EFFECT OF VARIATION OF STRESS FREQUENCY ON FATIGUE CRACK PROPAGATION IN TITANIUM

Abstract

The effect of variation of stress frequency on fatigue crack propagation in 99.5% pure titanium, which has remarkable strain rate dependence in the plastic region, was studied. Fatigue crack propagation tests were carried out under four stress frequencies: two constant stress frequencies (20Hz, 0.02Hz) and two stress frequencies changed step-wise (20→0.02Hz, 0.02→20Hz). An elasto/visco-plastic analysis of fatigue crack propagation was performed by the finite element method (FEM), and the comparison between the change of crack propagation rate due to variation of stress frequencies and the visco-plastic strain behavior at the crack tip calculated by the analysis was made. The results obtained in this study are summerized as follows:(1)It was found from the experiments that the crack propagation rate was characteristically changed depending on the variation of stress frequency.(2)A parameter closely related to the fatigue crack growth rate is the visco-plastic strain range at the crack tip.(3)The effect of variation of stress frequency on the fatigue crack propagation rate may be explained by variation of the visco-plastic strain range at the crack tip based on the strain rate dependence of the material.