Principal stress in plastic region
Principal stress in plastic region
(OP)
Good Morning,
I run a 3D non-linear analysis in Nastan using SOL106.
The volume is modeled with CTETRA elements while CTRIA3 exist on skin.
Part is loaded in traction.
Material used is a steel for which an elasto-plastic curve - like the one shown here below - is used:
Further, MAT1 and MAST1 with PLASTIC option at field 4 is used.

I run 4 analyses:
- @ 50% of load
- @ 100% of load
- @ 150% of load
- @ 2000% of load
At critical site, I extracted Von Mises and Max principal stresses,(sigma1).
Here is what I get when I superpose Von Mises stresses on material curve.

Here is what I get when I superpose principal stresses on material curve.

Could anyone explain why VM stress follows the curce and Principal stress does not, (in plastic region this is evident) ?
Do I need to rotate material properties in principal stress direction in the second graph?
Thank you for you help ans suggestions.
Marc
I run a 3D non-linear analysis in Nastan using SOL106.
The volume is modeled with CTETRA elements while CTRIA3 exist on skin.
Part is loaded in traction.
Material used is a steel for which an elasto-plastic curve - like the one shown here below - is used:
Further, MAT1 and MAST1 with PLASTIC option at field 4 is used.

I run 4 analyses:
- @ 50% of load
- @ 100% of load
- @ 150% of load
- @ 2000% of load
At critical site, I extracted Von Mises and Max principal stresses,(sigma1).
Here is what I get when I superpose Von Mises stresses on material curve.

Here is what I get when I superpose principal stresses on material curve.

Could anyone explain why VM stress follows the curce and Principal stress does not, (in plastic region this is evident) ?
Do I need to rotate material properties in principal stress direction in the second graph?
Thank you for you help ans suggestions.
Marc





RE: Principal stress in plastic region
noting that (however) von Mises is a linear stress failure theory.
the "odd" thing to me is if the part is loaded in simple tension, where are the other principal stresses coming from ? (as von mises is the combination of the three principal stresses).
But that said, von mises is the better failure theory (IMHO) if you have a multi-axis stress field, as a way to account for the interaction between the stresses; in your case you seem to have tension is the other directions (if you had compression then max principal would be less than critical). If you have a uni-axial stress then von mises collapses to max principal.
another day in paradise, or is paradise one day closer ?
RE: Principal stress in plastic region
that yielding occurs when vm stress exceeds yield strength.
https://en.wikipedia.org/wiki/Von_Mises_yield_crit...
In multiaxial loading conditions it is possible for maximum principal stress to exceed vm stress.
Check also for other principal stresses if they exist because of for example a fixed
boundary condition.
RE: Principal stress in plastic region
another day in paradise, or is paradise one day closer ?
RE: Principal stress in plastic region
RE: Principal stress in plastic region
Nicely described question.
I think what is happening, is that although the part is in tension in say the x-dir., there is developing a tensile stress in the perpendicular or y-dir. due to Poisson's ratio (it wants to contract but is restraint from doing so by say a strong shoulder). E.g. a s1=1 plus s2=0.3 would give you a Svm=0.9
Regards
RE: Principal stress in plastic region
disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.