Direct observation of a self-affine crack propagation

We study experimentally the propagation of an in-plane crack through a transparent Plexiglas block. The toughness is controlled artificially and fluctuates spatially like uncorrelated random noise. The system is loaded by an imposed displacement and cracks in mode I at low speed (10(-7)-5 x 10(-5) m/s). The crack front is observed optically with a microscope and a high resolution digital camera. During the propagation, the front is pinned and becomes rough. Roughness of the crack front is analyzed in terms of self-affinity. The roughness exponent is shown to be 0.55 +/- 0.05 in a static regime. No evolution of the roughness exponent is observed during the propagation even if the crack speed changes.