The Anisotropy of Fatigue Weaklink Density and Strength Distribution in an AA7075 T651 Al Alloy Plate

An AA7075 T651 Al Alloy plate was fatigued in four-point bend along the L-T (Rolling-Transverse), L-S (Rolling-Short transverse), T-L and T-S planes, respectively, at room temperature, 20 Hz, R=0.1, in air. The population of crack initiation sites (fatigue weaklinks), found on the sample surface, was Weibull-type function of the applied stress, which allowed determination of fatigue weaklink strength distribution. It was found that the fatigue weaklink density and strength distribution were profoundly anisotropic with the highest density being in the L-T samples and the lowest density in the T-S samples in the plate. Cross-sectioning of the crack initiating particles using a focus ion beam revealed that the thickness of the particles was the key factor controlling the behavior of the micro-cracks formed in these particles. Thinner particles were likely to cause non-propagating micro-cracks, while thicker ones could lead to long propagating cracks. Electron back scatter diffraction experiments indicated that the twist angle of crack deflection at the interface between a fractured particle and the matrix was another important factor controlling the growth behavior of the micro-cracks. A quantitative model has consequently been developed to simulate micro-crack growth in 3-D by taking into account both driving force and resistance in high strength Al alloys