COD measurements at various positions along a crack

Load versus crack-opening displacement (COD) was measured at various positions along the border of a fatigue crack as it grew from a small surface crack on the edge of an aluminum specimen into a through-the-thickness crack. Displacements were measured with a laser-based interferometric system with a gage length of 70 m and a resolution of 0.01 m. These load-COD curves can be used to determine opening loads and thereby investigate the effect of closure on the growth of cracks. In general, the opening loads decrease as the crack grows. The compliances after the crack is fully open can also be measured, and show generally good agreement with the predictions of linear-elastic fracture mechanics.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.