CYCLIC STRESS-FIELDS AHEAD OF TENSION FATIGUE CRACKS IN AMORPHOUS POLYMERS

Cyclic near-tip stress fields which evolve ahead of fatigue cracks are known to have a pronounced effect on the fracture response of materia ls. This region of reversed damage is responsible for the initiation o f fatigue cracks from stress concentrations under fully compressive cy clic loads, crack growth retardation following tensile overloads, and transient crack growth response under spectrum fatigue loading. By rec ourse to photoelasticity and laser interferometry, we provide direct a nd in situ measurements of cyclic near-tip stresses ahead of notches a nd fatigue cracks in amorphous polymers subjected to cyclic tension lo ading. The near-tip stresses are quantified during both the loading an d unloading phases of the fatigue cycle, as well as during various sta ges of crack advance, in an attempt to establish a link between the ev olution of near-tip stress fields and the advance of the fatigue crack . In addition, transmission electron microscopy analyses of the near-t ip region have been performed to characterize cyclic deformation mecha nisms.