Crack propagation in tailored welded blanks joined by friction stir welding

Kurzfassung

Tailored Welded Blanks (TWB) offer utilization of material properties appropriate to loading conditions in structural parts and thus provide high potential for weight savings. Friction stir welding (FSW) is generally accepted as a joining method with minimal negative effects on the mechanical properties of high strength aluminum alloys. For aircraft applications in particular fatigue properties and crack growth behavior are important design criteria and therefore have to be studied. AlMgSc-sheets of different thicknesses were joined by FSW to provide TWBs with a pronounced thickness transition. As cutting of FSW joints usually affects residual stresses, single edge notch tension (SENT) specimens were used to study crack propagation behavior in full size FSW TWB as applied in this work. Crack propagation tests were performed for base metal as well as FSW TWBs. Local stress intensity factors (SIF) were calculated using 3D finite element models. Non-uniform displacements of the clamping devices during fatigue experiments were measured with an optical deformation measurement system. Subsequently, these data were used to define realistic boundary conditions in the corresponding simulations. Residual stresses along the crack path and associated distortions of the FSW TWB were measured using the cut compliance method and the optical deformation measurement system. The resulting stress fields were used to calculate corrected SIFs and to study their effect on crack propagation under fatigue loading. The thickness transition in TWBs accounts for locally reduced SIFs resulting in decreased crack propagation rates. Non-uniform displacements of the clamps during SENT specimen testing additionally affect the SIFs. The consideration of these geometrical and experimental features in the 3D simulations in combination with residual stress and distortion allows for the determination of crack propagation rates as a function of SIFs. Finally, crack propagation under fatigue loading can be studied for TWBs with various thickness transitions.