Hello,
I'm modeling a high temperature pipe using API-579 (fitness-for-service) which requires the use of an elastic-perfectly-plastic model (EPP). ASME Section VIII, Division 2 requires small deflection when using this type of EPP material model. API 579, however, gives no guidance on small vs large deflection.
When I use the large deflection option, I have convergence problems. but the small deflection model allows convergence. I believe this is due to highly distorted elements from the near-zero tangent modulus and large strain effects.
Is it acceptable to use small deflection with an EPP material model? I can only find documents describing small vs large deflection results in an elastic-only context.
Thank you!
Usually, the EPP model is based on the additive decomposition of strain into an elastic and a plastic part which implies small strains and rotations (as well as deflections). Regards
In principle, the solution shall converge with large deflection for elastic perfect plastic models. Check the following
1) Which formulation do you use in the large deflection model; Total lagrangian OR Updated lagrangian? Updated is preferred in most cases.
2) Is there influence of initial stress on the stiffness? Try switching it OFF or try using influence of tensile stresses only.
3) Is the structure buckling? if yes, switch ON non-positive stiffness option to see the post buckling results.
4) Try reducing the load increment step size.
5) Check the large deflection simulation with a hardening material model. Does it work?
When the large deflection model gives approximately the same result as small deflection (within your own defined acceptability limits), then you can assume small deflection.
kind regards
Muhammad
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