Hill Engineering recently collaborated with the Worcester Polytechnic Institute Integrative Materials Design Center to publish an article in International Journal of Fatigue, titled Fatigue crack growth mechanisms in similar and dissimilar aluminum friction stir welds: Residual stress, microstructure, and microhardness effects. The results of this work can be used to inform design decisions to improve crack growth resistance and integrity. The abstract is presented below, along with a link to the publication.
Spatially varying microstructure, microhardness, and residual stress in friction stir welds cause crack growth rates to fluctuate near weld nuggets and complicate fatigue life predictions. Similar and dissimilar friction stir butt welds of 6061-T651 and 7075-T735 aluminum alloys were characterized by optical microscopy and microhardness traversals, and residual stress mapping by the contour method and fatigue crack growth testing were conducted across and within weld nuggets. Crack growth data were analyzed by the crack-compliance method to calculate residual stresses and consequent stress intensity factors (Kres), and contrasted with the contour method. Constant ΔK crack growth testing across the welds revealed suppression of growth rates relative to the base materials, being lowest in the thermo-mechanically affected zones. Crack growth within the weld nuggets was conducted at decreasing ΔK and resulted in uneven through-thickness crack growth. A method for crack propagation analysis of materials containing heterogeneous residual stresses is proposed using contour method data to calculate Kres profiles, indicating that crack growth rates increased in regions of greater Kres and microhardness in both crack-weld orientations. The results from these studies can be used to design weld configurations with beneficial residual stresses and enhanced fatigue crack growth resistance for structural applications.
The full publication can be read here.
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