抄録
The effect of the variation of stress frequency on fatigue crack propagation in pure titanium plate, which has remarkable strain rate dependence in the plastic region and notable orthotropy, was studied.
Fatigue crack propagation tests were carried out under four stress frequencies: two constant stress frequencies (20Hz, 0.02Hz) and two variable stress frequencies changed step-wise (20→0.02Hz, 0.02→20Hz). The test specimens were cut out from the rolling direction and transverse to the rolling direction of the plate. An anisotropic elasto/visco-plastic analysis of the fatigue crack propagation was performed by the use of the orthotropic elasto/visco-plastic overlay model, and a comparison between the change of crack propagation rate due to variation of stress frequency and the visco-plastic strain behavior at the crack tip calculated by the analysis was made.
The results obtained are summarized as follows:
(1) It was found from the experiment that the crack propagation rate changed characteristically depending on cutting out direction of the specimen because of the anisotropy of titanium plate and on the variation of stress frequency.
(2) A parameter closely related to the fatigue crack propagation rate is the visco-plastic strain range at the crack tip.
(3) The effect of the variation of stress frequency on the fatigue crack propagation rate may be explained by the variation of visco-plastic strain range at the crack tip based on the strain rate dependence and the anisotropy of the material.
