Development of a mathematical model for evaluating child occupant behaviour in the case of a vehicle side impact simulation

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  • This article was downloaded by: [The Aga Khan University]On: 30 October 2014, At: 23:25Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

    International Journal of CrashworthinessPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tcrs20

    Development of a mathematical model for evaluatingchild occupant behaviour in the case of a vehicleside impact simulationM-D Surcel & M Goua cole Polytechnique de Montreal Mechanical Engineering Department, Box 6079,Station CV, Montral (Qubec), Canada, H3C 3A7b cole Polytechnique de Montreal Mechanical Engineering Department, Box 6079,Station CV, Montral (Qubec), Canada, H3C 3A7Published online: 08 Jul 2010.

    To cite this article: M-D Surcel & M Gou (2005) Development of a mathematical model for evaluating child occupantbehaviour in the case of a vehicle side impact simulation, International Journal of Crashworthiness, 10:1, 111-118, DOI:10.1533/ijcr.2005.0330

    To link to this article: http://dx.doi.org/10.1533/ijcr.2005.0330

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  • Woodhead Publishing Ltd 0330 111 IJCrash 2005 Vol. 10 No. 1 pp. 111118

    Corresponding Author:Professor Michel GouEcole Polytechnique de Montral, PO Box 6079, Station Centre-villeMontral, Qubec, Canada, H3C 3A7Tel: +1 (514) 340-4669 Fax: +1 (514) 340-5867Email: michel.gou@polymtl.ca

    INTRODUCTION

    Child passenger safety is a major concern for regulatoryorganisation as well as for child restraint systems andvehicle manufacturers. Statistics show that road collisionsare the primary cause for the death and the injury ofchildren, despite continuous improvements incrashworthiness and child restraint system effectiveness.Child restraint systems offer a specialized protection forthe child occupant, whose body structure is still immatureand growing. Using the child restraint system, the childis linked to the vehicle structure and participates to theimpact in the same time as the structure. The tight link tothe car structure solves only part of the problem because,in order to optimize the body impact tolerance, theremaining load should be distributed as widely as possibleon the strongest region of the child body.

    Moreover, the effectiveness of child restraint systemshas been well demonstrated for frontal impact but theperformances of these protective devices in side-impactsituation were not, as yet, clearly demonstrated. Applicableregulations are only stipulating that the child passengershould not be ejected from the car in a side impact howevercrash data shows that there are side impact situations wherethe child restraint system does not offer sufficientprotection, resulting in serious injuries or even the deathof the child occupant.

    Therefore all the aspects concerning the requirementsand the test procedures for the child passenger side impactprotection are presently under discussions: test type (sledor vehicle body), acceleration pulse, child dummy,instrumentation and even the child restraint systemeffectiveness assessment criteria.

    Reliable and realistic procedures are necessary to decideabout these major issues and many related problems andin this context numerical modelling is a relativelyinexpensive and efficient tool that could be used to evaluatethe performance of child restraint systems.

    Thus this project was aimed at the development of anumerical method to simulate the behaviour of a child

    Development of a mathematical model forevaluating child occupant behaviour in thecase of a vehicle side impact simulation

    M-D Surcel* and M Gou***cole Polytechnique de Montreal Mechanical Engineering Department, Box 6079, Station CV, Montral(Qubec), Canada, H3C 3A7, E-Mail: marius-dorin.surcel@sympatico.ca**cole Polytechnique de Montreal Mechanical Engineering Department, Box 6079, Station CV, Montral(Qubec), Canada, H3C 3A7 E-Mail: michel.gou@polymtl.ca

    Abstract: The effectiveness of child restraint systems has been well proven for frontal collisions butthe performances of the protective devices in side-impact situations were not, as yet, clearly demonstrated.

    This research programme was aimed at the development of a numerical method to simulate thebehaviour of a child passenger, restrained in a protective device, in the case of a vehicle side impact.

    The MADYMO software was chosen to build the model using finite element and multi-bodymethods. The side wings of the child restraint system and the vehicle body have been modelled by thefinite-element technique, to allow for a better representation of the contacts and to allow the simulationof the vehicle body deformation.

    The model has been evaluated against similar test data. The simulation results were generally veryclose to the experimental data but differences could be observed for some injury criteria, mainly dueto a different initial position of the child dummy. The model exploitation aims to assess differentinstallation configurations, dummy model influence and consequences of vehicle intrusion.

    Key words: Child restraint system, ISOFIX, MADYMO, side impact, simulation.

    doi:10.1533/ijcr.2005.0330

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  • M-D Surcel and M Gou

    IJCrash 2005 Vol. 10 No. 1 112 doi:10.1533/ijcr.2005.0330 Woodhead Publishing Ltd

    passenger restrained in a protective device in the case ofa vehicle side impact.

    METHODOLOGY

    Child restraint system and vehicle body models have beenbuilt using finite element and multi-body techniques. TheMADYMO software was chosen to build the model becauseit reduces the computational time and the related costs,allows the use of already validated dummy models fromthe MADYMO library and makes possible the comparisonwith other simulations created with the same software. Toreduce the costs and analysis time, the model was mainlybased on a multi-body method. However the side wingsof the child restraint system and the vehicle body havebeen modelled by the finite-element technique, to allowfor a better representation of the contacts between thechild dummy and the restraining device and the structureof the vehicle and to make possible the simulation of thevehicle body deformation.

    The reverse engineering method was mainly used toobtain the necessary constructive data, because themanufacturer information is generally consideredconfidential and not easily accessible.

    The second step was to validate the model against similartest data. Table 1 presents the parameters of the modelsand simulation.

    both femurs; between each femur and the abdomen, thethorax, the neck and the head; between both tibias; betweeneach tibia and the neck and the head; between both arms;between each arm and the neck and the head. The modelused has been validated by TNO for frontal loading.

    A special program has been elaborated to position thechild dummy in the child restraint system by applyingthe gravitational force on the child dummy, which allowsfor an equilibrium state in order to avoid the initialaccelerations that could be caused by the joints stiffness.

    CHILD RESTRAINT SYSTEM MODEL

    For the chosen child dummy model, the required childseat is the convertible restraint system designed for useby infants and toddlers. The Cosco Touriva child seat waschosen, because both test results and a specimen wereavailable for analysis.

    A shell and a support that comes in contact with thevehicle bench cushion compose the child restraint system.The shell is mounted on the support, more or less rigid,depending on the desired seat functionality. The inside ofthe shell is covered with a padding that satisfies energyabsorption requirements. The seat is equipped also withan inclination adjustment button, a rear support and aharness.

    The shell is made from polypropylene, vitreous atambient temperature. Moreover, the polymer behaviouris very complex; the same material might differently reactat the same solicitation and it is very sensitive with thetemperature, hydrostatic pressure and deformation speed.It might behave as a rubber, at high temperatures, or as abrittle material at low temperatures. Various laws, the linear-elastic law, the Johnson-Cook elasto-plastic model or theelasto-viscoplastic model of Boyce, Parks and Argon, canbe used to describe the behaviour of this material butnone is really satisfactory. To characterize this material,the elasto-plastic isotropic model was chosen, being theavailable MADYMO model with the closest behaviour tothe properties described by Lefeuve, Verron, Peseux,Delcroix and Rabeony [3].

    The central region of the child seat has been builtusing sixty-eight very thin ellipsoids with nearly rectangularcross-sections and four cylinders. The child seat side wingshave been reconstructed using finite elements, to allowfor a better representation of the contacts between thechild dummy and the restraining device and between theside wings of the child restraint system and the vehicleinterior. Because MADYMO software has no meshingtools, the coordinates for each node must be inputted foreach element. For this reason the mesh has to be obtainedfrom other software and CATIA has been chosen for thispurpose. Coordinates of arbitrary points were measuredusing the OPTOTRAK - POLARIS equipment(optoelectronic digitizer with LED pointer). The wingshave been reconstructed using this cloud of points andmeshed with CATIA software and the nodes and the

    Table 1 Parameters of the models and simulation

    System Input data

    Vehicle body Dimensional characteristics Material properties Position and dimensions of anchorages Safety belts mechanical characteristics Vehicle body deformation in case of side

    impactChild Dimensional characteristicsRestraint Material propertiesSystem Harness mechanical characteristics

    Acceleration pulseChild dummy According to MADYMO library model

    Acceleration pulse

    CHILD DUMMY MODEL

    A majority of tests and studies have been done using threeyears old dummies and test results are readily availablefor the Hybrid III three years old child dummy. For thesereasons the Hybrid III three years old child dummy waschosen for the simulation.

    The Hybrid III 3 years old child dummy numericalmodel is available in the MADYMO Data Base [1]. Themodel consists of 28 ellipsoids while some head regionsare built using the finite elements method. The contactbetween head and thorax is defined by default andadditional contacts were defined for our model: between

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  • Development of a mathematical model for evaluating child occupant behaviour

    Woodhead Publishing Ltd doi:10.1533/ijcr.2005.0330 113 IJCrash 2005 Vol. 10 No. 1

    elements were inputted in the MADYMO programme(Figure 1) which was used to assemble together (Figure2) the multi-body central part and the finite elements sidewings.

    Vehicle body dimensional characteristics andconstitutive material properties were either measured orexperimentally determined on a similar vehicle and itscomponents. The vehicle safety belt characteristics werealso either measured or adapted from available literaturedata [2]. The vehicle body structure is illustrated in Figure4.

    Cloud CATIA surface CATIA mesh MADYMO mesh

    Figure 1 Child restraint system side wings digitization.

    The inertial properties of the complete child restraintsystem were computed by MADYMO, based of those ofthe side wings and of the central part of the seat. A specialprogram was elaborated to compute the inertial propertiesof the central part. For the finite element parts, MADYMOcalculated the inertial properties using the dimensionalcharacteristics and the material properties. A contactstiffness function was also defined for the padding of thechild restraint system.

    The harness straps were represented using MADYMObelt segments. The release button and the harness retainerclip were built by ellipsoids and the harness characteristicswere measured or adapted from the available literaturedata [2]. The installation of the harness on the child dummyis presented in Figure 3.

    VEHICLE MODEL

    Since readily available test results (vehicle side impacttest and child restraint system test) had been performedon a Pontiac Grand Am 1999, this vehicle model waschosen for the simulation.

    (a) COSCO Touriva child seat [4] (b) MADYMO model

    Figure 2 Child restraint system model.

    Figure 3 Child restraint system harness installation.

    Figure 4 Vehicle body structure (adapted from [5]).

    The rear bench and the front seats were representedusing ellipsoids and they were linked to the referencespace using point restraints (a combination of threemutually perpendicular parallel springs and dampers), inorder to allow for their displacement in the case of theside impact. The contact stiffness for the rear bench cushionand rear bench back was defined by force-deflection curves(Table 2).

    Vehicle side frame, rear doors, rear panel, rear shelf,rear glass and rear doors glasses were built using finiteelements, to allow for a better representation of the contactsbetween the vehicle interior and child dummy and childrestraint system side wings and to make possible thesimulation of the vehicle body deformation, based onavailable test data. Arbitrary points were picked to digitizethe interior panels of the rear door (using theVISUALEYEZ equipment, electronic digitizer with laserpointer) and the metallic parts of the rear door (using theOPTOTRAK POLARIS equipment)...

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