Experiments on 3-D crack growth in uniaxial compression

Abstract

A study of spatial crack propagation under uniaxial compression was undertaken in a brittle transparent plastic with artificially induced internal cracks. The specific issue addressed was whether there was a qualitative difference between 2-D and 3-D crack propagation. Fracture of heterogeneous materials (e.g., rocks) under uniaxial compression is produced by propagation of internal cracks towards the load, some of which propagate so extensively that they eventually split the material into columns. Experimental and theoretical studies of this phenomenon based on 2-D models (plates with through cracks.) showed that the extensive crack growth towards compression emerged from pre-existing (initial) defects such as through cracks in plates inclined to the compression axis, or cylindrical pores (e.g., [1-10]). The main point of these studies is that a single 2-D microdefect is capable of producing extensive crack growth sufficient to cause fracture. In reality however, pre-existing cracks are three-dimensional, which can result in a more complicated mechanism of their growth. A limited number of experiments: Adams and Sines [4] and Cannoo et al. [11], have been reported on studies of the growth of inclined disk-like cracks in uniaxially compressed transparent samples (from PMMA polimethilmethacrylate plastic). However, the dimensions of the PMMA samples used in these experiments were not sufficient to investigate the extensive crack growth. In order to study mechanisms of fracture in uniaxial compression, further experiments are required using sufficiently large samples which would provide enough room for extensive crack growth. In addition, the spatial interaction of several growing cracks has to be investigated. This paper reports the results of such experiments. 1. Sample preparation and experimental technique. In the experiments, parallelepiped samples made from transparent casting polyester resin "Polylite 61-209" with cross-section dimensions of 55 mm x 55 mm, and a height of 120 mm were used. When frozen to -17°C, this material is perfectly brittle, deforms without barrelling and has linear stress-strain behaviour up to its burst-like fracture. The mechanical properties evaluated during the tests are: Young's modulus _=_4GPa; uniaxial compressive strength _=_ 140MPa; fracture toughness _=_0.6MPa-m 1/2. Two methods of modelling the internal initial disk-like cracks were adopted: (1) embedding a thin disk-like inclusion into the resin block during casting. The inclusion consisted of two aluminium foil disks greased and put together and held within the sample by two cotton threads; (2) cutting two semi-circular slots of 0.3 mm thickness in two halves of the sample and then gluing the halves together (this method was used in [4]). In order to ensure the contact between the opposite faces of the crack, teflon or greased foil disks were inserted into the slots (According to the 2-D analysis [9], the initial cracks, i.e. voids with contacted lips are the strongest drivers of the extensive crack growth).