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Child safety analysis for forward-facing child restraintsystem in frontal impactA. Zhao a , S.-W. Hong a , C.-K. Park a , C. D. Kan a , S. H. Park b & H. Bae ba FHWA/NHTSA National Crash Analysis Center, Department of Civil and EnvironmentalEngineering , The George Washington University , 20101 Academic Way, Ashburn, VA, 20147,USAb Advanced Technology Center, Hyundai-Kia Motors , 772-1, Jangdeok, Hwaseong, Gyeonggi,445-706, South KoreaPublished online: 29 Apr 2009.

To cite this article: A. Zhao , S.-W. Hong , C.-K. Park , C. D. Kan , S. H. Park & H. Bae (2009) Child safety analysis forforward-facing child restraint system in frontal impact, International Journal of Crashworthiness, 14:2, 151-163, DOI:10.1080/13588260802614340

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International Journal of CrashworthinessVol. 14, No. 2, April 2009, 151–163

Child safety analysis for forward-facing child restraint system in frontal impact

A. Zhao,a S.-W. Hong,a∗ C.-K. Park,a C. D. Kan,a S. H. Parkb and H. Baeb

aFHWA/NHTSA National Crash Analysis Center, Department of Civil and Environmental Engineering, The George WashingtonUniversity, 20101 Academic Way, Ashburn, VA 20147, USA; bAdvanced Technology Center, Hyundai-Kia Motors, 772-1, Jangdeok,

Hwaseong, Gyeonggi 445-706, South Korea

(Received 23 October 2007; final version received 9 November 2008)

The objective of this paper is to investigate the design of a forward-facing child restraint system (CRS) and related vehiclecomponents through the study of three-year-old children in frontal impact. In this research, the finite element (FE) modelof a five-point harness child restraint system was developed and a three-year-old Hybrid III dummy in MADYMO formatwas used along with the extended coupling method with LS-DYNA and MADYMO. This paper specifically addresses theparametric study of a five-point harness CRS and LATCH (lower anchors and tethers for children) system for forward-facingCRS in frontal crashes. The parameters include a chest clip, lower anchor straps and a seat pad. Furthermore, a new LATCHtype is suggested, in which supplementary webbing was considered. The influence of preloading condition of a harnesson a child dummy was explored as well. These results demonstrate the feasibility of the improved safety by devising anappropriate CRS.

Keywords: forward-facing child restraint system (CRS); child safety analysis; LS-DYNA, MADYMO; lower anchors andtethers for children (LATCH); frontal impact

Introduction

Based on the findings from Partners for Child PassengerSafety (PCPS), it is evident that more than 1.5 million chil-dren in the United States were involved in crashes eachyear and more than 225,000 of these children were injured,25,500 of them seriously injured. The face and head arethe most commonly injured body regions [5]. These datafurther show half of fatalities occurred in frontal crashes.However, over the years child restraint system (CRS) hasplayed an important role in reducing child injury in col-lision; although its safety performance still needs to beimproved. For forward-facing CRS systems, head injuryhas been a serious concern [1]. From the database of Na-tional Automotive Sampling System (NASS) in NationalHighway Traffic Safety Administration (NHTSA), it wasdetermined that, in US New Car Assessment Program(NCAP) from 2001 to 2005 with three-year-old childrenin forward-facing CRS, 25.55% of passenger cars, 30.5%of SUVs and 32.6% of light trucks failed to keep the HIC15limit of 570 [2]. Therefore, it is necessary to study the ef-fectiveness of current lower anchors and tethers for chil-dren (LATCH) system for a forward-facing CRS in frontalcrashes.

The objective of this research is to investigate the effectof safety configurations of CRS for three-year-old children.In this study, a CRS finite element (FE) model was devel-

∗Corresponding author: Email: swhong@ncac.gwu.edu

oped with Hybrid III MADYMO model of a three-year-old.This work focuses mainly on parametric study of a forward-facing CRS, including a chest clip, lower anchor straps anda seat pad, and the influence of preloading was also con-sidered. Furthermore, the results of parametric study werediscussed in detail.

Model development and simulation analysis

CRS model configuration

As shown in Figure 1, the model was composed of a rearseat, a LATCH system with CRS (Evenflo Vangard) and Hy-brid III MADYMO dummy of a three-year-old. FE model ofCRS was digitised and modelled. A five-point harness hasa webbing strap over shoulders, pelvis and between the legswith all five coming together at a common buckle. LATCHsystem consists of two lower attachments on a child safetyseat that anchor and connect to lower anchors and a toptether is at the vehicle’s back seat in the baseline FE model.The CRS FE model also includes a pad on the seat and achest clip through which the webbing is threaded. The rearseat model taken from a sport utility vehicle was comprisedof a seat frame and a seat cushion. The material type ofthe cushion is a low-density foam. The dummy was posi-tioned on the CRS and wrapped by a five-point harness.The child seat was anchored at the lower anchorage of the

ISSN: 1358-8265Copyright C© 2009 Taylor & FrancisDOI: 10.1080/13588260802614340http://www.informaworld.com

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Figure 1. Isometric views of LS-DYNA/MADYMO model.

seat and the tether was anchored at the middle of the back ofthe seat.

Development of finite element model of child safetyanalysis in frontal impact

A frontal impact was simulated by specifying the acceler-ation field to the child model, while the vehicle seat wasmounted to a reference space as shown in Figure 2, whereZ-direction is θ = 0◦ and X-direction is θ = +90◦. Appliedpulses were extracted from the frontal NCAP test database[6]. In the test, the dummy was placed on the child seat(Evenflo Titan 5) in the left side of the rear seat. Actual X-component and Z-component of the acceleration measuredat the rear floor of the vehicle were plotted in Figure 3 andapplied at the mount location of FE seat frame model shownin Figure 2. All curves were filtered with SAE60 filter.

The extended coupling method with LS-DYNA andMADYMO was utilised in simulations defining the contact

Figure 2. Side view of model.

between MADYMO dummy and FE model in MADYMOside. In order to check the contact between vehicle compo-nents and the dummy, full car simulation with the dummyin CRS was performed first. From the simulation results ofthe head and foot excursions of the dummy, the interactionbetween vehicle components and the dummy was not found,therefore only the rear seat was selected from the vehiclefor the research of a three-year-old child in frontal impact.

The entire FE model consists of 146,200 elements with74,200 nodes. Detailed modelling of the chest clip wasconsidered in order to enable harnesses to slide through thechest clip smoothly with proper edge contacts. The chestclip was located at the armpit level of the child dummy. A5-mm thin rectangular box along with curved surfaces in-side with 2.5 mm gap was generated to represent the chestclip. The contact surface was defined between curved sur-faces and harnesses. Furthermore, the friction coefficientin this contact surface was considered to influence the per-formance of the chest clip. The comparison was presentedin the following section of this parametric study.

The baseline model (case 1) in all tables and figures ofthis paper was configured by (1) applying pre-analysis, (2)selecting the friction coefficient (0.1) of the chest clip and(3) attaching deformed lower anchor strap.

Two main modelling strategies were considered: one isto perform pre-analysis before applying the accelerationsin order to remove the slack among the CRS, the dummy,and the rear seat; the other is to choose an appropriate chestclip. The effect of these strategies is shown in the followingsection.

Model validation

In order to validate the simulation model, the dummyresponses in simulations are compared with those in theNCAP test #5315. In Figure 4(a), the accelerations ofdummy head are compared. Resultant and Z-accelerations

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Figure 3. Crash pulses at the rear floor pan of vehicle from NCAP test #5315 [4].

are very close, but the peak times and values of X-acceleration are somewhat different. Both HIC36 andHIC15 are compared in Figure 5(a). In Figure 4(b), the ac-celerations and deflections of dummy chest are compared.The peak values of chest acceleration are different but theirtrends are similar, especially after peak time. The initialdeflection of dummy chest in the simulation is higher thanone in the NCAP test since there is preloading caused bythe pre-analysis on the dummy chest. The peak value ofchest deflection in simulation is somewhat lower than theone in NCAP test. Figure 5(b) shows that the peak valuesof acceleration and deflection of chest are lower than thosein NCAP test. In Figure 4(c), the forces and moments ofdummy neck are compared. The Z-forces of dummy neckare very close, but the peak times and values of X-force ofdummy neck are different. The Y-moments of dummy neckare shown here for completeness, but they are not very sig-nificant in this study since their values are quite small all thetime. Comparable values of dummy neck are summarised inTable A1 in the Appendix. The values in simulation arelower than those in the NCAP test. Above comparisonsshow that the overall responses of the dummy in thesimulation are fairly analogous to those of the NCAP test.

Parametric study

The parametric study of the CRS includes finding the effectof the pre-analysis, the chest clip, the pad, and the lower an-chor strap. All cases in the parametric studies were modified

from the baseline. Table A1 in the Appendix summarisesall values of the injury parameters for all cases.

Effect of pre-analysis

In order to remove the slack between CRS and the dummyand between the dummy and the rear seat, pre-tensionedsprings were attached in each lower anchor strap, harnessesand tether. The pre-analysis was performed for 50 ms. Thestrap and tether had a tensional load of 20 N. The end of eachshoulder harness also had a pre-tensioned spring to removethe slack between the CRS and the dummy. Each harnessstrip had tensional load about 5 N after the pre-analysis. Asshown in Figure 2, the CRS is inclined backward about α =25◦ after the pre-analysis. Figure 6 depicts the effect of theslack on each calculated injury parameter. Table A1 furtherindicates that each injury parameter from the case with theslack (case 2) is almost twice the one from the baseline. Theresults agree with the tests of shoulder harness slack fromTransport Canada [4]. Table 1 indicates that there is a largedifference between case 1 and case 2 in head excursions aswell as foot excursions. Additionally, as shown in Table A1,the neck peak tension and Nte in case 1 are much lower thanthose in case 2. Therefore, it is necessary to eliminate theslack in simulations. Accordingly, it should be compulsoryto ask consumers to tighten CRS in the vehicle.

Effect of chest clip

The baseline model (case 1) includes the chest clip to retainshoulder harnesses in the appropriate position; however, it

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Figure 4. (a) Validation of accelerations of a dummy head. (b) Validation of responses (acceleration and deflection) of a dummy chest.(c) Validation of responses (X-, Z-neck force and Y-neck moment) of a dummy neck. (Continued.)

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Figure 4. (Continued.)

is excluded in case 4. Compared to the two cases in Figure 7,the accelerations of the head and the chest in case 1 and itschest deflection are smaller than those in case 4. Table 1indicates the chest clip has influences on the head excursion.Additionally, the neck peak tension and Nte are lower in case1 than in case 4. Therefore, the importance of the chest clipis clear from these comparisons.

In addition, the dummy performances are very sensitiveto the friction coefficients of the contact between harnessesand the chest clip. Case 1 is compared with the case ofhigh friction (case 3) listed in Table A1. Case 1 adoptsthe value of 0.1 as a static friction coefficient (FS) anddynamic (FD), while case 3 selects FS = 0.3 and FD =0.2. These differences result in a fairly different responseof the dummy since the chest clip does not slide down easilyalong the harness straps when the friction coefficient is toohigh. Figure 8 illustrates the trace history of the dummyhead during the impact. Accordingly, the values of injuryparameters in case 3 are greater than those found in case 1,

as illustrated in Figure 9. As indicated in Table A1, the neckpeak tension and Nte of case 3 are much higher than case 1.This suggests that the old harness straps should be examinedin order to ensure that the chest clip is slipping properly.

Effect of seat pad

Although there is a pad with the thickness of 10 mm inthe baseline (case 1), no pad is included in the model ofcase 5. A comparison of the first peak of head accelerationshown in Figures 10(a) and 10(b) reveals that the pad doesnot have a great influence on the dummy injury parameters.However, the responses of the dummy in the second peak ofhead acceleration are quite different in both cases. The padreduces maximum resultant acceleration of the dummy inthe second peak as the dummy’s head rebounds to the CRS,as shown in Figure 10(c). The material and the thickness ofthe pad need to be studied further.

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Figure 8. Loci of head during crash.

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Figure 9. Values of injury parameters of cases 1 and 3.

Effect of stiffness of lower anchor strap

Although the child seat is tightened to the rear seat byLATCH under preloading, it still can move along the vehi-cle seat during the frontal impact. If the movement can bereduced, it will enhance the CRS performance. Since thismovement was related to the connection of lower anchorstraps, the stiffness of the lower anchor strap was analysedby the following three cases. Case 1 is the baseline in whichthe lower anchor strap is connected with deformable beamelements. In Table 2, soft deformable beam elements (case6) and rigid beam elements (case 7) are connected to thelower anchor strap respectively. The longitudinal displace-ments in the bottom (ddown) and at the top (dup) are shownin Figure 11 and Table 3. The longitudinal displacementsof CRS in these three cases are different. Specifically, thesofter the lower anchor strap, the more the longitudinalmovement of the CRS. On the contrary, the stiffer loweranchor strap reduced the forward movement in the bottomof the CRS (ddown). Accordingly, this movement results indifferent dummy responses during the impact. Figure 12demonstrates the differences in three cases of the test re-sults. Figure 13 shows that the stiffer lower anchor strapindeed reduces the injury value of head, chest and neck. IfCRS moves less forward in the longitudinal direction, in-jury parameter values remain low. Therefore, it is necessaryto reduce the forward movement as much as possible.

Table 1. Head and foot excursion.

Excursion (mm)

Cases Head Foot

Maximum distance in NCAP #5315 720.0 850.01 Baseline 338.3 635.12 With slack 440.0 767.54 No chest clip 341.4 633.4

Table 2. Maximum movement of CRS during crash.

CRS movement (mm)

Cases ddown dup

1 Baseline 29.4 37.12 With Slack 133.0 94.06 Soft LATCH 40.1 43.17 Rigid LATCH 7.5 32.6

LATCH system improvement

The parametric study in the previous section concludes thatthe LATCH system can be further improved by reducing theforward movements of the CRS. Thus, an addition of sup-plementary webbings to lower anchorage is proposed in thispaper. Essentially, when two webbings fasten to the CRSwith 45◦ to the horizontal line, it can effectively restrainCRS in both upward and forward directions. However, ifthe angle of the lower anchor strap of the baseline is morethan 45◦ from the horizontal line as shown in Figure 14,then it cannot restrain the CRS from forward movement.If an auxiliary lower anchor strap is added in a horizontaldirection as depicted in Figure 14(a), it can help prevent theCRS from moving forward. Based on this concept, an im-proved LATCH system is suggested as shown in Figure 14,

Table 3. Maximum movement of CRS during crash.

CRS movement (mm)

Cases ddown dup

1 Baseline 29.4 37.18 Additional 14.9 35.98-1 webbing 11.5 37.1

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Figure 11. Specific positions of movement of CRS.

where additional webbing is attached to lower anchorages.Furthermore, study on the CRS with new LATCH system intwo preloading methods is performed. Two cases are studiedto consider the order of fastening lower straps and tetherby (1) applying preloading to two lower anchor straps andthe tether simultaneously (case 8), (2) applying preloadingto the lower anchor straps first and then to the tether (case8-1). Examples (1) and (2) are indicated in Figures 15 and16 and Table A1. The differences in the two cases can befound in the following section.

Preloading to two lower anchor straps and thetether simultaneously

Preloading causes the child seat to rise from the vehicleseat slightly. The accelerations of the head shown in Figure15 are lower than those in the test as specified in this paper.The head and chest response values of this case (case 8)

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are listed in Figure 16, Table 3 and Table A1. As predicted,the CRS movements are reduced in the modified LATCHsystem as seen in Table 3. All head, chest and neck pa-rameter values of case 8 are much lower than the baselineexcept for the chest acceleration that is slightly greater than

the baseline. Specifically, the HIC15 is 280 in the modifiedLATCH system, while HIC15 is 391 in the baseline. More-over, the neck peak tension and Nte are greatly reduced incase 8, where Nte goes down from 1.77 to 1.4, while neckpeak tension is reduced by 335 N (from 1882 N to 1547 N).

Figure 12. (a) Resultant acceleration profiles of dummy head in cases 6 and 7. (b) X-acceleration profiles of dummy head in case 6 and7. (c) Z-acceleration profiles of dummy head in cases 6 and 7. (Continued.)

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Figure 12. (Continued.)

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Figure 14. Additional belt locations.

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Preloading to the lower anchor straps first andthen to tether

In this case (case 8-1), the values of the injury param-eters including head, chest and neck are still lower than

the baseline. However, both HICs and neck injuries arehigher than case 8. This implies that the upright positionof the seat is better for keeping children safe in frontalcrashes.

Figure 15. (a) Resultant acceleration profiles of dummy head in cases 8 and 8-1. (b) X-acceleration profiles of dummy head in cases 8and 8-1. (c) Z-acceleration profiles of dummy head in case 8 and 8-1. (Continued.)

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Figure 15. (Continued.)

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Conclusions

A parametric study was performed and a modified LATCHof the forward-facing CRS was explored for the frontalimpact by FE analysis. The results provided a qualitativeindication of the sensitivity of a Hybrid III dummy for3-year-old to the configurations of the forward-facing CRSin frontal impact. The observations from this study are listedbelow:

1. A chest clip plays an important role in child safety inthe forward-facing CRS: a chest clip should slide downproperly. Therefore, old and worn harness straps shouldbe examined.

2. Stiffer lower anchor straps decrease the forward move-ment of the CRS in frontal impact and diminish dummyreadings, especially head acceleration.

3. Pad on the child seat lessens the maximum value ofthe acceleration of the head in second peak when thedummy’s head rebounds to the child seat in frontalcrashes.

4. The additional lower anchor strap can reduce the forwardmovement of the child seat along the vehicle rear seatso that it significantly decreases the values of injuryparameters of the dummy, especially in head and neck.It may also benefit other impact modes, for example,rollover and side impacts.

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5. The order of fastening lower straps and tether fairly af-fects the performance of CRS. It implies that the order oftightening to lower anchor strap and tether to vehicles isimportant. This should be emphasised in the instructionof CRS installation.

AcknowledgementsThis research was funded by Hyundai-Kia Motors.

References[1] K. de Jager and M. van Ratingen, Assessing new child dum-

mies and criteria for child occupant protection in frontalimpact, The 19th International Technical Conference on theEnhanced Safety of Vehicles (ESV), Paper no. 05-0157,Washington D.C., June 6–9, 2005.

[2] R. Eppinger, E. Sun, and S. Kuppa, Development of improvedinjury criteria for the assessment of advanced automotiverestraint systems — II, National Highway Traffic SafetyAdministration, National Transportation Biomechanics

Appendix

Research Center (NTBRC), Roger Saul, National HighwayTraffic Safety Administration Vehicle Research & Test Cen-ter (VRTC) Supplement, March 2000.

[3] Federal Motor Vehicle Standards, Child Restraint Systems,49 CFR Part 571, [Docket No. NHTSA-03-15351], FederalRegister/vol. 68, No. 121/Tuesday, June 24, 2003/Rules andRegulations.

[4] S. Lalande, F. Legault, and J. Pedder, Relative degradationof safety to children when automotive restraint systems aremisused, The 18th International Technical Conference on theEnhanced Safety of Vehicles (ESV) Proceedings, Nagoya,Japan, May 19–22, 2003.

[5] R. Menon, Y. Ghati, and S. Ridella, Evaluation of restrainttype and performance tested with 3- and 6-year-old HybridIII dummies at a range of speeds, SAE Technical PaperSeries 2004-01-0319, 2004.

[6] NHTSA database, http://www-rd.nhtsa.dot.gov/database/aspx/vehdb/querytesttable.aspx

[7] Q. Wang, T. Kapoor, and W. Altenhof, Use of rigid and de-formable child restraint seats in finite elements simulationsof frontal crashes, 2006 SAE World Congress, Paper no.2006-01-1141, Detroit, Michigan, USA, SAE International,April 3–6, 2006.

Table 1A. Injury criteria for Hybrid III 3-year-old child dummy and injury parameters of NCAP test and all simulation cases.

Head criteriaa Thoracic criteria Neck criteriaa

Nij

HIC HIC Chest Chest Peak Peak(15 ms) (36 ms) acceleration deflection Ntf Nte Ncf Nce tension compressiona

CASES < 570 – < 55 G < 34 mm <1.0 <1.0 <1.0 <1.0 <1130 N <1380 NNCAP TEST #5315 369 657 49.3 14.6 0.8 1.21 0.01 0.63 2052 315

Vehicle rear seat model with NCAP pulse1 Baseline 391 597 33.1 12.8 0.32 1.77 0 0.21 1882 312 With slack 754 1079 71.8 21.7 0.13 2.68 0 0.43 3041 413 High friction 513 729 44.0 14.5 0.18 2.70 0.45 0.22 2463 4784 Without chest clip 501 702 37.4 15.6 0.50 1.87 0 0.22 2039 455 Without pad 393 552 36.4 12.1 0.16 1.79 0 0 1897 06 Soft LATCH 416 620 35.4 14.0 0.40 1.83 0 0.19 1907 257 Rigid LATCH 310 506 31.2 10.7 0.23 1.68 0 0.23 1714 498 Additional belt 280 483 40.6 11.3 0.17 1.40 0 0.25 1547 788-1 367 534 33.4 9.8 0.21 1.60 0 0.24 1856 70

aValues are calculated within the period of 0–160 ms.bCriteria of FMVSS 213: HIC (36 ms) < 1000, chest acceleration < 60 Gs, and head and foot excursions < 720 mm and 915 mm, respectively. Otherinjury criteria are adopted from [2].cOnly acceleration curves of dummy head in Figure 6 and HIC (15 ms) are mentioned in the reference [7].dHead and foot excursions are 374 mm and 748 mm, respectively.

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