I think the iterations are indirectly sorting this out. In fact I found that the initial stiffness(0) doesn't influence the results at all. I've attached results of compression (comp) and tension (tens)  tests at rate of 1mm/sec as well as the script in its current state. Based on lab tests, comparative compression failure at this rate occurs at 6.6 N/mm2 whereas tension failure occurs at 3.0 N/mm2. So the script is accurately predicting compression failure but under-estimating tension failure. Still exploring ...


a

On Wed, Jan 26, 2011 at 10:19 AM, Robert Cimrman <cimr...@ntc.zcu.cz> wrote:
Well, this flag is used essentially only in the generic solver used by simple.py. In the strain rate solver script we solve every time step regardless this flag.

The first time step is different, however, as strain equals strain0 (no previous strain defined), and so initial dstrain is zero. Maybe this causes some problems?


r.

On Wed, 26 Jan 2011, Andre Smit wrote:

Alright, that fixes it! Now I understand the reason for quasistatic! In
the non-linear analyses, the time stepper is set programatically - is is
possible to force solving for the first time step here as well as in:


    for ii, disp in ds:
        #my_output(dformat % (ii + 1, ds.n_step, disp))

        force0 = 0.0

        pb.ebcs['Load'].dofs['u.2'] = disp

        pb.ts.is_quasistatic = True  # <<<=========== Force quasistatic
        pb.time_update(ts)




On Wed, Jan 26, 2011 at 9:57 AM, Robert Cimrman <cimr...@ntc.zcu.cz>
wrote:

     So the problem is the single dot outside the linear curve,
     right?

     What if you add

            'quasistatic' : True,

     to the time-stepper options? As it is now, the first time
     step is not solved but taken from the initial conditions
     (unspecified = zero) and boundary conditions (nonzero!), so
     it is not in equlibrium!


r.

On Wed, 26 Jan 2011, Andre Smit wrote:

Sorry - forgot to attach the figure - your point is taken re
the nodal
residual. Forces are calculated and summed on each of the
nodes in the
Top region. I checked this a while back and plotted in
Paraview - the
forces are zero at the other nodes within the model but equal
and
opposite in direction to the Top at the corresponding Bottom
nodes of the
model (as you'd expect).

a

On Wed, Jan 26, 2011 at 9:26 AM, Robert Cimrman
<cimr...@ntc.zcu.cz>
wrote:
     Where can I see the line? maybe you forgot to attach a
     figure?

     As for the forces, it might be better to compute them
using a
     (inner) surface integral of stress instead of the nodal
     rezidual. Btw. you look at the nodal force in the first
node
     of the Top region only, right? What are the values in
the
     other nodes?

     r.

     On Wed, 26 Jan 2011, Andre Smit wrote:

           Thanks Robert!

           I've attached a modification that shows the
           strain/stress output for the
           elastic case of the cylinder under compression.
           The slope of the line is
           Young's modulus as you'd expect. As with our
           non-linear analyses, the
           forces calculated after the first time step
           appear to be too high - not
           sure what the reason for this is. 

           On Wed, Jan 26, 2011 at 2:25 AM, Robert Cimrman
           <cimr...@ntc.zcu.cz>
           wrote:
                There was some old code in the
           stress_strain() function. The
                exception was caused by putting directly the
           traction
                function into the EBC definition, instead of
           its name. The
                attached file should work.

                r.


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