Abstract

Rocks are usually inhomogeneous and anisotropic materials. The presence of foliation planes, grain boundaries or even microcracks may alter the stress distribution. In order to identify whether unusual behaviours in rocks are due to these imperfections or result from other factors (e.g. experimental configuration), the analyses of homogenous and isotropic materials is an useful approach. We have performed a series of mode I fracture toughness (KIC) tests using polymethyl methacrylate (PMMA) samples, which has the advantage of allowing photoelastic stress analysis based on its birefringent nature. Three different testing configurations were considered in the study: S\ emi-circular bend (SCB) test, the pseudo-compact tension (pCT) test, and a new alternative configuration based on the previous two that we have called pseudo-SCB (pSCB) test. To perform the photoelastic analysis, all the experiments were complemented with a specially-designed experimental setup consisting in two orthogonally arranged circular polarizers placed on both sides of the tested specimens. Using a source of white (polychromatic) light on one end it is possible to record the stress distribution using a digital camera aligned with the samples on the other end. As the load increases, a distinct evolving pattern of colour fringes can be visualized in the samples illustrating the spatially distributed stress levels. Based on this analysis we observe in some of the tests performed non-symmetrical stress fields. Although this behaviour could be related with the testing configuration, results suggest that other features, such as the shape of the notch tip, imperfections in sample preparation, or the misalignment of the samples in the testing device may also have an influence in stress distribution.

Introduction

Rock materials are usually discontinuous at all scales [1]. At the microscale, the presence of pores, grain boundaries, and pre-existing fractures can lead to high stress concentrations under increasing load. At the macroscale, foliation or bedding planes are characterized by having lower resistance and represent zones of weakness. These discontinuities might be determinant in fracture growth and, therefore, play an important role in the success of engineering projects involving rock materials (e.g., hydraulic fracturing, nuclear waste disposal or geothermal energy). Mode I fracture toughness (KIC) represents the resistance of a material to the propagation of tensile cracks under the presence of pre-existing discontinuities. In KIC assessment in rocks, properties such as grain size, particle arrangement, or the degree of cementation can determine the propagation path of the cracks [2]. To identify additional factors the use transparent models made of materials such as polymers, resins or glass is an interesting approach specially to visualize fracture development. For instance, Wu et al [3] monitored the process of crack growth on hydraulic fracture tests using polymeric samples. Their results suggest that far-field stresses determine the orientation of the final fracture orientation but have little effect on fracture initiation. Although the materials used in this type of studies are usually homogeneous and have, therefore, a limited ability to improve the understanding about the behavior of heterogeneous samples, they are valuable to identify additional factors (e.g. geometrical features, loading conditions) that can have an effect on fracture propagation.

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