Friction
Both explicit cracks and multimaterial mode MPM can model frictional contact between the surfaces.
Introduction
Contact mechanics between surface can be model as frictionless sliding, Coulomb friction with a surface coefficient of friction, or stick contact (no sliding). In frictionless sliding, the tangential surface force is zero while the normal forces is determined by the amount of contact. In frictional sliding, the magnitude of the tangential force is related to the magnitude of the normal force by:
<math> f_t = \mu f_n<\math>
where μ is the coefficient of friction. But, if the induced tangential force from surface motion is less than this value (e.g., because fn is low) the the contact is modeled as stick contact instead. In stick contact, the two surface move in the center-of-mass velocity field as if there were one material, but only as long as they remain in contact.