Constraint Effects on Fracture Toughness of Impact Loaded, Pre-cracked Charpy Specimens

Impact loaded, pre-cracked Charpy specimens often play a crucial role in irradiation surveillance programs for nuclear power plants. However, the small specimen size (B = W = 10 mm) limits the maximum value of cleavage fracture toughness (Jc ) that can be measured under elastic-plastic conditions without loss of crack tip constraint. In this investigation, plane-strain impact analyses provide detailed resolution of crack-tip fields for impact loaded specimens. Crack-tip stress fields are characterized in terms of J-Q trajectories and the Toughness Scaling Model which is applicable for a cleavage fracture mechanism. Results of the analyses suggest deformation limits at fracture in the form of b >MJc/ao, where M approaches 25-30 for a strongly rate sensitive material at impact velocities of 3-6 m/ s. Based on direct comparison of the static and dynamic J-values computed using a domain-integral formulation, a new proposal emerges for the transition time, the time after impact at which inertial effects diminish sufficiently for simple evaluation of J using the plastic eta factor approach.U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research. Division of Engineering.Contract N00167-92-K-003