The behavior of fatigue crack growth under mode II loading has been investigated on steels (JIS SPCC and SM 50B) and aluminum alloys (7075-T6 and 2017-T4). Tests were made by a specially designed apparatus under various combinations of ΔKII and KIS, where ΔKII is the range of the repeated mode II loading and KIS is the statically applied KI. Under mode II loading, two modes of fatigue crack growth, mode II growth and tensile mode growth, were usually observed, though in some cases only one was observed. Here, mode II growth and tensile mode growth mean the fatigue crack growth in the plane of precrack and that in the plane which makes about 60°to the precrack, respectively. In the latter, the plane of crack growth may be taken as the plane of the maximum tensile stress.
Aluminum alloys showed continuous mode II growth in the region of ΔKII larger than ΔKII-threshold for mode II growth. In the case of mild steel (SPCC), however, tensile mode growth occurred in the region of ΔKII larger than ΔKII-threshold for tensile mode growth, and mode II growth was observed only in the narrow region of ΔKII smaller than the above ΔKII-threshold for tensile mode growth. In weldable structural steel (SM 50B) only tensile mode growth was observed under mode II loading. The mode II growth rate in aluminum alloys was much larger than that of mode I growth, while the mode II growth rate was very small in mild steel specimens.
