McHenry
Software, Inc.
In 1963 the U.S. Public
Health Service and the Automobile Manufacturer’s Association, Inc. funded
research at Calspan performed by Raymond R. McHenry to develop a mathematical
model of an automobile occupant in a longitudinal collision. The resulting
computer program was called the CAL-2D ([i],[ii],[iii]). The research was aimed at the development of
a response to a Consumer Reports issue ([iv])
and related reports ([v][vi])
that included the assertion that American belts failed under the Swedish test
conditions and "the major points of failure of the belts tested were the
webbing…and the floor brackets themselves".
The CAL-2D model was
created "in order to help improve understanding of the complex
relationships of force-acceleration-time-position-velocity that occur in the
impact and energy-absorbing cycle of automobile passenger restraint
systems". The study was performed
to provide guidance concerning "(1) fundamental differences in the results
obtained by static and dynamic testing and (2) the possible need for dynamic
acceptance testing of seat belts.
One of the results of
the study was the conclusion that "the use of a very short stopping
distance in a cart test of lap belts can produce a distorted comparison of the strength
(when belt loads are not measured) and the performance of webbing materials
with different load-elongation characteristics. A short (3") cart‑stopping
distance, from 25 mph, produces increases in the magnitudes of both primary and
secondary belt loading cycles over those obtained a more "realistic"
(17") stopping distance, as encountered in automobile crashes".
A subsequent follow-up
contract for a 3 dimensional crash victim simulation at Calspan was directed by
Dr.
The Articulated Total Body (ATB) Model is a public domain
computer program that is used to simulate the dynamic motion of jointed systems
of rigid bodies The ATB program is a
research tool which is used primarily to interpolate and extrapolate the
results of full-scale vehicle crash tests with anthropomorphic test dummies. A
typical input file for the ATB program can require anywhere from 500 to 7000 parameters The inputs for ATB require detailed
definitions of the occupant, the vehicle interior, the interaction of the
occupant with the vehicle interior and definition of the acceleration
environment. The appropriate values for input parameters are not readily
available and there are no recognized “default” or “typical” values. ([x])
"The
ATB model was developed to complement experimental research in automobile crash
environments and to provide a functional instrument for parametric
investigations" (viii)
Since
the advent of the PC and the availability of PC versions of the ATB computer
program, the ATB has been frequently encountered as an accident reconstruction
tool used for demonstrative purposes in litigation matters. This presentation
will include discussion and presentation of some of the types of applications
encountered. The presentation will also include discussion of the appropriateness
and applicability of the ATB to specific forensic investigations.
"The most common application of ATB is to model human
or dummy occupant motion in vehicle crashes" ([xi]). "ATB
can be a useful tool in the design process when used in conjunction with a
series of full-scale tests" ([xii]). Validation of the ATB occupant
simulation model consists of running full-scale tests and then comparing the
predictions of the mathematical model with the full-scale test results.
"Generally good
correlation was obtained with the results of the impact sled experiments. The
results of full-scale car-to-car crash tests...did not correlate with the
program predictions to the same degree of accuracy" ([xiii])
“Assumptions are
inherent in any mathematical model” and “a given model may yield better
predictions for one situation than for another” ([xiv], p 180) . “The Agreement achieved
between predictions and observed responses of a system, (“validity”) is … user
dependent” (xiv, p181). In view of variations of the use of the CVS
program “the authors have drawn no conclusions concerning the validity of the
CVS model that is demonstrated by the results obtained in this study” (xiv, p181).
The Human Biomechanics
and Simulation Standards Committee of the Society of Automobile Engineers (SAE)
created a proposed Validation Index which requires a quantitative comparison of
output data generated by models or tests. ([xv]).
Most attempts at validation of the
ATB/CVS have been by comparing the responses of the ATB/CVS with sled tests of adult
anthropomorphic dummies. A live human
being has responses which differ substantially from an anthropomorphic dummy.
The use of the Articulated
Total Body (ATB) Model for occupant simulation requires extensive data to
describe human and dummy geometric and inertial properties ([xvi]). The modeling approach used in the ATB model
considers the body as being divided into individual rigid segments, typically
15 or 17 segments. The segments are
joined at locations representing the physical joints of the human body and have
the mass of the body between body joints.
To satisfy the input requirement
for definition of the occupant, the GEBOD (GEnerator of BODy Data) was
developed to generate human and dummy data sets. ([xvii],
[xviii]). The data sets created by GEBOD include approximations
of the body segments' geometric and mass properties, and approximations for the
joints' locations and mechanical properties.
Regression equations from anthropometric surveys and stereo photometric
data are used in computing these data sets. ([xix],
[xx]). There are vast differences in the
proportions, musculature and mass distribution of individuals, particularly
children. Regression equations can only
roughly approximate the properties of an occupant or dummy in vehicle crash
tests. The properties created by Regression equations of GEBOD are for a passive
occupant or anthropomorphic dummy to be used in modeling vehicle crash
tests.
“The user should be
cautioned, however, in the use and limitations of percentiles to describe an
individual or class of individuals. As Daniels ([xxi])
has demonstrated it is virtually impossible to find an individual who is
“average” in more than a few body measurements.” (xx) “Anthropometrically, while the human body is the
same in qualitative appearance within the species, there are considerable
differences in the quantitative measures of the body. In statistical terms,
there are relatively few dimensions that are highly correlated (“r”>.70)
which means that the system varies in dimensional description within the same
body and population.”(xx)
“In the standard application of CVS or ATB,
the kinematics of the occupant, as well as the occupant’s initial position and
orientation, must be known beforehand in order to validate the program for its
specific application” ([xxii]). “For real world accidents, actual
observed kinematics are not available and there is thus no means to validate
the accuracy of the input data used for the program”( xxii). “Even
small deviations between input data and actual values may have significant
effects on the reliability of the results” (xxii).
“Force-deflection values which have been established for a specific contact area
of a specific vehicle have substantial variations depending on the location of
the loading, the angle of loading, and the rate of loading” (x). The “extreme
variability” of force-deflection values “is one of the most significant
problems with using ATB for accident reconstruction” (x). The ATB has
never been validated as a general predictive occupant kinematics simulation
model for any type of real-world accidents.
When two objects
interact in a collision, a deformation or compression phase is followed by a
restoration or restitution phase ([xxiii] (p346),[xxiv].).
The amount that the two objects deform during the initial deformation or
compression phase is determined by the stiffness or hardness of the two objects
interacting. The restitution phase is the time from the maximum
deformation condition to the instant at which the bodies separate. ([xxv]).
Restitution consists of two separate aspects: (1) a partial dimensional
recovery and (2) a partial restoration of kinetic energy.( [xxvi], [xxvii]). The
amount of restoration or restitution is a separate property of each object with
no direct relationship to the stiffness or hardness of an object. The coefficient of restitution depends
on the impact velocity and can approach unity as the impact velocity approaches
zero. (xxiv). The coefficient of restitution depends on the
types of materials, sizes, shapes and temperatures of both colliding bodies (xxiii).
Restitution in the ATB
is modeled by the R & G factors which originated in the CAL-2D program. The
R factor is the energy absorption function and it is used to specify the amount
of energy recovered at the end of unloading. The R factor ranges from 0 to 1.
The G factor it the permanent deflection function and it is used to model
permanent deformation due to contact force. The G factor ranges from 0 to 1.
The R & G factors in the ATB program are not a measure of the “the stiffness
of the contacting surfaces when they impact"([xxviii]).
Interactions between the
occupant and the environment are accomplished by the creation of functions
representing the force-deflection and frictional properties. Experimental and theoretical modeling of head
impact concluded that the ‘choice of head modeling greatly influences the
nature of the shock’ and demonstrates ‘the importance of a more realistic
modeling of the head in the theoretical and experimental study of shock
aggressiveness’ ([xxix]).
There exists a dataset
of joint properties for a seated adult male subject in a sled test. ([xxx]).
The ATB adult male joint properties may produce unrealistic oscillations
of the arms and legs when used for simulations other than a seated adult male
in a sled test.
ATB Model Version differences
Interior dimensions and properties
Use of the ATB to attempt to demonstrate "Mitigation"
is arbitrary and misleading
·
Examples
include varying the force deflection properties w/o varying the dimensions of
the objects,
o
"air-gap
" padding
o
“Padded” deflection of 2” on sill, 1.5” on
B-Pillar rear face ( For baseline run deflection 0.9” on sill, 0.70” on rear
face of sill) Can these deflections be accommodated? Why did they not increase the dimensions of the B-pillar to accommodate
the additional “padding” requirements?
·
Installation
and creation of properties for components that include component movement –
o
Timing of
movement
o
Rate of
movement
o
arbitrary
nature lends itself to sensitivities
o
extremely arbitrary
and sensitive
o
For some
runs, belt “anchor” points erroneously move forward and upward with the seat.
§
Arbitrary
belt Anchor movements can produce arbitrary slack in the belt which may not
have occurred in an actual crash.
o
For some other
runs, outboard anchor points of lap and 3-point belt erroneously do not move
inboard with intrusion
§
Anchors
penetrate the front of striking car
o
For 3-point
run, Upper anchor moves forward and upward with the seat movement.
·
HIC
"predications" are extremely sensitive, especially when moving
components contact the occupant
·
A moving
component striking the occupant head produces wild variations in the predicted
HIC
Conclusions and Recommendations
ATB should be used in forensics
and accident reconstruction only as a tool to assist in understanding gross
occupant kinematics. Any results or conclusions drawn from an ATB application
related to detailed occupant kinematics involve so many approximations,
estimates, and assumptions that they must be recognized as not being compatible
with sound engineering practices and principles and, therefore, not
scientifically supportable.
References
[i] McHenry “Analysis of the Dynamics of Automobile Passenger-Restraint Systems”,
7th Stapp Car Conference Proceedings, SAE, 1966
[ii] McHenry, Naab “Computer Simulation of the Crash Victim – A Validation Study”, SAE
paper 660792, SAE 1966
[iii] Cheng, Sens, Weichel, Gunther “An Overview of the Evolution of Computer
Assisted Motor Vehicle Accident Reconstruction”, SAE paper 87-1881
[iv] Consumer
Reports, October 1961, Articles by Michelson and Tourin
[v] Michelson, Torin, "Consumer
[vi] Michelson,
[vii] Bartz “Development and Validation of a Computer Simulation of a Crash Victim
in Three Dimensions”, SAE Paper 720961, SAE, 1972
[viii] Fleck, “An Improved 3-Dimensional Computer Simulation of Vehicle Crash Victims”,
NHTSA, April 1975, NTIS PB-241 693, PB 241 694, PB-241 695
[ix] Fleck, “Improvements in the ATB/CVS Body Dynamics Model”, 13th
International Technical Conference on Experimental Safety Vehicles, S8-W-20,
1991
[x] James, Nordhagen, Warner, Allsop,
Perl “Limitations of ATB/CVS as an
Accident Reconstruction Tool”, SAE paper 97-1045
[xi] http://www.orl.columbia.edu/~atbug/moreinfo.html
(ATB users Group site)
[xii] Digges, "Improvements in the Simulation of Unrestrained Passengers in Frontal
Crashes Using Vehicle Test Data", SAE Paper 860654
[xiii]
Bartz,
[xiv] Fleck,
[xv] Robbins, "Restraint Systems Computer Modeling and Simulation State of the Art and
Correlation with Reality", SAE paper 891976
[xvi] Cheng, Obergefell, Rizer “Generator of Body Data (GEBOD) Manual",
AL/CF-TR-1994-0051, March 1994
[xvii].
Baughman, L.D., 1983, "Development
of an Interactive Computer Program to Produce Body Description Data"
AFAMRL-TR-83-058, Aerospace Medical Research Laboratory, Wright-Patterson Air
Force Base,
[xviii]. Gross, M.E., 1991, "The GEBODIII Program User's Guide and
Description" AL-TR-1991-0102, Armstrong Aerospace Research Laboratory,
Wright-Patterson Air Force Base,
[xix]. Clauser, Charles E., Pearl E.
Tucker, John T. McConville, Edmund Churchill, Lloyd L. Laubach, Joan A.
Reardon, April 1972, "Anthropometry
of Air Force Women" AMRL-TR-70-5, Aerospace Medical Research
Laboratory, Wright-Patterson Air Force Base, Ohio.
[xx] Snyder, R.G., Schneider, L.W.,
Owings, C.L., Reynolds, H.M., Golomb, D.H., Sckork, M.A., May 1977, "Anthropometry of Infants, Children, and
Youths to Age 18 for Product Safety Design" UM-HSRI-77-17, Consumer
Product Safety Commission,
[xxi] Daniels, The “Average Man”?,
[xxii] Declaration of
[xxiii] Riley, Strurges “Engineering Mechanics – Dynamics”,
John Wiley and Sons, ISBN – 0-471-51242-7
[xxiv] Meriam, Engineering Mechanics – Volume 2 – Dynamics, John Wiley
& Sons, ISBN-0-471-59461-X
[xxvii] McHenry, McHenry “Effects
of Restitution in the Application of Crush Coefficients”, SAE paper 97-0960
[xxviii] From Deposition testimony of Expert utilizing ATB.
[xxix] Guimberteau, McLean, Andersen, Farmer “Experimental and Theoretical Modelling of
Head Impact – Influence of Head Modelling” , Proceedings of 1996
International IRCOBI Conference on the Biomechanics of Impact, 1996,
[xxx] Cheng, Rizer “Articulated
Total Body Model V Users’s Manual", Veridian,
[xxxi] Ashrafiuon, Colbert, “Introduction of Deformable Segments in the ATB Model" ,
Presentation at 1995 ATB Model Users’ Colloquium
©McHenry Software, Inc.