1994 年 43 巻 488 号 p. 539-544
The fatigue crack growth tests under constant amplitude and two-step program loadings were carried out to assess the effect of anisotropy on the crack growth of a forged Ti-6Al-4V alloy. CT specimens were cut from three different orientations with regard to the rolling direction of the material. Crack closure was measured by a compliance method to examine the overloading effect on the crack growth during the two-step program loading. Fracture surface morphologies were examined by a scanning electron microscope to assess the crack growth mechanism of the material tested. It was found that the crack growth rate was almost independent of the orientation of the material, regardless of loading condition. The crack opening stress intensity under the two-step program loading was almost the same in each block and was governed by the maximum stress intensity factor in the block. The fatigue crack growth rate under two-step program loading was correlated well with an effective stress intensity factor range estimated based on the linear accumulation of the stress intensity factor ranges of each step in the block. Cleavage facets occupied the major portion of fracture surface of low ΔK region under constant amplitude loading. Fracture surface under program loading was covered with striations corresponding to the loading sequence. Large striation due to overloading seems to be formed through the blunting and resharpening mechanism at crack tip.