On enhancing ductility of brittle-like materials by high confining hydro-pressure

Abstract

An explanation is sought for the substantial increase in apparent ductility of brittle-like solids (increasing their tensile strain to failure by three orders of magnitude) when surrounded by sufficiently high confining hydrostatic pressure (practically activated by fluid pressure). The magnitude of this hydro-pressure which is required to initiate the ductility enhancement is hypothesized to be the one that causes a complete closure of pre-existing micro-flaws (cavities, voids, etc). With further more hydro-pressure it resists slip between the flanks up to the order of the material grains boundaries slip. In such event, the solids behave as ‘flawless’ materials whose tensile failure is determined by necking instability preceded by large deformation, akin to ductile-like materials. When the confining hydro-pressure is insufficient for complete closure of all voids, it is supposed that even a minute amount of micro-voids may grow rapidly, link with neighbors, and cause brittle-like failure with barely noticeable strain. These geometrical events are traced here in detail in time-like scale (namely, by incremental increase of the tensile load at a given confining pressure). It is done by solving the incremental form of the governing equations that define, in our view, the mechanics of the considered situation. The constitutive equation of the net material matrix follows a power-law hardening, but the overall evolutionary behavior of the cavitated material is continuously perturbed by the presence of the shrinking/expanding cavities. The forthcoming solution leads to results which are generally in satisfactory agreement with experiments found in the open literature.