We analyse the statistical distribution function for the height fluctuations of brittle fracture surfaces using extensive experimental data sampled on widely different materials and geometries. We compare a direct measurement of the distribution to a new analysis based on the structure functions. For length scales δ larger than a characteristic scale δ * , we find that the distribution of the height increments ∆h = h(x + δ) − h(x) is Gaussian. Self-affinity enters through the scaling of the standard deviation σ, which is proportional to δ ζ with a unique roughness exponent. Below the scale δ * we observe an effective multi-affine behavior of the height fluctuations and a deviation from a Gaussian distribution which is related to the discreteness of the measurement or of the material.

We study experimentally the propagation of an in-plane fracture into a transparent and heterogeneous Plexiglas block. A stable crack propagation in mode I is monitored by an imposed displacement. The experimental setup allows a high resolution observation of the crack front in situ. Self-affine properties of the crack front are described over more than three decades using several techniques: variable bandwidth, return probability, Fourier spectrum, and wavelet analysis. The different methods lead to a roughness exponent of 0.63+/-0.03, consistent with a previous work.

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Abstract. It is proposed to use a discrete particle model as a complimentary "numerical testing machine" to identify the hydrostatic elasticity-damage coupling and the corresponding sensitivity to hydrostatic stresses parameter. Experimental tri-axial tensile testing is difficult to perform on concrete material, and numerical testing proves then its efficiency. The discrete model used for this purpose is based on a Voronoi assembly that naturally takes into account heterogeneity. Tri-tension tests on a cube specimen, based on a damage growth control, are presented. A successful identification of the hydrostatic sensitivity function of a phenomenological anisotropic damage model is obtained.

This paper considers a model of crack propagation taking place at the interface between a rigid support and an elastic plate. The interface is modeled using a fiber bundle model (i.e., describing a damage behavior using a discrete set of elastic brittle elements having a random strength). This paper studies the fluctuations of the force required to propagate the crack along the interface. The statistics of avalanches, defined as a series of elements that are broken simultaneously under a load that decreases with the crack advance, are studied numerically and analytically. Local fiber breakage kinetics is related to a correlation length, which sets the size of a fracture process zone.

The propagation of an interfacial crack through a weak plane of a transparent Plexiglas block is studied experimentally. The toughness is controlled artificially by a sand blasting procedure, and fluctuates locally in space like uncorrelated random noise. The block is fractured in mode I at low speed (10 À7 À 10 À4 m/s). The crack front is observed optically with a microscope and a high resolution digital camera. During the propagation, the front is pinned by micro-regions of high toughness and becomes rough. Roughness of the crack front is analyzed in terms of self-affinity. The in-plane roughness exponent is shown to be 0:63 AE 0:05. Experimental results are compared to a numerical model. The model reproduces the self-affine behavior of the crack front, i.e., long-range correlations of the roughness. Analogies between mode I and mode III are presented in order to discuss implications of the experimental results for creeping faults. Accordingly, correlations of the slip pattern are shown to exist over scales substantially larger than the asperity sizes.