A Study of the Shock Loading of Materials

A bstract Elastic and inelastic processes are involved in the propagation of any particular disturbance and the literature on wave propagation is reviewed. A laser interferometer was constructed to resolve the details in the wave structure and which had sufficient sensitivity to resolve the reverberations of the elastic precursor between the free surface and the oncoming plastic wave. This non‐contacting surface technique offers the advantage of excellent time resolution (∼5 nanoseconds) and space resolution (0.06‐micron) together with small surface area requirements. The stress pulse was generated in either 6061‐T6 aluminum or aluminum oxide with an exploding foil driven Mylar flyer. A stress wave analyzing program (SWAP) was written in Fortran to compute stress time histories in problems involving one‐dimensional strain configurations in elastic, plastic, and hydrodynamic materials. SWAP uses the method of characteristics approach and represents all stress wave shapes by a series of shock waves. A sufficient amount of experimental and theoretical evidence has now been gathered to support the assertion that the critical normal fracture stress criterion for spall must be at least extended, if not totally revised. There is a stress‐dependent fracture delay time which decreases with in‐ceasing stress, not only because more cracks are nucleated, but also because the acceleration of the crack tips increases. Similarly ductile materials require more time to fracture than brittle materials because stresses are relieved not only by fracture but also by flow. The results provide additional evidence for the necessity of including a time‐related parameter in the spall criterion.