Stress triaxiality is hydraustatic mean stress/von Mises effective stress

Below link may be helpful:

http://imim.pl/files/archiwum/Vol3_2009/8.pdf

The link below explain a little about this topic.

https://caeai.com/blog/incorporating-stress-triaxiality-factor-fea-simulations

Hello Ante.

In material Processing plastic strains is controlled mainly by deviatoric stress tensor (Von mises Yielding criteria). But hydrostatic term of stress tensor or pressure is with significant effect on damage, fracture or failure.

Compressive hydrostatic stress increses strain failure limit ( favorable in metal forming: deep drawing, forging, roll forming etc.

Tensile hydrostatic stress give lower strain limit ( favorable in cutting or thickness reduction in free forming or forging) .

triaxiality = hydrostatic pressure/ deviatoric stress.