The pseudo-compact tension (pCT) method recently proposed by Muñoz-Ibáñez et al. (2020) is a satisfactory approach to measure mode I fracture toughness (KIC) in rocks and other materials using disc-shaped samples loaded under pure tensile conditions. In contrast to other methods, such as the semi-circular bend (SCB) suggested by the ISRM (2014), the pCT test provides with good control after peak load, making it possible to further characterize the processes involved in fracture propagation. In this work we assess the influence of the testing configuration at the onset of unstable crack propagation. In order to extend the pCT concept to complementary geometries with potential interest we studied an alternative to the SCB specimen, which we call pseudo-SCB (pSCB). To compute KIC in this configuration we have derived the corresponding dimensionless stress intensity factor function (Y’) based on the finite element method. The results show that the pSCB test provides with consistent values of KIC and it also allows to control the propagation of the crack beyond peak load, which reinforces the idea that the loading conditions may be a more determinant factor than the sample geometry in controlling post-peak behaviour. In addition, an expression of Y’ is presented for cubic samples tested using the pCT approach. This configuration may be useful for testing other materials amenable of moulding such as mortar, concrete, ceramics, etc.
Mode I fracture toughness (KIC) measures the resistance of a material containing a pre-existing defect to the propagation of tensile cracks [1]. Since rocks are discontinuous at all scales, KIC is of great importance in geotechnical engineering (i.e., tunneling, mining), energy resources exploitation (i.e. geothermal energy, hydraulic fracturing) projects [2-4], etc. In the last years, a number of methods have been proposed to assess KIC in rocks. Worth mentioning among them are the suggested methods endorsed by the International Society for Rock Mechanics (ISRM) [5-7]. The semi-circular bend (SCB) test has favorable features such as simple specimen geometry and testing procedure. However, in this configuration tensile loads are indirectly generated via sample compression. In addition, the dynamic unstable fracture propagation prevents post-peak assessment. These drawbacks can be overcome using an alternative testing approach namely pseudo-compact tension (pCT), which allow determining KIC under pure tensile conditions with satisfactory control on fracture development after failure [8]. The better performance of the pCT method would not be associated to better electronic control or higher stiffness of the testing device but to a lower level of elastic energy storage in the sample during its loading [9]. To check this conjecture, in this study we assess the influence of the loading conditions and specimen geometry on unstable crack propagation. To extend the pCT concept to complementary geometries with potential interest we have studied an alternative to the SCB specimen which we refer as pseudo-SCB (pSCB). Finally, we extend the compact tension approach to cubic geometries (cubic-pCT), which may have a significant interest in materials that, for instance, can be molded such as mortars.