Crack Growth during Brittle Fracture

Title: Crack Growth During Brittle Fracture in Compression. Author: Bartlett W. Paulding, Jr. Submitted to the Department of Geology and Geophysics February 9, 1965 in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology. Photoelastic analysis of several two-crack arrays predicts that compressive fracture is initiated at cracks oriented in a particular en schelon manner. Observation of partially-fractured samples of Westerly granite, obtained during uniaxial and confined compression tests by stopping the fracture process, indicate that fracture is initiated by en echelon arrays of biotite grains and pre-existing, trans-granular, cracks. Crack growth is predominantly parallel with the direction of maximum compression and the partial fractures incorporate both grain boundaries and trans-granular cracks. The porosity due to crack growth was determined by compressibility tests. Crack growth was found to be time dependent. Certain characteristics of the volumetric strain vs maximum stress difference curves for several rocks indicate that crack growth is initiated at a stress difference of about half the compressive strength. Therefore, neither the Griffith theory nor the McClintockWalsh modification can be expected to predict the strength of common rocks. However, good agreement exists between the McClintock-Walsh modification and the stress difference at the onset of crack growth. Because of the volume increase due to crack growth about 15 per cent of the work required to fracture a specimen in compression goes into work against the pressure medium. Thesis Supervisor: William F. Brace Associate Professor of Geology Title: