Active control of passive safety in passenger motor vehicles : a feasibility study investigating dynamic denting of members using pyrotechnic devices

Master Thesis


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University of Cape Town

This report describes a feasibility study investigating dynamic denting of members using pyrotechnic devices to engineer favourable energy absorption characteristics into thin walled tubes. A tube of sufficiently low slenderness ratio and wall thickness, when loaded axially to failure, will collapse in the progressive buckling mode. After the ultimate buckling load has been exceeded, and as the tube continues to compress, the load oscillates between loads considerably lower than the ultimate buckling load. The object of introducing an advantageous deformation is to decrease the ultimate buckling load to a magnitude comparable with the subsequent peak loads, but at the same time avoiding a change in the buckling mode which is not advantageous. Testing was limited to thin walled square mild steel tubes. The test procedure began with a process to determine the limitations imposed on the geometric imperfections that could be achieved by the use of explosive. It was found that all the explosively induced deformations were rounded, i.e. the dents were hemi-spherical in shape. It was also found that a smooth edged round hole could be created in the centre of the dent with the use of a round, flat explosive charge. Geometric imperfections that could be induced explosively in the specimens (as well as other deformation shapes, tested for comparative purposes) were mechanically formed in the specimens. The tubes were then quasi - statically crushed to determine the energy absorption characteristics induced by the deformations. When spherical dents were induced, the deformation affected the tube beyond the immediate spherical dent and hence the distance between the plastic hinges was increased and instabilities in the crushing process were introduced. Holes (without any visible denting) decreased the distance between the plastic hinges and thus also induced instabilities. In both cases the tubes tended to skew over to one side and in extreme cases Euler buckling ensued.