Image of the fracture propagating from top to bottom. The cracked area is in light gray. The scale is given by the two white lines which are 2 mm long. Each pixel corresponds to an area of 20 ×20 μ m2. Image treatment allows a precise description of the fracture front (red line) for each image showing the details of the pinning of the front on the local asperities corresponding to local increase of the toughness. Using a series of images, one can reproduce the exact history of the fracture front propagation trough time.