This study concerns on the plastic behaviour of (Al-2%Cu) sheets under
simple tension, plane strain tension and balanced biaxial tension. Experimental
yield loci were extracted based on the constant strain energy theory, comparisons
with the proposed two yield criterion after R. Hill , showed that the experimental
yield loci does not follow very well the old yield criterion, but it can be fitted better
with the new yield criterion. The determined value of ''M'' in the new yield
criterion were ( m = 1.85 ).