I wanted to get some thought in non linear fea. Let me say there is a spring of initial stiffness k. Let me say in a non linear analysis the load is divided in to 20 steps of 1N each(Total load is 20N). 1.1st iteration.1N is applied and the displacement is arrived 2.2nd iteration 2N is applied. The stiffness matrix(tangent stiffness) is derived from iteration 1. The displacement(u) is calculated. F=ku is the calculated value of resisting force of the spring. Now i want to understand how any fea software is able to calculate the actual resisting force the spring produces.Because after this step only the software is able to calculate the residual force(residual force=Calculated resisting force-actual resisting force). can anyone explain this for a rod (Take the example of a axially loaded rod. The non linearity arrises from material non linearity).