VOLUME 87, NUMBER 10 PHYSICAL REVIEW LETTERS 3SEPTEMBER 2001 Dynamical Event during Slow Crack Propagation Knut Jørgen Måløy 1 and Jean Schmittbuhl 2 1 Fysisk Institutt, Universitetet i Oslo, P.O. Boks 1048 Blindern, N-0316 Oslo 3, Norway 2 Laboratoire de Géologie, UMR 8538 École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France (Received 29 January 2001; published 17 August 2001) We address the role of material heterogeneities on the propagation of a slow rupture at laboratory scale. With a high speed camera, we follow an in-plane crack front during its propagation through a transparent heterogeneous Plexiglas block. We obtain two major results. First, the slip along the interface is strongly correlated over scales much larger than the asperity sizes. Second, the dynamics is scale dependent. Locally, mechanical instabilities are triggered during asperity depinning and propagate along the front. The intermittent behavior at the asperity scale is in contrast with the large scale smooth creeping evolution of the average crack position. The dynamics is described on the basis of a Family-Vicsek scaling. DOI: 10.1103/PhysRevLett.87.105502 PACS numbers: 62.20.Mk, 46.50. +a, 61.43. –j, 81.40.Np Most studies on fractures have focused on homoge- neous materials. The role of heterogeneities has been ad- dressed more recently. For instance, heterogeneities lead to self-affine long range correlations of fracture roughness [1,2]. However, the physical influence of heterogeneities on the crack process is still not fully understood. Static elasticity leads to long range interactions along the crack front [3,4] but additional elastic waves, especially recently observed crack front waves [5,6], create dynamical stress overshoots which play an important role. In most model- ing of fracture dynamics in heterogeneous materials, espe- cially at low speed, the latter are ignored since a quasistatic assumption is used. Actually very few experimental data that describe the crack front propagation through hetero- geneous materials exist. This work presents the very first experiment where the detailed dynamics of a fracture front line in a heterogeneous material is investigated. At large scales, seismic inversions of slip history during an earth- quake [7] provide hints of the features of the rupture front but with a low resolution, especially compared to the het- erogeneity sizes (i.e., asperities). At laboratory scales, a 3D description of a crack front at rest in a heterogeneous aluminum alloy was obtained by Daguier et al. [2]. Interesting similarities exist between the propagation of crack fronts through a heterogeneous medium and the problem of propagation of dislocation lines through a field of solute atoms. At low temperatures, the dislocation lines become rough due to pinning by immobile solute atoms [8 –10] adjusting its shape to the local stress. We describe here an experiment where two Plexiglas plates are annealed together to create a single block with a weak interface [11]. Propagation of the crack front along the weak interface is directly observable because of the transparency of the material. The plates are as follows: 32 cm 3 14 cm 3 1 cm and 34 cm 3 12 cm 3 0.4 cm, and annealed together at 205 ± C under several bars of normal pressure. Before annealing, both plates are sand- blasted on one side with 50 mm steel particles. Sand- blasting introduces a random topography which induces local toughness fluctuations during the annealing proce- dure. The upper Plexiglas plate is clamped to a stiff alu- minum frame. A normal displacement is applied by a press to the lower plate which results in a stable crack propa- gation in mode I at constant low speed 68 mms [12]. The fracture front is observed with a microscope linked to a high speed Kodak Motion Korder Analyzer camera which records during 8.7 s at 500 images per second with a 512 3 240 pixel resolution (1 pixel covers an area of 10 mm 3 10 mm). In Fig. 1 is shown a sample image obtained with this setup where the front is observed from above. The uncracked part is seen as white (i.e., trans- parent), while the gray region represents the open fracture (i.e., nontransparent since the sandblasting procedure un- polishes the surfaces). The front is defined as the con- trast boundary. The x and y coordinates are defined in Fig. 1. The front line position is given by y  ax, t . In Fig. 1, samples of fronts separated with a constant time in- terval dt  200 ms are superimposed on the picture. Lo- cal fluctuations of the front movement can be seen from the variations in the distance between the front lines. We de- fine a front normal  n to be normal to the front line and FIG. 1. Sample of crack fronts. The background is an inverted raw image covering an area of 5.12 mm 3 2.4 mm where the in- tact material appears white. In contrast, the cracked zone is dark. Crack propagates in this case from bottom to top (x direction) because of a tensile mode I load. The 43 solid lines that cor- respond to the fracture front positions for later times are super- imposed on the picture. The time delay between each front is 200 ms. Totally 4367 pictures were taken with a time interval of 2 ms between each picture in this experiment. 105502-1 0031-9007 01 87(10) 105502(4)$15.00 © 2001 The American Physical Society 105502-1