Striation lines in intermittent fatigue crack growth in an Al alloy

Fatigue failure of crystalline materials is a difficult problem in science\nand engineering, and recent results have shown that fatigue crack growth can\noccur in intermittent jumps which have fat-tailed distributions. As fatigue\ncrack propagation is known to leave markings -- called striations -- on the\nfracture surface, the distances between these should also have fat-tailed\ndistributions, if the crack propagation is intermittent. Here, we combine\nmacroscale crack tip tracking in fatigue crack growth measurements of aluminum\n5005 samples with \\emph{post-mortem} scanning electron microscopy imaging of\nthe striation lines. We introduce two different methods for extracting the\nstriation line spacing from the images. What we find is a similar distribution\nof striation spacings as jump sizes using one of our methods, but the average\nstriation spacing does not correlate with the crack growth rate. We conclude\nthat we observe avalanche-like crack propagation, reflected in both the\nmacroscale crack tip tracking as well as the analysis of the fracture surfaces.\nOur results show that the fracture surfaces can be used to study the\nintermittency of fatigue crack propagation and in development of\ncrack-resistant materials. The advantages and disadvantages of the two methods\nintroduced are discussed.\n