Figure 1: Sample of crack fronts. The background is an inverted raw image where the intact material appears white. In contrast, the cracked zone is dark. Crack propagates in this case from bottom to top because of a tensile load. transparency of the material. Plates are : 32cm 14cm 1cmand annealed together at 205 Æ C under several bars of normal pressure. The annealed surface corresponds to a weak plane which the fracture will propagate along. Before annealing, both plates are sand-blasted on the side to be annealed with 50 msteel particles. The sand-blasting procedure introduces a random roughness which induces local toughness uctuationsduringtheannealingprocedure.Sincethesand-blastinggivesacuto ofthe structure in the plates of about 50 m, we do not expect correlated toughness above this length scale. Oneoftheplatesisclampedtoasti aluminumframe.Anormaldisplacementisappliedbyapress to the other plate which induces a stable crack propagation in mode I at constant low speed (68 m=s). The fracture front is observed with a microscope linked to a high speed Kodak Motion Korder Analyzer camera which records 8.7s at 500 images per second with a 512x240 pixel resolution. We also performed experiments with a normal speed Kodak DCS 420 CCD camera which has a resolution of 1536x1024 pixels. This camera was used after loading when the fracture had come to rest. The visualization setup is mounted on a translation table with possible movement parallel to the propagation plane (x;y) controlled by two stepping motors. In Fig. 1 is shown a sample image obtained with this setup. The uncracked part is seen as white whilethegrayregionrepresentstheopenfracture.Thefrontisde nedasthecontrastboundary.Let V be the local front velocity in the direction of the front normal. The velocities V at all front positions were calculated by measuring the distance Æl, along the normal, to the intersection with the front that is 20ms later. The distributions of the velocityV is shown in Figure 2. The distribution have clear long tails at large velocities. Very high front velocities compared to the average crack speed are observed locally.Figure3showsthe uctuationsofthecrackfrontposition: h(x;t) hh(t)ix in gray levels [1]. Light regions correspond to regions which are in advance of the average crack front. On the contrary, the dark gray correspond to pinned regions of the front that evolve slower than the average crack front. In the guretherearenumeroussharptransitionswhichappearclosetohorizontal.Thesetransitionsare triggered after pinning periods (dark regions) and correspond to fast propagations or local instabilities along the front. The fracture front line only evolves during these fast instabilities or bursts. Apart from 2
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