PATH AND KINETICS OF BRANCHING FROM DEFECTS UNDER UNIAXIAL AND BIAXIAL COMPRESSIVE LOADING

In order to understand the physical significance of branching features, especially path and kinetics, formed from defects in confined geological or mining conditions, a set of branch fractures obtained under uniaxial and biaxial loading at the tip of an isolated pre-existing oblique open slot was studied in PMMA (polymethylmethacrylate) plates. It was found that the branching always initiated perpendicular to the local plane tangent to the slot edge at the branch crack root while the branch crack root - crack tip distance increased with the slot - loading axis angle; the influence of biaxial loading is also discussed. The study of the stress field linked to an elliptical slot under shear conditions confirms these experimental observations and predicts the influence of the radius of curvature at the crack tip on the branching distance. The conditions of propagation are studied in terms of strain energy release rate along the stress trajectory starting from the point of maximum tensile stress at the slot edge, taking into account the presence of microcracks stemming from the slot tip induced by the sawing method. This allows one to describe the three successive regimes of propagation of the branch crack: spontaneous, catastrophic and controlled ruptures, according to the intensity of the applied compressive uniaxial stress and the size of pre-existing microcracks at the edge of the open slot. The computed variation of the branch crack length versus time agrees well with our experimental observations.