A B S T R A C T
A brittle crack is in a state of "semi-stable" equilibrium when its stress intensity factor, K0, is equal to the fracture toughness, Kc; any perturbation which makes K o > K c will cause crack extension. The influence of the actual value of this perturbation on the resulting motion is discussed. An approximation is introduced which provides some insight into the equation of motion of a Griffith crack, and allows it to be compared with equations proposed by earlier writers. The disturbance generated by either one of the moving crack tips is shown to have a generally negligible effect on the motion of the other. The dynamic stress intensity factor associated with a crack extension of the type observed experimentally is calculated and compared with estimates used in laboratory practice: the quasi-steady approximation is found to yield estimates which are accurate to within 5%. While a detailed analysis is presented only for anti-plane deformation, the relevance of the results to the more realistic case of plane deformation is emphasized throughout. In particular upper and lower bounds for the crack extension force in mode I are given in a form convenient for practical purposes.