Hi Prof. Taylor, I want to use an elastic user element to calculate the kinetic energy for a transient simulation. In order to do this, I need to calibrate the user element using FEAP solid element for transient simulations. The input files for FEAP solid element and the user element are included here. I need to get the permission of my advisor to post the user element. But the user element only uses the basic elastic theory. In the input files, only one element is used, and one time step simulation is run. I output consistent mass matrix, tangent matrix and other mass related matrix to compare their results. You know there are two ways to deal with the mass in transient simulations, either adding lumped mass or use consistent mass. I expect the user element gives me the same results as those from the FEAP solid element, but unfortunately not in all situations. However, using lumped mass in user element gave me close results to those from the FEAP solid element. I am confusing how to program the user element to obtain the same results as those from FEAP solid element, and how to evaluate the following findings. Thanks.
1. adding lumped masses will only change the diagonal mass value in the mass matrix.
2. if both of them use lumped masses, or don’t use lumped masses, the mass matrix are the same for the user element and the FEAP solid element, but the tangent matrix is different.
3. the maximum vertical displacement after 1 step of transient simulation of one simple testing case is summarized as follows for each situation.
solid user element different place
both use lumped mass 9.5763e-13 4.05e-12 tangent matrix are different, but the values have the same order
both consistent mass 1.35e-12 1.25e-2 tangent matrix have a huge difference
