extension test

What is an extension test?
Is it not just a compression test with reverted sign for the strain rate?
Why would you change stressMask?

For the extension test I have to:
- increase the lateral stress
- keep the vertical stress constant
Hence (I think but this is why I asked since something goes wrong) I have to
change stressMask because the control of stresses/strains is different than
in compression test (where we keep the lateral stress constant and we
increase the axial stress). Only changing the sign of the strain rate would
unload the sample. I want to _load_ the sample but in extension.

Chiara

On 17 October 2011 13:45, Chareyre <email address hidden>wrote:

> Your question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> What is an extension test?
> Is it not just a compression test with reverted sign for the strain rate?
> Why would you change stressMask?
>
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Actually you might be right. I mean, perhaps by doing as I said I reach the
point of failure and so I cannot maintain the equilibrium. Whereas if I did
the test just by reverting the sign of the strain rate, am still in
extension but unloading other than loading. However in this case I should be
able to maintain the equilibrium as the test is run in displacement and not
stress control.
Will try, C.

On 17 October 2011 13:45, Chareyre <email address hidden>wrote:

> Question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> What is an extension test?
> Is it not just a compression test with reverted sign for the strain rate?
> Why would you change stressMask?
>
> --
> You received this question notification because you are a member of
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Oh ok. I didn't imagine this variant.
No I didn't try that before. What are the values in goal?

On 17 October 2011 15:55, Chareyre <email address hidden>wrote:

> Your question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> Oh ok. I didn't imagine this variant.
> No I didn't try that before. What are the values in goal?
>
I tried with: triax.goal=(-0.30,-0.30,-1e5)
So I thought that I could increase the lateral pressure by simply applying a
constant strain rate whereas I would keep the vertical pressure constant.
The test at the beginning seems to work fine but then I always reach a point
of instability which means I loose equilibrium. This could be explained by
the fact that in such a case I control stress other than displacement as for
the common compression test (I think).
Chiara

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Would you be in a special case of imposed stress?
With strain imposed in two directions, I would say you don't loose controlability, but...
Did you set a low enough strain rate? maxStrainRate=(?,?,?)
How does the instability look like?

On 17 October 2011 18:15, Chareyre <email address hidden>wrote:

> Question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> Would you be in a special case of imposed stress?
> With strain imposed in two directions, I would say you don't loose
> controlability, but...
> Did you set a low enough strain rate? maxStrainRate=(?,?,?)
>
max strain rate is 1e-4 1/s. I do not think that that is the problem though.

> How does the instability look like?
>
There is a sudden increase in kinetic energy while the deviatoric stress
drops down (in absolute value). I apply the correct stress path, I have
checked that. At the moment I cannot explain it, maybe someone else could
express an opinion if had experienced the same problem before.

Chiara

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Unless you normalize it, it is difficult to tell if a peak in kinetic energy is "big" or "small".
Deviatoric stress decrease is expected. Why do you call this instability? Is there a big unbalanced force?

On 18 October 2011 09:16, Chareyre <email address hidden>wrote:

> Your question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Chareyre posted a new comment:
> Unless you normalize it, it is difficult to tell if a peak in kinetic
> energy is "big" or "small".
> Deviatoric stress decrease is expected. Why do you call this instability?
> Is there a big unbalanced force?
>
Yep, unbalanced force increases a lot. I suppose it is an instability but I
do not know the cause of it. I have also tried to just invert the sign of
strain rate (so stressMask remains the same and everything else) but I have
the same problem...

>
> --
> You received this question notification because you asked the question.
>

> Yep, unbalanced force increases a lot. I suppose it is an instability but I
> do not know the cause of it. I have also tried to just invert the sign of
> strain rate (so stressMask remains the same and everything else) but I have
> the same problem...

I did that before (reverting sign of rate) and I didn't find problems.
Hard to tell what's the problem in your case.

On 19 October 2011 11:15, Chareyre <email address hidden>wrote:

> Your question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
>
> > Yep, unbalanced force increases a lot. I suppose it is an instability but
> I
> > do not know the cause of it. I have also tried to just invert the sign of
> > strain rate (so stressMask remains the same and everything else) but I
> have
> > the same problem...
>
> I did that before (reverting sign of rate) and I didn't find problems.
> Hard to tell what's the problem in your case.
>
Ok, thanks to let me know. I will double check the sample I use for the
test.

>
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On 19 October 2011 11:15, Chareyre <email address hidden>wrote:

> Your question #174878 on Yade changed:
> https://answers.launchpad.net/yade/+question/174878
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
>
> > Yep, unbalanced force increases a lot. I suppose it is an instability but
> I
> > do not know the cause of it. I have also tried to just invert the sign of
> > strain rate (so stressMask remains the same and everything else) but I
> have
> > the same problem...
>
> I did that before (reverting sign of rate) and I didn't find problems.
> Hard to tell what's the problem in your case.
>
Oh I found the problem, it was a silly mistake in my py script! Otherwise
the test itself works just fine (at least in unloading case as you suggested
but I will try the loading case as well although the control is stress
controlled there and this "could" generate instabilities).
C.

>
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