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 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.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?

'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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