Experimental investigation of near crack tip creep deformation in alloy 800 at 650°C

Using a novel, noncontacting experimental method, the values for creep fracture parameters C(t) and C* have been experimentally quantified from full field, time-dependent surface deformation measurements in the crack-tip region for the first time. Near-tip experimental deformation results were obtained for a stationary crack in alloy IN800 at 650 degrees C and compared to predictions based on both far-field measurement and finite element analyses. Results indicate that (a) for short times, the C(t)-integral is time and path dependent; (b) for long times, the C(t)-integral approached a constant for all paths which is agreement with C* values obtained from far-field experimental data. Comparisons of experimental results with FEM solutions were illustrated by means of radial and angular variations, full-field contour plots, as well as line integration form. Results from these comparisons indicate that (c) the experimental strain data portrays trends similar to those observed in the FEM solutions; (d) experimental, near-tip strain values were different in magnitude from the predicted values for all angles, and (e) the transition time obtained from the tests was about 10 times longer than that calculated from a formula proposed by Riedel and Rice (1980), indicating that primary creep cannot be neglected for this material.