Separating the influence of K{sub max} from closure-related stress ratio effects using the adjusted compliance ratio technique
In a common interpretation of crack closure, the crack is visualized as peeling open as stress is applied. This interpretation, while useful, has led to the assumption that the driving force for crack propagation, {Delta}K{sub eff}, exists only when the crack tip is fully open (above K-opening). However, evidence of significant crack-tip cyclic strain below the K-opening will be presented. The exclusion of this additional driving force can yield misleading values of {Delta}K{sub eff}. This is particularly so in the near-threshold regime where opening loads are typically high. The new analysis technique for estimating {Delta}K{sub eff} is referred to as the adjusted compliance ratio (ACR) method and is based on an interpretation of crack closure as a stress redistribution (or load transfer) on a relatively compliant crack wake. The ACR method is evaluated using the results of fatigue crack growth tests on 6013-T651, 2324-T39 and 7055-T7751 aluminum alloys using the center crack tension M(T) specimen geometry and stress ratios ranging from {minus}1.0 to above 0.96. The experimental results of this study indicate that the fatigue crack growth rate is not determined solely by {Delta}K{sub eff} but also depends on K{sub max}. It was observed that this K{sub max} dependence takes the form of a power law with the magnitude of the exponent being a measure of K{sub max} sensitivity. As a result, {Delta}K{sub eff} curves from all test conditions could be collapsed to a unique intrinsic FCGR curve using a simple modification to the Paris Law. It is expected that continued research in this area will lead to improvements in fatigue life prediction methodology.
- Research Organization:
- Alcoa Technical Center, PA (US)
- OSTI ID:
- 20005696
- Country of Publication:
- United States
- Language:
- English
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