THE EFFECT OF SPECIMEN THICKNESS AND STRESS RATIO ON THE FATIGUE BEHAVIOR OF POLYCARBONATE

The cyclic residual stresses ahead of fatigue cracks are known to play an important role in the fatigue fracture response of engineering mat erials. The size of these residual stresses are directly affected by t he stress state of the component. In this paper, we examine the effect of stress state for fully compressive and fully tensile cyclic loadin g of polycarbonate. The role of stress state is studied using two diff erent specimen thicknesses, one thickness will represent a near-plane stress condition and the other will represent a near-plane strain cond ition. In cyclic compressive loading, it will be shown that the near-p lane stress specimen with its larger zone of residual tension will exh ibit enhanced crack saturation lengths. While for cyclic tensile loadi ng, the larger zone of residual compression upon unloading will result in crack retardation for the same unloading stress intensity. A serie s of systematic experiments on the effects of mean stress on fatigue f racture is reported, and the results of the experiments are rationaliz ed with the aid of scanning electron microscopy of the fracture surfac es. The results of this study have strong implications for both consta nt amplitude fatigue loading and variable amplitude fatigue loading as well as applicability to other engineering materials.