EXPERIMENTAL CHARACTERIZATION OF CRACK-TIP DEFORMATION FIELDS IN ALLOY-718 AT HIGH-TEMPERATURES

A series of structure mechanics tests were conducted at temperatures o f 650 degrees C and 704 degrees C in air using Inconel(R) 718.(1) A no ncontacting measurement technique, based on computer vision and digita l image correlation was applied to directly measure surface displaceme nts and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures ale presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutio ns. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 degrees C during crack growth, and confirm that K-1 is a viable continuum-based fracture par ameter for creep crack growth characterization, Postmortem fractograph ic analyses indicate that grain boundary embrittlement lends to crack extension before a significant amount of creep occurs at this temperat ure. At higher temperatures, however, no crack growth was observed due to crack rip blunting and concurrent stress reduction after load appl ication.