Interior energy focusing within an elasto-plastic material

In this paper we consider the problem of focusing acoustic energy within a subsurface volume in a solid by means of a designed surface pattern, and ask how that focusing is affected by plastic yield of the material. Surface force patterns that yield efficient subsurface acoustic focusing have been designed using optimal control theory, based on a linear elastic model of a solid. The acoustic waves generated by these forces then are propagated, via numerical algorithms, in a model solid that exhibits plastic yield. Numerical results indicate that as the amplitude of the force increases, yield begins to develop at the focus, with the formation of an expanding region of permanent plastic deformation. Despite the occurrence of yield near the focus, acoustic energy still can be delivered to the focal volume with good efficiency.