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# How to consider the effect of a rough substrate during compression test

## How to consider the effect of a rough substrate during compression test

(OP)
I am doing a compression test on a sample which is placed on a rough substrate. Any idea how to consider the effect of rough substrate? (just approximately, Can I consider multiple point boundary consitions for the positions im touch with the rough substrate?)
Thanks

### RE: How to consider the effect of a rough substrate during compression test

How rough is this substrate (what is the scale of its roughness) ? Can you show a picture of it ? Depending on the anawer to the first question roughness can be modeled either explicitly (as rough geometry) or using contact interaction with proper frictional behavior (can be anisotropic).

### RE: How to consider the effect of a rough substrate during compression test

(OP)
Assume 2um roughness. I want to model as contact intercation with friction behvaior. Can you please explaint a bit how to do it?
Thanks

### RE: How to consider the effect of a rough substrate during compression test

It depends what data you have from experiment or literature. Apart from frictionless the following friction formulations are available as Tangential Behavior for mechanical contact interaction:
- Penalty - here you define a single friction coefficient, it's the easiest approach since you can even assume some value and then compare results with real life to adjust it properly
- Static-Kinetic Exponential Decay - here you specify static and kinetic friction coefficient separately, the transition between these two occurs in exponential decay manner with increasing slip rate
- Rough - this one means infinite friction coefficient (no slip), not interesting in this case
- Lagrange Multiplier (only in Abaqus/Standard) - here you also specify a single friction coefficient (like in case of Penalty) but the formulation is different and it's harder to achieve convergence, so Penalty is usually recommended
- user defined - you can also define complex frictional behavior using FRIC subroutine if classical Coulomb model is not enough

For Coulomb model there's also an option to include dependency of friction coefficient on variables such as pressure, slip rate and temperature. Or other user defined properties (utilizing FRIC_COEF subroutine).

To sum up I would start with Penalty formulation (assume and adjust a value of friction coefficient). Only if you have sufficient data you can succesfully model more advanced contact behavior.

### RE: How to consider the effect of a rough substrate during compression test

(OP)
I don't want to create a rough geometry (not the explicit method). So if I want to define a contact interaction, How should I do that?

### RE: How to consider the effect of a rough substrate during compression test

(OP)
I am saying when there is no rough geometry in the modeling, how is it possible to define interctaion? between which surfaces?

### RE: How to consider the effect of a rough substrate during compression test

The sample is placed on the substrate, right ? In such case you model the substrate as flat (no surface roughness) and then apply surface to surface contact interaction between touching surfaces of the substrate and the sample. You have to assign contact property to this interaction. The choice of formulation for this property is described in my previous post (in short - select Penalty).

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