Rock-like Specimens Research Articles

Investigations on the fracture mechanisms of Z-shaped fissured rock-like specimens

Complex shapes of cracks exist in natural rock masses, however, present research has simplified it into straight line-shaped fissures. In view of this, three-dimensional (3D) printing is employed to make Z-shaped fissured specimens with different main fissure angle α and secondary fissure angle β. The compress fracture experiments are conducted, and full-field strain distributions during crack propagations are obtained using Digital Image Correlation (DIC) technology. Traditional Smoothed Particle Hydrodynamics (SPH) method is improved to examine damage process as well as fracture mechanisms in Z-shaped fissured specimens. It can be concluded that: Wing Crack (WC), Main Crack (MC), Outer Crack (OC) and Inner Crack (IC) are observed in the experiment. The secondary fissure angle β guides the initiation of MC, while the main fissure angle α guides the initiation of WC. Stress–strain curve of 3D printing samples with Z-shaped fissures shows similar trends to that of real rock, experiencing four typical parts. Numerical results show high similarities with experimental results. We have also discussed the crack initiation mechanisms in various main fissure angle α and secondary fissure angle β in the end, and concluded the stress distribution characters. The findings of this research will offer some references for correct understanding of the crack evolution processes and fracture mechanics of Z-shaped fissured rock masses.

Study on Failure Mechanism of Rock-Like Specimen with Closed Non-Persistent Fractures Under Confining Pressure

Study on Failure Mechanism of Rock-Like Specimen with Closed Non-Persistent Fractures Under Confining Pressure

Cracking evolution and failure mechanism of brittle rocks containing pre-existing flaws under compression-dominating stresses: Insight from numerical approach

Comprehending the cracking evolution and failure mechanism of flawed rocks under compression-dominating stresses is essential to evaluating the stability of rock engineering. In this work, a newly developed geometry-constraint-based non-ordinary state-based peridynamic (GC-NOSBPD) theory that can eliminate the numerical oscillation is applied to predict the development of new cracks of flawed rocks under compression-dominating stresses. The failure of bonds between particles is judged by the bond-associated stress-based fracture criteria. The cracking evolution trajectories of semi-disc and disc specimens containing flaws are traced. The effects of flaw inclination angle on the cracking trajectories are analyzed. The cracking evolution paths of rock-like specimens with two flaws under uniaxial and biaxial compression are molded. The effects of confining stress on the growth of new cracks are investigated. The confining stress restrains the propagation of tensile cracks, but it is easy to promote the growth of shear cracks. The concentration and transfer effects of stress can plainly revel the failure mechanism of flawed rocks. The present numerical method is reasonable to predict the tensile and shear failure modes of flawed specimens under compression-dominating stresses.

Study on Failure Characteristics and Acoustic Emission Laws of Rock-like Specimens under Uniaxial Compression

Complex underground conditions make it challenging to conduct extensive coring, and it is difficult for laboratories to carry out a large number of rock mechanics experiments due to the limited number of cores. Rock-like specimens are commonly used in the laboratory to replace coal-rock specimens. In this paper, rock-like specimens with different proportions are produced to study the mechanical properties, failure characteristics, and acoustic emission laws of rock-like specimens under uniaxial compression. The results show that when the rock-like specimen does not contain gypsum, the stress of the specimen decreases rapidly after reaching the peak stress, which is similar to the mechanical properties of hard brittle rock. When the rock-like specimen contains gypsum, the stress of the specimen decreases slowly after reaching the compressive limit, and the failure of the specimen is gentle, which is similar to the mechanical properties of soft rock. When the specimen lacks gypsum, the strength of the rock-like specimen is larger, and the strength of the specimen is positively correlated with the proportion of cement. When the specimen contains gypsum, the strength of the rock-like specimen decreases sharply, and the strength of the rock-like specimen is negatively correlated with the proportion of gypsum. The maximum acoustic emission ringing count increases with higher cement content but decreases with increased gypsum content. The cumulative count change of acoustic emissions approximately underwent four stages, aligning well with the four stages of uniaxial compression failure observed in typical rock. The research results have important reference value for the selection of rock-like materials to replace the original rock materials for laboratory research.

Experimental research on the glass fiber-reinforced effect and mechanisms of sand powder 3D-printed rock-like materials

Conducting laboratory tests on natural rocks has significant value for the scientific and practical advancement of various types of rock engineering. The investigation of a rock-like materials that is similar in characteristics and structure to natural rock is key to the further advancement of laboratory rock experiments. The rapid development of sand powder 3D printing technology in recent years has provided a solution for fabricating rock-like materials. However, the low strength and elastic modulus of sand powder 3D-printed materials limit their application in simulating natural rocks. Therefore, this paper proposes a method to enhance the mechanical properties of sand powder 3D-printed materials utilizing glass fiber reinforcement by incorporating glass fiber into sand powder 3D-printed rock-like specimens. The mechanical properties and microstructural variations of the specimens with varying glass fiber contents are investigated utilizing mechanical testing, acoustic emission, scanning electron microscopy, etc. The results indicate that (1) it is feasible to utilize glass fiber to enhance the mechanical properties of sand powder 3D-printed materials; (2) The mechanical properties of sand powder 3D printing materials are significantly enhanced by the addition of glass fiber; and (3) the mechanisms controlling the reinforcement effect of glass fiber addition on sand powder 3D-printed specimens are determined by analyzing the microstructural characteristics of the specimens. This study improves the applicability of sand powder 3D-printed materials in simulating natural rock, thereby promoting the further application of 3D printing in the field of rock mechanics.

Experimental and numerical studies of rock-like specimens with different hole shapes under compressive-shear loading

Experimental and numerical studies of rock-like specimens with different hole shapes under compressive-shear loading

Effect of Bedding Angle on Energy and Failure Characteristics of Soft–Hard Interbedded Rock-like Specimen under Uniaxial Compression

To investigate how bedding planes affect the energy evolution and failure characteristics of transversely isotropic rock, uniaxial compression tests were conducted on soft–hard interbedded rock-like specimens with varying bedding angles (α) using the RMT-150B rock mechanics loading system. The test results indicate that throughout the loading process, the energy evolution shows obvious stage characteristics, and the change of α mainly affects the accelerating energy dissipation stage and the full energy release stage. With the increase of α, the ability of rock to resist deformation under the action of energy shows the characteristics of “strong–weak–strong”. The energy dissipation process is accelerated by medium angle bedding planes (α = 45°~60°). The precursor points of the ratios of dissipation energy to total energy (RDT) and elastic energy to dissipation energy (RED) can be used to effectively predict early failure. With the gradual increase of α, the difficulty of crack development is gradually reduced. The changes of energy storage limitation and release rate of releasable elastic energy are the immanent cause of different macroscopic failure modes of specimens with varying α.

Mechanical characteristics and damage model for rock-like specimen with two parallel grout-filled cracks

To investigate the mechanical properties of rock-like specimens with two parallel cracks filled with grout, uniaxial compression tests in combination with digital image correlation (DIC) and acoustic emission (AE) techniques have been performed on grout-filled cracked specimens with different rock bridge dip angles β. Results indicated that with the increase of β, the uniaxial compression strength, cumulated AE count, AE energy, as well as shear crack proportion initially decreases and subsequently increases. The overall AE b-value and the DIC-based maximum principal strain initially increases and subsequently decreases as β increases. The failure mode also varies from tensile to mixed tensile-shear mode. Furthermore, a particle flow code (PFC) model, which considers the grout filling, grout-rock interface, and rock, has been proposed. Numerical results showed that the range of microcracks inclination angle first decrease from 70°−100° to 70°–90° and then increases to 70°–110°. Besides, existence of grout filling and changes of bridge angle leads to significant difference in PFC particle displacement, which can cause the changed of crack initiation positions and failure modes. Finally, a damage model considering stress intensity factor for the grout-filled specimen was established. Both of pre-crack damage and loading damage were considered in this model, and a damage repair factor was proposed to describe effect of grout filling. The results showed that grouting could can effectively suppress stress concentration caused by pre-existing crack, thereby changes the strength of cracked specimen.

Strain-rate effect on mechanical properties of rock-like specimens with narrow crack under uniaxial compression

Uniaxial compression tests were carried out on rock-like specimens containing prefabricated narrow crack under various strain rates. Experimental observations and numerical results show that narrow crack will close or partially close or expand under low compressive stress. Due to the unidirectional loading of loading device, the deformation degree of crack tip near the loading end was different with that of crack tip near the bearing end. With the increase of strain rate, the cracking modes can be divided into 3 categories, near vertical splitting failure affected by horizontal crack (α = 0°), anti-N-wing crack (15°≤α ≤ 60°) and wing crack (60°<α ≤ 90°). Both of the initiation angle θ1 of crack tip near the loading end and θ2 of crack tip near the bearing end decreased with the increase of α and strain rate. Specially, when α = 90°, both θ1 and θ2 rose with the increase of strain rate. Moreover, the value of θ2 was greater than that of θ1 under the same α value because of the different deformation characteristics at two tips. Besides, the mechanical properties of fissured rock including peak compressive stress, residual compressive stress, strain at peak stress, elastic modulus and post-peak softening modulus were significantly influenced by the strain rate and inclination angles.

Effect of Joint Angle on the Failure Behavior of Rock-Like Specimens Under Unilateral Restrained Compression

Effect of Joint Angle on the Failure Behavior of Rock-Like Specimens Under Unilateral Restrained Compression

Physical model test and application of 3D printing rock-like specimens to laminated rock tunnels

Weak structural plane deformation is responsible for the non-uniform large deformation disasters in layered rock tunnels, resulting in steel arch distortion and secondary lining cracking. In this study, a servo biaxial testing system was employed to conduct physical modeling tests on layered rock tunnels with bedding planes of varying dip angles. The influence of structural anisotropy in layered rocks on the micro displacement and strain field of surrounding rocks was analyzed using digital image correlation (DIC) technology. The spatiotemporal evolution of non-uniform deformation of surrounding rocks was investigated, and numerical simulation was performed to verify the experimental results. The findings indicate that the displacement and strain field of the surrounding layered rocks are all maximized at the horizontal bedding planes and decrease linearly with the increasing dip angle. The failure of the layered surrounding rock with different dip angles occurs and extends along the bedding planes. Compressive strain failure occurs after excavation under high horizontal stress. This study provides significant theoretical support for the analysis, prediction, and control of non-uniform deformation of tunnel surrounding rocks.

Mechanical behavior of rock-like specimens with 3D nonpenetrating and nonpersistent rough joints under uniaxial compression: experimental study

Mechanical behavior of rock-like specimens with 3D nonpenetrating and nonpersistent rough joints under uniaxial compression: experimental study

Dynamic mechanical properties and failure mechanisms of rock-like materials containing main and auxiliary holes with various arrangement inclinations and spacing ratios

As a typical rock defect, circular holes significantly influence rock masses, diversifying their failure modes and ultimately weakening their bearing capacities. To study the effects of hole arrangement inclination and spacing ratio on the mechanical properties and failure mechanisms of rock-like specimens, dynamic uniaxial compressive tests were conducted by employing a modified split Hopkinson bar apparatus and digital image correlation. The results reveal a V-shaped trend in the peak stress, Young’s modulus, and absorbed energy proportion as the hole arrangement inclination increases from 0° to 90°. However, the peak strain is insensitive to changes in the hole arrangement inclination and spacing ratio. The hole spacing ratio has no significant effect on the Young’s modulus, but it causes a gradual reduction in the proportion of energy absorbed in the specimens. Moreover, shear and tensile cracks emanating from the main hole dominate the failure mode of specimens with main and auxiliary holes, as demonstrated by the resultant displacement and vertical strain contours. Rock bridges experience shear failure as the hole arrangement inclination increases from 15° to 75°, whereas the rock bridges of specimens with various hole spacing ratios tend to fail owing to tensile cracks. The interaction between main and auxiliary holes with an arrangement inclination of 0° is the weakest, suggesting that tunnel groups should be arranged perpendicularly to the direction of dynamic loading.

Quantitative Analysis and 3D Visualization of Crack Behavior in 3D-Printed Rock-Like Specimens with Single Flaw Using In-Situ Micro-CT Imaging

3D printing technology allows for precise control of preparing complex geometries and internal defects in printed rock analogs, while in-situ Micro-CT imaging enables real-time observation of crack behavior. The combination of these technologies offers a new research approach for studying rock crack behavior. In this study, 3D-printed rock-like specimens containing a pre-existing flaw were prepared using a gypsum powder-based 3D printer. An advanced in-situ Micro-CT system equipped with a loading device was used to quantitatively and visually investigate the crack behavior in 3D-printed specimens under uniaxial compression testing. 2D CT images obtained from in-situ compression testing at different deformations could be used to reconstruct a 3D model and visually identify the crack patterns of the extracted cracks in 3D-printed specimens. The initiation angle of cracks, volume of the pre-existing flaw, volume of newly formed cracks, and damage value with respect to strains were analyzed to quantitatively investigate crack behavior. The results indicated that within the 3D-printed specimens, tensile cracks were first initiated near the internal flaw, followed by the occurrence of shear cracks or tensile-shear mixed cracks at the flaw tips. Additionally, there was a negative linear correlation between the initiation angle of newly formed cracks and the initial flaw angle. For flaw angles in the range of 0° ≤ α ≤ 45°, a higher number of newly formed cracks were observed in the 3D-printed specimens, and the rates of increase in crack volume and damage values with strain were faster. However, for flaw angles in the range of 45° < α ≤ 90°, the results showed the opposite trend. Furthermore, through comparison with the crack behavior of natural rocks containing a single flaw, it was found that the failure modes and crack behavior of the 3D-printed specimens exhibit certain similarities with natural rocks.

A hybridized DDA-DDM for modeling jointed rock masses

The complexity of geo-materials’ behaviors surpass the capabilities of conventional numerical methods alone, particularly when realistically modeling rock structures. Coupling methods can enhance the realism of numerical modeling. Discontinuous deformation analysis (DDA) and displacement discontinuity method (DDM) are combined to model block displacement and crack propagation mechanisms in a blocky rock mass. DDA calculates stresses, strains, rotations, and displacements of the blocks, while DDM predicts crack propagation paths based on specified boundary conditions. The displacements result from DDA are transformed into stresses, followed by investigating crack propagation mechanisms within each block using Kelvin's solution. Boundary stresses are adjusted as cracks propagate, updating stress boundary conditions in DDA. This iterative process continues until crack propagation halts or a new block emerges. In this paper, the complete failure process of a rock slab with one pre-existing non-persistent joint located on the crest of a rock slope and compression test on the rock-like specimen with a single random crack with a 45° slope angle is simulated using the proposed numerical approach. The proposed numerical solution is compared with existing numerical results. This comparison confirms the accuracy and effectiveness of the proposed procedure, as the predicted crack propagation paths from the coupled numerical method align well with the corresponding numerical results from the rock-like samples.

Proportioning optimization of transparent rock-like specimens with different fracture structures

Clarifying the principles of proportioning optimization for brittle transparent rock-like specimens with differential fracture structures is crucial for the visualization study of the internal fracture and seepage evolution mechanisms in rock masses. This study, utilizing orthogonal experimental methods, uncovers the influence mechanisms, extents, and patterns by which the ratios of resin, hardener, and accelerator, along with the freezing duration, impact the mechanical characteristics of transparent rock-like specimens. Notably, it was observed that as the accelerator ratio and freezing time are increased, there’s a general decline in the uniaxial compressive strength, tensile strength, and elastic modulus of the specimens. In contrast, an increase in the hardener ratio initially leads to an enhancement in these mechanical properties, followed by a subsequent decrease. Under uniaxial compressive loading, the specimens exhibit four typical modes of failure: bursting failure, splitting failure, single inclined plane failure, and bulging failure. As the hardener and accelerator ratios increase, the mode of failure gradually shifts from bulging to bursting, with freezing time having a minor overall impact on the evolution of failure modes. The study proposes a method for inducing random three-dimensional closed fractures within the specimens and further clarifies the principles for optimizing the proportions of specimens with different fracture structures, such as intact, embedded regular, and random three-dimensional fractures. This research facilitates the in-depth application of transparent rock-like materials in various scenarios and provides theoretical guidance and technical support for visualizing the evolution of fracture and seepage characteristics within the fractured rock mass.

Compression Mechanical Properties and Constitutive Model for Soft–Hard Interlayered Rock Mass

The properties of soft–hard interbedded rock masses are significantly impacted by the strength of rock layers and the characteristics of interface surfaces. This study investigates the mechanical properties of soft–hard interlayered rock masses by preparing rock-like specimens with different interface angles. Uniaxial and triaxial compression tests were conducted to examine the compression mechanical characteristics of the specimens. Experimental results demonstrated that in the uniaxial compression tests, the peak strength of the two-layer rock-like specimen exhibits an initial decrease followed by an increase as the interface angle increases. Similarly, the peak strength of the three-layer rock-like specimen also follows a “U-shaped” pattern. The failure of both specimens shifts from tensile failure to shear failure. In the triaxial tests, the strength of the two-layer rock-like specimen initially increases and subsequently decreases as the interface angle increases. In contrast, the intensity of the three-layer rock-like specimen exhibits a decreasing trend, transitioning from shear dilation or tensile failure to shear failure. By utilizing the damage constitutive model to compute the compressive strength of the composite specimen, it was observed that the deviation from the experimental value did not exceed 2.5%, and the overall shape of the curves was in good agreement. Consequently, it is affirmed that the damage constitutive model developed in this study can accurately capture the pre-peak phase of the stress–strain relationship in soft–hard interlayered rock-like specimens, thus providing a valid representation of their mechanical behavior.

Mechanical and energetic properties of rock-like specimens under water-stress coupling environment

AbstractSoft rock has the properties of low strength, poor integrity, and difficulty in core extraction. In order to study the deformation and failure of soft rock, this study used fine river sand as aggregate, cement and gypsum as bonding materials, and borax as a retarder to produce cylindrical rock-like samples (RLS) with a sand cement ratio of 1:1. Uniaxial compression tests were conducted on RLS under DIT (different immersion times) (0, 4, 8, 12, 24, and 48 h) in the laboratory. The mechanical and energy properties of RLS under water-stress coupling were analyzed. The results showed that the longer the IT of the RLS, the higher their water content (WC). As the moisture time increases, the uniaxial compressive strength, elastic modulus (EM), and softening coefficient (SC) of the sample gradually decrease, while the rate of change of EM is the opposite. The fitted sample SC exhibits a good logarithmic function relationship with WC. During the loading process of the sample, more than 60% of the U (total energy absorbed) during the loading process of the sample is accumulated in the form of Ue (releasable elastic energy), while less than 40% of U is dissipated by the newly formed micro cracks during the compaction, sliding, and yield stages of the internal pores and cracks of the sample. The U before the peak and the Ue of the RLS decrease exponentially with the moisture content; the relationship curves of Ue/U (released elastic energy ratio) and Ud/U (dissipated energy ratio) of RLS during uniaxial compression with the σ1/σmax (axial stress ratio) can be divided into three stages of change, namely the stage of primary fissure compaction and closure (σ1/σmax &lt; 0.25), continuously absorbing energy stage (0.25 &lt; σ1/σmax &lt; 0.8), and energy dissipation stage (σ1/σmax &gt; 0.8); the D (damage variable) was defined by the ratio of Ud (dissipated energy) to the Udmax (maximum dissipated energy) at failure time of RLS, the fitting of the relationship between the damage variable and axial strain conforms to the logistic equation.

Repairing Performance of Polymer-Modified Cement-Based Thin Spray-On Liners on Pre-Cracked Rock-like Specimens

With the development of coal mining and the increase in excavation depth, the stress on roadway surrounding rock is also increasing. This creates conditions for crack development in the roadway, so it is urgent to develop rock repair materials with excellent performance. The ability of thin spray-on liner (TSL) to repair rock and concrete opens up the possibility of reusing abandoned roadways. The ability of TSL to support the surrounding rock is also important in preventing the generation of roadway waste. In this paper, styrene–acrylic emulsion (SAE), vinyl acetate–ethylene copolymer emulsion (VAE), and polyvinyl alcohol powder (PVA) were used to prepare three TSLs. Rock-like materials were configured using cement mortar according to similar principles. Three types of TSLs were tested for basic properties such as viscosity and mechanical strength, which provided data to support the explanation of the repair performance of TSLs. Three TSLs were used to repair pre-cracked rock-like specimens (PR). The number of brushing times and the angle of PR’s cracks were regarded as test variables. Changes in the mechanical strength of repaired PRs were tested by compressive and flexural tests. TSL repair performance was evaluated with the help of mechanical strength changes. Results show that polyvinyl alcohol powder modified cement-based thin spray-on liner is most suitable for repairing rock cracks; as the thickness of the brush slurry increases, its repair performance continues to improve. This paper can provide experience and a theoretical basis for the research of other rock repair materials, and it is also instructive for repairing shotcrete in the roadway.

Experimental Investigation and Constitutive Modeling of Rock-Like Specimens’ Interface with the Effect of Roughness Based on 3D Printing

Experimental Investigation and Constitutive Modeling of Rock-Like Specimens’ Interface with the Effect of Roughness Based on 3D Printing