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ABSTRACT--The dynamic response of sheet metals at high strain rate is investigated with a tensile split Hopkinson bar test using plate type specimens. The tension split Hopkinson bar inevitably causes some errors in the strain at grips with the plate type specimens, since the grip and specimens disturb the one-dimensional wave propagation in bars. To validate the experiment, the level of error induced from the grips is estimated by comparing the waves acquired from experiments with the Pochhammer-Chree solution. The optimum geometry of the specimen is determined to minimize the loading equilibrium error. High strain rate tensile tests are then performed with auto-body sheet metals in order to construct their appropriate constitutive models for use in crash-worthiness evaluation.

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For the first time, we experimentally detected the formation of 6fold symmetry patterned microcrystals of AlN (aluminum nitride), such as simple prisms, stellar and sectored plates, and stellar and fernlike stellar dendrites. These crystals were formed during combustion of a solid mixture of Al + kAlF 3 (k = 0.05−0.1 mol) under a nitrogen atmosphere (P N2 = 2.5 MPa). The combustion temperature recorded with a thermocouple was in the 1700−1900 °C range. We examined the physical mechanisms governing the formation of AlN crystals, and we treated this problem as a case study of the dynamics of crystal growth from the liquid phase. Particular attention was given to the basic theoretical understanding of the subject, utilizing first-principles density functional theory calculations. The Wulff construction method was applied to identify equilibrium structures of nanoand microscale AlN crystals. Energy minimization arguments were used to show that certain crystal planes of AlN are more probable than others, giving the crystal 6-fold symmetry. The integrated experiments and computations form the basis for our proposal of the underlying mechanisms for nucleation and growth of the AlN crystals.