Hi all,

I am very sorry, if I bother with a too simple problem, but: I try to simulate behavior of a dental composite (polymer resin- spherical particles) , using ABAQUS CAE. For resin with asymmetric yield stresses I use Drucker-Prager plasticity model.

If I run the material model with a unit cube of resin, it converges very quickly, even by introducing of high value of uniaxial displacement load (up to 6% of strain).

But if used for 3D simulation, it refuses to converge. I have tried many techniques of improve convergence, including mesh refinement, analysis step refinement, using other elements, but everything increased only the computational time and did not help with the convergence.

I try to model the shrinkage process of dental composite, obtain the residual stresses, then apply far-field load and investigate, how the residual stresses influence the mechanical response. Shrinkage of the polymer resin is up to 7%, so it is far in the plastic region of the resins stress-strain curve. Shrinkage is modeled by thermal analogy, this means, defining expansion coefficient α,, and decreasing temperature in model.

For the reason of computational time I use linear tetrahedron elements, but even use of other elements does not help with the convergence. For the Drucker-Prager model I use the linear shear criterion, deffault Flow Potential Eccentricity Friction and Dilatation Angle both of 28˚,, Flow Stress Ratio 1 and tension-based hardening described by following stress plastic strain points

20 0

25 0.001

30 0.0014

35 0.0024

40 0.0033

45 0.0047

50 0.0076

55 0.0118

58 0.025

This describes the stress-strain curve well, even in plastic region, but only for uniaxial test-model of pure resin. By introducing three-dimensionality, even by a model of composite of 1 particle in block of resin and only shrinkage load, it needs more than 30 iterations, as the level of shrinkage reaches plastic value. Changing of the parameters of analysis step does not help.

In the stepmodule, I currently use static-general, Nl geom ON, increment size initial 0.1 , min 1E-005, max 0.2.

Please, would anyone of you be able to give me any suggestion or advice, how to improve the model to reach convergence, without significant increase of the simulation time? It will help me a lot with a a problem which I cannot solve for a long time!

Very many thanks,

Ondrej