MICROCRACK CLOSURE IN ROCKS UNDER STRESS - DIRECT OBSERVATION

Microcrack closure in rocks under increasing stress was observed directly with a scanning electron microscope. Uniaxial stresses to 300 bars were applied with a small hydraulic press to specimens of Westerly (RI) granite, both unheated and previously heat cycled to 500 degree C, and of Frederick (MD) diabase, heat cycled to 700 degree C. Closure characteristics (rate, final closure pressure, etc. ) depend on crack orientation, shape, surface roughness, and on the nature of fracture intersections and interactions. Closure characteristics varied significantly among the samples. The variations in closure rate and charactere were also observed in strain measurements.