5 Replies Latest reply on Sep 17, 2009 5:29 PM by Matthew Derov



      I am attempting to use non linear - tresca yield criterion with a elastic, perfectly plastic material mode.  Firstly, how do I setup the perfectly plastic material mat'l?  I have tried the following with the solver refusing to converge on a simple, one component beam geometry, where the problem solves easily for linear elastic, isotropic mat'l. mode. 

      1. set the the tangent modulus to 0 psi and kinematic hardening ratio to 1

      2. enter the (engineering) stress - strain line with first point at yield and second point at elongation (i.e., rupture) in tension only (the loading is logically in tension).

      The study is quasi-static non linear with the folllowing settings: time increment is automatic, large strain option only selected, direct sparse solver with force, NR, Newmark settings, loading period is 1 s (default).  I have also attempted to vary the load below and above the yield condition:  no luck even with a low load with the non linear solver!  As I mentioned, this is a very simple geometery/loading problem; What am I, generally, doing wrong?

          Loic Ancian


          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.

            Anthony Botting
            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.
              Matthew Derov



              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).


              Good luck!



              elastic-perfectly plastic1.tif