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 ( ${10}^{-7\mathrm{}}–5\times {10}^{-5\mathrm{}}\mathrm{m}/\mathrm{s}$). 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 $0.55\pm 0.05$ in a static regime. No evolution of the roughness exponent is observed during the propagation even if the crack speed changes.