hi i have a part that i want to use in a nonlinear static study. the part is made of an elastomer (room temperature vulcanized). i apply a load on the edge and i fix the other end. the study shows a warning that the Assembled Stiffness Matrix has a negative diagona and later it fails showing the message in the picture i have attached. i used fine curvature based mesh with a growth ratio of 1.2 for the part as it has some very small faces, and large problem direct sparce solver. i noticed that the material doesn't affect the error. can someone help me with this?

Run it with linear static you will see that even with the 1 N and 4 N loads applied it deflects about 1300 mm (for a rubber stiffer than the one defined, initial linear E modulus of 3 MPa). Nonlinear FEA is not a magic black box where everything can be solved, and it would be quite impressive if this can be solved for this loads and nonlinear elastic model with large deflections (geometric nonlinear), that wants to deflect many times it's original size (so apply smaller loads and see what happens, in nonlinear FEA loads are applied gradually, especially for rubber parts, in order to help the NR iterations to reduce the residuals to acceptable values below the convergence criteria).

Another point is that the mesh is not really good and that itself can lead to problems when trying to obtain a converged solution. Trying to improve/simplify the geometry and to obtain a better mesh is important (not sure how one can look on mesh quality parameters, like Jacobian determinants, tetrahedral volumes, aspect ratios, mixed products,..).

As suggested, if displacement can be used (then look on reactions to get total load applied), that might help also, especially if your part is to soft and we get a negative stiffness (load scaling cannot deal with that very well).