文档介绍：3 Yield criteria
General consideration
Yield surface and yield locus
Coordinate on -plane
Tresca yield criterion
Mises yield criterion
of the two yielding criteria
Experimental verification of yield criteria
Plastic work
Effective stress and effective strain
General considerations
Yield criteria
Material yields (plastic deformation starts to occur) when a particular stress level has been reached. Any yield criterion is a postulated mathematical expression of the stress states that will induce yielding or the onset of plastic deformation.
On one hand, it is related to stress state, on the other hand, it is related to material properties.
Yield
stress
Elastic deformation
Work hardening
reloading
unloading
Subsequent
yield stress
Total
deformation
o
General considerations
Yield criteria
where c is either a material constant in the case of initial yielding or a function of the amount of deformation for workhardening material. Any points along a workhardening stress-strain curve may be regarded as subsequent yield points.
The stresses attain a value which exceeds that required for elastic deformation and the material is said to have yielded.
Assumptions: the material is homogeneous and isotropic.
General considerations
A yield criterion is a postulated mathematical expression of the stress state that will induce yielding or the onset of plastic deformation. Any proposed yield criterion should be verified experimentally.
By means of a yield criterion it is then possible to decide whether plastic deformation takes place.
The general form of yield criterion is
(4-1)
Yield criteria
or
elastic state
not exist
In terms of principal stresses, yield criterion es
In terms of stress invariants,
It assumed that for a hydrostatic stress, pressive or tensile, and whether applied alone or superimposed on bined stress state, the yielding of a metal is unaffected.
General considerations
(4-2)
(