FINITE ELEMENT MODELING AND SIDE IMPACT STUDY OF A LOW-FLOOR
MASS TRANSIT BUS
A Thesis by
Vikas Yadav
B. E. Shivaji University, 2003
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 2006
FINITE ELEMENT MODELING AND SIDE IMPACT STUDY OF A LOW-FLOOR
MASS TRANSIT BUS
I have examined the final copy of this thesis for form and content, and recommend that it be
accepted in partial fulfillment of the requirements for the degree of Master of Science, with a
major in Mechanical Engineering.
Hamid M. Lankarani, Committee Chair
We have read this thesis and recommend its acceptance:
Kurt Soschinske , Committee Member
Krishna K. Krishnan, Committee Member
i
ACKNOWLEDGEMENTS
Completing this thesis was a challenge for me and would not have been possible without
the support, inspiration, encouragement, and contribution of many individual. First of all, I
would like to take this opportunity to express my profound thanks to my advisor Dr. Hamid M.
Lankarani for his excellent support, inspiration, and encouragement throughout my studies at
Wichita State University. Without him, things would never have worked better than this.
I would also like to thank Dr. Gerardo Olivares for all his support and guidance during
my thesis.
I would like to express my gratefulness to my parents and my brothers who have always
supported my vision and have given me timely guidance, support and inspiration in every aspect
of my education. Without their support, my stay and studies at Wichita State University would
not have been possible.
Finally, I would like to express my gratitude to Dr. Kurt Soschinske and Dr. Krishna K.
Krishnan for being on my thesis committee and for their valuable suggestions in directing my
efforts to present my work as a complete thesis.
ii
ABSTRACT
Mass transportation systems, specifically bus systems, are a key element of the national
transportation network. Buses are one of the safest forms of transportation; nonetheless, bus
crashes resulting in occupant injuries and fatalities do occur. According to Traffic Safety Facts
reports from 1999 to 2003, an average of 40 fatalities and 18,430 injuries of bus occupants have
occurred per year, with side impact accounting for 14 % according to type of impact and by
initial point of impact it accounts for 36%.
A full scale crash test is considered the most reliable source of information regarding
structural integrity and safety of motor vehicles. However, the high cost of such tests and
difficulties in collecting data has resulted in an increasing interest in the analytical and
computational methods of evaluation. With the advancement in computer simulations, full finite
element validated vehicle models are being analyzed for different impact scenarios to predict
vehicle behavior and occupant response.
This thesis research work presents the procedure for development of a finite element (FE)
model of a mass transit bus and the results of its crashworthiness and structural integrity analysis.
The finite element model is developed by extracting mid-surface from solid cad model. This
model is a detailed model with all parts. All parts are connected using different multi point
constraints and special links with failure to model actual types of structural connections such as
bolts and spot welds. LS-DYNA non-linear, explicit, 3-D, dynamic FE computer code was used
to simulate behavior of the transit bus under different side impact scenarios. A parametric study
is done to study structural response of transit bus when impacted by vehicles of different masses,
sizes and shapes. A multibody analysis is done to study occupant response to different side
impact crash conditions
iii
TABLE OF CONTENTS
Chapter
1.
INTRODUCTION ...............................................................................................................1
1.1
1.2
1.3
2.
2.2
3.2
3.3
3.4
3.5
Modeling Guidelines-General................................................................................34
3.1.1 Element Quality Check ................................................................................36
Procedure ...............................................................................................................37
3.2.1 Importing Geometry Data ............................................................................37
3.2.2 Meshing........................................................................................................41
3.2.3 Procedure to Create Joints ...........................................................................46
Element Formulation .............................................................................................50
Material Model and Properties...............................................................................51
Implicit Analysis....................................................................................................61
FINITE ELEMENT MODEL VALIDATION ..................................................................62
4.1
4.2
4.3
5.
Preprocessor...........................................................................................................20
2.1.1 Hypermesh ...................................................................................................20
2.1.2 Oasys Primer................................................................................................22
Analysis Software ..................................................................................................23
2.2.1 LS-DYNA ....................................................................................................24
2.2.2 MADYMO...................................................................................................27
FINITE ELEMENT MODELING.....................................................................................32
3.1
4.
Background ..............................................................................................................2
1.1.1 Statistical Analysis of Mass Transit Bus .......................................................3
1.1.2 Finite Element Model Generation..................................................................6
1.1.3 Side Impact Test Standard for Transit Bus ....................................................7
1.1.4 NHTSA/Crashworthiness and Side Standards...............................................7
1.1.5 Injury Biomechanics ....................................................................................12
Objective ................................................................................................................15
Methodology ..........................................................................................................15
COMPUTATIONAL TOOLS ...........................................................................................20
2.1
3.
Page
Side Impact Validation ..........................................................................................62
Rear Impact Validation ..........................................................................................66
Frontal Impact Validation ......................................................................................68
SIDE IMPACT STUDY OF TRANSIT BUS ...................................................................71
5.1
Crash Conditions....................................................................................................71
5.1.1 FMVSS 214 .................................................................................................74
iv
TABLE OF CONTENTS (continued)
Chapter
6.
MADYMO MODELING OF VEHICLE INTERIOR.......................................................89
6.1
6.2
6.3
7.
Page
5.1.2 Dodge Neon .................................................................................................79
5.1.3 Chevy 2500 Pickup ......................................................................................81
5.1.4 Ford 800 Truck ............................................................................................85
MADYMO Model .................................................................................................89
Side Impact Standard for Transit Bus....................................................................92
FMVSS 214 Side Impact Standard ........................................................................96
CONCLUSIONS AND RECOMMENDATIONS ..........................................................100
REFRENCES...................................................................................................................104
APPENDICES .............................................................................................................................108
A. Description of Variables .............................................................................................109
B. Chevy Test Results......................................................................................................110
C. F800 Test Results ........................................................................................................116
D. MAMDYMO Simulation According to SBPG...........................................................121
E. MAMDYMO Simulation According to FMVSS 214 .................................................132
v
LIST OF FIGURES
Figure
Page
1.1
Buses involved in crashes with Injuries, by initial point of impact .....................................2
1.2
Typical mass transit bus.......................................................................................................3
1.3
Bus occupant fatalities 1975-2003.......................................................................................5
1.4
Bus occupant injuries 1988-2003.........................................................................................5
1.5
FMVSS 214 side impact test................................................................................................9
1.6
ECE 95 side impact test .....................................................................................................10
1.7
FMVSS 214 barrier and EU 96/27/EC barrier...................................................................12
1.8
IIHS side impact test configuration ...................................................................................12
1.9
Flow chart ..........................................................................................................................16
1.10
Coupon testing ...................................................................................................................17
1.11
Validation of frontal bumper test.......................................................................................17
1.12
Side impact validation test .................................................................................................18
1.13
Rear bumper validation test ...............................................................................................18
2.1
Primer interface..................................................................................................................22
2.2
FE dummy positioning in primer.......................................................................................23
2.3
LS_DYNA Interface ..........................................................................................................24
2.4
MADYMO 3D models ......................................................................................................27
2.5
Hybrid III 50th percentile dummy model Ellipsoidal.........................................................31
3.1
FARO Arm – portable digital tool .....................................................................................32
3.2
Finite element modeling flow chart ...................................................................................33
3.3
Element mesh is to be orthogonal to the centerline of the part..........................................34
vi
LIST OF FIGURES (continued)
Figure
Page
3.4
Transition mesh..................................................................................................................34
3.5
Flange.................................................................................................................................35
3.6
Mesh with holes .................................................................................................................35
3.7
PRO- E menu .....................................................................................................................38
3.8
Hypermesh Interface for importing CAD IGES file..........................................................39
3.9
Hypermesh interface for importing CAD ..........................................................................39
3.10
Chassis IGES file ...............................................................................................................40
3.11
Mid surface meshing..........................................................................................................41
3.12
Simplified CATIA mode of engine block..........................................................................42
3.13
Meshed model of engine block ..........................................................................................43
3.14
Chassis and side panels......................................................................................................43
3.15
Side structure .....................................................................................................................43
3.16
Structural parts ...................................................................................................................44
3.17
Front suspension ................................................................................................................44
3.18
Rear suspension .................................................................................................................44
3.19
Petrol tank and radiator......................................................................................................45
3.20
Structural members ............................................................................................................45
3.21
Finite element model shaded views of non-structural components ...................................46
3.22
Frontal axel kinematics joints ............................................................................................46
3.23
Rear axel kinematics joints ................................................................................................46
3.24
Joint description .................................................................................................................47
vii
LIST OF FIGURES (continued)
Figure
Page
3.25
Types of joints....................................................................................................................48
3.26
FE bus model .....................................................................................................................49
3.27
Curve for strain rates 0.00001mm/s, 0.1mm/s, 100mm/s ..................................................54
3.28
Curve for strain rates 0.1 mm/s, 20 mm/s, 4000mm/s .......................................................54
3.29
Stress strain curve for aluminum .......................................................................................55
3.30
Implicit analysis of roof top...............................................................................................61
4.1
FMVSS 214 and modified barrier......................................................................................63
4.2
Modified FMVSS 214 barrier 35 MPH (56km/hr) ............................................................64
4.3
Bus 40 km/hr side impact test validation...........................................................................65
4.4
Side impact test vehicle kinematics ...................................................................................65
4.5
Displacement (mm) and von mises stress (MPa)...............................................................66
4.6
Rear impact test validation.................................................................................................67
4.7
Frontal bumper test validation ...........................................................................................69
4.8
Front bumper cross section and frontal view von mises stress..........................................69
4.9
Front bumper test validation vehicle kinematics ...............................................................70
5.1
Real life crash condition for side impact ...........................................................................73
5.2
DYNA – Hybrid III 50 th % dummy .................................................................................74
5.3
FMVSS 214 barrier............................................................................................................75
5.4
FMVSS 214 test setup .......................................................................................................75
5.5
FMVSS 214 frames............................................................................................................76
5.6
Structural deformation and dummy kinematics.................................................................77
viii
LIST OF FIGURES (continued)
Figure
Page
5.7
Displacement fringes for bus .............................................................................................78
5.8
Displacement velocity and acceleration.............................................................................78
5.9
Dodge neon ........................................................................................................................79
5.10
Test setup for neon 90 deg impact .....................................................................................79
5.11
Frames for dodge neon 90 deg impact...............................................................................81
5.12
Chevrolet c2500 .................................................................................................................81
5.13
C-2500 ± 30 and 270 deg impact angel .............................................................................82
5.14
Test Setup C 2500 90 degimpact .......................................................................................83
5.15
Frames for 90 deg impact ..................................................................................................84
5.16
F 800 truck .........................................................................................................................85
5.17
Test Setup F-800 -30 deg impact angel 25 mph ................................................................86
5.18
Frames for F 800 30 deg impact ........................................................................................87
6.1
MADYMO model of transit bus ........................................................................................89
6.2
Ellipsoidal dummy models ................................................................................................90
6.3
Facet model........................................................................................................................91
6.4
Dummy titles and position .................................................................................................93
6.5
Side impact setup using SBPG ..........................................................................................93
6.6
Acceleration pulse..............................................................................................................93
6.7
SBPG simulation frames....................................................................................................94
6.8
FMVSS 214 test setup .......................................................................................................96
6.9
Acceleration curve for FMVSS 214 simulation ................................................................97
ix
LIST OF FIGURES (continued)
Figure
6.10
Page
FMVSS 214 simulation frames..........................................................................................97
x
LIST OF TABLES
Table
Page
1.1
Comparison between US and European Standards For Side Impact................................11
3.1
Mesh Quality......................................................................................................................37
3.2
Element Type .....................................................................................................................49
3.3
Model Size .........................................................................................................................50
3.4
Units...................................................................................................................................53
3.5
Front Axel Damping Function ...........................................................................................57
3.6
Rear Axel Damping Function ............................................................................................58
3.7
Front Axel Spring Function ...............................................................................................59
3.8
Rear Axel Spring Function ................................................................................................59
4.1
Side Impact Validation Test Conditions ............................................................................63
4.2
Rear Impact Validation Test Conditions............................................................................67
4.3
Front Impact Validation Test Conditions ..........................................................................68
5.1
Test Matrix.........................................................................................................................72
5.2
Model Size of Dodge Neon................................................................................................79
5.3
Element type for Dodge Neon ...........................................................................................79
5.4
Chevy Model Size..............................................................................................................82
5.5
Element type for Chevy Model..........................................................................................82
5.6
Test Results........................................................................................................................85
5.7
Model Size of F 800 Truck ................................................................................................86
5.8
Element type for F800 Model ............................................................................................86
5.9
Simulation Results for F 800 .............................................................................................88
xi
LIST OF TABLES (continued)
Table
Page
6.1
FMVSS 208 Injury Criteria’s.............................................................................................91
6.2
Side Impact Test Standard (SBPG)....................................................................................92
6.3
Injury Values for SBPG Simulation ..................................................................................94
6.4
Injury Values for SBPG Simulation ..................................................................................95
6.5
Side Impact Test Standard (FMVSS 214) .........................................................................96
6.6
Injury Values for FMVSS 214 Test Setup.........................................................................98
6.7
Injury Values for FMVSS 214 Test Setup.........................................................................99
xii
CHAPTER 1
INTRODUCTION
Mass transportation systems, specifically bus systems, are a key element of the national
transportation network. Buses are one of the safest forms of transportation, nonetheless, bus
crashes resulting in occupant injuries and fatalities do occur. Therefore, crashworthiness research
is a continuing effort. Transit bus usage, in terms of passenger-miles, averages 20.6 billion miles
per year. From 1992 to 2002, transit motor bus rider ship has increased by 11% in terms of
unlinked trips. From 1990 to 2002, the number of transit motor buses in the U.S. has increased
by 30% [1]. Clearly, transit buses are an integral part of the national transportation system.
According to the TRAFFIC SAFETY FACTS reports from 1999 to 2003 [2-6], an
average of 40 fatalities and 18,430 injuries of bus occupants occurred per year. In these reports,
buses are defined as “large motor vehicles used to carry more than ten passengers, including
school buses, inter-city buses, and transit buses”. According to traffic safety bus crashes
involving fatalities side impact accounts for 14% and by initial point of impact it is 34%. As
shown in Figure 1.1.
Side impacts are frequent and extremely harmful crashes. The likelihood of being killed
or seriously injured is very high in side impact crashes. Twenty-five percent of vehicle casualties
(28 percent of fatalities) occur from these crashes, accounting for roughly one-third of occupants
harm on our roads [28]. According to Traffic Safety Facts it’s evident that side impact as initial
point of impact plays major role in occupant injury. To address this crucial safety question
involving mass transit bus this thesis work will help to better understanding the structural
response of transit buses and dummy kinematics during side impacts.
1
Buses Involved in Crashes with Injuries,
by Initial Point of Impact, 1999-2003
Total: 64,000; Average: 12,800/year
0%
7%
35%
24%
Front
Side
Rear
Noncollision
Other/Unknown
34%
Figure 1.1 Buses involved in crashes with Injuries, by initial point of impact, 1999-2003
summary.
This thesis work describes the modular finite element (FE) modeling and validation
methodologies of a typical low-floor US transit bus. A side impact study was carried out to do
the following (1) Characterize the crashworthiness structural response of mass transit buses and
(2) characterize the occupant kinematics and injury mechanisms.
1.1 Background
In the United States the primary focus to date has been placed on school bus safety. The
introduction of Motor Vehicle Safety Standards in the mid-1980’s resulted in the passive safety
systems or “compartmentalization” in school buses. This passive system, largely dependent on
seat spacing and padded seat backs has worked well in preventing injuries during collisions. The
effectiveness of lap/torso seat belts has been recognized, however there are concerns regarding
installation costs and maintenance issues as well as their proper use [31].
Europe currently has regulations that apply to the strength of the superstructure, and
strength of seats and their anchorages. In the United Kingdom, regulations require seat belts to
2
be fitted in all new intercity and minibuses have been introduced. Fitting of seat belts in other
European countries varies from country to country. A three-year research program, Enhanced
Coach and Bus Occupant Safety (ECBOS), was initiated in Europe in January 2000 and aimed to
reduce injuries through the development of new bus regulations and standards [31].
1.1.1
Statistical Analysis of a Mass Transit Bus
Transit – It is an entity providing passenger transportation over fixed, scheduled routes, within
primarily urban geographical areas. Figure 1.2 shows a typical transit bus.
Figure 1.2 Typical mass transit bus.
According to Buses Involved in Fatal Accidents (BIFA) report that presents aggregate
statistics for buses involved in traffic accidents and compiled by the University of Michigan
Transportation Research Institute (UMTRI) an average of 111 transit buses are involved in fatal
traffic accidents each year a total of 246 fatalities resulted from transit bus involvement from
1999 to 2000 of which 43% of the fatalities were drivers of other vehicles, 37% were
pedestrians, 13% percent were passengers of other vehicles. According to Buses Involved in
Fatal Accidents (BIFA) 80% of transit bus fatal involvements occur during the work week. The
lowest percentage of involvements, 7.8%, occur on Sunday of which 62% of fatal transit bus
involvements occur in daylight and 29% in dark but lighted conditions. Relative to type of road
58% of fatal transit bus involvements occur on local streets (township or municipality), 15% on
3
state highways, and 8% on county roads. Low platform buses accounted for 65% of fatal transit
bus involvements. Buses with 25,001 to 30,000 lb. empty weights accounted for 70% of fatal
transit bus involvements.
The Traffic Safety Facts report is an annual compilation of motor vehicle crash data
presented by the National Highway Traffic Safety Administration (NHTSA). Data from the
Fatality Analysis Reporting System (FARS) and the National Automotive Sampling System
General Estimates System (GES) is combined to create Traffic Safety Facts. Data presented in
the Traffic Safety Report is often grouped by crash severity, with the following categories:
(1) Fatal Crash. A police-reported crash involving a motor vehicle in transport on a trafficway in
which at least one person dies within 30 days of the crash. (2) Injury Crash. A police-reported
crash that involves a motor vehicle in transport on a trafficway in which no one died but at least
one person was reported to have: (i) an incapacitating injury; (ii) a visible but not incapacitating
injury; (iii) a possible, not visible injury; or (iv) an injury of unknown severity.
According to 1999 to 2003 Traffic Safety Facts and synopsis [7] 40 bus occupants were
killed and 18,430 injured per year. An average of 12,000 bus occupants per year is injured in
two vehicles crashes while 8,800 occupants were injured per year of other vehicles .
According to the safety facts side impact also plays a major role in crash fatalities with
36% of bus occupant injuries resulting from side crashes. By initial point of impact in bus
crashes involving injuries side crashes accounts for 36% in all types of impacts. Twenty-eight
percent of bus occupant fatalities resulted from occupant ejection, 53% from non-ejected fatal
impacts and 19% were unknown. Non-ejected injuries account for more fatalities because the
occupant comes in contact with the seatback structure or walls of the bus on impact.
4
Bus Occupant Fatalities, 1975-2003
80
70
Occupant Fatalitities
60
50
40
30
20
10
2003
2001
1999
1997
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
0
Year
Figure 1.3 Bus occupant fatalities, 1975-2003.
Bus Occupant Injuries, 1988-2003
35,000
30,000
20,000
15,000
10,000
5,000
Year
Figure 1.4 Bus occupant injuries, 1988-2003.
5
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
0
1988
Occupant Injuries
25,000
1.1.2 Finite Element Model Generation
Full-scale crash testing is still the only method for final certification of crashworthiness
for the vehicles but engineers have become increasingly reliant on computer simulation programs
[18]. The first analytical methods using computer simulation programs were developed at the
Cornell Aeronautical Laboratory [18]. These methods involved the development of dynamic
models using springs, dashpots, beams, and links to study vehicle-barrier collisions.
Modifications to this work led to the development of the Highway Vehicle Object Simulation
Model (HVOSM) in the late 1960s. It was initially developed for simulating vehicle handling
models [17]. This program simulated the three-dimensional behavior of vehicle/roadside object
interaction. This program was used to develop many improvements in guardrail design [16]. The
program uses a total of 11 degrees of freedom (DOF); a 6-DOF sprung mass, 1-DOF for each tire
and one steering DOF [17]. This program had some limitations such as inability to model tire
penetrations into soft soil and instability of vehicle rotation directions during an impact. HVOSM
uses a thin-disk tire model that does not have any lateral flexibility. This feature prevents the
model from accurately simulating tire deflections and tire/barrier contact forces. HVOSM's is not
suiTable for modeling suspension damage since it can not predict this kind of situation
accurately [17]. Modified forms of HVOSM’s are still widely used for specific applications [19].
In 1970 Graham Powell developed two-dimensional codes; BARRIER V and BARRIER
VI for the Federal Highway Administration. They were originally designed for impact
simulations with flexible barriers. Though very useful, this code had severe limitations due to its
two-dimensional nature [20]. In 1973 Powell published the next version, BARRIER VII, which
included an energy balance computation. The program has accurately simulated the snagging
effect of wheels with posts or other snagging obstacles which leads to unacceptably excessive
6
vehicle decelerations. The program however becomes unsTable in the presence of very large
deformations and can not account for the snagging of vehicle and barrier components. BARRIER
VII, owing to its two-dimensional nature, does not allow prediction of vehicle vaulting or under
riding of the barrier [17]. In 1991 FHWA sponsored three independent projects to make
recommendations for improving analytical capabilities for simulating impact problems. All
projects recommended general purpose finite element code DYNA3D in place of other specialpurpose codes such as NARD, GUARD, Barrier VII and HVSOM [20]. DYNA3D is a generalpurpose non-linear explicit three-dimensional finite element code. It was developed as a
classified product at the Lawrence Livermore National Laboratory (LLNL) in 1976 [21]. The
FHWA and NHTSA have been promoting the use of FE analysis over the last several years.
General finite element codes like DYNA3D, PAMCRASH, LS-DYNA, Mecalog/RADIOSS and
their improved versions have gained wide popularity in the federal agencies, universities and
private companies specializing in the finite element analysis field [22]. LS-DYNA, the
commercial version of DYNA3D is discussed in more detail in section 2.2
1.1.3 Side Impact Test Standard for Transit Bus
According to Standard Bus Procurement Guideline (SBPG) the bus shall withstand a 25mph impact by a 4,000-pound automobile at any point, excluding doorways, along either side of
the bus with no more than 3 inches of permanent structural deformation at seated passenger hip
height. This impact shall not result in sharp edges or protrusions in the bus interior.
1.1.4 NHTSA/Crashworthiness and Side Impact Standards
The
National
Highway
Traffic
Safety
Administration
(NHTSA),
under
the
U.S. Department of Transportation, was established by the Highway Safety Act of 1970, as the
successor to the National Highway Safety Bureau, to carry out safety programs under the
7
National Traffic and Motor Vehicle Safety Act of 1966 and the Highway Safety Act of 1966.
The Vehicle Safety Act has subsequently been recoded under Title 49 of the U. S. Code in
Chapter 301, Motor Vehicle Safety. NHTSA also carries out consumer programs established by
the Motor Vehicle Information and Cost Savings Act of 1972, which has been recoded in various
Chapters under Title 49. NHTSA is responsible for reducing deaths, injuries and economic losses
resulting from motor vehicle crashes. This is accomplished by setting and enforcing safety
performance standards for motor vehicles and motor vehicle equipment, and through grants to
state and local governments to enable them to conduct effective local highway safety programs.
NHTSA investigates safety defects in motor vehicles, sets and enforces fuel economy standards,
helps states and a local community reduce the threat of drunk drivers, promote the use of safety
belts, child safety seats and air bags, investigate odometer fraud, establish and enforce vehicle
anti-theft regulations and provides consumer information on motor vehicle safety topics. NHTSA
also conducts research on driver behavior and traffic safety, to develop the most efficient and
effective means of bringing about safety improvements.
FMVSS 214: The Federal Motor Vehicle Safety Standard (FMVSS) 214, "Side Impact
Protection" was amended in 1990 to assure occupant protection in a dynamic test that simulates a
severe right angle collision [23]. It is one of the most important and promising safety regulations
issued by the National Highway Traffic Safety Administration (NHTSA). In 1993, side impacts
accounted for 33 percent of the fatalities to passenger car occupants. Extensive research was
done and amendments to FMVSS 214 were suggested to avoid such high rate of fatalities in this
particular crash event. The current FMVSS 214 is the culmination of many years of research to
make passenger vehicles less vulnerable in side impacts, and especially to reduce fatality risk to
the nearside occupant when a vehicle is struck in the door area by another vehicle - the
8
configuration responsible for the majority of side-impact fatalities. In the side impact crash test
the vehicle is stationary and a Moving Deformable Barrier (MDB) hits the side of the vehicle
[24]. Weight, speed, dimensions and material for MDB is governed by the specifications given in
FMVSS 214. Figure 1.5 shows the test setup for side impact test [24]. The procedure and test
equipment differ for European Standard from those in FMVSS 214 in many significant ways.
The MDB impacts the target vehicle at 50 kph (31 mph) and 90 degrees with no crab angle, as
shown in Figure 1.6. This differs from FMVSS 214 in that no attempt is made at simulating the
movement of the target vehicle. The lateral striking position is aligned with the occupant seating
position rather than the vehicle wheelbase.
Figure 1.5 FMVSS 214 side impact test (illustration by NHTSA).
ECE95 side impact test: The dimensions of the European barrier face are given in Figure 1.7.
The barrier face is segmented into six blocks with specific force deflection characteristics. The
barrier face is smaller and much softer than the U.S. barrier on the blocks closest to the sides.
The bottom edge is the most forward part of the European MDB and is 300 mm (11.8 in.) from
the ground. This is in comparison to the 280 mm (11.0 in.) high bottom edge and 330 mm (13
9
in.) bumper height in the U.S. barrier face. The European barrier has a mass of 950 kg (2095 lbs)
compared to 1367 kg (3015 lbs) for the U.S. barrier.
Figure 1.6 ECE 95 side impact test (illustration by NHTSA).
As in FMVSS 214, for EU Directive 96/27/EC successful test performance is determined
by dummy injury criteria. However, both the test dummy and injury criteria differ from those in
FMVSS 214. SID is capable of measuring acceleration of the ribs, spine and pelvis. These
readings are the bases for the U.S. injury criteria. EUROSID-1 has the capability of measuring
more parameters than SID, including force and displacement as well as acceleration based
readings. The EU Directive places limits on five dummy measurements to determine vehicle
performance. The head injury criterion (HIC) is derived from head acceleration and must remain
below 1000. A rib deflection of 42 mm (1.7 in.) is allowed in the thorax along with a Viscous
Criterion (V*C) of 1 m/s. The Viscous Criterion is calculated from combined rib displacement
and velocity. The abdominal force is limited to 2.5 KN (562 lbs). Finally, the Pubic Symphysis
force, which is in the pelvic region, must be less than 6 KN (1350 lbs). The dynamic test
10
simulates the 90 degree impact of a striking vehicle traveling 48.3 kph (30 mph) into a target
(test) vehicle traveling 24.2 kph (15 mph). This is achieved by a moving deformable barrier
(MDB), with all wheels rotated 27 degrees (crab angle) from the longitudinal axis, impacting a
stationary test vehicle with a 54 kph (33.5 mph) closing speed. For a typical passenger car, the
left edge of the MDB is 940 mm (37 in.) forward of the mid point of the struck vehicle wheel
base. The MDB has a total mass of 1367 kg (3015 lbs). The aluminum honeycomb of the barrier
face is specified by design. The bottom edge of the MDB is 280 mm (11 in.) from the ground.
The protruding portion of the barrier simulating a bumper is 330 mm (13 in.) from the ground
(25) .Differences between US standards for side impact and European standards for side impact
are given below in Table1.1.
EU 96/27/EC
FMVSS 214
MDB Mass
950 kg
1367 kg
Velocity Vector
50 kph/90
54 kph/63
Ground Height
300 mm
279 mm Bumper 330 mm
Face Width
1500 mm
1676 mm
Table 1.1 Comparisons between US Standard and European Standard.
The side impact standards fro vehicle is given in Figure 1.8. It shows side impact test setup
according insurance institute of Highway Safety (IIHS).The standard is 1500 kg bullet vehicle
impacting target vehicle with 50 km/hr at 90 deg. This test is done using IIHS side impact
barrier. The specification for this barrier is different to FMVSS 214 barrier.
11
FMVSS 214 Side Impact
Deformable Barrier Face.
EU 96/27/EC Side Impact
Deformable Barrier Face
Figure 1.7 FMVSS 214 barrier and EU 96/27/EC barrier.
Figure 1.8 IIHS side impact test configuration.
1.1.5 Injury Biomechanics
Injury biomechanics describes the effect mechanical loads have on the human body,
particularly impact loads. Due to a mechanical load, a body region will experience mechanical
and physiological changes, the biomechanical response. Injury occurs if the biomechanical
response is so severe that the biological system deforms beyond a recoverable limit, resulting in
damage to anatomical structures and altering the normal function. The mechanism involved is
12
called injury mechanism; the severity of the resulting injury is indicated by the expression
“injury severity” [29].
Injury parameter: An injury parameter is a physical parameter or a function of several physical
parameters that correlates well with the injury severity of the body region being examined. Many
schemes have been proposed for ranking and quantifying injuries. Anatomical scales describe the
injury in terms of its anatomical location, the type of injury and its relative severity. These scales
rate the injuries instead of the results of the injuries. The most well-known, widely accepted
anatomical scale is the Abbreviated Injury Scale (AIS). Although originally intended for impact
injuries in motor vehicle accidents, the updates of the AIS allow its application also for other
injuries such as burns and penetrating injuries [29].
Abbreviated Injury Scale: The AIS distinguishes the following levels of injury: 0 no injury, 1
minor, 2 moderate, 3 serious, 4 severe, 5 critical, 6 maximum injury (causes death) and 9
unknown. The AIS is a “threat to life” ranking. The numerical values have no Significance other
than to designate order
Head Injury Criteria:
(1.1)
Injury criteria for head where T0 is the starting time of the simulation, TE is the end time of the
simulation, R (t) is the resultant head acceleration in g’s (measured at the head’s centre of
gravity) over the time interval T0 ≤ t ≤ TE, t1 and t2 are the initial and final times (in s) of the
interval during which the HIC attains a maximum value [29].
13
HIC limitations: An injury criterion and associated tolerance level should relate to the injury
severity. Limitations of the HIC are:
-
HIC is only valid for a hard contact, thus the time duration of impact is limited,
-
HIC is based on the WSTC, which is derived from subjects loaded in anterior-posterior
direction only.
Despite these drawbacks, HIC is the most commonly used criterion for head injury in automotive
research and is believed to be an appropriate discriminator between contact and non-contact
impact response
Neck Injury Criteria: Neck injury is often assessed by peak forces and moments in the upper
and lower neck. Neck injuries are defined in tension-extension (NTE), tension-flexion (NTF),
compression-extension (NCE), and compression-flexion (NCF).The equation for the calculation
of Nij is given by
(1.2)
Fzc and Myc are constant force and moment which depend on dummy neck loading condition
(compression/tension and flexion/ extension). Only the injury predictor of the applicable loading
condition can be greater than zero. The sum of all predictors may not exceed a value of one [29]
Chest3 ms criterion (CON3MS and CUM3MS): The thorax contains, after the head, the next
most critical organs to protect from injuries. The bony cage structure of the thorax consists of
twelve thoracic vertebrae (numbered T1 to T12), the sternum and twelve pairs of ribs which form
a relatively rigid and also flexible shell. A commonly stated human tolerance level for severe
chest injury (AIS ≥ 4) is a maximum linear acceleration in the centre of gravity of the upper
14
thorax of 60 g, sustained for 3 ms or longer. Thus the criterion is not based on a single maximum
value, but on a sustainable level of linear acceleration [29].
Femur Force Criterion (FFC): The Femur Force Criterion (FFC) is a measure of injury to the
femur. It is the compression force transmitted axially on each femur of the dummy as it is
measured by the femur load cell.
1.2 Research Objective
•
To generate a detailed FE bus model, and validate Model using the standard bus
Procurement guidelines.
•
To structural Analysis of some real life Side impact crash conditions with other vehicles:
passenger vehicles and light trucks to extract crash pulses and vehicle intrusions at
various occupant locations during side impact.
•
To generate a simplified multibody model of a Bus to study the passenger kinematics and
biomechanical performance for standard crash configurations.
•
To study occupant kinematics and provide design guidelines to mitigate the risk of
passenger injuries during side impact.
1.3 Methodology
The primary goal of this research was to develop a finite element model of the mass
transit bus. The Flow chart shown in Figure 1.9 details the process and software used to develop
this model. Design data/geometric data required for building the FE model was available. This
data was preprocessed for cleaning geometry in Pro-E. Geometric data was converted to IGES
file and imported into a Hypermesh preprocessor for finite element mesh generation. The
geometry was cleaned to provide continuity in meshing and constant mesh density. The
documentation was carried out to keep track of all parts and their mesh quality and size.
15
Figure 1.9 Flow chart for model generation.
Material testing: Coupon tests for some of the materials were carried out by a third-party
contractor to determine their mechanical properties. Coupon tests were carried out for different
loading condition. There are three types of testing systems required for crashworthiness
applications:
•
Mechanical or Servo-Hydraulic: Quasi-static condition and strain rates bellow
0.1/s.
•
Servo-Hydraulic: Strain rate range 0.1 to 500/s.
•
Bar System: Strain rate range 100 to 1000/s, and higher
Strain rates increments 1, 10,100,250,500 /s are sufficient for describing the strain rate sensitivity
These results were used to create strain rate dependent material. Once the finite element model
was complete with all material properties, crash analysis was carried out to validate the FE
16
model. The validated model was used to study different crash events. Various load and boundary
conditions were applied to the FE model, based on each specific crash event under consideration.
Figure 1.10 Coupon testing.
Frontal impact validation: The structural integrity of the bus during frontal impact was analyzed
using the Standard Bus procurement guidelines (SBPG). The bus model was validated using data
obtained from actual testing, i.e., front bumper test with 5.5 mph speed in to a rigid barrier
(FMVSS 208) as shown in Figure 1.11.
Figure 1.11 Validation of front bumper test.
17
Side impact validation: According to Standard Bus procurement guideline transit bus was tested
for side impact using bullet vehicle of 4000 Lbs impacting at 25 mph. The actual testing data was
available for side impact test condition. The vehicle used for actual testing was 1983 Chevrolet
caprice. Finite element model for 1983 caprice is not available. Hence FMVSS 214 barrier is
modified to represent the crush strength and geometry of caprice front structure. The barrier
model is first validated using NHTSA new car assessment program data for caprice model. This
modified barrier is used for side impact validation of transit bus.
Figure 1.12 Side impact validation test
Rear impact validation: According to SBPG actual bus is tested 2.2 mph in to rigid barrier for
rear bumper test. This test data was available from actual testing. FE model of bus was validated
using the same test set-up.
Figure 1.13 Rear bumper validation test
18
Madymo Model Generation: The facet model of the Transit Bus was developed to study
occupant kinematics and to study various injury parameters. This model was used with
multibody dummies to study the effect of real life side impact crash conditions on occupants.
19
CHAPTER 2
COMPUTATIONAL TOOLS
2.1 Preprocessors
Finite element modeling was done using preprocessing software such as Hypermesh and
Patran. To generate the transit bus model Hypermesh is used as the preprocessor. To check the
syntax of the DYNA file preprocessors like Primer were used.
2.1.1 Hypermesh
Altair HyperMesh is a high-performance finite element pre- and postprocessor for major
finite element solvers, allowing engineers to analyze design conditions in a highly interactive and
visual environment. The Hypermesh user-interface is easy to learn and supports the direct use of
CAD geometry and existing finite element models, providing robust interoperability and
efficiency. Advanced automation tools within Hypermesh allow users to optimize meshes from a
set of quality criteria, change existing meshes through morphing, and generate mid-surfaces from
models of varying thicknesses.
Benefits
•
Reduce time and engineering analysis cost through high-performance finite element
modeling and post-processing.
•
Reduce learning time and improve productivity with an intuitive user-interface and bestin-class functionality.
•
Customizable to fit seamlessly in any environment.
•
Reduce redundancy and model development costs through the direct use of CAD
geometry and existing finite element models.
20
•
Simplify the modeling process for complex geometry through high-speed, high-quality
automeshing.
•
Broadest support of commercial solvers by providing direct interfaces to a wide array of
analysis codes, ensuring the best code is used for specific situations
•
Cost-effective pricing to deliver maximum functionality for your software investment.
HyperMesh provides import/export access to a variety of industry-leading CAD data
formats (such as Pro-E, STEP, IGES, UG, Catia, etc.) for generating finite element models.
HyperMesh contains a series of tools for cleaning up or ‘mending’ imported geometry entities.
Imported geometry can contain surfaces with gaps, overlaps and misalignments that may prevent
an automesher from creating a quality mesh. By eliminating misalignments and holes, and
suppressing the boundaries between adjacent surfaces, users can mesh across larger, more logical
regions of the model. This improves meshing speed and element quality. In addition, users can
apply boundary conditions to their surfaces for future mapping to underlying element data.
HyperMesh presents users with a sophisticated suite of easy-to-use tools to build and edit
models. For 2D and 3D model creation, users have access to a variety of mesh generation panels
besides HyperMesh powerful automeshing module. Automatic midsurface generation, a
comprehensive laminate modeler, and morphing offer new levels of model manipulation. The
surface automeshing module in HyperMesh provides users with a robust tool for mesh
generation and gives users the ability to interactively adjust a variety of mesh parameters per
surface or surface edge. These mesh parameters include element density, element biasing, mesh
algorithm and more. Element generation can be automatically optimized from a set of quality
criteria. HyperMesh can quickly automesh a closed volume with high-quality first or second
order tetrahedral elements. The tetra automesh module uses the powerful AFLR algorithm.
21
Control element growth for structural or CFD modeling requirements by selecting options for
tria or quad elements for tetrahedral generation.
HyperMesh supports a host of different solver formats for both import and export. Along
with fully supported solvers, HyperMesh provides the flexibility to support additional solvers via
a complete export template language and C libraries for development of input translators:
OptiStruct,
ABAQUS,
NASTRAN,
ANSYS,
MoldFlow/Cmold,
LS-DYNA,
Radioss,
PAMCRASH, MADYMO, Marc, IDEAS, and more.
2.1.2 Oasys Primer
The Oasys Primer preprocessor is designed to make preparation and modification of LSDYNA models fast and as simple as possible, improving user productivity and efficiency, and
reducing the time spent manipulating and developing models suitable for LS-DYNA [30].
Primer is designed specifically for preprocessing with LS-DYNA. Therefore the user interface is
clear, simple and tailored towards LS-DYNA, without any compromises. All of the common
keywords can be created, modified, and graphically visualized to help users understand exactly
what a model contains and how the various entities are inter-related.
Figure 2.1 Primer interface.
22
Primer’s dummy positioning facility detects stop angles and rotational degrees of
freedom. Primer reads in LS-DYNA keyword, NASTRAN, RADIOSS, SAP 2000, and IDEAS
input files. It creates, edits, copies and deletes all common keywords. It has a control card and
database editing facility.
Figure 2.2 FE dummy positioning in primer.
Primer has a mesh-independent spotweld creation option, which helps infixing,
reprojecting, and checking. It can create spotwelds from a weld data file, either interactively or in
batch mode. It helps in contact visualization, penetration checking with automatic or manual
fixing, and advanced model deletion, renumbering, and merging. Primer has quick-pick
modification and editing of LS-DYNA keywords [30].
2.2 Analysis Software
Different finite element packages are available. Some of the important solvers are as
follows:
•
LS-DYNA
•
MADYMO
•
PAM-CRASH
23
•
RADIOSS
•
DYTRAN
•
ABAQUS
2.2.1 LS-DYNA:
Since the 1950’s, when the finite element method was first applied for linear problems,
the method has been continuously developed and is now an essential component of
computeraided design. The history of nonlinear finite element methods is tied with the evolution
of the computer age and is well represented by the development of codes. Many of today’s
commercial nonlinear codes were developed at universities and national research laboratories
[26]. The Figure 2.3 shows some application of DYNA.
Figure 2.3 LS-DYNA interface.
A new era of programming explicit finite element codes began at Lawrence Livermore
National Laboratories by John Hallquist. The first version of his DYNA code was released in
1976. Very soon, DYNA-2D and DYNA-3D were widely used at universities and research
laboratories throughout the world. Hallquist’s improvements including effective contact-impact
algorithms, application of one-point quadrature elements with consistent hourglass control and
the high degree of vectorization, opened new avenues for engineering simulations. The code
contains over a hundred material models and numerous contact-impact algorithms, links, multi
point constraints (MPCs), and special elements such as airbag and seat belt, which makes it a
24
leading analytical tool in the automotive industry worldwide. The DYNA codes were first
commercialized in the 1980s, by a French firm, ESI Group. ESI’s product called PAMCRASH
included many routines from WHAMS. Livermore Software Technology Corporation (LSTC)
distributes LS-DYNA, a commercial version of DYNA-3D. LS-DYNA is an explicit, 3-D,
nonlinear finite element code suitable for studying high strain rate impact problems. There are
three major, commercial explicit codes, used for crash simulations. LS-DYNA is used at GM,
Daimler-Chrysler, Ford-Australia and Jaguar, while a French code Mecalog/RADIOSS
(introduced in 1986) is used at Ford, Mazda, Volvo, PSA Peugeot, Citroen, Opel and Renault
[16]. ESI’s PAMCRASH is widely used in Europe and Japan.
LS-DYNA is a general-purpose, explicit finite element program used to analyze the
nonlinear dynamic response of three-dimensional inelastic structures. Its fully automated contact
analysis capability and error-checking features have enabled users worldwide to solve
successfully many complex crash and forming problems.
An explicit time integration scheme offers advantages over the implicit methods found in
many FEA codes. A solution is advanced without forming a stiffness matrix (thus saving storage
requirements). Complex geometries may be simulated with many elements that undergo large
deformations. For a given time step, an explicit code requires fewer computations per time step
than an implicit one. This advantage is especially dramatic in solid and shell structures. In
extensive car crash, airbag and metal forming benchmark analyses, the explicit method has been
shown to be faster, more accurate, and more versatile than implicit methods.
LS-DYNA has over one hundred metallic and nonmetallic material models like Elastic,
Elastoplastic, Elasto-viscoplastic, Foam models, Linear Viscoelastic, Glass Models, Composites,
etc. The fully automated contact analysis capability in LS-DYNA is easy to use, robust, and
25
validated. It uses constraint and penalty methods to satisfy contact conditions. These techniques
have worked extremely well over the past twenty years in numerous applications such as full-car
crashworthiness studies, systems/component analyses, and occupant safety analyses. Coupled
thermo-mechanical contact can also be handled. Over twenty-five different contact options are
available. These options primarily treat contact of deformable to deformable bodies, single
surface contact in deformable bodies, and deformable body to rigid body contact. Multiple
definitions of contact surfaces are also possible. A special option exists for treating contact
between a rigid surface (usually defined as an analytical surface) and a deformable structure.
One example is in metal forming, where the punch and die surface geometries can be input as
IGES or VDA-surfaces which are assumed rigid. Another example is in occupant modeling,
where the rigid-body occupant dummy (made up of geometric surfaces) contacts deformable
structures such as airbags and instrument panels.
Some of the prime application areas of LS-DYNA are as follows:
•
Crashworthiness simulations: automobiles, airplanes, trains, ships, etc
•
Occupant safety analyses: airbag/dummy interaction, seat belts, foam padding, etc
•
Bird strike
•
Metal forming: rolling, extrusion, forging, casting, spinning, ironing, superplastic forming,
sheet metal stamping, profile rolling, deep drawing, hydroforming (including very large
deformations), and multi-stage processes
•
Biomedical applications and many more.
LS-DYNA runs on leading UNIX workstations, supercomputers, and MPP (massively
parallel processing) machines. Computer resource requirements vary depending on problem size.
Simulations with more than 1.200.000 elements have been run using 250 million words of
26
memory and 3.5 GB of disk space. On supercomputers, the code is highly vectorized and takes
advantage of multiple processors.
2.2.2 MADYMO
MADYMO (Mathematical Dynamical Models) is a general-purpose software package,
which can be used to simulate the dynamic behavior of mechanical systems. Although originally
developed for studying passive safety, MADYMO is now increasingly used for active safety and
general biomechanics studies. It is used extensively in industrial engineering, design offices,
research laboratories and technical universities. It has a unique combination of fully integrated
multibody and finite element techniques.
Figure 2.4 MADYMO 3D Models.
MADYMO combines in one simulation program the capabilities offered by multibody,
for the simulation of the gross motion of systems of bodies connected by complicated
kinematical joints and finite element techniques, for the simulation of structural behavior, Figure
2.4 It is not necessary to include both in a model, i.e. a model with either finite elements or
multibodies can be used.
27
The multibody algorithm in MADYMO yields the second time derivatives of the degrees
of freedom in explicit form. The number of computer operations is linear in the number of
bodies if all joints have the same number of degrees of freedom. This leads to an efficient
algorithm for large systems of bodies. At the start of the integration the initial state of the
systems of bodies has to be specified (initial conditions). Several different kinematic joint types
are available with dynamic restraints to account for joint stiffness, damping and friction. Joints
can be (un) locked or removed based on a user defined criterion.
In MADYMO a finite element module is available. The finite element method divides
the actual continuum into finite volumes, surfaces or line segments. The continuum is then
analyzed as a complex system, composed of relatively simple elements where continuity should
be ensured along the interface between elements. These elements are interconnected at a discrete
number of points, the nodes. The initial nodal positions and velocities, the nodes corresponding
to each element, the connectivity, as well as the element properties, e.g. the material behavior,
must be specified at the start of the simulation. Material models are available for metals, fabrics,
foams, composites, rubbers and honeycomb.
The way the interaction between bodies and finite elements is modeled allows the use of
different time integration methods for the equations of motion of the finite element part and the
multibody part.
All used integration methods are conditionally stable and therefore put
limitations on the time step that can be used. To increase the efficiency of the entire analysis the
finite element module is being sub-cycled with respect to the multibody module using a different
constant time step for each module.
Dummy Database: Two MADYMO dummy model types are distinguished. These model types are:
1. Ellipsoid dummy models
28
2. Facet dummy models
The main difference between the model types lies in the modeling techniques applied to
represent the geometry and the mechanical properties of the dummy components. The geometry
of the models is based on detailed contour maps of the dummy segments. These contour maps
were mostly derived from digital surface measurements. Often, additional information was
retrieved from technical drawings.
All MADYMO dummy models have a similar basis, consisting of chains of rigid bodies
with inertial properties, which are connected by kinematic joints. This basis allows for a general
positioning procedure for all models of all three model types. Instrumentation is also modeled
the same way (with some exceptions) in the three model types. Load cells are modeled by
bracket joints at the sensor locations. Accelerometers and displacement transducers are modeled
as output defined for points on bodies. The inertial properties of the standard dummy
instrumentation are included in the inertial properties of the related bodies. Inertial properties of
optional instrumentation are not included in the models. Also the inertial properties of sensor
cables and dummy clothing are not included in the models. If desired, the user could add these
properties, although it is noticed from user applications and dummy validations that in general
satisfactory results are obtained without such modifications. Below, the specific features of the
three types of models are described.
Ellipsoidal Models: Ellipsoid models are models that are based fully on MADYMO’s rigid body
modelling features. The inertial properties of the dummy components are incorporated in the
rigid bodies of the model. Their geometry is described by means of ellipsoids, cylinders and
planes. Structural deformation of flexible components is lumped in kinematic joints in
combination with dynamic restraint models. Deformation of soft materials (flesh and skin
29
components in the dummy) is represented by force-based contact characteristics defined for the
ellipsoids. These characteristics are used to describe contact interactions within the dummy and
between dummy and environment.
Facet Models: Facet models are also multibody models, but compared to the ellipsoid models
they benefit from more advanced multibody technology. The inertial properties of the dummy
are incorporated in the rigid and deformable bodies of the model. In facet models the outer
surfaces of the dummy are described with meshes of shell type, mass less contact elements
(further referred to as facet surfaces). These facet surfaces are fully connected to rigid bodies
and/or deformable bodies. They allow a more accurate geometric representation in comparison
with ellipsoids. Although the facet surfaces are defined in FE_MODEL elements, facet dummy
models are still multibody models, since no FE solver is used in the simulations. Consequently,
they require only a MADYMO multibody solver license and not a MADYMO structural license.
Structural deformation of flexible components such as ribs is represented by deformable bodies.
These deformable bodies enable a more realistic representation of structural deformation than the
joints and restraint models used in the ellipsoid models. Deformation of soft materials (flesh and
skin components in the dummy) is represented by stress-based contact characteristics defined for
the facet surfaces. Using these contact characteristics in contact definitions, soft material
deformation is represented accurately through the contact interactions within the dummy model
and between the dummy model and its environment.
The Hybrid III 50th percentile male is the most widely applied dummy for the evaluation
of automotive safety restraint systems in frontal crash testing. The size and weight of the dummy
represent an "average" of the USA adult male population. It is accepted in several standards
(FMVSS 208, ECE R. 94) and used in global NCAP programmes.
30
Figure 2.5 Hybrid III 50th percentile Dummy model: Ellipsoid model (left) Facet model (right).
31
CHAPTER 3
FINITE ELEMENT MODELING
The procedure for development of finite element meshes begins with importing IGES
geometry data, upon which FE meshes are developed. All FE meshes are then verified for
inconsistencies and refined to improve the mesh quality. Connecting all refined and modified FE
model components completes the FE model building process.
All automotive companies owning detailed vehicle FE models hold proprietary rights on
them. Therefore, to develop a FE model reverse engineering (RE) techniques are followed.
Reverse Engineering is a process by which a complex CAD model of an existing part or product
is created from a set of measurements aided by digitizing tools. Reverse engineering begins with
a final product, which is disassembled into individual parts. The parts are taped, scanned,
digitized, and mapped into a computer for further analysis of the entire design and manufacturing
process. The developed model describes all the relevant characteristics of the part or product,
such as geometry, tolerances, etc. [33].
Figure 3.1 FARO Arm – portable digital tool.
32
The first objective of RE methodology is to generate a conceptual model (example:
surface triangulated) starting from a physical model: a sample (part or tool); or prototype.
Therefore 3D-scanning digitizing techniques aided by specialized software for model
reconstruction are necessary. The usual 3D-scanning digitizing technique to capture 3D
geometries consists of generating a points cloud matrix (3D-coordinates) starting from a surface
geometry of a physical object. The FARO arm is used for this purpose. Data generated during
3D-scanning, i.e., the digital points cloud data in x, y, z coordinates, is exported to model
reconstruction system software to be transformed into conceptual model supported by a
triangulated surface geometry or by a CAD surface data [27], the geometry required to develop a
finite element model of the mass transit bus. To build a full FE model, CAD geometry of the
model is required. CAD models are primarily proprietary models of companies typically they are
not available for reference while modeling FE model.
To develop the full FE model of a mass transit bus a CAD model was provided by a local
manufacturer. This CAD model saved a considerable amount lot of time that would have been
required to create geometry using RE.
Figure 3.2 Finite element modeling flow chart
33
3.1 Modeling Guidelines – General
To generate an acceptable FE model, some basic guidelines that should be followed. To
create mesh with proper quality criteria the geometry should be cleaned to avoid bad meshes.
The connection between adjacent parts should be considered during meshing same mesh along
flanges. Avoid fast transition from small elements to large elements. Check model for free node,
free edges undesired mesh overlap between parts. The element mesh should be on the mid-plane
of the cross section of the component thickness. The mid-plane can be obtained from solid
geometry from preprocessors. The element mesh must be orthogonal to the center line of the part
as shown in Figure 3.3. Element size should be uniform wherever possible, in order to maintain a
smooth transition from coarse to fine mesh, as shown in Figure 3.4. There should be no
component thickness penetration and part intersection. The element size is given below for
modeling different parts during model generation
•
10 – 15 mm element size recommended for modeling main structural areas.
•
30 – 50 mm element size recommended for modeling nonstructural areas.
Figure 3.3 Element mesh is to be orthogonal to the centerline of the part.
Figure 3.4 Transition Mesh.
34
Flange: Flanges of crash models are modeled with three (3) elements across the flange, or Two
(2) elements can be used if the flange width is less than 15 mm.
Figure 3.5 Flange.
Holes: The geometry with hole is always difficult to mesh since the hole distorts mesh
generation therefore holes with a diameter less than 10 mm are ignored. Holes with a diameter
greater than 10 mm are included. When meshing around holes, a minimum of 6 elements is
required.
Figure 3.6 Mesh with Holes.
Notches: For notches greater than 20 mm, the notch and split element to are kept to a minimum
of two across the width. Notches less than 20 mm are eliminated and replaced with two
35
triangular elements. Notches less than 6 mm are eliminated and replaced with two triangular
elements (larger than 6 mm).
Mesh Transition: Uniform mesh transition is an important aspect while modeling. Triangular
elements are used for smooth transition. Triangular elements are stiffer than quad elements for a
coarser mesh. The aspect ratio of triangular elements should be checked, since this will affect
time step in the analysis. The Co option in LS –DYNA (*CONTROL_SHELL, full sorting)is
used.Triangular elements should be less than 10% of total elements.
3.1.1 Element Quality Check
Some quality criteria’s that can be set as the default in Hypermesh while meshing
components are as follows:
ƒ
Min Size: It is length of the smallest side of an element.
ƒ
Max length: It is length of the largest side of an element.
ƒ
Aspect ratio: It is the Ratio of longest side to the shortest side of an element.
ƒ
Warpage: It is maximum angle between the two planes of triangles created by splitting a
quad element diagonally. This check is performed only for quad elements.
ƒ
Max angle quad: It is maximum internal angle of a quad element.
ƒ
Min angle Quad: It is minimum internal angle of a quad element.
ƒ
Max angle Tria: It is Maximum internal angle of a triangle element.
ƒ
Min angle Tria: It is Minimum internal angle of a triangle element.
ƒ
Skew: For quads, it is calculated by finding the minimum angle between two lines joining
opposite mid-sides of the element. In trias, it is calculated by finding the minimum angle
between the vector from each node to the opposing mid-side and the vector between the
two adjacent mid-sides at each node of the element.
36
ƒ
Jacobian: It is a measure of the deviation of a given element from an ideally shaped
element. The check is performed by mapping an ideal element in parametric coordinates
onto the actual element
ƒ
Chordal dev: It is the farthest distance between the middle of an edge to its project on the
corresponding surface (or inferred surface).
ƒ
Percentage of Trias: Ratio of number of trias to total number of elements (displayed).
These are quality a criteria are required for modeling good-quality mesh, which will not
only save computational time of the model but also give accurate results. Table 3.1 shows the
maximum and minimum limits of quality parameter.
No Quality Parameter
1
2
3
4
5
6
7
8
9
Min Side Length
Max Side Length
Max Aspect Ratio
Min Quad Internal Angle
Max Quad Internal Angle
Min Tria Internal Angle
Max Tria Internal Angle
Max Warp Angle
% of Trias
Allowable
Min./Max.
5.0
100
5.0
45.0
145.0
15.0
120.0
15.0
5.0
Table 3.1 Mesh Quality.
3.2 Procedure
3.2.1 Importing Geometry Data
The CAD model required to build an FE model of a mass transit bus was available. The
CAD model was in PRO-E 2.0. Transporting the CAD geometric data to the preprocessor is a
two step process. The first step is to create an IGES file from PRO-E 2.0. The data is imported
into a preprocessor Altair/Hypermesh in the IGES format. IGES is an abbreviation for Initial
37
Graphics Exchange Specification, which is a standard format providing a way of representing
geometric entities in such a way that several CAD programs can read and process them. The
model available was in inches it was converted to millimeters while writing the IGES file. To
export the part IGES geometric data from PRO-E 2.0, the user must simply select the File > save
a copy out menu; this prompts the window with different format option select type IGES (see
Figure 3.7). The user then selects the name of the directory where the IGES file should be
exported.
Figure 3.7 PRO-E menu.
The IGES file exported from PRO-E 2.0 is imported in Altair/Hypermesh. The Figure 3.8
shows the importing procedure of IGES file in Hypermesh. The Figure 3.9 shows different
38
importing options: HM model, Geometry, FE, WELD, CUSTOM. The FILE to be imported is a
geometry file click on the option GEOM.
Figure 3.8 Hypermesh interface for importing CAD IGES file.
Figure 3.9 Hypermesh interface for importing CAD IGES file
39
The options available for importing CAD data are as follows CATIA, IGES, DXF, STL,
PDGS, VDAFS, HM ASCII, UG, PRO-E, and STEP files with this extension can be imported in
to Hypermesh. To import the generated IGES geometric data in to Hypermesh, the user needs to
perform the
Following steps:
1. Open ALTAIR/HYPERMESH.
2. Check on IMPORT option in main menu at bottom left corner as shown in Figure 3.8.
3. Select GEOM option on import window.
4. Select type IGES file.
5. Click on import option that is displayed in green this opens window with different directories.
6. click on the directory in which the IGES file is saved and accept the selection. This imports
the geometry from the IGES file, displaying solid geometry with curves, and surfaces.
Figure 3.10 shows solid model for the chassis of bus as an example of geometry
imported in Hypermesh. Once the part geometry is imported in Hypermesh, the process of mesh
development begins.
Figure 3.10 Chassis IGES File.
40
The exported geometry file in IGES format is cleaned to remove fillets and hole.
Geometry should be cleaned to remove small feature which can create small element size.
3.2.2 Meshing
Building the FE mesh is the first step in developing a finite element model. Next,
decisions regarding element formulations, material models, material characteristics, contact
algorithms, MPCs and connections, loading and boundary condition formulations, solution
parameters and others should be determined to create a valuable, concise model. Only a well
defined FE model, with carefully established parameters, can realistically represent the structural
behavior of the system.
Development of FE meshes was the most important phase of this research project. The
majority of the research time was spent to creating, verifying, and refining FE meshes that could
give reliable and realistic information about crash analysis.
Figure 3.11 Mid surface meshing.
41
Figure 3.11 show the method followed. First the geometry is cleaned. The mid surface is
extracted using option extract mid-surface of solid geometry in Hypermesh. According to the
meshing guidelines for most parts Belytschko say elements are used as they save computational
time as well as are recommended for crash analysis. The auto mesher option of Hypermesh can
be used to create mesh with the required quality criteria and element size. The extracted midsurface is meshed. When the part is meshed using auto mesher, some elements fail to fit in to the
quality criteria due to complex geometry. These elements are corrected using the edit option in
Hypermesh, which provides the perfect environment for meshing complex geometry by
simplifying the features to create a good quality mesh.
The CAD model shows a detailed drawing of an engine block. This complex geometry is
difficult to be meshed with the required quality criteria, so the geometry is simplified before
meshing. It was not possible to simplify this geometry in Hypermesh so a simple model was
created using CATIA. The IGES file was exported to CATIA by taking external dimensions and
creating simplified geometry with same external shapes. This CATIA model was imported in
Hypermesh and meshed using solid elements. The FE model of the transit bus was not used for
doing any structural analysis of the engine so it was modeled as a rigid part.
Figure 3.12 Simplified CATIA model of engine block.
42
Figure 3.13 Meshed model of engine block.
Figure 3.14 Chassis and side panels.
Figure 3.15 Side structure.
43
Figure 3.16 Structural parts
Figure 3.17 Front suspension.
Figure 3.18 Rear suspensions.
44
The Figures 3.17 and 3.18 show detailed components of front and rear suspension
modeling. The detail joints and links were modeled of suspension to imitate the actual motion of
suspension during impact since force on a tire plays a major role during the side impact of a
vehicle. Since suspension parts absorb some of the impacting energy during accident conditions,
suspension was modeled in detail.
Figure 3.19 Petrol tank and radiator.
Figure 3.20 Structural members.
45
Figure 3.21 Finite element model shaded views of the non-structural components.
3.2.3 Procedure to create Joints
Figure 3.22 Front axel kinematics joints.
Figure 3.23 Rear axel kinematics joints.
46
Figure 3.24 Joint descriptions.
Joints were created using the Hypermesh menu for FE joints. The FE joints panel allows
one to create, review, or update joint elements. A joint element is a defined as the connection
between two rigid bodies. Joint elements store property and orientation information.
Location:
1D page
2D page – safety module
Tool – safety module
Joint elements are config 22 and are displayed with lines between the appropriate nodes
and the letter J between nodes 1 and 3 of the element. The type of a joint element controls the
number of nodes contained in the element. The type also controls the orientation information
formats available. The type of an element cannot be changed or selected using the Element
Types panel. The Figure 3.24 above shows procedure to create a revolute joint in Hypermesh.
Joints are created only between two rigid parts so body A and body B have to be rigid. To create
a Revolute joint it requires four nodes which are coincident to each other. Node J1 and J4 belong
to rigid body A, and node J2 and J3 to body B. They are connected to body using the
47
Constrained Extra node feature in LS-DYNA. Figure 3.25 show the type of joints that can be
created in Hypermesh [34].
Figure 3.25 Types of joints created in Hypermesh.
48
FE model of the transit bus consists of 32 joints, which consist of revolute, spherical, and
translational joints.
•
REVOLUTE
- 18 joints
•
SPHERICAL
- 2 joints
•
TRANSLATIONAL
- 12 joints
Figure 3.26 FE bus model.
The final bus FE model consists of 302,227 elements, 298,833 nodes, 1,405 components,
43 sub-assemblies, 6 control volumes (Tire model), 1,348 section properties, 20 materials, 32
kinematic joints, and 20219 spot welds. Table 3.3 shows the summary of model with number of
parts and elements in model.
Number of
element
Beam
Quad
Triangular
Solid
Spring
Accelerometer
1
205,077
8,420
2,009
12
25
Table 3.2 Element Type.
49
Number of Nodes
298,833
Number of Elements
302,227
Number of Parts
1,405
Number Of Mass Element
1,931
Control Volumes (Tyres)
6
Spotwelds
20,219
Kineamtic Joint
32
Table 3.3 Model Size.
3.3 Element Formulation
Various element formulations were used for bus components. For most of the parts a
shell element with the element formulation Belytschko-Lin-Tsay (2) is used. Since there was no
structural analysis for the engine, it was modeled as rigid body. To obtain perfect mass and
inertia, the engine is meshed with solid elements with element formulation 1. The air spring and
damper require having a force displacement function, so these parts were modeled using discrete
element as it perfectly represents a suspension of vehicle.
Belytschko-Lin-Tsay shell element developed by Belytschko and Tsay in 1981 was
implemented in Ls-Dyna as an alternative to the Hughes-Liu element formulation. Since
integration points through the thickness increase, the number of mathematical operations
required for the Hughes-Liu formulation as compared to Belytschko-Lin-Tsay element. For
example if there are five integration points through the thickness of shell element, Hughes-Liu
requires 4,066 mathematical operations, whereas Belytschko-Lin-Tsay requires only 725
mathematical operations. Thus the Belytschko-Lin-Tsay element formulation is more cost
effective and efficient as compared to the Hughes-Liu formulation. More details of deformation,
50
i.e., impacting areas were given three integration points through the thickness of the shell
element. For remaining area two integration points were provided [14].
Constant Stress Solid Element (1):
This element formulation is used for all solid
elements mostly engine parts. This element formulation is used for three dimensional structural
calculation of eight node element. As the entire solid element has given rigid material property
so Ls-Dyna skips calculations for this elements thus reducing the analysis time.
Spotweld Beam (9): Element formulation is used for beam element. This beam element is
used as propeller shaft, which gives output from engine to differential.
3.4 Material Model and Properties
The main bus structure is predominately made up of steel and aluminum members. Since
a single model will be used to study various crash configurations (impact velocities from 0 km/hr
up to 60 km/hr), it is important to define the strain rate effect on the mechanical properties of the
main structural materials. In general, for the strain rate region of interest to crashworthiness
applications there are three types of testing systems available:
-
Mechanical or Servo-Hydraulic: Quasi-static condition and strain rates bellow 0.1/s.
-
Servo-Hydraulic: Strain rate range 0.1 to 500/s.
-
Split Hopkinson Bar System: Strain rate range 100 to 1000/s, and higher.
Strain rates increments 0.1, 1, 10, 100, 250, and 500 /s are sufficient for describing the
material strain rate sensitivity in this application, therefore a Mechanical and a Servo-Hydraulic
System were used to extract the material data. The results of the material testing show that the
steel materials used are more strain rate sensitive than the aluminum materials. In order to define
the input properties for the FE model, the following procedure to convert the tension test data
was adopted
51
a. Obtain the engineering stress and strain from the tension test:
(3.1)
b. Calculate the true stress and true strain:
(3.2)
c. Calculate the effective stress and strain curves that will be used as input for the FE
model:
(3.3)
The LS-Dyna material model used for all the structural members is material type 24. It is
an elasto-plastic material with an arbitrary stress versus strain curve and arbitrary strain rate
dependency [14]. Windshield and passenger window glass properties were also modeled with
material type 24 with a defined plastic strain failure model. For the discrete spring and damper
suspension elements LS-Dyna material DS2 (Spring Nonlinear Elastic) and DS5 (Damper
Nonlinear) were used. Components of the bus with negligible deformations such as the engine
block and transmission were modeled using LS-Dyna material type 20 (Rigid), and inertial
properties were defined per component as specified in the engineering documentation. For the
tires the LS-Dyna material type 1 (linear elastic material model) was used. Spot-welds were
modeled with LS-Dyna material type 100. The final bus FE model contains twenty material
definitions. In this section all the material models used in bus are discussed. The method of
defining a material in LS-DYNA is given below. The units used for defining material properties
are given in Table 3.4.
52
No.
Property
Units
1
Length
millimeter
2
Time
second
3
Mass
tone
4
Force
Newton
5
Young’s modulus of steel
210.0E+03
6
Density of steel
7.85E-09
7
Yield stress of Mild Steel
200.0
8
Acceleration due to gravity
9.81E+03
9
Velocity equivalent to 30 mph
13.4E+03
Table 3.4 Units.
1) Steel Parts: This material card is used for most of chassis parts. This is material type 24. An
elasto-plastic material with an arbitrary stress versus strain curve and arbitrary strain
dependency can be defined using this model. Material type 24 is used for all chassis and steel
parts. This card is defined as follows:
*MAT_PIECEWISE_LINEAR_PLASTICITY
Variabl
e
MID
RO
E
PR
SIGY
ETAN
FAIL
Type
1002
7.89e-9
2.1e+5
0.3
270
0.01
0.0
C
P
LCSS
LCSR
VP
80.0
4.5
1017
0
0
Variabl
e
Type
To define a strain rate dependent material a Table id must be defined which consist of
all strain rates and their corresponding curves defined in the define curve cards. Load curve
53
of effective stress versus effective plastic strain is defined. Above card is used to define the
two different steel materials. Two different stress strain curves were used for defining above
materials.
Figure 3.27 Curves for Strain Rates 0.00001mm/s, 0.1mm/s, 1000mm/s.
Figure 3.28 Curve for Strain rates 0.1mm/s, 20mm/s, 4000mm/s.
2) Aluminum Parts:
*MAT_PIECEWISE_LINEAR_PLASTICITY
Variable MID
Type
RO
E
PR
1081150 2.76e-9 69000 0.33
SIGY ETAN FAIL TDEL
300
Variable C
P
LCSS
LCSR VP
Type
0.0
1017
0
0.0
54
0
0.0
0.0
0.0
Figure 3.29 Stress strain curve for aluminum.
2) Tire: This material card is used to define tire properties. This is material type 1. This is an
isotropic elastic material and is available for beam, shell and solid elements in LS-Dyna. This
card is defined as follows:
*MAT_ELASTIC
Variable
MID
RO
Type
10811921 1.3e-9
E
PR
DA
DB
10000
0.3
0
0
K
3) Glass: This material card is used to define material of glass. This is material type 24.
*MAT_PIECEWISE_LINEAR_PLASTICITY
Variable
MID
RO
E
PR SIGY ETAN FAIL TDEL
Type 1081172 2.5e-9 70000 0.22
30
1000
0.01
0
3) Engine: This material card is used to define material for engine parts. This is material type
20. Parts made from this material are considered to belong to a rigid body. This material type
provides a full way of turning one or more parts comprised of beam, shells, and solid
elements into a rigid body. Elements which are rigid are bypassed in the element processing,
55
and no storage is allocated for storing history variables. Thus rigid material type is very cost
efficient.
*MAT_ RIGID
Vari
MID
RO
Type 1081165 2.7e-9
Vari
CMO
Type
1.0
E
PR
210000
N
COUPLE M
0.3 0.0
0.0
0.0
ALIAS/R
0
CON 1 CON 2
0
0
6) ACCELEROMETER:
*MAT_ RIGID
Vari
MID
RO
E
PR
Type 1081181 7.89e-9 210000
Vari
CMO
Type
1.0
CON 1
0.3
N COUP M ALIAS/R
0.0
0.0
0
0
CON 2
0
0
7) SPRINGS & DAMPERS: Shock absorbers are used in bus suspension system. This is material
type 5 used for dampers. This material provides a viscous translational damper with an arbitrary
force versus velocity dependency. In this card, user can define force versus rate of displacement
curve.
Variable
MID
LCDR
Type
1081200
1009
56
A force versus rate of displacement curve is defined in this card. From local bus
manufacturer front and rear axle damping function were obtained. Damping force versus piston
velocity curve is used to define curve in this card.
Front Axle Damping
Damping
Force (N)
-650
Piston
Velocity
-500
-550
-400
-450
-300
-350
-200
-250
-100
0
0
250
100
350
200
450
300
550
400
650
500
Table 3.5 Front Axel Damping Function.
57
Rear
Axle
Damping
Damping
Piston
Force (N)
Velocity
-958.32
-500
-843.75
-400
-708.33
-300
-562.4
-200
-375
-100
0
0
375
100
562.4
200
708.33
300
843.75
400
958.32
500
Table 3.6 Rear Axel Damping Function.
This card is used to define spring element. This is material type 4 for discrete spring and
dampers. This material provides nonlinear elastic translational and rotational spring with
arbitrary force versus displacement.
*MAT_SPRING_NONLINEAR_ELASTIC
Variable MID
Type
LCD LCR
1081203 1007 0
From available data for front and rear spring functions were obtained. The functions used in Fe
model are as follows this function are applied to discrete element which will behave like an air
spring
58
Front Axle Spring
Function
Load (N)
Height (mm)
-38.1
-53378.7
-25.4
-31137.6
-12.7
-11120.6
0
0
12.7
11120.55
25.4
19600
38.1
26689.33
Table 3.7 Front Axel Spring Function.
Rear Axle Spring Function
Load (N)
Height (mm)
-35.56
-18237.7
-22.86
-7561.98
-10.16
-2668.93
0
0
15.24
2668.933
27.94
4448.222
40.64
6227.51
Table 3.8 Rear Axel Spring Function.
8) SPOTWELDS: This is material type 100. This material model applies to beam element type 9
and to solid element type 1. The bus parts are connected using spotwelds.
*MAT_SPOTWELD
This card is used to define a spotweld material and failure criteria can be given in this card.
59
Variable MID
Type
RO
E
PR SIGY ET
1081198 7.89e-9 210000 0.3 200
DT TFAIL
1000 0
0
9) BUMPER: This card is used to define material properties of bumper. Bumper is made of
plastic.
*MAT_PIECEWISE_LINEAR_PLASTICITY
Variable MID
Type
RO
E
PR SIGY ETAN FAIL TDEL
1081208 1.2e-9 2800 0.3 45
420
Spotwelds: A card is used to give mass less spot welds between noncontiguous nodal pairs. The
spot weld is a rigid beam that connects the nodal points of the nodal pairs. Thus, nodal rotations
and displacements are coupled. Spotwelds must be connected to nodes having rotary inertias e.g.
beam or shell.
Failure of spotwelds occurs when addition of shear force and normal force at the
spotweld is greater than effective plastic strain. We can specify this value in constrained
spotweld card and if this value is reached then spotwelds will fail.
(3.4)
Where fn and fs are the normal and shear interface force.
All parts are assembled in Hypermesh and parts are connected by spotwelds and beam
elements. Damper and air springs are modeled using discrete elements. Tires are modeled using
simple airbag properties. All parts are assigned their material properties and section properties.
60
3.5 Implicit Analysis
To check stability as well as connectivity of parts an implicit analysis was done. To run a
explicit analysis of the full model requires 35 hours with 2 CPU while an implicit analysis runs
in a few seconds thus reducing checking time as well as speeds up the process of modeling. The
cards required to carry out an implicit analysis is given below [14]:
*CONTROL_IMPLICIT_GENERAL
IMFLAG – Solution Type
•
0 Explicit (default)
•
1 Implicit
*CONTROL_IMPLICIT_EIGENVALUES
NEIGV – Number of Eigen value
These are LS-DYNA key words required to do implicit analysis. There are some limitations to
implicit analysis.
•
High Memory Requirements
•
Very expensive Time Step calculations
•
Contact Inexperience
•
Need Efficient Linear/Non-Linear solver
Figure 3.30 Implicit analysis of roof top.
61
CHAPTER 4
FINITE ELEMENT MODEL VALIDATION
The model has been validated for a variety of impact conditions specified in the Bus
Procurement Guidelines [11]. The test data for the Bus Procurement Guidelines test conditions
was provided by the bus manufacturer, additional higher speed crashworthiness evaluations were
compared to data from previous publications of similar class transit buses [8]. The validation
parameters were limited to the test data provided; these data included Bus CG displacements,
velocities, acceleration, rigid wall reaction forces, and measurements of permanent structural
deformations.
4.1 Side Impact Validation
Per Bus Procurement Guidelines section 5.4.1.2 [11]; the bus shall withstand a 25-mph
(40.4 km/hr) impact by a 4,000-pound (1814 kg) automobile at any point, excluding doorways,
along either side of the bus with no more than 3 inches (76 mm) of permanent structural
deformation at seated passenger hip height. This impact shall not result in sharp edges or
protrusions in the bus interior.
Due to the unavailability of a FE model for a 1983 Chevrolet Caprice, a modified
FMVSS 214 type deformable barrier model was created. In order to create the model the
required inertial parameters such as the vehicle’s wheelbase, track width, bumper height, and
vehicle weight were obtained from the NHTSA Light Vehicle Inertial Parameter Database [12],
and data from vehicle specifications. The FMVSS 214 type barrier overall dimensions were
modified accordingly as shown in Figure 4.1. Consecutively the 1983 Caprice maximum
dynamic crush displacements (0.838 m), vehicle weight (1869 kg), weight distribution (54.2 %
font and 48.8 % rear), and front structure linear stiffness values (662.5 kN/m) were derived from
62
NHTSA’s 56.8 km/hr front crash test number 515 [12,13]. The modified barrier model
performance for a 56.8 km/hr frontal test was compared with the results of a 1983 Chevrolet
Caprice crash test [13].
Parameter
Physical Test
Finite Element Model
Test Type
Side Impact
Side Impact
Target Vehicle
Transit Bus
Transit Bus
Target Vehicle Weight
21290 Lbs (9656 kg)
21290 Lbs (9656 kg)
Impact Speed
25.1 mph (40.4 Km/hr)
25.1 mph (40.4 Km/hr)
Impact Angle
2700
2700
Bullet Vehicle Weight
4018 Lbs (1822 kg)
4018 Lbs (1822 kg)
Bullet Vehicle
1983 Chevrolet Caprice
Modified FMVSS 214
Occupants
None
None
Bus Tire Pressure
110 psi (7.6 bar)
110 psi (7.6 bar)
Table 4.1 Side Impact Validation Test Conditions.
Figure 4.1 FMVSS 214 barrier and modified FMVSS barrier.
63
As shown in Figure 4.2, the modified deformable barrier (MDB) model captures the
overall phasing and amplitude of the vehicle’s CG velocity and displacement test profiles. Figure
4.3 shows the comparison with the test of the bus center of gravity velocity and displacement
time histories. There is a difference in bus CG y-velocity after 60 ms; this discrepancy is
attributed to the differences in linear stiffness characteristics, and structural load path of the
MDB with respect to the actual 1983 Caprice frontal structure. Figure 4.5 shows that the post–
test intrusion in the bus compartment is 12 mm at the end of the 150 ms event; the maximum
intrusion measured minutes after the physical test was 8 mm.
Figure 4.2 Modified FMVSS 214 barrier 35mph (56 km/hr) frontal impact validation.
64
Figure 4.3 Bus 40 km/hr side impact test validation.
Figure 4.4 Side impact test vehicle kinematics.
65
Figure 4.5 Displacement (mm) and von Mises stress (MPa) contour plots.
4.2 Rear Impact Validation
Per Bus Procurement Guidelines section 5.4.3.9.2 [11]; no part of the bus, including the
bumper, shall be damaged as a result of a 2-mph (3.2 km/hr) impact of the bus at curb weight
with a fixed, flat barrier perpendicular to the longitudinal centerline of the bus. The bumper shall
return to its pre-impact shape within 10 minutes of the impact. As shown on Figure 4.6, the FE
model parameters correlate with the results from the physical test. The stress levels do not
exceed the yield strength of the bumper or any of the rear structure components; therefore it can
be assumed that the bumper cover should return to its pre-impact shape. The Table 4.2 shows the
physical testing specification. The bus impact a flat barrier with 2 Mph at zero degrees.
66
Parameter
Physical Test
Finite Element Model
Test Type
Rear Bumper Barrier Test
Rear Bumper Barrier Test
Bus Weight
21800 lbs (9888 kg)
21800 lbs (9888 kg)
Impact Speed 2 mph (3.2 km/hr)
2 Mph (3.2 km/hr)
Impact Angle 0 degrees
0 degrees
Fixed Target
Fixed Flat Barrier
Fixed Flat Barrier
Test Setup
Rear Bumper Contacts Flat
Rear Bumper Contacts Flat
Barrier
Barrier
Table 4.2 Rear Impact Validation Test Conditions.
Figure 4.6 Rear impact test validation.
67
4.3 Frontal Impact Validation
Per Bus Procurement Guidelines section 5.4.3.9.2 [11]; no part of the bus, including the
bumper, shall be damaged as a result of a 5-mph (8 km/hr) impact of the bus at curb weight with
a fixed, flat barrier perpendicular to the bus' longitudinal centerline. The bumper shall return to
its pre-impact shape within 10 minutes of the impact. As shown in Figure 4.9, the FE model
parameters correlate with the results from the physical test. The front bumper test vehicle
kinematics is shown on Figure 4.8. As shown in Figure 4.9 the stress levels do not exceed the
yield strength of the bumper or any of the front structure components, therefore it can be
assumed that the bumper cover should return to its pre-impact shape. The Table 4.3 shows all the
test specification fro physical testing.
Parameter
Physical Test
Finite Element Model
Test Type
Frontal Bumper Barrier Test
Frontal Bumper Barrier Test
Bus Weight
21800 lbs (9888 kg)
21800 lbs (9888 kg)
Impact Speed 5 mph (8.0 km/hr)
5 mph (8.0 km/hr)
Impact Angle 0 degrees
0 degrees
Fixed Target
Fixed Flat Barrier
Fixed Flat Barrier
Test Setup
Front Bumper Contacts Flat
Front Bumper Contacts Flat
Barrier
Barrier
None
None
Occupants
Table 4.3 Frontal Validation Test Conditions.
68
Figure 4.7 Front bumper test validation.
Figure 4.8 Front bumper cross section and frontal view von Mises stress (MPa).
69
Figure 4.9 Front bumper test validation vehicle kinematics.
70
CHAPTER 5
SIDE IMPACT STUDY OF TRANSIT BUS
The finite element model of low-floor transit bus is used to study real life side impact
crash conditions. To carry out this study different Finite element models that are built by NCAC
were used. Using FE models of cars it has become possible to carry out different structural and
occupant safety studies. These models not only help in finding design flaws of the structure but
help in studying different crash protections without destruction of the actual model.
Crashworthiness and occupant injury simulations are often used to evaluate the effectiveness,
and potential limitations of proposed test procedures and safety countermeasures. Occupant
safety is the most important issue in the design and development of a vehicle. Depending on
occupant response, the reliability and performance of a vehicle can be evaluated.
5.1 Crash Conditions
There are no finite element models developed for transit buses according to the literature
review, and even very less amount of work done on side impact of a transit bus. Therefore this
study was developed to determine the effect of side impact on the structure of a transit bus and
occupants in the bus. There was no data available for particular side impact collision from any of
the databases so a test matrix was developed by studying the road construction. Test matrix given
in Table 5.1 is design to imitate a real life crash conditions for side impact. Finite element
simulations are carried out according to the test matrix given above to do structural and
kinematic analysis of transit bus. To study occupant response to various crash condition (LSTC)
Finite element models of dummies are used. These dummy models are not validated so they are
just used for kinematic study of occupant. Different injury parameters are also recorded on these
dummies but cannot be accounted for real life injures as the models are not validated.
71
Sr. No.
Condition
1 Fmvss 214
2
3
90 deg
4
5
6
7
30 and -30 deg
8
impact
9
10 side impact test
Source
NHTSA
Test Procedure
Bus - 0mph : Fmvss Barrier - 31mph
Bus - 0 mph : chevy-s10 - 25 mph
Bus - 10 mph : chevy-s10 - 30 mph
Bus - 10 mph : chevy-s10 - 30 mph
Bus - 0 mph : neon - 35 mph
Bus - 0mph :Neon - 25 mph
Bus - 0 mph : F-800- Truck - 25 mph
Bus - 0mph : chevy-s10- 30 mph
Bus - 0 mph : F-800-Truck - 25 mph
Bus - 0 mph : Modified Barrier
Table 5.1 Test Matrix.
Primary application of structure in vehicle is to absorb impact energy with no cabin
intrusion. This will reduce acceleration to occupant cabin and in turn reduce injury values. This
is a parametric study where different size vehicles with different structural configuration are
used. These models are impacted in a range of speeds to see the effect of mass and impacting
velocity on the bus structure. Impacting angel and side of impact is varied to study bus structure
when it’s loaded from both sides and different angels. The models used for side impact study are
down loaded from NCAC site. Theses models were validated to actual testing results.
Interior modeling is carried out to study occupant behavior in FE environment. The FE
dummies used are rigidised so the don’t add to time step of FE simulation. The Figure 5.1 shows
some possible crash conditions on actual road. These pictures are streets of Chicago and Miami
taken form goggle earth. The test setup used for finite element analysis is overlaid on these
pictures to show the real life conditions. These are 90 degree and 30 degree crash conditions
explained in test matrix.
72
Figure 5.1 Real life crash conditions for side impact.
73
Figure 5.2 LS DYNA hybrid III 50th % dummy.
Figure 5.2 shows dummy set up for finite element analysis. These dummies are finite
element rigidised Hybrid III 50th % male dummy models. Dummy 1 is used to study effect of
seat back on femur and effect of side wall on occupant. Dummy 2 is used to study injuries due to
initial point of impact and occupant to occupant contact. Dummy 3 is used to study injuries due
to ejection of occupant. Dummy 4 is used to study effect of modesty panels on femur and effect
of side wall on the head injury. Dummy 5 is used to study effect of wheel cover and initial point
of impact on occupant. The injury values off all dummies will change depending on which side
the bullet vehicle impacts.
5.1.1 FMVSS 214 Test Condition
This analysis is done to find vehicle crush and occupant kinematics when a side impact
standard barrier with 1350 kg (3015lb) impacts a transit bus. The FMVSS 214 is a moving
deformable barrier impacts the target vehicle with 33.5 Mph with 27 degrees angle. This test
represents what happens to a passenger vehicle when it is struck by a car or SUV from side.
Structural performance of a target vehicle is based on measurement indicating the amount of
intrusion in passenger cabin. Less intrusion assures that occupant of other size and different
seating position will also have less injury risk. This is side impact standard for SUV and
74
passenger vehicles whose GVWR is below 10000 Lbs. This side impact standard is used for
vehicles with side airbags.
Figure 5.3 FMVSS 214 barrier.
Figure 5.4 FMVSS 214 test setup.
75
Figure 5.5 FMVSS 214 frames.
The side impact analysis for FMVSS 214 is done using a moving deformable barrier with
standard specification. This analysis is done for 300 ms using finite element solver LS-DYNA.
The Figure 5.5 shows frames taken from simulation at different interval. This Figure gives better
understanding of dynamic event during a side impact crash condition. When moving deformable
barrier impact the bus it dips down as the suspensions are loaded there is certain amount of crush
in bus side wall and as barrier model collapses the bus starts sliding. The bus comes to original
height when the suspensions are unloaded. The severity of impact is high as the bullet vehicle is
at 33.5 Mph and the crush distance is very less on side as compared to front side of the bus.
76
Figure 5.6 Structural deformation and dummy kinematics.
The Figure 5.6 shows dummy kinematics during the side impact test. Dummy2 and
dummy 5 are seated on side facing seats they show sudden neck rotation and deflection of chest.
Dummy 2 comes in contact with dummy 3 as dummy 3 is ejected from its seat there is possibility
of tibia injuries and femur injuries to both dummies. Dummy 1 and dummy 4 are seated on
forward facing seats dummy 1 comes in contact with forward seat and it is ejected towards point
of impact possibility of head neck femur and tibia injuries in that seating position. Dummy 4 is
moves on left side of bus possibility of impacting the wall of bus or can be ejected from the
window.
77
Figure 5.7 Displacement fringes for bus structure
The Figure 5.7 shows structural deformation of bus structure during impact. The
deformation in bus structure is 2.7 inches as shown in the Figure 5.7. According to Standard Bus
Procurement Guideline the maximum allowable permanent deformation is 3 inches in side
impact crash.
Figure 5.8 Displacement velocity and acceleration at cg.
The Figure 5.8 shows Acceleration at the CG of bus. The maximum acceleration noted is
about 15G’s on the CG accelerometer. This standard show larger acceleration compared to
SBPG side impact standard for transit bus.
78
5.1.2 Dodge Neon
The Figure 5.9 shows detail model of Dodge neon. It is 2689 lb (1220 kg) and is stable
for 25 to 40 Mph for full frontal impact according to NCAC report. The Table 5.3 and 5.4 gives
properties of model in detail. This model represents a passenger vehicle that is mostly found on
streets. This model is used to study its effect on the bus structure when it impacts from the side.
The test conditions as shown in Figure 5.10 are 90 degrees impact with two different speeds 25
and 35 Mph.
Figure 5.9 Dodge neon.
Number of Nodes
285910
Number of Elements
272485
Number of Parts
370
Table 5.2 Model size of Dodge Neon.
Number of element
Beam
63
Quad
269249
Triangular
-
Table 5.3 Element type for Dodge Neon.
79
Solid
2860
Spring
8
Figure 5.10 Test setup for neon 90 deg impact.
The Figure 5.11 shows different frames at different time of simulation it shows crushing
of neon frontal part as it impacts the side of bus. The dodge neon is impacting at 35 Mph and the
amount of crush in bus is 14.4 inches. The cabin crush quite high as the chassis of neon comes in
contact with bus structure at initial stage. The acceleration measured at CG is 35 Gs’. The
acceleration is high because there is direct loading of chassis of neon model which gives less
crushing and low energy absorption. The results from this analysis show that vehicle shape and
structural design do affect the acceleration on vehicle. Neon model being a mid size vehicle it
produces larger deformation.
80
Figure 5.11 Frames for Dodge Neon 35 Mph 90 Degrees impact.
5.1.3 Chevy 2500 Pickup
The Figure 5.13 shows setup used to study bus crashworthiness when a pickup impacts a
bus from side. The test matrix given in Table 5.1 shows different setups used. Chevy pickup is a
detail model shown in Figure 5.12 is 4030 Lbs and is stable for 25 to 40 Mph full frontal impact.
The Table 5.5 and Table 5.6 give details about the finite element model. This model is
downloaded from NCAC database. The dummy setup used is same as used for FMVSS 214
simulation. The Figure 5.12 shows structural deformation of bus during 90 degrees side impact
where bus is stationary and bullet vehicle moves at 25 Mph. this simulation shows same dummy
81
movement as shown in FMVSS 214 simulation. The injury values might vary compared to
FMVSS 214 as the mass of bullet vehicle is greater.
Figure 5.12 Chevrolet C2500.
Number of Nodes
66050
Number of Elements
57742
Number of Parts
248
Table 5.4 Chevy Model Size.
Number of element
Beam
153
Quad
54028
Triangular
-
Solid
3561
Table 5.5 Element Type for Chevy Model.
Figure 5.13 C-2500 ± 30 and 270 deg impact angel.
82
Spring
0
Figure 5.14 Test Setup C-2500 90 deg impact angels.
The Figure 5.15 shows the structural deformation of bus when a bullet vehicle of 4030
Lbs hits the target vehicle with 25 Mph speed. The maximum amount of deformation observed
was along the side aluminum panel. Appendix C shows detail of crush of bus. The maximum
crush in bus structure was 9 inches on the impacted area in this crash condition. Acceleration on
bus was recorded at the CG of Bus it was 22 g’s. Different test setups were set using the C-2500
model. The results obtain from them are given in Table 5.7. The bullet vehicle is C-2500 and
target vehicle is bus.
83
Figure 5.15 Frames for 90 deg impact.
According to Table 5.6 it shows that as mass is constant of bullet vehicle the acceleration
recorded at CG is proportional to impacting speed. Deformation increases with impacting speed
and area of contact of two vehicles. During side impact structural crush plays a major role as the
crash distance is minimal in side structures. Test conditions where the bus is moving the
occupant kinematics observed is quite high. This shows that dynamic events are quite harmful so
while designing side impact standards dynamic events should also be considered. The results for
above simulations are plotted in Appendix B. It is observed from this simulation that as crush
increases the acceleration decreases. Crush in bus observed when C 2500 impacts is quite less
84
compared to neon this shows that impacting speed also plays major role compared to mass of
impacting vehicle.
Test Condition
Acceleration Deformation
At CG (G’S) (in)
Bullet 25Mph- Target 0 Mph -90 Deg
Bullet 25 Mph- Target 0 Mph - +30Deg
Bullet 30 Mph- Target 30 Mph – 90 Deg Door Side
Impact
Bullet 30 Mph- Target 30 Mph – 90 Deg Driver Side
Impact
Bullet 30 Mph- Target 10 Mph – +30 Deg Driver Side
Impact
22
9
25.4
5.9
29
15.9
28.42
15.31
20.977
6.5
Table 5.6 Test Results.
5.1.4 Ford 800 Truck
The Figure 5.16 shows finite element model of F 800 truck model it is downloaded form
NCAC site. This is model is 11049 Lbs (5012 kg). This is simple model of F 800 with 147 parts
just to represent mass and outer structural profile. This model is used to study effect impact of
large vehicle on bus structure. The properties of finite element model are given in Table 5.8 and
5.9. This model has less number of elements so being larger in size it has coarser mesh it takes
less time for analysis. This model cannot be used for larger speed of impacts as it is not stable for
larger speeds.
Figure 5.16 F800 Truck.
85
Number of Nodes
35778
Number of Elements
28583
Number of Parts
147
Table 5.7 Model Size F 800 Truck.
Number of element
Beam
124
Quad
18471
Triangular
1638
Solid
8350
Spring
0
Table 5.8 Element Type for F 800 Model.
Figure 5.17 Test setup F-800 -30 deg impact angel 25 mph.
The CG of truck is higher to bus the results of such impacts could lead to rollover of bus
in high speed impacts and the deformation will be larger as the mass of this vehicle is large.
Looking at the front structure of truck the crush distance is quite large so energy absorption will
take place. This energy absorption mostly reduces cabin accelerations for both bus and truck.
This model is used for three different setups it is ± 30 degrees and 90 degrees impact with 25
86
Mph speed. The dummy setup is similar like other tests. Setup shown in Figure 5.17 is
negative30 degrees side impact.
Figure 5.18 Frames for F 800 30 deg impact.
The Figure 5.18 shows frames recorded at different instants in simulation. It shows the
side impact event of F 800 impacting bus with 25 Mph and bus is stationary. There is quite large
cabin intrusion of bus. This simulation shows as the mass of bullet vehicle increases the cabin
intrusion shows increase for same impacting speed. The amount of structural deformation
measured in bus is 19.3 inches. The amount of acceleration recorded at CG is 15 G’s. The
acceleration is quite low because of larger deformation there is large energy absorption. Table
87
5.9 shows that acceleration produced by F 800 at CG is same in all cases of impact. The
permanent deformation on bus structure is high in angular impact compared to perpendicular
impact. The result of these accidents may lead to occupant fatalities.
Test Setup
Acceleration at CG (G’s) Deformation (in)
Bullet vehicle -25Mph 90 Degree
15
15.59
Bullet vehicle -25Mph -30 Degree
14
19
Bullet vehicle -25Mph +30 Degree
15
20.3
Table 5.9 Simulation Results for F 800.
88
CHAPTER 6
MADYMO MODELING OF VEHICLE INTERIOR
6.1 MADYMO Model
Safety of occupant is the most important issue that concerns during the design and
development of vehicle. Depending on the occupant response the reliability and performance of
the vehicle can be evaluated. In order to understand the occupant response in side impact of a bus
we have created a detail vehicle interior in MADYMO (Figure 6.1)
Figure 6.1 MADYMO model of transit bus.
The multibody model for bus is constructed using finite element model. This mutlibody
model consists of all interior parts and bus side walls and floor. The model is constructed by
converting the finite element parts in to facets. MADYMO model of Low floor transit bus is
used to study dummy kinematics and different injury parameter. The acceleration pulses will be
89
extracted from DYNA structural analysis and will be used as input for MADYMO model. The
Figure 6.2 shows MADYMO model with seating location of all ellipsoidal dummy models used
for these simulations. The dummy model used are 50th % Male dummy, 95th % Male dummy and
5th % Female dummy. The reason behind using different size of occupant is to study the effect of
occupant size on bus interior occupant to occupant interaction. The two forward facing seats at
the low floor region shows 95th % with 5th % and 50th % with 5th %, this is used to observe
interactions of different size occupants.
Figure 6.2 Ellipsoidal dummy models.
This seating arrangement shown in Figure 6.2 is mostly design to study occupant
response, when occupant comes in contact with seat structure, side walls, modesty panels and
occupant to occupant impact. The results from this simulation will be injury values which can
helps us understand an accident situation and possible occupant injuries. The model is not
updated with actual functions of some parts so the injury value will not resemble the real life
injury values. This model can be used to see injury severity on each occupant.
90
Figure 6.3 Facet model.
MADYMO model is used to compare the difference between side impact standards used
for side impact validation of buses and vehicles. Transit buses are validated for side impact by
using standards from Standard Bus Procurement Guide line and vehicles with GVWR less than
10000 Lbs use FMVSS 214 standard. The Table 6.1 gives FMVSS 208 injury Criteria and limits
that are used to study these simulations.
Table 6.1 FMVSS 208 Injury Criteria’s.
91
Figure 6.4 Dummy Titles and Position.
The injury values are compared to standard injury limits to observe severity of injuries.
Figure 6.4 shows location and reference names of all dummies.
6.2 Side Impact Standard for Transit Bus
Parameter
Physical Test
Test Type
Side Impact
Target Vehicle
Transit Bus
Target Vehicle Weight
21290 Lbs (9656 kg)
Impact Speed
25.1 mph (40.4 Km/hr)
Impact Angle
2700
Bullet Vehicle Weight
4018 Lbs (1822 kg)
Table 6.2 Side Impact Test Standard (SBPG).
According Standard Bus Procurement Guide line the side impact test standard is given in
Table 6.1. The Figure 6.4 shows setup according to SBPG the target vehicle is modified FMVSS
214 barrier with 4000 Lbs weight. Finite element simulation is carried out according to side
92
impact standard the acceleration pulse is recorded on CG. Figure 6.5 shows pulse extracted from
FE analysis which is used as input for MB analysis.
Figure 6.5 Side Impact setup using SBPG.
Figure 6.6 Acceleration pulse.
93
T = 0 sec
T = 0.05 sec
T = 0.1 sec
T = 0.15 sec
Figure 6.7 SBPG simulation frames.
Injury Criteria
D2
D4
HIC (15ms Max.)
0.38 2.7
Chest acc (3ms) (G’s)
4.47 21.1 5.9
Chest Deflection (mm)
0.38 1.4
Femur Load (N)
56.3 88.9 242
102
Neck Peak Tension (mm)
44.9 170
233
119.7 70.9
69
179
177
NIJ
0.04 0.04 0.04 0.04
0.02
Neck Flexion(NM)
3.19 3.71 2.6
3.76
Neck Extension (NM)
1.28 3
12.7 8.7
2.94
Neck Shear (N)
50
108
57
Neck Peak Compression (mm) 99.7 219
44
D5
D6
D7
0.57 2.9
0.8
12.7
0.85 1.8
4.13
48.5
Table 6.3 Injury Values for SBPG Simulation.
94
6.3
0.53
977
Injury Criteria
D8
HIC (15ms Max.)
D9
D10 D11 D12
1.49 1.40 1.19 1.20 0.44
Chest acc (3ms) (G’s)
11.2 12.3 4.5
Chest Deflection (mm)
1.85 1.95 0.95 0.45 0.28
Femur Load (N)
6.6
Neck Peak Tension (mm)
65.3 85.6 46.0 34
Neck Peak Compression (mm) 279
40.2 72
7.5
173
4.5
177
22.4
137
180
293
NIJ
0.09 0.1
0.1
0.05 0.02
Neck Flexion(NM)
12.6 13.6 13.7 3.4
2.9
Neck Extension (NM)
12.9 12.7 7.9
3.2
2.12
Neck Shear (N)
188
62.6 51.4
194
168
125
Table 6.4 Injury Values for SBPG Simulation.
Post processing is carried out in Motion-view the results are plotted in Appendix D. The
summary for the results is provided in Table 6.3 and Table 6.4.Summary Tables shows that no
injury criterion exceeds the limit given in Table 6.1. This shows that according to SBPG the
structure of bus can sustain an impact of 4000 Lbs vehicle at 25 Mph without any major injury to
occupant due to any interior parts. Considering seating position of dummy the possible injury
causing parts for dummy 2 are modesty panel which affects femur injuries and side walls affect
head and neck injuries. Dummy 4 and 6 are affected by occupant to occupant impact and side
wall impact. Possibilities of injuries are head injury, neck injury and thorax injuries. Dummy 8,
dummy 9 and dummy 10 are seated on side facing seats. The possible injuries to them are due to
initial point of impact and occupant impact due to ejection. Possible injuries are femur and tibia
95
fractures and high neck injuries. Dummy 11 and 12 possible cause of injury to them are due to
ejection from seats.
6.3 FMVSS 214 Side Impact Standard.
According to FMVSS standard vehicle are tested for side impact using guidelines given
in Table 6.2. Test setup is shown in Figure 6.8 FMVSS 214 barrier impacting bus with 23
degrees angle and 33.5 Mph speed. Acceleration curve extracted from this simulation is shown in
Figure 6.9
Parameter
Physical Test
Test Type
Side Impact
Target Vehicle
Transit Bus
Target Vehicle Weight
21290 Lbs (9656 kg)
Impact Speed
33.5 mph (53.9 Km/hr)
Impact Angle
2700
Bullet Vehicle Weight
3015 Lbs (1350 kg)
Table 6.5 Side Impact Test Standard (FMVSS 214).
Figure 6.8 FMVSS 214 test setup.
96
Figure 6.9 Acceleration curve for FMVSS 214 simulation.
T = 0 sec
T = 0.05 sec
T = 0.1 sec
T = 0.15 sec
Figure 6.10 FMVSS 214 simulation frames.
97
Injury Criteria
HIC (15ms Max.)
D2
D4
D5
D6
D7
5.9
8.09
5.9
5.9
5.9
Chest acc (3ms) (G’s)
14.2 29
16.4 30.4 16.4
Chest Deflection (mm)
0.4
1.7
0.96 2.9
Femur Load (N)
45
146.6 334
253
394
Neck Peak Tension (mm)
101
338
127
348
164
Neck Peak Compression (mm) 2.7
5.3
2.8
5.4
5.3
0.75
NIJ
0.06 0.06
0.05 0.05 0.05
Neck Flexion(NM)
2.7
2.8
Neck Extension (NM)
1.28 3.9
21.4 6.9
3
Neck Shear (N)
49
125
78
5.3
41
5.4
38
5.3
Table 6.6 Injury Values for FMVSS 214 Test Setup.
Table 6.6 and Table 6.7 are summary Tables for the simulation results. Injury values are
below the limit given in FMVSS 208 Table. FE analysis shows that there is no cabin intrusion so
possibility of injury to occupants is due to impact with interior parts. The reasons for injuries to
occupant are similar to Side impact simulation carried out according to SBPG. The injury values
are higher as the input acceleration value is high. Femur loads in dummy 5 dummy 7 and dummy
10 are recorded higher compared to other dummies. Neck peak tension is quite high in dummy 2
to 7 and neck compression is high for dummy 8 to 11. This shows that as input acceleration
increases the injury values also increases so as severity of accident increases the occupant
injuries will be high.
98
Injury Criteria
HIC (15ms Max.)
D8
D9
D10 D11 D12
5.49 5.5
4.5
6.1
6
Chest acc (3ms) (G’s)
22
24
12
14
14
Chest Deflection (mm)
3.9
3.8
2.3
0.52 0.31
Femur Load (N)
36
66.6 105
227
198
Neck Peak Tension (mm)
161
169
153
38
22.4
Neck Peak Compression (mm) 565
364
253
375
148.9
NIJ
0.15 0.11 0.13 0.07 0.03
Neck Flexion(NM)
13.9 12.6 16.3 3.7
Neck Extension (NM)
21.9 21.4 21.5 4.25 2.6
Neck Shear (N)
250
259
308
3.9
72.9 54.2
Table 6.7 Injury Values for FMVSS 214 Test Setup.
The multibody analysis shows that depending on seating layout of bus and seating
position of occupant the type of injuries occurred will vary. The severity of injury is proportional
to acceleration values. Maximum injury observed are to neck of occupants seating on side facing
seats. To reduce this injury head rest can be best possible solution looking at design standards for
buses.
99
CHAPTER 7
CONCLUSIONS AND RECOMMENDATIONS
The objective of this research was to investigate the crashworthiness and passenger safety
of a Low Floor Mass Transit Bus, in order to build a detailed finite element model of a Mass
Transit Bus, validate the model for different crash standards, and use this model to extract
acceleration pulses from different real life crash scenarios and study different dummy kinematics
and injury. The study focused on a Low Floor Mass Transit Bus, which served as a sample
representing a wider range of similar transit buses. The research attempted to evaluate the
crashworthiness of the selected bus through computational mechanics and impact analysis
(simulated crash-testing).
A reliable finite element model of the transit bus was developed in order to perform the
crashworthiness and structural integrity evaluation. All parts were connected using different
multi-point constraints and special links to model the actual types of connections such as bolts
and welds, including failure. The model represents geometrical and material properties for all
structural parts of the actual vehicle and connections between all parts. The contact among
elements during deformations is also represented. This is a working model, which can be directly
used for LS-DYNA numerical simulations. All internal errors terminating computer runs and
faults (like initial penetration of nodes) and causing numerical noise have been removed. The FE
model is a good geometric representation of all major parts of the bus. Material properties for the
model were obtained from coupon testing and available resources.
The FE model was validated according to the Standard Bus Procurement Guidelines for
side impact test conditions. The permanent crush on the bus body was well below 3 in limit. The
velocity and displacement plot show that the simulation results match the actual test results. The
100
multibody simulation was carried out using the acceleration pulse from the FE simulation that
was carried out for validation. Results shows that the injury levels on occupants were well
below the limits. There is no major injury to any of the occupants sitting in different locations as
shown in the multibody simulation. This shows that the finite element model is capable of
predicting actual injury types in real-life crashes. This model can be used to study different side
impact crash conditions.
This side impact study was carried out on different accident conditions using models of
different sized vehicles. Results obtained from the finite element simulation show that the
amount of crush in a vehicle structure and the acceleration at the CG is affected by the following:
-
Size of vehicle impacting the target vehicle.
-
Impacting velocity and mass of bullet vehicle.
-
Angular or perpendicular type of impact.
As the size of the vehicle increases, the amount and crush of the target vehicle increases. Crush
and acceleration are also affected by the type of loading on the bullet vehicle. If the crush area is
small for the bullet vehicle and the chassis is loaded in initial moment of impact then the amount
of crush and acceleration increases. Comparing models like the Neon and the F800 truck, the
Neon model produces more acceleration than the F800 truck, because the chassis of the Neon
model is loaded in initial moment of impact. Crush is mostly affected by the impacting vehicle
mass and impacting angle since the surface area of contact increases, the amount of crush also
increases.
The multibody simulation shows that 4000 lbs mass vehicle with impacting speed of 25
mph produces injuries well below maximum injury limits. According to the literature review it is
seen injury due to the initial point of impact is 36%. The simulation does show higher injury
101
values to occupants seated near the impacting area. Another reason for injury is due to ejection.
The simulation shows the ejecting of occupants and possible type of injury. To avoid injury due
to such conditions possible solutions are lap belts and compartments, which are mostly used in
school buses. The amount of neck injuries can be prevented by using head restraint. Femur force
and tibia injury can be reduced using lap belts and energy absorbing cushions on the seat backs.
The limitation is whether it is feasible to put lap belts on transit buses since the amount of fatality
rate is quite low. The cost would be high for such restraints and the question is whether
passenger would use the restraints if the journey is of a shorter distance. Looking at this
perspective it is clear that instead of using restraints on bus the buses structure should be
designed to absorb more energy at a high impacting speed without cabin intrusion. The transit
bus design is quite compatible to the latest standard, but as the impacting speed increases, this
structure starts failing. The structure can be made safer for impact by making changes in side
impact standards. Since the crush distance is lowest on the side of bus, this area should be more
focused for energy absorption without intrusion. This design change can only take place when
the side impact validation test speed will be increased above 25 Mph since the bus structure
starts behaving differently as the loading speed increases.
The existing model can be easily converted and modified to reflect potential changes in
the body structure of new buses for detailed parametric studies. Comparison of results for models
with different material properties or structural components can assist engineers in developing
recommendations regarding optimal and safer design. The FE model of the Transit Bus,
developed in this project, can closely resemble the behavior of an actual bus during various
accidents. Numerical data provided from computational analysis can be very helpful in
establishing new standards for bus body builders. Continuation of the project beyond the
102
presented scope would allow for model validation and application of the model for other transit
buses. It would also serve as research evidence in efforts to improve the Standard Bus
Procurement Guidelines.
MADYMO model can be used to do a parametric study to find out the safest seating
layout for particular impact conditions. The finite element model can be used to study vehicle
compatibility. Pulses extracted from structural analysis can be used for sled testing.
103
REFERENCES
104
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107
APPENDICES
108
APPENDIX A
Description of Variables
Variable
Description
MID
RO
E
PR
LCID
LCSS
N
COUPLE
Identification number, Unique identification number to be selected.
Mass density
Young's modulus
Poisson’s ratio
Load curve ID, see *DEFINE-CURVE, defining the stress versus
Engineering strain.
Initial tensile force. If FO is defined, an offset is not needed for an initial
tensile force.
Yield stress
Tangent modulus
Hardening parameter
Failure flag, when the plastic strain reaches this value, the element is deleted
from the calculations.
Load curve ID or Table ID
MADYMO3D coupling flag
Coupling option if applicable
M
CMO
CON1
CON2
MADYMO/CAL3D Coupling option flag
Center of mass constraint option
First constraint option
Second constraint option
F0
SIGY
ETAN
BETA
FAIL
109
APPENDIX B
Chevy Test Results
t=0 sec
t=0.05sec
t=0.1 sec
t=0.15 sec
Figure B.1 Chevy 30 Degrees impact at 25 Mph and Bus 0 Mph
110
Figure B.2 Chevy 30 Degrees Impact Results.
111
t=0 sec
t=0.1 sec
t=0.15 sec
t=0.2sec
t=0.25 sec
t=0.3 sec
Figure B.3 Chevy 30 Mph and bus 10 Mph 30 degrees Impact
112
Figure B.4 Chevy 30 Mph and Bus 10 Mph Results
113
t=0 sec
t=0.1 sec
t=0.15 sec
t=0.20 sec
t=0.25 sec
t=0.3 sec
Figure B.5 Chevy 30 Mph Bus 30 Mph Door Side Impact
114
t=0 sec
t=0.1 sec
t=0.15 sec
t=0.2 sec
t=0.25 sec
t=0.3 sec
Figure B.6 Chevy 30 Mph Bus 30 Mph Driver Side Impact
115
APPENDIX C
F800 Test Results
Figure C.1 F 800 Negative 30 Degree Impact Results
116
t=0 sec
t=0.05 sec
t=0.10 sec
t=0.15 sec
Figure C.2 F 800 – 90 Degree impact
117
Figure C.3 F 800 90 Degree Impact Results
118
t=0 sec
t=0.05 sec
t=0.1 sec
t=0.15 sec
Figure C.4 F 800 –Positive 30 Degree impact
119
Figure C.5 F 800 Positive 30 Degree Impact Results
120
APPENDIX D
MAMDYMO Simulation According to SBPG
Figure D.1 Results for Side impact by SBPG
121
Figure D.2 Head and Thorax Acceleration
122
Figure D.3 Thorax Acceleration
123
Figure D.4 Thorax Acceleration and Chest Deflection
124
Figure D.5 Chest Deflection
125
Figure D.6 Neck Forces
126
Figure D.7 Neck Forces and Moments
127
Figure D.8 Neck Moments
128
Figure D.9 Femur Forces
129
Figure D.10 Femur Forces
130
Figure D.11 Femur Force
131
Figure D.12 Femur Force
132
APPENDIX E
MAMDYMO Simulation According to FMVSS 214
Figure E.1 Head Acceleration
133
Figure E.2 Head and Thorax Acceleration
134
Figure E.3 Thorax Acceleration
135
Figure E.4 Thorax Acceleration and Chest Deflection
136
Figure E.5 Chest Deflection
137
Figure E.6 Neck Force
138
Figure E.7 Neck Force and Moments
139
Figure E.8 Neck Moments
140
Figure E.9 Neck Moment and Femur Forces
141
Figure E.10 Femur Forces
142
Figure E.11 Femur Forces
143
Figure E.12 Femur Forces
144