Stability of cracking

Quasistatic cracking entails a time rate of increase of crack area A ̇ such that generation of kinetic energy in the structure is negligible compared with the work of cracking. For splitting of a beam like structure, the limit of the quasistatic concept corresponds to a time rate of increase of crack length of about one thousandth of the velocity of a longitudinal wave in the material. Continuous quasistatic cracking is called stable, and when achieved, the fracture toughness (R) may be found experimentally by the method of Gurney and Hunt. Variation of R with A ̇ may then be found within the limits of the quasistatic concept. Variables affecting stability and within the control of the operator are, stiffness of testing machine and its speed of operation, geometry of test piece, and the introduction of rate sensitive devices operated by displacement of parts of the test piece.