Biaxial Compression Experiments Research Articles

A double tunnels model test study on mechanical properties of surrounding rock during tunnels excavation and creep stages

The tunnel engineering has become increasingly common with the needs of urban development. However, excavation sequence has a significant effect on the stability of the tunnel perimeter rock, which can lead to more unpredictable perimeter rock hazards and more difficulty in maintaining perimeter rock stability. In this study, to investigate the effect of different excavation sequences on tunnel deformation and creep, biaxial compression experiments were conducted on muddy siltstone specimens by using the acoustic emission (AE) and digital image correlation (DIC) techniques. The results indicated that, as the distance from the tunnel portal increases, the amount of stress and strain change decreases, with the largest values of stress changes occurring in the tunnel vault and middle area of twin tunnels. The specimen undergoes two stages in the creep process: deceleration creep and steady-state creep. The steady-state creep rate of the measurement points depends on the degree of excavation disturbance impact it experiences. The AE events during excavation can be divided into two phases: the quiet period when the specimen is not excavated and the active period when the specimen is excavated, and the cumulative counting curve shows a “stepwise” increase with the number of excavations. There is a sudden drop in the b-value during excavation and a decrease in the b-value during the deceleration creep phase. The results of this study can reflect the creep patterns of the surrounding rock under different excavation sequences, which can help to choose a more suitable excavation plan in terms of rock creep stability.

Mechanical Behavior and Crack Evolution of Goaf Surrounding Rock with Different Roof-Contacted Filling Rates.

The goaf formed by mining is filled and treated, which greatly improves the safety and stability of the surrounding rock. During the filling process, the roof-contacted filling rates (RCFR) of goaf were closely related to the stability control of the surrounding rock. The influence of the roof-contacted filling rate on the mechanical characteristics and crack propagation of the goaf surrounding rock (GSR) has been studied. Biaxial compression experiments and numerical simulation experiments were conducted on samples under different operating conditions. The results were as follows: (1) The peak stress, peak strain, and elastic modulus of the GSR are closely related to the RCFR and the goaf size; they increase with the increase of the RCFR, and decrease with the increase of the goaf size; (2) In the initial loading stage, a small number of cracks are generated, and the acoustic emission ringing count increases slowly. The mid-loading stage is the crack initiation and rapid expansion, and the cumulative ring count curve shows a "stepwise" growth. In the later loading stage, cracks continue to propagate and form macroscopic fractures, but the number of rings significantly decreases; (3) Shear cracks are prone to occur in the rock part of the GSR; tensile cracks are prone to occur in the backfill; and the crack propagation speed in the rock is faster than in the backfill. Stress concentration is the direct cause of GSR failure. The maximum concentrated stress of rock mass and backfill is 1~2.5 times and 0.17~0.7 times of the peak stress of the GSR, respectively.

Implications for identification of principal stress directions from acoustic emission characteristics of granite under biaxial compression experiments

The rock fracture characteristics and principal stress directions are crucial for prevention of geological disasters. In this study, we carried out biaxial compression tests on cubic granite samples of 100 mm in side length with different intermediate principal stress gradients in combination with acoustic emission (AE) technique. Results show that the fracture characteristics of granite samples change from ‘sudden and aggregated’ to ‘continuous and dispersed’ with the increase of the intermediate principal stress. The effect of increasing intermediate principal stress on AE amplitude is not significant, but it increases the proportions of high-frequency AE signals and shear cracks, which in turn increases the possibility of unstable rock failure. The difference of stress in different directions causes the anisotropy of rock fracture and thus leads to the obvious anisotropic characteristics of wave velocity variations. The anisotropy of wave velocity variations with stress difference is probable to identify the principal stress directions. The AE characteristics and the anisotropy of wave velocity variations of granite under two-dimensional stress are not only beneficial complements for rock fracture characteristic and principal stress direction identification, but also can provide a new analysis method for stability monitoring in practical rock engineering.

Preliminary Study on High-Energy and Low-Energy Microfracture Event Evolution Characteristics in the Development Process of Rock Failure

The evolution characteristics of high-energy and low-energy microfracture events play an important role in the brittle failure mechanism of rock and reasonable microseismic (MS) monitoring and acoustic emission (AE) monitoring. The bimodal distribution (BMD) model is commonly used to observe the evolution characteristics of high-energy and low-energy MS events; however, its precise mechanism remains unclear. The evolution characteristics of high-energy and low-energy microfracture events are assessed in this study based on a BMD model. MS monitoring results from the No. 22517 working face of the Dongjiahe Coal Mine are studied, and AE monitoring results of a biaxial compression experiment of a granite specimen are analyzed. High-energy MS events in the No. 22517 working face are found to be generated by an increase in the failure scale of the overlying rock mass upon exiting the insufficient mining stage and entering the sufficient mining stage. The change characteristics of the high-energy AE hits are positively correlated with crack evolution characteristics in the granite specimen and negatively correlated with changes in the Gutenberg-Richter b value. A precise high-energy and low-energy AE hit evolution mechanism is analyzed based on the microscopic structure of the granite specimen. Similarities and differences between high-energy MS events and low-energy AE hits are determined based on these results. Both are found to have bimodal characteristics; an increase in the failure scale is identified as the root cause of the high-energy component. The bimodal distribution of AE hits is far less obvious than that of MS events.

Experimental investigation on rockburst process and failure characteristics in trapezoidal tunnel under different lateral stresses

In general, the in-situ stress increases with the increase of excavation depth, and it significantly affects the safe and long-term stability of tunnels and chambers. To study the influence of the in-situ stress on the rockburst process and failure characteristics, three different lateral stresses were loaded to perform biaxial compression experiments on sandstone specimens with a trapezoidal opening. The results indicate that the high lateral stress accelerates the formation of localized damage around the trapezoidal opening, which can effectively consume the elastic strain energy. The deep surrounding rock at the roof and floor are basically in a compressive stress state, and the tangential compressive stress in the deep surrounding rock of the sidewall is greater than that in the deep surrounding rock of the roof and floor, which can cause the deep surrounding rock of the sidewall to be damaged more severely. For the corners of the trapezoidal opening, the high lateral stress can restrain the rotation of the principal stress, which lead to the reduction of the crack density around the corners. AE signals and CT scanning show that the failure mode of the model specimen shifts from tensile fracture to mixed tensile and shear fracture with the increase of the lateral stress. Under different lateral stresses, the outlines of failure zones on both sidewalls of trapezoidal opening are V-shaped, and the damage degree at the sidewall of the trapezoidal opening increases with the increase of the lateral stress when the rockburst occurs. Additionally, the failure zones on both sidewalls are larger than those of circular opening, which indicate that the supporting area on the sidewall of trapezoidal opening needs to be larger than that on the sidewall of circular opening.

Crack growth analysis for rock-like materials with ordered multiple pre-cracks under biaxial compression

The pre-burying iron sheets approach was used to prepare rock-like materials with ordered multiple pre-cracks. 60 specimens in total were prepared in these experiments. Through biaxial compression experiments, the influence of both the number of pre-cracks and pre-cracks angles to crack growth was analyzed. Meanwhile, species of rock bridge failure were summarized, namely, wing crack, secondary shear crack between horizontal pre-cracks and secondary shear crack between vertical pre-cracks. The wing crack plays a significant role in crack growth. Furthermore, fractal dimension was adopted to describe quantitatively the crack growth during the failure process. The results indicate that with the failure of specimens, corresponding fractal dimension for specimen monotonically increases, which indicates that the fractal dimension can be considered to the failure of the specimens quantitatively.

Experimental Study of Crack Growth in Rock-Like Materials Containing Multiple Parallel Pre-existing Flaws Under Biaxial Compression

Cracks and joints are common in rock masses and play a crucial role in rock mass stability. This study prepared specimens with multiple parallel pre-existing flaws by embedding iron sheets in rock-like materials and used the samples to investigate the crack growth characteristics of these materials. Biaxial compression experiments were performed on sixty specimens, and the influences of the number of pre-existing flaws, their angles and the lateral stress on crack growth were investigated based on video recordings of the crack growth. The results demonstrate that structural failure will occur due to crack growth when the sample contains a small number of pre-existing flaws and that as the number of cracks increases, the specimens will fail due to local failures. In addition, the types of rock bridge failures are summarized, including wing cracks, secondary shear cracks between horizontally-separated pre-existing flaws and secondary shear cracks between vertically-separated pre-existing flaws. Wing cracks play a significant role in the failure of the specimens. The results increase the understanding of crack growth in brittle materials that contain multiple parallel pre-existing flaws under biaxial compression.

Discrete Element Simulations of Shallow Plate-Load Tests

AbstractShallow plate-load tests were simulated using a two-dimensional discrete element procedure together with parameters calibrated from a series of biaxial compression experiments on aluminum rods. The deformation and force transmission characteristics within ground materials were investigated first, and then attentions were focused on studying the influences of the interparticle friction coefficient, the roughness of the loading plate, and the size of loading plate on the testing results, including the ultimate bearing capacity, the deformation behavior, and the failure mode. The well-known scale effect in plate-load tests was also analyzed from the perspectives of both continuum mechanics and discrete element method simulations. It was proven that the nonlinear strength behavior of the materials beneath the foundation is a possible source of the scale effect; however, this effect may be counterbalanced by the boundary restriction effect in laboratory experiments.

Systematic study of the effects of mass and time scaling techniques applied in numerical rock mechanics simulations

Numerical modeling is a well established tool in rock mechanics studies investigating a wide range of problems. Implicit methods for solving linear equations have the advantage of being unconditionally stable, while explicit methods, although limited by the time step, are often used because of their limited memory demand, their scalability in parallel computing, and simple implementation of complex boundary conditions. In numerical modeling of explicit elastoplastic dynamics where the time step is limited by the material density, mass scaling techniques can be used to overcome this limit and significantly reduce computation time. While often used, the effect of mass and time scaling and how it may influence the numerical results is rarely-mentioned in publications, and choosing the right scaling technique is typically performed by trial and error. To our knowledge, no systematic studies have addressed how mass scaling might affect the numerical results. In this paper, we present results from an extensive and systematic study of the influence of mass and time scaling on the behavior of a variety of rock-mechanical models. We employ a finite difference scheme to model uniaxial and biaxial compression experiments using different mass and time scaling factors, and with physical models of increasing complexity up to a cohesion-weakening frictional-strengthening model (CWFS). We also introduce a normalized energy ratio to assist analyzing mass scaling effects. We find the tested models to be less sensitive to time scaling than to mass scaling, so mass scaling has higher potential for decreasing computational costs. However, we also demonstrate that mass scaling may lead to quantitatively wrong results, so care must be taken in interpreting stress values when mass scaling is used in complicated rock mechanics simulations. Mass scaling significantly influences the stress–strain response of numerical rocks because mass scaling acts as an artificial hardening agent on rock deformation.

In-plane biaxial compression and tension testing of thin sheet materials

Detailed experimental data on the behavior of textured sheet metals under compressive loading is important to describe their tension–compression asymmetry. This is particularly needed for materials that exhibit a strength-differential effect, or in cases where the Bauschinger effect occurs. So far, there is no systematic work describing the third quadrant in the 2D stress space under biaxial compressive loading. This paper presents a new device for biaxial, compressive in-plane testing of thin sheets. Biaxial and uniaxial compression experiments are carried out in the strain controlled device, analyzing the behavior of deep drawing steel sheets with and without skin-pass treatment. Moreover, in order to allow for the experimental description of the yield surfaces, biaxial tensile tests are performed. Detailed numerical validations and experimental strain analysis both for the new specimen for biaxial compressive testing and for the cruciform specimen for biaxial tensile testing show that reasonably homogeneous strain distributions can be achieved. The combined experimental and numerical method presented here allows to evaluate the tension–compression asymmetry of thin sheet materials. The results for the skin-passed condition clearly exhibit a tension–compression asymmetry, which highlights the necessity of biaxial compression tests already in the as-received material condition. The biaxial compression test opens a pathway to a more detailed analysis of the flow behavior of thin sheets under biaxial compression loading.

Multi-Modal Monitoring of Slip Along Frictional Discontinuities

Seismic wave transmission and digital image correlation (DIC) are employed to study slip processes along frictional discontinuities. A series of biaxial compression experiments are performed on gypsum specimens with non-homogeneous contact surfaces. The specimens are composed of two blocks with perfectly mated contact surfaces with a smooth surface with low frictional strength on the upper half and a rough surface with high frictional strength on the lower half. Compressional, P, and shear, S, wave pulses were transmitted through the discontinuity while digital images of the specimen surface were acquired during the test. A distinct peak in the amplitude of transmitted wave occurs prior to the peak shear strength and is considered a “precursor” to the failure. Precursors indicate that slip initiates from the smooth surface and extends to the rough surface as the shear load is increased. From the DIC data, slip is identified as a jump in the displacement field along the fracture that initiates from the smooth surface and propagates to the rough surface. Precursors are associated with an increase in the rate of slip across the discontinuity and are a measure of the reduction in the fracture shear stiffness.

DEM Simulation of Biaxial Compression Experiments of Inherently Anisotropic Granular Materials and the Boundary Effects

The reliability of discrete element method (DEM) numerical simulations is significantly dependent on the particle-scale parameters and boundary conditions. To verify the DEM models, two series of biaxial compression tests on ellipse-shaped steel rods are used. The comparisons on the stress-strain relationship, strength, and deformation pattern of experiments and simulations indicate that the DEM models are able to capture the key macro- and micromechanical behavior of inherently anisotropic granular materials with high fidelity. By using the validated DEM models, the boundary effects on the macrodeformation, strain localization, and nonuniformity of stress distribution inside the specimens are investigated using two rigid boundaries and one flexible boundary. The results demonstrate that the boundary condition plays a significant role on the stress-strain relationship and strength of granular materials with inherent fabric anisotropy if the stresses are calculated by the force applied on the wall. However, the responses of the particle assembly measured inside the specimens are almost the same with little influence from the boundary conditions. The peak friction angle obtained from the compression tests with flexible boundary represents the real friction angle of particle assembly. Due to the weak lateral constraints, the degree of stress nonuniformity under flexible boundary is higher than that under rigid boundary.

Micro-Deformation Mechanism of Granular Materials under Different Boundary Conditions Based on DEM Simulation

Series of biaxial compression simulations are carried out to investigate the effects of boundary condition on the deformation of granular materials by using DEM. The parameters used in DEM are validated by the biaxial compression experiments on elliptical steel bars. The effects of boundary condition on the stress-strain relationship are analyzed. And special focus are put in the analysis of particle displacement, particle rotation, void distribution, particle long axis orientation and contact force with the development of deformation.

Influence of Joint Set with Different Angles on Rock Behaviour and Cavern Stability

Using DDARF method, the uniaxial and biaxial compression experiments of double-fractured rock samples are numerically investigated. Influences of different fracture angles and confining pressures on behaviour of rock samples are studied. Moreover, the fracture process, the crack initiation stress, and the peak strength of the fractured rock under both uniaxial and biaxial compression tests are obtained. The results are used to obtain the strength envelope and to analyze the rock stability of an underground project as a case study.

Experimental Study on Concrete under Uniaxial and Biaxial Compression

In order to study on mechanical character and damage mechanism of the concrete under different loading conditions, the uniaxial and biaxial compression experiment of concrete is investigated by Isotropic Biaxial Loading System of Henan Polytechnic University. The experimental results show the damage of the concrete under uniaxial compression is shear damage, however the damage of the concrete under biaxial compression is tension-shear damage; biaxial compression stress-strain curves of concrete are similar with those of concrete and can equally be divided into three stage; as the result of the intermediate principal stress, 1:1 biaxial compression strength increased 56.12% and 13.30% than that under 2:1 biaxial compression and uniaxial compression, at the same time 2:1 biaxial compression strength increased 37.80 % than that of the uniaxial compression.

The influence of constant axial compression pre-stress on the fatigue failure of torsion loaded tube springs

This paper reports the results of a series of biaxial static compression and torsion experiments performed to evaluate the effects of static compression stress on the fatigue life those smooth tubes made of high strength spring steel. Compression pre-stress was introduced by a solid steel bar inserted into a hollow spring and loaded with a screw-joint. The experimentally obtained results show a significant extension of fatigue strain life as a result of combining axial compression loading with torsion. Cracking behavior was observed and it was noted that compression pre-stresses contribute to retardation of the fatigue crack initiation process and, consequently, contribute to the extension of fatigue life. The fatigue shear crack initiated in a transverse direction. This crack continues to propagate in the same direction until it starts to propagate as a macro-crack on the maximum shear plane.

Multiscale investigation of shear bands in sand: Physical and numerical experiments

AbstractIn many geotechnical systems, it is not uncommon to observe failure in zones of high localized strain called shear bands. The existing models predict the existence and the extent of these localizations, but provide little insight into the micromechanics within the shear bands. This research captures and compares the variation in microstructure both inside and outside of shear bands that formed in physical laboratory plane strain and companion numerical two‐dimensional discrete element method (DEM) biaxial compression experiments. Unsheared and sheared laboratory specimens of Ottawa 20–30 sand of varying dilatancy were solidified using a two‐stage resin impregnation procedure. The solidified specimens were sectioned and the resulting surfaces were prepared for microstructure observation using optical bright‐field microscopy and stereological analysis. Statistical properties of microstructural parameters for sub‐regions in a grid pattern and along predefined inclined zones were determined. Similar measurements were performed on 2D DEM simulation specimens at varying strain levels to characterize the evolution of microstructure with increasing strain. The results showed how differences evolved in the mean, standard deviation, and entropy of void distributions with increasing global strain levels. The results indicate how disorder increases and that the material within the shear band does not adhere to the classical concept of critical state, but reaches a terminal void ratio that is largely a function of initial void ratio. Furthermore, there appears to be a transition zone between the far field and the fully formed shear block, as opposed to an abrupt delineation as is traditionally inferred. Copyright © 2010 John Wiley & Sons, Ltd.

3326 工業用純チタンのひずみ経路変化を伴う二軸圧縮塑性変形における力学的挙動(S18-2 結晶塑性解析,S18 材料の強度と組織)

The understanding of the multiaxial flow behavior of polycrystalline metals is important not only for the process planning but also for the quality assurance of metal working products. In the present study, the biaxial compression experiments with strain path change were performed on industrial pure titanium using a newly developed testing apparatus. The influences of strain path and abrupt path change upon the mechanical behavior were then investigated. The results show that both the strain level at path change and the directional relation between the sequential paths affect the stress-strain relation of the material. It is also found that the plastic anisotropy has influences on the mechanical behavior with regard to the strain histories.

Experimental Study of Buckling Strength for Short Square Pipe Column of Carbon Steel Subjected to a Biaxial Bending Load (3rd Report, Upper Eccentric and Lower Coaxial Compressive Loding)

This paper is concerned with a biaxial bending compression experiment for the case of eccentric at the upper and axial at the lower ends on carbon steel square pipes in the short column range, supported with spherical seat. An empirical formula for calculating the eccentric bending compressive buckling stress σcre* of the carbon steel square pipes is presented as follows ; σcre*/σcr =1/ (1+ ao), where σcc, is equal to axial compressive buckling stress, ao is equal to factor of the eccentricity and biaxial ratio. The measurement reveals that the results calculated by this formula are in good agreement with those obtained experimentally for the short column range.

Spatio‐Temporal Characteristics of Acoustic Emission During the Deformation of Rock Sample Containing Fault and Barrier

AbstractPhysical model for regular fault containing a columnar barrier has been designed and conceptually been used to simulate the barrier or strong area concerning earthquake preparation. Biaxial compression experiment with a middle‐size rock sample of this model has been done and AE spatio‐temporal developing characteristics have been studied. Results show that (1) Due to complex action between the hanging wall and lower wall, as well as the barrier, alternate AE activities in different areas become the most remarkable feature during deformation process of the sample. (2) The pre‐condition for instability of regular fault containing a barrier is that the barrier should fail firstly with the increase of loading. The AE spatio‐temporal distribution features are very different before and after the barrier failure. (3) The total deformation amount of this sample is large and its cumulated AE frequency increases exponentially. During the weakening period, there are only a few AE events distributed in barrier and surrounding area. Abrupt instability happens under such a relative quiet background. (4) b values of G‐R relationship for different pre‐setting tectonic patterns have some obvious differences, the regional differences of b value interrelated to different tectonics are remarkable.