# Discussion Forum

## Shape factor of multi-linear separation of decohesive setting & penalty factor

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Dear Comsol users,

I noticed that for decohesive settings tag, there is a parameter λ called shape factor when multi-linear seperation is selected. Could anyone please give some explanation on this? What does this factor do?

Also, the contact pressure penalty factor in penalty method is interpreted as the stiffness of a distributed spring connecting two contact boundaries. Why does it have the unit of N/m^3?

Any suggestion is appreciated.

Best, Xiaokang

3 Replies Last Post 26 Nov 2018 04:50 GMT-05:00

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Posted: 5 months ago

Hi,

Lambda is described in the user's guide as:

"This law requires one more material parameter lambda, describing the width of the “plastic” region. The Shape factor lambda is the ratio between the plastic (constant stress) part of Gic and the total “inelastic” part of Gic."

Looking at the figure just above that text (in the section 'Multilinear Separation Law' in the theory chapter) will make things clear.

As for the penalty factor unit, an ordinary spring has a stiffness with unit N/m. If that is distributed over an area, you divide by m^2.

Regards,
Henrik

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Posted: 5 months ago

Hello Henrik,

Thank you so much for the explaination.

I have a follow-up question for you. The multi-linear decohesive law uses both ERR (J/m^2) & interficial strength (Pa) together as the criterion for separation. I'm wondering how can I use interfial strength only as the criterion, meaning, separation occur right after shear stress exceeds interficial shear strength.

Thank you!

Best, Xiaokang

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Posted: 5 months ago
Updated: 5 months ago

Hi Xiaokang,

The problem with using a material law in which the layer breaks instantaneously at a certain peak stress is that it may be difficult to obtain convergence in such a model.

But if you want to try it, you can just use the linear separation law, and set GIc such that it matches the energy in the layer exactly at peak stress, that is 0.5*max_stress^2/penalty_factor. Probably that value should be multiplied by a small number, like 1.001 or so to avoid division by zero.

Regards,
Henrik

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