GS(2)-6(GSW0153) CTOA of a Tunneling Crack-a Hybrid Analysis

Abstract

A hybrid moire-finite element analysis (FEA) method was used to determine the crack tip opening angle (CTOA) along a tunneling crack front in a three-point bend (SEB) specimen. The specimen was machined from a ductile 2024-T351 aluminum plate of 8.1 mm thickness and was pre-fatigued to an initial crack length, α_0,of α_0/W=0.45. The specimen was subjected to stable crack growth varying Δα=0.5 to 5.5 mm, after which the specimen was post fatigued to mark the final crack front and then loaded to failure. The changes in the μ-and ν-displacement fields on the specimen surface with increasing load and crack growth were recorded by moire interferometry. The crack profiles from five broken specimens were assembled to compose sequential tunneling crack profiles in a single specimen. A quarter segment of the SEB specimen was modeled with a truncated 3-D elastic-plastic FEA model for computational efficiency. A preliminary 3-D elastic-plastic analysis of the full half segment with α/W=0.45 was used to determine the minimum distance from the crack plane at which the von-Mises stress is uniform through the specimen thickness. The span-wise length of the truncated FEA model, with a minimum element of 0.25 mm cube, was set to this distance. The measured surface displacements, which were obtained by moire analysis, with successively increasing load and incremental stable crack growth were prescribed on the truncated edge. The CTOA was obtained from the computed crack opening displacement, approximately 1 mm behind, and normal to the crack front. The figures below show the CTOA variations with local crack extension on the surface, length-wise quarter-and mid-planes of the SEB specimen. CTOA obtained directly from the moire data is included for comparison. The good agreement between the measured and computed surface CTOA are indicative of the accuracy of the procedure.