Measurement and simulation of energy absorption behaviour of motorcycle helmet materials

Abstract

Head injuries are one of the most frequent injuries resulting from a motorcycle accident,nhighlighting the importance of motorcycle helmets. The effect of repetitive minor impacts, nsuch as from drops or knocks, on the ability of a motorcycle helmet to absorb energy isnan area that has not been heavily researched in literature. Thus, the aim of this thesis isnto investigate the effects of these repetitive impacts on the ability of motorcycle helmetnmaterials to absorb impact energy. In addition, this thesis is aimed at evaluating thenappropriateness of material models and finite element techniques for the application ofnsimulating degradation of energy absorption of helmet materials. ABS plastic outer shellsnand expanded polystyrene foam liners were selected as the materials to be investigatedndue to their prevalence as motorcycle helmet materials.Drop-weight impact testing was carried out on flat plates of ABS and expandednpolystyrene, these plates were impacted a total of 10 times each for 15J of impact energy.nTwo grades of expanded polystyrene were investigated, VH (28kg/m3) and VVHn(38kg/m3). Energy absorption behaviour was investigated thorough processing of forcetimendata and impactor velocity. An FE model of the impact testing was created and thenresults analysed and compared with experimental results. A calibration process was usednto calibrate the MAT057 material model used to simulate expanded polystyrene.Analysis of the results of experimental impact testing indicated a degradationnof the energy absorption ability of the helmet materials due to repetitive minor impacts.nDegradation was observed to be significant for the first impact with a reduction of approximatelyn14%-15% observed for all plates. Percentage absorption was observed to tapernoff to a constant level after the first 2 impacts for all plates, with the VH liner platesntapering off to an average percentage energy absorption of 35% while VVH liner platesntapered off to an average of 28%.nResults from the created impact testing FE model showed good agreement withnthe first three impacts of experimental impact testing for VH foam liner plates. Energynabsorption degradation was over predicted by the FE model after the third impact asndecreasing to an average of 17%. FE model simulations of VVH foam liner plates showndecreased accuracy due to unrealistic loading and unloading speeds.nResearch into other material models is recommended to more realistically modelnthe energy absorption degradation of ABS and expanded polystyrene helmet materials. Innaddition, full scale helmet impact testing is suggested as a future study to more accuratelyndetermine the effects of minor impacts on energy absorption ability.n