Different rupture modes and different fracture toughness values were obtained on an aluminium alloy subjected to four different heat treatments. The correlation functions of the bidimensional cuts of the four fractured surfaces were determined. It was found that these surfaces were fractal, and that their fractal dimensions were identical, within experimental error.

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We present theoretical ideas which allow us to understand part of the scaling laws recently observed on branched cracks. We argue that some features are common to all critical branched structures, which barely survive when propagating. These ideas are illustrated by the directed percolation problem, which serves as an excellent toy model. Finally, we propose a Langevin equation for unbranched cracks in three dimensions, which naturally leads to self-affine structures.