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Dynamic fracture is a time-dependent phenomena in which the spall stress depends on the stress pulse duration. It is shown that the empirical relation between the spall stress and the square root of the stress gradient is equivalent to a simple impulse criterion. A more general criterion based on the concept of cumulative damage is proposed, and good agreement is obtained between calculated and observed values of the spall layer thickness in aluminum.

Experimental data on the shock-wave compaction of 78% dense porous aluminum are presented and compared with theoretical predictions from a mathematical theory of time-dependent pore closure for ductile materials with isolated spherical voids. A recently modified form of the theory was used which includes deviatoric stresses and material viscosity. Incorporation of the work-hardening properties of the solid resolved previous discrepancies between measured hydrostats and predictions assuming elastic-perfectly plastic deformation of the matrix. Generally good agreement was also observed between experimental quasistatic and shock data, although there was some evidence that the quasistatic data were strain-history dependent. Finally, a satisfactory check of the theory was obtained by comparing predicted steady-wave rise times with measured stress-wave profile data. These results suggest that the current version of the theory is a significant improvement over past models, because it permits a more accurate estimate of the influence of both pore size and the plastic deformation characteristics of the metal matrix on shock compaction.

An exampleof the applicationof the power law relationshipfor sampleconsolidationwith time, Equation2.3.3.1, to data representingthe changein densitywith time of a mixtureof PVC, polyethyleneparts,and surgicalglovesundera constantaxial stress of 13.