The yield function is very important in establishing the plastic constitutive relation and analyzing the plastic deformation.
Hence, expanded on the general yield function in its Taylor to six-order plastic tensors, this article gives a new
yield function on isotropic metal. Based on independent properties of the hydrostatic pressure for metal yield, such as derivation
character, mechanical property invariance of isotropic metal etc., these plastic tensors are analyzed as traceless, totally
symmetric and objective. And the yield function for isotropic metals can be degenerated to the one for identical and
different property of tension-compression yield. Finally, by means of the results of Lode test, it is proved that the new
yield function is quite suitable for those metal materials having both the identical and different properties of tensioncompression
yield. There are many merits of this yield function, such as it includes only 2 parameters for material, i.e.
simple form and generality etc. And this yield function will lay a theoretical foundation for analyzing mechanical properties
of metal materials.