The normal contact problem of two spheres was studied to investigate the contact regime of curvature joint interface. The force applied to the circular contact region formed by the point higher pair contact of two spheres was analyzed through utilizing the virtual material thickness of joint interface. The Majumdar-Bhushan plane model and classic Hertz theory were combined in the analysis. It was demonstrated that the entire definite range of fractal dimension satisfies 1≤D<2 in the two-dimensional Weierstrass-Mandelbrot fractal function by adopting Hardy's non-differentiable condition at a point anywhere. Numerical simulation exhibits that the spherical generalized contact area ratio is not greater than one. The spherical generalized contact area ratio in inner contact is larger than that of outer contact. The spherical generalized contact area ratio may increase by increasing the compressive force or reducing the virtual material thickness of joint interface. The true contact area in internal contact is higher than it in external contact. As the fractal roughness
material hardness or virtual material thickness of joint interface increases
the true contact area decreases. As the fractal roughness increases
the compressive force required to produce a specified true contact area increases. This accounts for the fact that an increase in the fractal roughness implies an increase in microcontact's normal deformation
which therefore requires a higher compressive force. As the value of fractal dimension increases from 1.4 to 1.5
the actual contact area first increases for a given compressive force. As the value of fractal dimension increases from 1.5 to 1.9
the true contact area decreases. The Hertz stress in internal contact is smaller than that in external contact. These research findings may provide a basis for further research on the contact strength calculation of spherical bearing. The spherical contact fractal model possesses the universality and practicality to expand the mechanical part contact strength theory in machine design.
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