A numerical investigation into the effect of CRS misuse on the injury potential of children in frontal and side impact crashes

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Accident Analysis and Prevention 43 (2011) 1438–1450

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numerical investigation into the effect of CRS misuse on the injury potential ofhildren in frontal and side impact crashes

anya Kapoora, William Altenhofa,∗, Anne Snowdona, Andrew Howardb, Jim Rasicoc,uchun Zhuc, Daniel Baggioa

Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 410 Sunset Avenue, Windsor, Ontario N9B 3P4, CanadaThe Hospital for Sick Children, Division of Orthopedic Surgery, 555 University Avenue, Toronto, Ontario M5G 1X8, CanadaFirst Technology Safety Systems, Plymouth, MI 48170, USA

r t i c l e i n f o

rticle history:eceived 12 December 2009eceived in revised form2 December 2010ccepted 23 February 2011

eywords:hild safetyead and neck injury potential

njury criteriaountermeasuresRS misuse

a b s t r a c t

This research focuses on an investigation into the head and neck injuries sustained by toddlers due toCRS misuse under frontal and side impact crashes. A fully deformable FE model incorporating a HybridIII 3-year-old dummy was developed which has been previously validated for frontal impacts underCMVSS 208 and FMVSS 213 testing conditions. Furthermore, this model has also been validated undernear-side impact conditions in accordance to crash tests carried out by NHTSA. In addition, numericalmodels incorporating a Q3/Q3s prototype child crash test dummies were developed. The objective of thisresearch was to study the effect of seatbelt slack and the absence of the top tether strap on the head andneck injuries sustained by toddlers in a vehicle crash. Numerical simulations were conducted under fullfrontal and near side impact crash testing conditions in accordance with FMVSS 213 for the Hybrid III3-year-old dummy and Q3/Q3s dummies in the absence and presence of slack in the seatbelt webbing,and in the absence and presence of the top tether strap. In addition, the effect of using a cross-shapedrigid ISOFIX system was also investigated. An analysis of the head and chest accelerations, neck loadsand moments was completed to investigate the potential of injury due to CRS misuse. An increase inHIC15 by approximately 30–40% for the frontal impact and 10–20% for the near-side impact respectivelywas observed for the Q3 child dummy due to both forms of CRS misuse. In the absence of the top tetherstrap the forward head excursions were observed to be increased by approximately 70% for the Hybrid
III 3-year-old dummy and 40% for the Q3 dummy, respectively. Use of the cross-shaped rigid ISOFIXsystem illustrated a reduction in head and neck injury parameters, for both frontal and side impactconditions, in the absence and presence of CRS misuse. CRS misuse results in a significant increase ininjury parameters and potential for contact related head injuries. Use of a rigid ISOFIX system to restraina CRS provides better CRS and dummy confinement and reduced injury potential than a flexible ISOFIX system.
Abbreviations: AIS, Abbreviated Injury Scale; ANPRM, Advanced Notice ofroposed Rule-making; AOD, atlanto-occipital dislocation; ATD, Anthropometricesting Device; CAD, Computer Aided Design; CMVSS, Canadian Motor Vehicleafety Standard; CRS, Child Restraint Seat; FARS, Fatality Analysis Reporting System;EMB, Finite Element Model Builder; FE, Finite Element; FMVSS, Federal Motor Vehi-le Safety Standard; FTSS, First Technology Safety Systems; LATCH, Lower Anchoragend Tether for Children; MVC’s, motor vehicle collisions; NCSA, National Centeror Statistics and Analysis; NHTSA, National Highway Traffic Safety Association;RV’s, Protection Reference Values; UMTRI, University of Michigan Transportationesearch Institute.∗ Corresponding author. Tel.: +1 519 253 3000x2619; fax: +1 519 973 7007.

E-mail address: [email protected] (W. Altenhof).

001-4575/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.aap.2011.02.022

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Traffic related injuries are a major public health challenge glob-ally that requires intensive efforts for effective and sustainableprevention (WHO, 2004). Worldwide, 1.2 million people are killedand 50 million injured annually in road crashes. According toBureau of Reproductive & Child Health, and statistics from Fatal-ity Analysis Reporting System (FARS), motor vehicle crashes arethe leading cause of deaths for children in Canada and the UnitedStates (Statistics Canada, 2003; NHTSA, 2005). In the year 2005,
there a total of 103 deaths and 13,649 injuries to children underthe age of 14 due to vehicle crashes (Transport Canada, 2006). Chil-dren under the age of 4 accounted for 23.3% of traffic fatalities and19.4% of injuries to children in that year. According to National Cen-ter for Statistics and Analysis (NCSA) in the year 2005 there were
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T. Kapoor et al. / Accident Analysis and P

Nomenclature

Fresultant resultant force/loadFx force in the x-directionFy force in the y-directionFz force in the z-directionHIC15 head injury criteria in a 15 ms windowHIC36 head injury criteria in a 36 ms windowHICt2–t1 head injury criteria in a t2–t1 ms windowMresultant resultant momentMx moment in the x-directionMy moment in the y-direction

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haccmtc

Mz moment in the z-directiont time

348 deaths and 271,000 injuries to children under the age of 15n automobile crashes in the United States (NHTSA, 2006). Childrennder the age of 5 accounted for 25% (590) of these traffic fatalitiesnd 20.6% (56,000) of all the children injured in vehicle crashes inhe United States.

Anatomical differences between children and adults place chil-ren at an increased risk of head and neck injury as compared todults. Tingvall (1987) established the most notable anatomicalifference between children and adults is body segment propor-ions. This dissimilarity in body segment proportions leads to aigher center of gravity in a child, which affects the body kine-atics in the event of an accident which may make children more

rone to acceleration derived head and neck injury in high accel-ration environments often experienced in Motor Vehicle CrashesMVC’s). In addition, the biomechanical structure and properties ofhe upper cervical spine of children place them at an increased riskor acceleration induced injury. The ligaments are lax, the vertebraere not ossified and are more likely to separate, the facets are pre-ominately horizontal providing limited restriction of subluxationnd the posterior-lateral contours of the vertebral bodies are noteveloped to restrict flexion forces. Developmental differences inhildren also play a role in injury. In children, cervical vertebrae areonnected by synchondrosis, which is a temporary cartilaginousoint that exists during normal growth and development. As thehild grows older, this soft tissue undergoes ossification. In infantsnd toddlers, there is curtailed ossification of the synchondrosis,hich makes their necks more prone to fracture (Yoganandan et al.,

999). In a study released by the University of Michigan Transporta-ion Research Institute (UMTRI), Weber (1995) determined thatoung children are at risk for devastating head and neck injuriesue to the neck’s fragile physiology and weak neck musculature.hese factors reduce the tolerance of a child to withstand high crashorces. In a study done by Gotschall et al. (1999), it was determinedhat head injuries are the leading cause of death among childrenges birth to five in the United States. In children younger than 11,otor vehicle crashes account for 38% of all cervical spine injuries

Mousny et al., 2001). Injuries to the head and neck account for areater number and severity of abbreviated injury scale (AIS) 2+njuries (Arbogast et al., 2002). According to a case study carriedut by Sherwood et al. (2003), more than 80% of the fatalities tooddlers occurred due to head trauma.

The most common mechanisms of neck injury include extremeyper flexion which leads to rupture of the tectorial membranend separation of vertebrae which leads to atlantooccipital dislo-
ation (AOD) (Dublin et al., 1980). The disproportionate size of thehild’s head relative to the rest of the body increases the relativeoment of inertia per unit of acceleration for children as compared
o adults. This in combination with the under developed mus-uloskeletal pediatric cervical spine put children at an increased

revention 43 (2011) 1438–1450 1439

risk for cervical injuries in MVC’s. The head injuries include bothcontact induced injuries as well as inertial injuries. Injuries suchas skull fracture, epidural hematoma, and frontal lobe contusionare contact injuries (Gennarelli, 1986) that are most likely due toexcursion of the head and subsequent impact with the vehicle inte-rior. Arbogast et al. (2002) demonstrated that substantial intrusionalters the space available for the child, thus allowing for contact.The primary concern of head trauma is injury to the brain causedby acceleration induced contusions to the cerebral cortex.

Child restraint laws came into effect between 1982 and 1987.Child safety seats are affixed to car seats using various anchoragedevices. According to the Motor Vehicle Safety Act, Standard 210.2(Transport Canada, 2002), all new Canadian vehicles, manufacturedon or after September 1, 2002, are to be equipped with lower uni-versal anchorage bars. Whereas, in older vehicle models, the childsafety seats are secured to the car seat using seat lap belt. A toptether strap is mandatory in both cases. Top tethers provide addi-tional protection by limiting the motion of the Child RestrainingSystem (CRS) during a severe crash. Weber at UMTRI found thatuse of a top tether keeps the child head from traveling beyond asafe limit during a severe frontal crash (Weber, 2000). Accordingto Weber, CRS provide specialized protection for small occupantswhose body structures are still immature and growing (Weber,2000). The primary goal of an occupant protection system is to pro-tect injury to the central nervous system. Rapid deceleration of thebody and the impact of the vehicle’s structure on body surfaces areboth associated with severe injury during collisions. All CRS workon the principle of creating a tight coupling of the restrained childand the crushing vehicle, and to distribute the remaining load aswidely as possible over the child’s strongest anatomical structures.Restraints in vehicles are designed to limit and control the body’srate of deceleration during a crash, thus reducing the forces actingon the body’s surface to minimize the differential motion betweenthe skeleton and the internal organs (Weber, 2000). Child safetyseats are extremely effective when correctly installed and used inpassenger cars, reducing the risk of death by 71% for infants and 54%for children ages 1–4, and reducing the need for hospitalization by69% for children ages 4 and under (National and Campaign, 2004).Forward facing CRS effectively reduce the risk of serious injury by78% in MVC’s compared to children restrained by using only seatbelts (Arbogast et al., 2004).

According to Chouinard and Hurley (2005), the number of fatal-ities for children under the age of 14 in Canada has been under100 per year since 1998 and major injuries have been under 700since 1999. There were a total of 402 fatalities for child occupantsof light duty vehicles between 1998 and 2002. For children whowere fatally injured, unsuitable restraint was found to be approx-imately 66% for infants (under 1 year old), 50% for toddlers (1–4years old), 97% for school-age children (4–8 years old) and 31%for older children (9–14 years old) (Chouinard and Hurley, 2005).Weber determined that in North America, 80% of child restraintswere not being used as intended (Weber, 2000). Different types ofmisuse can have different effects on child restraint performance.The most common forms of misuse are relatively minor, resultingin sub-optimal levels of protection. Some extreme forms of mis-use can themselves lead to injury in a crash where otherwise noinjury would have occurred (Weber, 2000). If all child passengersaged 4 years and younger were restrained, each year an additional162 lives could be saved and 20,000 injuries could be prevented.Approximately 29% of children aged 4 years and younger do notride in appropriate restraints, placing them at twice the risk of fatal
and nonfatal injuries of those riding restrained. In addition, approx-imately 85% of children riding in child safety seats are improperlyrestrained (Zaza et al., 2001).
The most common form of misuse for CRSs is loose harnessstraps and loose vehicle safety belts (Eby and Kostyniuk, 1999;

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440 T. Kapoor et al. / Accident Analysi

ecina and Lococo, 2005; Decina and Knoebel, 1997). Presence oflack in the restraint systems, incorporating both child restraintsnd the use of adult seatbelts for children, generally leads to areater extent of head excursion (displacement of the head rela-ive to the torso) putting both the head and spinal regions at greaterisk of injury. In a study completed in 1998, Transport Canada foundhat at least 32% of all CRSs installations were incorrect. Of thesencorrect installations 26% were installed without the top tether orhe tether attachment connected in the wrong location. Other sig-ificant factors associated with misuse of the top tether included a

arge amount of slack and twisting of the top tether (Rubin-Brownt al., 2007). The National Highway and Traffic Safety Administra-ion (NHTSA) completed a six state study in 2002 and found thatritical CRS misuse accounted for 72.6% of all installations. Spe-ific to forward facing convertible CRSs approximately 82% of allnstallations involved some form of misuse. Loose vehicle seat beltttachments (fastening the CRS to the vehicle) and CRS webbingsed to constrain the child to the CRS were noted as the mostignificant misuse rationale (Decina and Lococo, 2003).

Brown et al. (2005) carried out an analysis of injury data fromedical records from two specialist pediatric hospitals, in combina-

ion with an interview with the vehicle driver and a car inspectionn Australia. This study focused on children 3–8 years of age. Analy-is of injury mechanisms showed that head injuries were typicallyaused by head contacts with the vehicle interior. These head con-acts generally occurred when there was excessive head excursionecause of poor upper body restraint, due to misuse of a restraint,r when there was intrusion. The more serious spinal injuries alsoccurred in conjunction with excessive head motion, and were alsossociated with restraint misuse. Spinal injuries mostly occur inhe cervical and thoracic spine and are associated with poor fitf the seatbelt, poor pre-crash posture and misuse (Brown et al.,005). However, the most severe spinal injuries occur when childestraints are misused, due to which excessive head excursionesults. In another study carried out by Arbogast et al. (2002), headnjuries were observed to occur due to looseness of both the vehi-le seatbelt attaching the child restraint and of the child restraintarness. With loose vehicle belt attachment, the time and space

or “ride down” is reduced. With a loose CRS harness, the thoracicpine is allowed to flex and there is relative movement betweenhe torso of the child and the back of the child seat (Arbogast et al.,002).

This research focuses on an investigation into the head and necknjuries sustained by toddlers due to CRS misuse under frontal andide impact crashes. A fully deformable Finite Element (FE) modelncorporating a Hybrid III 3-year-old dummy (LS-DYNA, 2000) waseveloped which has been previously validated for frontal impactsnder Canadian Motor Vehicle Safety Standards (CMVSS) 208 andederal Motor Vehicle Safety Standards (FMVSS) 213 testing con-itions. Furthermore, this model has also been validated underear-side impact conditions in accordance to crash tests carried outy NHTSA. In addition, numerical models incorporating a Q3/Q3sQ Advanced, 2005; Carlson et al., 2007; LS-DYNA, 2006; LS-DYNA,007) prototype child crash test dummies were developed. Thebjective of this research was to study the effect of seatbelt slacknd the absence of the top tether strap on the head and neck injuriesustained by toddlers in a vehicle crash. These two types of mis-se represent the most common type of observed misuse of childafety seats. Yet, modelling the injury outcomes of such misuse hasot been systematically investigated. Numerical simulations wereonducted under full frontal and near side impact crash testing
onditions in accordance with FMVSS 213 for the Hybrid III 3-year-ld dummy and Q3/Q3s dummies in the absence and presence oflack in the seatbelt webbing, and in the absence and presence ofhe top tether strap. In addition, the effect of using a cross-shapedigid ISOFIX system was also investigated. An analysis of the head
Prevention 43 (2011) 1438–1450

and chest accelerations, neck loads and moments was completedto investigate the potential of injury due to CRS misuse.

2. Numerical methods

The CRS, as well as all other components of the numerical modelincluding the seat buck, the CRS webbing and the CRS foam padwere meshed using Finite Element Model Builder (FEMB). Thenumerical model incorporated complete deformability of the CRSincluding material non-linearities and contact definitions amongstdifferent components of the CRS. The details of the numericalmodeling of different components of the CRS incorporated in thisresearch are presented in Appendix A. No changes to the numericalmodels of the Anthropometric Testing Devices (ATDs) were com-pleted in this investigation. The widespread use of the Hybrid III3-year-old dummy by CRS manufacturers and safety institutes, inboth frontal and side impact conditions, was the main reason for theuse of this ATD in this investigation. Furthermore, this study alsoincorporated the Q3 and Q3s to consider the relative performanceof these advanced ATDs to the Hybrid III under the simulated crashconditions.

Tightening of the front-adjusting harness strap was simulatedquasi-statically to properly position the child dummy into the CRS.This process was achieved prior to the application of the accelera-tion pulse to the seat buck through dynamic relaxation. The numberof iterations between the convergence checks and the convergencetolerance for dynamic relaxation were specified as 1000 and 0.0001,respectively. In this stage, the front-adjusting harness strap as wellas the upper tether and lower LATCH anchor were preloaded. Loadsassigned to the LATCH and the top tether in the dynamic relaxationphase was of a magnitude of 60 N (FMVSS, 2001) respectively in thedirection of the tethers.

Contact between the Hybrid III and the CRS, including the web-bing, buckles, foam padding and surfaces of the CRS was achievedthrough a penalty based contact algorithm. Values for the staticand dynamic coefficients of friction were specified as 0.25 and 0.2respectively for all contact definitions. Similar contact definitionswere incorporated between the Q3/Q3s and the CRS model.

Experimental sled testing was completed at the Graco Cor-poration’s sled testing facilities in accordance with FMVSS 213standards (Turchi et al., 2004). Fig. 1(a) illustrates the FMVSS 213acceleration pulse observed from these sled tests. The numericalmodel incorporating the Hybrid III 3-year-old child dummy hasbeen previously validated utilizing FMVSS 213 and CMVSS 208acceleration pulses (Wang et al., 2006). Further, sled tests wereconducted by NHTSA in 2001 at Test Research Center (TRC). Thistest, labelled 4585 was a sled test without a vehicle body and inthe presence of a side rigid wall (VCTD, 2006). The deformableCRS model incorporating the Hybrid III 3-year-old dummy has alsobeen validated in near side impact conditions (Kapoor et al., 2008).Fig. 1(b) illustrates the ANPRM FMVSS 213 (Test No. 4585) acceler-ation pulse observed from these sled tests.

The purpose of this research was to numerically investigate theeffect of two major types of misuse, namely absence of the toptether and presence of slack in the seatbelt webbing, on the injurypotential of toddlers in frontal and side impact crashes. The toptether strap was deleted for simulating the effect of “no top tether”condition. For the second type of misuse (seatbelt slack), tighten-ing of the front-adjusting harness strap was not completed. Theamount of slack, measured between the chest of the dummy and
the seatbelt webbing was observed to be approximately 2.5 cm.
Numerical simulations were conducted in the absence and pres-ence of the CRS misuse utilizing both flexible LATCH and thecross-shaped rigid ISOFIX system (Fig. 2A) in both frontal and near-side impact situations. The acceleration pulse obtained from the

T. Kapoor et al. / Accident Analysis and Prevention 43 (2011) 1438–1450 1441

0

5

10

15

20

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4585 Acceleration Pulse

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Sled AccelerationFMVSS 213 Upper Acceleration LimitFMVSS 213 Lower Acceleration Limit

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0 20 40 60 80 10Time (ms)

Fig. 1. (a) FMVSS 213 sled test acceleration pul

xperimental sled tests (Fig. 1(a)) was prescribed to the rigid por-ion of the seat buck in the positive X direction for the frontal impactests. The acceleration pulse illustrated in Fig. 1(b) was utilized inhe simulation of the pure side condition. The acceleration pulsein Fig. 1(b)) was prescribed in the negative Y direction for theide impact tests. The near-side impact tests were completed inhe presence of a rigid wall. The rigid wall was meshed using shelllements on the seat buck which represents the location of a vehi-le’s side structure, positioned 508 mm from the point Z1, adjacento the CRS (ANPRM, 2002). The rigid wall was meshed extendingrom the seat cushion to a height of approximately 762 mm. Theall extended forward a distance of approximately 690 mm, to

nsure that head contact would only be with a flat surface. The wallas modeled using a rigid material definition in LS-DYNA. Fig. 2

llustrates the Q3s child dummy, seated in the forward facing CRSnder the near-side impact configuration in the presence of theigid wall. All numerical simulations incorporating the Hybrid IIInd the Q3/Q3s child dummies were conducted using LS-DYNA ver-ion 970 revision 5434a (Hallquist, 1998) on a personal computerquipped with dual 2.6 GHz AMD Athlon processors with 4 GB ofAM.

. Data analysis

Child occupant injury data was extracted from the instrumenta-
ion, including accelerometers and load cells, equipped within theummy model. Tri-axial accelerometers were mounted in the head,hest and pelvis of the Hybrid III 3-year-old and the Q3/Q3s childummy models which provided acceleration data in the local x, ynd z-directions. Load cells within the dummies’ neck provided all
ig. 2. Configuration of the Q3s child dummy in the CRS for the near side impactest in the presence of the rigid wall.

0 25 50 75 100 125Time (ms)

(b) Test No. 4585 sled test acceleration pulse.

six output channels consisting of three forces and three momentsfor the upper and lower neck of the dummy in the local x, y and zdirections.

The resultant head and chest accelerations were computedusing Eq. (1) where x, y and z-axes are the local coordinate systemlocated at the head and spine locations, respectively.

aresultant =√

a2x + a2

y + a2z (1)

The resultant neck forces and moments were computed usingEqs. (2) and (3) respectively where the x, y, and z-axes are the localcoordinate systems located at each neck location.

Fresultant =√

F2x + F2

y + F2z (2)

Mresultant =√

M2x + M2

y + M2z (3)

All the data was measured with respect to the dummy’s localcoordinate system as outlined in SAE J211 (SAE, 2003) and wassampled at rate of 10 kHz. The numerical results were filtered inaccordance to SAE J211 (SAE, 2003). Filtering was conducting usinga Butterworth 4-pole phaseless filter as described in reference(LS-DYNA, 2000). Data filtering was completed prior to any dataanalysis.

3.1. Head injury criteria

The resultant head acceleration (Eq. (1)) was used in calculatingthe HIC using Eq. (4).

HICt2−t1 =[

1t2 − t1

∫ t2

t1

aresultant · dt

]2.5

· (t2 − t1) (4)

Here the resultant acceleration is expressed as a multiple of localacceleration due to gravity (g units) calculated at the center of grav-ity of the dummy’s head. Time is measured in seconds. The HICwas analyzed using both 15 and 36 ms time interval. The proposedlimiting values of HIC15 and HIC36 by NHTSA are 570 and 1000,respectively (Eppinger et al., 1999).

4. Results and discussion

Numerical simulations, utilizing the identical FMVSS 213 testingconditions for frontal and near-side impact situations were com-pleted in the absence and presence of CRS misuse; for the Hybrid
III 3-year-old dummy and prototype Q3/Q3s dummies. In addi-tion, the effect of using a cross-shaped rigid ISOFIX system wasalso investigated. The simulation results were compared in termsof resultant head and chest accelerations as well as neck loads andmoments. Tables 1–4 tabulate the peak values observed for all the

1442 T. Kapoor et al. / Accident Analysis and Prevention 43 (2011) 1438–1450

Table 1Summary of peak values of various injury parameters observed for the Hybrid III 3-year-old child dummy in the absence and presence of CRS misuse for the frontal impact.

Flexible LATCH Rigid ISOFIX

No misuse Seatbelt slack No top tether No misuse Seatbelt slack No top tether

Resultant head acceleration (g’s) 59.5 63.2 60.5 47.9 54 48.5Resultant chest acceleration (g’s) 59.2 55.5 44 41.9 54.5 32.7Head injury criteria (HIC-15) 304.7 335.56 332.5 173.4 217.84 179.5Head injury criteria (HIC-36) 325.2 339 415.8 174.2 280 283.8Resultant upper neck force (N) 2260 2267 2380 1937.7 1955 1780Resultant lower neck force (N) 1473.9 1530 1589.3 776 899 754Resultant upper neck moment (N m) 33.2 30 25.1 26.8 33.9 20.5Resultant lower neck moment (N m) 143.9 143 174.2 143.4 158 137Forward head excursion (mm) 235 283 397 209 247 250

Table 2Summary of peak values of various injury parameters observed for the Q3 child dummy in the absence and presence of CRS misuse for the frontal impact.



Resultant head acceleration (g’s) 57.5 68.2 64.5 60 57 56.5Resultant chest acceleration (g’s) 55.4 50 41 45.5 52.2 37.2Head injury criteria (HIC-15) 310 430 382 256 247 242Head injury criteria (HIC-36) 345 530 578 256 411 303Resultant upper neck force (N) 2366 2324 2924 2431 2401 2398Resultant lower neck force (N) 891 1537 2055 824 931.8 764Resultant upper neck moment (N m) 16.7 19.7 13.5 18 26.5 14.9Resultant lower neck moment (N m) 123 133.3 134 120 122.25 110Forward head excursion (mm) 331 346 457 282 305 326

Table 3Summary of peak values of various injury parameters observed for the Hybrid III 3-year-old child dummy in the absence and presence of CRS misuse for the near-side impact.



Resultant head acceleration (g’s) 60.6 55.6 64.2 48.4 52.2 50.6Resultant chest acceleration (g’s) 80.5 85.2 69 42 41 42Head injury criteria (HIC-15) 239 203.8 288 172.4 180.2 172Head injury criteria (HIC-36) 147.4 133 182.5 174.6 175.3 167.2Resultant upper neck force (N) 1449.6 1547 1561 1117 1005 1041

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Resultant lower neck force (N) 1770 1987Resultant upper neck moment (N m) 102.6 97Resultant lower neck moment (N m) 193.9 179.6Lateral head displacement (mm) 213 220

njury parameters for the Hybrid III 3-year-old dummy and the3/Q3s child dummies.

.1. Qualitative analysis

Fig. 3 illustrates the configuration of the Q3 dummy at the time
hen the maximum forward excursion of the dummy’s head was
bserved for both the anchoring configurations, in the absencend presence of the CRS misuse for the frontal impact condition.

slight increase in the forward displacement of the dummy’sead was observed due to the presence of slack in the seatbelt

able 4ummary of peak values of various injury parameters observed for the Q3s child dummy

Flexible LATCH

No misuse Seatbelt slack

Resultant head acceleration (g’s) 156.5 178.4Resultant chest acceleration (g’s) 99.4 97Head injury criteria (HIC-15) 735 750Head injury criteria (HIC-36) 400 376Resultant upper neck force (N) 2159.8 1783Resultant lower neck force (N) 2134 1825Resultant upper neck moment (N m) 37.8 39.4Resultant lower neck moment (N m) 44.6 52.36Lateral head displacement (mm) 226 237

1919.5 986 1025 1047.2115.6 56.5 55.6 55.6196.5 134.3 138 141.6217 169 189 169

webbing. For the flexible LATCH configuration without top tetherstrap, a greater forward displacement of the CRS accompanied withimmense crushing of the seat buck was observed. Greater forwardexcursion of the head and neck rotation was observed for the flexi-ble LATCH configuration without the top tether strap compared tothe rigid ISOFIX configuration in the absence of the top tether.

The configuration of the Q3s dummy at the times when maxi-mum lateral displacement of the head was observed for all the CRSconfigurations is exhibited in Fig. 4. For the condition of slack in thewebbing, slightly greater lateral displacement of the dummy’s headwas noted. Contact between the CRS and the dummy’s head was

in the absence and presence of CRS misuse for the near-side impact.

Rigid ISOFIX

No top tether No misuse Seatbelt slack No top tether

156.6 58.5 53.3 5686.5 54.3 51 50.5

919 237 228 237437.7 231 175 172.8

2472.4 1125 1197 11402465 714 963 733

37.4 17.9 19.2 1643.6 37.6 34.7 35.5

227 196 209 196


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ig. 3. Maximum forward head and CRS displacement observed for the Q3 dummyf CRS misuse, under a simulated frontal impact.

bserved for the flexible LATCH configuration for both the absencend presence of the CRS misuse conditions. This contact was notbserved due to incorporation of the cross-shaped rigid ISOFIX sys-em. Foot contact with the rigid wall was observed for both theexible LATCH and the rigid ISOFIX configurations.

.2. Head and chest response

Figs. 5 and 6 illustrate respectively the resultant head and chestcceleration profiles as a function of time for both the anchoringonfigurations (with and without the CRS misuse), for the Hybrid IIInd the Q3/Q3s child dummies, under different impact situations.

.2.1. Frontal impactFor the frontal impact, for the flexible LATCH configuration,

imilar time profiles were observed for the resultant head acceler-tion due to the presence of seatbelt slack. An increase in the peakesultant head accelerations by approximately 20% for the Q3 childummy and 8% for the Hybrid III 3-year-old dummy was observedue to the presence of slack in the seatbelt webbing. Due to theresence of slack, there is an increase in the forward displacementf the head of the child, as illustrated in Fig. 3, which increases theead acceleration in the local y-axis acceleration. Tables 1 and 2 tab-late the maximum forward head excursion for the Hybrid III andhe Q3 child dummies, respectively. The forward head excursionas increased by approximately 20% for the Hybrid III 3-year-old
ummy due to the presence of slack in the seatbelt webbing.
For the numerical simulations conducted in the absence of theop tether strap, similar peak head accelerations were noted forhe Hybrid III 3-year-old dummy. For the Q3 child dummy, a 10%ncrease in the resultant head accelerations was observed due to

oth the flexible LATCH and rigid ISOFIX configuration, in the absence and presence

the absence of the top tether strap. A delay in the peak valuesoccurred for the simulations in the absence of the top tether due togreater forward displacement of the CRS. Additionally, these peakvalues were observed for a longer duration of time compared tothe non-misuse condition. Greater forward head excursions wereobserved for both the child dummies. The forward head excursionswere observed to be increased by approximately 70% for the HybridIII 3-year-old dummy and 40% for the Q3 dummy, respectively.This enhances the probability of a child to procure contact basedhead injuries. Similar time profiles with similar peak values of theresultant chest acceleration were observed for both the Hybrid III3-year-old and the Q3 child dummies, in the absence and presenceof CRS misuse.

Tables 1 and 2 also tabulate the values of head injury criteriaevaluated using both a 15 ms and a 36 ms window. An increase inHIC15 by approximately 10% for the Hybrid III 3-year-old dummyand 30 to 40% for the Q3 child dummy was noted due to both formsof CRS misuse. This increases the probability of a child to acquireinertia based head injuries.

A reduction in the resultant head accelerations by approxi-mately 20% was observed for both the child dummies due to theuse of cross-shaped rigid ISOFIX instead of the flexible LATCH sys-tem, in the absence and presence of CRS misuse. Specifically, forthe numerical simulations conducted in the absence of the toptether strap, a reduction in the forward head excursion by approxi-mately 60% for the Hybrid III 3-year-old dummy and 40% for the Q3
child dummy was observed due to the use of rigid ISOFIX systeminstead of the flexible LATCH configuration. Incorporation of thecross-shaped rigid ISOFIX system exhibited a reduction the resul-tant chest accelerations. In the absence of the top tether strap, areduction in the resultant chest accelerations by approximately 10%


F for boo

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ig. 4. Maximum lateral head and CRS displacement observed for the Q3s dummyf CRS misuse, under a simulated near-side impact.

or the Q3 dummy and 25% for the Hybrid III 3-year-old dummyespectively was observed for the rigid ISOFIX system. In addition,reduction in HIC15 by approximately 30–50% was observed for

oth the child dummies, due to incorporation of the rigid ISOFIXystem for both the presence of slack and absence of top tether con-itions. Therefore, the cross-shaped rigid ISOFIX effectively reduceshe probability of a child to acquire both acceleration and contactnduced head injuries. Rigid ISOFIX system also reduces the effectf CRS misuse on the injury probability of a child compared to theexible LATCH system.

.2.2. Near-side impactFor the Hybrid III 3-year-old dummy in the near-side impact

ondition, similar time profiles with similar peak values werebserved for the resultant head accelerations. For the Q3s childummy, an increase in the peak resultant head acceleration bypproximately 15% was noted due to the presence of slack in theeatbelt webbing. Comparatively higher values of head accelera-ion were observed for the Q3s dummy due to the contact madeetween the dummy’s head with the rigid wall (Fig. 4). In addition,n increase in the lateral displacement of the dummy’s head bypproximately 5% was observed for both the child dummies due to
he presence in slack in the system. The absence of the top tethertrap did not significantly affect the lateral displacement of the heador both the dummies. For the Hybrid III 3-year-old dummy, anncrease in the head acceleration by approximately 7% was noted.ables 3 and 4 tabulate the peak values for the head injury cri-
th the flexible LATCH and rigid ISOFIX configuration, in the absence and presence

teria for both the child dummies in the absence and presence ofCRS misuse. The absence of the top tether strap increased the HIC15by approximately 10–20% for both the Hybrid III 3-year-old andthe Q3s child dummies. Therefore, presence of slack in the seatbeltwebbing and the absence of the top tether strap slightly increasethe probability of a child to acquire contact based and inertia basedhead injuries.

Incorporation of the cross-shaped rigid ISOFIX system illus-trated a significant reduction in the lateral displacement of thedummy’s head and the CRS compared to the flexible LATCH con-figuration. A reduction in the resultant head accelerations ofapproximately 25% for the Hybrid III 3-year-old dummy and 65% forthe Q3s dummy, respectively, was observed for the cross-shapedrigid ISOFIX configuration in the absence of the top tether strap.Use of the cross-shaped rigid ISOFIX system exhibited a reductionthe resultant chest accelerations for the near-side impact condition.A reduction of approximately 50% in the resultant chest accelera-tions was observed for both the child dummies, in the absence andpresence of misuse, due to incorporation of the rigid ISOFIX systemcompared to the flexible LATCH configuration.

In addition, a reduction in the HIC15 by approximately 65–75%was observed for the rigid ISOFIX configuration in the absence and
presence of CRS misuse, compared to the flexible LATCH config-uration. Use of rigid ISOFIX system also illustrated a reduction inthe lateral displacement of the dummies’ head by approximately20% in the absence and presence of the CRS misuse. Thus basedon the head response of both the dummies it can be stated that


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Fig. 5. Resultant head acceleration as a function of time, for the Hybrid III 3-year-old and Q3/Q3s child dummies, for both the flexible LATCH and rigid ISOFIX configuration,in the absence and presence of CRS misuse, under a simulated frontal and near-side impact.

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Fig. 6. Resultant chest acceleration as a function of time, for the Hybrid III 3-year-old and Q3/Q3s child dummies, for both the flexible LATCH and rigid ISOFIX configuration,in the absence and presence of CRS misuse, under a simulated frontal and near-side impact.


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ig. 7. Resultant upper neck forces as a function of time, for the Hybrid III 3-year-oln the absence and presence of CRS misuse, under a simulated frontal and near-side

he use of rigid ISOFIX system reduces the probability of a childo acquire contact based and acceleration induces head injuries inhe event of a side impact. It also limits the effect of CRS misusen the injury probability of a child compared to the flexible LATCHystem.

.3. Neck response

Figs. 7 and 8 respectively illustrate the resultant upper neckorces and moments as a function of time for both the anchoringonfigurations, in the absence and presence of the CRS misuse, foroth the Hybrid III 3-year-old and the Q3/Q3s child dummies, underifferent impact situations.

.3.1. Frontal impactFor the frontal impact condition, an increase in the resultant

pper neck forces by approximately 5% for the Hybrid III 3-year-oldummy and 25% for the Q3 dummy respectively was observed dueo the absence of the top tether strap. The presence of the seatbeltlack did not significantly affect the forces recorded at the uppereck load cell for both the dummies. Tables 1 and 2 also tabulatehe peak values for the resultant lower neck forces for both theybrid III 3-year-old and the Q3 child dummies. For the Q3 dummy,

he presence of the CRS misuse increased the resultant lower neckorces by approximately 75–85%. Similar time profiles with similareak values were observed for the resultant upper neck momentsor both the dummies, in the absence and presence of CRS misuse.n increase in the resultant upper neck moments by approximately
0–15% was observed for both the child dummies in the absence ofhe top tether strap. Greater amount of neck rotation was observedor the Q3 dummy (Fig. 3) due to the absence of the top tethertrap. Extreme hyper flexion leads to rupture of the tectorial mem-rane and separation of vertebrae which leads to AOD (Dublin et al.,
Time (ms)

Q3/Q3s child dummies, for both the flexible LATCH and rigid ISOFIX configuration,ct.

1980). Therefore, the absence of the top tether strap increases theprobability of a child to acquire inertia induced neck injuries.

Use of the cross-shaped rigid ISOFIX system illustrated a reduc-tion in the resultant upper neck forces by approximately 20–25%and the resultant lower neck moments by approximately 20% forboth the child dummies, in the absence and presence of the CRSmisuse. Therefore, the cross-shaped rigid ISOFIX effectively reducesthe probability of a child to acquire inertia based neck injuries inthe absence and presence of CRS misuse.

4.3.2. Near-side impactFor the near-side impact condition, an increase in the resultant

upper and lower neck forces by approximately 8% for the Hybrid III3-year-old dummy and 15% for the Q3s dummy respectively wasobserved due to the absence of the top tether strap. For the HybridIII 3-year-old dummy, an increase in the resultant upper neck forcesby approximately 15% was observed due to the presence of the slackin the seatbelt webbing. Similar time profiles were observed forthe resultant upper neck moments for both the child dummies inthe absence and presence of the CRS misuse. Comparatively highervalues of neck moments were observed for the Hybrid III 3-year-old child dummy compared to the Q3/Q3s dummies. This can beattributed to the difference in the geometry and the stiffness ofthe neck for both the dummies. The neck of the Q3/Q3s dummies isapproximately 14 mm longer than the neck of the Hybrid III 3-year-old dummy. In addition, the neck cable in the Hybrid III 3-year-olddummy is approximately 10 times stiffer than the neck cable in theQ3/Q3s child dummies.

A reduction of approximately 35% for the Hybrid III 3-year-olddummy and 55% for the Q3s child dummy in the resultant upperneck forces was observed due to incorporation of rigid ISOFIX sys-tem. Additionally, use of cross-shaped rigid ISOFIX system reducedthe resultant upper neck moments by approximately 50% for both


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ig. 8. Resultant upper neck moments as a function of time, for the Hybrid III 3-year-n the absence and presence of CRS misuse, under a simulated frontal and near-side

he dummies, in the absence and presence of CRS misuse. There-ore, a rigid ISOFIX system decreases the probability of a child tocquire severe neck injuries in the event of a near-side impact.

. Conclusions

Numerical simulations were conducted incorporating a HybridII 3-year-old dummy and Q3/Q3s prototype dummies, in thebsence and presence of CRS misuse for both frontal and near-sidempact conditions. Two types of misuse were investigated: pres-nce of slack in the seatbelt webbing and absence of the top tethertrap. In addition, the effect of using a cross-shaped rigid ISOFIX sys-em was also investigated. Through the quantitative and qualitativenalysis, following conclusions can be stated:

. For CRS configurations having a flexible LATCH system, anincrease in the peak resultant head accelerations by approxi-mately 12% for the frontal impact condition and approximately15% for the near-side impact condition (considering only the Q3sdummy) was observed due to the presence of slack in the seat-belt webbing. Utilizing a rigid ISOFIX system resulted in either aminor increase or reduction in the peak resultant head acceler-ations when averaging the response of both types of dummiesconsidered in this study.

. For the Q3 child dummy, a 12% increase in the resultant headaccelerations was observed due to the absence of the top tetherstrap using a flexible LATCH configuration. In the case where arigid ISOFIX system was utilized, peak resultant head accelera-
tions were found to reduce by a minor extent due to the absenceof the top tether strap.
. The forward head excursion was increased by approximately20% for the Hybrid III 3-year-old dummy due to the presence ofslack in the seatbelt webbing. For the “no top tether” condition,

Time (ms)

d Q3/Q3s child dummies, for both the flexible LATCH and rigid ISOFIX configuration,ct.

the forward head excursions were observed to be increased byapproximately 70% for the Hybrid III 3-year-old dummy and 40%for the Q3 dummy, respectively. This enhances the probabilityof a child to acquire contact based head injuries.

4. An increase in HIC15 by approximately 30–40% for the frontalimpact and 10–20% for the near-side impact respectively wasobserved for the Q3 child dummy due to both forms of CRS mis-use. Therefore the presence of slack in the system and absence ofthe top tether strap increase the probability of a child to acquireacceleration induced head injuries.

5. For both frontal and near-side impact conditions, an increasein the resultant upper neck forces by approximately 5–10% forthe Hybrid III 3-year-old dummy and 15–25% for the Q3 dummyrespectively was observed due to the absence of the top tetherstrap. In addition, an increase in the resultant upper neck forcesby approximately 15% was observed due to the presence of theslack in the seatbelt webbing for the near-side impact situa-tion. Greater amount of neck rotation was observed for both thedummies in the absence of the top tether strap.

6. Use of cross-shaped rigid ISOFIX system reduced the resultanthead accelerations by approximately 20% for the frontal impactsand 25–65% for the near-side impacts, respectively. In addition,the forward head excursions were reduced by approximately40–60% during a frontal impact and the lateral displacement ofthe head was reduced by approximately 20% for near-side impactcondition, for both the absence and presence of the CRS misuse.Thus a cross-shaped rigid ISOFIX system effectively decreasesthe probability of a child to acquire acceleration and contact
based head injuries.
7. Use of the cross-shaped rigid ISOFIX system illustrated a reduc-tion in the resultant upper neck forces by approximately 20–25%and the resultant lower neck moments by approximately 20%for both the child dummies, in the absence and presence of the

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CRS misuse for frontal impacts. For the near-side impacts, use ofcross-shaped rigid ISOFIX system reduced the resultant upperneck moments by approximately 50% for both the dummies,in the absence and presence of CRS misuse. Thus the cross-shaped rigid ISOFIX effectively reduces the probability of a childto acquire inertia based neck injuries in the absence and presenceof CRS misuse.

. In similar testing conditions, it was observed that the Q3 and Q3sdummies experienced greater amounts of neck flexion than theHybrid III 3-year-old dummy. This leads to greater head excur-sions and the increased possibility of contact related head andinertia neck injuries, both of which are main reasons for childinjuries in automobile crashes. This finding indicates that theQ3/Q3s dummies may better predict child kinematics in vehic-ular crashes.

cknowledgements

Financial support provided by the Auto21 Network Centres ofxcellence is gratefully acknowledged. The in-kind contributionsrom Graco Corporation are greatly appreciated. The conclusions,pinions and analyses expressed herein do not necessarily state oreflect those of Graco Corporation.

ppendix A. Numerical model development

.1. CRS modeling

The child seat was modeled using Computer Aided Design (CAD)urfaces provided by the Century/Graco Corporation. Tensile test-ng was completed in accordance to ASTM D638M (ASTM, 2004)n specimens extracted from various portions of the CRS in ordero determine the mechanical characteristics of the CRS polypropy-ene material under large deformation. The thickness of the CRS

as measured using a vernier calliper and was observed to be 3.5nd 4.5 mm, respectively for different panel sections. Two differentection properties were generated for the CRS, both incorporatinghe Belytschko-Tsay shell elements that were assigned a thicknessf 3.5 and 4.5 mm for both the regions of the CRS. An isotropiclastic–plastic material model that utilizes the Von Mises yield cri-erion was selected to model the material behaviour of the CRSolypropylene. Values for the density, Young’s modulus and Pois-on’s ratio were 800 kg/m3, 0.842 GPa and 0.3, respectively. Theesh of the child seat was comprised of 12,728 nodes and 13,379

hell elements.

.2. Seatbelt restraints and anchoring techniques

The seatbelt was modeled to pass through a series of open-ngs and channels of the CRS and to fit around the child dummy

hich had the same configuration as outlined in the FMVSS 213tandards (FMVSS, 2001). To reduce computational time, a portionf the seatbelt which generally contacts the back of the CRS wasodeled using one-dimensional seatbelt elements. Connectivity to

he one-dimensional seatbelt elements and the two dimensionalhell elements used for modeling contact between the seatbelt andummy was completed through the use of nodal rigid bodies. The

ower ends of the three portions of the seatbelt that passed throughhe child dummy’s groin and wrapped around the child dummy’segs were connected directly to the CRS.

To assess the mechanical characteristics of the seatbelt webbing,ensile testing was completed on specimens extracted from the CRSnder a loading/unloading condition in order to obtain informationn the behaviour of the seatbelt. These tests were completed on aydraulic Tinius-Olsen testing machine at room temperature at a

Prevention 43 (2011) 1438–1450

nominal crosshead speed of approximately 25 mm/min. The load-ing/unloading behaviour of the seat belt webbing as extracted fromthe tensile specimens was incorporated into the material model forthe CRS webbing. The loading/unloading behaviour of the seat beltwebbing as extracted from the tensile specimens is illustrated inFig. A.1(a) and was incorporated into the material model for theCRS webbing.

A fabric material model within LS-DYNA was selected to modelthe material behaviour of the two dimensional seatbelt finiteelements. The density, elastic modulus and Poisson’s ratio werespecified as 890.6 kg/m3, 2.068 GPa and 0.3, respectively. In addi-tion, a stress versus strain relationship for the material wasgenerated from experimental observations and implemented intothe material model. For the 1 dimensional seatbelt finite elements,a material model specifically developed for this element formula-tion was utilized. The experimental loading/unloading behaviourwas incorporated in the material model for the seatbelt elements.A fully integrated Belytschko-Tsay membrane element formulationwas utilized for the two dimensional shell elements of the seatbelt.An element formulation specifically developed for 1 dimensionalseatbelt elements was used for the top tether, lower anchor-age and tethers for children (LATCH), and portion of the CRSseatbelt.

The buckle in front of the dummy’s chest was modeled to per-mit sliding motion of the webbing. The buckle at the waist of thedummy was modeled on the assumption that there was no relativemovement between the waist buckle and the seatbelt; thus, thenodes at the ends of the waist buckle were merged with the nodesin the seatbelt. The material model and element formulation of thebuckles were identical to the CRS. Fig. A.1(b) illustrates the finiteelement model of the seatbelt, LATCH, top tether, and the five-pointrestraint system.

In addition to the flexible LATCH system, use of cross-shapedISOFIX system has also been investigated in this research. Our pre-vious work (Kapoor et al., 2008) has illustrated advantages of usinga cross-shaped ISOFIX system for near side impacts by limitingthe lateral displacement of the CRS more compared to the rect-angular shaped ISOFIX system outlined in the UNECE Regulation14 (UNECE, 2001). The ISOFIX system was modeled using fullyintegrated shell elements and was assigned material propertiesof steel (elastic modulus = 207 GPa, density = 7.8 g/cm3, Poisson’sratio = 0.3). An elastic/plastic behaviour was defined for the ISOFIXsystem with the yield strength of 210 MPa and the ultimate ten-sile strength of 475 MPa. The cross-shaped section ISOFIX systemcomprised of two mutually perpendicular rectangular sections atthe center, with the linear dimensions of 67 mm length, by 30 mmheight, by 3 mm thickness. The ISOFIX was constrained to the CRSand the vehicle seat using constrained extra nodes card in LS-DYNA.Fig. A.2 illustrates the geometry of the cross-shaped rigid ISOFIXsystem.

A.3. Modeling of the foam insert

A foam pad inserted between the polypropylene shell and theseat fabric of the CRS was also incorporated into the FE model of theCRS. The foam was modeled using a selectively reduced solid ele-ment formulation. A highly compressible low density foam materialmodel was selected to model the material behaviour of the solidelements representing the foam pad. Density and the stress/strainrelationship for the foam were experimentally determined from
compressive foam specimens. Values for the density and elasticmodulus of the material were defined as 50.2 kg/m3 and 5.463 MPa,respectively. The nonlinear stress/strain relationship (for loadingonly) was also incorporated into the material model for the foam.This material model was obtained from Turchi et al. (2004).


the finite element model of the seatbelt, LATCH, top tether, and the five-point.

A

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.4. The seat buck

The 213 seat bench consists of two different densities of foamaterial. Both the foam specimens were obtained from Magna

nternational that were acquired from an actual 213 seat bench.ompressive testing was completed on both the foam speci-ens and corresponding material models, identical to the foam

nsert, were developed. The thickness of the low density foam was00 mm, and for the high density foam was 50 mm, respectively.alues for the density of the material were defined as 23.2 kg/m3

or the low density foam and 40.1 kg/m3 for the high density foam,espectively. Fig. A.3 illustrates the stress versus strain behaviour ofoth the foam specimens. These curves (loading only) were incor-
orated in the input file to define the material model. Both theeformable parts of the seat buck were modeled using solid ele-ents and an under-integrated element formulation was specified
or the solid elements of the seat buck. In addition, a rigid seat buck

ig. A.2. Different anchoring methods: (left) flexible LATCH, and (right) rigid ISOFIXross-shaped section.

Fig. A.3. Engineering stress versus engineering strain curves for the low density andthe high density 213 seat bench foam.

frame was modeled using shell elements. A rigid material defini-tion was selected to model the seat buck frame. The seat buck foamand frame were connected by constraining the outermost nodes ofthe seat pad to the frame. The kinematics of these nodes on the seatbuck pad will be identical to the seat buck frame.

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Documents

A numerical investigation into the effect of CRS misuse on the injury potential of children in frontal and side impact crashes