Macro and microscopic observations of fatigue crack growth in friction stir welded aluminum joints

This paper presents the macro and microscopic fractography performed on fractures from fatigue cracks through friction stir welded joints. The welds were placed under different angles in the various specimens to study the influence of the yield strength and residual stress on fatigue crack growth. As a result, different behavior was observed at the macro level, depending on the type of alloy and orientation of the weld. The variations in rotation of the crack plane raised a number of questions regarding the mode of loading, i.e. mode I or mode II. The purpose of this study was to investigate the fracture surfaces at microscopic level to find explanations for the local macro behavior. Special focus was placed on the fracture surfaces on which features were observed indicating mode II fatigue crack growth.