Friction and adhesion in rigid surface indentation of nitrile rubber

When a rigid body in the form of a plane strain indentor is forced into an elastomer, the asperities on the surface of the indentor are filled by the softer material. As depth of ingress increases, the rubber displaced into the indentor asperities exhibits stick–slip behaviour. The rubber adheres to the rigid body and if the depth of ingress is held at a maximum, the level of adhesion remains constant despite short-term load relaxation occurring in the rubber.This text describes the influence of a range of factors on indentation forces and adhesion in rigid indentation of hydrogenated nitrile rubbers. Blocks of rubber in four hardness grades were subjected to plane strain indentation using mild steel plate indentors. The edges forced into the elastomers were radiused to produce ingress of a semi-circular profile into the blocks and this allowed subsequent finite element modelling of the indentor as a continuum. During physical testing, indentation rates and indentor surface finish were varied and load/displacement characteristics, adhesion and short-term load relaxation were measured. The correlation between indentation loads at the common maximum depth of ingress and the adhesion theory of friction for different surface finishes was examined. Non-linear finite element stress analyses, employing adaptive meshing, alternative friction algorithms and competing strain energy density functions were used to model the indentation process and comparisons of surface profiles with test results are included.