THE HIGH-STRAIN CYCLIC FATIGUE AND FRACTURE-BEHAVIOR OF 2090 ALUMINUM-ALLOY

A study has been made to understand the high strain low cyclic fatigue properties and cyclic fracture behavior of aluminum alloy 2090 (Al-2.85Cu-2.04Li-0.12Zr; where the composition is in weight percent). The alloy was cycled over a range of plastic strains giving lives of less than 10(4) cycles. The specimens were cycled using tension-compression loading under total strain control. The alloy, in the peak-aged condition, exhibited a distinct discontinuity or break in the Coffin-Manson curve. The abnormal plastic strain-fatigue life behavior is attributed to possible differences in the homogeneity of deformation as a function of plastic strain amplitude, and to a change in fracture mode from ductile monotonic type to brittle crystallographic as a function of plastic strain amplitude. Cyclic deformation was found to produce softening at all strain amplitudes. The presence of shearable matrix precipitates in the peak-aged microstructure results in a local decrease in resistance to dislocation movement, leading to loss of ordering contributions to hardening and plastic strain inhomogeneity. The observed softening is due to dislocation-precipitate interactions, and is a mechanical effect. The cyclic fracture behavior of the alloy is discussed in terms of plastic strain amplitude, response stress, intrinsic microstructural features and matrix slip characteristics.