Cracks produced in several samples of Westerly (RI) granite by uniaxial stress have been examined by measuring high-precision strain as a function of hydrostatic pressure over several pressure cycles to 2 kbars. These new data show: (1) Uniaxial stressing to 85 per cent of failure strength increases total crack porosity by 30 to 45 per cent. Most of this increase is due to the formation of cracks in the rift plane (which in our experiments was parallel to the stress direction). (2) The closure pressure of most of the new cracks is 100 bars. (3) Stressing decreases the closure pressure (aspect ratio) of some pre-existing cracks. (4) Scanning electron microscope observations show that some cracks are elongated by stress in a direction parallel to maximum compression. (5) Hydrostatic pressure cycling of virgin and stressed rocks decreases total crack porosity by 20 to 60 per cent between the first and second cycles, but less than 20 per cent between the second and third cycles. (6) Stressing reopens cracks closed by hydrostatic pressure. (7) Repeated stressing increases the total crack porosity after each stress cycle.

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