抄録
The conditions of the initiation of mode II fatigue crack growth have been investigated on the specimens with a fatigue precrack or a machined slit whose notch tip radius was 0.05mm or 0.5mm. Tests were made on 7075-T6 and 2017-T4 rolled plate and 2017-T4 extruded bar specimens. According to the test results, all the specimens of 7075-T6 and 2017-T4 with fatigue precracks and a 0.5mmR slit showed mode II growth under repeated mode II loading in the region of ΔKII larger than a certain critical value of ΔKII, where ΔKII of the crack of the same length is used for a slit with finite tip radius. The critical ΔKII's for mode II growth mentioned above depended on the material and on the slit tip radius. No mode II growth was observed on any of the 0.5mmR slit specimens of all the aluminum alloys tested. On 7075-T6 and 2017-T4 rolled plate specimens, tensile mode growth was observed in the region in which the ΔKII was smaller than that in the region in which mode II growth was observed. A fractographic investigation on mode II fatigue crack growth was made with special attention to the area of initial growth. In some cases similarities in fractographs of mode II growth and stage I growth in tensile mode growth were observed.
