Multiscale Optical Methods to Measure the Fatigue Crack Closure

In this work, two different high-resolution optical tools are used to measure in-plane displacements on the crack flanks close to the crack tip, which can then be used to determine the presence of crack closure in propagating fatigue cracks. Middle tension (MT) AA6082-T6 aluminum alloy specimens were used with an initial notch, submitted to a cyclic fatigue-loading to open the crack with a pre-determined length. Then, the cracked specimen is monotonically loaded, with defined load increments, to reach the peak load. Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometry (ESPI) are employed to collect the experimental data at different resolution, calculating the displacement and deformation fields while the specimen is statically loaded. The force-displacement variation on the vertical virtual gauges defined on the crack flanks adjacent to the crack tip is also meticulously monitored. Results from both techniques are compared, and a complementary multiscale methodology is proposed for the crack closure phenomenon.