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  • 1.
    Juntikka, Rickard
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Energy Absorbing Material Concepts for Automotive Accident Injury Prevention2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    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.

  • 2.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Selection of energy absorbing materials for automotive head impact countermeasures2004In: Cellular polymers, ISSN 0262-4893, E-ISSN 1478-2421, Vol. 23, no 5, p. 263-297Article in journal (Refereed)
    Abstract [en]

    Material candidates for energy absorption in head impact countermeasures for automotive applications are evaluated using both quasi-static and dynamic test methods. Ranking of different materials turns out to be difficult since the mechanical response of a material could vary considerably with temperature, especially for polymers. Twenty-eight selected materials, including foams, honeycombs and balsa wood are tested and evaluated. The materials are subjected to a sequence of tests in order to thin out the array systematically. Quasi-static uni-axial compression is used for initial mapping of the selected materials, followed by quasi-static shear and dynamic uni-axial compression. The quasi-static test results show that balsa wood has by far the highest energy absorption capacity per unit weight but the yield strength is too high to make it suitable for the current application. The subsequent dynamic compression tests are performed for strain rates between 56 s(-1) and 120 s(-1) (impact velocities between 1.4 and 3 m/s) and temperatures in the range -20 - 60 degreesC. The test results emphasize the necessity of including both strain rate and temperature dependency to acquire reliable results from computer simulations of the selected materials.

  • 3.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Shear characterization of sandwich core materials using four-point bending2007In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 9, no 1, p. 67-94Article in journal (Refereed)
    Abstract [en]

    A new shear test method for sandwich core materials is proposed and evaluated. Sandwich beams are loaded in four-point bending, and the shear deformation is measured with two rotary sensors. Conditions of idealized sandwich theory are assumed to prevail, and the accuracy of the proposed methodology is thus dependent on a few mechanical and geometric relations between the sandwich constituents. The stress-strain responses for two polymer foam core materials, one relatively brittle and one relatively ductile, are extracted and compared with results from single-block shear tests of the same material batch. The new method provides several benefits with respect to the block shear test. It does not suffer from extreme stress concentrations and the specimens are tested under in-service conditions. Problems arise, however, for the ductile material, predominantly related to large deformations during the test eventually resulting in bending failure of the face sheet instead of shear failure of the core.

  • 4.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Weight-balanced drop test method for characterization of dynamic properties of cellular materials2004In: International Journal of Impact Engineering, ISSN 0734-743X, E-ISSN 1879-3509, Vol. 30, no 5, p. 541-554Article in journal (Refereed)
    Abstract [en]

    A novel weight-balanced drop rig used to evaluate the response of cellular materials subject to dynamic compression is presented. The testing method utilizes approximately constant velocity throughout the major part of the compression phase and the results compare well with results from other methods, reported in the literature. The repetitiveness is excellent, the rig is simple and the results are easily extracted. The applicability of the method for determination of elastic modulus is however limited to materials with relatively low stiffness. Accurate modulus measurements for stiff materials at high strain-rates require a very rigid and lightweight test set-up.

  • 5.
    Juntikka, Rickard
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Neuronic Engineering (Closed 20130701).
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Optimization of single skin surfaces for head injury prevention - a comparison of optima calculated for global versus local injury thresholds2004In: International Journal of Crashworthiness, ISSN 1358-8265, E-ISSN 1754-2111, Vol. 9, no 4, p. 365-379Article in journal (Refereed)
    Abstract [en]

    This paper describes optimizations of material properties for a bonnet-like plate using finite element calculations and the Euro-NCAP pedestrian head impact test. Four different head models were used for the impact simulations, a Euro-NCAP dummy head, a Hybrid III dummy head and two biomechanical head models exhibiting different mechanical properties for the brain tissue. The objective function was to minimize the displacement of the bonnet plate while satisfying constraints on the head injury criterion (HIC), the resultant contact force and, for the human head models, the strain in the brain tissue. An investigation was also conducted of the kinematics of the head models during impact, evaluating the energy distribution and the apparent mass. The analysis gave at hand that optimization of the plate with respect to impact with the Euro-NCAP and Hybrid III head models reached substantially, different results compared to impact with the biomechanical head models. For the latter case, the stiffness of the brain tissue influenced which constraints were active in the final solution. The investigation of the kinematics at impact showed that a substantial portion of energy was confined within the brain during impact for the biomechanical head models. The apparent mass at impact coincided with the actual mass for the rigid dummy heads while for the human head models it was roughly the mass of the skull only.

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