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From the viewpoint of improving both the crash safety and the fuel efficiency, various shaped thin-walled tubes have been utilized as energy absorbers of automobiles such as front side members, crash boxes and so forth. In the axial crushing test of the regular polygonal tube, if the number of angles was small enough, it showed a certain inherent wrinkle mode, and the mean buckling load increased with that number, while it showed the mode of cylindrical tube if that number became larger. In the oblique crushing test, the same tendency was also shown within the range that the transition from axial collapse to bending collapse did not occur. This transition considerably decreases the mean buckling load so that it is important to know the threshold crush angle for the transition. Then, the crushing behavior of regular 4-12 angled tubes were investigated with changing the crush angle mainly by the experiment. The threshold angle is sensitively influenced by the initial imperfection and the boundary condition so that both the threshold angles obtained by the experiment and by the calculation for the square tube are 8-13° smaller than that predicted by the equation proposed by Han and Park. For the carbon steel S25C, the increase of the strain rate in the axial collapse mode raises the mean buckling load, while it shows little strain rate dependence once the transition occurs.

A new method of applying a dynamic load directly to a tensile specimen was developed. By combining the new method with the testing method based on a sensing block [1-5], we improved the testing method to obtain the entire tensile stress-strain curves for a wide range of strain rates on the order of 10 -3 ∼10 3 s -1 , with remarkable accuracy, especially for the high strain rates range. Based on numerical simulations and experiments, the accuracy of the measurement of the non-coaxial Hopkinson bar method [6,7] was discussed. By performing a series of numerical simulations, the relation between the gauge length of a specimen and the testing strain rate, necessary for obtaining an accurate dynamic stress-strain curves corresponding to a testing error of approximately 2% or 4%, was also obtained for a wide range of strain rates from 10s -1 to 10 4 s -1 .