COMPATIBILITY STUDY IN FRONTAL COLLISIONS - MASS AND STIFFNESS RATIO

SaadA.W. Jawad ACME Department,University of Hertfordshire United Kingdom PaperNumber 98-Sl-O-14

problem of compatibility is a problem of mass, structural and geometrical interaction between the two colliding ABSTRACT vehicles. The question of compatibility between vehicles Compatibility of vehicles of different mass and involved in frontal collisions has been revitalised in stiffens in head-oncollisions is studied in this paperwith Europe. Recent developmenthas introduced the concept the aid of an eight-degreesof freedom, two dimensional of the ultra small and light vehicle for economic and lumped-mass simulation model. The model takes into environmental reasons. This development coupled with considerationmass and stiffness ratio as the main factors alarming fatality rate of -to-car accidentshas brought contributing to compatibility of the two colliding the problem of compatibility - i.e. safety implication vehicles. Other factors like length of crumple zone, offset when large and small collide together - into the overlap and speedare also considered.Three injury risk forefront. European statistics indicate those 60-65% of criteria have been consideredin this study; delta V or fatalities in truck accidentsare car occupants.Some 4200 change in of vehicle after impact, maximum of them die every year in car-to-truck frontal collisions. acceleration sustained by the passenger compartment Some2900 die yearly in car-to-carfrontal collisions. throughout impact, and length of deformation sustained Despite publication of numerous research by the car front. The most crucial compatibility programmeon compatibility, conflicting claims are being parameteris the mass ratio for the delta V criterion. The made regarding various factors affecting compatibility secondand third compatibility parametersare mass ratio and aggressivityof road vehicles. It is generally accepted and stiffness ratio for deformation length criterion. A that injury risk to the occupants is higher in lighter longer crumple zone is proposedfor heavier vehicles to vehicle involved in head-on collision with larger one. provide the required protection for smaller vehicles This is consideredso on the basis of the effect of mass involved in head-on collision. It was found that both ratio betweenthe two colliding vehicles. Becausemass is mass and stiffness ratio have no aggressivity on the a surrogate measurefor other factors like size, shape, partner vehicle when maximum accelerationcriterion is length of crumple zone and stiffness too, the effects of usedat low impact speeds. these individual factors on compatibility are overshadowedby the effect of the mass2. It is envisaged that a comprehensivefast running simulation programme is required to achieve a thorough INTRODUCTION study involving all important factors affecting compatibility. This researchfocuses on the development The safety of car occupantsdoes not only dependon of such a simulation programme, and to perform a the safe design of the car they ride, but dependalso on thorough study and sensitivity analysis of main factors the aggressivity of the design of the partner car involved affecting compatibility of vehicle in head-on collisions. in head-on collision. An aggressivedesign may provide Mass and stiffness ratio are the main factors considered good protection to the car in question and pausea threat in this study. Other factors like length of crumple zone, to occupants of the other car involved in the collision. offset overlap and speedare also considered.Geometric This raisesthe questionof compatibility betweenvehicles compatibility is not considered in this study. It is where the level of protection of the occupantsof a certain assumedthat the two colliding structures geometrically vehicle does not only depend on its crashworthiness interact with eachother. performance, but on that of the other car too. The

269 lumped-massmodel using two stagedeformations is used MATHEMATICAL MODEL in this study.

The objective of the model is to enable quick The full model include vehicle mass, engine mass simulation of the crush processand allow at the same and front assembly structure mass for each of the time independentvariation of important parametersthat colliding cars. Two levels of plastic springs is assumed might influence compatibility. The parametersto take on both sides of the engine. The left and right into considerationare: mass, stiffness, crash overlap, longitudinal are replacedwith plastic spring to simulate engine mass and engine location. A two-dimensional plastic deformation of the structure.Figure 1 shows all massesand stiffnesselements for both cars.

Vehicle1 Vehicle2 Frontassembly Engine1 Engine2

Ml ml ml1 m21 m2 M2

kn kz3 k24

L L

Figure 1 Schematic model of the crush system deformation zone at 350 mm. The secondary deformation zone starting at 350 mm is a constant deformation (zero rate) for the rest of the deformation zone. The simulation solves 24 equations The stiffness rates of the right and left hand sides and 24 unknowns:

are taken to be symmetricalfor both cars. For eachcar, x, 8, Xl, 01, X2,%, x15, x25 the two levels of ‘spring’ stiffness are increasingwith FII, FIZ, F13, FM, FZI, Fzz, F23, Fu

deformationdistance. Combining the primary stiffness x11, x12, x13, x11, x21, x22, x23, x24 kll, k13,kZ1, kU and the secondarystiffness k12, ki4, k12, k24, the Initial conditionsfor all masses,except for ml1 and overall force displacementcharacteristics for a ‘standard m21,were assumedto be that of impact of the specification‘ car (total mass 1500 kg) has primary vehicles. The central assemblymasses ml1 & stiffness and secondarystiffness. One input for the rnzl wereassumed to be in contactthroughout the crush stiffnessvalue for eachcar was usedto determineboth processand havea unified velocity and displacement.Its primary and secondary stiffness. The deformation initial velocity is calculatedusing momentumequation displacementpoints were fixed. Although stiffness and of two massescolliding head-onwith zero coefficient of mass distribution are symmetrical, an asymmetrical restitution impact (overlap~1) producesasymmetrical loading and henceasymmetrical deformation for both cars.The front assemblyparts, ml1 & mzl, act as a rigid body of two massesstuck together,therefore providing a mechanism Various combination of mass ratio, stiffness ratio, of load transfer from one longitudinal ‘spring’ to the impact speedratio have simulated. Crash displacement, other. passengercompartment acceleration, ‘spring ‘ The primary part of the stiffness cun’e is a lower havebeen plotted versustime to check and validate the rate stiffness for the first 270 mm of deformation, simulation results.Standard data havebeen tried as well followed by higher stiffness rate for the nest 80 mm of as comparison with other published work (” (13)to deformation - before reaching the end of engine validate the simulation. Typical simulation resuits of

270 acceleration, displacement and force versus time are shown in figures 2 and 3 and 4. Figure 5 also shows accelerationversus displacement curve. Crash acceleration- The simulation results show quasi equilibrium crash dynamics when ignoring effective mass of the structure. Only variation in stiffness is taken into consideration thus eliminating any high frequency oscillation from the results. The simulation data are that of 15 tons vehicie versus 1.5 tons both colliding at 30 mph speed 100% overlap.

Crush acceleration Signiture 0 10 20 30 40 50 60 70 80 90 -act displacement . . :. Figure 5 acceleration versus displacement

!.. INJURY RISK CRITERIA -t ___I 6C) 120 Various criteria have been used by different

,. researchersranging from the maximum or average I rime (ms) -accl -accZ ’ accelerationof the passengercompartment, to the length of deformationattained during the crash. Other criterion Figure 2 Crash acceleration signature involved occupants kinematics model by calculating accelerationand speedof various parts of the torso up to Crush displacement -total I the time of the secondaryimpact betweenthe occupant I and car interior. The question of modelling the occupant’s motion has been ruled out on the ground of the accuracy required. Three criteria have been identified as most relevantfor this purpose;

i) Delta V change sustainedby each vehicle at the end of the crash. I ii) Maximum acceleration sustained by the 0 20 40 60 80 100 120 passengercompartment during the crash. iii) Maximum length of deformation sustainedby -CL134 -CL212 time (ms) the fronta structure of the car.

Figure 3 Crash deformation signature The weightings of these criteria are believed to depend mainly on the closing collision velocity at the Crush forces -1 moment of impact. It can be said that the first criterion - delta V is more relevant at lower closing velocities than ( 8ooT j j : the other two criteria as acceleration and deformation length are not expected to attain critical levels. At intermediate closing velocities the second criterion - maximum acceleration becomes more relevant, whilst the third criterion - deformation length is relevant at high closing velocities only. What define the boundaries of these closing velocity categories depend on the 0 20 40 60 80 100 12c combination of three design parameters:mass, stiffness -F13 -F147 time (m s: and crumple zone. It is therefore considered that all these criteria are important to be consideredand treated Figure 4 Primary and secondary forces in the priority cited above.

271 I! Crush zone Agressivity Mass ratio compatibility The first parameter investigated was mass ratio Cl-llsh with regardto all three injury risk criteria. Figures 6, 7 zone and 8 show variation of injury criteria with mass ratio ranging from 0.5 to 10. Stiffness characteristicswere w-0 assumedstandard for both vehicles while offset overlap were taken to be 100%. Initial velocities for both 0 2 4 6 8 IO vehicleswere fixed at 13.33m/s (30 mph). Vehicle 1 is ! *CLlm *CL2m Mass Ratio the standardfixed characteristicsvehicle while vehicle 2 is the varied characteristics one. Compatibility is Figure 8 Deformation versus mass ratio measuredon changesin injury criteria of the standard vehicle (no 1) due to variation in characteristicsof the partnervehicle (no 2).

Crush zone - 0.5 overlap Crush 800 T :

0 2 4 6 8 10 *CLlm *CL2m Mass Ratio

Figure 9 Deformation versus mass ratio Figure 6 delta V ratio versus mass ratio

Figure 6 showsclear aggressivityof massratio for the delta V criterion. This is a direct implication of the laws that result in the lower mass vehicle enduring higher velocity change.Figure 7 shows very low aggressivity for the accelerationcriterion because stiffness of both vehicles is the same. The maximum crush force causing the decelerationof both vehicles is not changed,and thereforeacceleration of the standard 0 2 4 6 8 10 vehicle (no 1) does not change with variation of the partner vehicle’s mass. The latter would experience Mass Ftatio smaller acceleration with increasing mass as the masimum force is fixed in all cases.For offset crashes Figure 7 maximum acceleration versus mass ratio of 50% overlap, the behaviours of delta V and accelerationcriteria are almost the sameas that of 100% overlap. However, accelerationlevels in offset crashes are slightly lower due to concentrationof load on one longitudinal. Aggressivity of the mass ratio for the deformation length criterion is clearly illustrated in Figure 8 with the increasing deformation length of the standard vehicle versus increasing mass ratio. The behaviour of the partnervehicle (no 2) is shown to increasefirst with the

272 mass ratio up to a ratio of about 3, beyond which Figure 10. For 50% overlap, Figure 11 shows that- deformation length of the partner vehicle decreaseswith stiffness ratio has some level of acceleration mass ratio. This behaviour of the partner vehicle is aggressivity, particularly for ratio less than 3. This is attributed to energy absorption which become less for clearly evident from the fact that the maximum the higher mass vehicle. Since the first 350 mm of accelerationof the standardvehicle (no 1) in figure 11 deformation has lower crush force, higher deformation increasesfrom 25 g, at a ratio of 0.5 to 40 g at a ratio of length is expectedin this region. For offset crashesof 3. 50% overlap, aggressivity of mass ratio for the deformation length criterion is demonstratedto be lower than 100% overlap. This is clearly demonstrated in Figure 9 where little change in deformation length of Acceleration-l 00% overlap the standardvehicle (no 1) is shown versus increasein massratio. This behaviour in offset crashesis attributed to the existenceof load transfer mechanismbetween the two longitudinal ‘springs’, transferring deformation from the highly loaded longitudinal to the less loaded longitudinal in the region when deformation reaches about 700 mm. This behaviour demonstrates how 0123456 homogenousdistribution of stiffness across car front contributesto Compatibility in head-oncrashes. *alm + a2m Stiffness Ratio

Figure 10 maximum acceleration versus Stiffness ratio compatibility stiffness ratio, 100% overlap The second parameter investigated was stiffness ratio. Figures 10, 11, 12 and 13 show variation of injury criteria with stiffness ratio ranging from 0.5 to 6. The II 1 maximum ratio of 6 is dictated by the maximum --Acceleration - 50% overlap possible rigid structure (equivalent maximum crush force of 1250 kN). Mass characteristics were assumed standard for both vehicles while offset overlap were taken to be 100%. Initial velocities for both vehicles were fixed at 13.33 m/s (30 mph). Vehicle 1 is the standardfixed characteristicsvehicle, while vehicle 2 is the varied characteristicsone. Compatibility is measured 0 1 2 3 4 5 6 on changesin injury criteria of the standardvehicle (no 1) due to variation in characteristics of the partner *aim +dh StiffnessRatio vehicle (no 2). i Primary stiffness ratio kil/kzl, k1Jkz3, and Figure 11 maximum acceleration versus stiffness ratio, 50% overlap secondary stiffness klJk?2, b4k4 have been investigated separately. Very similar results were obtained for their effects on all three injury criteria. It Stiffness ratio has no delta V aggressivity at all as was decided to lump the two stiffness ratios in one and far as the mass ratio remains constant. This is the case vary both primary and secondarystiffness parametersat becausethe maximum delta V ratio (to closing velocity) the same time and according to one common stiffness of the standardvehicle (no 1) staysconstant at about 0.5 ratio input. Figure 10, 11 show the acceleration injury regardlessof the stiffness ratio. This behaviour is almost criterion versus stiffness ratio for 100% and 50% repeated for 50% overlap crashes. As for the overlap respectively.For 100% overlap,Figure 10 shows deformation length criterion, Figure 12 (100% overlap) that stiffness ratio has no accelerationaggressivity at all. shows clear aggressivity of the stiffness ratio where the This is the case becausethe masimum acceleration of deformation length increasesfrom 400 mm, at a ratio of the standardvehicle (no 1) stays constant at about 44 g 0.5, to 600 mm at a ratio of 6. Similar results are regardlessof the stiffness ratio. On the contrary higher obtainedfor a 50% overlap crash. stiffness of the partner vehicle increases acceleration injury risk of its own occupants as demonstrated in

273 ratio for deformation length criterion. These two / Crush zone agressivity 1 compatibility factors could be made to compensatefor eachother. A slightly softer and longer crumplezone is required for heavier vehicles to achieve this compatibility. As far as accelerationinjury criterion is concerned, both mass and stiffness ratio have no aggressivity on the partner vehicle. Acceleration is determinedby the vehicle’s own massand stiffness.

Delta V Accelera Deformation tion length Qure 12 Deformation versus stiffness ratio Mass High None Medium ratio Stiffness None None Medium SUMMARY AND CONCLUSIONS ratio Table 1 Summary of aggressivity 100% overlap The aggressivity of mass ratio and stiffness ratio for three injury risk criteria is summarisedin Table 1. Aggressivity rating is given one of four scores;high, medium,low & none. Two scores are given for each category; 100% overlapand 50% overlap. The most crucial compatibility parameteris the mass ratio for the delta V criterion. Nothing could be done Table 2 Summary of aggressivity rating 50% overlap about this incompatibility factor apart from geometric compatibility measures where structural interaction betweenthe vehicles is prevented.The secondand third compatibility parametersare mass ratio and stiffness

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