Striation lines in intermittent fatigue crack growth in an Al alloy

Funding Information: M.J.A. and T.M. acknowledge support from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 857470 and from European Regional Development Fund via Foundation for Polish Science International Research Agenda PLUS programme Grant No. MAB PLUS/2018/8. M.J.A. acknowledges support from the Academy of Finland (Center of Excellence program, 278367 and 317464). J.K. acknowledges funding from Academy of Finland (308235) and Business Finland (211715). I.V.L. acknowledges funding from Academy of Finland (341440 and 346603). The authors acknowledge the computational resources provided by the Aalto University School of Science “Science-IT” project and the provision of facilities and technical support by Aalto University at OtaNano–Nanomicroscopy Center (Aalto-NMC). | openaire: EC/H2020/857470/EU//NOMATEN Fatigue failure of crystalline materials is a difficult problem in science and engineering, and recent results have shown that fatigue crack growth can occur in intermittent jumps that have fat-tailed distributions. As fatigue crack propagation is known to leave markings - called striations - on the fracture surface, the distances between these should also have fat-tailed distributions if the crack propagation is intermittent. Here, we combine macroscale crack tip tracking in fatigue crack growth measurements of aluminum 5005 samples with postmortem scanning electron microscopy imaging of the striation lines. We introduce two different methods for extracting the striation line spacing from the images. What we find is a similar distribution of striation spacings as jump sizes using one of our methods, but the average striation spacing does not correlate with the crack growth rate. We conclude that we observe avalanchelike crack propagation, reflected in both the macroscale crack tip tracking as well as the analysis of the fracture surfaces. Our results show that the fracture surfaces can be used to study the intermittency of fatigue crack propagation and in development of crack-resistant materials. The advantages and disadvantages of the two methods introduced are discussed.