CONSTRAINT EFFECTS ON FRACTURE-TOUGHNESS OF IMPACT-LOADED, PRECRACKEDCHARPY SPECIMENS

Impact-loaded, precracked Charpy specimens often play a crucial role i n irradiation surveillance programs for nuclear power plants. However, the small specimen size B = W = 10 mm limits the maximum value of cle avage fracture toughness J(c) that can be measured under elastic-plast ic conditions without loss of crack tip constraint. In this investigat ion, plane strain impact analyses provide detailed resolution of crack tip fields for impact-loaded specimens. Crack tip stress fields are c haracterized in terms of J-Q trajectories and the toughness-scaling mo del which is applicable for a cleavage fracture mechanism. Results of the analyses suggest deformation limits at fracture in the form of b > MJ(c)/sigma(0), where M approaches 25-30 for a strongly rate-sensitiv e material at impact velocities of 3-6 m s(-1). Based on direct compar ison of the static and dynamic J values computed using a domain integr al formulation, a new proposal emerges for the transition time, the ti me after impact at which interial effects diminish sufficiently for si mple evaluation of J using the plastic eta factor approach.