Precracking of fracture-toughness specimens of hardmetals by wedge indentation

Abstract

A technique has been devised for precracking fracture-toughness specimens of hardmetals whereby a sharp crack is introduced into bend-test specimens using a wedge indenter and residual stresses are relieved by removing the surface layer containing the indentation. The technique is reproducible, simple to use, and does not suffer from the inaccuracies and difficulties of interpretation inherent in using the spark-machining and pyramid-indentation techniques. It is found that annealing is not a satisfactory way of removing residual stresses from specimens precracked by indentation since it produces effects that give too high a value for fracture toughness. Spark-machined specimens do not give reliable fracture-toughness data because there are practical problems in introducing a notch sufficiently narrow to simulate the effect of a thin crack and because thermal damage caused by the spark discharge probably alters the properties of the material at the notch root.

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