Bar, Left -91 879 178 0 0 0 901 0 Sway Bar, Right -96 -864 57 0 0 0 871 0

Table 16: Maximum Torque in Reverse Direction Loading Components

94 4.11 Reverse Bump

The Reverse Bump loading scenario simulates both rear wheels hitting a bump simultaneously while the vehicle is traveling in the reverse direction, similar to the loading seen when a vehicle travels over a speed bump at high speed in reverse. The loading forces and moments for this scenario are shown in Table 17.

(14) Reverse Bump Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left -315 2309 -489 -10 -14 -24 2381 30 Toe Link, Right -315 -2309 -488 10 -14 24 2381 30 Camber Link, Left -214 868 -101 -17 -32 -36 900 51 Camber Link, Right -214 -868 -101 17 -32 36 899 51 H-Arm, Rear, Left -2263 -13455 -1980 -29 -8 -23 13787 38 H-Arm, Rear, Right -2263 13455 -1980 29 -8 23 13787 38 H-Arm, Front, Left -3776 8500 -157 -32 -10 -13 9302 36 H-Arm, Front, Right -3776 -8500 -157 32 -10 13 9302 36 Sway Bar, Left -217 1414 174 0 0 0 1441 0 Sway Bar, Right -217 -1414 173 0 0 0 1441 0

Table 17: Reverse Bump Loading Components

4.12 Forward Impact

The Forward Impact loading scenario simulates the loading transmitted into the cradle when the vehicle impacts a large stationary or slow moving object while traveling in the

95 forward direction, similar to those forces seen when a car crashes head on into a concrete barrier. The loading forces and moments for this scenario are shown in Table 18.

(15) Forward Impact Load Point Fx [N] Fy [N] Fz [N] Tx [Nm] Ty [Nm] Tz [Nm] Fm [N] Tm [Nm] Toe Link, Left 1558 -6705 1872 -14 6 33 7133 36 Toe Link, Right 1558 6705 1872 14 6 -33 7133 36 Camber Link, Left -456 7413 -1732 -19 -4 50 7626 53 Camber Link, Right -456 -7413 -1732 19 -4 -50 7626 53 H-Arm, Rear, Left 2484 17611 -9921 -28 -1 39 20365 48 H-Arm, Rear, Right 2484 -17612 -9920 28 -1 -39 20366 48 H-Arm, Front, Left 9683 -19969 7366 -33 -6 25 23383 42 H-Arm, Front, Right 9683 19969 7366 33 -6 -25 23383 42 Sway Bar, Left -73 890 113 0 0 0 900 0 Sway Bar, Right -73 -890 112 0 0 0 900 0

Table 18: Forward Impact Loading Components

CHAPTER 5: OE MODIFIED CRADLE DESIGN

5.1 OE Modified Rear Sub-frame

The OSU EcoCAR 2 design requires the rear electric motor coupled to a single-speed transmission be integrated into the rear cradle structure. The vehicle architecture design requires the sockets for the output shafts of the single speed transmission to be centered between the wheels. The location also must be such that, when the drive shafts are connected to the wheels and single-speed transmission and the suspension is exercised from fully extended to fully compressed, the shafts will not interfere with cradle structure.

The rear subframe from the 2013 base vehicle mounts to the vehicle chassis in four places using a bolted rubber bushing interface. The rear subframe supports a suspension that utilizes a lower A-arm linkage, a toe linkage, a camber linkage, and a sway bar linkage. In order to maintain the Original Equipment (OE) suspension, all rear subframes produced by GM that utilized the same suspensions and chassis mount geometries are identified. After examining the geometries of each compatible subframe, the rear cradle used in the 2013 Buick LaCrosse with all-wheel drive is selected because it provides the least interference for integration with the rear electric powertrain, of the subframes considered.

97 5.2 Component Integration

Despite having the fewest interferences, the OE LaCrosse cradle does need modified in order to be compatible with the electric powertrain components. Figure 54 and Figure 55 show how, when optimally located, the electric machine is able to narrowly avoid interference with the OE cradle, but the single speed transmission intersects with both the front and rear cross members.

Figure 54: Rear View of Powertrain Interference on Stock Cradle

Figure 55: Front View of Powertrain Interference on Stock Cradle

99 5.3 Material Removal

In order to modify the OE cradle to be compatible with rear powertrain components the two cross members highlighted in Figure 56 are removed as shown in

Figure 57.

Figure 56: OE Cradle with Highlighted Crossmembers for Removal

100

Figure 57: OE Cradle with Cut Out Sections Removed

5.4 Reinforcing

After removing the two lateral cross-members, 1/4" thick laser-cut strips of A36 steel are added to the model to connect the two halves remaining of the OE cradle without infringing in the space claim of the rear electric powertrain. In addition to structurally connecting the two halves of the OE cradle, steel plates are oriented vertically in the proper location and with the needed hole patterns to mount the rear electric powertrain components in the proper place. The structure is then analyzed using FEA, and additional reinforcements are incorporated as needed to achieve the desired Factor of Safety (FOS) for the modified cradle relative to the OE cradle design. The final modified cradle geometry is shown in Figure 58 and Figure 59.

101

Figure 58: OE Modified Cradle Final Design - Rear View

Figure 59: OE Modified Cradle Final Design - Front View

102 5.5 Simulations

In order to determine if the modification to the OE cradle maintains the structural integrity of the original cradle design, FEA is performed on both original and the modified designs. The original cradle as manufactured by GM is used as a baseline for stress and displacement values to be maintained or improved upon by the modified OE cradle design. Both OE baseline cradle and OE modified cradle are analyzed using the 15 loading scenarios described in Chapter 4. The methodology and setting used to setup the

FEA simulations is described in detail in 3.2.

5.6 Result Processing

After the FEA solver completes processing the simulations for the various loading scenarios, the results are analyzed to determine the maximum values for each loading scenario. Due to the slight differences between the simplified model used in the FEA simulation and the actual geometry of the fabricated cradle, the results must be scrutinized to identify stress values that are higher than the actual part will experience in reality. One cause for these unrealistic high stress points is unusual element geometries in the FE mesh that create a sharper angle than the majority of elements due to the mesh interacting with a sharp point in the model geometry. This most often occurs at the intersection of two plates where the sharp corner occurring in the FEM is actually a fillet weld in the fabricated assembly. This is caused because the welds are modeled as rigid elements with infinite yield strength. For these localized stress peaks, the fillet weld and some micro strain hardening at the stress peak will cause the actual stress to be more distributed and lower than represented by the FEA displayed results. Siemens, who 103 created the FEA program used for this analysis, considers the first and second nodal rows to be unrealistic and dismissible when using rigid elements to model welds. An example occurrence of this unrealistic stress concentration is shown in Figure 60.

Figure 60: Unrealistic Weld Stress Occurance

Another cause of unrealistic stress concentrations occurs at the bolt mounting points of the suspension components. Since rigid elements are used to connect the loading point to the edge of the bolt interfacing hole, the elements apply tensile loading to the back side of the hole that in reality the bolt would shift away from and load the least instead of the most. Figure 61 shows an example of this bolted connection stress concentration occurrence. In Figure 61, the white arrow show the direction of the resultant force applied at the loading point.

104

Figure 61: Mounting Point Stress Concentration

Both of these types of unrealistic stress concentrations were dismissed for the establishing the maximum stress results in the analysis of the OE cradle, the OE modified cradle and the reduced mass aluminum cradle. The numerical results of the analysis for all three cradle designs is shown in 6.4.

105

CHAPTER 6: REDUCED MASS ALUMINUM CRADLE

6.1 Approach

In order to reduce the total weight of the OE modified cradle designed and created for year two of the EcoCAR 2 competition, the drivers of the designs weight are considered.

Two main drivers of weight in the design that can be altered by creating a new design are the strength to weight ratio of the material used in the construction of the cradle and the structure geometry. The structure of the modified cradle design has many areas that are overly reinforced because the OE portion of the modified cradle was designed to be optimal in its original state. With the removal of the two lateral crossmembers and several additional plates added to the structure, the load paths of the structure are changed resulting in portions that were optimally reinforced in the OE state of the cradle to become overly reinforced in the modified design. Thus, the year three cradle design is created from scratch using aluminum alloy 6061-T6 instead of steel, for its increased strength to weight ratio, roughly three times that of steel. This new aluminum cradle is also designed to provide structure for the necessary loading paths without having overly reinforced regions driving increased weight unnecessarily.

106 6.2 Component Integration

In order to maintain the original suspension geometry the new design is built by first overlaying all the mounting points that attach the cradle to both the chassis and the suspension linkage from the OEM model as shown in Figure 62.

Figure 62: Model Mounting Points and Constraints Multiple Viewing Angles

6.3 Model Development and Simulation

After defining all the mounting points that must be maintained, a combination of aluminum tube and 1/4" thick plate was modeled in to connect all the mounting locations 107 while maintaining all clearances provided by the previous cradle design to insure that the new cradle design will integrate into the rest of the Ohio State EcoCAR 2 vehicle design.

The first iteration was modeled with the minimal number of plates and tube necessary to create and connect the mounting points for the suspension, powertrain and vehicle chassis. This new design also incorporated part of the adapter plate, coupling the electric machine to the single speed transmission into the cradle structure. This helps distribute the weight of the powertrain more evenly across the cradle, and it utilizes the single- speed transmission as a structural member instead of leaving it as a structural avoidance area as is done in the OE modified cradle design.

After creating an initial structure the aluminum cradle design is processed using the same

FEA methods used on the previous OE modified and baseline cradle designs. After analyzing the new cradle structure and comparing the stresses and displacements of the aluminum design to both the other two cradle designs and yield strength of 6061 aluminum, some areas are reinforced by either thickening the plate to 1/2" thick material or including additional gusseting components to the design. The objective is to have the maximum stress in the aluminum cradle design be lower than the maximum stress identified in either of the other two cradle designs for each loading scenario. The other main objective for the aluminum cradle design is to have a FOS relative to material yield strength greater than or equal to two for all 15 loading scenarios. With these objectives driving the process, the aluminum cradle design undergoes nine design iterations with a

FEA conducted between each iteration and the results of each analysis driving the changes to the design structure for the next iteration. The ninth iteration FEA results 108 show that the objectives are met by this design and no additional design changes are needed prior to fabrication. Figure 63 through Figure 66 show the final design generated during the ninth design iteration of the aluminum cradle design.

Figure 63: New Design Comprised of Aluminum Plate and Tube - Upper Front View

109

Figure 64: New Design Comprised of Aluminum Plate and Tube - Lower Front View

Figure 65: New Design Comprised of Aluminum Plate and Tube - Upper Rear View

110

Figure 66: New Design Comprised of Aluminum Plate and Tube - Lower Rear View

6.4 Results and Comparison

The maximum established stress for the baseline OE cradle, the modified OE cradle, and aluminum cradle designs are listed in Table 19 and plotted graphically in Figure 67. This is called the maximum established stress because it is the highest stress value measured after dismissing the unrealistic stress values described in section 5.5.

111 Established Maximum Stress (Mpa) Loading Scenario OE Stock OE Modified Aluminum 1 Two Wheel Bump 226.1 76.55 103.2 2 One Wheel Bump – Left Hand Side 228.2 75.46 96.43 3 One Wheel Bump – Right Hand Side 237 75.17 101.8 4 Twist – LHS Jounce RHS Rebound 231.4 67.62 116.7 5 Twist – LHS Rebound RHS Jounce 270.5 79.62 112.6 6 Forward Braking 167 131.5 90.67 7 Reverse Braking 225 135.5 94.81 8 Cornering – Left Turn 239.1 60.63 117.6 9 Cornering - Right Turn 224.6 79.32 108.4 10 Forward Acceleration 190.7 112.6 60.07 11 Reverse Acceleration 286.5 48.23 94.1 12 Max Torque – Forward 250.4 137.7 83.72 13 Max Torque – Reverse 142.8 81.22 73.13 14 Reverse Bump 250.2 137.4 87.13 15 Forward Impact 218.8 143 118.4

Table 19: Established Maximum Stress

112

Figure 67: Maximum Established Stress Comparison Plot

Both the OE modified and aluminum cradle designs have maximum established stress values occurring in the 50 MPa to 150 MPa range, which is roughly half of the stress range of the baseline OE stock cradle, which has maximum established stress values in the 140 MPa to 290 MPa range.

Table 20 and Figure 68 show the ratios of the maximum established stress for the baseline stock cradle relative to the modified and aluminum cradle designs, numerically and graphically, respectively. The FOS, relative to the yield strength of the materials for the two new designs, is also shown in this table and figure. For both the stress ratio and

FOS high values indicate lower stress in the new designs relative to the OE baseline cradle stress values and material yield strength. 113

Stress Ratio to Stock Material FOS Loading Scenario OE Modified Aluminum OE Modified Aluminum 1 Two Wheel Bump 2.95 2.19 3.27 2.34 2 One Wheel Bump – Left Hand Side 3.02 2.37 3.31 2.51 3 One Wheel Bump – Right Hand Side 3.15 2.33 3.33 2.38 4 Twist – LHS Jounce RHS Rebound 3.42 1.98 3.70 2.07 5 Twist – LHS Rebound RHS Jounce 3.40 2.40 3.14 2.15 6 Forward Braking 1.27 1.84 2.32 2.67 7 Reverse Braking 1.66 2.37 2.25 2.55 8 Cornering – Left Turn 3.94 2.03 4.12 2.06 9 Cornering - Right Turn 2.83 2.07 3.15 2.23 10 Forward Acceleration 1.69 3.17 2.22 4.03 11 Reverse Acceleration 5.94 3.04 5.18 2.57 12 Max Torque – Forward 1.82 2.99 2.21 2.89 13 Max Torque – Reverse 1.76 1.95 3.08 3.31 14 Reverse Bump 1.82 2.87 2.22 2.78 15 Forward Impact 1.53 1.85 2.13 2.04

Table 20: Stress Ratio and FOS of OE Modified and Aluminum Cradle Designs

114

Figure 68: Stress Ratio to Stock and Material FOS Plot

Figure 68 shows that the OE modified cradle design has stress ratios and FOS ranging between 1 and 6. The aluminum cradle design ratios and FOS occur in a narrower range between 2 and 3. This is an indicator that the aluminum cradle is a more optimal design relative to the OE modified design.

The primary motivation for redesigning the cradle out of aluminum is to reduce the total weight of the rear cradle design. As part of the vehicle system wide weight reduction, the target weight reduction for the aluminum cradle design is to weigh 30% less than the OE modified cradle, used for year two of the competition. The projected weights of both the

115 rear cradle designs based on CAD data, and the actual weights from the fabricated assemblies are shown in Table 21.

Projected Weight Actual Weight Weight Relative to Design (lbs.) (lbs.) OE Modified (%) Baseline Cradle 60 60 54% OE Modified Cradle 107 112 100% Aluminum Cradle 65 66 59%

Table 21: OE Modified vs. Aluminum Weight Comparison

Table 21 indicates that the aluminum cradle design achieved an actual weight reduction of 41% relative to the OE modified cradle design. This is an 11% improvement beyond the target of 30% set for the design. Table 21 also indicates that the aluminum cradle is within 6 lbs. of the baseline cradle.

The maximum displacement values for each of the three designs across all 15 loading scenarios is summarized in Table 22.

116 Maximum Displacement (mm) OE Stock OE Modified Aluminum Maximum 1.84 0.77 1.80 Minimum 0.36 0.27 0.33 Average 0.91 0.54 0.88

Table 22: Maximum Displacement Summary

Based on the values in Table 22 the OE modified cradle had the least displacement at approximately 40% less on average than the OE stock cradle design. The aluminum cradle design is more similar in displacement to the OE stock design only averaging approximately 5% less displacement.

6.5 Vehicle Testing

For both the OE Modified and aluminum cradle, suspension testing is performed on the

OSU EcoCAR 2 vehicle at the Transportation Research Center (TRC) in their Vehicle

Dynamics Area (VDA). The vehicle is instrumented to measure the vehicle speed, driver line, three linear accelerations, and 3 rotational speeds of the vehicle through the test maneuvers. Some of the testing maneuvers conducted at TRC include Max Acceleration,

Maximum Braking, Maximum Lateral Acceleration, Steady State Turn, Double Lane

Change, and Large Bump Profile at Varying Speeds. [14]

117

Figure 69: The EcoCAR Vehicle Driving the Double Lane Change Test

Figure 70: The EcoCAR 2 Vehicle Driving Over a the Large Bump Ride Profile

118 These test maneuvers generate loading similar to the scenarios simulated in the FEA validation, so this is a good method for validating the FEA simulations results. For both the OE modified and aluminum cradle designs, after completing the suspension testing, the cradles are visually examined for any cracking or deformation as a result of the stress induced for testing. On both designs no issues were found reinforcing the validation done using FEA.

119

CHAPTER 7: CONCLUSIONS AND FUTURE WORK

7.1 Conclusions

In an effort to reduce weight in vehicle cradle designs, the use of 6061 aluminum alloy demonstrates great improvement in both strength to weight ratio and stiffness properties.

By changing from a steel weldment to an aluminum alloy weldment, the OSU EcoCAR 2 rear cradle design shows a weight reduction of 41%. This weight reduction is achieved while simultaneously reducing the maximum stress in the cradle and achieving a displacement that is closer to the stock baseline cradle produced and tested by GM.

Achieving a displacement more similar to the stock cradle with the aluminum cradle versus the OE modified steel cradle is an improvement because stiffness is a defining characteristic for the crashworthiness of a structure. Since all three cradle designs were analyzed using the same loading scenarios, the stiffness is directly proportional to the stiffness for comparative purposes. Therefore, because the baseline cradle has been crash test validated it serves as a good gauge for determining the crashworthiness of the other cradle designs. The aluminum design being within a few percent of the same displacement average is concluded to be the more crashworthy design relative to the OE modified steel cradle which is roughly twice as stiff as the baseline cradle design.

120 The method of using the constraints from an existing cradle design, and adding additional constraints to accommodate changes in powertrain design, and iteratively designing and validating using FEA proves a useful technique for creating a roadworthy, functioning prototype in a relatively short timeframe for vehicle component design. This method for designing and validating can be repeated by future EcoCAR teams for reducing time from concept to functional prototype and reducing total vehicle weight. The requirements for each EcoCAR competition will determine the powertrain selection, but the methodology highlighted in this study for creating the aluminum cradle design can be adapted to future designs. The next challenge is to determine a more robust and automated method for optimizing the design structure, rather than relying on human interpretation of the FEA results to drive changes from one iteration to the next.

7.2 Future Work

Many aspects of the design, optimization, and validation process identified for this cradle design can be improved. The development of the design constraints could be made more robust and efficient by conducting the process of creating a boundary diagram and conducting design Failure Modes and Effects Analysis (FMEA) per SAE J1739. This standard provides a framework for the method used by the automotive industry to identify and prioritized mitigation of potential failure modes in a design based on probability and severity of the failure modes. [15]

121 Another area with the potential to improve upon the design optimization method employed for the development of the aluminum cradle design is utilizing a topology optimization program, such as OptiStruct 11.0, after the cradle constraints are established, but before designing any of the cradle structure. The use of such a program at this phase of the design process could drastically reduce the number of iterations needed to reach an optimal design. This program was used in a study conducted by Chao Li and shows much potential.[4]

Lastly, investigating the potential structural benefits of using aluminum extrusions to reduce the amount of welding and improve the strength to weight ratio for the structure may also benefit the future development of cradle designs for a functional prototype like the OSU EcoCAR 2 competition vehicle. The cost of tooling for metal working procedures like stamping and forging is too high for use in prototyping, but aluminum extrusion tooling cost are significantly lower and the extrusion process improves the material properties of 6061 aluminum for use in structural applications.

122

BIBLIOGRAPHY

[1] Advanced Vehicle Technology Competitions, "About EcoCAR 2," Argonne National Laboratory, 2011. [Online]. Available: http://www.ecocar2.org/about- ecocar2. [Accessed May 2015]. [2] E. Woisetschlaeger, "Keine Monkultur," in Automobile Entwicklung, 2001, p. 130. [3] H. Wallentowitz, "Structural Design of Vehicles," Institute fur Kraftfahrwesen, Aachen, 2004. [4] C. Li, I. Y. Kim and J. Jeswiet, "Conceptual and Detailed Design of an Automotive Engine Cradle by Using Topology, Shape, and Size Optimization," Springer, Berlin, 2014. [5] D. Triantos and M. Michaels, Design and Fabrication of an Aluminum Engine, Detroit, Michigan: SAE International, 1999. [6] C.-M. Ni, C. J. Bruggemann, T. Hassan and W. Hall, Application of Hydroformed Aluminum Extrusions to Vehicle Sub-Frame with Varied Wall Thickness, SAE International, 1999. [7] J. Oldfield, "Electrochemical Theory of Galvanic Corrosion," ASTM STP 978, 1988, pp. 5-22. [8] G. J. Courval, J. Allin and D. P. Doyle, Galvanic Corrosion Prevention of Steel- Aluminum Couples, SAE International, 1993. [9] I. D. Harris, "Arc Welding Automotive Aluminum," Advanced Materials & Processes (USA), vol. 159.9, pp. 52-54, 2001. [10] V. K. Banthia, J. M. Miller, R. R. Valisetty and E. F. M. Winter, "Lightweighting of Cars with Aluminum for Better Crashworthiness," in SAE Technical Paper Series, Detroit, 1993. [11] X. Chen, H. Mahmood, W. A. Wagner and M. R. Baccouche, Aluminum Subframe Design for, Detroit, Michigan: SAE International, 2004. [12] Siemens Product Lifecycle Management Software, "Structural Analysis," 2012. [Online]. Available: http://www.plm.automation.siemens.com/en_us/products/nx/for- simulation/structural-analysis/#lightview%26uri=tcm:1023-4481%26title=NX-

123 Nastran%E2%80%93Basic-Fact-Sheet-2661%26docType=.pdf. [Accessed May 2015]. [13] T. D. Gillespie, Fundamentals of Vehicle Dynamics, Warrendale, PA: Society of Automotive Engineers, Inc., 1992. [14] M. J. Organiscak, "Model Based Suspension Calibration for Hybrid Vehicle Ride and Handling Recovery," The Ohio State University, Columbus, 2014. [15] SAE J1739, "Potential Failure Mode and Effects Analysis in Design (Design FMEA), Potetial Failure Mode and Effects Analysis in Manufacturing and Assembly Processes (Process FMEA)," in Surface Vehicle Standard, SAE International, JAN 2009.

124

APPENDIX A: LIST OF SYMBOLS AND ABBREVIATIONS

AVTC Advanced Vehicle Technology Competitions

CAD Computer Aided Design

DOE Department of Energy

DOF Degrees of Freedom

EWI Edison Welding Institute

E85 85% ethanol and 15% gasoline fuel by volume

FEA Finite Element Analysis

FEM Finite Element Model

GM General Motors

GMAW Gas Metal Arc Welding

MIG Metal Inert Gas

NVH Noise, Vibrations, and Harshness

OE Original Equipment

OEM Original Equipment Manufacturer

PHEV Plug-in Hybrid Electric Vehicle

125 TRC Transportation Research Center

VDP Vehicle Development Process

126

APPENDIX B: OE CRADLE AND OE MODIFIED CRADLE FEA FIGURES

Figure 71: Two-Wheel Bump Stress

Figure 72: Two-Wheel Bump Displacement

127

Figure 73: One-Wheel Bump LHS Stress

Figure 74: One-Wheel Bump LHS Displacement

Figure 75: One-Wheel Bump RHS Stress

128

Figure 76: One-Wheel Bump RHS Displacement

Figure 77: Twist - LHS Jounce, RHS Rebound Stress

Figure 78: Twist - LHS Jounce, RHS Rebound Displacement

129

Figure 79: Twist - LHS Rebound, RHS Jounce Stress

130

Figure 80: Twist - LHS Rebound, RHS Jounce Displacement

Figure 81: Forward Braking Stress

Figure 82: Forward Braking Displacement

131

Figure 83: Reverse Braking Stress

Figure 84: Reverse Braking Displacement

Figure 85: Cornering - Left Turn Stress

132

Figure 86: Cornering - Left Turn Displacement

Figure 87: Cornering - Right Turn Stress

Figure 88: Cornering - Right Turn Displacement

133

Figure 89: Forward Acceleration Stress

Figure 90: Forward Acceleration Displacement

Figure 91: Reverse Acceleration Stress

134

Figure 92: Reverse Acceleration Displacement

Figure 93: Max Torque - Forward Stress

Figure 94: Max Torque - Forward Displacement

135

Figure 95: Max Torque - Reverse Stress

Figure 96: Max Torque - Reverse Displacement

Figure 97: Reverse Bump Stress

136

Figure 98: Reverse Bump Displacement

Figure 99: Forward Impact Stress

Figure 100: Forward Impact Displacement

137

APPENDIX C: REDUCED MASS ALUMINUM CRADLE FEA GIGURES

Figure 101: Two-Wheel Bump Max Stress

Figure 102: Two-Wheel Bump Max Deflection

138

Figure 103: One-Wheel Bump LHS Max Stress

Figure 104: One-Wheel Bump LHS Max Deflection

139

Figure 105: One-Wheel Bump RHS Max Stress

Figure 106: One-Wheel Bump RHS Max Deflection

140

Figure 107: Twist - LHS jounce, RHS rebound Max Stress

Figure 108: Twist - LHS jounce, RHS rebound Max Deflection

141

Figure 109: Twist - LHS rebound, RHS jounce Max Stress

Figure 110: Twist - LHS rebound, RHS jounce Max Deflection

142

Figure 111: Forward Braking Max Stress

Figure 112: Forward Braking Max Deformation

143

Figure 113: Reverse Braking Max Stress

Figure 114: Reverse Braking Max Deformation

144

Figure 115: Cornering - Left Turn Max Stress

Figure 116: Cornering - Left Turn Max Deformation

145

Figure 117: Cornering - Right Turn Max Stress

Figure 118: Cornering - Right Turn Max Deformation

146

Figure 119: Forward Acceleration Max Stress

Figure 120: Forward Acceleration Max Deformation

147

Figure 121: Reverse Acceleration Max Stress

Figure 122: Reverse Acceleration Max Deformation

148

Figure 123: Max Torque - Forward Max Stress

Figure 124: Max Torque - Forward Max Deformation

149

Figure 125: Max Torque - Reverse Max Stress

Figure 126: Max Torque - Reverse Max Deformation

150

Figure 127: Reverse Bump Max Stress

Figure 128: Reverse Bump Max Deformation

151

Figure 129: Forward Impact Max Stress

Figure 130: Forward Impact Max Deformation

152