Propagation of an inter-facial crack front in an hetrogeneous medium: experimental observations

We study experimentally the propagation of an in-plane crack through a weak plane of a transparent plexiglas block. The toughness is controlled artificially by sand blasting surfaces and fluctuates spatially like an uncorrelated random noise. The block is fractured along the weak plane in mode I at low speed. The crack front is observed optically with a microscope and a high resolution digital camera for pinned fronts or a fast digital camera for local dynamics of fronts. 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 roughness exponent of trapped fronts is shown to be 0.63. During propagation, bursts of the front are described as an interface growth process. The dynamical exponent of a Family-vicsek scaling is measured to: z=1.15. Velocity fluctuations are measured in the space-time domain.