Measuring fatigue crack deflections via cracking of constituent particles in AA7050 via in situ x-ray synchrotron-based micro-tomography

Abstract Brittle constituent particles play a significant role in the fatigue behavior of 2XXX and 7XXX series aluminum alloys. In this work, in situ fatigue crack growth combined with x-ray synchrotron computed micro-tomography (μXSCT) characterization is used to observe and quantify three dimensional propagating fatigue cracks interacting with constituent particles. A relationship is observed with the size of the constituent particle ahead of the crack tip and the maximum distance the crack path deflects to interact with the particle. This crack deflection behavior agrees with the Murakami and Endo model, for the effect of small defects on fatigue performance. A stress field analysis, based on (i) an Eshelby inhomogeneity solution of a perfectly bonded particle, and (ii) a cracked particle, based on the Westergaard solution, indicates the stress concentration developed by the presence of a constituent particle contributes to crack path deflections. This finding has implications for modeling fatigue crack growth in AA7050-T7451, specifically in determining the actual crack length in terms of a tortuous crack topography, as opposed to a flat projected length.

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