Abstract: Rough surface asperity parameters are characterized based on fractal geometry theory. Asperity normal contact stiffness model is developed based on contact mechanics that the deformation transitions from elastic, through elastic-plastic, to eventually to the plastic, respectively. The mechanical joints normal contact stiffness calculation model is presented based on fractal geometry and contact mechanics theory. In different plastic index, the model is developed the relationship between the joint normal contact load and contact stiffness. The result shows that plastic index is lesser，asperity is mainly elastic deformation, and the relationship between normal contact load and stiffness is the approximate linear, that, with the plastic index increase, asperity is mainly plastic deformation, and the relationship between normal contact load and stiffness is the strongly nonlinear. At last, normal contact stiffness which machining surface are milling and grinding are calculated and analyzed for existed experimental parameters used the model. The comparison result indicates that the present model is consistent with experiment result.

Key words: Asperity, Elasticoplastic deformation, Fractal geometry, Mechanical joints, Normal contact stiffness