CRASHWORTHINESS of a VEHICLE for ROLLOVER IMPACT PROTECTION USING IMPAXX and ALUMINUM FOAM MATERIALS a Thesis by Sanjay Narayan

CRASHWORTHINESS OF A VEHICLE FOR ROLLOVER IMPACT PROTECTION USING IMPAXX AND ALUMINUM FOAM MATERIALS

A Thesis by

Sanjay Narayan

Bachelor of Engineering, Bangalore Institute of Technology, 2014

Submitted to the Department of Mechanical Engineering and the faculty of the Graduate School of Wichita State University in partial fulfillment of the requirements for the degree of Master of Science

December 2019

CRASHWORTHINESS OF A VEHICLE FOR ROLLOVER IMPACT PROTECTION USING IMPAXX AND ALUMINUM FOAM MATERIALS

The following faculty members have examined the final copy of this thesis for form and content, and recommend that it be accepted in partial fulfillment of the requirement for the degree of Master of Science with a major in Mechanical Engineering.

______Hamid M. Lankarani, Committee Chair

______Rajeev Nair, Committee Member

______Saideep Nannapaneni, Committee Member

iii DEDICATION

To my Parents

ACKNOWLEDGEMENT

I would like to express my gratitude to advisor, Dr. Hamid M. Lankarani, for all his guidance

and support throughout my entire period of stay at graduate school and research. His guidance, support and patience helped me to complete this thesis. I would like to thank my committee members

Dr. Rajeev Nair and Dr. Saideep Nannapaneni for their time in reviewing this thesis. I would also

like to thank my friends Muthyala Saketh Reddy, Sandeep Vijayan and Kishan for their continuous

support and care in making my stay at Wichita memorable.

Finally, I would like to thank my parents Mrs. Savithri, Mr. Narayan and Shilpa Shree for

their helping hand during my master’s studies.

ABSTRACT

Rollover crash events frequency accounts for only 3% among all types of accidents, but in terms of fatality rate, it accounts for 33% of fatalities in automobile crash accident events. Rollover events are complex and unpredictable, making them quite difficult to design a standard testing procedures. The angle of contact with the ground, the side coming into contact, height of fall, speed of vehicle, cause for rollover, etc., are all parameters and factors which could affect rollover crash responses. Presently the standard testing procedure for rollover is a quasi-static roof crush test procedure.

This study details the development, analysis and computational modelling and simulation of the performance of an IMPAXX and aluminum foam and their potential as an energy absorbing material to improve the roof strength of a vehicle. First the IMPAXX foam and Aluminum foam are modelled using the LS-DYNA code, and their performances are evaluated using a drop tower test and a free motion headform test methods. Next, the IMPAXX foam and Aluminum foam are modelled for insertion in the roof A-pillar of a typical sports utility vehicle (Toyota Rav4). Quasi- static roof crush computational tests, per two different standards FMVSS 216 and IIHS are conducted on the vehicle and solved using the LS-DYNA for vehicles with and without the

IMPAXX and aluminum foam. The results are analyzed and compared to quantify the effectiveness of the IMPAXX foam and aluminum foam as energy absorbing materials for rollover crashworthiness responses of vehicle and its occupant protection.

TABLE OF CONTENTS

Chapter Page 1. INTRODUCTION ...... 1

1.1 Crashworthiness and Mechanism of Rollover ...... 1 1.2 Standards and Regulations ...... 7 1.3 Foam as High Energy Absorbing Materials and their characteristics ...... 10 1.4 Deformation Mechanism of Foam Structures ...... 11

2. LITERATURE REVIEW ...... 13

3. OBJECTIVES AND METHODOLOGY ...... 17

3.1 Objectives ...... 17 3.2 Methodology ...... 18

4. DESIGN AND VALIDATION OF IMPAXX AND ALUMINUM FOAM MATERIAL MODELS ...... 20

4.1 Introduction to Foam Validation Tests ...... 20 4.2 Drop Tower Testing Finite Element Analysis ...... 20 4.2.1 Comparison of Drop Tower Test Results ...... 25 4.2.2 Validation of Drop Tower Test Results ...... 25 4.3 Free Motion Headform Finite Element Analysis ...... 27 4.3.1 Analysis of Headform Acceleration Results without Foam ...... 28 4.3.2 Analysis of Headform Acceleration Results with IMPAXX Foam ...... 29 4.3.3 Analysis of Headform Acceleration Results with Aluminum Foam ...... 30 4.3.4 Analysis of Head Injury Criteria Results with and without Foam ...... 31

5. VEHICLE ROOF CRUSH MODELLING AND ANALYSIS ...... 33

5.1 Finite Element Model of Sport Utility Vehicle Toyota Rav4 ...... 33 5.2 National Highway Traffic Safety Administration (NHTSA) Roof Crush Test .....35 5.2.1 Finite Element Modelling in LS-DYNA...... 36 5.2.2 Simulation Results with Current A-Pillar ...... 37 5.2.3 Simulation Results of IMPAXX Foam Padded A-Pillar ...... 38 5.2.4 Simulation Results of Aluminum Foam Padded A-Pillar ...... 40 5.2.5 Comparison of Force – Displacement Results ...... 41 5.2.6 Comparison of Internal Energy Results ...... 42 5.3 Insurance Institute for Highway Safety Roof Crush Test ...... 43 5.3.1 Simulation Results with Current A-pillar ...... 44

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TABLE OF CONTENTS

Chapter Page 5.3.2 Simulation Results of IMPAXX Foam Padded A-Pillar ...... 45 5.3.3 Simulation Results of Aluminum Foam Padded A-Pillar ...... 46 5.3.4 Comparison of Force – Displacement Results ...... 48 5.3.5 Comparison of Internal Energy Results ...... 49 5.4 Summary Comparison of Roof Crush Test Results ...... 49

6. CONCLUSIONS AND RECOMMENDATIONS ...... 51

6.1 Conclusions ...... 51 6.2 Recommendations for Future Study ...... 52

REFERENCES ...... 53

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LIST OF TABLES

Table Page 4.1 Finite Element Summary of Foam Model ...... 21

4.2 Material Properties of IMPAXX and Aluminum Foam ...... 23

4.3 Material Properties for Aluminum Foam model ...... 24

4.4 Sprague and Gear Error Estimation ...... 26

4.5 Free Motion Headform Test FE Model Summary ...... 28

4.6 Comparison of the Head Injury Criteria for with and without Foams ...... 31

5.1 Modelling Details of Sport Utility Vehicle (Toyota-Rav4 1997) FE Model ...... 34

5.2 Finite Element Summary for Loading Plate ...... 37

5.3 Summary of Roof Crush Simulation Test Results ...... 50

LIST OF FIGURES

Figure Page 1.1 Top View of a Car Making a Right Turn ...... 3

1.2 Front View of a Car Showing the Center of Gravity of the Car ...... 4

1.3 Front View of a Car Showing Equivalent Lateral Force Acting on C.G of Vehicle ...... 4

1.4 Front View of a Car Rolling Action Start with its Component Forces ...... 5

1.5 Car Undergoing Tripped Rollover due to Impact with Guardrail ...... 6

1.6 Car Undergoing Un-tripped Rollover ...... 6

1.7 FMVSS 216 Test Configuration for Quasi Static Roof Crush Test ...... 8

1.8 Uniaxial Stress-Strain Curve for Foams ...... 11

2.1 Volvo car corporation Volvo v40 Crash Structures Body in White ...... 13

2.2 Metal Foam Classes Based on Manufacturing Route ...... 14

3.1 General Methodology of this Study ...... 18

4.1 Finite Element Model Setup of Drop Tower Test ...... 21

4.2 Input Curve for IMPAXX Foam model ...... 23

4.3 Stress – Strain plots for IMPAXX and Aluminum Foams ...... 25

4.4 Drop Tower Test Validation for IMPAXX Foam ...... 25

LIST OF FIGURES (continued)

Figure Page 4.6 Drop Tower Test Validation for Aluminum Foam ...... 26

4.7 Headform test Simulation without foam and with Foam ...... 28

4.8 Head Acceleration of the Head form without foam ...... 29

4.9 Head Acceleration of the Head form with IMPAXX foam ...... 30

4.10 Head Acceleration of the Head form with Aluminum foam ...... 31

5.1 Toyota Rav4 Finite Element Model ...... 34

5.2 FMVSS 216 Regulation Configuration for Roof Crush Resistance Test ...... 35

5.3 FMVSS 216 Regulation Configuration of Loading Plate ...... 36

5.4 Finite Element Model setup for FMVSS 216 Rav4 Roof Crush Test ...... 37

5.5 End of Test for FMVSS 216 Rav4 Roof Crush Test ...... 38

5.6 A-Pillar cavity without and with Foam Padding to Car Model ...... 38

5.7 Toyota Rav4 Finite Element Model before FMVSS 216 Test ...... 39

5.8 Toyota Rav4 Finite Element Model after FMVSS 216 Test ...... 39

5.9 A-Pillar cavity without and with Foam Padding to Car Model ...... 40

5.10 Toyota Rav4 Finite Element Model before FMVSS 216 Test ...... 41

LIST OF FIGURES (continued)

Figure Page 5.11 Toyota Rav4 Finite Element Model after FMVSS 216 Test ...... 41

5.12 Toyota Rav4 Finite Element Model Force vs Displacement Comparison ...... 42

5.13 Toyota Rav4 Finite Element Model Internal Energy Comparison ...... 43

5.14 IIHS Regulation Configuration for Roof Crush Resistance ...... 43

5.15 Finite Element Model setup for IIHS Rav4 Roof Crush Test ...... 44

5.16 End of Test for IIHS Rav4 Roof Crush Test ...... 45

5.17 A-Pillar cavity without and with Foam Padding to Car Model ...... 45

5.18 Toyota Rav4 Finite Element Model before IIHS Test ...... 46

5.19 Toyota Rav4 Finite Element Model after IIHS Test ...... 46

5.20 A-Pillar cavity without and with Foam Padding to Car Model ...... 47

5.21 Toyota Rav4 Finite Element Model before IIHS Test ...... 47

5.22 Toyota Rav4 Finite Element Model after IIHS Test ...... 48

5.23 Toyota Rav4 Finite Element Model Force vs Displacement Comparison ...... 48

5.24 Toyota Rav4 Finite Element Model Internal Energy Comparison ...... 49

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LIST OF ABBREVIATION

1. NHSTA – National Highway Traffic Safety Administration

2. IIHS - Insurance Institute for Highway Safety

3. FMVSS - Federal Motor Vehicle Safety Standards

4. DOD – Department of Defense

5. FAA – Federal Aviation Administration

6. NASA - National Aeronautics and Space Administration

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CHAPTER 1

INTRODUCTION

1.1 Crashworthiness and Mechanism of Rollover

Vehicle designs have undergone major changes throughout history since its inception,

Occupant safety is foremost requirement in any vehicle design. Crashworthiness is the concept

which focuses on protection of occupant to reduce the number of fatal and serious injuries occurred

due to accidents. Crashworthiness is a research program responsible for developing and upgrading

test procedures for evaluating motor vehicle safety. Crashworthiness research encompasses new

and improved vehicle design, safety countermeasures and equipment to enhance safety of occupant

[1].

Crashworthiness prime concept of occupant safety especially in automobile vehicle is

achieved by making sure that during accidents vehicle deformation occurs in such way that

occupant has enough space to survive the deaccelerating loads at time of crash.

At present age crashworthiness research had advanced in leaps and bounds with the advent

of supercomputers and many crash dynamic simulation software packages such as LS-DYNA,

MADYMO, PAM-CRASH, MSC Dytran, etc. Simulation software packages have helped in

recreating real time scenarios and assessing various criteria for occupant injury and vehicle structural response parameters.

To assess and regulate crashworthiness of the vehicles today numerous regulatory agencies

have been established and these bodies have developed their own authoritative safety requirements

and conducted extensive research in the field:

• National Highway Traffic Safety Administration (NHTSA)

• Federal Aviation Administration (FAA)

• National Aeronautics and Space Administration (NASA)

• Department of Defense (DOD)

Rollover crash events are the most unpredictable and life threatening to the safety of the

occupant and the vehicle. Rollover crash events frequency accounts only 3% among all types of

accidents but when it comes down to fatality rate it accounts to 33% of fatalities in automobile

crash accident events. Rollover are events which are very complex in motion and unpredictable

making it very tough to design a standard testing procedures.

Using simple Physics the mechanism of a rollover incident can be explained. A rollover

incident occurs when there is sufficient centripetal force generated by friction between tires and

road. Generally vehicles with high center of gravity, vehicles on a narrow track tend to be

unstable and easily rollover.

Consider a vehicle in motion taking a turn for vehicle to turn the wheels of vehicle must

create a lateral force “Centripetal Force” to oppose Newton’s First Law of Motion. The

Centripetal force is being generated due to friction between tires and road, when there is enough

friction the force generated will be sufficient to cause the rollover situation.

Consider a vehicle turning right, friction between tire and road is acting to the right and

to counter this, there is a force called centrifugal force that seems to exist in the opposite direction, as shown in Figure 1.1. There is no centrifugal force it is just the vehicle obeying

Newton’s Law’s and this force is exactly equal in size with the centripetal force allowing the

vehicle to take a right turn. It is interesting to note that the driver feels as there is a force driving

him towards the left side (Centrifugal Force) in reality it is just the driver obeying the Newton’s

Laws of him trying to go straight while the vehicle and seat is moving to the right underneath

him.

Right Turn

Centripetal Force

Figure 1.1: Top View of a Car Making a Right Turn [27].

According to Newton’s law Force equals mass times acceleration. The Force acting on

the vehicle for circular motion, is F = m*a. The F = ma for a circular motion is in the (familiar)

form F = (W * v * v)/(g * r), where [W = vehicle weight; v = velocity/speed; r = circle radius; and g = acceleration due to gravity]. This force is the force which must be exerted on the vehicle sideways to make it turn in a circle rather than it going straight the way it was intending to go.

This force can be visualized better to be acting at one point of the vehicle Center-of-

Gravity (CG) which is a point on the centerline of vehicle. Mathematically we can assume the entire mass of vehicle to be concentrated on this point. Figure 1.2 shows vehicle location of

Center-of-Gravity (CG) point on the centerline of the vehicle and through the length on which the entire vehicle could be balanced. The whole weight (mass) of vehicle seems to be acting at the height of the center of gravity, and the lateral force (Centripetal force) by tires and road being applied at the height of the road surface to vehicle weight, acting to the left. The horizontal force is not acting directly at center of gravity of vehicle as it is at wrong height, therefore this force is

pushing the vehicle weight/center of gravity sideways but also tends to try to twist the vehicle

around the center of gravity this twisting act is the cause for rollover initiation.

Vehicle Center- of-Gravity (CG)

Track Width

Figure 1.2: Front View of a Car Showing the Center of Gravity of the Car [27].

The lateral force is acting on the vehicle at the height of the bottom of tires to the left in our front view drawing, for easier analysis we can say an equivalent identical force is acting to the right and on the center of gravity causing the same effect of twisting on the vehicle. Figure

1.3 shows the equivalent identical lateral force acting on center of gravity of vehicle.

Figure 1.3: Front View of a Car Showing Equivalent Lateral Force Acting on C.G of

Vehicle [27].

The lateral horizontal force, shown in Figure 1.3, can be further seen as a combination of two separate component forces. One of the component forces is towards the thread of tire

(downward right direction) this component of force is the tires friction to road enabling vehicle to make turns. The other component of the force is towards the upwards right direction this component of force which acts to rollover the vehicle. Figure 1.4 shows vehicle with component forces with vehicle rolling action start.

Figure 1.4: Front View of a Car Rolling Action Start with its Component Forces [27].

Rollover accidents can be categorized into two types:

• Tripped Rollover

This type of rollover occurs when vehicle is striking an object e.g. curb, guardrail

etc. The high reaction force striking applied to the tires upon impact causes

vehicle to rollover, as shown in Figure 1.5. According to National Highway

Traffic Safety Administration (NHTSA) 95% of all single-vehicle rollovers are

tripped [2].

Figure 1.5: Car Undergoing Tripped Rollover due to Impact with Guardrail [2].

• Un-tripped Rollover

This type of rollover occurs usually when vehicle is executing high speed

collision avoidance maneuvers which serves as the tipping mechanism, as shown

in Figure 1.6. This type of rollover is uncommon occurring less than 5% and

mostly experienced by top heavy vehicles, vehicles with very high center of

gravity [2].

Figure 1.6: Car Undergoing Un-tripped Rollover [2].

1.2 Standards and Regulations

Federal Motor Vehicle Safety Standards (FMVSS) are United States (U.S) federal regulations requiring design, construction, and durability requirements for motor vehicles, automobile safety components, features, and systems. These regulations are counterpart to United

Nation (UN) regulations recognized to varying degree by most countries and developed at World

Forum for Harmonization of Vehicle Regulations. Statutory authorization was granted to National

Highway Traffic Safety Administration (NHTSA) by National Traffic and Motor Safety Act of

1966. National Highway Traffic Safety Administration (NHTSA) develops and enforces Federal

Motor Vehicle Safety Standards (FMVSS). Federal Motor Vehicle Safety Standards (FMVSS) can

be separated into three classes:

• Crash Avoidance 100-Series

• Crashworthiness 200-Series

• Post-Crash Survivability 300 Series

Manufacturer’s need to comply with Federal Motor Vehicle Safety Standards set by

National Highway Traffic Safety Administration for civilian and automobile safety. Roof Crush

Resistance to vehicle rollover is an important safety concern to both civilian and automotive

safety community. Original FMVSS 216 was developed and enforced in 1971 to decrease roof

collapse in rollover events [28].

FMVSS 216 – Roof Crush Resistance standard shown in Figure 1.7, specifies for roof

crush resistance requirements over passenger compartments for passenger cars (not including

convertibles), multi-purpose vehicles (MPV), trucks and buses (not including school buses) with

a gross vehicle weight of 6000 pounds or less. Vehicle roof structure must crush 5 inches or less

with a minimum force of 1.5 times the unloaded vehicle weight (UVW) or 5000 lbs, whichever

is less.

Figure 1.7: FMVSS 216 Test Configuration for Quasi Static Roof Crush Test [4].

National Highway Traffic Safety Administration published a Notice of proposed rulemaking on August 23 2005 to upgrade FMVSS 216 and after receiving comments to this rule finally NHTSA on April 30 2009 established FMVSS 216a standard for roof crush testing [28].

There were four major changes proposed in FMVSS 216a [28]:

1. The maximum applied force must equal three times the unloaded vehicle weight for

vehicles with a gross vehicle weight rating of 6000 pounds or less.

2. The standard expanded to include vehicles with gross vehicle rating between 6000

pounds and 10000 pounds.

3. Head room maintenance is monitored through the use of a head form representing a 50th

percentile male seated in the front occupant position.

4. The platen force, displacement, and head form contact requirements to be met on both

sides of vehicle roof structure (Driver and Passenger Side).

NHTSA set a phase-in for the FMVSS 216a over a period of six years, the phase-in was

based on fleet percentage [5]:

Vehicles with a gross vehicle weight rating (GVWR) of 6000 lbs or less – passenger cars,

multipurpose passenger vehicles, trucks and buses [28]:

• 25% of vehicles manufactured during period of September 1, 2012 to August 31,

2013.

• 50% of vehicles manufactured during period of September 1, 2013 to August 31,

2014.

• 75% of vehicles manufactured during period of September 1, 2014 to August 31,

2015.

• 100% of vehicles manufactured during period of September 1, 2015 to August 31,

2016.

Passenger cars, multipurpose passenger vehicles, trucks and buses with gross vehicle weight rating (GVWR) between 6000 lbs and 10000 lbs were phased in for 100% of vehicles manufactured on or after September 1, 2016 [28].

1.3 Foam as High Energy Absorbing Materials and their Characteristics

Foam is formed by trapping pockets of gas in a liquid or solid. Foams have cellular core

structure created by expansion of blowing agent. Foam structures consist at least two phases,

gaseous voids and polymer matrix. Open cell or Closed cell foam structure is formed by cellular

wall enclosing gaseous voids. Closed cell foams have cell walls which completely enclose the

gaseous voids. Open cell foams do not enclose the gaseous voids. Foams can be flexible or rigid,

generally open cell foams are flexible and closed cell foams are rigid.

Many Innovative ideas and concepts have been thought up and implemented to minimize

or eliminate injury to occupants in a vehicle during impact or crash events. Application of Foam

in vehicles is one such idea. Foam materials have gained significant momentum and attention for

use in structural applications in both the automobile and aircraft industries. Foams are light

weighted have excellent insulation and energy absorbing properties during impact. Currently

Polyurethane foams have gained a lot of attention and are used in a lot of industries.

Polyurethanes have found uses in major appliances such for refrigerator and freezer thermal

insulation systems. In automotive car seats, interior ceiling sections, spoilers all have foam

paddings to make car lighter and fuel efficient, energy absorbers in rails, pillars bumpers etc.

1.4 Foam Structures and their Deformation Mechanism

Foam structures as per mechanics of cell crushing can absorb impact energy while undergoing deformation, its ability to absorb energy during impact, lightweight, damping, thermal insulation all these properties have made foam structures to be in extensive use in different applications. Mechanical properties of foams are closely linked to their complex microstructure and their cell wall deformation [8]. Properties of foam like young’s modulus,

10 yield stress and polymer density depend upon characteristics like relative density, degree to which the cell walls are open/closed and the geometric anisotropy of the foams.

Figure 1.8: Uniaxial Stress-Strain Curve for Foams (a) Elastomeric Foam upon Compression, (b) Elastic-Elastic Foam upon Compression, (c) Elastomeric Foam upon Tension and (d) Elastic-Plastic Foam upon Tension [11].

Figure 1.8 shows stress-strain response for Elasto-plastic and Elastomeric foams under tension and compression. Foams show brittle properties under tension and plastic behavior under compression. The brittle properties are due to stress concentration effect at porous sites which causes fracture in tension [9].

As seen from graphs Figure 1.8 foams under compression, foams exhibit a region of linear elasticity for low stresses and followed by a plateau where stress does not change by a lot, followed by a region of densification where there is steep rises in stress. The linear elastic region

11 is due to bending of cell walls, walls of cell are stretched to withstand the load during this phase foams show low strains and have elastic response. Plateau region is the phase where foam is exhibiting plastic deformation behavior this region is different for all foams. Plateau region failure is stopped by cell collapse, cell walls have completely collapsed and allow no further deformation, at this stage stresses rises rapidly and this region is known by densification. Foams with higher density have high Young’s modulus this results in increase of the plateau stresses and reduction in the strain at point of densification origin. The origin point of densification for foam is called its shoulder point, shoulder point varies from foam to foam and depends on their density and strain rates.

CHAPTER 2

LITERATURE REVIEW

In automotive industry, the main concept behind the design, structure and shape of vehicle is aimed towards crashworthiness where it is the ability of the vehicle structure to deform plastically yet maintain sufficient space for survival for occupants inside vehicle during crash events. Secondary purpose can be defined as to reduce damage to vehicle structure.

Usually high Stiff structures are considered to be favorable for deformation but they have poor crashworthy deformation characteristics, this increases occupant injury rate during crash events. Considering this a structure is supposed to be stiff at certain portions to protect sensitive areas of passenger compartment of a vehicle and soft at certain areas to absorb the maximum amount of forces (energy) before it reaches stiff portions of vehicle. This is achieved by building areas in the structure of the vehicle where deformation can be controlled and predicted. Figure

2.1 shows a car where steel beams are incorporated to the vehicle structure. Weak points are engineered for predictable deformation to the steel bars allowing dissipation of energy through motion of buckling, heat and noise rather than transfer of energy to cabin ultimately leading to occupant risk.

Figure 2.1: Volvo car corporation Volvo V40 Crash Structures Body in White (BIW) [6].

These several past few decades there has been an increasing development in using various countermeasures and foams such as polymeric foams. Foams especially polymeric foams are playing a pivotal role in vehicle safety and protection parameters. Among all the different types of foams, polyurethane foams are finding widespread use in automotive industry as high energy absorbing material. Via mechanics of cell crushing polyurethane foams have the ability to absorb energy during deformation. During Impact energy absorption on a microscopic scale cell walls deform plastically and get damaged [7].

Metal foams are lightweight cellular materials inspired by structures of materials in nature like wood, bones, sea sponges etc. are examples of some structures. Metal foams can be categorized into two distinctions which are closed cell foams and open cell foams (metal sponges) as illustrated in Figure 2.2. Metal foams structure is more or less homogenous and has different characteristic features which determine its properties and application field [10].

Closed cell metal foams can be divided into two classified based on manufacturing processes: the melt (ML, direct foaming) and the powder metallurgical (PM, Indirect foaming).

Open cell foams or metal sponges can be classified into polymeric sponge structure and placeholder [10].

Figure 2.2: Metal Foam Classes Based on Manufacturing Route [10].

Polymeric foams has been well established in the market and has become a part of daily

lives. Some other foams such as food foams, ceramic foams, are also popular and well

established. Metal foams on the other hand are less common and not wide spread in the market

although there are number of application to this product. Metal foams are more costly compared

to polymeric foams and ergonomically they cannot be implemented on a commercial scale as of

now but we can see a forecast of a slow but continuous growth [10].

Kenneth, et al., [15] investigated specific application of aluminum foam in high speed rail

equipment. Author’s main focus was improving the crush energy absorption of rail passenger car

structures. The author discussed and explored four application areas of aluminum foam through

analytical studies, sub scale testing. Based on the test results author conducted a full scale 8g

pulsed test with 50th percentile male instrumented dummies with sliding rail filled with

aluminum foam and found reduction in Head Injury Criterion (HIC) values by 80%.

Saketh, et al., [16] investigated application of IMPAXX and Polyurethane foam materials

for improvement in energy absorption and occupant protection in side impact collision events.

The author conducted various validation studies on IMPAXX and Polyurethane foams. The

author performed simulations of side impact tests of IMPAXX and polyurethane foams padded

b-pillar of Toyota Yaris with 50th percentile dummy model in LS-DYNA according to FMVSS and IIHS regulations and found reduction in injury parameters such as rib deflections, HIC, CSI and TTI for car occupant.

Zhu, et al., [18] investigated into response of sandwich square panels with aluminum foam core under blast loading following up with a Finite Element (FE) simulation in LS-DYNA.

In this research tests were conducted on two face sheets of identical thickness with an aluminum foam core of various core density under blast loading with a TNT charge, oscillation amplitude,

15 impulse was measured using laser displacement transducer, pressure time history was measured using Polyvinylidene fluoride sensor (PVDF). It was found energy absorption was increased with higher core density.

Garcia et al., [19] discusses production processes, properties and industrial applications of metal foams in our society. Author shows us the present drawbacks in technology towards a streamlined production process of aluminum foams for wide scale commercial application.

Author discusses how high end premium cars are using aluminum foams in frames for increased stiffness, energy absorption in cars such as Ferrari 360, 430 spider and Audi Q7 SUV. Author discusses various sectors where aluminum foams have found use and highlights promising forecast towards use of aluminum foams in commercial sector.

Examination of the literature review on this topic shows aluminum foams is a promising high energy absorbing material for implementation in vehicles for crashworthiness.

The study is conducted for understanding and analyzing the effects of aluminum foam as well as

IMPAXX foam padded roof under rollover tests.

CHAPTER 3

OBJECTIVES AND METHODOLOGY

3.1 Objectives

Foam structures have shown good promise towards improvement of automobile vehicle crashworthiness. Extensive research has been conducted to understand the energy absorption characteristics and material model behavior of foam structures. In this study, finite element

methods are used to analyze the energy absorption properties and mathematical model behavior

of IMPAXX and aluminum foam material, tests at component level using a drop tower test and a free motion headform test and the effect of foam material effectiveness as a high energy absorber was studied by conducting roof crush resistance tests utilizing NHTSA and IIHS roof crush test safety regulations.

The specific objectives of this research are:

• To analyze and evaluate IMPAXX foam and Aluminum foam material models for stress-

strain behavior using Drop Tower Test method

• To validate component level simulation tests results with experimental results

• To Analyze the behavior of the same foam in free-motion headform test according to

FMVSS 201U.

• To evaluate and compare the dynamic response of a sport utility vehicle (Toyota-Rav4)

using Federal Motor Vehicle Safety Standards 216 and Insurance Institute of Highway

Safety regulatory safety standards.

• To quantify the effect of IMPAXX and Aluminum foam materials in rollover

crashworthiness and occupant protection of vehicle.

3.2 Methodology

This research starts with development of aluminum foam material models in LS-PrePost.

The aluminum foam and IMPAXX foam material models are tested for validation through drop

tower test according to test regulations of ASTM D3574 C and results are observed for

mechanical behavior and energy absorbing characteristics for the IMPAXX foam and aluminum foam material models. Next IMPAXX foam and aluminum foam models are tested for validation

through free motion headform test according to test regulations safety standards FMVSS 201U.

Figure 3.1: General Methodology of this study.

The roof crush test for a representative Sport Utility vehicle (SUV) car Toyota Rav4 is

subsequently conducted as per NHTSA and IIHS safety regulations with current and new foam

18 padded A-pillar. The roof crush tests for Sport Utility vehicle (SUV) car Toyota Rav4 are simulated in LS-DYNA followed with post-processing and comparison of results. The methodology is illustrated in Figure 3.1. The full scale simulation tests are conducted in

LS_DYNA and post-processed for results.

CHAPTER 4 DESIGN AND VALIDATION OF IMPAXX AND ALUMINIUM FOAM MATERIAL

MODELS

4.1 Introduction to Foam Validation Tests

Broad research has been contributed to the engineering of polymeric foam’s properties, it

was seen that foam cellular materials exhibit different mechanical properties for different foam

core densities, strain rates loading and failure mechanisms. To study and evaluate crush response

of components in the interior of car roof, it is necessary to understand simulation model and its

response in crash environment [20]. Material data are required to perform finite element analysis simulation of component level tests. Material properties of foam depend on failure mechanisms

like cell wall cracking or cell wall bending it is very demanding to understand their behavior

with one model, it is necessary to model number of foams with various material models to

understand their behavior. It is very cost intensive to perform all these test hence for practicality

and cost effectiveness all these are virtually tested in finite element modelling software, as LS-

Dyna, Hypermesh, etc. There are various component level tests to study the characteristics of

material models. In this research to study the mechanical behavior of IMPAXX foam and

Aluminum Foam two methods are used:

• Drop Tower Tests

• Free Motion Head-Form Tests

4.2 Drop Tower Testing Finite Element Analysis

Testing regulations ASTM D3574 C Foam force deflection is generally used for drop

tower tests. The drop tower test finite element modelling is setup entirely and tested in LS-Dyna.

The finite element model setup consists of foam model supported onto to a fixed plate and an impactor plate. The foam model has dimensions 107mm x 107mm x 75mm is modeled with solid elements, Element type 2 – which is formulation with fully integrated solid element. The foam model is supported on a rigid fixture base. The steel plate impactor hits with an initial velocity of

12.5 mph in downward z-direction. Figure 4.1 shows test setup model in LS-Dyna.

Figure 4.1: Finite Element Model Setup of Drop Tower Test

In simulation, fixture plate acts as a support plane and its motion is restricted in all directions, the impactor plate is restricted in all directions except z-direction. The impactor plate is kept close to foam model to reduce solver runtime of the simulation. The details of foam model setup such as number of elements, number of nodes and the foam element size is discussed below in Table 4.1.

Table 4.1: Finite Element Summary of Foam Model.

Number of Elements in Foam 995 Number of Nodes in Foam 2551 Foam Element Length 10

Impactor plate and the fixed plate are of steel material. In Ls-Dyna material card

MAT_20 (MATERIAL_RIGID) has been used to define the fixed plate and impactor plate, and represent its material properties. Impactor plate and fixed plate both have four node quadrilateral shell element property 1-mm thick assigned from element formation 2 Belytschko-Tsay shell elements.

Aluminum foam model material properties is represented and defined using *MAT_154,

MAT_DESHPANDE_FLECK _FOAM has been used for compressible, low density and elastic foams. Generally MAT_57 and MAT_63 are used for representing foams with low density, compressible and elasticity. MAT_154 has been used to represent the aluminum foam model,

MAT_63 has been used to represent the IMPAXX foam model. Table 4.2 lists the material properties assigned to the IMPAXX foam and aluminum foam model for simulation of drop tower test. Contact is defined under SOFT 2 and used generally for soft materials to control the element intrusion. Hour glass is an important element to control solid element deformation during simulation. For the simulations HOUR_GLASS 0.5 value is used.

INTERIOR_CONTACT has been defined for a set of nodes of the foam block to avoid any negative volume errors.

Table 4.2 Material Properties of IMPAXX and Aluminum Foams.

Foam Type Density Young’s Tension cut- Material Damping Shape (Ton/mm3) Modulus off Stress Card Coeffecient Factor (N/mm2) (MPa)

IMPAXX 3.700e-11 10.5 1.000e+020 MAT_57 0.225 15

ALUMINUM 2.50e-14 590.0 1.000e+020 MAT_154 FOAM

Simulation termination was set to 0.025 s with a time step (dt) of 1.000e-06 s. All components are defined with contact algorithm AUTOMATIC_SINGLE_SURFACE. Interior contact algorithm is defined to avoid any negative volume error in runtime. Impactor plate to

Foam contact is defined using FORCE_TRANSDUCER_PENALTY Contact algorithm.

IMPAXX foam is sensitive towards compression hence proper densification is needed to be defined for input stress-strain curve, LS-Dyna refers this process to as curve fitting. Figure 4.2 shows the densified curve used for IMPAXX foam during testing.

Figure 4.2: Input Curve for Impaxx Foam model [30].

Aluminum foam model uses material card MAT_154 Deshpande Fleck foam and this material card does not require input densification curve but material card MAT_154 requires several other factors in material info for foam validation such as shape of yield surface

(ALPHA), curve fit parameter (GAMMA), densification strain (EPSD), curve fit parameter

(ALPHA2), volumetric strain (CFAIL) and these properties are listed in Table 4.3.

Table 4.3 Material Properties for Aluminum Foam Model [29].

3.1.1 Comparison of Drop Tower Test Results

Stress-strain response comparison curves for IMPAXX and Aluminum foam models for the given boundary conditions is shown in Figure 4.3. It can be seen that IMPAXX foam shows higher load till half the duration (plateau region) but Aluminum curve has overall higher load

(plateau). Aluminum curve has higher densification region than the IMPAXX curve. Both response curves shows they exhibit linear elastic regions, stress plateau regions and densification regions as exhibited in mechanics of foams under deformation. Aluminum foam has a higher density than IMPAXX foam, hence its shoulder point higher than IMPAXX foam.

Figure 4.3: Stress – Strain Plots for IMPAXX and Aluminum Foams

4.2.2 Validation of Drop Tower Test Results

IMPAXX and Aluminum foam models are simulated in LS-Dyna and respective response curves are obtained and compared with experimental drop tower test results respectively. The experimental and analytical force vs displacement response curve for IMPAXX foam are shown in Figure 4.4.

Figure 4.4 Drop Tower Test Validation for IMPAXX Foam

The experimental and analytical force vs displacement response curves for Aluminum foam are shown in Figure 4.5. Both force vs displacement curves for IMPAXX and Aluminum foam show close relation with simulation results. The Experimental Elastic Young’s Modulus is higher than the Young’s Modulus from simulation for IMPAXX foam. Densification process begins at 55-mm displacement for experimental test while densification process begins at 50-mm displacement. The IMPAXX foam has 20 kN as maximum load bearing capacity from simulation results and nearly the same for both experimental and simulation.

Figure 4.6 Drop Tower Test Validation for Aluminum Foam

Table 4.3 Sprague and Gear Error Estimation

Foam Test Magnitude Phase Total Type Deviation Deviation Deviation

IMPAXX Experimental 15% 5% 16%

Simulation 13% 5% 14% AL Foam Experimental 13% 7% 15%

Simulation 15% 7% 16%

Sprague and Geer method [31] was used to calculate the deviation for magnitude, phase

and cumulative total deviation, Sprague and Geer method equations was coded and implemented

in MATLAB and the results are calculated and tabulated in Table 4.3. Overall results show a sound correlation between experimental and simulation results. Total deviation for IMPAXX

foam material model is 16% and 14% for experimental and simulation test respectively, total

deviation for Aluminum foam material model is 15% and 16% for experimental and simulation

test respectively.

4.3 Free Motion Headform Testing Finite Element Analysis

FMVSS 210U safety regulations are used to conduct Free Motion Headform tests. In the

experimental setup Hybrid head structure has been used, where head accelerations for frontal

head impacts are measured for foam samples of different core thickness and density with a

constant thickness mounted onto a steel structure at low (2.72 m/s), intermediate (4.0 m/s) and

high (6.72 m/s) impact speeds. For current study the Free Motion Headform model Hybrid

dummy used was developed by Livermore Software Technology Corporation (LSTC) [22]. The

headform model consists of eight different parts and defined under material card MAT-RIGID.

The outer skin features of the dummy model are defined under material card

MAT_OGDEN_RUBBER. The center of gravity of headform location is at node 1, an

accelerometer is placed at this node from where all output injury parameters are calculated from

this node through nodeout output file. Figure 4.7 shows the finite element setup for frontal

impact of headform both with foam and without foam. Table 4.4 shows the finite element model

summary for headform.

Figure 4.7 Headform Test Simulation without Foam and with Foam

Table 4.4 Free Motion Headform Test FE Model Summary

Number of nodes for Headform 19572

Number of elements for Headform 16786

Number of Shell Elements for Headform 4044

Number of Solid Elements for Headform 16514

4.3.1 Analysis of Headform Acceleration Results without Foam

Head Acceleration of the Headform response curves at three different speeds, 2.72 m/s, 4.0 m/s, 6.72m/s is shown in Figure 4.8. From graphs we can observe that peak acceleration for

6.72m/s is 1181 G’s, peak acceleration for 4.0 m/s is 440 G’s and peak acceleration for 2.72 m/s is 250 G’s. The three curve plots at the three impact speeds of 2.72 m/s, 4.0 m/s, 6.72 m/s are similar in passion. From graph we see that maximum acceleration is reached at 1.5 msec for 6.72 m/s impact speed to headform, similarly maximum acceleration is reached at 2.0 msec for 4.0 m/s impact speed to headform and similarly maximum acceleration is reached at 2.75 msec for 2.72 m/s impact speed to headform.

Figure 4.8 Head Acceleration of the Head form without foam 4.3.2 Analysis of Headform Acceleration Results with IMPAXX Foam

Figure 4.9 shows Head Acceleration response curves for the Headform at three different

speeds, 2.72 m/s, 4.0 m/s, 6.72 m/s when the IMPAXX foam model is padded between steel structure and head form. From graphs we see that peak acceleration at 2.72 m/s is 150 G’s, for peak acceleration at 4.0 m/s is 80 G’s and for peak acceleration at 6.72 m/s is 55 G’s. Inclusion of foam material shows significant drop in acceleration. From graph we see that maximum acceleration is reached at 0.5 msec for 6.72 m/s impact speed to headform, similarly maximum acceleration is reached at 0.6 msec for 4.0 m/s impact speed to headform and similarly maximum acceleration is reached at 0.7 msec for 2.72 m/s impact speed to headform.

Figure 4.9 Head Acceleration of the Head form with IMPAXX foam

4.3.3 Analysis of Headform Acceleration Results with Aluminum Foam

Figure 4.9 shows Head Acceleration response curves for the Headform at three different speeds, 2.72 m/s, 4.0 m/s, 6.72 m/s when the Aluminum foam model is padded between steel structure and head form.

Figure 4.10 Head Acceleration of the Head form with Aluminum foam

From graphs we see that peak acceleration at 2.72 m/s is 46 G’s, for peak acceleration at

4.0 m/s is 64 G’s and for peak acceleration at 6.72 m/s is 104 G’s. Foam material inclusion

shows significant drop in acceleration further drop in acceleration can be seen from Aluminum

foam.

4.3.4 Analysis of Head Injury Criteria Results with and without Foam

Free motion headform test observations are analyzed and tabulated in Table 4.5. The component level tests show good correlation with experimental tests conducted by LSTC [25].

Table 4.5 Comparison of the Head Injury Criteria for with and without Foams

HIC With Foam Experimental Without without Acceleration HIC with Foam Results Foam Foam (G) Acceleration Acceleration (G) (G) No Foam IMPAXX AL IMPAXX AL

Peak Resultant Acceleration, (G) at 225-275 237 689 56 46 106 78 2.72m/s

Peak Resultant Acceleration, (G) at 437 444 2,484 83 64 264 182 4.0m/s

Peak Resultant Acceleration, (G) at 1067 1181 14,590 148 102 1,072 594 6.72m/s

It can be inferenced from the headform tests that there is significant drop in acceleration

with padded IMPAXX and Aluminum foams. There is a reduction of acceleration of around 90%

with Aluminum padded foam at 6.72 m/s. It is also seen that there is substantial reduction in

Head Injury Criteria with inclusion of foam paddings both IMPAXX and Aluminum foams. In all cases of tests head injury is prevented except for one case of IMPAXX test at 6.72 m/s where head injury criteria is observed to be 1072, and it is close to the injury window, but this is minimal injury and it is negligible.

CHAPTER 5

VEHICLE ROOF CRUSH MODELLING AND ANALYSIS

In this study, vehicle roof crush tests are conducted, by following NHTSA and IIHS safety regulations. Mathematical Finite Element models of Toyota RAV4 and Impactor plate are used to perform simulation of roof crush testing. In this chapter, roof crush tests simulation is performed for Toyota Rav4 car model with current roof and IMPAXX and Aluminum foam padded roof are analyzed under National Highway Traffic Safety Administration (NHTSA) and

Insurance Institute for Highway Safety (IIHS) regulations.

5.1 Finite Element Model of Sport Utility Vehicle Toyota Rav4

Finite Element Model of 1997 Toyota Rav4 sport utility vehicle (SUV) was used for this study which was developed by National Crash Analysis Center (NCAC) of George Washington

University. Car model developed with meticulous finite element modelling to include full functional of both steering and suspension systems. Car finite element modelling was aided by using reverse engineering process where a Toyota Rav4 car is disassembled part by part, systematically each part was catalogued and classified for material type, scanned for its geometry and measured for its thickness. All data obtained was entered into a computer and using finite element modelling software all parts were modelled and assembled together to form the car model with all structural and mechanical features represented in digital form.

The Toyota Rav4 model was able to be digitalized during time period between 2003 -

2005. Important parts of car model was updated to current values through coupon testing of samples of the vehicle parts.

Figure 5.1 Toyota Rav4 Finite Element Model [23].

Table 5.1 Modelling Details of Sport Utility Vehicle (Toyota-Rav4 1997) FE Model [23].

Number of parts 577

Number of nodes 478,624 Number of shells 472,423 Number of Beams 2 Number of Solids 21,539 Total Number of Elements 494,117 Weight 1266 Kgs, 2791 Lbs Center of Gravity (CG) 1135 mm Rearward of Front Wheel Engine Type 2.0 L4SFI DOHC 16V Tire Size P215/70R16 Wheelbase 2415 mm

Table 5.1 shows Finite Element Model summary, the finite element was modelled after a

1997 Toyota rav4 and validated in different ways to be a true representation of actual vehicle in digitized form. Modelling summary, mass, center of gravity (CG) location, wheelbase and various other parameters of the finite element model are listed in Table 5.1.

5.2 National Highway Traffic Safety Administration (NHTSA) Roof Crush Test

In United States vehicles manufactured after 1971 must satisfy FMVSS safety regulations set by National Highway Traffic Safety Administration (NHTSA). NHTSA has setup certain safety regulations and must be followed for each crash scenarios. This study follows the FMVSS

216 regulations for roof crush resistance test. Figure 5.2 illustrates FMVSS 216 safety regulations for roof crush resistance testing.

Figure 5.2: FMVSS 216 Regulation Configuration for Roof Crush Resistance Test [24].

Force is applied quasistatically at the top of side rail on vehicle top. The quasistatic load

is applied through a rigid rectangular plate with dimensions 762mm X 1829mm. The rigid plate

is oriented at a pitch of 5deg and roll of 25deg. Test vehicle is kept stationery through tie-down

by securing chassis frame to I-beam structures with epoxy or welding.

5.2.1 Finite Element Modelling in LS-DYNA

Figure 5.3 shows the finite element model of the loading plate. A rectangular plate of

dimensions 762 mm X 1829 mm is modelled in LS-Dyna as per NHSTA FMVSS 216

regulations. The loading Plate is oriented at an angle of 25° roll and 5° pitch as stated in the

FMVSS 216 regulations.

Figure 5.3: FMVSS 216 Regulation Configuration of Loading Plate.

The loading plate is defined with material card MAT_RIGID (MAT_20). The loading plate finite element model summary is listed in Table 5.2. Table shows the number of shell elements, rigid elements and nodes in model.

Table 5.2 Finite Element Summary for Loading Plate.

Model Information MDB Number of shell elements 22680 Number of Rigid elements 22680 Number of nodes 23023

5.2.2 Simulation Results with Current A-Pillar

The Toyota Rav4 finite element model is quasistatically loaded by loading plate

according to FMVSS 216 regulations. The full scale roof crush test simulation is conducted in

LS-Dyna for 0.15 seconds using multi parallel processing technology. The distance between

Toyota Rav4 vehicle model and loading plate contact is kept as minimal as possible to reduce the test runtime. The finite element model test set up for vehicle model Toyota Rav4 is shown in

Figure 5.4 and the end of test simulation for vehicle model Toyota Rav4 with current A-Pillar is shown in Figure 5.5.

Figure 5.4: Finite Element Model setup for FMVSS 216 Rav4 Roof Crush Test.

Figure 5.5: End of Test for FMVSS 216 Rav4 Roof Crush Test.

5.2.3 Simulation Results of IMPAXX Foam Padded A-Pillar

The created foam model is inserted into the A-pillar cavity as show in Figure 5.6. The A- pillar inner and A-pillar support shell elements are extracted as new component. Dimension for available free space between the parts at A-pillar is calculated. The shell elements are dragged toward A-pillar inner with appropriate thickness.

Figure 5.6: A-Pillar cavity without and with Foam Padding to Car Model.

The modelling of foam block is such that complete care is taken that there is no intrusion to elements. Material properties of steel is assigned to foam outer surface. Figure 5.6 shows cavity in A-pillar where foam is extracted and padded to A-pillar.

Figure 5.7: Toyota Rav4 Finite Element Model before FMVSS 216 Test.

Figure 5.8: Toyota Rav4 Finite Element Model after FMVSS 216 Test.

Simulation results of Toyota Rav4 with IMPAXX foam padded A-pillar before and after

FMVSS 216 test is shown in Figure 5.7 and Figure 5.8 respectively.

5.2.4 Simulation Results for Aluminum Foam padded A-Pillar

The created foam model is inserted into the A-pillar cavity as show in Figure 5.9. The A- pillar inner and A-pillar support shell elements are extracted as a new component. Dimension for available free space between the parts at A-pillar is calculated. The elements are dragged toward

A-pillar inner with appropriate thickness.

Figure 5.9: A-Pillar cavity without and with Foam Padding to Car Model.

The modelling of foam block is such that complete care is taken that there is no intrusion to elements. Foam model is then sandwiched between two Aluminum plates. Figure 5.9 shows cavity in A-pillar where foam is extracted and padded to A-pillar. Figure 5.9 shows only the outer A-pillar and some of the other components of the A-pillar are blanked to show foam insertion area.

Simulation results of Toyota Rav4 Finite element with Aluminum foam padding before and after FMVSS 216 Roof crush test is shown in Figure 5.10 and Figure 5.11 respectively.

Figure 5.10: Toyota Rav4 Finite Element Model before FMVSS 216 Test.

Figure 5.11: Toyota Rav4 Finite Element Model after FMVSS 216 Test.

5.2.5 Comparison of Force against Displacement Results

Figure 5.12 shows the response curves comparison of z-force against displacement in

Toyota Rav4 finite element model with current, IMPAXX foam padded onto A-pillar and

Aluminum foam padded onto A-pillar. From plot we can observe that the resultant force of current A-pillar is less than the resultant force of IMPAXX and Aluminum foam padded A-

Pillar. Area for Force v/s displacement graph plot can be observed to evaluate the energy

characteristic of a system. From the graph outline of the Rav4 force vs displacement plot we can

observe that the area under curve for current A-Pillar is least than compared to the area under

IMPAXX foam and Aluminum foam padded A-Pillar. Energy absorbed by IMPAXX and

Aluminum foam padded A-Pillar roofs are more than the current A-pillar.

Figure 5.12: Toyota Rav4 Finite Element Model Force vs Displacement Comparison.

5.2.6 Comparison of Internal Energy Results

Figure 5.13 shows global internal energy comparison plot for Toyota Rav4 model with

current, IMPAXX padded A-pillar and Aluminum foam padded A-pillar models. It can be

observed that models with IMPAXX and Aluminum foams padded onto A-pillar shows a slight increase in internal energy, which signify that the modified A-pillar with IMPAXX and

Aluminum foams show significant improvement in energy absorption. It is observed that energy

absorbed by Aluminum foam padded A-pillar is slightly greater than IMPAXX foam padded A-

pillar model.

Figure 5.13: Toyota Rav4 Finite Element Model Internal Energy Comparison.

5.3 Insurance Institute for Highway Safety Roof Crush Test

Figure 5.14 shows Insurance Institute for Highway Safety (IIHS) regulation configuration

for roof crush test.

Figure 5.14: IIHS Regulation Configuration for Roof Crush Resistance [13].

The regulations for IIHS involves the loading plate have a maximum travel displacement

of 10 inches (254 mm) and maximum force is considered between first five inches (127 mm) of

plate travel. The load applied to the vehicle is at a rate of 0.2 inches per sec.

5.3.1 Simulation Results with Current A-Pillar

The Toyota Rav4 finite element model is quasistatically loaded by loading plate

according to IIHS regulations. The full scale roof crush simulation test carried out in LS-Dyna is conducted for 0.15 seconds using multi parallel processing technology. The distance between

Toyota Rav4 vehicle model and loading plate contact is kept as minimal as possible to reduce the test runtime. The finite element model test set up for vehicle model Toyota Rav4 is shown in

Figure 5.15 and the end of test simulation for vehicle model Toyota Rav4 with current A-Pillar is shown in Figure 5.16.

Figure 5.15: Finite Element Model setup for IIHS Rav4 Roof Crush Test.

Figure 5.16: End of Test for IIHS Rav4 Roof Crush Test.

5.3.2 Simulation Results of IMPAXX Foam Padded A-Pillar

The created foam model is inserted into the A-pillar cavity as show in Figure 5.17. The

A-pillar inner and A-pillar support shell elements are extracted as new component. Dimension

for available free space between the parts at A-pillar is calculated. The shell elements are

dragged toward A-pillar inner with appropriate thickness. The modelling of foam block is such that complete care is taken that there is no intrusion to elements. Material properties of steel is

assigned to foam outer surface. Figure 5.17 shows cavity in A-pillar where foam is extracted and padded to A-pillar.

Figure 5.17: A-Pillar cavity without and with Foam Padding to Car Model.

Figure 5.18: Toyota Rav4 Finite Element Model before IIHS Test.

Figure 5.19: Toyota Rav4 Finite Element Model after IIHS Test.

Simulation results of Toyota Rav4 with IMPAXX foam padded A-pillar before and after

IIHS test are shown in Figure 5.18 and Figure 5.19 respectively.

5.3.3 Simulation Results for Aluminum Foam padded A-Pillar

Foam model is created and inserted into the A-pillar cavity as show in Figure 5.20. The elements of A-pillar inner and A-pillar support shell elements are extracted as a new component.

Dimension for available free space between the parts at A-pillar is calculated. The shell elements

are dragged toward A-pillar inner with appropriate thickness. The modelling of foam block is

such that complete care is taken that there is no intrusion to elements. Foam model is then

sandwiched between two Aluminum plates. Figure 5.20 shows cavity in A-pillar where foam is extracted and padded to A-pillar.

Figure 5.20: A-Pillar cavity without and with Foam Padding to Car Model

Figure 5.21: Toyota Rav4 Finite Element Model before IIHS Test.

Figure 5.22: Toyota Rav4 Finite Element Model after IIHS Test.

Simulation results of Toyota Rav4 Finite element with Aluminum foam padding before and after IIHS Roof crush test is shown in Figure 5.21 and Figure 5.22 respectively.

5.3.4 Comparison of Force against Displacement Results

Figure 5.23 shows the response curves comparison of z-force against displacement for finite element model with current, IMPAXX foam padded onto A-pillar and Aluminum foam padded onto A-pillar.

Figure 5.23: Toyota Rav4 Finite Element Model Force vs Displacement Comparison.

From plot we can observe that the resultant force of current A-pillar is less than the

resultant force of IMPAXX and Aluminum foam padded A-Pillar. Area for Force v/s displacement graph plot can be observed to evaluate the energy characteristic of a system. From the graph outline of the Rav4 force vs displacement plot we can observe that the area under curve for current A-Pillar is least than compared to the area under IMPAXX foam and

Aluminum foam padded A-Pillar. Energy absorbed by IMPAXX and Aluminum foam padded A-

Pillar roofs are more than the current A-pillar.

5.3.5 Comparison of Internal Energy Results

Figure 5.24 shows global internal energy comparison plot for Toyota Rav4 model with current, IMPAXX padded A-pillar and Aluminum foam padded A-pillar models.

Figure 5.23: Toyota Rav4 Finite Element Model Internal Energy Comparison.

It can be observed that models with IMPAXX and Aluminum foams padded onto A-pillar shows a slight increase in internal energy, which signify that the modified A-pillar with

IMPAXX and Aluminum foams show significant improvement in energy absorption. It is

observed that energy absorbed by Aluminum foam padded A-pillar is slightly greater than

IMPAXX foam padded A-pillar model.

5.4 Summary Comparison of the Roof Crush Test Results

Table 5.3 Summary of Roof Crush Simulation Test Results

Test Type Foam Type Peak Load (N) Magnitude Magnitude Deviation Increase % No Foam 22400 - -

FMVSS 216 IMPAXX 24800 2400 10%

AL 26200 3800 17%

No Foam 30400 - -

IIHS IMPAXX 32800 2400 8%

AL 34000 3600 11%

Table 5.3 shows summary of test results. It can be observed that peak load is higher in

both foam cases for both FMVSS 216 and IIHS tests. Higher resultant force is experienced by

IMPAXX foam, load is similar and in range under both type of testing. Peak load observed in

Aluminum foam, load is similar and in range under both types of testing. Aluminum foam

padding yields higher resistance force in both FMVSS 216 and IIHS tests. Aluminum foam

proves to be the better combination for both cases of tests.

Area under the Force v/s displacement graph plot can be observed to evaluate the energy

characteristic of a system. From the graph outline of the Rav4 force vs displacement plot we can

observe that the area under curve for current A-Pillar is least as compared to the area under

curves for IMPAXX and Aluminum foam padded A-Pillar.

CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The objective of this research was to investigate the effectiveness of foam materials

IMPAXX and Aluminum use as energy absorbing materials, when padded into A-pillar cavities of a sports utility vehicle to increase roof crush resistance thereby reducing risk to occupant during rollovers. A mathematical model of the foam block was designed with which component level simulation tests were conducted in LS-DYNA to validate the foam model performances.

Several full scale roof crush tests were then simulated according to the NHSTA FMVSS 216 and then the IIHS test regulations using sports utility vehicle (Toyota Rav4) finite element model.

The following conclusions can be inferenced from this study.

• The foam exhibited quasi-static elastic and elastoplastic behavior in Drop Tower Tests

for the thickness of 6.5 cm.

• From Free Motion Head-form tests, it was seen that Head Injury Criteria and Head

acceleration values were reduced. Up to 90 % of head acceleration was reduced by using

Aluminum foam padding at 6.72 m/s test. In all test scenarios for headform tests with

IMPAXX and Aluminum foam, the Head injury is prevented. Observations showed that

up to 87% acceleration was reduced by using IMPAXX foam.

• From NHSTA FMVSS 216 roof crush test, It was noticed that there is slight increment in

the internal energy for both IMPAXX and aluminum foam, which shows that there is

increase in energy absorption with inclusion of foams. There is an increase in force

plateau region and maximum force for IMPAXX and Aluminum foam, vehicle can

withstand higher loading till roof collapse.

• From the IIHS roof crush test, it was observed that force plateau region and maximum

forces was increased for padded A-pillar with both IMPAXX and Aluminum foams as

compared to no foam A-pillar. There is also higher energy absorption as observed from

increase in internal energy.

Overall, the study demonstrates that with the insertion of a small piece of foam IMPAXX and

Aluminum material into the A-pillar of a vehicle could significantly improve the roof crush resistance and thus improve the vehicle crashworthiness responses to rollover conditions.

6.2 Recommendations for Future Study

To extend the current study in future the following recommendations are suggested:

• Experimental tests should be performed for full scale roof crush tests for current A-pillar,

IMPAXX padded A-pillar and Aluminum padded A-pillar to validate the full-scale roof

crush tests.

• Using design optimization tools the design of foam model can be better optimized.

• For further improvements to foam materials, performance Carbon Nano materials can be

included into foam model and tested.

• Further studies can be conducted both experimental and simulation with other vehicle

models

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