Could you explain why you set up tangent modulus to 0 psi??
because it means that you material curve is flat after yield point...
Usually we use Tangent modulus = 10 percent of youngs modulus.
Perfectly plastic model has zero tangent modulus by definition. One can also have a scenario where a negative tangent modulus may apply (i.e., the mat'l. weakens to elongation): there appears to be no allowance in Simulation for this, however.
Indeed, in sw sim, one cannot define such material model. No flat tangent modulus nor negative.
But have you tried defining your own stress strain curve by entering points?
Yes I do not believe the tangent modulus can be zero or even have negative slope in Nonlinear imulation. Since it's a displacement-based solver, E cannot get too close to zero or displacements go infinite. I don't believe the software has error traps to examine the ETAN prior to running, so it may just stop altogether.
To define an elastic-perfectly plastic material, you can just enter 0 as the tangent modulus.
For your case, if it is a simple model and the deformation is nearly one-dimensional, it will be necessary to use displacement control to overcome the convergence issues when the slope of the equilibrium path is zero.
In general, however, ETAN = 0 is not a necessary condition to switch the control method from "force" to "displacement." In fact, it is recommended that users always start with force control. If difficulty in convergence is encountered, you can display a few force-displacement curves around the critical area to confirm there is a "nonlinear buckling" type of behavior which would cause the force control to fail. If that's the case, then displacement or arc-length control should be used.
See the image attached. In the model, fixed geometry was defined at the bottom and a load applied at the top. Because the deformation is one-dimensional displacement control was used with a y-displacement prescribed at the point on the top of the bar. You can verify the elastic-perfectly plastic material definition by viewing the response plot (see where the slope of the equilibrium path goes to 0).