James K. Knowles Lecture and Caltech Solid Mechanics Symposium

Knowles was interested in the motion of shocks and phase boundaries in nonlinear materials such as shape memory alloys and rubbers. A crucial issue in these problems is the idea of a kinetic relation that connects the "driving force" and the speed of the discontinuity. In this presentation, I will explore this problem through a series of experiments in a few different systems – stretching of rubbers, elastomers, and shape memory alloys, and compression of foams. We perform specially designed experiments, with appropriate diagnostic tools to obtain spatio-temporally resolved strains and particle velocities, and explore the formation and propagation of shocks and phase boundaries. We also exploit the hysteretic response of rubbers to form shocks upon unloading from a highly stretched state. These experiments show that shocks are difficult to generate in rubbers and elastomers under tension, but are easily generated upon unloading. Metallic low-density foams provide another type of specimen for examination of compression shocks. Interpretation of these experiments also shows that the dynamic behavior of these materials differs significantly from the quasi-static behavior, leading to the need for inverse methods for identification of the material response.