CYCLIC STRESS-FIELDS FOR FATIGUE CRACKS IN AMORPHOUS SOLIDS EXPERIMENTAL MEASUREMENTS AND THEIR IMPLICATIONS

The evolution cyclic residual stress fields within the damage zone ahead of fatigue cracks is known to have a significant influence in the fatigue fracture response of materials during constant-amplitude and variable-amplitude cyclic loading. In this paper, we present direct and in situ measurements of cyclic stress fields ahead of fatigue flaws in a model amorphous solid subjected to far-field cyclic compression loading. Photoelasticity and laser interferometry experiments conducted on single-edged-notched plates of a photoelastic resin are used to determine the residual stresses within the near-tip damage zone during the inception and subcritical growth of the fatigue flaw from the stress concentration. Quantitative analyses of the relevant stress components are used to establish a link between the evolution of cyclic near-tip fields and the conditions for the onset and advance of fatigue flaws. Transmission electron microscopy observations are presented for a rubber-toughened polystyrene to illustrate how the residual tensile stresses developing within the cyclic damage zone cause crazes to form along the plane of the fatigue crack, in a direction normal to the far-field compression axis. A series of systematic experiments on the effects of mean stress on fatigue fracture is reported, and the results of the experiments are rationalized with the aid of the near tip cyclic stress measurements. The applicability of the results of this study to a broad range of materials, including crystalline metals and ceramics, semicrystalline and amorphous polymers, and composites, is demonstrated. Implications of this work for a variety of fatigue phenomena involving constant- and variable-amplitude fatigue are also addressed.