Observations of grain level damage accumulation in fatigue

Measurements of fatigue crack opening levels are made at multiple scales using three methods based on the technique of digital image correlation (DIC). The first method uses two-point DIC displacement gages at the microscale (0.33 μm/pix) to find local crack opening levels. Crack tip opening levels are also be determined from these measurements with the ASTM compliance offset method. The second and third methods use full-field DIC displacements at the macroscale (1.1 and 3.9 μg/pix). Effective stress intensity factors are then calculated by performing a least squares regression on full-field displacements perpendicular to the crack tip. The second method (the effective K method) calculates opening loads by comparing regression stress intensity factors to theoretical stress intensity factors while the third method (the K compliance change method) determines crack opening loads by detecting a compliance change in the load vs. K curve. The effective K method is found to give crack opening values far from the crack tip while the K compliance change method gives crack opening values close to near crack tip local opening levels. The crack tip opening level predicted by the methods are around 35% of peak load for grade 2 titanium (with R=0.05 loading). DIC at 0.33 μm/pix is also used to observe crack growth in a titanium specimen with large grains (obtained by heat treating). Large grain sizes will allow for quantitative local measurements of grain level deformations. In this work, a wake of plastically deformed material related to the crack closure phenomenon is observed as the crack tip propagates through the field of view.