SCC crack growth behavior under dynamic mixed mode loading conditions was investigated in a high-strength Al-Zn-Mg alloy immersed in 3.5% NaCl solution. Mode II dynamic loads including high frequency (30Hz) small vibratory loads at a high stress ratio of 0.95 (dynamic SCC) and low frequency (0.002-5Hz) variation at a stress ratio of 0.1 (cyclic SCC) were applied to specimens with superposition of static Mode I loads of KIS=10MPa·m1/2 larger than the crack opening stress intensity factor and KIS=30MPa·m1/2 larger than the Mode I static SCC threshold, KISCC. In the case of static and dynamic Mode II SCC tests, crack extension was not observed; oblique cracks at an angle of about 50-60 degrees against the direction of shear stress were initiated and propagated, which was dominated by Mode I intergranular dissolution. On the other hand, subcritical Mode II cyclic SCC crack extension was observed at high ΔKII levels with higher crack growth rates than dry air values. The enhancement of crack growth rate was dependent on the values of Mode I static loads and Mode II stress cycle frequency: the increase in crack growth rate from the dry air value at KIS=10MPa·m1/2 was larger than that at KIS=30MPa·m1/2, and had maximum at a stress cycle frequency of 0.1Hz. In the case of Mode II fatigue, transgranular failure including elongated dimples and slips was dominant, whereas transgranular dissolution in addition to those observed in Mode II fatigue existed. The fraction of transgranular dissolution fairly well corresponded with an enhancement of cyclic SCC crack growth rates.
