Dear Jorgen and others:

I wrote a Umat of Drucker-Prager constitutive model. When I tested it using one element (compression and extension), it worked well, even for large strains (say 50%). But when I used it on multile elements, it can only work on small-strains. I checked the mamual and found the for pressure-dependent plasticity, large volume changes with geometric nonlinearity is frenquently encountered. My problem is simulating a plate load test, generally in geotechnical engineering. The displacement of the plate is generally 0.1m. Is it true that this problem is a kind of geometric nonlinearity?

If it is, exact Jacobian matrix is needed. Here I have another question. For Umat, is it enough only using exact Jacobian matrix, for the problem of large volume changes with geometric nonlinearity? I think in the integration of the rate-form constitutive laws, the stress update should be different compared to that under small-strains. Is it right?

If Vumat is used, according to the manual, the stress strain etc are stored in orientation of the local material axes. Is it necessary to consider deformation gradient in the Vumat, or just convert directly the model for small strains to that in vumat? I really confused on how tho use deformation gradient.

Best regards,

willness