Fatigue and Crack Growth under Constant- and Variable-Amplitude Loading in 9310 Steel Using “Rainflow-on-the-Fly” Methodology

Fatigue of materials, like alloys, is basically fatigue-crack growth in small cracks nucleating and growing from micro-structural features, such as inclusions voids, or at micro-machining marks, large to failure. Thus, the traditional nucleation stage (Ni) microcracks (initial flaw sizes 1 30 μm about 250 μm) metal alloys. crack-growth tests were conducted on a 9310 steel under laboratory air room temperature conditions. Large-crack-growth-rate data obtained compact, C(T), specimens over wide range rates threshold fracture for load ratios (R) 0.1 0.95. New test procedures based compression pre-cracking used near-threshold regime because current ASTM method (load shedding) has been shown cause load-history effects with elevated thresholds slower than steady-state behavior constant-amplitude loading. High load-ratio approximate small-crack-growth-rate behavior. A crack-closure model, FASTRAN, was develop baseline crack-growth-rate curve. single-edge-notch-bend, SEN(B), both Cold-Turbistan+ spectrum Under loading, model “Rainflow-on-the-Fly” subroutine account damage. Test results compared fatigue-life calculations made loading establish initial microstructural size predictions FASTRAN code using same micro-structural, semi-circular, surface-flaw (6-μm). unified fatigue approach, crack (small-crack growth) large-crack failure mechanics principles. The validated predictions. In general, agreed well data.