Load transmission by elastic, elasto-plastic or fully plastic deformation of rough interface asperities

Abstract

The normal contact between two nominally flat, parallel rough surfaces is analysed on the microgeometric scale from a theoretical point of view. The microgeometry is described using a probabilistic model based on the quantification of the variability in the summit altitude and the asperity radius. This model takes for its basic data six standardised parameters for roughness and waviness defects measured on both surfaces in contact. For a given separation between two surfaces, this model allows us to determine the number of asperities in contact and their respective indentations. For each asperity in contact, realistic indentation laws are developed taking into account either elastic, elasto-plastic or plastic deformation. The continuous sum of the loads at each asperity gives the global load at the contact and the force transmitted through elastic, elasto-plastic or fully plastic deformed asperities. The theoretical analysis of classical surfaces possessing various mechanical characteristics confirms the combined influence of microgeometry (roughness and waviness parameters) and mechanical characteristics (Young's modulus, Poisson's ratio and yield stress) on load transmission. The association of these parameters into a classical index for plasticity confirms previous predictions. The last necessary parameter is the normal load applied through the contact. This parameter does not appear in previous theories for the plasticity index but can drastically change the degree of plasticity in the contact. The main contribution of our research consists in the quantification of the proportion of plastification in the contact at a given normal load.