Experimental setup. (a) Side view. Samples are made of two plates that are annealed together (20×10×1 cm3 for the upper one and 23×2.8×0.5 cm3 for the lower one). One of the plates is longer and undergoes a cantilever load from the vertical displacement of a rod (white circle on the left) that bends the plate. This bending with respect to the other plate induces a mode I fracture that propagates along the weak annealing interface (green zone). A camera (blue) is sitting over the sample and takes an optical image of the front propagation using a lighting by transmission located below the sample and reflected through a mirror (blue). Two acoustic arrays of sensors (red) are also attached to the fixed plate and record acoustic emissions during propagation. The sensors have a central frequency of 500 kHz. The array along the propagation direction is made of 64 sensors. The array perpendicular to the front is made of six sensors (see also Figure 7). (b) Bottom view of the sample. The rod is loading the longest plate of the sample. The green area corresponds to the broken zone behind the fracture front. The blue zone shows the zone where pictures are taken. The red zones are transmission views of the acoustic arrays.