To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on ...sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.
In numerous engineering applications, the occurrence of highly rapid loading conditions, coupled with significant temperature rise, is prevalent. Under these harsh conditions, shear localization is a ...prominent material failure mechanism, which may result in catastrophic failure. Generally, materials possessing higher strength, limited work hardening rate, and lower thermal conductivity are more prone to mechanical instability. However, in the current study, a cryo-rolled CoCrNi alloy, characterized by much higher strength and limited work hardening, shows remarkable room and high-temperature mechanical performance under impact loading except at one temperature domain∼600 °C where this material displays mechanical instability by exhibiting adiabetic shearband. To shed light and understand this abnormal mechanical response, an in-depth microstructural study revealed that the ASB formation is closely linked to the recrystallization behavior of this alloy that is invariantly related to the stored energy of cold work, test temperature, and strain rate. Uniquely, the amplified nano-twinning activity at high strain rate and critical temperature promoted the fragmentation of grains and facilitated recrystallization, respectively. Once the recrystallization is completed, the alloy regains work hardening ability even at very high test temperatures, reaching 900 °C due to the deformation of the newly recrystallized matrix. The present work is valuable in delineating the alloy's application domain for rigorous impact and crash-worthiness applications, as there exists a dearth of literature on this material for dynamic scenarios.
•Cryo-rolling introduces extensive twinning in CoCrNi.•Dynamic hot compression from room temperature to 900 °C.•Formation of shear band at only one critical temperature ∼600 °C,neither below nor above this temperature.•Dynamic strain rate induced secondary and tertiary twins fragment the deformed matrix into nano-sized blocks.•Shear band formation and propagation linked to stored energy of cold work and ambient test conditions.
The fractal characterisation of stress and acoustic emission (AE) of coal and rock failure may provide quantitative guidance for analysing the stability of rock mass during excavation engineering. ...The correlation between stress and AE data was calculated using fractal theory methods and Grassberger-Procaccia (G-P) algorithms to evaluate the damage and degree of flaw in coal and rock materials under multilevel dynamic loading (MDL). First, the mechanical properties of the testing device were developed to obtain synchronous data regarding the stress and AEs of coal and rock specimens under MDL. Second, strength deterioration behaviour and the Felicity effect during coal and rock failure are compared under different levels of dynamic loading. The results show that cumulative dynamic loading are less than the uniaxial compression strength of coal and rock. The Felicity effect is notable, and Felicity ratios decrease to 1 with increased loading velocity. Finally, stresses and AE signals are considered as the data mining specimens of correlation dimensions based on the G-P algorithm. Fractal characteristics on stress and AE coupling properties become more notable as the correlated dimensions become larger. However, the Felicity effect is less prominent, reflecting a lesser degree of damage and flaws in coal and rock materials. Therefore, this study suggests a data mining method for the correlation dimension to be applied in an in-situ monitoring system for rock mass excavation engineering.
Rock failure phenomena, such as rockburst, slabbing (or spalling) and zonal disintegration, related to deep underground excavation of hard rocks are frequently reported and pose a great threat to ...deep mining. Currently, the explanation for these failure phenomena using existing dynamic or static rock mechanics theory is not straightforward. In this study, new theory and testing method for deep underground rock mass under coupled static-dynamic loading are introduced. Two types of coupled loading modes, i.e. “critical static stress t slight disturbance” and “elastic static stress t impact disturbance”, are proposed, and associated test devices are developed. Rockburst phenomena of hard rocks under coupled static-dynamic loading are successfully reproduced in the laboratory, and the rockburst mechanism and related criteria are demonstrated. The results of true triaxial unloading compression tests on granite and red sandstone indicate that the unloading can induce slabbing when the confining pressure exceeds a certain threshold, and the slabbing failure strength is lower than the shear failure strength according to the conventional Mohr-Column criterion. Numerical results indicate that the rock unloading failure response under different in situ stresses and unloading rates can be characterized by an equivalent strain energy density. In addition, we present a new microseismic source location method without premeasuring the sound wave velocity in rock mass, which can efficiently and accurately locate the rock failure in hard rock mines. Also, a new idea for deep hard rock mining using a non-explosive continuous mining method is briefly introduced.
•A comprehensive review about effects of volume fraction and shape of steel fibers in SFRC under dynamic loading has been conducted in this article.•The obtained results from previous studies have ...been compared and reached a noticeable outcome for future studies.
Among the properties of steel fiber-reinforced concrete (SFRC), the volume fraction and shape of steel fibers play an important role when the concrete is subjected to impact and high-velocity loading. In this study, material responses obtained through impact tests, such as the drop weight test, the split Hopkinson pressure bar test, and the swinging pendulum test, are classified and briefly illustrated. The structural responses of several types of fiber-reinforced concrete with steel fibers are introduced and compared. Given the significance of this topic, an up-to-date review of available literature is provided to enhance our current understanding of the behavior of SFRC with varying volume fractions and shapes of steel fibers under different loading rates. The areas where further research is required are highlighted.
Geomaterials are increasingly being subjected to the quasi-static confinements and dynamic loadings, and thus it is critical to characterise dynamic behaviour under the coupled static-dynamic loading ...conditions. In this study, a series of dynamic biaxial compression tests was performed on cubic specimens of sandstone by using a triaxial Hopkinson bar system, high-speed three-dimensional digital image correlation (3D-DIC) and synchrotron-based micro-computed-tomography (μCT). This testing apparatus allows for determining dynamic stress-strain behaviour of the specimen under biaxial static pre-stress (σ1, σ2) conditions and different impact velocities (i.e. 15, 20 and 26 m/s), corresponding to average axial strain rates of 80, 200 and 250 s−1. Both σ1 and σ2 applied in the tests varied from 0 to 40 MPa with an interval of 10 MPa. Dynamic strength and fracture behaviours of biaxially confined rocks were systematically characterised from macroscopic to microscopic, surface to interior and 2D to 3D by using the 3D-DIC and synchrotron-based μCT techniques. Experimental results show that, under the same biaxial pre-stresses, dynamic biaxial compressive strength of sandstone increases with strain rate, i.e. rate dependence. At a given impact velocity, dynamic biaxial compressive strength decreases with increasing pre-stress values (σ1) along the impact direction, while it enhances with the increase of intermediate principal stress (σ2) in the lateral direction, which is so-called confinement dependence. Besides, two types of dynamic stress-strain curves of rocks are observed under biaxial compression, which is associated with post-peak fracturing characteristics. High-speed 3D-DIC shows that rock fragments are ejected out of the free surface (σ3 = 0) with certain velocities including the translation and rotation, which resembles the rock ejection observed during drilling and blasting and even strong rockbursts. μCT reveals that the double “V” shapes of shear fractured zones are symmetrically generated under biaxial compression. The crack surface area, volume and dynamic fracture energy are also estimated under different confinements to characterise the damage of impacted rock.
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•Biaxial static-dynamic coupled tests were conducted using a triaxial Hopkinson bar.•The effects of strain rate and confinement on stress-strain curves and failure modes.•Ejection velocity, displacement field and kinetic energy are measured by 3D-DIC.•Micro-cracks of impacted specimens are identified by the synchrotron-based μCT.•Fracture energy is calculated with the combination of high-speed 3D-DIC and μCT.
The fracture behaviour and crack propagation features of coal under coupled static-dynamic loading conditions are important when evaluating the dynamic failure of coal. In this study, coupled ...static-dynamic loading tests are conducted on Brazilian disc (BD) coal specimens using a modified split Hopkinson pressure bar (SHPB). The effects of the static axial pre-stress and loading rate on the dynamic tensile strength and crack propagation characteristics of BD coal specimens are studied. The average dynamic indirect tensile strength of coal specimens increases first and then decreases with the static axial pre-stress increasing. When no static axial pre-stress is applied, or the static axial pre-stress is 30% of the static tensile strength, the dynamic indirect tensile strength of coal specimens shows an increase trend as the loading rate increases. When the static axial pre-stress is 60% of the static tensile strength, the dynamic indirect tensile strength shows a fluctuant trend as the loading rate increases. According to the crack propagation process of coal specimens recorded by high-speed camera, the impact velocity influences the mode of crack propagation, while the static axial pre-stress influences the direction of crack propagation. The failure of coal specimens is a coupled tensile-shear failure under high impact velocity. When there is no static axial pre-stress, tensile cracks occur in the vertical loading direction. When the static axial pre-stress is applied, the number of cracks perpendicular to the loading direction decreases, and more cracks occur in the parallel loading direction.
This work proposes a meshless generalized finite difference method (GFDM) with supplementary nodes for the solution of thin elastic plate bending under dynamic loading. The first- and second-order ...time derivatives in equilibrium equation are firstly discretized with the Houbolt method. The numerical solution of the resulting spatial problem at each time step is then calculated by the GFDM. The supplementary nodes on the boundary of the problem domain are introduced in the GFDM to yield the well-determined linear system of equations. Numerical example is provided to verify the accuracy and the stability of the developed approach, and these preliminary results reveal the potential of the method.
To investigate the disturbance‐induced shear instability mechanism of structural catastrophe in the deep rock mass, MTS 815 material testing machine was used to carry out quasi‐static loading tests ...and disturbance shear tests on symmetrical regular dentate joints of two materials at three undulation angles under specific initial static stress, disturbance frequency, and peak value. The test results indicate that: (i) the total ultimate instability displacement is only related to the intrinsic properties of the joints but not to the initial static stress and disturbance parameters; (ii) the cumulative irreversible displacement required for the disturbance instability conforms to the logistic inverse function relationship with the number of disturbances, displaying the variation trend of “rapid increase in the front, stable in the middle, and sudden increase in the rear”; (iii) the accumulation of plastic deformation energy is consistent with the evolution law of irreversible displacement of joints and the overall proportion of hysteretic energy is not large; (iv) the dissipated energy required for the instability of each group of joints is basically the same under various disturbance conditions, and this energy is mainly controlled by the initial shear stress and has no connection with the disturbance parameters. The stability of the total disturbance deformation and the disturbance energy law of the joints revealed in the tests provide data support for reasonably determining the disturbance instability criterion of joints.
In this study, to investigate the effect and underlying mechanism of the failure process under disturbance, a series of quasi‐static loading tests and disturbance shear tests in the vicinity of the strength limit is conducted on symmetrical regular dentate joints. Combined with the “locked patches” theory of typical gradual sliding, the failure law and morphology of joints under quasi‐static loading are summarized. Further, the variation rule of irreversible displacement under disturbance and the fitting prediction is also examined. Besides, the energy evolution process of joints under disturbance is analyzed based on the energy damage and energy storage mechanism. Overall, the findings of this study can provide test support for reasonably determining the disturbance instability criterion of joints under high initial stress
Highlights
The total ultimate instability displacement is only related to the intrinsic properties of the joints.
The cumulative irreversible displacement required for the disturbance instability is related to the initial static stress.
The dissipated energy required for the instability of each group of joints is basically the same under various disturbance conditions.
Understanding the mechanical properties and fracturing behaviours of coal is significant for the safety of underground mining engineering. Coal failure characteristics are highly influenced by the ...anisotropy and loading conditions. In this study, the coupled biaxial static and dynamic tests are conducted on coal specimens with five bedding orientations θ (i.e., 0°, 30°, 45°, 60°, and 90°) with respect to the normal direction to loading. A Triaxial Hopkinson bar (Tri-HB) system is adopted to apply the biaxial quasi-static stress first and then dynamic loading at four impact velocities (i.e., 10, 13, 17, and 21 m/s). Real-time processes of coal fracturing are recorded by two high-speed cameras, and accordingly, full-field deformation and ejection velocities are identified by the three-dimensional digital image correlation (3D-DIC) technique. Moreover, the internal fracture morphology of coal specimens is characterised using synchrotron-based X-ray computed tomography (CT). Experimental results show that at similar strain rates, the peak stress against θ shows a “U” shape with the lowest value at θ = 60°. The peak stress increases with increasing impact velocity, while its growth rate exhibits a downward trend revealing a decreasing sensitivity to strain rate. Coal ejection velocities are positively rate-dependent, and the highest ejection velocity is found shifting from θ = 45° to θ = 90° with increasing impact velocity. The average fragment size of coal specimens is negatively related to impact velocities and energy absorption, and the finest fragmentations are observed at around θ = 45°. Dynamic behaviours of coal under biaxial pre-stresses are dependent on bedding structures and strain rates, while the bedding effect becomes weak as strain rate increases.
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•Coal bursts under coupled biaxial static-dynamic loads by Triaxial Hopkinson bar.•Effect of strain rates and anisotropy on strength and fragmentation.•Deformation process and ejection velocity in coal bursts are revealed by 3D-DIC.•The internal fracture morphology is characterised by synchrotron-based μCT.
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