MECHANISMS OF FRACTURE PROPAGATION IN EXPERIMENTALLY EXTENDED SIOUX QUARTZITE
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abstract
Notched samples of Sioux quartzite have been extended to failure at temperatures T between 25 and 710 C, strain rates {greater-than with dot}e from 5.2 10-6 to 6.7 10-4 s-1 and 100 MPa confining pressure, and fracture morphologies examined by optical and scanning electron microscopy. In all the extension tests performed, sample failure occurred by the formation of a tensile fracture in or near the notch mid-plane. Differential stress magnitudes at failure are insensitive to variations in T and {greater-than with dot}e and calculated tensile strengths are comparable to previously reported values for unconfined tests at room temperature. Force-displacement records show a change in mechanical response, from linear elastic behavior at temperatures between 25 and 412C to significant yielding in compression prior to failure in the 500 to 710C tests. In all cases, fractures consist almost entirely of intragranular cracks (IGC) which exhibit characteristic surface features which can be used to infer local crack propagation directions. In one test in which fracturing was initiated at an edge flaw, IGC propagation directions show no overall trend. The observations indicate that the mechanism of fracture propagation involves nucleation, growth and coalescence of IGC which are themselves propagating in all directions in the plane of the main fracture. The onset of inelastic yielding prior to failure at elevated temperatures is attributed to the formation of thermally induced GBC which act as nucleation sites for IGC during extension. 1990.
