Discontinuous Crack Fronts of Three-Dimensional Fractures

– The relation between fracture surface morphology and the three-dimensional structure of crack fronts is investigated through direct observation of brittle cracks in gels. A key notion in this investigation is the discontinuity of the crack front, whose advancement creates the fracture surface. We discuss the significance of our findings in the studies of general three-dimensional brittle fractures. The crack front of a three-dimensional fracture is a one-dimensional object whose advancement into a sample creates a crack. The presence of the third dimension here allows for phenomena which are not seen in two-dimensional fractures [1]. Actual crack fronts are usually not straight lines; many step lines are found on fracture surfaces found in rock [2], glass [3, 4, 5, 6, 7], ceramics [8], rubber [9], gel [10] (including edible jelly), etc. Frechette [8] has pointed out that a crack often becomes an aggregation of crack segments which become interconnected during the course of fracture propagation and thus that such a crack front does not define a continuous line. Theoretically, the emergence of non-planar cracks from the front of planar cracks has been studied by considering sinusoidally varying perturbation applied to a straight crack front [11, 12] and the ‘planting’ of planar crack segments ahead of a parent planar crack [13, 14, 15]. Little is known, however, about actual fronts of well-developed cracks [16, 17]. Because of the variety of complicated forms exhibited by general three-dimensional fracture, it would be instructive to begin a study of such phenomena by carrying out a geometrical characterization of the most elementary structures of genuine three-dimensional cracks, just as two-dimensional fractures have been characterized in the terms of elementary mode-I, II, and III cracks. Mechanical characterization and molecular theory may follow a geometrical characterization. It is the aim of this paper to present the most elementary geometrical characterization of three-dimensional cracks, based on direct observation. For this purpose, the choices of a suitable material system and investigative method are crucial. Some experiments have been conducted using dense glasses, but in these systems crack growth is very fast, and it is therefore difficult to observe crack fronts in situ. We have previously reported [10] on pattern formation involving steps which appear on fracture surfaces of gels and on the related dynamical transition with a critical crack growth Typeset using EURO-LaTEX 2 EUROPHYSICS LETTERS