Frustrated element deletion in Explicit, resulting in nonsense partially dead element

**jevanf** · 2010-10-11, 11:39

I'm running a simulation of time dependent deformation and failure at a notch root in polyethylene. I'm using Jorgen's VUMAT, the TNM. When the model reaches the defined critical state (critical chain strain or ultimate true strain) it throws the failure flag that Abaqus sees, and Abaqus unloads the offending integration point. The problem is, the remaining integration points in the element survive and are apparently protected from failing by the unloaded "dead" material point. The result is that I get these organized stacks of elements that are partially dead, and this partial death spreads through the model instead of any elements being removed. Eventually the element stacks start deforming in impossible ways and the simulation breaks down. Has anyone seen this sort of frustrated deletion problem? Anyone have insight?

Also, I am scaling the mass a great deal because the elements are tiny (L=1e-6), but the process is essentially quasi-static so I think this is OK unless it brings in a problem that I don't recognize. I'm running it in double precision, so I think the calculations are OK, and therefore the problem is systemic.

Jevan

**Jorgen** · 2010-10-17, 19:23

Hi Jevan,

What element type are you using?
I have done run many FE models with element deletion from within a VUMAT and I have never seen that problem.

-Jorgen

**jevanf** · 2010-10-18, 20:53

Hi Jorgen,

I'm using C3D8 elements in Abaqus 6.9EF. I tried quadratic full integration elements at one point, but I believe the same thing happened and the simulation took forever. I'm using the September'10 build of the PolyUMod. I get the same problem if I have a relatively coarse or fine mesh, but remember that I'm modeling highly triaxial tension at the root of a blunt crack, so things could be pretty strange there compared to more regularly shaped specimens. Also, the deformation there is self-similar, so the blunt crack in the simulation goes from say 5 micron wide to 20 micron wide, so you don't see the blunting to create a "second dogbone" effect like Mike Soberaj saw with his notched cylindrical specimens.

Jevan

**lumpwood** · 2010-10-20, 09:53

I've used element deletion with VUMATs very often and had never seen anything like this...until today! I've started using Abaqus 6.10 with my VUMATs for the first time today (upgraded from 6.9)and am seeing the same as you. Will check them in 6.9 again to make sure but looks like it may be a bug.

**brunda** · 2012-10-25, 23:59

Dear all,

I am facing issues with element deletion. I am really hoping that some of you can provide help.

I have a working VUMAT for elasto-plastic material. During my simulation, I delete elements once a failure criterion (of plastic strain) is satisfied. The simulation runs until few elements are deleted and then throws excessive deformation error. I have tried ALE, changed mesh sweeps, tried quadratic elements, distortion control. But nothing seems to work. I am running the simulation for 2D pstrain. Currently, I am using CPE3 elements.

Has anybody encountered this problem? Can you please provide any insights.

Thanks in advance
Brunda

**Jorgen** · 2012-11-04, 06:15

Yes, that is commonly observed. You can often improve the post failure convergence using different stabilization or progressive damage approaches.

-Jorgen