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Static fatigue of graphite Hodkinson, Pauline H.

Abstract

Subcritical crack growth has been studied in two different grades of a fine-grained isotropic graphite and in pyrolytic graphite. The double torsion technique was used. Room temperature tests revealed a stress intensity-crack velocity behaviour similar to the third stage of crack growth in soda-lime glass. Thus, it appears that graphite undergoes static fatigue at room temperature only at large fractions (^92%) of the critical stress intensity. Structural characterisation studies have enabled the slow crack growth behaviour to be related to microstructural features. Transverse rupture tests have shown that fracture in graphite is initiated at surface flaws, mainly grain pull-outs produced by the polishing process or machining operation employed. Subsidiary cracking or microcracking occurs in the regions around these flaws on loading prior to failure. A mechanism of slow crack growth has been discussed which takes into account the formation of micro-cracks and their link-up to form a propagating crack. Slow crack growth tests were also performed at 500°C on the isotropic graphites to reveal the effect of temperature and environment on crack growth. These tests were complemented by thermo-gravimetric analysis studies. The KT-V diagrams at 500°C still showed stage III characteristics but the slopes were about half the values obtained at room temperature, indicating that thermal and/or environmental assistance of slow crack growth in graphite occurs at this temperature.

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