Experimental and analytical estimates of fatigue crack closure in an aluminium-copper alloy 1: Laser interferometry and electron fractography

Results of an experimental estimation of the crack closure stress, using electron fractography and laser interferometry for CT specimens of an aluminium-copper alloy are presented in this paper. Crack closure stress was estimated at stress ratios of 0.1, 0.3, 0.5 and 0.7 using programmed load sequences. In addition, the crack closure stress was estimated for a simple random load sequence. Scanning and transmission electron fractography provided a similar estimate of crack closure, with the later technique providing better quality fractographs. Laser-interferometry based estimates were closer to fractographic estimates when the crack-tip is very close to the micro-indent axis. In addition, laser-interferometry based estimates of the crack opening stress for a random load sequence, confirmed the possibility of a cycle-by-cycle variation of the crack opening stress