Sheet metal behaves differently for different stress and strain ratios. For instance, the material tolerates less deformation at plane strain compared with uniaxial tension. Hill's yield criteria can be used to describe sheet material's deformation behavior. Hill's original yield criterion for plane stress, assuming that there is no planar anisotropy is given. The equation shows better agreement with experimental results when r > 1 (steel 4010), while there is no agreement with experimental results when r < 1 (Al-5052). Hill proposed, therefore, a new yield criterion, which is given. The value of a can be determined by comparing the experimental work-hardening characteristics for uniaxial tension and balanced biaxial tension; 1 < = a < = 2. For soft Al, having an average r-value of 0.723, the value of a has been found to be 1.8. The parameter a has its greatest effect at balanced biaxial tension. Graphs