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

[Brad Aagaard]

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
>
>
>
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