Possible explanation for unexpected departures in hydrostatic tension-fracture strain relations

For materials which fail by void coalescence and growth, and possibly for solids failing by other mechanisms, it is commonly held that increasing tensile hydrostatic stress states at fracture produce lower strains to fracture. In tests on sheet material there is a marked change in strain ratio before fracture if plane-strain necking occurs. Then it may be shown that at the instant of fracture the current hydrostatic stress is most tensile for the greatest strains to fracture. These contrary observations may be explained by means of, path-dependent damage functions which involve not only the hydrostatic stress and effective strain but also the current strain ratio.