Fatigue crack retardation in PM aluminum alloys AA8022 and AA5091. Ph.D. Thesis
An important goal of fatigue research is the prediction of fatigue crack propagation rate under service loading conditions. Intermittent high peak tensile loads can cause crack retardation in subsequent load cycles. The mechanisms responsible for crack retardation must be characterized in each alloy or class of alloys so that appropriate predictive models can be developed and utilized. The objective of this investigation was to identify the mechanisms responsible for crack retardation in PM aluminum alloys AA8009 and AA5091. The results of single tensile overload tests and computer simulations with FASTRAN, a plasticity-induced closure model, indicate that plasticity-induced closure is the primary cause of retardation in AA8022 at both low and high R. Both plasticity and roughness-induced closure contribute to crack retardation in AA5091. The relative contributions of the surface and interior region to crack retardation were determined by performing surface removal experiments and examining crack front profiles at various positions through the affected zone. The results of these experiments indicate that the plasticity-induced closure mechanism is through-thickness in nature. However, the magnitude of the retardation effect is greater in the surface regions than in the interior region. The effect of an overload was significantly reduced by subsequent elevated temperature exposure. This behavior results from reductions in plastic strain and compressive residual stresses ahead of the crack tip. These changes significantly reduce the contribution of closure mechanisms to crack retardation.
- Research Organization:
- Virginia Univ., Charlottesville, VA (United States)
- OSTI ID:
- 237322
- Report Number(s):
- N--96-23117; NIPS--96-07630
- Country of Publication:
- United States
- Language:
- English
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