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Strange automatic decohesion occurs though no decohesion law is specified
Posted Jan 16, 2024, 4:42 a.m. EST Structural Mechanics, Modeling Workflow Version 6.1 5 Replies
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Hi there, I am trying to understand how the cohesive model works in Comsol.
A simple model including square A and B in contact with each other is built. B is assigned as rigid material and pulled along the vector (u,v)=(0.4,0.4), while the top boundary of A, which is in contact with B, is fixed, so that both shear and tension occurs between the contact pair.
For the contact, the adhesive activation criterion is set to 'Always active'. The adhesive stiffness is set to be 0 for kt1 and Et/ts for kn. There is NO decohesion sub-node.
The results are strange. Shear stress remains 0 (blue line), which is consistent with the 0 shear stiffness. But the normal tension stress (green line) doesn't follow the theoretical value (red line), and finally reduces to 0.
It seems that decohesion does happen, though no decohesion law is specified.
The adhesive stress distribution on the contact boundary is give at v=0.001,0.014,0.070,0.167,0.181, as follows
As shown above, the contact pair is not totally adhesive. There is a decohesion site at x=0 initially. As the load increases, the zone of decohesion expands. And then, much more new decohesion sites emerge and expand progressively. Finlly, the totally decohesion happens.
But the adhesion activation criterion is set to 'Always active' and no decohesion law has been specified yet.
This is disturbing. Is there something that I did not do it right?
Best, HCL
All the pictures are attached below, in case that they are not properly shown above.
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Without a model, it is only possible to guess. But my first guess is that you need to increase the search distance in the contact pair. Even though there is adhesion, that contribution is only computed between points that are inside the search distance from each other.
Note that if you slide the block in the horizontal direction, you will still loose some adhesion stiffness, since there is only a contribution from the parts actually in contact with each other.
-------------------Henrik Sönnerlind
COMSOL
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Thank you, Henrik!
I increased both the width of B and the search distance, so that A is always fully in contact with B and that the distance between the contact pair is less than the search distance. It works.
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Without a model, it is only possible to guess. But my first guess is that you need to increase the search distance in the contact pair. Even though there is adhesion, that contribution is only computed between points that are inside the search distance from each other.
Note that if you slide the block in the horizontal direction, you will still loose some adhesion stiffness, since there is only a contribution from the parts actually in contact with each other.
Do I always have to set the search distance manully? Is there some guide about when the automatic setting fails? It concerns me that the computation may be slowed down if I manully set a too large search distance with many contact pairs.
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The default search distance is 1% of the size of the geometry (to be more specific: 1% of the diagonal of the bounding box). For most contact problems that will work. In your case, you need interaction even when the objects are a bit away from each other.
It is actually more common that the search distance needs to be decreased, because there may be an ambiguity when deciding which boundaries that will interact.
-------------------Henrik Sönnerlind
COMSOL
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Thank you, Henrik, for your clarification!