You can model a situation where two boundaries stick together once they get into contact by adding an Adhesion subnode to Contact. Adhesion can only be modeled when the penalty contact method is used. The adhesion formulation can be viewed as if a thin elastic layer is placed between the source and destination boundaries when adhesion is activated.

Using the effective gap distance gn and the slip Δgt, the adhesion formulation defines an incremental displacement jump vector u in the local boundary system as

where Tb-T contains the transform from the global system to the boundary system. In the above expression, the Macaulay brackets indicate the positive parts operator such that

Using the displacement jump vector, the adhesive stress vector f is defined as

where k is the adhesive stiffness. For negative values of gn, the normal component of f is zero, and the contact condition is resolved by the penalty contact formulation. Notice that a different sign convention is used for the normal stress in the adhesion and contact contributions, where Tn is positive in compression.

The adhesive stiffness k can be defined using three different options: From contact penalty factor, User defined, and Use material data. For the first option, the normal stiffness is set equal to the contact pressure penalty factor pn. The two tangential stiffness components are then assumed to be related to the normal stiffness, so that the stiffness vector equals

where nτ is a coefficient with the default value 0.17. This coefficient can either be input explicitly, or be computed from a Poisson’s ratio. A plane strain assumption is used for this conversion, giving

For the Use material data option, k is calculated from the elastic constants of a fictive layer with a thickness equal to ds.