The elastic or viscoelastic force would add a quadratic term to the dependency between external indenter load and indentation depth [68]. The model‟s initial, nongeneralized dependency is for an adhesion-free interface where the net external load equilibrates solely the elastic force. When indenter-sample adhesion is not negligible (such as the case in testing asphalt binder or mastic), a term describing the adhesive force variation during the unloading process is added to the external loading force.