Von Mises Criterion limitations...

[SofieneFEA]

Hello Everybody,

As I know, the Von Mises Criterion is based on energy. “Von Mises” say that a point of the material (an elastic linear isotropic material) reach the plasticity when the energy (calculated at this point) resulting from the deviatory part of the stress tensor is equal to the energy needed to attain plasticity in a simple axial traction test.

The problem is that the Von Mises Criterion (the VMC) don’t limit (care about) the hydrostatic (same load in all directions in same time) compressions and tractions

The VMC assume that the material used have symmetric behaviour in traction and compression.

My question is how to determine the plasticity limit in case of use of other materials? (non symmetric behaviour for example..)

I think even ANSYS calculate only Von Mises Stress…People who work with ANSYS for example, work with the principal stresses? Or use the principal stresses to calculate other Criterions?

But, thank you “Von Mises”…and thank you for All.

 

[gfbotha]

Von Mises stress is very much the preferred yield/failure criterion for relative ductile metals like steel. The fact that it "oversees" the hydrostatic component during say, compression, is in my opinion a good thing because it actually matches with what will happen in reality... (imagine: you pressurise a steel cube using hydraulic oil).

Of course, for each category of materials one has to decide on the most applicable failure criterion (Surely VM is not always suitable!).

If a material in general follows VM yield criterion, and it hardens as well, then specifying the kinematic hardening rule will in fact take care of different yield limits during tensile/compressive cycling. But, more likely what you like to hear about is something like the Coulomb-Mohr theory which considers different plastic limits for positive and negative stresses.

Gert

 

[EdR]

Suggest you check out a Mechanics of Materials reference book....There are a number of different yield criterion (Principal stress, Maximum Shear, tresca, etc...) and each may behave differently depending on the specific criteria selected....Some are more appropriate for brittle materials than VM (and not all produce "plastic" strains i.e. strains comming from a flow rule)....Selection of the yield criteria appropriate to the material is why the analyst's get the big bucks....

Ed.R.

 

[gfbotha]

Thanks for your remark Ed.R - because I should not have used the word "plastic" in my sentence about Coulomb-Mohr!

G