Two-dimensional Hurst index of joint surfaces

The three-dimensional roughness morphology of joint surfaces affects their mechanical behavior. The one-dimensional joint profiles provide an incomplete and biased characterization of joint surface morphology. A two-dimensional quantitative description of surface roughness is thus needed. This paper extends the Hurst index applied on the one-dimensional profile to the two-dimensional joint surfaces. The two-variable fractal Brownian motion theory is employed successfully to describe the whole degree of roughness on the joint surface. To find this two-dimensional roughness index, two base dimensions and asperity height of the joint surface are enlarged by different ratios to achieve the requirement of statistical self-affinity properties. The Hurst index and thus fractal dimension of two joint surfaces cored from sandstone and schist is examined using the triangular-prism-surface-area method as well as this new method. It is shown that for distinguishing the whole roughness difference between these joint surfaces the new method is better. The asperity height distribution of the joint surface is Gaussian and behaves like a self-affine fractal, while the sampling points are enough. For practical application, a two-dimensional Hurst index HID, that is directly expressed in the form of Fourier series represented by the profiles in two property orthogonal directions is proposed to describe the anisotropy of a whole joint surface.