I want to define variable properties on the interface using the Mohr-Coulomb criterion (so, I need to enter as input the cohesion, friction, normal and shear stiffness of the interface, but i have these properties depending on the normal stress on each interface). Also, i want to know if exist a variable to select the stresses at interface, in order to be able to create a FISH for this purpose.
The shear stiffness of elements can be initially defined as very high to ensure the compatibility of displacements of nodes on either side of the interface.Once the shear stress exceeds the shear strength of the interface (tf), as defined by Mohr-Coulomb relation as shown below, the shear stiffness is reduced to a small value that is 1000 times less than the initial value.
tf = ca + sn tanfa
in which ca and fa are shear strength properties of interface determined from large direct shear box tests, and sn is the normal stress at the interface.
Generally speaking, compresion stiffness of the interface can be defined verty high to prevent the penetration. and tension stiffness is defined a low value. shear stiffness is often correlation with stress state.You can reter to the artical "Clough GW, Duncan JM. Finite element analysis of retaining wall behaviour. Journal of Soil Mechanics and Foundations Engineering".
I assume you refer to the interface between the surface and underlying asphalt layer in a road or other pavement.
As you say, the properties you require are a function of the normal or confining stress. You can estimate this based on finite element or layered elastic theory. I have regularly used the layered elastic tool mePADS (from South Africa) for such calculations, which is adequate for studies based on 'relative comparison' (eg. comparing braking to non-braking conditions). If you don't have access to such tools, you could estimate the normal stress using very simple Boussinesq equations.
It is important to remember that the normal and the shear stress varies under the tyre footprint. Stress-to-strength ratios can be calculated over a grid to determine the critical location. Near the surface, this is generally just in front of the cente of the tyre, where the shear stress is high and the normal stress is low.
It is also important to acknowledge that the interface properties (strength, modulus, cohesion, adhesion) vary widely based on tack coat materials, interface texture, curing conditions and temperture. It is essential to characterise your specific interfaces by direct shear testing, which generates cohesion and adhesion properties, as well as modulus and post-elastic friction.
There are many publications on this topic, and I am happy to provide the citations if you make direct contact.
Cheers, Greg
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