On the influence of single and multiple peak overloads on fatigue crack propagation in 7075-T6511 aluminum

Abstract

The phenomenon of fatigue crack retardation due to variable amplitude loading spectra was studied in 7075-T6511 aluminum alloy. Selected precracked compact tension specimens were subjected to single and multiple peak applications of overload stress intensity values and subsequently cycled at lower stress intensities. Detailed characteristics of the fatigue crack propagation process were measured directly on the fracture surfaces using electron microscopy replicating techniques. The degree of retardation was shown to depend strongly on: 1. A. The relative amplitudes of the peak stress intensity ( ΔK 2 ΔK 1 ) 2. B. The number of stress applications N 2 at the peak stress intensity ( ΔK 2) 3. C. The magnitude of the constant amplitude crack growth rate at the lower stress intensity range ( da dn ) ΔK1 4. D. The number of fatigue cycles N 3 at the lower stress intensity level after the last peak stress is applied. A comparison was made between the experimentally observed magnitude of retardation and that predicted using: (1) a model where the retardation is proportional to the ratio of peak stress plastic zone size to secondary stress plastic zone size and (2) an effective stress intensity model. It appears that no present model can accurately predict many of the complex features of the cycle by cycle crack growth behavior experimentally observed. For example, immediately following the last application of the high stress, the crack growth rate at the lower stress levels was accelerated over the value normally observed at constant amplitude cycling (at that same lower stress level). After a period of several cycles, the retardation increased. This is the previously reported delayed retardation effect. After further cycling the crack growth rate slowly increased to what would normally be observed had the peak stress not been applied.