[CIG-SHORT] sensitivity of Drucker-Prager yielding to time step

Tue Oct 7 14:29:27 PDT 2014

Hi Charles,

We talked briefly about this issue last month at SCEC, and I would still
love to find a resolution despite having dropped it for a few months. I've
attached the full set of input files so you can take a look yourself. The
parameter I've been varying is "dt" in pylithapp.cfg, and earlier I
reported the odd results when I vary this in combination with the input
file "plastic_nofault_noshear.cfg". Some other input files are also
included -- elastic or plastic, with or without a fault, and with or
without an initial shear stress applied.

Let me know if you're able to reproduce my issue, and have any ideas how to
fix it.

Thanks!
Eric

On Tue, Jul 22, 2014 at 3:32 PM, Charles Williams <willic3 at gmail.com> wrote:

> Hi Eric,
>
> I’ve been on vacation for the past month, but I was curious whether you
> had made any more progress on this problem.  If not, could you send us all
> of your input files, and we will have a look at it?
>
> Cheers,
> Charles
>
>
> On 12/07/2014, at 1:15 pm, Eric Lindsey <elindsey at ucsd.edu> wrote:
>
> Hi Brad,
>
> In all cases (except possibly the dt=1yr case) it's yielding significantly
> before the yield condition that I calculate. My initial stress state has no
> shear stress, so it should be plenty far from the yield condition. The
> initial isotropic compression is -100MPa, and the friction angle is 30
> degrees -- for an inscribed Drucker-Prager yield condition, I found that
> this implies yielding when the shear stress reaches 69.3 MPa. None of the
> models yielded at that value (i.e. at the first time step that exceeded
> that level).
>
> I do have one confusion about the initial stress state. I was setting
> sigma_zz_initial equal to the other two compressional stresses (isotropic
> compression), but this technically violates the plane strain assumption
> that sigma_zz = poisson_ratio*(sigma_xx + sigma_yy). Is this a problem? My
> poisson ratio is 0.25, so I re-ran a few of the models and the calculation
> of equation 5.101 for the case where sigma_zz_initial=-50MPa, in which case
> I predicted that yielding should occur at a shear stress of 111.8 MPa. But
> I get yielding at 40MPa for the dt=50years case, and 41MPa for dt=10years.
>
> Sorry to send so much detail about this issue... your help is much
> appreciated!
>
> Eric
>
>
> On Fri, Jul 11, 2014 at 1:41 PM, Brad Aagaard <baagaard at usgs.gov> wrote:
>
>> Eric,
>>
>> How close is your initial stress state to the yield condition? At what
>> time do you expect yielding to occur? Have you computed the yield condition
>> (equation 5.101 on page 88/252 in the PyLith manual) from the PyLith output
>> to see if it matches your expectations of when yielding should occur?
>>
>> My guess is that your load increment might be too big, so larger time
>> steps push the response much further outside the yield surface. Intuitively
>> I would think this would give a higher shear stress at yielding at larger
>> time steps but that is the opposite of what you see. This is why I suggest
>> going back to the yield criterion and starting from there.
>>
>> Brad
>>
>>
>>
>>
>> On 07/10/2014 06:11 PM, Eric Lindsey wrote:
>>
>>> Hi all,
>>>
>>> I'm getting some strange results with the DruckerPragerPlaneStrain
>>> material. In short, I'm finding that the value of shear stress at which
>>> yielding/failure of the material occurs depends on my timestep...
>>>
>>> I have a 2D box and apply Dirichlet boundary conditions on all 4 sides,
>>> and
>>> an initial isotropic compression -- including the zz component as
>>> required
>>> for models with plasticity. There is zero initial displacement, then I
>>> apply simple shear to the domain, such that the stresses remain uniform
>>> everywhere as shear stress increases.
>>>
>>> It turns out that the point at which yielding occurs depends
>>> significantly
>>> on the time step. Initial compression is -100MPa. The shear modulus
>>> (30GPa), displacement rate (1m/yr), and domain size (150km) implies a
>>> stressing rate of 0.2MPa/year. Here is a table of some results:
>>>
>>> timestep dt (yr)         shear stress at yielding (MPa)
>>> 50                               30
>>> 10                               36
>>> 5                                 58
>>> 1                                 76.6
>>>
>>> The log file doesn't indicate convergence problems, but could this be a
>>> tolerance issue? Or a more subtle solver problem? My .cfg files and a log
>>> of the screen output are attached.
>>>
>>> Thanks,
>>> Eric
>>>
>>>
>>>
>>> _______________________________________________
>>> CIG-SHORT mailing list
>>> CIG-SHORT at geodynamics.org
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>>>
>>>
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>
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>
> Charles A. Williams
> Scientist
> GNS Science
> 1 Fairway Drive, Avalon
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