400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760
SAE TECHNICALPAPER SERIES 1999-01-0072
Frontal Crash Feasibility StudyUsing MADYMO 3D Frame Model
Suk-jae Hahm, Yong-hee Won and Dong-seok KimDAEWOO Motor Co. Ltd.
Reprinted From: Vehicle Aggressivity and Compatibility in Automotive Crashes(SP-1442)
International Congress and ExpositionDetroit, Michigan
March 1-4, 1999
The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.
SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.
Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.
To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.
No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.
ISSN 0148-7191Copyright 1999 Society of Automotive Engineers, Inc.
Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.
Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.
Printed in USA
All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database
1
1999-01-0072
Frontal Crash Feasibility Study Using MADYMO 3D Frame Model
Suk-jae Hahm, Yong-hee Won and Dong-seok KimDAEWOO Motor Co. Ltd.
Copyright © 1999 Society of Automotive Engineers, Inc.
ABSTRACT
The purpose of this study is to evaluate and compare twofront body structure designs of a new car program usingthree dimensional lumped spring-mass model so called‘MADYMO 3D frame model’ in the view of frontal crashwith concept drawing only. A MADYMO 3D frame model,composed of a number of bodies and joints, was built fora current vehicle model and correlated with full car crashanalysis results from explicit finite element analysis forFMVSS 208 and AMS(Auto motor und sports) crash con-ditions. Then the same method was applied to a new carstructure. The new 3D frame models of two front struc-ture design concepts were built and the result was com-pared for two crash conditions. Two models wererequired to reduce footwell intrusion to meet the designtarget. Several design modifications were tried to reducefootwell intrusion for both models. The footwell intrusionwas reduced quite much by introducing connecting con-cept and reducing front rail bending stiffness for two mod-els, respectively.
INTRODUCTION
WHAT IS 3D FRAME MODEL? – 3D Frame model is akind of lumped-mass spring model which is composed ofrigid bodies connected by joints, point restraints and car-dan restraints of MADYMO in 3 dimensional coordinates.The rigid bodies represent undeformed areas of vehicle.The joint positions and connections are chosen at thelocations where structural deformation can be expectedfrom experience or known crash modes shown by simula-tion or crash test. The point restraint and CardanRestraint characteristics of joints and connections areextracted by means of explicit finite element calculation.
EXTRACTING JOINT CHARACTERISTICS – As the firststep of modeling, vehicle front structure concept isreviewed and hard points are prefixed. By this way, thelocations of joint and Cardan restraint are selected. Twoor more bodies are connected by several types of joint tobe a system and the systems are joined by CardanRestraints and Point restraints to be a full vehicle 3Dframe model. The shear effect of panels e.g. roof paneland floor are replaced with Kelvin elements.
The body masses and inertia data are calculated usinggeometrical data from the full vehicle finite elementmodel. The area in which bending occurs is cut out fromfull vehicle finite element vehicle model and modified tobe a component non-linear finite element model. Jointcharacteristics are calculated from these componentmodels. For calculating the bending moment, one end issimply supported and the other section is welded with alinear beam . Then the linear beam is loaded by rotatingmotion and the torque and rotation angle of the beam areextracted. This calculation was repeated for the otheraxis as well. As an example, a component model andextracted bending stiffness curves are shown in Figure 1and Figure 2.
Figure 1. A component model for calculating joint characteristics.
Figure 2. Example of bending stiffness of a joint from joint component model.
2
For the axial crush, one end is clamped and the otherend is axially loaded by a moving rigid wall. The rigid wallforce and displacement is extracted as axial collapsingcharacteristics.
MODELLING AXIAL CRUSHING AND BENDING FREEJOINT – Both of bending moment and axial loadingcurves are used to simulate bending and axial crushingbehavior of the sections such as the front end of front rail.4 Point restraints are used to model complex bending andclamping of main sections. These Point restraints arepositioned on the free joint section with proper distance(moment arm) to resist against bending. In order to getthe point restraint loading curve, axial compression crushpeak force and bending moment curve are reproduced tobe a force vs. deflection curve which represents axial andbending stiffness with a specific distance between pointrestraints.
VALIDATION OF 3D FRAME MODEL
The model was correlated with a full vehicle finite ele-ment model for FMVSS208 and AMS offset crash condi-tions. The acceleration on B pillar lower and the footwellintrusion are compared with those of finite elementmodel. The 3D frame model used in this paper can befound in Figure 3. A lot of ellipsoid, Point Restraints,Kelvin elements are composed to be a frame model.
Figure 3. The full vehicle 3D frame model used in this study.
The model has most of front structural parts such as frontrail, bumper, front wheel suspension assembly, subframe, engine, transmission etc.. The components havetheir own mass and inertia data and the lumped mass atthe end of vehicle has extra mass which represents thebehind part of vehicle.
FMVSS 208 VALIDATION – The acceleration and vehiclevelocity time history curves at B-pillar lower are plotted inFigure 4 and Figure 5 respectively. The peak value andtrend of acceleration history are similar to the finite
element model. The vehicle rebound time is also similarto the full car finite element model result.
Figure 4. B pillar lower acceleration time history comparison between Finite element and frame model in FMVSS 208.
Figure 5. Vehicle velocity time history comparison between Finite element and frame model in FMVSS 208.
AMS VALIDATION – The validation of AMS crash isfocused on the footwell intrusion and occupant compart-ment deforming shape. Basically, the AMS 3D framemodel is exactly same as the FMVSS 208 model excepta special care for Bryant angles of the joints in largedeforming area.
The comparison of B-pillar lower acceleration vs. timecurves of the finite element and the frame model are plot-ted in Figure 6. The 3D frame acceleration pulseincreases relatively higher than that of finite elementmodel. However, the trend of the pulse shows good cor-relation with that of finite element model. The footwellintrusion time history is shown in Figure 7. Although thereis a time shift about 7 milliseconds between tow results,the maximum intrusion value is within just 10% differ-ence. Looking at the deformed animation of 3D framemodel in Figure 8 and Figure 9, deformed shapes of frontrail, short-gun, extension of front rail and wheel carrier
3
are similar to the finite element model. Short-gun collaps-ing mode of the finite element model is expressed byTranslate joint which allows axial deformations by specificstiffness characteristics. Wheel carrier moved upward inboth models because of upward bending behavior of thefront rail. Therefore it can be concluded that the generaldeformation characteristics of 3D frame model used inthis paper show good correlation with the full vehiclefinite element model.
Figure 6. Comparison of B-pillar lower left acceleration time history of finite element and frame model in AMS.
Figure 7. Comparison of Footwell intrusion time history of finite element and frame model in AMS.
TWO FRONT STRUCTURE DESIGN CANDIDATES
TWO DESIGN CANDIDATES – Two design candidates offront structure for a new car model were suggested and itis required to predict the crash performances of two mod-els to get design idea and select one of them. The twomodels are named as MODEL1, MODEL2 in this studyand have same hard points, test weight conditions andexternal dimensions. Main differences between MODEL1and MODEL2 are in front structure such as front side rail,brace wheel house, cross member front floor, front siderail extension and sub-frame(wheel carrier) etc..
Figure 8. Deformed view of 3D-frame model in AMS.
Figure 9. Deformed view of finite element model in AMS.
Figure 10. Front main structure of MODLE1 3D frame model.
4
All parts of two models except the marked in Figure 10are exactly the same and engine and chassis systemsare also shared by two design candidates. The frontstructure of MODEL1 can be found as 3D frame model inFigure 10 and the side view of MODEL1 and MODEL2are compared in Figure 11 and 12 respectively.
Figure 11. Side view of MODEL1front structure.
Figure 12. Side view of MODEL2 front structure.
The front side rail angle from horizontal line in side viewof MODEL1 in Figure 11 is a little slanted but zero inMODEL2 of Figure 12. Only MODEL2 has front rail outerwhich connects the end of front rail to A-pillar lower asindicated in Figure 12. The section of front rail and shortgun of MODEL1 are different from those of MODEL2.The extension of front rail floor and cross member frontfloor also have different sections and reinforcements.
MODEL DESCRIPTIONS – The frame models forMODEL1 and MODEL2 were created by the modificationof pre correlated 3D frame. The front structure dimen-sions, hard points and joint positions were modifiedaccording to the given vehicle dimension. The sub-frame(wheel carrier), vehicle total weight and joint propertieswere changed. The joint properties were calculated bymeans of the explicit finite element calculation. The finiteelement models were built using concept section draw-ings. The radiator mounting support bracket attached tothe front end of front side rail was suggested to scissorstype to increase the axial collapsing area at the front partof front rail The sub-frame(wheel carrier) can be found inFigure 13. Control arm is connected to the sub-frame
with Point Restraint and the sub-frame has severalspherical joints to allow bending during crash. Frontshock absorber, left engine mounting, power steeringgear box and driving shaft were also included in themodel.
Figure 13. sub-frame and front suspension model.
The number of MADYMO components used for one 3Dmodel are listed below. It takes about 40minites to runone calculation on one CPU of SGI power challenge.
SYSTEM 13
BODY 208
JOINT 388
KELVIN ELEMENT 177
POINT RESTRAINT 142
BASELINE MODEL RESULTS
Acceleration peak value and vehicle rebound time of twomodels are compared for FMVSS208 crash. Footwellintrusion and deformation of occupant compartment aremainly considered for AMS evaluation. The accelerationtime history in FMVSS208 are shown in Figure 14. Ascan be seen in Figure 14, the first peak acceleration ofMODEL1 is quite similar to that of MODEL2 because ofsimilar front end structure which has same scissors typeconnecting concept between radiator support and frontrail end. The maximum acceleration peak of MODEL2 ishigher and the vehicle rebound time is shorter thanMODEL1. The reason why MODEL2 has harder pulsecan be found in the front rail concept. The front rail shapeof MODEL2 (Figure 12) is so straight that the front railwas not bent much as shown in Figure 16. Only front endof front rail collapsed and first peak of acceleration is over20g near 13msec. In case of MODEL1, However, thefront rails have slight angle from the horizontal line sothat it bent quite much more than MODEL2 (Figure 15).The engine and transmission go upward because thefront rails on which transmission mounted were bentupward. Collapsing of front ends of front rails occurred in
5
both models. It was possible because scissors typebrackets do not take much collapsing space of front railend. It is positive trend for restraint system development.
Figure 14. Acceleration time history of MODEL1 and MODEL2 in FMVSS208.
Figure 15. Deformed view of MODEL1 in FMVSS208.
Figure 16. Deformed view of MODEL2 in FMVSS208.
The footwell intrusions in AMS are compared for bothmodels. Maximum footwell intrusion of MODEL1 is about240mm and MODEL2 is 325mm. Although the intrusionof MODEL1 is smaller than MODEL2, it is still required tobe decreased to take upper third range of safety rankingin European market. In case of MODEL1 in Figure 17, alocal large bending occurred behind the engine mounts.All space of engine room was consumed and the occu-pant compartment kept it’s initial shape. On the contrary,engine room of MODEL2 did not show big deformationbut occupant compartment deformed much because ofrocker and A-pillar bending and the front rail was notwholly bent but bending was just initiated as shown inFigure 18. Load from barrier was transferred to A pillarlower via front rail outer section and this load induced
large moment on rocker and A pillar sections. It meansthe stiffness of front structure is excessive to prevent therocker from large bending and it is needed to find out theproper stiffness level to reduce footwell intrusion andoccupant space deformation.
Figure 17. Deformed shape of MODEL1 in AMS at 100 millisecond.
Figure 18. Deformed shape of MODEL2 in AMS at 100 millisecond.
CASE STUDY RESULTS AND DISCUSSIONS
According to the baseline results, it is needed to reducethe footwell intrusions in both of models. Therefore theauthors conducted several case studies for AMS crashcondition. FMVSS 208 was not studies because 30mphrigid barrier crash is not severe test in the view of vehiclestructure. In order to find out design concept for decreas-ing footwell intrusion in AMS crash condition, severalcase studies were conducted for two models by changingfront structure joint stiffness and introducing new struc-tural concept.
CASE STUDY FOR MODEL1 – MODEL1 case study aresummarized in table 1.
In Case1, the stiffness of engine mounted area on frontrail was scaled up by 30% to reduce more local bending.However the footwell intrusion increased by 46% compar-ing baseline result. Although front rail bending decreaseda lot, much bending occurred at brace of front rail areaand footwell intrusion increased. Therefore it was under-stood that 30% increasing of front rail stiffness is notproper for reducing footwell intrusion in MODEL1. In thesecond study, Case2, a diaphragm was added in front railsection at engine mounting area where local large
6
bending occurred. The new stiffness function for the jointof this area was extracted by calculating finite elementjoint model to which a diaphragm was added. Twomoment vs. angle functions before and after adding adiaphragm can be found in Figure 19. The diaphragmmade average moment higher but peak moment isalmost same as previous one. The footwell intrusiondecreased by 6% after adding diaphragm inside front rail.Therefore it can be said that the diaphragm works a littlefor improvement in AMS.
Figure 19. Calculated moment vs. angle curves with (solid line) and without(dashed) a diaphragm.
In order to get a better improvement, new concept whichconnects the front rail and A-pillar lower, was introducedin Case3 as show in Figure20. This concept is similar tobaseline of MODEL2 but the position in z axis is higher inside view and the angle between x axis and this connect-ing section is smaller than MODEL1 in top view. There-fore, load from the front rail to A-pillar is bigger thanMODEL2. The connecting section has about 40mmX40mm section size which is bigger than MODEL2. Asthe result, the footwell intrusion improved about 35%comparing baseline. Instead of decreasing in footwellintrusion, however, occupant compartment deformation alittle increased (Figure 21). But the deformation of occu-
pant compartment is still acceptable. Therefore it is rea-sonable to accept this connecting concept and it is alsorequired to find out more proper section size for both ofintrusion and compartment deformation through adetailed study by the full vehicle finite element analysis.
Figure 20. Connecting concept between front rail and A-pillar (case3 of MODEL1).
Figure 21. Deformed shape of Case3 of MODEL1 in AMS.
CASE STUDY FOR MODEL2 – MODEL2 case studiesare summarized in table 2.
The extension of front rail was stiffened by 30% scalingup the joint functions in Case1. The purpose was toreduce intrusion by means of decreasing bending at theextension area of front rail. However it was found that theextension front rail did not attribute much to footwell
Table 1. MODEL1 case study descriptions and results in AMS.
Case Study baseline Case1 Case2 Case3
Modifica-tion
Front Rail Stiffness 30% Scale up
Add Diaphragmto Front Rail
Case1 +ConnectionFront rail to A-Pillar lower
Maximum Dynamic Footwell Intrusion
(mm)
240 350 225 157
Table 2. MODEL2 Case study description and results in AMS.
Case Study baseline Case1 Case2 Case3
Modifi-cation
Extension Front Rail Stiffness 30% Scale up
Move Front Rail20mm downward
Front RailStiffness30%Scale down
Maximum Dynamic Footwell Intrusion
(mm)
325 325 310 200
7
intrusion. As the next step, the height of front rail wasmoved 20mm downward to minimize bending moment onA-pillar caused by the force which passes through frontrail. This modification also gave no major change in foot-well intrusion. In order to increase deformation of engineroom area, the stiffness of engine mounting area wasscaled down by 30% in Case3. As a result, the footwellintrusion reduced by almost 40% and the occupant com-partment kept the initial space as shown in Figure22. It isoutstanding improvement in AMS crash comparing withbaseline.
Figure 22. Deformed view of case3 of MODEL2 in AMS.
CONCLUSION
In this study, the 3D frame modeling technique was intro-duced for concept feasibility study of a vehicle front struc-ture. A 3D frame model was built and the result wascompared with full vehicle finite element analysis. A goodcorrelation was shown in FMVSS208 and AMS crash.
Using the same approach, The 3D frame models werecreated for two front structure design alternatives. Basedon simulation results of baseline design, The footwellintrusion were required to be reduced for both of twomodels, MODEL1 and MODEL2. The front rail deformingmode of MODEL1 showed local large bending at enginemounting area because of slanted front rail. On the con-trary, large deforming in occupant compartment occurredin MODEL2 due to straight and stiff front rail concept.
Several case studies for reducing footwell intrusion wereconverged on one structural concept. For MODEL1, aconnecting concept between front rail and A-pillar wasrecommended as new design idea. In the MODEL2, frontrail bending stiffness was scaled down to induce largerdeformation in engine room and to reduce footwell intru-sion and occupant compartment deformation. The con-cepts recommended will be confirmed and tuned by adetailed finite element simulation.
The frame model can be built at the early stage of carprogram when only section and concept drawings areavailable. Therefore it is possible to evaluate and getdesign concept before a full vehicle finite element simula-tion. Furthermore, model updating and computing timeare so much short that faster simulation actions will bepossible.
ACKNOWLEDGMENTS
The 3D frame model validation work in this paper isbased on a project with TNO, in The NETHERLANDS.
REFERENCES
1. J. Huibers, J.J. Nieboer, P. De Coo, “Design Tools for Frontand Side Impact Protection”, 5th international madymouser’s meeting, 1994.
2. Alexandra C. Carrera, Stuart G. Mentzer, Randa RadwanSamaha, “Lumped-Parameter Modelling of Frontal OffsetImpacts”, SAE950651.
3. Dusan Kecman and Nigel Randell, “The role of calculationin the development and type approval of coach structuresfor rollover safety”, ESV paper 96-S5-O-05, 1996.
4. Rajiz Pant, James Cheng, Chris O’Connor, David Jacksonand Aravind Mellireri, “Light Truck Concept Models andTheir Applications”, ESV96-S1-W-19.
5. TNO, “Madymo User’s and Theoretical Manual”, 1997.
