Near-threshold propagation of mode II and mode III fatigue cracks in ferrite and austenite |
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Authors: | T. Vojtek R. Pippan A. Hohenwarter L. Holáň J. Pokluda |
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Affiliation: | 1. Faculty of Mechanical Engineering, Brno University of Technology, Technická 2, CZ-61669 Brno, Czech Republic;2. Central European Institute of Technology, Brno University of Technology, Technická 10, CZ-61600 Brno, Czech Republic;3. Austrian Academy of Sciences, Erich Schmid Institute of Materials Science, Jahnstr. 12, A-8700 Leoben, Austria;4. Department of Materials Physics, Montanuniversität Leoben, Jahnstr. 12, A-8700 Leoben, Austria |
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Abstract: | The near-threshold behavior of mode II and mode III long fatigue cracks in ferritic (ARMCO iron) and austenitic (X5CrNi18-10) steel were experimentally studied using various samples specially prepared to obtain the effective threshold values ΔKIIeff,th and ΔKIIIeff,th. In both investigated materials, the effective thresholds for mode III were ~1.7 times higher than those for mode II. Three-dimensional topological data obtained by the examination of fracture surfaces using stereophotogrammetry were utilized to identify crack growth micromechanisms. In austenite, mode I branching of both the mode II and mode III cracks started at the very onset of crack growth. On the other hand, all cracks in ferrite propagated in crystallographically assisted local mixed mode I + II + III with mode II dominance. These experimental results can be understood in terms of crack growth micromechanisms according to a deformation model in ferrite and a decohesion model in austenite. The dissimilarity of growth mechanisms in ferrite and austenite may be attributed to a different number of available slip systems in body-centered cubic and face-centered cubic metals. |
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Keywords: | Effective threshold Shear-mode fatigue cracks Ferrite Austenite Dislocation crack growth models |
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