The effect of stress frequency on fatigue crack initiation in 99.5% pure titanium plate, which has remarkable strain rate dependence in plastic region and notable orthotropy, was studied.
Fatigue crack initiation tests were carried out under three kinds of stress frequencies (0.2Hz, 1Hz, 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 was performed by the use of the orthotropic elasto/visco-plastic overlay model, and the comparison between the cycles to crack initiation and the viscoplastic strain range at the notch root obtained by the analysis was made.
The results obtained in this study are summarized as follows:
(1) It was found from the experiment that the cycles to fatigue crack initiation depended on the cutting-out direction of the specimen because of the anisotropy of titanium plate, and was approximately in proportion to fn(n>0), where f is the stress frequency.
(2) A parameter related closely to the fatigue crack initiation is the visco-plastic strain range at the notch root, and the cycles to fatigue crack initiation is approximately in inverse proportion to the nth power of the visco-plastic strain range at the notch root.
(3) The effect of stress frequency on fatigue crack initiation may be explained by the variation of visco-plastic strain range at the notch root based on the strain rate dependence and the anisotropy of the material.
