Pedestrian Impact Testing: Modelling the Effect of Head-form Mass and Speed

Transcription

1 Pedestran Iact Testng: Modellng the Effect of Head-for Mass and Seed D. Searson, R.W.G. Anderson Centre for Autootve Safety Research, Unversty of Adelade, AUSTRALIA, 55 Eal: Abstract Pedestran act testng s used to assess the relatve level of rotecton fro a vehcle to a edestran n the event of a collson. Testng s conducted as art of new car assessent rogras (Euro NCAP, ANCAP), and for colance wth regulatons n Euroe and Jaan. A key coonent of edestran act testng s the head-for test, n whch a duy head-for s fred nto the front of the vehcle n free flght, at secfc locatons tycally on the bonnet or wndscreen. The acceleraton of the head-for s easured and s used to assess the relatve level of rotecton at that locaton through calculaton of the Head Injury Crteron (HIC). Alternatve rotocols secfy dfferent test head-for asses and seeds. Ths aer resents a odel of the acceleraton resonse of the head-for n any gven test condton. Gven a test wth a known result, the odel can be used to estate the outcoe of a test on the sae structure usng a dfferent head-for ass and/or seed. The odel s a non-lnear daed Hertz odel of contact. Valdaton data showed that the odel estates the HIC to wthn 1% of that obtaned fro test results. Sulaton of a seres of generc act scenaros was conducted under the condtons of the Australasan New Car Assessent Progra (ANCAP) and the draft Global Techncal Regulaton (GTR) on edestran rotecton, whch stulates a dfferent head-for ass and seed. The results ndcate a large dscreancy exsts between erforance n an ANCAP test and erforance under the GTR, such that a structure that would ass the GTR ay be rated very oorly under the ANCAP test. Keywords Pedestran Testng, Contact Modellng Introducton Pedestran act testng sulates contact between the edestran and the front of the vehcle usng subsyste actors, each of whch reresents a dfferent art of the body: the head, tba and knee jont and the uer leg. Durng testng, the vehcle reans statonary, and the actor s fred nto the vehcle at a gven seed and angle. Head-for testng s tycally conducted on the bonnet surface and wndscreen. Testng s conducted as art of new car assessent rogras, ncludng the Australasan New Car Assessent Progra (ANCAP), the Euroean New Car Assessent Prograe (Euro NCAP), and for colance wth regulatons n Euroe and Jaan [1]. Slar tests are also the bass for a roosed Global Techncal Regulaton (GTR) on edestran rotecton, whch s currently beng drafted by Workng Party 29 of the Unted Natons Econoc Cosson for Euroe (UNECE) [2]. Australa s a sgnatory to the UNECE 1958 Agreeent concernng the Adoton of Unfor Techncal Prescrtons for Vehcle Safety and exects to sgn the 1998 Agreeent on Global Techncal Regulatons [3]. As such, t s exected that a fnal GTR wll be consdered n Australa wth a vew to creatng a haronsed Australan Desgn Rule. The head-for conssts of an alunu base late and shere, whch s covered n a thck rubber skn. Deendng on the wra-around dstance to the locaton beng tested, ether a chld head-for or a heaver adult head-for are used. The wra-around dstance s easured fro the ground along the surface of the car to the locaton beng tested. Durng the test, data s recorded fro a traxal acceleroeter ounted at the centre of gravty of the head-for. The resultant total acceleraton of the head-for s used to calculate the Head Injury Crteron (HIC), whch deternes the score n the case of a Euro NCAP/ANCAP test, or whether the test asses or fals n the case of a regulaton test. Noveber 28, Adelade, South Australa 23

2 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed Testng for Euro NCAP/ANCAP s conducted accordng to the Euro NCAP edestran testng rotocol [4]. The head-for actors used n Euro NCAP/ANCAP testng are a 4.8 kg adult head-for, and a 2.5 kg chld head-for, and tests are conducted at 4 k/h. The locatons tested wth the chld head-for le wthn a regon bordered by wra-around dstances of 1 and 15. The locatons tested wth the adult head-for le further u the vehcle, between wra-around dstances of 15 and 21. Any tye of structure that les wthn these wra-around dstances ay be tested, ncludng the bonnet and the wndscreen. Maxu onts are receved for a HIC of 1 or less, and zero onts are receved f the HIC exceeds 135. Between these two values, a lnear sldng scale s used. For exale, a HIC of 1175 receves half of axu onts [5]. The current draft of the GTR [2] secfes a 4.5 kg adult head-for and a 3.5 kg chld head-for, and tests are conducted at 35 k/h. Under the GTR, only the bonnet s tested. Locatons that le between wra-around dstances of 1 and 17 are tested wth the chld head-for, and locatons that le between wra-around dstances of 17 and 21 are tested wth the adult head-for. To ass the regulaton, 2/3 of the tested area ust have a HIC of less than 1, and the reanng area ust have a HIC of less than 17. Addtonally, half of all of the chld head-for tests ust have a HIC of less than 1. Consequently, vehcles desgned to eet the GTR ay not necessarly erfor well under Euro NCAP/ANCAP testng, where the lghter ass of the chld head-for and ncreased test seed ncrease the act acceleraton. Addtonally, onts that le between wra-around dstances of 15 and 17 would be tested wth an ANCAP adult head-for, whle under the GTR a chld head-for wll be used. Regulatons are often consdered as nu benchark standards, whle consuer tests often seek to encourage erforance beyond that whch the regulaton requres. And so soe dfferental n the Euro NCAP/ANCAP erforance and erforance under the roosed GTR ght be exected. Nevertheless, wth soe exerence of consuer testng, t s of nterest to see how erforance under the Euro NCAP rotocol corresonds wth erforance under the roosed GTR. It s ossble to characterse the force on the head-for throughout an act usng a atheatcal odel. The odel can then be used to sulate the outcoe of a test on the sae structure under dfferent act condtons. That s, the force on the head-for n a Euro NCAP/ANCAP test can be used to estate the force on the head-for n a GTR test on the sae structure. One such odel s a Hertzdaed odel, whch has been successfully used to odel uer leg-for to buer acts [6]. For certan classes of head-for act, ths odel ay also be alcable. Once a sutable odel has been generated for a artcular test locaton, the nfluence of the ntal seed and ass of the head-for can be exaned by nuercally sulatng the act under any condton. Thus, t s ossble to take an arorate test conducted under the ANCAP rotocol, develo a odel that fts the data, and gve an aroxate result f the test was conducted under the condtons of the GTR. Method Estatng the noral acceleraton n a head-for test A scheatc of the head-for actng a flat surface s shown n Fgure 1. If the angle s known, then the noral acceleraton a n and the tangental acceleraton a t can be calculated, as the orentaton of each acceleroeter relatve to s known. Thus, the goal s to know throughout the act. The ntal angle s known fro easureents, and so the ntal acceleraton coonents a n, and a t, are known also. Noveber 28, Adelade, South Australa 24

3 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed Fgure 1 - Scheatc of head-for actng the vehcle surface We assue that the head-for rolls along the surface of the bonnet wth no slage. Thus, for the th te-ste, ncludng =, the angular acceleratonω can be calculated as: ω = a t, r Where r s the radus of the head-for. The resultng angular velocty and the angle calculated as follows for te-stes > wth te dfference t: can be ω = ω θ = θ 1 + ω Δt 1 + ω Δt At the end of ths rocess, the noral acceleraton a n s known at each te-ste. Nuercal ntegraton can be used to obtan the noral dslaceent and noral velocty. Valdaton of the estate of the noral acceleraton Ths ethod of extractng the noral coonent was valdated usng a MADYMO sulaton. MADYMO s a ult-body sulaton software ackage develoed by TNO Autootve Safety Solutons (TASS). In the MADYMO sulaton, a shere wth the sae oent of nerta, ass and daeter as a chld head-for was acted nto a flat late at an angle. The noral force-deflecton resonse of the late was set usng a tycal odel. The condtons of the sulaton are suarsed below. Table 1 - MADYMO valdaton odel condtons Condton Value Coents Daeter 13 As for Euro NCAP chld head-for Mass 2.5 kg As for Euro NCAP chld head-for Moent of nerta 3.6 x 1-3 kg. 2 As for Euro NCAP chld head-for Velocty 11.1 /s Euro NCAP head test velocty (4 k/h) Angle to horzontal 65 As for Euro NCAP adult head test Surface angle 1.6 Tycal bonnet angle The acceleraton of the centre of the head-for was easured n the sulaton, wth the three easureent axes fxed relatve to the orentaton of the head-for. The head-for rolled durng the sulated act as would be exected n a hyscal head-for test. Ths acceleraton data was then rocessed to derve the noral coonents usng the ethod descrbed earler. The actual noral coonent of acceleraton was also stored as an outut fro the sulaton. The results were coared, as shown n Fgure 2. Although the results dverge slghtly towards the end of the act, the results ndcate that the ethod for extractng the noral coonent s arorate. Noveber 28, Adelade, South Australa 25

4 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed 15 Noral force (kn) Noral velocty (/s) Noral dslaceent () -5 Te (s) Fgure 2 - Derved noral data (dashed lne) coared wth data fro a MADYMO sulaton (sold lne) Contact odel descrton and alcablty The Hertz-daed odel s resented n Anderson et al. [6]. The odel gves the contact force F as a functon of the enetraton and enetraton velocty. The odel s based on the Hertz contact law for undaed elastc contacts: F n = K (1) where K s a stffness constant and n deends on the geoetry of the act. Hunt & Crossley [7] extended the law to nclude a dang ter that s deendent on the enetraton velocty : F n q = K + b where b s a dang constant, and t s usual to set = n and q = 1 [7]. Introducng a new constant c = b/k, the equaton reduces to: F n K ( 1+ c ) = (2) However, the above equaton does not account for eranent deforaton as the force wll only be zero when the enetraton s zero. Lankaran & Nkravesh [8] used the axu enetraton and eranent deforaton to descrbe the unloadng hase of undaed contact. Ther equaton for unloadng can be reduced to: ( ) F = K n (3) Whch has the sae for as (1), and as such dang can be added n an analogous way to (2). As a result, the odel has two fors, one for the loadng hase, when the enetraton velocty s ostve, and one of the unloadng hase, when the enetraton velocty s negatve. To suarze, durng the load hase, when : Noveber 28, Adelade, South Australa 26

5 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed (, n F ) = K (1 + c ) And, durng the unloadng (rebound) hase, when : ( ) (, F ) = K (1 + c ) Fro the test data, and can be found. The araeter n s related to the geoetry of the act and should be equal to 1.5 for a shere actng a flat late [7]. Two addtonal araeters, the stffness K and dang araeter c deend uon the characterstcs of the structure beng acted. At the te of axu enetraton, the enetraton velocty wll be zero, and as such we can rearrange to fnd K fro the axu enetraton value and the force at axu enetraton F : n K F n = (4) Through tral and error, the araeter c can be chosen to atch exerental data. Ths s done by calculatng an estate of the elastc force by dvdng the actual force by the dang ter ( 1+ c ) and coarng wth the theoretcal elastc force gven by (1) durng loadng and (3) durng unloadng. An addtonal value J can be ntroduced to ensure that the average sloe of the unloadng curve reans constant. J can be calculated fro exstng test data as follows: J F = Once values for K, n, c and J are obtaned, t s ossble to nuercally sulate acts under any test condtons. The ntal condtons for the nuercal sulaton are as follows: = = =, act Where s the desred act seed. If the desred head-for ass s and F(, ), act descrbed above, then for each te ste, = 1 + 1Δt = 1 1Δt F(, ) = s as Noveber 28, Adelade, South Australa 27

6 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed Durng the sulated act, the value of s taken at the frst te ste where the enetraton velocty s found to be less than zero. The eranent ndentaton s calculated fro J, whch s assued to be constant across all act condtons: = F J Note that the orgnal act can be sulated by settng the actual head-for ass for., act to the actual act velocty, and usng There are several ltatons on ths odel, whch do not ake t alcable for all head acts. Naely: The odel ales to a sngle contact force nteracton, and so cannot be used for acts nvolvng ultle structures for exale, the bonnet, followed by the engne block. Energy losses and the assocated forces due to vbraton are not accounted for these are often resent n lower severty bonnet acts. Only the noral coonent of the act s sulated. The next secton s dvded nto three arts. In the frst art, the odel s used to sulate the results of a seres of act tests that were conducted at the sae locaton on the bonnet of a artcular vehcle. In the second art, an exale s gven of how the act seed and ass of the head-for affect the results of a artcular test. In the thrd, a seres of general act scenaros are odelled, and the results under the Euro NCAP rotocol are coared wth results under the GTR testng rotocol. Results Coarson wth actual act data To valdate the odel, a locaton on the bonnet of a artcular vehcle was tested under varous condtons varyng both the act seed and the ass of the head-for wth each test. A new bonnet was used for each test. Test condtons are gven n Table 2. A odel was constructed usng the sae values of the araeters K, n, c and J for all of the acts. The values of these araeters were selected through traland-error to best estate the HIC calculated fro test data, across the range of acts. A tycal exale of the ft of the odel to a artcular test s shown n Fgure 3. Note that the axu dslaceent s slghtly overestated n the sulated case. Ths s ost lkely due to the sall ntal ske n the actual act data not beng relcated n the odel. 2 7 Noral acceleraton (g) Noral force (kn) Te (s) Dslaceent () Fgure 3 - Actual act data (sold lne) coared wth sulated act data (dashed lne) Noveber 28, Adelade, South Australa 28

7 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed The HIC was calculated for each test, based on the noral coonent of acceleraton. The reason for ths s that the odel sulates only the noral coonent of the act. The tangental coonent of acceleraton has soe effect on the overall HIC, but t s generally sall, tycally on the order of 5-1%. A coarson between the HIC fro the real act data, and the HIC fro the sulated data, s gven n Table 2. Table 2 - HIC n actual vs sulated acts Iact seed, /s Head-for ass, kg Actual HIC Sulated HIC Error, % The sulated HIC estates the actual HIC, wth an error of u to around 8%. The data n Table 2 are also shown n Table 3, but are stated relatve to the test conducted at 11.1 /s wth a 3.5 kg head-for ass. The relatve dfference n HIC for both the real acts and the sulated acts was calculated. Table 3 - HIC relatve to baselne act, n actual vs sulated acts Iact seed, /s Head-for ass, kg Actual HIC, % of reference Sulated HIC, % of reference The data n Table 3 ndcates that the odel s useful for redctng the general trend n HIC wth a varaton n ass and/or act seed, when easured relatve to the HIC calculated at a reference set of condtons. An exale of the effect of ass and act seed on HIC Usng the odel as descrbed n the revous secton, t s ossble to choose a set of araeters and sulate an act for any act seed and head-for ass. Thus, the change n HIC under dfferent act condtons can be estated. An exale of ths can be seen n Fgure 4. The araeters used to construct the curves were the sae as those used to odel the real-world tests n the revous secton. Each curve reresents a dfferent headfor ass. The act seed, n k/h, s on the x-axs, and the sulated HIC on the y-axs. The condtons for a Euro NCAP chld head-for test (2.5 kg at 4 k/hr) and a GTR chld head-for test (3.5 kg at 35 k/hr) are hghlghted. In ths case, the HIC for a chld head-for test under the Euro NCAP rotocol s aroxately 25, whle the HIC for a chld head-for test conducted under the GTR rotocol s aroxately 14. Accordng to the odel, ths test locaton would have well exceeded the ANCAP axu allowable HIC of 135, whle t would have et the axu HIC requreent of 17 n the GTR (although a HIC of between 1 and 17 s only allowed for 1/3 of the total test area, and 1/2 of the chld tests, n the current draft). Noveber 28, Adelade, South Australa 29

8 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed HIC Euro NCAP chld kg 2 15 GTR chld Iact seed (k/h) Fgure 4 - Effect of act seed and head-for ass on HIC for a artcular test case (n = 1.4, c =.21, K = 21 x 1 3, J = 241 x 1 3 ) Corresondence between Euro NCAP erforance and GTR erforance for a varety of structures Several sets of araeters were used to coare, ore generally, the dfference between values of HIC under the Euro NCAP rotocol and the GTR rotocol. The results of these sulatons are shown n Table 4 and Table 5. The araeter n was set to ts theoretcal value of 1.5, and the araeter c was set to.25, whch, fro our exerence, s a tycal value for head-for to bonnet acts. The loadng stffness K affects the HIC the ost strongly, and so ths was vared to generate secfc HIC values under the Euro NCAP test condtons. The unloadng sloe J was vared also, but had very lttle nfluence on the HIC result. After an arorate value of K was deterned, the ass and act seed were set to the condtons of the GTR and the act was sulated agan, and the HIC recalculated. Noveber 28, Adelade, South Australa 3

9 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed Table 4 - Sulated HIC n a chld head test under the Euro NCAP rotocol and the GTR (wth n = 1.5, c =.25) HIC, Euro NCAP chld head test (2.5 kg, 4 k/h) HIC, GTR chld head test (3.5 kg, 35 k/h) K J Table 5 - Sulated HIC n an adult head test under the Euro NCAP rotocol and the GTR (wth n = 1.5, c =.25) HIC, Euro NCAP adult head test (4.8 kg, 4 k/h) HIC, GTR adult head test (4.5 kg, 35 k/h) K J Note that every sulated test conducted under the GTR rotocol fell under the allowable HIC of 17. (Note though that 2/3 of the total test area ust receve HICs of less than 1, and 1/2 of the chld head tests ust also have HICs of less than 1.) In the case of the sulated chld head-for tests, 1 was only exceeded under the GTR condtons when the Euro NCAP HIC was 2. In the case of the sulated adult head-for tests, 1 was exceeded under the GTR condtons when the Euro NCAP HIC was 15. The data n Table 4 and Table 5 les that the HIC under the Euro NCAP rotocol s lnearly related to the HIC under the GTR rotocol. Under ths set of condtons, the HIC under the GTR s 56% of the Euro NCAP HIC for a chld head test, and 71% of the Euro NCAP HIC for an adult head test. Dscusson Ths aer detaled relnary fndngs of a study on the corresondence between the erforance of head-for tests under the Euro NCAP edestran rotecton rotocol and the roosed GTR rotocol. Pedestran head-for act testng s a core coonent of Euro NCAP and ANCAP edestran testng, and also for regulatons, such as the roosed GTR on edestran rotecton. Due to the dfferences n test condtons between the dfferent rotocols, a ethod of scalng results to dfferent test condtons ay be useful. Ths aer has resented such a ethod, whch utlses a Hertz-daed contact odel to odel the noral coonent of a head-for act. A ethod of extractng the noral coonent of acceleraton fro exstng test data has also been resented. When coared wth data fro a real test, the odel estated the HIC wth an error of wthn 1%. A seres of generc acts were constructed by choosng arorate araeters for the odel. The acts were sulated under the condtons stulated by the Euro NCAP testng rotocol, as well as the condtons secfed n the ost recent draft of the GTR. The HIC under the GTR test condtons was found to be sgnfcantly less than the HIC for the equvalent Euro NCAP test. In both chld and adult head-for tests, locatons that scored zero onts under the Euro NCAP rotocol were found to ass the equvalent GTR test. Thus, the results of the sulaton ly that t s ossble that a vehcle that scores zero head test onts under the Euro NCAP rotocol ay ass the GTR requreents. Furtherore, the results ndcate that for a fxed set of condtons, the HIC under the Euro NCAP rotocol has a lnear relatonsh to the HIC under the GTR. There are, however, ltatons to ths study. The odel can only be used to relably sulate a certan class of acts that nvolve a sngle structure and no vbraton effects. Ths tye of act s relatvely rare. Noveber 28, Adelade, South Australa 31

10 Pedestran Iact Testng: Modellng the Effect of Head-For Mass and Seed It s also worth notng agan that only the noral coonent of the act s beng sulated. As such, only the noral coonent of the orgnal test data has been used to calculate the HIC n ths reort. Ths s vald only f the head-for s contactng the vehcle surface at 9 degrees, for exale, a chld headfor act on the leadng edge of the vehcle. In any, f not ost, acts, the act angle s less than 9 degrees, and as such there s an addtonal tangental and rotatonal acceleraton of the head-for whch s not beng consdered. A relnary look at the data ndcates that ths ay account for an ncrease of u to 1% of the HIC value. However, t would be exected that slar trends n the data would stll be resent, as the tangental coonent would be exected to rse wth the noral coonent. Future work n ths area wll nvolve develong a ore unversal odel that accounts for ultle structures n an act, and for vbraton effects. Test data for valdatng such a odel has been collected. A ore unversal odel wll be able to be used to characterse any new or exstng head acts, and therefore coare erforance under alternatve test reges. Ths wll allow for a ore corehensve coarson of results under dfferent rotocols, and ay generate further nsght nto the donant echanss that affect the test results. Conclusons Ths relnary study ndcates large otental dscreances between the results of headfor act tests under the roosed GTR and tests conducted accordng to the Euro NCAP rotocol. If confred, t ndcates that soe cars that erfor oorly n Euro NCAP/ANCAP assessents ay need no odfcaton to ass the roosed GTR. Ths tentatve concluson wll be tested n a future analyss. Acknowledgeents The Centre for Autootve Safety Research receves core fundng fro both DTEI and South Australa s Motor Accdent Cosson. The vews exressed n ths reort are those of the authors and do not necessarly reresent those of the Unversty of Adelade or the sonsorng organsatons. References 1. McLean, A.J. (25). Vehcle desgn for edestran rotecton. CASR Reort #37. Adelade: Centre for Autootve Safety Research. 2. Unted Natons Econoc Cosson for Euroe (UNECE) (27). Fnal Reort on the develoent of a global techncal regulaton concernng edestran rotecton. Reort of the Workng Party on Passve Safety on ts forty-frst sesson (7 11 May 27). Retreved June 27 fro htt:// 3. Deartent of Infrastructure, Transort, Regonal Develoent and Local Governent (28). Australan Desgn Rules. Retreved July 27 fro htt:// 4. Euroean New Car Assessent Prograe (Euro NCAP) (28). Pedestran Testng Protocol. Verson Euroean New Car Assessent Prograe (Euro NCAP) (28). Assessent Protocol and Boechancal Lts. Verson Anderson, R.W.G., Long, A.D., Serre, T. & Masson, C. (28), Deternaton of boundary condtons for edestran collson reconstructons, Acceted for resentaton at ICrash 28, Kyoto, Jaan, July Hunt, K.H. & Crossley, F.R.H. (1975), Coeffcent of resttuton nterreted as dang n vbroact, Journal of aled echancs, 42(Seres E), Lankaran, H.M. & Nkravesh, P.E. (1994), Contnuous contact force odels for act analyss n ultbody systes, Journal of nonlnear dynacs, 5, Noveber 28, Adelade, South Australa 32