Existing methods for compression tests have enabled the observation of class-II post-peak or snap-back behaviour. However, capturing tensile behaviour in rock testing is still challenging due to stronger snap-back in tension. This work adopts the recently developed Advanced Universal Snap-Back Indirect Tensile test (AUSBIT) to obtain the complete tensile load-displacement behaviour of granite and sandstone Brazilian discs in the post-peak stage through controlled lateral displacement. Digital Image Correlation (DIC) and Acoustic Emission (AE) were employed to analyse the progressive failure mechanisms when compared with the conventional Brazilian tests. Results show that AUSBIT enables controlled failure at extensive lateral displacements by allowing the stable propagation of microcracks. AE data further reveal that cracks formed in AUSBIT release significantly less energy compared to conventional tests. These phenomena were more significant in granite, showing the effectiveness of AUSBIT for controlling the tensile failure of brittle class-II rocks and in attaining the post-peak behaviour.

We have investigated the mode I fracture toughness (K_IC) of granite samples in the presence of various fluids pseudo-compact tension (pCT) test. Prior to the fracture toughness tests, several granite specimens were immersed in deionized water (DIW), hydrochloric acid (2.7M HCl), and sodium hydroxide (0.2M NaOH) solutions for 8 days to evaluate the effects of different reactive environments on K_IC. In addition, a group of specimens of the same rock were submitted to a 24-day immersion cycle using the same fluids (starting with the caustic NaOH solution, then by acidic HCl and finally with DIW), which consecutively soaked the samples every 8 days. The experimental results have been analyzed to assess the corresponding dissipation energies and to identify chemo-mechanical couplings eventually involved in crack initiation and propagation. Results show that, when compared with dry samples, those soaked in any fluid are weakened with a reduction in the threshold energy for crack initiation. However, the different fluids do not result an identical impact over K_IC. We observe that lowest affection is induced by the acidic solution, followed by the caustic one, and finally by DIW (K_IC,dry > K_IC,HCl > K_IC,NaOH > K_IC,DIW). Thus, deionized water exhibits the greatest reduction in fracture toughness (up to 30 %) and the specimens also have a lower average stiffness than the other immersion methods. According to available research (experiments performed with glass materials and molecular dynamics simulations), this may be attributed to micro-scale (molecular scale) processes by which the smaller kinetic diameter of water molecules makes them more accessible to the crack tip, facilitating the hydrolysis of siloxane bonds. Interestingly, despite the fact that the specimens were eventually immersed in DIW in the cyclic test, the K_IC obtained in this case is still higher than that resulting from the individual exposure to the single DIW fluids (K_IC,cycle > K_IC,DIW). That results from the residual hydrolyzing species (Na⁺/H₃O⁺/OH^-, etc.) at the crack tip of the specimens during the cyclic test. The assessments of bulk mechanical energies reveal that, regardless of the chemical environment, approximately 40% of the energy delivered to the samples in each test is consumed by crack initiation, while about 60% is consumed by crack propagation. These results highlight that the chemical environment at the crack tip is the primary factor influencing the subcritical fracture behavior and provide important insights for accurately assessing the fracture toughness of rocks in the presence of fluids.

Direct tensile testing is generally accepted as the most accurate method of determining tensile strength, but indirect methods are commonly employed due to the difficulty and precision re-quired to obtain viable results with the direct method. Accurate values of tensile strength are important, especially regarding design. Thus, values obtained from the direct tensile test are beneficial to be able to utilize and compare them to other experimental values and different methods. This paper presents the development and implementation of the direct tensile test at the Geotechnical Engineering Laboratory of the University of Cantabria, following the ASTM standards. The lessons learned are highlighted. The results of direct tensile tests on Moleanos limestone are here compared with the results of previous indirect tensile tests, namely the Brazilian (splitting tensile) test. The fracture pattern of the direct tensile tests is also presented and analyzed.

The ring test, which is one adaptation of the Brazilian test, was proposed in the 1940s. The laboratory test consists of the application of a diametric load on a ring specimen, in order to estimate the tensile strength indirectly. Its great advantage over the traditional test (disk samples) is that by having a point of weakness (the hole), it is guaranteed that the failure begins in the center of the specimen, a necessary condition for the Brazilian test to be valid. However, the ring test could not be used until the new empirical solution is proposed, because the result of the analytical solution is quite distant from the expected. The analytical equation is based on the theory of elasticity and does not consider the non-linearity of the rocks. According to this analytical solution, in the case of a specimen with a tiny hole, its tensile strength would be six times greater than that obtained with the Brazilian test. The empirical solution is independent of the rock type and correlates the load necessary to break a disc with the load that should be applied in a ring, is based on more than a hundred tests carried out on different materials and laboratories. In the present paper, the empirical solution is analyzed and the advantages/disadvantages of the different internal hole sizes are commented on. Also, how to prepare the ring sample is explained. Being highlighted that the sample preparation is very simple and does not need any special device. In the laboratory, sandstone blocks (lithic arkose) from Burgos (Spain) have been tested with different sizes of rings. The proposed empirical solution takes into account a change in the failure mechanism observed at certain hole sizes.

This review is focused on an innovative and non-destructive laboratory approach, referred to as IRTest, to estimate the rock porosity by Infrared Thermography. Based on the positive out-comes achieved through the use of Infrared Thermography for rock mass characterization, the study of the cooling behavior of rocks has suggested that the porosity grade is linked to the cooling speed of a previously heated rock specimen. The technique has been applied to different rock types, in terms of both porosity grade and lithology. Achieved results demon-strate that there is a positive linear relationship between rock porosity and CRI10 (Cooling Rate Index), corresponding to the infrared thermal monitoring of the rock cooling within a 10 minutes time window. IRTest was applied to differently shaped and sized rock samples, prov-ing the suitability of this technique on a variable statistical population, and suggesting the in-novative potential of Infrared Thermography for the rock laboratory characterization.

Rock mass shows bi-modularity behavior, consisting that elastic modulus having different ratios in compression and tension. Being the compressive elastic modulus always greater than the tensile modulus. In practice, the most common is to estimate the elastic modulus in compression, given that the uniaxial compression test is widely carried out. The direct tensile test requires a specific device, so the tensile strength usually is estimated by the indirect tensile test (Brazilian test). Ye et al. (2009) proposed an equation to estimate the tensile elastic modulus with the Brazilian test, using strain gauges attached in the horizontal direction on both sides of the specimen. However, they did not have how to perform the direct tensile test to compare the results. In the present paper, the elastic modulus is obtained in three different ways: in the compression test, in the direct tensile test, and in the Brazilian test (diametric load). A series of laboratory tests has been done in anisotropic sandstone (lithic arkose), from Burgos, Spain. Being remarkable that the mechanical behavior of the anisotropic rock mass is dependent on the inclination of the foliation planes, the tests were carried out with inclinations of 0°(horizontal) and 90° (vertical). These two configuration are the extremes of tensile strength.