Energy Absorbing Material Concepts for Automotive Accident Injury Prevention

Sammanfattning: Although automotive safety has experienced a rapiddevelopment during the last decades, traffic related accidentsrank very high in many countries in terms of cause of death.Statistics show that the head is the body region mostfrequently injured in motor vehicle accidents and the primarycause of death is head injuries. Standards have been developedwhich through instrumented dummies, accelerometers anddeformation sensors measure the severity of a car accident.Head impacts are simulated by using a headform propelled towardspecified targets such as the B-pillar and bonnet to simulatevehicle-to-vehicle side impact and vehicle-pedestriancollisions. These standards require the design and applicationof energy absorbing countermeasures.The work presented herein incorporates an extensive study ofenergy absorbing materials, foams and honeycombs, and testmethods to determine their properties. The test methodsincluded quasi-static uni-axial compression and shear tests anddynamic uni-axial compression experiments. The dynamicexperiments were performed using a novel weight-balanced droprig, which utilises thespecific property of cellular materialsof a plateau level in compression. The quasi-static testsshowed that balsa possesses the by far best energy absorbingcapability per unit weight and that properties of foams andhoneycombs are possible to estimate when good bulk materialdata are available. The dynamic experiments illustrate thenecessity to include both temperature and strain-ratedependence in transferring the results into finite elementmaterial models.The shear properties of two different PVC sandwich cores,one rigid (Divinycell H60) and one relatively ductile(Divinycell HD100), were evaluated using a new test methodbased on four-point-bending (FPB) and compared with block sheartests. The new method takes advantage of the FPB method in thatit does not induce severe stress concentrations in theinvestigated core material. The core is tested under conditionssimilar to in-service use of the sandwich material making themeasured stress-strain response relevant even in a homogenisedsense when modelling the mechanical behaviour of the core. Theresults show that the FPB method has several advantagescompared with the shear block test method. It produced a highershear modulus, a more distinct yield point and a higher yieldstress than the shear block test results. The FPB method didhowever fail to characterise the complete stress-strainresponse of the relatively ductile HD100 core material. Largedeformations caused local bending of the laterally compressedface sheet, eventually leading to premature failure of thespecimen.Finally, a finite element optimisation study of theEuro-NCAP bonnet pedestrian head impact was performed usingfour different head models, a Euro-NCAP dummy head, a HybridIII dummy head and two biomechanical head models exhibitingdifferent mechanical properties for the brain tissue. Theobjective was to minimise the bonnet deflection and theconstraints were the head injury criterion (HIC), the resultantcontact force and, for the human head models, the strain in thebrain tissue. The results illustrate the deviation betweenimpacts using rigid and non-rigid representations of the humanhead. The analysis showed that optimisation of the bonnet withrespect to impact with the Euro-NCAP and Hybrid III head modelsreached substantially different results compared to impact withthe biomechanical head models.