Photoelastic study of initial stages of brittle fracture in compression

Abstract

Initial stages of crack growth within various arrays of elongate open flaws were studied in photoelastic models subjected to uniaxial compression. The growth of inclined flaws into stable configuration with respect to stress for crack growth, with certain exceptions, is similar to that described previously (Bombolakis, 1964). Critical arrays for initial crack growth, however, include certain sets of closely spaced parallel flaws inclined 45° to the stress axis, in contrast to the 30° orientation predicted by Griffith theory. Despite the fact that flaws of various orientation and en-échelon configuration were located very close to one another in each array before crack growth began, coalescence of flaws did not occur during the first stage of crack growth within any array. Coalescence occurred during the second stage of crack growth, but this was in only one of the arrays less favorable for initial crack growth. It resulted in a partial fracture surface inclined approximately 36° to the stress axis. Characteristic features of the first and second stages of crack growth are the occurrences of small “shocks” and minor drops in the applied stress that causes crack growth. Such features were not observed in the several arrays in which subsequent stages of crack growth were obtained; crack growth was gradual instead of spasmodic, and it could be controlled by the loading. These results suggest a possible mechanism for some of the seismic events associated with shallow-focus earthquakes. Tests on several duplicate arrays indicate that the initial stages of crack growth depend in a remarkable way on the past history of deformation. Inclined flaws in those arrays either grew more or less parallel to the axial direction or perpendicular to it during the second stage of crack growth, depending on whether the stress for first-stage crack growth was increased directly or cycled to the stress for second-stage crack growth. This kind of dependency on history of deformation is of potential importance in interpretations of rock-deformation experiments and of fracture patterns in the field.