Observation of Low-Cycle Fatigue Crack Initiation and Propagation in Anisotropic Rolled Steel under Biaxial Stressing

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Crack initiation and propagation in anisotropic rolled steel, the anisotropy of which was caused by a laminated structure of ferrite and pearlite bands, were studied by means of micro- and macroscopic observations of specimen surface under biaxial stress conditions. Slip bands and microcracks were initiated at an early stage of fatigue life in ferrite bands. Their direction coincided with that of the maximum shear stress, independently of the directions of the principal axes of anisotropy. However, growth of the microcracks to macrocracks and propagation of these macrocracks were much affected by anisotropy. When the direction of ferrite bands coincided with one of the two maximum shear stress directions, the micro- and macrocracks grew, coalesced and propagated fast within and in the direction of the ferrite bands. When the direction of ferrite bands intersected the maximum shear stress directions, growth of the microcracks in a ferrite band was halted at the ferrite-pearlite interface. Their growth to a macrocrack and its propagation were caused by coalescence of the halted microcracks across the pearlite bands after a certain number of stress cycles, resulting in intermittent and slower propagation of macrocracks with zig-zag paths which coincided with the directions of the maximum shear stresses.
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