| This thesis examines both qualitatively and quantitatively the failure mechanisms of a series of aluminum alloy (AA5754 and AA5182) tailor-welded blanks (TWBs) when subjected to plane-strain extensions along the weld-line direction. With this weld-line orientation, the limits of TWB ductility are reached when a through-thickness shear band develops within the weld-metal. This process is facilitated by the existence of both external (i.e. surface roughness) and internal (i.e. porosity) defects along the weld-line and the constitutive material behaviour of the weld-metal. A series of FEM models were constructed with representative defects to predict the reduction in ductility arising from the interaction of these weld-line defects. Failure was detected in the FEM models through the use of a bifurcation analysis which delineated the formation of a macroscopic shear band. An added benefit of this failure criterion was its insensitivity to nominal element size. The predicted failure strains were found to be strongly sensitive to both the surface defect amplitude and macroporosity diameter, while surface defect wavelength was found to influence the failure morphology. Interactions between surface defects and internal pores lead to a reduction in weld-line ductility as a result of the enhanced formation of localized shear bands. When these defects act independently, significantly larger defects are required to achieve the same reduction in ductility. However, the measured defect population distributions favour smaller defects, and therefore, for a given failure strain, the probability that the failure site is associated with interacting defects is much higher than for larger defects acting independently. This finding was confirmed with the correlation of pre-existing weld-defects and failure locations of specimens subjected to plane-strain tension. Knowing the defect populations along the weld-line, and their influence on the ductility, a Blank Ranking Index was developed to predict the relative formabilities of these blanks when subjected to in-plane plane-strain tension conditions. Experimental ductility rankings of the same candidate materials are in agreement with those determined using this ranking index. |
