Quantification of the interaction within defect populations on fatigue behavior in an aluminum alloy

The interaction between fatigue crack evolution and porosity in A356-T6 alloys was characterized using both X-ray microtomography and finite-element analysis. The formation and propagation of fatigue cracks are dominated by the highest local stress–strain concentration (kg=kσkε) caused by the presence of porosity adjacent to the free surface and the crack tip, respectively. The kgkg factor proved to be a suitable parameter to quantify the influence of porosity on fatigue cracking and to predict the fatigue crack growth rate (da/dNda/dN). Although microporosity has a complex, tortuous morphology, a simple method for mapping the shape to an idealized sphere was proposed. It was found that using a sphere with equal projected area in the plane normal to the loading direction was a good approximation, allowing an equivalent kgkg to be calculated.