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

The evolution cyclic residual stress fields within the damage zone ahe ad of fatigue cracks is known to have a significant influence in the f atigue fracture response of materials during constant-amplitude and va riable-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 loa ding. Photoelasticity and laser interferometry experiments conducted o n single-edged-notched plates of a photoelastic resin are used to dete rmine 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 rub ber-toughened polystyrene to illustrate how the residual tensile stres ses developing within the cyclic damage zone cause crazes to form alon g the plane of the fatigue crack, in a direction normal to the far-fie ld compression axis. A series of systematic experiments on the effects of mean stress on fatigue fracture is reported, and the results of th e experiments are rationalized with the aid of the near tip cyclic str ess measurements. The applicability of the results of this study to a broad range of materials, including crystalline metals and ceramics, s emicrystalline and amorphous polymers, and composites, is demonstrated . Implications of this work for a variety of fatigue phenomena involvi ng constant- and variable-amplitude fatigue are also addressed.