Sheldon Imaoka
Ansys Employee

For your question on nonlinear behavior of FRP composites, while there are such effects present, but what you should consider is whether or not such nonlinear effects are important to your analysis objectives. For example, if you don't want your FRP composite structures to undergo nonlinear response, you can use failure criteria with orthotropic linear elastic material definition, for example, to determine the appropriate factors of safety for your design.
Composite failure can be quite complex, but there are damage models available in Ansys Mechanical to account for damage and anisotropic behavior. It may be useful to review texts on composites if you are not familiar with orthotropic stress/strain limits, but the idea is that you have different strengths in different directions (e.g., aligned with fiber orientation, for example). There may be different modes of failure (e.g., fiber failure, matrix failure, separation), and this can be accounted for by different failure criteria, where max strain or max stress criteria are simple ones, but one should have an understanding of possible modes of failure and how to account for that first, as that will help you determine which material models and/or failure criteria are suitable for your analysis purposes.
Regards Sheldon