small equation solver pivot terms 2

[jiligeo]

I am analysing a 3-D model of pile foundation under lateral load. The soil is modelled as DP (Draucker-Prager) material model with contact element between pile and soil. The solution converges to the true results for small loads (approximately for 20 % of ultimate loads) but for larger loads does not. The warning message is “there are 7 small equation solver pivot terms”.
Please help me if you have any idea.

 

[Drej]

>The warning message is “there are 7 small equation solver pivot terms”.

This is generally a warning because of large rotations, displacements, etc. being detected in your model. In your case it might mean that some bifurcation level has been reached or exceeded. I don't know for sure because you haven't provided enough information about your model/type of analysis/etc. What happens when the load goes above 20%, are your displacements to big, too small or what?

 

[jiligeo]

Dear Drej,
The type of analysis is static and the large deformation is possible (nlgeom, on).
The elements which I use are solid95 (for soil and pile) and conta174 and targe170 (for contact elements).
With increasing the load, the displacements will be big.

 

[Drej]

Plot the displaced shape of your model. Check that it is not experiencing buckling. To check for this, plot the displacement vs. time at the location where the greatest displacement takes place and note down the values and the shape of the graph. If the displacement increases drastically over a very short period of time, then your model is probably experiencing collapse. Again, how much do your displacements differ from the measured displacement (%) after the 20% of ultimate load is applied?

 

[jiligeo]

Dear Drej,
Thank you for your help. As you said, the reason of small equation solver pivot term was large deformation. I used DP material model and in this model weight of material is very important. I defined density for material but I forgot to define gravity (using Acel command).
Regards
Jili

 

[YanLiu]

Hi, Jili:

I have similar problem like yours (3D mesh, DP model, 170/174 contact pair) and same problem that calculation did not converge (a few small equation solver pivot terms after about 10% of load). Did you solve your problem after turn Acel on?

Thanks.

Yan

 

[jiligeo]

Dear Yan, after I turn acel on, the convergence was better but I have still problem with convergence for large load in my model.
At the moment I do not work on that project but I will start that again soon. For the same problem at the other project (but without contact elements), I have solved the problem using the finer mesh and changing the solution method. I have the better convergence with full Newton Raphson method (nropt, full).

Regards
Jili