International Journal of Rock Mechanics and Mining Sciences
Numerical Simulation Of The Brazilian Test And The Tensile Strength Of Anisotropic Rocks And Rocks With Pre-Existing Cracks
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Progressive highly stressed volume for size effect analysis
2023, Construction and Building MaterialsThe tensile strength of concrete and rock is vital for structural design, but variations in measurements arise from different experiments and specimen sizes. These discrepancies are influenced by the size effect and the highly stressed volume (HSV). Previous studies have relied on empirical formulations that provided valuable insights but overlooked fracturing's impact. In this study, we develop upon these approaches, emphasizing the significance of incorporating fracturing for improved accuracy. We develop relationships between progressive HSV and tensile strength, validated by experimental data. Furthermore, we consider the size effect using an equation relating progressive HSV, fracture quantification, and tensile strength.
Numerical and experimental investigation of the anisotropic tensile behavior of layered rocks in 3D space under Brazilian test conditions
2023, International Journal of Rock Mechanics and Mining SciencesBrazilian disc (BD) testing is a typical method for measuring the indirect tensile strength of rock materials. Both experimental and theoretical studies involving BD testing of anisotropic rocks typically focus on the influence of the 2D bedding effect, where a specimen is parallel to the bedding planes, whereas the influence of the 3D bedding effect combining both the orientation and loading angle has rarely been investigated. Therefore, in this study, BD tests considering the 3D bedding effect were conducted on layered Longmaxi shale, and 3D BD models of layered shale were established using 3D globally embedded cohesive elements with zero thickness. The numerical results were compared to testing results. The 3D BD numerical model was then applied to simulate Asan gneiss and Silurian silty slate to verify the applicability. Finally, the influence of the 3D bedding effect on both the apparent tensile strength (ATS) and failure modes was analyzed. We also considered the influence of the spacing of bedding planes and bedding strength. The results indicated that the numerical results for both the ATS and failure modes were in good agreement with the test results with a maximum error of 17.8% for the ATS. The applicability of the developed 3D BD numerical model to different layered rocks was confirmed by the numerical results for both Asan gneiss and Silurian silty slate. Strong dependencies of the ATS and failure modes on both the orientation (α) and loading (β) angles were observed in the layered Longmaxi shale. The normalized ATS for various orientation and loading angles achieved high values of 3.15 and 2.90, respectively, indicating that the influence of the orientation angle is slightly stronger than that of the loading angle. The Longmaxi shale exhibited a distinct tensile failure mode at α = 0° @ β = 0–15°, α = 0° @ β = 75–90°, and α = 90°, but a more complicated mixed mode of both tensile and shear failure was observed under other conditions. The contribution of shear failure increased significantly at 0° < α < 90° compared to α = 0°. The spacing of the bedding planes had a limited influence on the ATS and failure modes, but the bedding strength had a significant influence. There may be a linear relationship between bedding strength and the spatial distribution parameter of the critical plane approach criterion.
Experimental investigations of static mechanical properties and failure characteristics of damaged diorite after dynamic triaxial compression
2022, International Journal of Rock Mechanics and Mining SciencesTo investigate the static mechanical properties and failure characteristics of damaged rocks after impact under realistic in-situ stress, dynamic triaxial compression tests with different strain rates and confining pressures were conducted on diorite by a modified split Hopkinson pressure bar. The damage factor was calculated according to the change of the longitudinal wave velocity of rocks before and after the impact, and the residual strengths of the damaged diorite were obtained by uniaxial compression and Brazilian tensile tests. In addition, three brittleness indexes based on the residual strength were adopted to evaluate the brittleness of damaged specimens, and fractal dimension was introduced to study the fragmentation characteristics. The results show that the dynamic peak stress, strain, elastic modulus, damage factor and plastic deformation of diorite all increase with increasing strain rate, i.e., rate dependence, and the increasing rate decreases with increasing confining pressure. The increment in confining pressure leads to the increase of dynamic peak stress, elastic modulus, static residual strength and brittleness, and the increasing rate increases with increasing strain rate, while other physio-mechanical parameters decrease with confining pressure. The post-peak behavior of dynamic stress-strain curves is divided into two classes according to the strain rate and fracture state of rocks under dynamic loading. Upon the strain rate increases above 87 s−1, the brittleness of damaged specimens generally decreases to moderately brittle or even low brittle, and the corresponding drillability becomes much easier. The fractal dimension of damaged specimens increases with the increase of confining pressure and decreases linearly with the increase of strain rate.
Experimental investigation and numerical simulation of chip formation mechanisms in cutting rock-like materials
2022, Journal of Petroleum Science and EngineeringIn this study, the effects of tool geometry such as rake angle, and cutting parameters such as depth of cut on the cutting forces were studied and correlated with the built-up edge during the material removal process of a rock-like workpiece. Cutting or scratch tests were performed on low and high strength simulated rock-like materials using a tungsten carbide tipped orthogonal drag tool with three different rake angles (0°, 10° and 20°) in a custom-made machining set-up incorporating a high-speed video camera. Force data were measured by a tri-axial dynamometer and a compatible data acquisition system, and specific cutting energy was calculated to assess the material removal performance. Experiments showed that a cutting tool with a 20° rake angle produced an efficient cut. The high-speed video at the cutting edge were analysed to comprehend the formation and growth of the built-up edge. Novel insight was gained by characterising the shape and was observed that the constantly evolving shape was unique to each rake angle used, this creates an apparent rake angle. By varying the rake angle and cutting parameter, the measured cutting force and thrust force showed that the material strength, cutting tool geometry and depth of cut played important roles in removing materials. Higher cutting efficiency was indicated by lower specific cutting energy at higher depth of cut for all cutting conditions. The formation of the crushed zone in relation to the cutting force revealed that the cutting force increased with the size of the crushed zone having two types of chip formation modes: shearing and fracturing. Numerical simulations were performed using a commercially available tool called ELFEN, a hybrid finite-discrete element software package. The simulations correlated well with experimental investigation. The simulations also showed the formation of crushed zone and crack growth as observed experimentally through the use of high-speed video and also shed light on the state of stress state at the cutting edge.
Anisotropic elastoplastic phase field fracture modeling of 3D printed materials
2021, Computer Methods in Applied Mechanics and EngineeringA phase field model for anisotropic, elastoplastic fracture model in layered structures obtained by 3D printing processes is proposed. An extension of anisotropic phase field to elastoplasticity model is developed. The model is able to describe a transition from quasi-brittle to elastoplastic fracture behaviors depending on the angle of layers in the microstructure with respect to the external loading. Such feature is of special interest to describe the anisotropic fracture behavior in layered 3D printed materials. The present model introduces two phase field variables, one bulk fracture damage and one micro interfacial damage variables, describing two different micro damage mechanisms. Finally, we have proposed an original methodology to identify the macroscopic strain density as a function of the micro interfacial damage variable using numerical homogenization on Representative Volume Elements. Numerical investigations show that the present model is convergent with respect to mesh refinement, and allows to describe complex crack initiation and propagation in layered elastoplastic structures. An experimental comparison is provided to validate the use of such model for 3D printed polymer materials.
Experimental verification of the boundary conditions in the success of the Brazilian test with loading arcs. An uncertainty approach using concrete disks
2020, International Journal of Rock Mechanics and Mining SciencesThe present work analyses the reliability of the Brazilian test with loading arcs. A new testing set up has allowed to determine in an effective way the real load of the failure initiation as this moment was not always or correctly detected by the universal testing machine. The instrumentation used is a simple and low-cost method that allows to know the possible pressure distribution in the contact zone as well as the final contact angle. It has been observed that the success of the test depends mainly on the surface finish of the parts involved, their geometric tolerances and the symmetry of the applied load. These boundary conditions have a direct effect in the contact pressure distribution. The possible failure modes observed experimentally have been simulated with the finite element methods. For this, the contact boundary condition has been changed and the possible stress distribution in term of Griffith equivalent stress has been obtained. The numerical analysis allows to study the influence of the initial contact condition on the success of the test and agrees with the experimental results. Furthermore, an uncertainty analysis in the expression of the tensile strength confirms that, when the test is valid, a crack appears suddenly in the central area of the disk, as observed experimentally, so there is no need to determine if the starting point is in the centre. Additionally, it has been observed that the initial crack length depends on the type of pressure distribution in the contact zone. Finally, a series of recommendations are given in order to minimize both the variability of the final contact angle and the risk of premature failure of the Brazilian disk.