Abstract
Fatigue of materials, like alloys, is basically fatigue-crack growth in small cracks nucleating and growing from micro-structural features, such as inclusions and voids, or at micro-machining marks, and large cracks growing to failure. Thus, the traditional fatigue-crack nucleation stage (Ni) is basically the growth in microcracks (initial flaw sizes of 1 to 30 μm growing to about 250 μm) in metal alloys. Fatigue and crack-growth tests were conducted on a 9310 steel under laboratory air and room temperature conditions. Large-crack-growth-rate data were obtained from compact, C(T), specimens over a wide range in rates from threshold to fracture for load ratios (R) of 0.1 to 0.95. New test procedures based on compression pre-cracking were used in the near-threshold regime because the current ASTM test method (load shedding) has been shown to cause load-history effects with elevated thresholds and slower rates than steady-state behavior under constant-amplitude loading. High load-ratio (R) data were used to approximate small-crack-growth-rate behavior. A crack-closure model, FASTRAN, was used to develop the baseline crack-growth-rate curve. Fatigue tests were conducted on single-edge-notch-bend, SEN(B), specimens under both constant-amplitude and a Cold-Turbistan+ spectrum loading. Under spectrum loading, the model used a “Rainflow-on-the-Fly” subroutine to account for crack-growth damage. Test results were compared to fatigue-life calculations made under constant-amplitude loading to establish the initial microstructural flaw size and predictions made under spectrum loading from the FASTRAN code using the same micro-structural, semi-circular, surface-flaw size (6-μm). Thus, the model is a unified fatigue approach, from crack nucleation (small-crack growth) and large-crack growth to failure using fracture mechanics principles. The model was validated for both fatigue and crack-growth predictions. In general, predictions agreed well with the test data.
Highlights
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Test results were compared tocompared fatigue-life fatigue-life calculations made under constant-amplitude loading to establish the initial microstruccalculations made under constant-amplitude loading to establish the initial microstructural flaw turaland flaw size and predictions under spectrum from thecode code using the size predictions made undermade spectrum loading fromloading the FASTRAN
Like alloys, is basically the fatigue-crack growth in small cracks nucleating at micro-structural features, such as inclusions and voids, or at micro-machining marks, and large cracks growing to failure
Summary
Fatigue and crack-growth tests and room room temperature temperature conditions. Large-crack-growth-rate data data were were obtained obtained from from compact, compact, C(T), C(T), and specimens over a wide range in rates from threshold to fracture for load ratios (R). High load-ratio (R) used data to were used to approximate small-crack-growth-rate behavior. A crack-closure crack-closure model, FASTRAN, was used to develop the baseline crack-growth-rate curve. FASTRAN, was used to develop the baseline crack-growth-rate curve. Fly” subroutine to account for crack-growth damage. Test results were compared tocompared fatigue-life fatigue-life calculations made under constant-amplitude loading to establish the initial microstruccalculations made under constant-amplitude loading to establish the initial microstructural flaw turaland flaw size and predictions under spectrum from thecode. The model was validated for both fatigue and crack-growth predictions. Publisher’s Note: MDPI stays neuPublished: 15 May 2021 tral with regard to jurisdictional claims in published maps and instituPublisher’s Note: MDPI stays neutral tional affiliations
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