Fatigue Crack Growth Mechanisms At the Microstructure Scale in Al-Si-Mg Cast Alloys: Mechanisms in Regions II and III

The fatigue crack growth behavior in Regions 11 and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions 11 and III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively through AI-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress ratio, R = 0.1, in ambient conditions

Ductile Damage and Constraint in Components with Embedded and Surface Semi-Circular Cracks

Semi-elliptical surface cracks are among the most common defects in welded components subject to fatigue loading. Embedded cracks are also an important type of defects to be considered in welded structures. Control of the static capacity of structures with such defects is important in order to determine load levels corresponding to crack growth initiation and tearing. In the present investigation, the Gurson model based approach has been applied in studying the ductile crack initiation and constraint effect in tension components with semi-circular surface cracks and embedded circular cracks. 3D models of 3 different crack sizes but with exactly the same crack tip element arrangement have been analyzed using the micromechanical parameters established in an earlier study. The results show that for a relatively large embedded crack, the initiation of crack growth will start at the point with the smallest ligament, however, the largest amount of crack growth will occur at the point with the largest ligament. The initiation and growth of surface cracks are rather consistent, with a distinct "canoeing" effect. The influence of crack size and growth on the ductile fracture behaviour is explained by means of constraint