applied sciences Article EffectArticle of Seat Condition on Abdominal Injuries to Effect of Seat Condition on Abdominal Injuries to VehicleVehicle Occupants inin FrontalFrontal ImpactImpact AccidentsAccidents Yasuhiro Matsui 1,* and Shoko Oikawa 2 Yasuhiro Matsui 1,* and Shoko Oikawa 2 1 National Traffic Safety and Environment Laboratory, 7-42-27 Jindaiji Higashi-machi, Chofu, 1 National Traffic Safety and Environment Laboratory, 7-42-27 Jindaiji Higashi-machi, Chofu, Tokyo 1820012, Japan Tokyo 1820012, Japan 2 Faculty of Systems Design, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino-city, Tokyo 1910065, 2 Faculty of Systems Design, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino-city, Tokyo 1910065, Japan; Japan; [email protected] [email protected] * Correspondence: [email protected]; Tel.: +81-422-41-3371 * Correspondence: [email protected]; Tel.: +81-422-41-3371

Received: 21 September 2018; Accepted: 20 October 2018; Published: 25 October 2018


Received: 21 September 2018; Accepted: 20 October 2018; Published: 25 October 2018
 Abstract: Vehicle occupants were killed in 33% of all traffic accidents in Japan in 2017. Of the vehiclesAbstract: inVehicle vehicle occupants-to-vehicle were accidents killed in, 54% 33% ofwere all traffic impacted accidents from in the Japan front. in 2017. In frontal Of the vehicles impact accidents,in vehicle-to-vehicle when the lap accidents, belt moves 54% away were from impacted the iliac fromcrests the of the front. pelvis In of frontal a vehicle impact occupant, accidents, the beltwhen moves the lapdirectly belt movesinto the away abdomen. from Here, the iliac we investigated crests of the causes pelvis of of a abdominal vehicle occupant,injuries to thevehicle belt occupantsmoves directly, because into the the abdomen abdomen. is Here,associated we investigated with the highest causes rates of abdominalof severe injury injuries and to fatality. vehicle Theoccupants, purpose because of this study the abdomen was to clarify is associated the correlation with the between highest downward rates of severe movement injury andof the fatality. seat andThe of purpose the lap ofbelt this away study from was the to iliac clarify crests the correlationof a human betweenoccupant downward of a car, in movementthe event of of a thefrontal seat impact.and of theWe lap investigated belt away fromthis phenomenon the iliac crests by of conducting a human occupant simulations of a using car, in an the anthropomorphic event of a frontal 50thimpact. percentile We investigated male (AM50 this) phenomenonhuman model by wearing conducting a three simulations-point seatbelt. using an We anthropomorphic set two deformable 50th spercentileeat conditions: male V (AM50)ertical movement human model and wearinglean forward a three-point movement. seatbelt. Our results We set revealed two deformable that the seatlap beltconditions: came off Vertical from both movement of the iliac and crests lean forwardduring lean movement. forward Ourmovement results but revealed only from that theone lap of beltthe iliaccame crests off from during both vertical of the iliac movement. crests during We concluded lean forward that movement abdominal but injuries only from can one be caused of the iliac by downwardcrests during movement vertical movement. together with We concludedforward rotation that abdominal in the seat injuries during can vehicle be caused-to-vehicle by downward frontal impacts.movement together with forward rotation in the seat during vehicle-to-vehicle frontal impacts.

KKeywords:eywords: humanhuman model; model; abdominal abdominal injury; injury; frontal frontal impact impact accidents; accidents; simulation simulation

1.1. Introduction Introduction VehicleVehicle occupants occupants were were kill killeded in in 33% 33% (1221) (1221) of of all all traffic traffic accidents accidents (3694) (3694) in in Japan Japan in in 2017 2017 [1] [1] ((FigureFigure 11).). Therefore, Therefore, countermeasures countermeasures to to reduce reduce fatal fatal traffic traffic accidents accidents are required. are required. In Japan, In Japan, when roadwhen-users road-users are killed are killedor injured or injured in traffic in traffic accidents, accidents, police police officers officers and medical and medical doctors doctors determine determine the mainthe main body body regions regions associated associated with with severe severe injuries injuries that that led led to to death. death. R Resultsesults of of these these investigations investigations revealreveal serious serious and and fatal fatal injury injury rates rates for for each each main main body body region, region, presented presented in in Table Table 11.. The abdominal regionregion is is associated withwith thethe highesthighest raterate of of severe severe injury injury and and fatality fatality of of vehicle vehicle occupants occupants (Table (Table1)[ 1)1]. [1].The The same same trend trend has has also also been been observed observed in Francein France [2]. [2]. Therefore, Therefore, in in this this study, study, we we focus focus on on causes causes of ofabdominal abdominal injuries injuries to to vehicle vehicle occupants. occupants. 2017年：状態別死亡者

Car Pedestrian 1221 1347 33% 36% 3694

Motor Bicycle cycle 480 448 Moped 13% 12% 184 5% FigureFigure 1 1.. DistributionDistribution of of車 traffic traffic自転車 fatalities fatalities原付 自動二輪 by by crash crash歩行者 type type in in Japan Japan in in 201 2017.7.

Appl. Sci. 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2047; doi:10.3390/app8112047 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2047 2 of 13

Table 1. Severe injury and fatality rates for individual body regions of vehicle occupants involved in traffic accidents in Japan in 2017.

Number Rate (%) Main Body Region (a) Minor Injured (b) Serious Injured (c) Fatality Serious Injury Fatality Rate Occupants Occupants Occupants Rate b/(a+b) c/(a+b+c) Head 9047 738 305 8 3 Face 3507 304 14 8 0 Neck 302,051 2325 67 1 0 Chest 12,348 2836 444 19 3 Abdomen 1558 511 147 25 7 Back 3009 141 3 4 0 Hip 22,329 937 36 4 0 Arms 8334 985 5 11 0 Legs 7259 1251 28 15 0 Total 369,442 10,028 1049 - -

Fifty-four percent of vehicles struck in vehicle-to-vehicle accidents in Japan were impacted from the front [3]. A US accident investigation database of occupant injuries in the front passenger seat of impacted vehicles showed that 59% of abdominal injuries were directly caused by seatbelts [4]. Two detailed accident case studies revealed that lap belts caused serious abdominal injuries and residual deformation marks were left behind on seat surfaces, which suggests that the seat moved vertically during the frontal impact [4]. Some studies have attempted to evaluate the relationship between lap belt condition and abdominal injury. Zhu et al. performed simulations using a vehicle cabin model with a rigid-seat model to investigate injury risk, considering the abdomen, for the anthropomorphic 50th percentile male (AM50) and obese male models during frontal impact [5]. In addition, Steffan et al. investigated the relationship between lap belt loading and abdominal response using an AM50 dummy and post mortem human subjects (PMHSs), by employing a rigid seat during frontal impact to facilitate the design and development of seat belt tensioning systems [6]. However, only a few studies have focused on the movement of a seat during frontal impact. Kitamura et al. investigated two accidents in which drivers sustained abdominal injuries owing to car-to-car frontal impacts [7]. These authors indicated the possible cause of abdominal injuries may be due to deformation of the seat during impact. Matsui and Oikawa investigated the relationship between seat height vertical movement and lap belt upward movement, in which an anthropomorphic 50th percentile male Hybrid III dummy model (hereafter referred to as an AM50 Hybrid III model) was seated in a passenger seat model [8]. Results revealed that when the AM50 Hybrid III model moved 150 mm vertically downward, the lap belt exhibited significant upward movement from its initial position on the dummy’s waist. This phenomenon resulted in the abdomen being directly compressed, owing to tightening of the lap belt. In the upper body structure of a Hybrid III dummy, there is a space between the thoracic and waist part [9] because the dummy does not have internal organs. Currently, Toyota Motor Corporation and Toyota Central Research and Development Labs. Inc. have jointly developed a finite element model, the total human model for safety (THUMS) version four, which includes an internal organ finite element model [10]. It is possible that dynamic movement of a seatbelt on a Hybrid III dummy may be different from that of a seatbelt worn by a human, i.e., after a lap belt moves away from the iliac crests of the human pelvis it moves deeply into the abdomen. Thus, the purpose of this study is to clarify the correlation between different downward movement conditions of the seat and those of the lap belt from the iliac crests of a human occupant in the front passenger seat of a car in the event of a frontal impact. We investigate this phenomenon by conducting simulations using an AM50 human model. Figure2 shows a photograph of residual deformation of the front passenger car seat after a Japan new car assessment program (JNCAP) full frontal rigid barrier impact test, in which a Hybrid III dummy was seated in the passenger seat during the test. The frontal edge of the seat exhibits residual Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 12

Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 12 dummyAppl. Sci. 2018 was, 8 seated, 2047 in the passenger seat during the test. The frontal edge of the seat exhibits residual3 of 13 deformationdummy was seated of 100 in mm, the whichpassenger suggests seat during the seat the movedtest. The downward frontal edge along of the with seat aexhibits small forwardresidual rodeformationtation. Therefore, of of 100 100 mm, in mm, this which whichstudy, suggests suggestswe select the thetwoseat seat movedconditions: moved downward 150 downward mm along vertical alongwith downward a withsmall a forward small movement forward rotation. of theTherefore,rotation. seat [8] Therefore,in and this 150 study, mmin this wevertical study, select downward twowe select conditions: two movement conditions: 150 mm in vertical combination 150 mm downward vertical with downward forward movement rotation. movement of the seat [of8] theand seat 150 [8] mm and vertical 150 mm downward vertical downward movement inmovement combination in combination with forward with rotation. forward rotation.

Figure 2. Photograph of residual deformation of the front passenger seat in a car after a Japan new carFigure assessment 2. Photograph program ofof residual(residualJNCAP) deformation deformationfull frontal rigid of of the the barrier front frontpassenger impact passenger test. seat seat in in a cara car after after a Japan a Japan new new car carassessment assessment program program (JNCAP) (JNCAP full) full frontal frontal rigid rigid barrier barrier impact impact test. test. 2. Materials and Methods 2. Materials and Methods 2. MaterialsAn AM50 and THUMS Methods ver.4 model (hereafter referred to as the AM50 human model) [10] was placed on a Anseat AM50 fixed THUMS to a vehicle ver.4 cabin model mod (hereafterel (Figure referredreferre 3). Thed toto AM50 as the AM50human human model model) is composed [[1010] was of placed finite elementson a seat for fixedfixed all tobody a vehiclevehicle parts. cabinThecabin height modelmod eland (Figure(Figure mass3 3).of). ThetheThe AM50 AM50AM50 human humanhuman model modelmodel were isis composedcomposed 1.78 m and ofof finite74finite kg respectively.elements for all The body thorax parts. and The whole height -body and response mass of theof the AM50 AM50 human human model model were seated 1.78 m in anda vehicle 74 kg impactedrespectively. from The The the thorax front were and validatedwhole whole-body-body through response a comparison of the AM50 with human responses model of post seated mortem in a human vehicle subjectsimpacted (PMHSs) fromfrom the front [10]. were All simulations validated through were performed a comparison using with the responses LS-DYNA of post commercial mortem human finite elementsubjects (PMHSs)software (PMHSs) [11] [ 10 [10].. ].The All All simulationsvehicle simulations cabin were model were performed was performed fixed using to the using the vehicle LS-DYNA the coordina LS-DYNA commercialte system. commercial finite Normally, element finite aelementsoftware seat belt software [ 11is ].the The only [11] vehicle system. The cabinvehicle that model restrains cabin wasmodel the fixed upper was to fixed thebody vehicle to of the an vehicle coordinateoccupant. coordina The system. AM50te system. Normally, human Normally, model a seat woreabelt seat is abelt the three onlyis the-point system only seatbelt.system that restrains that The restrains vehicle the upper the cabin bodyupper model of body an andoccupant. of an three occupant.- Thepoint AM50 The seatbelt humanAM50 (model human model number woremodel a 100112_V1.0)worethree-point a three seatbelt. - werepoint developed Theseatbelt. vehicle The by cabin Livemore vehicle model cabin Softw and model three-pointare Technology and seatbelt three Corporation-point (model seatbelt number (LSTC) (model 100112_V1.0) [12] number. In the Nationalwere100112_V1.0) developed Highway were by LivemoreTraffic developed Safety Software by Administration Livemore Technology Softw (NHTSA) Corporationare Technology New (LSTC) Car Corporation Assessment [12]. In the (LSTC) NationalProgram [12] Highway(NCAP). In the testTrafficNational report, Safety Highway load Administration limiters Traffic for Safety drivers (NHTSA) Administration and New front Car-seat Assessment (NHTSA) passengers ProgramNew were Car identical (NCAP) Assessment testat approximately report, Program load (NCAP) limiters 5 kN [13]testfor drivers.report, Forman andload et front-seatal. limiters reported for passengers that drivers vehicle and were occupants front identical-seat with atpassengers approximately a 5 kN wereload limiter 5identical kN [13, without]. at Forman approximately an etairbag al. reported, could 5 kN experience[13]that. vehicleForman multiple occupantset al. reported rib withfractures that a 5 vehicle kN[14] load. Foret occupants limiter,-Bruno without withet al. asuggested 5 ankN airbag, load that limiter could a load, without experience limiter an of airbag multiple4 kN ,would could rib provideexperiencefractures reasonable [14 multiple]. Foret-Bruno safetyrib fractures etwhen al. suggested used[14]. Foretin combination that-Bruno a load et limiteral. wit suggestedh an of 4airbag kN that would [15] a load. provideIn limiterthis study, reasonable of 4 kNthe wouldupper safety limitwhenprovide of used thereasonable inforce combination limiter safety was when with regulated anused airbag in to combination [3.315 ].kN. In thisWe study,excludedwith an the airbag the upper airbag [15] limit. fromIn of this the the study, force current limiter the vehicle upper was cabinlimitregulated ofmodel the to force 3.3in order kN. limiter We to excludedinvestigate was regulated the potential airbag to from3.3 for kN. theabdominal We current excluded vehicleinjury the under cabin airbag modelthe from most in the ordersevere current to conditions investigate vehicle, wherepotentialcabin model there for was abdominalin order a large to injuryinvestigate movement under potentialof the the most hip for severe and abdominal lower conditions, extremities. injury where under The there the seat was most-back a largesevere and movement floorconditions of the of, vehiclewherethe hip therecabin and lowerwas model a extremities.large were movement rigid. The seat-back of the hip and and floor lower of theextremities. vehicle cabin The modelseat-back were and rigid. floor of the vehicle cabin model were rigid. Seatbelt Seatbelt

Knee-panel Seat-back Knee-panel Seat-back

Seat z Seat y x z Floor y x Figure 3. Anthropomorphic 50th percentile male totalFloor human model for safety ver.4 (AM50 human Figure 3. Anthropomorphic 50th percentile male total human model for safety ver.4 (AM50 human model) [10] seated in the vehicle cabin model [12]. modelFigure) 3[10]. A nthropomorphicseated in the vehicle 50th cabin percentile model male [12] . total human model for safety ver.4 (AM50 human model) [10] seated in the vehicle cabin model [12]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 12

The vehicle cabin model used in our research is shown in Figure 4, together with springs representing seat and knee-panel characteristics. The seat was set to move downwards (z-axis) with an increase in applied forces from the thigh and breech. In this study, the center of the seat was moved downward by a maximum displacement of 150 mm, as previously obtained results indicated that this amount of vertical downward movement resulted in upward movement of the lap belt from its initialAppl. Sci. position 2018, 8, x at FOR th ePEER waist REVIEW of the AM50 Hybrid III model [8]. 4 of 12 We set two seat movement conditions: (a) vertical movement and (b) lean forward movement. For verticalThe vehicle movement, cabin model we assigned used in one our spring research in the is shown seat to in represent Figure 4, vertical together seat with movement springs (Figure 4a). The seat moved vertically downwards according to force-displacement characteristics in Appl.representing Sci. 2018, 8 ,seat 2047 and knee-panel characteristics. The seat was set to move downwards (z-axis)4 with of 13 onean increase spring model in applied of the forces seat, from as shown the thigh in Figure and breech. 5a. For In leanthis study,forward the movement, center of the we seat assigned was moved two springsdownward in the by seat a maximum to reproduce displacement forward rotation of 150 ofmm the, asseat previously (Figure 4b). obtained The seat results moved indicated downwards that togetherthis amountThe with vehicle of forward vertical cabin rotation downward model according used movement in our to force research resulted-displacement is in shown upward characteristics in movement Figure4, together inof thethe frontlap with belt spring springs from and its backrepresentinginitial spring position models seat at th and eof waist the knee-panel seat, of the as AM50shown characteristics. Hybrid in Figure III 5b.model The seat [8]. was set to move downwards (z-axis) withThe anWe increase setknee two panel inseat applied wasmovement set forces to move conditions: from in the the thigh (a)forward vertical and breech.direction movement In (x this-axis) and study, (b)with thelean an center forwardincrease of the movement.in seatapplied was forcesmovedFor vertical from downward the movement, knee by through a maximum we assignedthe axial displacement force one spring of the of thigh in150 the mm, of seat the as previouslytoAM50 represent human obtained vertical model. results seatThus, movement indicatedthe knee panelthat(Figure this was 4a). amount arranged The seat of verticalto moved move downward forward,vertically as downwards movement shown in Figure resulted according 6. inThe upwardto initialforce- displacement movementseat angle and of thecharacteristics seat lap-back belt angle from in wereitsone initial spring set to position 16°model and atof 62° the the respectively waist seat, ofas theshown (Figure AM50 in HybridFigure4). 5a. III modelFor lean [8 ].forward movement, we assigned two springs in the seat to reproduce forward rotation of the seat (Figure 4b). The seat moved downwards Slip-ring together with forward rotation according to force-displacement characteristics in the front spring and back spring models ofShoulder the seat,-belt as shown in Figure 5b. (b) One "Spring" in knee-panel (b) One "Spring" in knee-panel The knee panel wasSeat -setback to move in the forward direction (x-axis) with an increase in applied forces from the knee throughLap-belt the axial force of the thigh of the AM50 humanKnee-panel model. Thus, the knee panel was arranged toKnee move-panel forward, as shown in Figure 6. The initial seat angle and seat-back angle Seat Seat 62° were set to 16° and 62° 16respectively° (Figure 4).

Floor Retractor Slip-ring

Z (a) One "Spring" in seat Z Shoulder-belt (a) Two "Springs" in seat X X (b)Y One "Spring" in knee-panel a) (b)Y One "Spring" in knee-panel b) Seat-back Figure 4. Vehicle cabinLap model-belt and modified spring model used in this study:Knee (a)-panel vertical seat movement Figure 4. Vehicle Knee cabin-panel model and modified spring model used in this study: (a) vertical seat and (b) lean forwardSeat seat movement. Seat movement and (b) lean forward seat62 movement.° 16° We set two seat movement conditions: (a) vertical movement and (b) lean forward movement. Floor Front spring Single spring Retractor For vertical movement, we assignedシートクッション特性 one spring in the seat to representBackシートクッション特性 spring vertical seat movement 6.00 6.00 (FigureZ 4a). The seat6 moved vertically(a) One downwards "Spring" in seat accordingZ6 to force-displacement characteristics in (a) Two "Springs" in seat 5.00 5.00 X 5 5 X

one springY model of the seat, as shown in Figurea) 5a. ForY lean forward movement, we assigned twob) )

springs in the seat) 4.004 to reproduce forward rotation of the4.004 seat (Figure4b). The seat moved downwards

kN kN (

( 前バネ (kn) togetherFigure with 4. forward(kn)Vehicle3.003 cabin rotation model according and modified to force-displacement spring3.003 model used characteristics in this study: in the (a) front vertical spring seat and

沈み込み150ｍｍ 後バネ 荷重

movement荷重 and (b) lean forward seat movement. Force back spring modelsForce 2.002 of the seat, as shown in Figure5b.2.002

1.001 1.001 Front spring Single spring 0.000 シートクッション特性 0.000 シートクッション特性 00 5050 100100 150150 200200 250250 00 50 100100 Back 150150 spring 200200 250250 6.00 ストローク(mm) 6.00 ストローク(mm) 6 Displacement (mm) a) 6 Displacement (mm) b)

5.005 5.005 )

Figure 5. Force) 4.004 -displacement characteristics used in the4.004 spring models of the seats: (a) vertical seat

kN kN (

( 前バネ (kn) movement and(kn) 3.003 (b) lean forward seat movement. 3.003

沈み込み150ｍｍ 後バネ

荷重

荷重 Force Force 2.002 2.002

1.001 1.001

0.000 0.000 00 5050 100100 150150 200200 250250 00 50 100100 150150 200200 250250 ストローク(mm) ストローク(mm) Displacement (mm) a) Displacement (mm) b)

FigureFigure 5.5. Force-displacementForce-displacement characteristicscharacteristics used in thethe springspring modelsmodels ofof thethe seats:seats: ((aa)) verticalvertical seatseat b movementmovement andand ((b)) leanlean forwardforward seatseat movement.movement. The knee panel was set to move in the forward direction (x-axis) with an increase in applied forces from the knee through the axial force of the thigh of the AM50 human model. Thus, the knee panel was arranged to move forward, as shown in Figure6. The initial seat angle and seat-back angle were set to 16◦ and 62◦ respectively (Figure4). Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 12 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 12 44 44 33 33 Appl. Sci. 2018Appl., Sci. 8, x2018 FOR, 8, 2047PEER REVIEW 22 5 of 13 5 of 12

Force(kN) 22

Force (kN) Force 11 Force(kN) 44 (kN) Force 11 00 0 2020 4040 6060 8080 100100 00 33 Displacement (mm) 0 2020 4040Stroke (mm) 6060 8080 100100 Displacement (mm) Stroke (mm) Figure 6. Force-displacement characteristics22 used in the spring models in the knee panels.

Figure 6. Force-displacement characteristics used in the spring models in the knee panels.

Force(kN) Force (kN) Force To simulate impact, we used11 acceleration-time history of the side-sill, which represents the crash pulseTo of simulate a small- impact,sized sedan we used—where acceleration engine - displacementtime history of is the 1490 side cc-sill,, curb which mass represents is 1130 the kg, crash and 00 maximumpulse of a number small-sized of passengers sedan—where is0 5 — impacting engine 2020 displacement4040 a rigid 6060 barrier is8080 1490at 55 100 100 km/h cc, curb according mass is to 1130 JNCAP kg, test and maximum number of passengers is 5—impactingDisplacement a rigid (mm) barrier at 55 km/h according to JNCAP test procedure [16]. Figure 7 shows acceleration and velocityStroke (mm)-time-histories applied to the vehicle cabin procedure [16]. Figure 7 shows acceleration and velocity-time-histories applied to the vehicle cabin model in thisFigure study. 6. Force-displacement The velocity-time characteristics history in Figure used in 7b the was spring obtained models from in the a kneesingle panels. integration of Figureaccelerationmodel 6. Forcein this-time- displacementstudy. history The velocityin Figurecharacteristics-time 7a. historyThe dummyused in Figure in themodel 7b spring was was obtained modelsplaced ininfrom thethe a vehicle kneesingle panels. integrationcabin model of underaccelerationTo normal simulate-time conditions impact,history described in we Figure used in acceleration-time7a. the The frontal dummy rigid model historybarrier was impact of theplaced side-sill,test inprocedure the which vehicle by represents JNCAPcabin model [16] the. To Thecrashsimulateunder vehicle pulsenormal impact, cabin of conditions a small-sizedand we dummy used described sedan—where model acceleration in are the shown frontal engine-time in r igidFigure history displacement barrier 8. I nitialof impact the clearance is side 1490test- proceduresill, cc, between curb which mass bythe represents JNCAP knee is 1130 panel [16] kg, . the crash pulse ofandandThe a the maximumsmallvehicle AM50- sizedcabin model number andsedan kneedummy of passengers— waswhere model 70 mm. engine isare 5—impactingIn shown this displacementstudy, in Figure athe rigid 8.center I barriernitial is of clearance 1490the at 55 seat km/h cc , height between curb according was mass the set knee to 150is JNCAP panel 1130mm kg, and and the AM50 model knee was 70 mm. In this study, the center of the seat height was set 150 mm maximumlowertest number procedure than its oforiginal [16 passengers]. Figure position7 shows isby 5 accelerationforce—impacting application. and velocity-time-historiesa rigid barrier at 55 applied km/h toaccording the vehicle to cabin JNCAP test modellowerUnder than in this it thes study.original two Theseat position velocity-time conditions by force (vertical historyapplication. movement in Figure 7 b and was lean obtained forward from movement), a single integration we first procedureinvestigatedof acceleration-time [16]Under. Figure dynamic the two 7 historyshows seatbehaviors conditions inacceleration Figure of the 7 (verticala.AM50 The and human dummy movement velocity model. model - and tNext,ime was lean- placed historieswe forwardobserved in the applied movement), vehiclethe instant cabinto the at we modelwhich vehi first cle cabin model intheunderinvestigated this lap normal study.belt detached dynamic conditionsThe velocity from behaviors described an -ilitime acof crest.the in history theAM50 In frontal addition, humanin Figure rigid model.we barrier investigated7b Next,was impact obtainedwetest movementobserved procedure from the of instantathe by single JNCAPlap at belt, whichintegration [16 as]. of accelerationwellThethe lap vehicleas-time tensilebelt cabinhistorydetached force and of in dummyfrom the Figure lap an modelbelt,ili ac7a. crest.shoulder areThe shownIn dummy addition, belt, in and Figure wemodelretractor8 investigated. Initial was belt clearance placed(Figure movement between 9)in under the of thethe vehiclemovable kneelap belt, panel cabinseat as model under normalheightandwell theas conditions. conditionstensile AM50 modelforce ofdescribed knee the lap was belt, 70 in mm. shoulderthe Infrontal this belt, study, r igidand theretractor barrier center beltimpact of the (Figure seat test height9) procedure under was movable set 150by JNCAP mmseat [16]. lowerheight than conditions. its original position by force application. The vehicle cabin and dummy600 model are shown in 100100Figure 8. Initial clearance between the knee panel 600 100 ) 100 and the AM50 model2 knee was 70 mm. In this study,8080 the center of the seat height was set 150 mm

400 ) 2 8080 lower than its original position400 by force application.6060 200 6060 Under the two seat conditions (vertical movement40 and lean forward movement), we first

200 40 Velocity (km/h) Velocity

investigated dynamic behaviors0 of the AM50 human(km/h) Velocity 40 model. Next, we observed the instant at which Acceleration (m/sec

Acceleration (m/sec2) Acceleration 2020 Velocity (km/h) Velocity

0 (km/h) Velocity Acceleration (m/sec the lap belt detached from(m/sec2) Acceleration an iliac crest. In addition,2020 we investigated movement of the lap belt, as --200200 00 0 5050 100100 150 200200 0 5050 100100 150150 200200 well as tensile force of--200 200the lap belt, shoulder belt, and00 retractor belt (Figure 9) under movable seat Time (msec) (ms) a) Time (msec)(ms) b) 0 5050 100100 150 200200 0 5050 100100 150150 200200 height conditions. Time (msec) (ms) a) Time (msec)(ms) b)

Figure 7. Time-series of (a) acceleration and (b) velocity applied to the vehicle cabin model. Figure 7. Time-seriesTime-series of (a)) accelerationacceleration andand ((bb)) velocityvelocity appliedapplied toto thethe vehiclevehicle cabincabin model.model.

600 100100 ) 2 8080 400

6060 200 70 mm40

70 mm Velocity (km/h) Velocity

0 (km/h) Velocity Acceleration (m/sec Acceleration (m/sec2) Acceleration 2020

--200200 00 0 5050 100100 150 200200 0 5050 100100 150150 200200 Time (msec) (ms) a) Time (msec)(ms) b)

Figure 7. Time-series of ((a)a) acceleration and (b) velocity(b) applied to the vehicle cabin model.(c) (a) Applied acceleration (b) (c) Applied acceleration Figure 8. Vehicle cabin and AM50 human models from (a) front, (b) side, and (c) top views. Figure 8. Vehicle cabin and AM50 human models from (a) front, (b) side, and (c) top views. Figure 8. Vehicle cabin and AM50 human models from (a) front, (b) side, and (c) top views. Under the two seat conditions (vertical movement and lean forward movement), we first investigated dynamic behaviors of the AM50 human model. Next, we observed the instant at which the lap belt detached from an iliac crest. In addition, we investigated movement of the lap belt, as well as tensile force of the lap belt, shoulder belt,70 mm and retractor belt (Figure9) under movable seat height conditions.

(a) (b) (c) Applied acceleration

Figure 8. Vehicle cabin and AM50 human models from (a) front, (b) side, and (c) top views. Appl. Sci. 2018, 8, 2047x FOR PEER REVIEW 6 of 1312 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 12

Slip-ring Slip-ring

Tensile force of a Tensileshoulder force-belt of a shoulder-belt Tensile force of a Tensileretractor force-belt of a retractor-belt

Buckle Buckle Tensile force Retractor Tensile force Retractor of a lap-belt Anchor of a lap-belt Anchor Figure 9. Tensile force activated on various restraining belts. Figure 9 9.. Tensile force activated on various restraining belts. 3. Results 3. Results DynamicDynamic behaviors of the AM50 human model under tw twoo considered seat conditions are shown Dynamic behaviors of the AM50 human model under two considered seat conditions are shown in Figure 1010.. D Dynamicynamic movements of the AM50 human model were were similar for for both both seat seat conditions. conditions. in Figure 10. Dynamic movements of the AM50 human model were similar for both seat conditions. SeatSeat angle angle of of the the lean lean forward forward condition condition (−4◦ ) (− was4°) smaller was smaller than that than of thethat vertical of the movement vertical movement condition Seat angle of the lean forward condition (−4°) was smaller than that of the vertical movement (16condition◦) at 90 (16°) ms. Dynamicat 90 ms. behaviorsDynamic behaviors of the lap beltof the in lap the belt two in seat the conditions two seat conditions are shown are in Figures shown 11in condition (16°) at 90 ms. Dynamic behaviors of the lap belt in the two seat conditions are shown in andFigures 12. 11 In theand vertical12. In the movement vertical movement condition, condition, the lap belt the detached lap belt detached from the rightfrom iliacthe right crest iliac at 80 crest ms Figures 11 and 12. In the vertical movement condition, the lap belt detached from the right iliac crest (Figureat 80 ms 12 (Figurea), but did12a), not but detach did not from detach the left from iliac the crest left duringiliac crest impact. during In theimpac leant. In forward the lean movement forward at 80 ms (Figure 12a), but did not detach from the left iliac crest during impact. In the lean forward condition,movement thecondition, lap belt detachedthe lap belt from detached the left from iliac crestthe left at 75 iliac ms crest (Figure at 7512 b)ms and (Figure then detached12b) and fromthen movement condition, the lap belt detached from the left iliac crest at 75 ms (Figure 12b) and then thedetached right iliacfrom crest the atright 90 msiliac (Figure crest at 11 90b). ms Thus, (Figure the lap 11b). belt Thus detached, the lap from belt both detached iliac crests from during both iliac the detached from the right iliac crest at 90 ms (Figure 11b). Thus, the lap belt detached from both iliac leancrests forward during movement.the lean forward movement. crests during the lean forward movement.

16 16 Seat angle: 16 Seat angle: 16 Clockwise from Clockwisehorizontal planefrom + a) horizontal plane + a) 0 ms 90 ms 0 ms 90 ms

16 - 4 16 - 4

b) b) 0 ms 90 ms 0 ms 90 ms Figure 10. Dynamic behaviors of AM50 dummies under two seat conditions: (a) vertical seat movement Figure 10. Dynamic behaviors of AM50 dummies under two seat conditions: (a) vertical seat andFigure (b) lean10. Dynamic forward seat behaviors movement. of AM50 dummies under two seat conditions: (a) vertical seat movement and (b) lean forward seat movement. movement and (b) lean forward seat movement. Appl. Sci. 2018, 8, 2047 7 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 12 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 12

Timing of lap-belt coming away from left iliac crest Timing of lap-belt coming away from left iliac crest

Iliac Lap-belt Iliaccrest Lap-belt crest

75 ms 80 ms 90 ms a) 75 ms 80 ms 90 ms a)

75 ms 80 ms 90 ms 75 ms 80 ms 90 ms b) b)

Figure 11. Dynamic behavior of the lap belt under two seat conditions (view from left): (a) vertical FigureFigure 11.11. DynamicDynamic behaviorbehavior of of the the lap lap belt belt under under two two seat seat conditions conditions (view (view from from left): left): (a) vertical (a) vertical seat seat movement and (b) lean forward seat movement. movementseat movement and (andb) lean (b) forwardlean forward seat movement.seat movement.

Timing of lap-belt coming away from right iliac crest Timing of lap-belt coming away from right iliac crest

Lap-belt Lap-belt

Iliac Iliaccrest 75 ms crest 80 ms 90 ms a) 75 ms 80 ms 90 ms a)

75 ms 80 ms 90 ms b) 75 ms 80 ms 90 ms b)

FigureFigure 12.12.Dynamic Dynamic behaviorbehavior ofof thethe laplap beltbelt underunder twotwo seatseat conditionsconditions (view(view fromfrom right):right): ((aa)) verticalvertical Figure 12. Dynamic behavior of the lap belt under two seat conditions (view from right): (a) vertical seatseat movementmovement andand ((bb)) leanlean forward forward seat seat movement. movement. seat movement and (b) lean forward seat movement. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 12

The time-series of seat height at the center of the seat, seat angle, and knee-panel displacement under the two seat conditions are shown in Figure 13. Maximum knee-panel displacement occurred at 75 ms for both vertical movement and lean forward movement (Figure 13c), which occurred 10 ms and 25 ms prior to the maximum seat-height displacement at 85 ms and 100 ms, respectively (Figure 13a). For the lean forward movement condition, the seat angle of 16° at 28 ms decreased to −4° at 102 ms (Figure 13b). For the vertical movement condition, the seat angle remained stable at 16° during impact. Appl. Sci. 2018, 8, 2047 8 of 13 Time histories of tensile forces on the retractor belt, shoulder belt, and lap belt are shown in Figure 14. Tensile forces of the retractor belt under two seat conditions were almost consistent (3.3 kN afterThe time-series51 ms) because of seat the height force atlimiter the center in the ofretractor the seat, was seat regulated angle, and to knee-panel3.3 kN in the displacement three-point underseatbelt the model. two seat Tensile conditions forces are of shown the shoulder in Figure belt 13. in Maximum two seat knee-panel conditions displacement were also almost occurred equal at 75(between ms forboth 4.2 kN vertical and 4.7 movement kN after and 51 leanms) forwardbecause movementthey were connected (Figure 13c), to whichthe retractor occurred belt. 10 msTensile and 25forces ms prior of the to lap the maximumbelt in two seat-height seat conditions displacement were almost at 85 ms consistent and 100 prior ms, respectively to 75 ms. Furthermore, (Figure 13a). Formaximum the lean tensile forward forces movement of the lap condition, belt were the similar seat angleunder of the 16 two◦ at 28seat ms conditions: decreased 9.0 to kN−4◦ forat vertical 102 ms (Figuremovement 13b). and For 8.9 the kN vertical for the movement lean forward condition, movement. the seat angle remained stable at 16◦ during impact.

Vertical movement Lean forward movement

200200 2020 16 deg 150 mm 1515

mm) 150150 ) 10

Z Z ( 10

-

deg (

100100 5

angle [deg] angle -

Y 0

Displacement [mm] Displacement Angle - 50

Z 50 -5-5 Min -4 deg at 102 ms Displacement 0 0 --1010 0 20 4040 6060 8080 100100 120 00 2020 4040 6060 8080 100100 120 TimeTime [msec](ms) (a) TimeTime [msec](ms) (b)

120120 100100 Max 101 mm

mm) at 75 ms

8080

X X ( - 6060 Max 100 mm

4040 at 75 ms

Displacement [mm] Displacement

- X

2020 Displacement 0 00 20 4040 6060 8080 100100 120120 TimeTime [msec](ms) (c)

Figure 13. Time histories of (a) seat height, (b) seat angle, and (c) knee-panel displacement under two Figure 13. Time histories of (a) seat height, (b) seat angle, and (c) knee-panel displacement under two seat conditions. seat conditions. Time histories of tensile forces on the retractor belt, shoulder belt, and lap belt are shown in Figure 14. Tensile forces of the retractor belt under two seat conditions were almost consistent (3.3 kN after 51 ms) because the force limiter in the retractor was regulated to 3.3 kN in the three-point seatbelt model. Tensile forces of the shoulder belt in two seat conditions were also almost equal (between 4.2 kN and 4.7 kN after 51 ms) because they were connected to the retractor belt. Tensile forces of the lap belt in two seat conditions were almost consistent prior to 75 ms. Furthermore, maximum tensile forces of the lap belt were similar under the two seat conditions: 9.0 kN for vertical movement and 8.9 kN for the lean forward movement. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 12 Appl. Sci. 2018, 8, 2047 9 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 12 Vertical movement Lean forward movement Vertical movement 1010 10

Lean forward movement ) 88 ) 88

1010 10

kN

kN ) 66 ) 66

88 88

kN

kN

force( force(

4 4 Force[kN] 4 Force[kN] 4

66 66 force(

22 force( 22 Tensile Tensile

4 Tensile 4 Force[kN] 4 Force[kN] 4

00 00

22 0 20 40 60 80 100 22 0 20 40 60 80 100 Tensile Tensile 0 20 40 60 80 100 Tensile 0 20 40 60 80 100 TimeTime [msec](ms) (a) TimeTime [msec](ms) (b) 00 00 00 20 4040 6060 8080 100100 00 2020 40 60 8080 100100 TimeTime [msec](ms) (a) TimeTime [msec](ms) (b)

1010 ) 88

1010 kN

) 66

88

kN force( 4 Force[kN] 4 66

force( 22 Tensile Tensile 4 Force[kN] 4

00 22 0 20 40 60 80 100 Tensile Tensile 0 20 40 60 80 100 TimeTime [msec] (ms) 00 (c) 00 20 40 60 8080 100100 TimeTime [msec] (ms) (c) Figure 14. Time histories of the tensile forces on the (a) retractor belt, (b) shoulder belt, and (c ) lap belt. FigureFigure 14.14. TimeTime histories histories of of the the tensile tensile forces forces on on the the (a) ( retractora) retractor belt, belt, (b) shoulder(b) shoulder belt, belt, and (andc) lap (c belt.) lap 4.4. DiscussionDiscussionbelt.

4. DiscussionInIn this this study, study, we we investigated investigated behavior behavior of of the the seatbelt seatbelt and and AM50 AM50 human human model model under under two two seat seat conditions:conditions: verticalvertical movementmovement andand leanlean forwardforward movement.movement. OurOur resultsresults indicateindicate thatthat thethe laplap beltbelt In this study, we investigated behavior of the seatbelt and AM50 human model under two seat detacheddetached fromfrom bothboth of the iliac iliac crest crestss during during the the lean lean forward forward movement. movement. Under Under this this condition, condition, the conditions: vertical movement and lean forward movement. Our results indicate that the lap belt thelap lap belt belt first first detached detached from from the the left left iliac iliac crest crest at 75 at 75ms ms and and then then from from the the right right iliac iliac crest crest at at90 90ms. ms. In detached from both of the iliac crests during the lean forward movement. Under this condition, the Inaddition, addition, under under this this condition, condition, tensile tensile force force of of the the lap lap belt belt reached reached a a maximum (8.9 kN)kN) atat 7070 msms lap belt first detached from the left iliac crest at 75 ms and then from the right iliac crest at 90 ms. In beforebefore decreasing. decreasing. AfterAfter thethe laplap beltbelt detacheddetached fromfrom bothboth ofof thethe iliaciliac crests,crests, tensiletensile forceforce ofof thethe laplap beltbelt addition, under this condition, tensile force of the lap belt reached a maximum (8.9 kN) at 70 ms reducedreduced more more steeply steeply after after 90 90 ms ms (Figure (Figure 15 15).). ItIt waswas consideredconsideredthat that thethe relativelyrelatively large large contact contact force force before decreasing. After the lap belt detached from both of the iliac crests, tensile force of the lap belt wouldwould act act on on the the abdomen abdomen as as a a result result of of the the large large tensile tensile force force generated generated by by the the lap lap belt. belt. reduced more steeply after 90 ms (Figure 15). It was considered that the relatively large contact force

would act on the abdomen as a result of theRetractor large-belt tensile forceLap -generatedbelt by the lap belt. Shoulder-belt 1010 Retractor-belt Lap-belt Shoulder-belt

) 101088 kN 66

) 88

kN force(

6464 force(

Tensile Tensile 2 442

00 Tensile Tensile 22 0 2020 4040 6060 8080 100100

00 Time (ms) 0 2020 4040 6060 8080 100100 Figure 15. Time history of tensile forces of the seatbeltTime under(ms) the lean forward movement seat condition. Figure 15. Time history of tensile forces of the seatbelt under the lean forward movement seat condition. Next,Figure we 15. focusedTime history on contact of tensile force forces and of thedeformation seatbelt under to the the abdomen lean forward of themovement AM50 seat human model. In this study, we measured resultant contact force (Fc) applied to the abdomen. Additionally, condition.Next, we focused on contact force and deformation to the abdomen of the AM50 human model. we measured deformation of the abdomen (D) shown in Figure 16. Deformation of the abdomen is In this study, we measured resultant contact force (Fc) applied to the abdomen. Additionally, we definedNext, as thewe distancefocused betweenon contact the force abdominal and deformation surface corresponding to the abdomen to the of centerthe AM50 of the human lap-belt model. and measured deformation of the abdomen (D) shown in Figure 16. Deformation of the abdomen is In this study, we measured resultant contact force (Fc) applied to the abdomen. Additionally, we measured deformation of the abdomen (D) shown in Figure 16. Deformation of the abdomen is Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 12

Appl.defined Sci. 2018 as , th8, ex FORdistance PEER REVIEWbetween the abdominal surface corresponding to the center of the lap10 -ofbelt 12 and the sacrum on the median sagittal plane. The resultant contact force reached a maximum value definedof 15.0 kNas that e70 distance ms (Figure between 17a) andthe abdominaldeformation surface of the correspondingabdomen reached to the a maximum center of valuethe lap of-belt 104 Appl.Appl. Sci.Sci.2018 2018,,8 8,, 2047 x FOR PEER REVIEW 1010of of 1312 andmm theat 90 sacrum ms (Figure on the 17b). median Strain sagittal of the plane.deformed The abdomen resultant wascontact 68% force (=104 reached mm/154 a mm),maximum because value the of 15.0 kN at 70 ms (Figure 17a) and deformation of the abdomen reached a maximum value of 104 initialdefined distance as th(D)e distance was 154 between mm. These the abdominalphenomena surface suggested corresponding that a large to the resultant center offorce the waslap- beltapplied mm at 90 ms (Figure 17b). Strain of the deformed abdomen was 68% (=104 mm/154 mm), because the to thetheand abdomen, sacrum the sacrum on resulting the on median the median in sagittalthe lapsagittal plane.belt plane. slipping The The resultant from resultant one contact contactiliac force crest. force reached After reached the a maximum alap maximum belt slipped value value of from initialboth15.0of iliacdistance 15.0 kN crests,kN at 70 at(D) ms70 thewas (Figurems abdomen (Figure154 17 mm.a) 17a) and These experienced and deformation deformationphenomena extreme of the ofsuggested abdomenthe deformation, abdomen reachedthat reached a large a which maximum resultanta maximum indicates value force value of possibility was 104 of mm applied104 for tocritical theatmm abdomen, 90 abdominal at ms 90 (Figure ms (Figureresulting injury. 17b). 17b). Strain in Sthetrain of lap the of bel the deformedt deformedslipping abdomen fromabdomen one was wasiliac 68% 68% crest. (=104 (=104 After mm/154 mm/154 the mm),lap mm), belt because because slipped thethe from bothinitial initial iliac distancedistance crests, the(D) (D) waswas abdomen 154154 mm.mm. experienced TheseThese phenomenaphenomena extreme suggestedsuggested deformation, thatthat aa largelarge which resultantresultant indicates forceforce was was possibility appliedapplied for criticaltoto thetheabdominal abdomen,abdomen, injury. resultingresulting inin thethe laplap beltbelt slippingslipping fromfrom oneone iliaciliac crest.crest. AfterAfter thethe laplap beltbelt slippedslipped fromfrom bothboth iliac iliac crests, crests, the the abdomen abdomen experienced experienced extreme extreme deformation, deformation,Sacrum which which indicates indicates possibility possibility for critical for D abdominalcritical abdominal injury. injury. Sacrum Fc D

Sacrum Fc D Ft Fc Figure 16. Illustration of the resultant contact forceFt (Fc) between lap belt and abdomen, deformation

of the abdomen (D), and tensile force of the lap beltFt (Ft). Figure 16. Illustration of the resultant contact force (Fc) between lap belt and abdomen, deformation Figure 16. Illustration of the resultant contact force (Fc) between lap belt and abdomen, deformation of of theFigure abdomen 2016. Illustration (D), and tensileof the resultant force of contactthe lap force belt (Fc)(Ft).120120 between lap belt and abdomen, deformation the abdomen (D), and tensile force of the lap belt (Ft). Max 104 mm at 90 ms of the abdomen (D), and tensile Maxforce 15.0 of kNthe lap belt (Ft). at 70 ms 100100 2015 120

mm) 120 ) 8080 Max 104 mm at 90 ms 20 Max 15.0 kN 120120 kN 100 ( 100 Max 104 mm at 90 ms 10 at 70Max ms 15.0 kN 60 1510 10010060

at 70 ms mm) ) 15 80

Force [kN] Force 15 80

Stroke [mm] Stroke Force

mm) 40 )

kN 8040

( 80 5 kN 60 105 Deformation ( 10 ( 60

10 60602020

Force [kN] Force Stroke [mm] Stroke

Force 40 Force [kN] Force

Stroke [mm] Stroke 40 Force 40 50 4000 5 00 5 20 4040 6060 8080 100100 Deformation ( 00 2020 4040 6060 80 100 5 Deformation ( 20 2020 TimeTime [msec] (ms) (a) TimeTime [msec] (ms) (b) 0 0 0 0 00 00 00 20 20 40 40 40 40 606060 60 80808080 100100100100 000 20202020 40 404040 6060 60 60 80 80 100 100 Figure 17. Time history ofTimeTime (a Time[msec]) (msresultant (ms) ) contact(a)(a) force (Fc) and (b) TimedeformationTimeTimeTime [msec] ( [msec]ms ()ms) of the(b) (b)abdomen (D) Time [msec] under lean forward movement conditions. Figure 17. Time history of (a) resultant contact force (Fc) and (b) deformation of the abdomen (D) FigureFigure 17. Time17. Time history history of of(a )( aresultant) resultant contact contact forceforce (Fc) andand ( b(b) )deformation deformation of ofthe the abdomen abdomen (D) (D) under lean forward movement conditions. underThe underdefinitio lean leanforwardn forward and movement time movement history conditions. conditions. of the vertical distance of lap belt movement are shown in Figure The definition and time history of the vertical distance of lap belt movement are shown in 18. The lapThe beltdefinitio slippedn and upwards time history by of a themaximum vertical distance distance of lapof 4belt mm movement at 50 ms are and shown maintained in Figure this FigureThe definitio 18. Then lap and belt time slipped history upwards of the byvertical a maximum distance distance of lap ofbelt 4 mm movement at 50 ms are and shown maintained in Figure upward18. The movement lap belt slippeduntil 80 upwards ms. We by consider a maximum that distancealthough of slipping 4 mm at distance50 ms and was maintained small, the this pelvis 18. Thethis upwardlap belt movement slipped upwards until 80 ms. by We a maximum consider that distance although of slipping 4 mm distance at 50 ms was and small, maintained the pelvis this couldupward be rotated movement forward until along 80 ms. with We the consider lean forward that although movement slipping of the distance seat from was smtheall, initial the pelvisseat angle could be rotated forward along with the lean forward movement of the seat from the initial seat angle upwardof 16°.could T imemovement be rotatedhistory forward untilof the 80 seat along ms. angle Wewith considerindicates the lean forward that that althoughit movementreached slipping 0°, of i.e., the seatadistance horizontal from the was initialcondition sm all,seat the angle, at pelvis 76 ms of 16◦. Time history of the seat angle indicates that it reached 0◦, i.e., a horizontal condition, at 76 couldand of4° be 16°. at rotated 90Time ms historyforward (Figure of alongthe13b seat). Duringwith angle the indicates these lean forwardperiods, that it reachedmovement the iliac 0°, ci.e., ouldof thea horizontal be seat rotated from condition theforward, initial, at seatleading76 ms angle to ms and 4◦ at 90 ms (Figure 13b). During these periods, the iliac could be rotated forward, leading to ofslipping 16°.and Time 4°of atthe history 90 lap ms belt of(Figure the from seat 13b the ).angle iliacDuring indicatescrests. these periods, that it reachedthe iliac 0°,could i.e., be a horizontalrotated forward, condition leading, at to76 ms slipping of the lap belt from the iliac crests. and 4°slipping at 90 ofms the (Figure lap belt 13b from). theDuring iliac crests.these periods, the iliac could be rotated forward, leading to slipping of the lap belt from the iliac crests. Center of lap-belt 1515

Center of lap-belt 1515

mm)

( ) ) mm) 10 Center of lap-belt ( 151510 ) ) 10

Dz 10 (

Dz Max 4 mm

( mm)

( Max 4 mm 5 at 50 ms ) ) 105

105 at 50 ms

Dz (

Dz deitance Displacement [mm] Displacement Max 4 mm

Dz deitance

-

Displacement [mm] Displacement Z - 0 Z 050 at 50 ms Min -Min1 mm -1 mm

Dz deitance

Vertical Vertical Vertical Vertical Displacement [mm] Displacement at 90 atms 90 ms - -5 Z -5-50-5 0 ms0 ms 6060 ms ms 00 0 20 20 40 40 60 60 80 80 100 100 0 0 2020 40 40 60 60 80 Min80 - 1001 mm100 a) TimeTime [msec] (ms [msec] )(ms) b) b) Vertical Vertical at 90 ms -5-5 0 ms 60 ms 0 20 40 60 80 100 Figure 18. Definition and time history of vertical distance (Dz)0 of lap20 belt movement: 40 60 (a) definition80 100 and FigureFigure 18. Definition18. Definition and and time time history history of of verticalverticala) distance (Dz) (Dz) of of lap lap belt beltTime movement: [msec] (movement:ms) (a) definition(a)b )definition (b) time history. and and(b) time(b) time history. history. Figure 18. Definition and time history of vertical distance (Dz) of lap belt movement: (a) definition Results indicated that the lap belt detached more easily from the iliac crests during the lean andResults (b) time indicated history. that the lap belt detached more easily from the iliac crests during the lean forward movement. A photograph of the front passenger seat after the full-frontal rigid barrier forward movement. A photograph of the front passenger seat after the full-frontal rigid barrier Results indicated that the lap belt detached more easily from the iliac crests during the lean forward movement. A photograph of the front passenger seat after the full-frontal rigid barrier Appl. Sci. 2018, 8, 2047 11 of 13

Results indicated that the lap belt detached more easily from the iliac crests during the lean forward movement. A photograph of the front passenger seat after the full-frontal rigid barrier impact test exhibits residual deformation of the frontal edge of the seat (Figure2). A similar residual seat deformation was observed in car-to-car real-world frontal impact accidents [7]. Based on this residual seat deformation, a seat would be rotated during frontal impact to a car. Thus, the lean forward movement condition is estimated to be similar to the deformation situation generated by real-world accidents. Findings of the present study suggest that to evaluate an occupant’s behavior in detail in future work, a seat model considering rotation should be incorporated into the vehicle cabin model when simulations are performed. In this study, we noted that the lap belt detached from both iliac crests during lean forward movement when two springs were used in the seat model and one spring used in the knee-panel model. We believe that our results derived using springs are consistent with those obtained using finite element models of the seat and knee panel. Finite element models of the seat can represent deformation of the cushion and framework structure in detail and can be compared with force-displacement spring characteristics. In the same manner, finite element models of the knee panel can represent details of deformation of the rib structure with relatively more or less rigid parts. Thus, in the future, finite element analyses of the seat and knee panel should be performed to understand these phenomena in more detail. In our method, we employed limited conditions: only two seat deformable conditions (vertical movement and lean forward movement), one frontal impact situation (full-frontal rigid barrier impact test at 55 km/h), one passenger car (engine displacement of 1490 cc), without airbag deployment, and an AM50-sized human model. However, for the case of airbag deployment, correlation between pelvis and lap belt movement would be different than that obtained in this study, because airbag deployment may prevent movement of the head and upper body of a human during impact. In addition, when we consider an initial seating condition, an occupant may wear a seatbelt with some amount of looseness. Under such conditions, the lap belt may detach from the iliac crests. In the future, simulation must be carried out considering a combination of factors such as forward rotation and looseness of the seat belt, by employing a wide variety of crash types and occupant seating conditions. Subsequently, a statistical analysis can be applied to develop the extracted results. Another limitation of the present study is that we performed simulations considering only an AM50-sized human model because current safety assessment tests usually involve performance of the full frontal rigid barrier impact test of the Japanese type approval test, using an AM50 dummy. However, people of several different sizes and body shapes may ride in vehicles. In the future, in combination with the AM50 sized human model, data of 95 percentile males and 5 percentile females should be used to represent large male and small female occupants when conducting such simulations in studies focusing on lap belt movement.

5. Conclusions We investigated the correlation between downward movement conditions of the seat and movement of the lap belt away from both iliac crests of a human occupant in the passenger seat during a frontal impact, by using an AM50 human model. We employed two deformable seat conditions: vertical movement and lean forward movement. The major finding was that the lap belt detached from both of the iliac crests during the lean forward movement of a seat, whereas the lap belt detached from only one of the iliac crests during vertical movement. We conclude that abdominal injuries can be caused by downward movement together with forward rotation in the seat during vehicle-to-vehicle frontal impacts. The finding of the present study suggests that when simulations are performed to evaluate the occupant’s behavior in detail, in the future, a seat model considering rotation should be incorporated into the vehicle cabin model. Abdominal injury simulations considering different model sizes of vehicle occupants must be considered along with various crash types and occupant Appl. Sci. 2018, 8, 2047 12 of 13 seating conditions. Furthermore, in the future, it is necessary to develop a design of a seat or a seat belt, such that the seat does not rotate during frontal impact.

Author Contributions: Conceptualization, Y.M.; investigation, S.O.; writing-original draft, Y.M.; writing-review and editing, S.O. Funding: This research was funded by the Japan Railway Construction, Transport and Technology Agency (JRTT), between 2012 and 2013. Acknowledgments: The authors are indebted to Junichi Inokuchi, P.E. Japan, Kenji Tamura, Norihisa Nomura, and Hiroaki Nishikori of the former Toyota Technical Development Corporation for supporting analysis of the vehicle cabin impact test model. Conflicts of Interest: The authors declare no conflict of interest.

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