In-situ observation of strain evolution and ratchetting of growing fatigue cracks

Crack-tip strain localization and accumulation has long been recognized while the mechanics and physics of fatigue are yet to be understood for complicated cracking modes, microstructural damage, and cross-scale fatigue crack growth modeling. In this work, near-tip strain was characterized at high-resolution and statistically analyzed to develop a strain-based yielding criterion for crack-tip plasticity evaluation which was found applicable to both tensile and fatigue loadings. The near-tip strain of face centered cubic 304 stainless steel satisfied Lognormal distribution whereas body centered cubic 25Cr steel obeyed Gauss distribution, and was found evolving Logistic-Gauss-Lognormal with fatigue crack growing and plasticity increasing. The relative strain accumulation was proposed and found well representing branch or stable growth mode, and drove fatigue crack growth when relative strain accumulation was beyond 1.3. The cracking speed parameter in terms of strain ratchetting per crack extension was established, which indicated that different cracking at macro-scale behaved similarly at the atomic scale for a given material. The microstructural damage analysis near crack-tip illustrated that fatigue crack growth of 25Cr steel was governed by breakdown of lathy martensite and formation of fine grains while the cracking of 304ss was accompanied with austenite-to-martensite transformation that was occurred at a near-tip strain of approximately 13%.