Weld Metal Ductility: Reduction in Area

It follows from the Chapter 7 that, except for very short gauge lengths, percent elongation is mainly influenced by uniform elongation, and is thus dependent upon the strain-hardening capacity of the matrix. In contrast, reduction in area is more a measure of the deformation required to produce fracture, and its chief contribution arises from the necking process. The work of Groom (1971) illustrates this. Groom studied the effects of various nominal prestrains on the behaviour of copper and steel tensile tests. Rolling and swageing were used to form embryonic voids. It was found that for all pre-strains, the reduction in area associated with necking alone was essentially constant (Fig. 8.1), showing that for a given inclusion spacing there is a critical level of triaxial tension that must be reached before fracture can occur by void coalescence and the "cup" stage of fracture can propagate. (The slight decrease at large prestrains can be attributed to the inclusion spacing being significantly reduced by large amounts of cold work, since it is the inclusion spacing in the plane through the minimum section of the neck that has to be considered). It is, therefore, correct to consider the uniform and non-uniform components of ductility, and their effects on elongation and reduction in area separately. In summary, therefore, the magnitude of €u, the uniform elongation, is a function of the strain hardening capacity of the material. In contrast, non-uniform elongation is expected to depend on the concentration and distribution of the stress-concentrating particles which influence deformation behaviour after necking.