Clearance Vs penetration

Hello,

Manual states: “The relative contact displacements constitute a vector with three components.
The first component is the clearance (entity [COPEN]), i.e. the distance between the slave node and the master surface. Only negative values are stored.
they correspond to a penetration of the slave node into the master surface."

In the attached example with mortar contact, there is clearly no penetration in between surfaces, but a large COPEN is reported in the slave Surface.
Should we reconsider the correspondence between clearance and penetration for Mortar contact or are those values unexpected?

This doesn’t happen with Surface-to-Surface contact.

Regards

inp file

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The research paper “Variationally consistent quadratic finite element contact formulations
for finite deformation contact problems on rough surfaces” by S. Sitzmann et al. (mentioned in the CalculiX manual as one of the references for the mortar contact formulation) discusses so-called “dual gap” problem. I guess it could explain this behavior:

That means that a slave node can be set active, even if there is still a gap between the bodies and some non-physical pushing apart occurs. This situation can occur due to high gradients in the model or high curvatures modelled with very few elements or sharp edges in the continuum model. […] Also this can be triggered by a coarse curved slave mesh and a finer curved master mesh. These problems arise due to the non-positive part of the dual basis functions […]. In many situations, the problems linked to the dual basis functions can be avoided by simply switching the master and the slave side or by refining the mesh in this region, since the severity of the induced numerical artefacts are h-dependent.

the manual does not recommend the use in these situations, maybe it would work properly with a 3D model.

It is advised to use the mortar method for contact between genuine 3-
dimensional elements only. Usage for contact in between 1-d or 2-d el-
ements will cause problems. In general, the mortar method is not well
suited if the contact areas are too much constrained by extra multiple
point constraints.

Yeah, I thought that mortar contact was not usable with 2D elements (at least plane stress/strain ones) at all. We should check if the same problem occurs for solid elements.

Same with one layer of 3D solids (C3D8 and C3D20):

So it’s probably that issue discussed in the paper.

Yep. Same result here once extruded and refined.

Do you mean that was anticipated in some cases? Some kind of limitation of Mortar under certain conditions.

It doesn’t seem to affect the contact pressure.

Regards

Only one element trough thickness lead to edge to edge contact and known problematic; Abaqus failed also in hard contact type in eliminating penetration.

Yeah, I mean the effect described here: Clearance Vs penetration - #2 by Calc_em

Basically, a slave node being active even when there’s a significant gap. This may happen for large curvatures and could be the case here.

I suspect you’re right. I hope Disla has time to check it out.

I think the issue I’m expossing is not well understood (probably not well explained from my side).

In the contact area the penetration and surface pressure are ok. The problem is that on areas where master and slave are more than 350mm appart there is COPEN reported on the slave surface.

I have refined with more elements per thickness and that unexpected behaviour persist.

my previous test shown a penetration is not okay at the edge of hard contact type. I try to avoid case of edge-to-edge contact condition whenever possible. make models with bigger ones for the part and soften material for comparison before going further another issue.

Yeah, these could be the falsely active slave nodes mentioned in the paper:

That means that a slave node can be set active, even if there is still a gap between the bodies

I just wonder why CPRESS is zero there.

that’s exactly what I understood.

It seems that the “solution” I offered is not a real fix. Thanks for taking the time to check it.

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It seems to be caused by the nature of the mortar contact formulation. It could go away when surfaces are swapped. And I guess it’s specific to large curvatures.

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Unfortunately swapping master and slave doesn’t help.

The main concern is about this comment on the paper :“some non-physical pushing apart occurs”.

but if I read it properly, the implementation in CCX is the one in the paper that fixes that issue, right?

I rebuilt the model again but with same results. By the way it recalls me the issue we had here.

model in pmx (3D): https://drive.google.com/file/d/1BSR52p_7AgoBgQcWtdTB3R2UizV87Tcu/view?usp=sharing

Hard to say if the fix was also finally implemented in the official builds of CalculiX. There are 3 papers from this author mentioned in the CalculiX documentation. Also, perhaps this issue was fixed only partially or a similar issue may arise in some cases. Anyway, it must be something about the Mortar contact formulation since it doesn’t happen with surface to surface contact.

@Disla Can you report it on GitHub ?

1 Like