CRASHWORTHINESS OF FORD F250® TRUCK “ROLL OVER –INVERTED VEHICLE DROP TEST (SAE J996)”

by

Pritesh K ShahB.E. Mechanical Engineering, January 2003, Pune University, INDIA

A Project Report Submitted to the Faculty of Old Dominion University inPartial Fulfillment of the Requirement for the Degree of

MASTER OF ENGINEERINGin

DESIGN & MANUFACTURING

MECHANICAL ENGINEERING

OLD DOMINION UNIVERSITYAugust 2011

ME-11-0082

CRASHWORTHINESS OF FORD F250® TRUCK “ROLL OVER –INVERTED VEHICLE DROP TEST (SAE J996)”

by

Pritesh K ShahB.E. Mechanical Engineering, January 2003, Pune University, India

A Project Report Submitted to the Faculty of Old Dominion University inPartial Fulfillment of the Requirement for the Degree of

MASTER OF ENGINEERINGin

DESIGN & MANUFACTURING

MECHANICAL ENGINEERING

OLD DOMINION UNIVERSITYAugust 2011

Approved By:

___________________________ (Advisor)

___________________________

ABSTRACT

A finite element analysis of roof strength for Ford F250®® truck as per SAE J996

standard is performed using LS-DYNA®. A detailed finite element model was taken from

NCAC (National Crash Analysis Centre) at George Washington University Virginia.

NCAC is one of the prominent leaders in vehicle safety research. Efforts were taken to

subject the F250 FEM (finite element model) for roll over impact study with the

procedure suggested by SAE J996.

After running the F250 FEM for roll over, key areas for reduced impact on the

roof structure of the truck were addressed and were redesigned. Further, a detailed study

is carried out to compare the results between the original F250 model with the redesigned

model. A comparison chart was made to illustrate the reduced impact on the predefined

nodal points on the roof structure.

TABLE OF CONTENTS

CHAPTER 1........................................................................................................................7

INTRODUCTION...............................................................................................................7

1.1. United States standards applicable to rollover..........................................................8

CHAPTER 2......................................................................................................................11

CASE SETUP AND ANALYSIS......................................................................................11

2.1. F250 Model Environment.......................................................................................11

2.2. Frontal Impact Analysis..........................................................................................14

2.3. LS- DYNA simulation...........................................................................................15

2.4. Dummy Database...................................................................................................18

CHAPTER 3......................................................................................................................24

ROLLOVER – INVERTED VEHICLE DROP TEST......................................................24

3.1. Case setup...............................................................................................................24

3.2. Results of rollover impact of Ford F250® (SAE J 996)........................................27

3.2.1. Total Energies.....................................................................................................27

3.2.2. Roof intrusion/deflections........................................................................................28

3.3.3. Force V/s Deflection................................................................................................29

3.4. Redesign of Roof structure Ford F250®...............................................................30

3.5 Results: Redefined Roof Ford F250®....................................................................34

3.5.1. Total Energies..........................................................................................................34

3.5.2. Roof Intrusion/deflection.........................................................................................35

3.5.3. Force V/s Deflection................................................................................................36

3.6. Comparison.............................................................................................................37

CHAPTER 4......................................................................................................................39

CONCLUSIONS................................................................................................................39

REFERENCES..................................................................................................................40

LIST OF FIGURES

Figure 1 Highway rollover fatalaties...................................................................................7

Figure 2.1 NCAC FORD F250® Finite Element Model...................................................12

Figure 3 Ford F250® with frontal impact with a rigid barrier (no dummy) @ 35 mph....14

Figure 4 Different stages of simulation for frontal impact (no dummy) @35mph...........17

Figure 5 Hybrid III 50th percentile rigid Fe dummy.........................................................18

Figure 6 Positioned Hybrid III dummy.............................................................................19

Figure 7 Ford F250® with frontal impact with 50th percentile Hybrid III dummy @ 35

mph....................................................................................................................................23

Figure 8 Different stages of deflection of roof Ford F250® (SAE J 996).........................26

Figure 9 Total Energies plot for Ford F250® FEM...........................................................27

Figure 10 Roof displacement V/s Time plot for Ford F250® FEM..................................28

Figure 11 Wall force V/s Displacement plot for original F250 FEM................................29

Figure 12 Support plate finite element model...................................................................30

Figure 13 Different stages of deflection of redesigned roof Ford F250® (SAE J996).....33

Figure 14 Total Energies plot for redesigned roof F250 FEM..........................................34

Figure 15 Roof displacement V/s Time plot for redesigned roof F250 FEM...................35

Figure 16 Wall force V/s Displacement plot for redesigned roof F250 FEM...................36

Figure 17 Comparison of deflection for original and redesigned roof F250 FEM............37

Figure 18 Comparison of Wall Force V/s Displacement for original and redesigned roof

of F250 FEM......................................................................................................................38

NOMENCLATURE

FEM – Finite Element Model

FMVSS – Federal Motor Vehicle Safety Standards

SAE – Society of Automotive Engineers

NHTSA – National Highway Traffic Safety Administration

NCAC – National Crash Analysis Centre

ms- Milliseconds

N- Newton (Force)

DOE – Design of experiment

CHAPTER 1

INTRODUCTION

Every year more than 10,000 people die in rollover crashes. Rollover crashes

constitute to a meager 2 percent of all collisions and yet account for 24 percent of

passenger fatalities. Rollovers are one of the most dangerous forms of vehicle crashes

because of the high occurrence of occupants catastrophic head injuries and fatalities.

Figure 1 Highway rollover fatalaties

In 2007, the National Highway Traffic Safety Administration (NHTSA) made it

mandatory that all vehicles to be equipped with electronic stability control (ESC). By the

2008 model year, ESC was standard on 65 percent of passenger cars, 96 percent of SUVs

and only 11 percent of pickups. The ESC technology helps minimize skidding, as well as

maintains control when drivers swerve. ESC senses when a driver may lose control and

automatically applies brakes to individual wheels (Outer front wheel to counter over steer

or inner rear wheel to counter under steer) to help stabilize the vehicle and avoid a

rollover [6].

Considerable research has been undertaken over the years to differentiate rollover

according to severity and to develop a standard rollover test. In most cases the studies

are applicable to passenger cars. However, many of the principles are generally

applicable to all vehicles. In recent years, the increase in rollover casualties from the

growing population of pickups and SUV’s has emphasized the need to examine these

vehicles as separate classes.

1.1. United States standards applicable to rollover

The following FMVSS (Federal Motor Vehicle Safety Standards) are practiced in

application of rollover safety features. [1]

Interior Protection -FMVSS 201

Glazing Materials -FMVSS 205

Door Locks and Retention - FMVSS 206

Occupant Crash Protection - FMVSS 208

Windshield Mounting - FMVSS212

Side Impact Protection - FMVSS 214

Roof Crush Resistance - FMVSS 216

Fuel System Integrity - FMVSS 301 [1]

Rollover crashes have been differentiated by a large number of parameters that

may influence rollover outcomes. Examples of such parameters are the terrain

topography, roadway grade, curvature and vehicle type. Further, a basis for selecting test

procedures and relating compliance with these procedures to real world benefits is vitally

needed.

The problem of assessing countermeasures in rollover is perplexed by the lack of

dummies and test procedures to study rollover. There is no rollover test dummy which

has been validated in a manner similar to the Hybrid III in frontal crashes, or the SID in

side impacts.

As mentioned earlier many aspects contribute to the occurrence of rollover

crashes. Rollover occurrences are mainly related to the vehicle type, unsafe and

irresponsible driving behaviors such as hard steering maneuvers, bad road design, etc.

Certain class of vehicles, such as Sport Utility Vehicle (SUV) and pickup trucks, are

more prone to rollover than other classes of vehicles. Most vehicles do not have adequate

roof strength during a rollover.

The quasi-static roof crush test mandated by the FMVSS 216 subjects the vehicle

to a maximum force significantly less severe than would be applied to the vehicle during

a multiple rollover. The Society of Automotive Engineers (SAE) recommended practice

J996, Inverted Drop Test, is also a test of rollover crashworthiness, and was developed by

SAE in the late 1960s. Since it is a more severe test, numerous engineers prefer it to the

quasi-static FMVSS 216 test.

The SAE J996 test was designed, “…to obtain as closely as possible deformation

of a vehicle roof or roll bar structure which occurs in a vehicle rollover.” In this test, the

vehicle is inverted, given a roll angle, pitch angle, and drop height that are representative

of the assumed loading at rollover. The angles present ensure that the majority of

potential energy is transferred directly to the A-pillar structure. This standard does not

specify any crush measurement methodology, permanent or dynamic [7].

The project focuses on the roof strength of the Ford F250® in event of a rollover

by testing the FEM using the “Inverted Vehicle Drop Test Procedure – SAE J996.”

CHAPTER 2

CASE SETUP AND ANALYSIS

2.1. F250 Model Environment

A Finite Element Model (FEM) of Ford F250® was used from the National Crash

Analysis Centre (NCAC) [2]. NCAC at The George Washington University's Virginia

Campus is one of the nation's leading authorities in automotive and highway safety

research.

The finite element model of a 2006 Ford F250® pick-up truck was developed at

the NCAC for the National Highway Traffic Safety Administration (NHTSA). The Ford

F250® truck is a multi-purpose pickup truck. The vehicle obtained by the NCAC is an

Extended-Cab, with a wheel base of 3610 mm (142.12 inches) and a maximum width of

2030mm (79.92 inches). The F250 has a total kerb weight of 3016 kilograms (6072.2 lbs)

with a 5.4 litre V8 engine. The pickup truck has a 5 speed manual transmission with a

four wheel drive configuration. However, several other models exist, such as higher

engine capacity, automatic transmission and drive configuration, with no change in the

general geometry.

Figure 2.1 NCAC FORD F250® Finite Element Model

The model consists of 871 parts with 738165 nodes made up of 726759 elements.

The following table illustrates different type of elements constituting the total number of

elements.

Table 1 Types of elements in Ford F250® FEM

Shell element Beam element Solid element

698501 2353 25905

Specifically, the properties of each component are defined by a set of material cards

with four types of materials being used in the model. Each of the components is

subdivided into either shell elements, beam elements or hexahedron elements.

Two types of shell elements are used in the finite element model, viz. quadrilateral

shell elements and triangular shell elements. The formulation of both types of shell

elements used is based on Belytschko-Tsay theory. Table 2.1 enlists the material models

used in LS- DYNA® along with the number of components.

Table 2 LS-DYNA material models

Material type number

Material Type No. of Components

7 Blatz–Ko Rubber 18

S02 Damper Viscous 4

1 Elastic 70

57 Low Density Foam 12

9 Null 48

24 Piecewise Linear Plasticity 598

20 Rigid 106

100 Spotweld 1

S01 Spring Elastic 5

S04 Spring Non Linear Elastic 5

6 Viscoelastic 4

2.2. Frontal Impact Analysis

The F250 FEM (finite element model) was subjected to a frontal impact study with

the parameters suggested by Federal Motor Vehicle Safety Standard (FMVSS) 208 which

states

35 mph (56kph) into fixed barrier.

50th percentile Hybrid III adult male dummy in front seat.

Initially a full frontal impact without the dummy was simulated to check the accuracy

of the model from the NCAC reports. After successful model verification a 50 th percentile

Hybrid III dummy was defined in the vehicle model and the interaction of dummy with

the vehicle environment & the injury values were analyzed.

Figure 3 Ford F250® with frontal impact with a rigid barrier (no dummy) @ 35 mph

2.3. LS- DYNA simulationThe simulation was run for 150 milliseconds of simulation time with a time step

of 9.00 E-07. The total computational time required for the run was around 32 hours. The

simulation is done using a rigid barrier.

Table 3 LS-DYNA simulation parameters

Version LS- DYNA_971

Revision 7600.1224

Precision Single (I4R4)

Feature SMP

OS level Linux

Number of processors 8

Total elapsed time 32 hours

Simulation time 150 milliseconds

The truck model comes in contact with the barrier at roughly about 5

milliseconds; the front hood starts to deform at 20 milliseconds. Maximum crush occurs

at 110 milliseconds. Figure below shows different stages of deformation of truck during

the simulation time.

Simulation results for Ford F250® with frontal impact with a rigid barrier (no dummy) @ 35 mph

(a) @ 24ms

(b) @ 40 ms

(c) @ 70 ms

(d) @ 150 ms

Figure 4 Different stages of simulation for frontal impact (no dummy) @35mph

2.4. Dummy Database

The Hybrid III is the most widely applied dummy for frontal impact. The series

includes a 5th percentile female, a 50th percentile male, and a 95th percentile male. The 50th

percentile is a five feet six inch dummy model with a weight of 170lbs, for the analysis

the 50th percentile male dummy is used. The details of the dummy can be found in the

LS-DYNA ftp site [5] which includes the calibration tests, positioning details, post

processing, Part ID details and the transformation details. The dummy model used in the

analysis is shown below.

Figure 5 Hybrid III 50th percentile rigid Fe dummy

The dummy model is imported in LS-DYNA® and positioned in the driver seat of

the vehicle via H point operations. By doing this we can get the responses of the dummy

and the interactions with vehicle environment along with the injury values.

The figure below shows the final positioned dummy on the vehicle’s driver seat.

Contacts are defined with the help of contact ids obtained from the report from LS-

DYNA®.

Figure 6 Positioned Hybrid III dummy

Since all LSTC (Livermore Software Technology Corporation) dummies use the

mm‐ms‐kg‐kN unit system, we use the following conversion to convert to mm-s-tonne

unit system.

Figure 2-6 Transform keyword file

LS-DYNA simulation for Ford F250® with frontal impact with Hybrid III 50 th

percentile dummy @ 35 mph

The simulation was run for 150 milliseconds of simulation time with a time step

of 9.00 E-07. The total computational time required for the run was around 32 hours 29

minutes. The truck model comes in contact with the barrier at roughly about 5

milliseconds; the front hood starts to deform at 20 milliseconds. Maximum crush

occurred at 110 milliseconds. Figure below shows different stages of deformation of

truck during the simulation time.

Table 4 LS-DYNA simulation parameters

Version LS- DYNA_971

Revision 7600.1224

Precision Single (I4R4)

Feature SMP

OS level Linux

Number of processors 8

Total elapsed time 32 hours 29 minutes

Simulation time 150 milliseconds

(a) @ 24ms

(b) @ 40 ms

(c) @ 70 ms

(d) @ 150 ms

Figure 7 Ford F250® with frontal impact with 50th percentile Hybrid III dummy @ 35 mph

CHAPTER 3

ROLLOVER – INVERTED VEHICLE DROP TEST

3.1. Case setup

For rollover - inverted vehicle drop test as per SAE J996 the Ford F250® truck

was inverted with a roll angle defined at 25° and a pitch angle defined at 5°.A rigid plate

for impact was defined and the drop height was set at 12 inches (300mm) . Computation

time was reduced by giving an initial velocity to the vehicle and was brought right above

the rigid plate. The initial velocity was defined in LS-DYNA input deck via

*INITIAL_VELOCITY_GENERATION.

A total of 10 nodes are defined to obtain the database history to monitor the

amount of roof intrusion. The nodes are defined via node set in the

DATABASE_HISTORY_NODE_SET. The averages of the 10 nodes are used to

calculate the amount of roof deflection. Figure 3.1 shows the deformation of the roof at

different stages of simulation

(a) @ 40 ms

(b) @ 75 ms

(c) @ 100 ms

(d) @ 120 ms

Figure 8 Different stages of deflection of roof Ford F250® (SAE J 996)

3.2. Results of rollover impact of Ford F250® (SAE J 996)

3.2.1. Total Energies

Figure 9 Total Energies plot for Ford F250® FEM

Figure 9 shows the global internal, kinetic & the total energies of the system. It

can be seen that at the start of the solution, the internal energy is zero and the kinetic

energy is equal to the total energy. When the F250 truck hits the rigid wall, the kinetic

energy of the system decreases and is converted to internal energy of the materials. The

kinetic energy and the internal energies meet at roughly 70 milliseconds, and the kinetic

energy continues to decrease whereas the internal energy continues to increase. The

sudden spike in the internal energy is due to the hourglass energy which occurs due to

under integration of elements. The roof structure being made up of very thin shell

elements excites the hourglass energy mode causing the internal energy to spike up.

3.2.2. Roof intrusion/deflections

Figure 10 Roof displacement V/s Time plot for Ford F250® FEM

Figure 10 shows the average nodal displacement for the rollover impact analysis of

Ford F250® truck. The maximum average displacement of the nodes is 267.64 mm,

which means the roof structure had intruded in the driver compartment by approximately

10 inches. The amount of roof intrusion has to be reduced to avoid any interface between

the driver and the roof structure to avoid catastrophic head injury or fatalities.

3.3.3. Force V/s Deflection

Figure 11 Wall force V/s Displacement plot for original F250 FEM

Figure 11 shows the Force V/s Displacement curve for the Ford F250® truck.

From the graph we can see that the truck experiences a maximum wall force of

approximately 35Kn and yields a total intrusion/deflection of roof structure by 265 mm.

3.4. Redesign of Roof structure Ford F250®

As seen above the F250 truck when subjected to a rollover impact gave a maximum

roof deflection of 267.64 mm. To make the car safer for rollover impact, the roof of the

vehicle had to be redesigned. This was done in three parts.

The roof material was changed from mild steel to ST 44 grade steel

A support plate was added between the A and the B pillar for roof support.

The thickness of the roof was changed from 0.8mm to 1.8mm

Figure 12 Support plate finite element model

Figure 3.5 shows the support plate used between the A and the B pillar to support

the deformation of the roof structure. The section is defined as Belytschko –Tsay shell

element [8]. The plate has a thickness of 2 mm with holes and slots similar to the

connection plate in the basic model. The plate is spot welded at 15 points to the roof rail

structure via spot weld beam sections and acts as an additional support to the roof rail.

The total increase in the mass of the roof by this redesign was in the tune of 45.15 lbs

(20.32 kgs) with the roof thickness change constituting to approximately 40 lbs.

(a) @ 55 ms

(b) @ 55 ms

(c) @ 100 ms

(d) @ 55 ms

Figure 13 Different stages of deflection of redesigned roof Ford F250® (SAE J996)

3.5 Results: Redefined Roof Ford F250®

3.5.1. Total Energies

Figure 14 Total Energies plot for redesigned roof F250 FEM

Figure 14 shows the global internal, kinetic & the total energies of the system. It can

be seen that at the start of the solution, the internal energy is zero and the kinetic energy

is equal to the total energy. When the truck hits the rigid wall, the kinetic energy of the

system decreases and is converted to internal energy of the materials. The kinetic energy

and the internal energies meet at roughly 50 milliseconds, and the kinetic energy

continues to decrease whereas the internal energy continues to increase. The internal and

kinetic energy should meet roughly at half the total energy and continue to stay at that

point without crossing for the rest of the analysis. The remaining energy, other than the

sum of kinetic & internal energies is the hour glass energy.

3.5.2. Roof Intrusion/deflection

Figure 15 Roof displacement V/s Time plot for redesigned roof F250 FEM

Figure 15 shows the average nodal displacement for the Redesigned Roof Ford

F250® truck. The maximum average displacement of the nodes is 193.73mm. There is a

significant reduction in the amount of roof displacement approximately 28 %( 73.91

mm).

3.5.3. Force V/s Deflection

Figure 16 Wall force V/s Displacement plot for redesigned roof F250 FEM

The redesigned roof structure gives a maximum intrusion of 193mm for

approximately 25 Kn of wall force.

3.6. Comparison

Figure 17 Comparison of deflection for original and redesigned roof F250 FEM

Figure 17 shows the comparison of deflection for the basic model (Line B) and

the redesigned roof (Line A). From the graph we can see that at the end of 120 ms the

total deformation of roof for the redesigned roof F250 model was 194 mm as compared

to the original F250 model which has a roof deformation of 264 mm.

Figure 18 Comparison of Wall Force V/s Displacement for original and redesigned roof of F250 FEM

Figure 18 shows the wall force versus displacement for the original Ford F250®

and the redesigned roof FEM. From the graph we can see that the force acting upon the

roof has reduced from approximately 35 Kn to 25 Kn (28.57 %). The maximum

deflection of the roof has been minimized to the tune of 70mm

CHAPTER 4

CONCLUSIONS

The rollover impact test procedure as suggested by SAE J996 was performed on

finite element model of a Ford F250® truck and the redesigned roof of Ford F250® and the

results were compared. During the study of typical rollover, the goal is to achieve

minimum roof intrusion to avoid contact between the roof structure and the passenger

head to avoid and catastrophic head injuries or fatalities. The parameters that affect the

outcome of a rollover impact are stiffness and geometry of roof structure, A and B pillar

reinforcement, occupant restraint system characteristics and stiffness and properties of

interior components. With this model all of the above mentioned parameters can be

varied to have a DOE study for various rollover scenarios including varying velocities

and angle of impact.

To protect driver in the driver compartment following things should be

considered. Defining certain accident situations (standard accidents), providing a well-

defined safe zone space into which no structural elements penetrate during the collision

and during the structural deformation and damage, keeping the driver in the safe zone

space during the collision. In current regulation, driver safety is not adequately

considered so this issue needs to be addressed during specifying the rollover impact

regulations.

Most of the fatal injuries in the rollover impact are caused between the passenger

head and the roof structure so it is recommended that more importance should be given to

the stiffness, geometry and the area of impact locations. This developed LS-DYNA

model is not validated and it just gives the estimate of the roof structure intrusion. In

future by validating this model by incorporating a certified rollover test dummy actual

injury values in case of a rollover impact can be accurately predicted.

A detailed study can be performed to assess more severe test conditions to

develop complete strong regulations. Furthermore, various other occupant positions like

passenger side, rear side can be considered for study.

REFERENCES

[1] National Highway Traffic Safety Administration, "Federal Motor Vehicle Safety

Standards and Regulations," in Crashworthiness, ed. Washington, DC, 1999.

[2] National Crash Analysis Centre. (2007). Finite Element Model Archive.

Available: http://www.ncac.gwu.edu/vml/models.html

[3] Livermore Software Technology Corp. (2007). LS-DYNA Theory 2006.

Available: http://www.lstc.com/manuals.htm

[4] Livermore Software Technology Corp. (2007). Keyword 971 2 volumes

Available: http://www.lstc.com/manuals.htm

[5] Livermore Software Technology Corp. (2008). Index of /user/lstc-dummies.

Available: http://ftp.lstc.com/user/lstc-dummies/

[6] D. Shepardson, "Newer SUVs now safer than cars in a crash," in Detroit News,

ed. Detroit, 2011.

[7] P. D. Stephen A. Batzer, P.E. & Robert M. Hooker, "DYNAMIC ROOF CRUSH

INTRUSION IN INVERTED DROP TESTING."

[8] Belytschko & Tsay, "Explicit Algorithms for Nonlinear Dynamics of Shells","

vol. AMD-Vol.48, ASME, 209-231, 1981.

[9] K. Elitok, "Explicit Dynamic Analysis of Vehicle Roll-Over Crashworthiness

Using LS-DYNA ", Mechanical Engineering, Istanbul Technical University,

2006.