Anomalous scaling of mortar fracture surfaces

Scaling properties of mortar crack surfaces are studied from a mode I fracture test. Fracture surfaces initiated from a straight notch are shown to display an anomalous dynamic scaling of the crack roughness emphasizing the different crack developments between the directions parallel and perpendicular to the crack propagation direction. This anomalous roughening involves the existence of two different and independent roughness exponents. The first one, called the local roughness exponent zeta(loc), drives the self-affine scaling properties of the roughness perpendicular to crack propagation direction and can be considered as a universal roughness index zeta(loc) approximately 0.8. The second one, called the global roughness exponent, estimated to zeta approximately 1.3, is used to described the growth of the roughness at large length scales as a function of the distance to the initial notch and appears as a material-dependent parameter. We argue that the anomalous scaling of the roughness development could be an inheritance of the microcracked fracture process zone, quite large in quasibrittle materials. Finally, in the case of such an anomalous roughening, we argue that the fractal dimension appears insufficient to characterize the fracture surface morphology as a whole.