The Active System

MIME 1501 - 1502

Technical Design Report

Capstone Design Course Report Project #9

Final Report

Design Advisor: Prof. Blucher

Design Team Matt Bonnell, Seth Pointer Erik Bodurtha, Ralph Castro

April 13, 2004

Department of Mechanical, Industrial, and Manufacturing Engineering College of Engineering, Northeastern University TABLE OF CONTENTS

ABSTRACT...... il •• � ••••••••••••••••• * •••••••••••••• " ..······*··· iv

1.0 PROJECT OVERVIEW...... 1

1.1 Introduction...... 1

1.2 Problem Statement...... 1

1.3 Data...... 1

2.0 REQUIREMENTS...... 3

2. 1 Extended Collapsible Structure ...... 0...... 3 2. 1.1 and Acceleration...... 3

2. 1.2 Structure Collapse...... 4

2. 1.3 Durability ...... 0 •• 0 ...... 4 2.2 Sensing System ...... 4

2.3 Deployment...... 4

2.4 Locking...... 5

3.0 CURRENT TECHNOLOGIES...... 5

3. 1 Sensing Systems ...... 5 3.2 Crumple Zones ...... 6

3.3 Aluminum Foam...... 6

4.0 ANALYTICAL INVESTIGATIONS...... 6

4.1 Target Energy Absorption...... 6

5.0 EXPERIMENTAL INVESTIGATION...... 8

5.1 Test Procedure...... 8 5.2 Test Results and Analysis...... 8

5.3 Application of Results ...... 14

5.4 Effect of System...... 15

6.0 DESIGN DETAILS...... 15 6. 1 Final Design Assembly ...... 15 6.2 Extended Sub-Assembly ...... 16 6.3 Base Sub-Assembly...... 17

7.0 POST-DESIGN ANALYSIS...... 18 7.1 Bending Moment and Shear ...... 18

7.2 Spring Plunger Stress...... 19

8.0 RECOMMENDATIONS...... 20

8.1 Metal Types...... 20 8.2 Geometries ...... 20 8. 3 Foam Considerations ...... 20 8.4 Deployment Force ...... 20 8.4.1 Frictional Force...... 21

8.4.2 Explosive Force...... 21

9.0 CONCLUSIONS...... 22

APPENDIX A...... 24 APPENDIX B...... 27 APPENDIX C...... 29 APPENDIX D...... 32 APPENDIX E...... 33 LIST OF FIGURES AND TABLES

FIGURES:

1. Chest g's vs. Injury Probability...... 2

2. System Deployment Threshold...... 5

3. Kinetic Energy vs. ...... 7

4. Force vs. Distance for Metal Samples...... 9

5. Cumulative Energy...... 10

6. Foam Force vs. Displacement...... 11

7. Foam Cumulative Energy...... 12

8. Force vs. Distance Foam/Skin Assembly...... 12

9. Cumulative Energy Foam/Skin Assembly...... 13

10. Crushed Samples...... 14

11. Final Design Assembly...... 15

12. Column Sub-Assemblies...... 16

13. Bending and Shear Stress on Structure...... 19

14. Spring Plunger Stress...... 19

15. Frictional Force...... 21

TABLES

1. Full Frontal Impact Deceleration Data...... 2

2. Energy Absorbed by 12 inch Specimens...... 10

3. Percentage of Residual Kinetic Energy Absorbed...... 11 Abstract

This report describes the development of an active front bumper system for small automobiles. This system anticipates a collision and extends a structure from the front of the , increasing the length over which the collision takes place. The increased length dissipates the energy of the accident over a greater amount of time and reduces the force transferred to the vehicle's occupants, lowering their risk of severe injuries. The main components of this design are a sensing system and a collapsible structure that extends from the front of the automobile prior to impact. This project focuses on the design of the collapsible structure. The final design recommendation for the structure is a series of six columns that collapse in much the same way as a modem vehicle's front frame, or "crumple zone'. Each column has an aluminum foam center surrounded by a thin steel frame. The columns are 12 inches in length with a 3 inch square cross section, and are connected to the front bumper of the automobile. 1.0 Project Overview

1.1 Introduction

Safety has been at the forefront of for several decades. From the first seatbelts to and anti-lock brakes, new technologies are continually being used to protect occupants in automobiles. Thanks to these developments, people have a greater chance than ever of walking away from serious accidents. Yet despite the continuous efforts to make vehicles safer, there is still room for improvement. In recent years consumer demands have led to higher speed limits on highways and an increase in the number of large vehicles, such as SUV' s, on the roads. These circumstances increase the risk to drivers and passengers, especially those who still drive small . Safety technology needs to constantly adapt to the ever changing dynamic of daily travel, as there are still thousands of people severely injured or killed in automobile accidents every year.

1.2 Problem Statement

The majority of severe accident-related injuries occur as a result of frontal collisions. These types of accidents are extremely dangerous because when a car collides head-on with another vehicle or object it decelerates rapidly, which translates a large g-force to the occupants. A high enough g-force can cause severe spine and neck injuries, and even death. The development of energy-absorbing crumple zones in the early 90's helped reduce the g-force translated to occupants and their risk of serious injury. However, the crumple zone is limited in the amount of energy it can absorb because the front frame of a car is only a few feet long. Cars with larger front ends would be able to absorb more energy, but such vehicles are less practical and desirable for everyday use. Therefore, a structure that could be stored in the front frame of a car and extended before an accident to help absorb impact energy would strike an ideal balance between safety and design.

1.3 Crash Test Data

New cars go through several crash tests to determine the risk of injuries to occupants. The full frontal test is the most applicable to this project. The National Highway Traffic Safety Administration (NHTSA) performs this test as part of their New Car Assessment Program. In this test a vehicle crashes into a fixed barrier at 35 miles per hour, and a computer analyzes the effects on seat belted crash dummies. The results show high occupant decelerations as energy is absorbed by the front of the car. Table 1 shows deceleration data for various small cars, which the active bumper system would help the most. The decelerations from this test are simulations of head-on collisions with vehicles of the same size, with each traveling at 35 mph. If the other vehicle was larger or moving faster these values would increase.

1 Table 1 -Full frontal impact deceleration data [1] Deceleration Data from NHTSA Full Frontal Impact Tests (35mph collision into fixed barrier)

2004 Make & Model Driver's Side Chest Deceleration (g) Safety Rating (out of 5 stars) Acura RSX 2-DR. 43 5 BMW Z4 Convertible 46 4 Chevrolet Cavalier 2-DR. 44 4 Dodge Neon 4-DR. 55 4 Ford Focus 2-DR. 46 4 Honda Civic 2-DR. 40 5 Cooper 55 4 Mitsubishi Eclipse 2-DR. 52 4 Nissan Sentra 4-DR. 48 4 Pontiac Sunfire 2-DR. 44 4 Toyota Corolla 4-DR. 42 5 Volkswagen Jetta 4-DR. 42 5

Deceleration values obtained from crash test dummies can be correlated to a human's probability of sustaining a severe injury. The deceleration translates to chest g's felt by the occupants. High chest g's can cause serious injuries. Figure 1 shows the probability of at least an AIS (Abbreviated Injury Scale) 4 chest injury as a function of chest g's. The NHTSA classifies an AIS 4 injury as severe.

CHESTG Figure 1 -Probability of severe injury as function of chest deceleration [1]

The chest deceleration values of 40-55g from full frontal tests of the above vehicles give the occupant a probability of between approximately 6% and 15% of sustaining at least a severe chest injury. As noted above, these decelerations and probabilities are for 35 mph collisions into a fixed barrier, the equivalent of hitting a car of the same size that is also traveling at 3 5 mph. If the cars are traveling faster, or if the opposing car in a head-on collision is larger, the resulting deceleration would be greater, and the probability of

2 severe injury would increase. The active bumper system would reduce the deceleration of the occupant by initiating the collision sooner, at a greater distance from the occupant. The extended structure would absorb a significant amount of energy and allow the accident to take place over a greater period of time. The smaller deceleration would result in lower chest g' s and give the occupant a better chance of walking away from the accident without a severe injury.

2.0 Requirements

2.1 Extended Collapsible Structure

The majority of the report will focus on the development of the collapsible structure and the way it absorbs energy. The collapsible structure needs to extend from the frame of the car prior to a.11 accident and dissipate enough energy to reduce the effects of an accident on a vehicle's occupants. Also, for the structure to work, the front bumper of the car needs to be detachable from the rest of the frame.

2.1.1 Force and Acceleration

The kinematics equation for length can be stated as:

Equation 1 where (v0) represents the velocity of the car, ( dx) is the length of the structure, and (a) is the acceleration of the passenger. The equation means a longer structure reduces the deceleration of collision. This increases the safety of the driver. Equation 2 relates the time of collapse to the force on the car. The force felt by the vehicle decreases as the time of the collision increases. The driver feels less force from impact if the collision takes place over a greater period of time. Both of these equations demonstrate the need for the

Equation 2 vehicle to absorb collision energy through a larger distance. The active bumper system would need to remedy this problem.

3 2.1.2 Structure Collapse

The way the active bumper's structure collapses is as important as the amount of energy it can absorb. A smooth collapse that requires a relatively constant force is ideal. If the force required to collapse the structure varied significantly, the vehicle would not decelerate gradually, and excess g's would be transferred to the occupants. On a force vs. displacement graph, the curve for this scenario would appear as a horizontal line. This indicates that the force required to collapse each unit of length of the structure is the same. Ideally, the extended structure would collapse in this manner at a force similar to the vehicles main frame. This would transfer the least amount of energy possible to the vehicle's occupants.

2.1.3 Durability

The structure needs to stand up to the daily operation of the vehicle. This means the materials selected for the structure need to be sturdy and able to withstand the wear and tear a vehicle endures over its lifetime. Since the active bumper system is potentially a lifesaving device, extra care must be taken to ensure that it remains intact, regardless of the amount of time the vehicle has been in service before it is needed.

2.2 Sensing System

For the active bumper system to work properly accurate collision predictions need to be made. A sensing system would have to constantly survey the area around the vehicle. This includes keeping track of the position, speed, and direction of other objects on the road. The system should signal the collapsible structure to extend only when a dangerous collision is imminent. Low-speed collisions and minor -benders would not warrant a deployment, and cars traveling in other lanes on two-way streets need to be accounted for. The sensing system is beyond the scope of this report. Current technology in this field is discussed in later sections, and it is reasonable to assume that this technology could be modifiedto meet the goals of the active bumper system.

2.3 Deployment

The active bumper's collapsible structure needs to be deployed after a collision is deemed unavoidable. It has been determined in calculations shown in the appendix that an imminent collision cannot be avoided within approximately 0.5 seconds of impact for all possible automobile speeds. Therefore, the deployment mechanism will be triggered when distance and closure rate data of a large object signal the occurrence of a collision in 0.5 seconds. The system deployment thresholdis shown in Figure 2. The system will need to deploy in 0.2 seconds to ensure that it is locked in place for impact.

4 System Deployment Threshold

:2 a. .§. 70

$.. n:: 60 "� .. 0 0 50

40

30

20

10 0 10 20 30 40 50 60 70 80 90 100

Distance (ft.)

Figure 2 - System Deployment Threshold

2.4Locking

After the structure has been fully extended before impact, it needs to be securely locked into place. This would prevent the structure from being forced back into its housing instead of collapsing. Making sure the structure collapses completely is essential for maximum energy absorption. Therefore, the locking mechanism is a key element in the design.

3.0 Current Technologies

3.1 Sensing System

As mentioned before, the active bumper system would rely on sensors to determine the probability of an accident. Currently, sensing systems using ultrasonic radio waves are used in vehicles to alert drivers of approaching objects. These devices also help cars navigate in and out of tight spaces by surveying the area around the vehicle and judging the distances around objects. Other systems using laser beams adjust the speed of a car operating under cruise control in response to the location of other vehicles. The development of the sensing system for the active bumper is beyond the scope of this

5 report. However, it is reasonable to assume that current technology could be modified to meet the requirements of this system.

3.2 Crumple Zones

For many decades, automobile frames were built rigid and did not collapse easily during accidents. This meant that the car itself absorbed a small amount of collision energy and a large amount of energy was transferred to the vehicle's occupants. Around the 1980's, car manufacturers developed collapsible front and rear frames called 'crumple zones'. These crumple zones fold in on themselves in a controlled manner during collisions, dissipating energy throughout the length of the crash. They absorb more energy than rigid frames and therefore reduce the energy transferred to the occupants. Our design for the collapsible structure in the active bumper system is modeled after these crumple zones.

According to the Nationai Highway Traffic Safety Administration ('.NHTSA), a crumple zone must satisfy three criteria. First, it should withstand fatigue from the daily operation of the vehicle. This includes the vibration of the engine and the energy transferred to the frame from the vehicle' s shock absorbers. Second, the crumple zone cannot fracture under a large material stress. If it suddenly fractured, the collision energy would be transferred to the passenger instead of dissipated by the frame. Finally, the crumple zone must collapse in a controlled manner. This ensures constant energy dissipation and a reduction in the g-force translated to the vehicle' s occupants.

3.3 Aluminum Foam

Stabilized Aluminum Foam (SAF), is an innovative new material produced by injecting air into molten aluminum and drawing aluminum foam off of the surface. The end product can be sheet cast or poured into a mold. When crushed, this material has a force displacement curve that approaches the ideal horizontal line discussed above. This means that a relatively constant force is required to crush a uniform sample. SAF is available in a wide range of densities. It can be manufactured to meet specific strength requirements. Different combinations of SAF density, metal type, and thickness can yield different energy absorption abilities.

4.0 Analytical Investigation

4.1 Target EnergyAbsorption

In order to set goals for energy absorption, we used crash test data from a Ford Focus. We chose the Focus for two reasons. First, the Focus is a small car with a similar size and shape to other cars in its class. Our goal was to develop an active bumper system for

6 small cars because they are the least equipped to sustain a serious accident. Second, a large amount of crash test data for the Focus was available, making it easy to set goals for the project.

The final design depends on the kinetic energy of the Focus traveling at 35 mph (the speed a full frontal test is carried out at). In order to determine the kinetic energy distribution, the kinetic energy (ft lbs) has been plotted against the velocity up to 40 (mph). The distribution may be viewed in figure 3. The equation used for determining this plot is given in equation 3.

2 1/ 2)final (1/ 2 1/ 2)initial A KE otal (II mv v + I ffi mv + I ffi L\ t 12 V 12 t t - 12 Vv . 12 t t Equation 3

The translational energy has been determined by the difference between the first two terms. The second term, the rotational contribution, depends on the rotating ; this number is small, but has been included for greater precision. The total kinetic energy can then graphically be determined below as 105000 ft lbs.

Kinetic Energy of a Ford Focus

160000

- 140000 +--····�·-···-··-· ------,,-·----;

- -- ·· --·-· �--· ··· ····· ·-/'------· U) 120000 + · -· --- · · · · ·· -

------·-··--··-······--· -- ·-·---·-·· � 100000 + -/ - +-·---·------·-·-··-:F-·------' 21 80000 - a> 60000 i---·------7------·---·--··-� c: w 40000 +------·--/------··---·-··-·

20000 ·+·-· --··-�"""------·--·---�

0 +--=--·--·- -- · · ---- ·- - · - 0 10 20 30 40 50 Velocity(mph)

Figure 3 - Kinetic Energy vs Velocity

Our active bumper system was designed to absorb a specific amount of kinetic energy. Equation 2 was used to determine this quantity. A standard crumple zone absorbs 81000 ft lbs of impact energy.

Ectesign= Evehcile - Ecrumplezone Equation 4

This number is an average estimate; the number will deviate slightly for large and small vehicles, but gives a general idea of the energy absorbed by a crumple zone. The value is based on the energy absorbed from a 35 mph crash test, and at this speed the crumple zone is assumed to completely crumple. The value, therefore, approximates the total energy dissipated by the crumple zone, and not a fraction of dissipated energy. The design focuses on absorbing energy from a 35 mph crash. Restricting attention to a 30 to

7 40 mph crash interval, allows the distribution around 35 mph to be determined. The kinetic energy of the vehicle within these velocity ranges minus the average energy absorbed by a crumple zone, gives the energy to be dissipated by the active bumper within these limits. At 35 mph, for instance, the active bumper should absorb 24000 ft lbs. The distribution can be viewed below in figure 3.

5.0 Experimental Investigation

We tested metal samples of different cross sections and structures. Metal was investigated primarily because of its proven ability to withstand the stress of everyday automobile use. Testing was done on a variety of sizes and weights of steel shells and aluminum foam.

5.1 Test Procedure

In order to test the energy absorption of different specimens that could be used as the extension members of the active bumper system, they needed to be crushed in an Instron machine. A hole was made in a circular plate, and it was threaded in order to screw onto the head of the Instron machine. Another circular plate was used as the base for crushing the specimens. A twelve inch long specimen made of sheet steel, with a square cross­ section, was placed between the plates, and was centered with respect to the head plate above it. The head was lowered until it was in contact with the specimen. The values were zeroed on the data acquisition program, and the machine was started. The data acquisition system measured the force and displacement values as it crushed each specimen. Afterthe specimen was crushed so much that it bega..rJ. to act like a solid mass, with the force rising quickly, the machine was stopped. Some of the tests were videotaped in order to have some visual data about the crushing behavior of the test specimens. The force versus distance data was placed in Excel, and graphs of these values were plotted. The areaunder the force-distance curve is the energy absorbed, and this was calculated numerically in the spreadsheet.

5.2 Test Results and Analysis

The force versus distance crushing data is shown below. The force values for the 22 and 26 gauge samples are significantly lower than those of the 18 and 16 gauge specimens. However, the thinner-walled samples have much lower initial force peaks, which is desirable to prevent large initial decelerations. The area under the initial peaks is very small, so it is basically useless for energy dissipation, and would ideally be eliminated. The thicker specimens also have bigger hills and valleys after the initial peak. Ideally, the force would rise to a certain value and level-off, staying fairly consistent until the specimen is fully crushed.

8 Force vs. Distance

20000

18000

16000

14000

12000 -3x3x26ga 1

�---- 1 3x3x22ga g: 10000 : -�---- 4x4x22ga u. 1 4x4x18ga i 8000 [�4x4x16ga 1

6000

4000

2000

0 0 2 3 4 5 6 7 8 9 10 x (in)

Figure 4 - Experimental force vs. distance for crushing tests

A notable aspect of the force-distance curves is that they seem to have fairly similar shapes for the same cross-section, especially the 16 and 18 gauge 4 inch square samples. The 18-gauge 4x4 sample only followed the trend through the second peak, but this could have been because that sample had fewer rivets. Many of the rivets popped out, weakening the specimen.

The graph below shows the total energy absorbed by each of the specimens as a function of distance. The area under the force curve, up to each distance value, was found by calculating rectangular areas of differential elements between each distance value that was recorded by the data acquisition system. The graph shows how much more energy is absorbed by the thicker-walled specimens because of the sustained higher required to crush them. However, these same energy values could be obtained by a specimen with a more consistent crushing force, falling between the peaks and valleys seen on the above graph of those thick samples.

9 Cumulative Energy Absorbed vs. Distance

6000

5000

4000

I-3x3x26ga i I�--- j :a 3x3x22ga : I ;s 3000 +------I ---- 4x4x22ga 1 w 4x4x18ga! I��· 4x_'lx_�(J

0 2 3 4 5 6 7 8 9 10 x (in)

Figure 5 - Cumulative Energy Absorbed vs. Distance

In order to specify an energy absorption value for each sample, a crushed distance of eight inches was chosen for consistency. At this point the specimens are beginning to act like more of a solid mass, as the force is starting to rise rapidly. The energy dissipation values up to eight inches are shown in the table below. Also, the amount of energy that would be absorbed by multiples of these specimens, arranged in parailel as they would be in the active bumper system, is shown.

Table 2 Energy Absorbed by 12 in. Specimens (ft-lb) x-section gauge number of specimens 1 2 3 4 5 4x4 16 4371 8742 13114 17485 21856 4x4 18 3241 6483 9724 12965 16206 4x4 22 1009 2018 3028 4037 5046 3x3 22 1031 2061 3092 4123 5153 3x3 26 756 1512 2268 3024 3780

The goal for energy absorption by the active bumper system is to absorb the amount of a vehicle's kinetic energy that is not absorbed by the crumple zone in a 35 mph collision with a fixedwall. This energy will be called the residual energy, and what is absorbed by the active bumper system will not be transmittedto the rest of the car andthe passengers. The percentage of this residual energy that is absorbed by each of the configurations is shown below.

10 Table 3 Percentage of Residual Kinetic Energy Absorbed by 12 in. Specimens (%) x-section gauge number of specimens 1 2 3 4 5 4x4 16 18.4 36.8 55.2 73.6 92.0 4x4 18 13.6 27.3 40.9 54.6 68.2 4x4 22 4.2 8.5 12.7 17.0 21.2 3x3 22 4.3 8.7 13.0 17.4 21.7 3x3 26 3.2 6.4 9.5 12.7 15.9

The configuration using 5 specimens that are4x4 and 16 gauge comes close to the goal of absorbing 100% of the residual energy, but there are problems with this configuration. The initial force required to crush these members at the same time would be about 94,000 lbs, which might be too high.

In order to solve the problem of high initial peak forces with less than adequate energy absorption, the use of aluminum foam was explored. First, a sample of the foam by itself was crushed in the Instron machine. The sample was about six inches long, and had a square cross-section with 2.75 inch sides. As was hoped, the initial force required to begin crushing the sample was not any larger than the forces needed to continue crushing it. The force stayed fairly consistent, hovering around 2000 lbs, until it had been crushed almost five inches and began behaving more like a solid mass. The foam absorbed about as much energy as the 4x4x22ga sheet steel specimen over the first four inches of crushing. The results were very encouraging because they showed that a thinner-walled steel specimen couid be filled with the foam to obtain high energy absorptions without large peak forces.

Force - Aluminum Foam 2124/04

8000

7000

6000

5000

4000 @: u.

3000

2000

1000

-1000

x(in.l

Figure 6 -Aluminum Foam Only

11 Cumulative Energy Absorbed -Aluminum Foam 2/24/04

1200

1000 � ��-��------

800 i---�-----��- --- �------�

:0 l 600 w

x(in.)

Figure 7 - Aluminum Foam Only Cumulative Energy

In the next tests, an assembly of aluminum foam skinned with 22 gauge sheet steel was crushed. That skin thickness was chosen because earlier test data had shown that it did not require extremely large initial peak forces but absorbed a significant amount of energy. Also, an identical piece of foam was crushed by itself, as was a duplicate of the steel skin. The goal was to determine whether the energy absorbed by the assembly was equal to the sum of the energy absorbed by its parts. This information could then be used to determine the energy absorption of larger members filled with foam. The test results for the three specimens are shown below in the graph of force versus crushed distance.

Force - 3/11/04

14000

12000

��-��--�----�------�------10000

8000 --assembly ----foam only i --�---skin only !

o•------,-----�----,----�-�--�----�--�---�-�---

0 0_5 1_5 2 2_5 3 4 x(in.)

Figure 8 - Force Displacement for Foam/Skin Assembly

12 The cumulative energy absorbed as a functionof crushed distance was then calculated in Excel and plotted for each specimen. The energy curves for the foam only and the skin only were added together for comparison with the energy curve of the assembly. The graph below shows that the energy absorption of the assembly is significantly more than the sum of the energy absorption of each of its components alone. When the energy curve representing the sum of the foam and skin is multiplied by a factor of 1.7, the resulting curve matches the energy absorption curve of the assembly fairly wen.

Cumulative Energy Absorbed - 3/11/04

4000 -,

2500 ,-­ !-assembly :a foam only 2000 1--- � ------skin only 1 - w ,I toam+s. k "1n i 1----1-'-�+_s) , 1500

1000

500

0 0,5 L5 2 2,5 3 3.5 4 4.5 5 x(in.)

Figure 9 - Cumulative Energy for Skin/Foam Assembly

The conclusion that the energy absorption of the assembly is almost twice the sum of its parts is somewhat surprising. It can be explained as a stabilizing effect that the foam has on the steel skin. According to a paper by CYMAT, the manufacturer of SAF, "not only does the foam absorb energy but it also causes the tube to behave differently (i.e. more folding) and absorb more energy than when empty." This effect of creating more folding in the steel skin can be seen in the picture below. The picture shows the crushed foam­ skin assembly on the left and the crushed skin on the right. The assembly has many folds in the steel, compared to the skin alone, which only has one or two large folds. The energy absorption of the steel in the assembly is greater because more work must be done to create more folds. An uncrushed piece of aluminum foam is also in the picture and shows the size of the specimens before they were crushed. In addition to the stabilizing effect described above, the skin may also cause the foam to absorb more energy because it cannot expand outward when it is being crushed. Each element basically makes the

13 other one stronger and more consistent, causing each to absorb more energy and collapse more evenly than they could by themselves.

Figure 10 - Crushed Samples

5.3 Application ofResults

The results of the crushing tests were used to determine a design that meets the energy absorption goals. As described above, it was determined that the energy absorbed by the foam-skin assembly was approximately 1.7 times the sum of the energy absorbed by the foam al one and that of the skin alone. This relationship will be a good estimate for similar cross-sections of aluminum foam to the one tested (2.75 inches square) that are skinned with the same thickness of sheet steel. The approximate energy absorption of a twelve inch long piece of foa..'ll with a three inch square cross section, skinned with 22 gauge steel was calculated, and this calculation is shown in the appendix. The test assembly was six inches long, and it crushed 3.5 inches before it began to undergo densification strain and act like a solid mass, with the force steadily rising. Therefore, a twelve inch long foam-skin assembly would crush about seven inches, and the energy absorption was calculated for this distance. The energy absorption of the foam is a function of volume, so this calculation was simple, and the data for a 3x3x22ga steel specimen had already been obtained. The energy absorptions of the foam and skin were added together and multiplied by 1.7, and the resulting energy value was 4320 ft-lb. This value is the estimated energy absorption of one twelve inch long member. Using CYMAT's equations for estimating average crush force and energy absorption of foam­ filled crashboxes, the energy absorption is estimated as 4790 ft-lb. However, the more conservative estimate of 4320 ft-lb will be used because it is based on actual experimental data instead of theoretical calculations. Multiple members can be configured in parallel for an active bumper system and the total energy absorption will be the sum of the energy absorbed by the members. The target for the system on a Ford

14 Focus is 24,000 ft-lb, which is the difference between the kinetic energy of the car at 35 mph and the amount of energy absorbed by the car' s crumple zones. To meet this target energy absorption, the active bumper system for a Ford Focus would consist of six parallel members. The total energy absorbed by this system would be approximately 25,900 ft-lb. The system could have fewer extension members than this and still reduce the chance of serious injury to the passengers in a frontal collision, but to meet the specified target energy absorption the system would need six members.

5.4 Effect of System

The fact that the active bumper system will absorb 25,900 ft-lb of a vehicle' s kinetic energy in a collision will dramatically reduce the chance of severe injury or death to passengers. For example, when a Ford Focus crashes into a wall at 35 mph the car will have approximately 79,100 ft-lb of kinetic energy after the active bumper system has been crushed and absorbed its energy. This means that the speed has been reduced to 30.4 mph when the crumple zones begin to be crushed. The driver and passengers experience the impact of a collision that is almost 5 mph slower than without the system. Using the kinematics equations it can be determined that the deceleration of the driver' s chest is reduced from 46g to 35g. This improvement has the dramatic effect of lowering the probability of a severe or fatal chest injury from 9% to 4%. The deceleration during crushing of the active bumper would only be 1 Og. The calculations that yielded this information can be seen in the appendix. The effects of the active bumper system on high speed collisions could mean the difference in life and death

6.0 Design Details

Figure 11 - Final Design Assembly

6.1 Final Design Assembly

The final design, shown above, is a series of six collapsible columns that connect to the back of the vehicle' s front bumper. Each column is a combination of two sub­ assemblies, which are pictured below. The structure on the left is the extended sub-

15 assembly. It consists of a column of aluminum foam encased inside a steel shell with a steel end plate. This is the collapsible structure that extends from the front of the car before a collision. The sub-assembly on the right is the housing and locking portion of the design. It consists of a steel housing, four steel brackets, twelve plungers and nuts, and an end cap. This portion stays inside the frame of the car and holds the collapsible structure before extension. At the end is a plunger-spring locking mechanism that ensures the extended structure collapses instead of getting pushed back. The parts in each sub-assembly are discussed in detail below, and the function of each part is explained.

Figure 12 - Column Sub-Assemblies

6.2 Extended Sub Assembly

Aluminum Foam 1 /

The aluminum foam is the central component of the design. It was cut to have a 3" by 3" cross section and a length of 12 inches. This piece is the major energy-absorbing component. Since energy absorption is the main factor of the design; the remaining components were built around this part. This ensures the foam behaves properly, crumpling uniformly during a collision.

The shell was built from a 22 gage steel sheet. The shell is constructed by bending the sheet to dimensions close to the 3x3 foam cross-section; and is cut to a 15 inch ]ength. The length of the shell exceeds the foam by 3 inches. The extra length provides room for the bored plunger holes and an explosive device for deployment. When the shell is closed around the foam, the overlapping seam is welded along the length of the shell. To ensure the pieces do not slide relative to each other, an adhesive was added to the inner face of the shell. This configuration also adds energy absorption to the design. Moreover, holes

16 have been drilled at a distance of approximately 12 inches from the front face. This ensures the plunger tips will rest against the back of the foam, and lock a large portion of the foam into the collision field (approximately 11 Y2 inches).The holes have also been elongated by Y2 inch to ensure that plunger tips project into the holes during deployment of the foam and shell assembly.

End Plate

The end plate was also made from 22 gage steel. The piece was cut to a 3x3 cross­ section. The cross-sectional face of the end plate coincides with the rear face of the aluminum foam. The piece serves as extra support for the plunger tips. In short, when the system deploys, the bored holes in the shell and the rear cross-section of the foam will experience less stress with the added piece. By decreasing the stress at these points, the bored holes and foam are less prone to tear, as the plunger tips lock behind the shell and foam.

6.3 Base Sub Assembly

Sleeve

The sleeve was constructed from 22 gauge steel. The piece acts as a holding device for the extendable structure. The sleeve is cut with a cross-section slightly larger than the shell and has a length of 15 inches. Twelve holes have been drilled to coincide with the holes in the shell, and provide bores for the plunger tips to project through. To explain the relationship to the last three components, when the shell and foam assembly are within the 15 inch sleeve, the plunger tips will be pressing against the surface faces of the shell, and only project into the shell holes upon deployment.

End Cap

The end cap attaches to the end of the sleeve. The piece acts like a cap, and is directly welded to the sleeve. The end cap produces a closed cavity at the back of the sleeve and therefore at the back of the shell. The dimensions of the cavity are 3 x 3 x 3, and provide a closed surface area for a propellant to act properly within.

Bracket

The bracket attaches to the end of the sleeve where the bored holes are positioned. There are four brackets: one attached to each face of the sleeve, and the hexagonal holes center with the bored holes through the sleeve. The bracket serves two purposes. The bracket

17 first acts as part that attaches to the frame of the vehicle; secondly, the bracket's hexagonal cuts position the hex nuts. The hexagonal face is welded to the sleeve along the external edges of the hex face, and the solid face is positioned flush with the edge of the sleeve. A stress calculation proving the member will not fail upon impact is presented in the appendix.

Nut

The hex nuts are class 2A nuts ( 5/8 - 11); the 5/8 represents the nominal diameter and the 11 stands for threads per inch. The hex nuts are welded to the edges of the hexagonal cuts in the bracket. The spring plungers are directly screwed into the hex nuts, and support the plunger as its tip undergoes shear stress from an impact.

Spring Plunger

The spring plungers act as a locking mechanism for the system. As stated above, the plunger rests within a hex nut, and a total of 12 plungers are positioned about the design. Before the shell and foam are deployed, each plunger exerts 1 Olbs of force against the surface face of the shell, and the tip rests within the plunger's casing. The plungers will induce friction along the shell when deployed. After deployment, however, the tip projects into holes bored in the shell, and the contact force is released. In addition, the plungers are a standard part. The casing extrudes a iength of 1.5 inches, and the tip project about .3 1 inches. These plungers do not fail u.11derthe force of collision.

7.0 Post Design Analysis

7.1 Bending Moment and Shear

When the six specimens are propelled out of the sleeves, there will be a bending moment and shear force acting on the cross-sectional area of each specimen. The bending moment a..11d shear force acting about the neutral axis of the cross-section is shown below in figure x. Furthermore, stress calculations have been performed on one specimen; the first calculation determines the max bending stress from the moment; whereas the second determines the max shear stress. The max bending stress turnedout as 1.184 ksi, and the max shear 3.49e-2 ksi; both calculations yield values below the yield stress 30 ksi, and prove that the design will not deform or fracture when fired into the collision field.The details of these calculations are presented in the appendix.

18 f.---- LSQQ00------1-' j 1------16.1200------1

Figure 13 - Bending and Shear Stress on Structure

7.2 Spring Plunger Stress

In the diagram below, each foam and shell sample will experience a max compressive force of 15000 lbs before crumpling. The force is supported by spring plungers tnserted into the outer sleeve through a nut, and the plunger tip projects into holes drilled through the internal shell, and rests behind the foam.

I 0--0:�� I __ � I )- - - , -4 I I H<>>Nut 1 I I � I I I 15000 tb,; I ' I I I I \:- -l 1 @ Bo•

_ u �Ex1:@-l"t"'O.I \_� _. Fig re I Shell SMll ana roor.

Figure 14 - Spring Plunger Stress

Using the critical force before crumpling, the stress on one plunger has been determined by the shear stress formula. This shear stress turns out to be 264.9 ksi. The shear stress on one plunger has been divided by the yield stress for steel or 30 ksi. The number of plungers required to prevent yielding is approximately 9 plungers. However, twelve plungers have been employed in the design. This yields a factor of safety of 1.4, a value close to the convention (F.S. = 2), to ensure failure does not occur. These calculations have been displayed in the appendix.

19 8.0 Recommendations

This project is a huge undertaking in engineering. We were able to prove a bumper that deploys in the instant before a collision is able to absorb significant amounts of collision energy. Our model meets the design goals we set for it. However, before a marketable active bumper system can be developed by major car companies several issues will have to be addressed. For starters, our research is for a specificcar, the Ford Focus. While our calculations are valid and can be used for energy absorption values of different vehicles, the exact design specifications will have to be changed to accommodate these vehicles. In addition, we developed a structure using research that was limited by our time and resources. Several suggested areas for further research are listed below.

8.1 Metal Types

Because of a supply of affordable steel, this project focused on steel to construct housings and sleeves. There is a wide array of metals that would also deform in a plastic manner and absorb energy. Steel is heavy and could have an effect on a vehicle's fuel economy. Other lighter metals may be able to provide the same strength and energy absorbing capabilities without an excess of added weight. More testing and research should be done to determine, if necessary, an ideal replacement for steel.

8.2 Geometries

The geometry of the energy absorbing member will most certainly have an effect on the amount of energy that it can absorb. While an array of thicknesses was tested, a limited number of cross section shapes was considered. While the final cross sectional design will absorb high amounts of energy, more geometry will have to be analyzed to findthe optimum metal thickness and cross section.

8.3 Foam Considerations

While the technology of skinned aluminum foam is still in the scholarly level of development, we feel that the benefits of its application in active bumper systems will prompt automakers in the future to use this new technology in their quest to provide people with the safest automobiles possible. To this end, further research on foam densities and types should be carried out as this industry develops.

8.4 Deployment Force

Deployment was not investigated in this report. However, we were able to develop some useful data for deploying our design.

20 8. 4.1 Frictional Force

Each specimen experiences friction during employment. The friction force is an important value that is related to the explosive force needed for deployment. Following figure x below, the net force in the y direction has been determined as 13.83 lbs; this is the net contribution from the foam, the shell, the sleeve, the bumper cover, and the bumper support acting on one member. In addition, there is 10 lbs of force applied by each plunger. This contributes 120 lbs of force to the internal shell and foam. The reaction force from all components acts normal on the internal member. The net normal reaction from these members is 133.8 lbs. Using the coefficient of static friction for steel on steel (J.ls .8), the friction force turns out asl07.064 lbs. The actual frictionis less than this value; the coefficient of kinetic friction should be used during deployment, but to simplify calculations and state the value as worst case 1-ls will only be used. ( ( �d����tr -- ·---- - ·- '-· ---' 0----"'- - -- _L �-1 F I .J

Figure 15 - Frictional Force

8.4.2 The Explosive Force

The explosive force measures the force needed to overcome friction acting on the sleeve, and meet the time requirement given by the closure rate calculation. The governing equation is:

* 2 Fexplosive-Fr = (mspecimen + l/6(mbumper + msupport)) (2L'lx)/t

Equation 5

Solving the equation for the explosive force, with the time equal to the approximate value of .2s, the explosive force turns out to be 128.5 lbs. The force can be divided by the cross-sectional area (9 in), and the required pressure turns out as 14.3 psi. Importantly, this force or pressure must be applied to the cross-section throughout the entire deployment. Many propellants are capable of producing such a force and deployment under the constraints (i.e. time andfriction) which will make deployment feasible.

21 9.0 Conclusion

The active bumper system would be a valuable safety improvement for dealing with head-on collisions. The technology behind the development of such a system is solid and can be manipulated to achieve the goals outlined in this report. Based on our design ideas and research, there are strong indications that an effective active bumper system can be manufactured in the near future.

22 References

[1] National Highway Traffic SafetyAd ministration, http://www.nhtsa.dot.gov

[2] Ericks, C (2001). Crumple Zone, http:// www.physics.northwestem.edu/classes/200 1 Spring/1 3 5-l/Projects/ 2/crumpy .html

[3] Ericks, C (2001). and Collision Analysis, http:// www.physics.northwestem.edu/classes/200 1 Spring/1 351/Projects/2/ momentum.html

[ 4] Crumple Zones in Automobiles, http://members.aol.com/phy sics/crumple.html

[5] CYMAT: Technical Manual for Stabilized Aluminum Foam, http:// www.cymat.com/Cymat_Techni cal.htm

23 Appendix A: Energy Calculations

Application of Experimental Results

Energy absorbed = E

from 3/11/04 data:

E 1.7 + foamskinassembly � (Efoam Eskin)

Volume of crushed foam in test: 2 vl = (2.75) (3.5) = 26.47 in3

Volume of crushed foam in 12" long 3x3 member: 2 v = (3) (7) 2 = 63 in3 E in design : foam V 63 = 2 = (694 ft ·lb) Efoam -Efoamtest V1 26.47 E 1 foam = 16 52 ft · b

from 1/30/04 data :

(12" long 3x3x22ga specimen crushed 7 inches)

E = 891 ft·lb

1.7(1652 + 891) = 4320 ft·lb Efoamskinassembly � 4320 ft ·lb is estimated energy absorbed by 12" long

3x3x22ga assembly (crushed 7 inches)

Target for entire system (for Ford Focus):

E > 24,000 ft ·lb

24,000 . --- = 5. 6 ----+ nee d 6 extensiOn memb ers to meet target E 4320

Energy absorbed by entire system design :

6( 4320) = 25,900 ft ·lb Esystem �

24 Effect of System on Collision

E 25,9QQft ·lb system � KE 105 QQQft ·lb vehicle �

KE=1 KE -E = 79,1OQft · lb vehicle system

1w '2 KE'= --v ;w = 2566lb(Focus at 35 mph) 2 g

2(KE')(g) v'= = 30.4mph w ,') v�') -v�') 0 -v·� �X= o 0 2a 2a' 2 v a , 0 a=-­2 v 0 Probability of AIS � 4 Chest injury

a=�· 462:: % 0 9 a'=35g:4%

2 2 v -v �X= ----- 2a a= lOg

Qualitative Methods- From CYMAT company literature Favg = F�vg +afAr + Cl �aoa f Ao

F!g average crush force of corresponding non-filled section

af = foam compressive strength

Af =foam core cross-sectional area

25 cl =dimensionless constant, cross-section dependent = ()o characteristic stress of the extrusion material taken as 12 (YS + UTS)

Ao = cross-sectional area of extrusion

Favg = F!g + afAf + Cl �aoa f Ao 13 Faov g = C o(/J2 ()0 A 0

Co = cross section dependant constant = A Af) : rp ( o -;- Solidity Ratio

Square Cross-Section Circular Cross-Section Co 1.3 2.15 cl 1.4 .9

s E = Stroke efficiency dmax = maximum stroke length of the absorber L = initial component length

(Fa�g + C1 �a0afA0)S� + (afaf )&n SE = ----"------Favg 0 S� = .76(1 -1.7 p .8)(Square) S� = .76(1 -1.7p 2)(Round)

& = & = 1-1 D densificationstrain of the foam: D .5 P

Energy= Favg x SE x L

26 Appendix B: Stress Calculations Stress on Support Bracket

The bracket experinces stress upon impact Depending on the bracket's attachment to the frame the rear face will be support a distributed load acting normal onto this fa ce. The plate has been modeled as a worst case scenerio; the distributed force has been applied to the top of the bracket face so the resultant force about the support will be a maximum. This resultant moment will determine the maximun bending stress at this location. A calculation covering the shear stress at the support will aslo be computed. This values will determine if the bracket fails under the resultant force.

Bro.cke--t SlMpllf'le-d Model

Moment calculation

M = Fx where x = 1.5 in F = 3750 lbs

M = 5625

Bending Stress from the Moment

Mel! M = 5625 !bs*in a max = I c= 1/16 in 4 I = .84375 in I

416.6 psi Ia max =

Shear Stress at Support

= VQ/It V= 3750 lbs "tmax Q = .01 1719 4 I = .84375 in

t = 3 in

17.36 psi "tmax =

Neither stress calculation yeilds a value close to the yield stress of 30 ksi.

27 Shear Stress Calculation for Spring Plungers

Figure 1 represents the basic design we wm use to eject the shell and foam. The following calculations focus on the locking mechanism to be used in the design; they prove that the plungers are sufficient for resisting a loading force of 15000 lbs, and indicate the number of plungers need to achieve this resistance. The plungers will be inserted through the hex nuts. {Note: the details for only one side are shown below)

S.he:l

Determining the minimum number of spring plungers to support a max load of 15000 lbs.

Shear stress is the largest stress contribution to the length of the spring plunger tip. The tip length is .3130 inches as shown below in figure 2.

For one spring plunger:

using tmax = VQ where: v = 15000 3 It = yA' = 2r Q ---- 3

tmax reduces to: 4V I = nr4 � t= 2r with r = .155 inches

Assu111ing no other stresses act on tip, the principal streSS equals: 1

2 cr = (crx+ cry!2) +/- { {(crx - cry)/2] + i}112

= 264.9 ksi a = +/- 'tmax

Compairing with the yield stress for steel:

cry = 30ksi

number of plungers = cr/cry = 8.8 plungers

using 12 plungers - 3 on each face - yields a factor of safety of 1.4

Checking the stress on one plunger with inserted gives 12

Tmax = VQ = 22.08 ksi < cry ---__ 1 21t

28 Appendix C: Sensing and Deployment Calculations:

Determining the Closure Rate

The following diagram is used to determine the time it takes for a vehicle to swerve out of the way of a collision. It will be shown that the velocity of the vehicle effects the closure rate very little.

This calculation assumes that the vehicle does not slide on the road or skid away from a circular path. The assumption means that an must not exceed 1g or skidding occurs.

v

I IL I ! D Au-to ]

v = velocity of vehicle x = length of the front end of a vehcile (constant = 1.52m = 60in) R = radius of circle a=max centripetal acceleration (constant 1 g = 9.81 m/s2) L = distance to stationary object s = arc length (approx. by pythagorean theorem)

Starting with the equation:

for v = 35 mph = 15.6 m/s for v = 70 mph = 31 .3 m/s

R = 24.8 m R = 99.8 m

At these speeds, the max length allowed for the car to swerve away is given by:

for v = 35 mph = 15.6 m/s for v = 70 mph = 31.3 m/s

l = 8.55 m L=17.35 m=

using the linear relationship: (This assumes no breaking occurs or deceleration during closure)

L = vt for v = 35 mph = 15.6 m/s for v = 70 mph = 31.3 m/s

t = .548 t= .554

29 Calculation of Friction Force:

A calculation of the friction force determines the amount of force required to propel! the foam and shell into the collision field. The force differential between the friction force and force produced from the propellant can be used to determine the firing speed.

In the diagram below, mg1 represent the weight of the shell, foam and end cap, mg2 represent the weight of the bumper support, mg3 depicts the weight of the bumper cover. All three of these forces produce the normal reaction that determines the friction force signified by F.

Also within each hole, the spring force exerted by the plunger contributes to the friction force F . (note: only three holes are depicted , but there are acually 12 each with a plunger inserted)

Friction Equation

Fr = ( 12Np + Nfs + 1/6[Nbumper + N supportD!ls !ls = .8 steel on steel the static coeficient has Np = 1 0 lbs of force fmm each plunger been used as worst case scenario Nfs = \Nfoam +INsteel + Wend cap

Wsteel = (pVg)steel = 1.45 lbs

Wtoam = (pVg)foam = 1.14 lbs

Wend cap=(pVg)end cap= .073 1bs

Nfs = 2. 66 lbs

Nbumper = 44.75 lbs - the specimen only holds 1/6 of this weight

Nsupport = 22.3 lbs - the specimen again holds 1/6 of this weight

these were determined by the mass property function in SolidWorks

using these terms:

F1 = 107.1 lbs friction force on each specimen

30 Explosive Force Requirment

The explosive force determines the force nessessary to propell the active bumper into the collision field. The calculation is based on the specimen and 1 /6 of the bumper cover and bumper support. This will determine the explosive force required at the base of one specimen.

The explosive force is also related to the closure rate; the time from this calculation yields the max time in which the system must deploy. That is, the system must deploy in (t<.55)

Choosing .20 s which is 36% of .55 is a reasonable number the equation used is:

Fexplosive - Ft = (mspecimen + 1 /6(mbumper + msupport))*(2�x)/e

I:m = 13.85 Ibm F1 = 107.1 lbs

�x = 12 in t = .2 s

Plugging in values yields:

I:F= 21 .49 lbs F explosive = 128.5 lbs

31 Appendix D: Qualitative Analysis

The following table shows results from the numerical analysis of the equations in Appendix A for multiple materials, cross sections, and thicknesses.

skin foam material thick. dim. x-section cr, (psi) rei. p cr1 (psi) (in) b (in ) h (in) d (in) Co Ao (in ) At(in ) F ovg (lb) Fovg (lb) (ft-lb) so I c,

32 Appendix E: Drawings and BOM

BOM-Active Bumper Assembly

Q!y Part

12 Spring Plunger McMaster Part #84975A75

12 Hex Nuts, 5/8-11 (standard)

1 Aluminum Foam

1 Shell

1 End Plate

1 Sleeve

1 End Cap

4 Bracket

33