DYNAMIC GROWTH OF MICROVOIDS UNDER COMBINED HYDROSTATIC AND DEVIATORIC STRESSES

This paper deals with the analytical study of the dynamic plastic growth of microvoids under the combined action of hydrostatic and deviatoric stresses. The results of this analysis are discussed with the help of a numerical study of the void growth relationship derived, and applied to the case of spall fracture- The conclusion is that void expansion may be affected in different manners by the presence of a field of deviatoric (purely distortional) strain rates. If the deviatoric plastic strain rate is not large compared with the rate of volumetric expansion, then, for void growth controlled fracture. the spall strength of the material tends to decrease with respect to a purely hydrostatic stress. The quantitative loss of strength may be important, depending upon the loading conditions. When void growth initiates in a state of very large deviatoric strain rates then, under the conditions of the analysis, the volumetric expansion of the voids may require excessive large stresses, so as to become very difficult in practice. Then, in such a situation a different mechanism, such as void nucleation for instance, might control the fracture process rather than plastic void growth.