UC San Diego

Heterogeneous materials under dynamic loading exhibit dramatic transformations resulting in self-organization and in situ microstructural changes due to the complex interaction of nonlinearity, dispersion, different mechanisms of softening, and fracture. We use numerical simulations to explain the diverse mechanisms of deformation, localization and in situ patterning exhibited by these materials and compare them to experimental results and phenomenological models.

In Chapters 2 and 3 we study dissipative Al-W laminates with unit cells 1, 2 and 4 mm under high velocity impact loading of different durations. Depending on the duration of the loading pulse, a qualitatively new solitarylike wave, a train of such waves or a quasistationary shockwave is formed in these dissipative laminates. A phenomenological model based on the Korteweg-de Vries equation is formulated introducing important space/time scaling for solitary and shock waves depending on materials properties.

In Chapter 4, Thick-Walled Cylinder experiments were conducted with highly heterogeneous porous mixtures of Al and W granular materials. Results were compared with numerical simulations to explore the effects of grain size and porosity on the mechanism of cavity instability. Numerical simulations demonstrated that a shear band like phenomena occurs along weak paths created by softer Al particles. The samples with initial porosity, delayed this mechanism of instability. Samples with grain sizes 400 and 100 micron demonstrated loss of cylindrical symmetry during collapse and sample with grain size 40 micron collapsed with mostly preserved cylindrical symmetry. This was consistent with experimental observation of cavity collapse in mixtures with 40 and 1 micron W particles.

In Chapter 5, explosively driven Thick-Walled Cylinder experiments were conducted with samples of 4340 steel having variable microstructures achieved through heat treatment. It was observed experimentally that the change in the initial microstructure resulted in completely different dynamic behavior and self-organization of shear bands/cracks. The heat treated sample, catastrophically failed due the formation and interaction of shear bands developing into cracks. Numerical simulations are compared to the experimental results. It was shown that the inclusion of artificial defects is necessary to reproduce self-organized pattern of shear bands and cracks observed in experiments.