Abstract
Specimens of two different high-strength, low-alloy steels were tested in both the longitudinal and the transverse directions to determine their resistance to fatigue loading at one stress level. These steels differed chiefly in the shape, number, composition, and size of constituent particles. Specimens cut at 0° from Steel 1 exhibited a 30–40% higher fatigue life than those cut at 90°, whereas Steel 2 showed practically equal fatigue lives in both 0° and 90° directions; the rate of crack propagation showed similar anisotropy in Steel 1 but not in Steel 2. Tests were also carried out on specimens cut from sheets prestrained at 0° and 90° before fatigue testing, and this straining considerably lengthened the fatigue life; presumably the increase in yield strength owing to strain hardening outweighed the effect of microstructural damage. The progress of the fatigue striations in a particular direction in the ferrite phase could be halted by both the inclusions and the carbides, the crack then propagating by ductile or shear fracture either inter- or transgranularly.
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