Application of a strip-yield model to predict crack growth under variable-amplitude and spectrum loading – Part 1: Compact specimens |
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Authors: | Y Yamada B Ziegler JC Newman Jr |
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Affiliation: | aOhio Aerospace Institute at NASA Glenn Research Center, Cleveland, OH 44135, USA;bMississippi State University, Aerospace Engineering, Mississippi State, MS 39762, USA |
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Abstract: | Fatigue-crack-growth tests were conducted on compact, C(T), specimens made of D16Cz (clad) aluminum alloy under constant-amplitude loading, a single spike overload, and simulated aircraft spectrum loading. Constant-amplitude tests were conducted to generate crack-growth-rate data from threshold to near fracture over a wide range of stress ratios (R = Pmin/Pmax = 0.1–0.75) using the new compression pre-cracking test methods. Comparisons were made between test data generated on the C(T) specimens with test data from the literature on middle-crack-tension, M(T), specimens machined from the same sheet. A crack-closure analysis was used to collapse the rate data from both specimen types into a narrow band over many orders of magnitude in rates using proper constraint factors. The constraint factors were established from constant-amplitude (CA) and single-spike overload tests. The life-prediction code, FASTRAN, which is based on the strip-yield model concept, was used to calculate crack-length-against-cycles under CA loading and a single-spike overload (OL) test, and to predict crack growth under simulated aircraft spectrum loading tests on C(T) specimens. The calculated crack-growth lives under CA loading were generally within about ±25% of the test results, but slower crack growth under the double-shear fatigue mode, unlike the single-shear mode (45o slant crack growth), may be the reason for some of the larger differences. The predicted results under the single-spike overload and the Mini-Falstaff+ spectrum were within 10% of the test data. |
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Keywords: | Cracks Fatigue-crack growth Aluminum alloy Crack closure Stress&ndash intensity factor Plasticity |
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