Growth of voids in biaxial stress fields

Abstract The relative density changes occurring in some commercial sheet metals as a result of plastic deformation have been determined using a differential method based on Archimedes' principle. Two batches of aluminium killed steel, soft 70 30 brass and soft commercial purity aluminium were stretched in balanced biaxial tension using Marciniak's technique and the density changes measured at increasing plastic strains. One of the steels was also tested in plane strain tension and at a ratio ϵ 2 ϵ 1 =0·5 . The largest relative density change, 0·005, occurred in one of the steels at a natural surface strain of approximately 0·4. It was found that the density changes are dependent on the amount of plastic work but independent of the applied strain ratio. Three stages are identified for relative density changes in steel. During the first stage existing voids expand and new voids are initiated. The second stage is the expansion of a constant number of voids while the third stage, leading to fracture, is characterised by local rather than uniform bulk deformation. Experimental results for the expansion of model voids are consistent with the observations of the second stage.