Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a pre-crack under mixed mode loading were divided into three regions-shear mode growth, tensile mode growth and no growth-on the ΔKI-ΔKII plane. The shear mode growth was observed in the region expressed approximately by ΔKII>3MPa√m and ΔKII/ΔKI>1.6. In 7075-T6, the condition of shear mode crack initiation was expressed by ΔKII>8MPa√m and ΔKII/ΔKI>1.6, and continuous crack growth in shear mode was observed only in the case of ΔKI/ΔKII≈0. The threshold condition of fatigue crack growth in tensile mode was described by the maximum tensile stress criterion, which is given by Δσθmax√2πr≈1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approached to the plane in which ΔKI decreases and ΔKII increases towards the maximum with crack growth. The da/dN-ΔKII relation of the curved cracks growing in shear mode under mixed mode loading agreed well with the da/dN-ΔKII relation of a straight crack under pure mode II loading.
