In order to investigate rock fracture and fragmentation mechanisms under dynamic loading, a cylindrical rock model with a centralized borehole is developed through the use of AUTODYN code. According ...to the material properties and loading conditions, four kinds of equation of state (EOS), linear, shock, compaction and ideal gas, are applied to the four kinds of materials employed in this numerical model. A modified principal stress failure criterion is applied to determining material status, and a well-behaved explosive, PETN, and a relatively homogeneous igneous rock, diorite, are used in this rock model. A single centrally located line source of explosive is fired numerically to produce the dynamic loadings operating on the surrounding rocks. This numerical model is applied to actual blasting conditions. The rock failure mechanism under dynamic loading is first analyzed, and then the influences of the following factors on rock fracturing are discussed: (a) coupling medium, (b) confinement, (c) boundary condition, (d) initiation location in an explosive column, and (e) air ducking. The results show that all these factors have significant effects on rock fracturing under dynamic loading.
In order to study fracture toughness of mode I crack under blast load, a new specimen of a rectangle plate with a crack and edge notches (RPCEN) was proposed in this study. The PMMA was selected to ...prepare the RPCEN specimens. The testing system consists of a dynamic strain amplifier, an oscilloscope, a constant source, strain gauges and crack propagation gauges. The strain gauges and crack propagation gauges (CPGs) were used to measure the blast load and crack propagation speed, respectively. The scanning electron microscope was applied in studying the property of fracture surface. The explicit dynamic software AUTODYN was applied in simulating crack dynamic propagation behavior. The JWL equation of state (EOS) was applied to the explosive detonation products, and a linear EOS was used to describe the relationship between pressure and density of PMMA. A modified principle stress failure criterion was employed to assess the material state. The experimental-numerical method was used to determine the dynamic initiation and propagation toughness of the PMMA. The dynamic stress intensity factors (DSIFs) were calculated by the finite element code ABAQUS. The experimental and numerical results show that: (1) The RPCEN specimen proposed in this paper has the function to minimize the reflected tensile stress waves; (2) During crack propagation the fracture surface roughness and the nucleation rate increase with crack propagation speed; (3) Crack propagation speed is not a constant, and propagation toughness is related to the crack propagation speed.
In order to study crack dynamic propagation behavior of cracked tunnels under dynamic loading, a new configuration specimen of a tunnel with single radial crack (TWSRC) emanating from tunnel edge was ...proposed and by using these specimens, drop weight impacting experiments were conducted in this paper. The study using TWSRC specimens could be directly applied to tunnel engineering, and could guide tunnel designers to enhance tunnel stability. Sandstone was selected to make TWSRC specimens and crack propagation gauges (CPGs) and strain gauges were used to measure crack initiation and propagation time and crack speed. Numerical models were established by using the finite difference code AUTODYN to simulate crack propagation behavior and propagation path. The finite element code ABAQUS was used to calculate dynamic stress intensity factors (SIFs). For cracks propagating with a speed, the dynamic SIFs were obtained by the corresponding stationary crack SIF times a universal function. According to the initiation time and propagation time, the critical SIFs (or fracture toughness) in initiation, propagation and arrest were determined by the experimental-numerical method. The results show that (1) in the process of crack propagation, crack speeds are not a constant, and the cracks may temporarily stopped for a period, and in this study, the maximum arrest period is 227.52μs; (2) the propagation toughness is related to crack speeds, and the arrest toughness is lower than the initiation toughness.
•A tunnel with single radial crack (TWSRC) specimen was proposed to determine the fracture toughness.•Crack propagation gauges (CPGs) were used to measure crack initiation and propagation time and crack propagation speed.•The finite difference code AUTODYN and the finite element code ABAQUS were used to simulate crack propagation path and to calculate stress intensity factors.•The fracture toughness were determined by the experimental-numerical method.
•A new sample of a tunnel with a radial crack (TWSRC) emanating from the tunnel edge was proposed.•Impact experiments by using a drop weight impact system and the TWSRC samples were conducted.•Crack ...propagation gauge (CPG) was applied to determine crack initiation time and crack propagation velocity.•The code AUTODYN and ABAQUS were applied to simulate crack propagation behavior and to calculate dynamic SIFs.•For mixed mode I/II cracks in tunnel model, their propagation properties and dynamic initiation toughness were analyzed.
Blast excavation may induce radial cracks around a tunnel, and these cracks may initiate and propagate under nearby mining activities. In order to study the dynamic fracture behavior of mode I and mixed mode I/II cracks in a tunnel under impact loads, a new large sample, i.e. a tunnel with a radial crack (TWSRC) was proposed. Impact tests by using the TWSRC samples and a drop-weight impact testing device were also implemented. The advantage by using the TWSRC samples is that the corresponding results could be directly employed to tunnel engineering practice, and could lead tunnel designers to improve tunnel dynamic stability and to prevent tunnel hazards. The pre-crack was designed in a tunnel roof and was parallel to the symmetrical axis of the tunnel. The ratio α of the distance between the crack and the symmetrical axis to the radius of tunnel roof was in the range between 0 and 1. The crack initiation time and the crack propagation characteristic were measured by strain gauges and crack propagation gauges. A finite difference commercial code AUTODYN was used in the simulation of crack propagation paths and the fracture mechanism was analyzed. A finite element commercial code ABAQUS was employed to calculate crack dynamic stress intensity factors (DSIFs), and the experimental-numerical method was used to measure crack initiation toughness. The results showed that for mode I cracks, the cracks propagate along the pre- exiting crack direction, whereas the mixed mode I/II cracks, they propagate with a certain angle to form a wing crack at the early stage, and finally they propagate along the major principal stress direction. For mixed mode I/II cracks, the critical mode I SIF increases slightly with the distance to the symmetrical axis, whereas the critical mode II SIF increases largely as compared to the mode I SIF.
It is widely accepted that T-stress near crack tip has a considerably effect on crack behavior. In the classic fracture mechanics, only Tx (T-stress component in x-direction) was addressed, and ...recently one researcher pointed out that both Tx and Ty exist for a crack in compression. In order to theoretically study the T-stress for an inclined crack under compression, analytical solution of stresses near crack tip was derived from the complex stress functions which were obtained according to the crack surfaces stress conditions and boundary conditions. The analytical solution of stresses consists of two parts: one is singular term which is the same as the classical stress expressed in terms of stress intensity factor, and the other is non-singularity term which can be expressed in terms of three components of T-stress, i.e. Tx, Ty and Txy, which indicates that the T-stress should have three components. In order to validate the result of T-stress, the crack initiation angle was calculated by the MTS criterion for four cases: (1) without T-stress, (2) Tx only, (3) Tx and Ty, and (4) Tx, Ty and Txy. The prediction results of the crack initiation angles by MTS criterion were compared to the previous test results of PMMA and the test results conducted in this study by using sandstone specimens with an inclined crack under uniaxial compression. The results show that the prediction results by considering the three components Tx, Ty and Txy can better fit both test results of PMMA and sandstone, which indicates that the T-stress near crack tip should have three components Tx, Ty and Txy, and all of them have effect on crack initiation behavior.
Fractured rock mass is a kind of complex rock mass widely existing in engineering projects, which is often subjected to erosion by different pH hydrochemical solutions such as surface water, acid ...rain, acidic and alkaline wastewater discharge, and deicing salts. When the rock project in a cold region suffers from the coupled effects of hydrochemical solutions and freeze-thaw (FT) cycles, the frost-heave-thaw-shrinkage characteristics of rocks are different from ordinary FT under dynamic loads, and the effect of FT cycles may be enlarged or reduced in different hydrochemical solutions. How to determine the effect of FT cycles on the expansion and changes of rock fractures, which is related to the safety and stability of many engineering rock masses in cold regions. In this study, the microstructure changes of specimens induced by hydrochemical solutions and cyclic FT were measured using nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). The dynamic fracture tests were conducted using single cleavage triangle (SCT) granite specimens. Crack propagation gauge (CPG) was applied to examine crack propagation speed (CPS). The findings indicate that the mineral composition of the granite did not vary large with the hydrochemical solutions and FT cycles, but the maximum diffraction intensity of each mineral composition changed. For the granite under the coupling effect of hydrochemical solutions and FT cycles, the granite specimen in NaOH solution with 100 FT cycles has the highest dynamic fracture toughness and is 10.76 MPa·m1/2, dynamic fracture toughness of H2O, HNO3 and Na2SO4 solutions are only 0.94, 0.91 and 0.93 of that of NaOH solution. With increasing FT cycle times, the dynamic fracture toughness and crack speed decrease, whereas the porosity increases. For the specimen in NaOH solution and 100 FT cycles, dynamic fracture toughness decreased by 18.12 % without FT cycles and crack speed decreased by 34.18 %, the porosity increases by 26.42 %.
A coupled material point model (MPM) based on the asymmetric generalized interpolation material point (aGIMP) and the tetrahedron convected particle domain interpolation (CPDI-Tet4) was established ...to simulate the dynamic splitting of a rock Brazilian disc under small deformation. Furthermore, a boundary-based contact algorithm was developed to accurately describe the contact force between the split bar's aGIMP particles and the rock's CPDI-Tet4 particles. The proposed aGIMP technology enabled particles to move on a non-uniform background grid by changing the original basis functions. A good simulation effect was achieved. It was concluded that the rock particle size plays an important role in the accuracy of the contact algorithm. Two shear failure bands were generated near the contact zones first after the rock specimen reached the force equilibrium state, and then the four arc fracture surfaces grew from the two sides of the shear failure bands and coalesced to a radial fracture surface on the mid-cross section of the specimen. In addition, the tensile strength calculated by the MPM model exhibited the typical rate effect of rock material and was slightly lower than the practical tensile strength of the particle on the radial fracture surface.
Tunnels in fractured rock masses are typically damaged by dynamic disturbances from various directions. To investigate the influence of blasting load directions on the stability of a tunnel with a ...pre-crack nearby, blasting tests were conducted on the physical models of an external crack around a tunnel (ECT) in this study. Failure modes of the tunnels were analysed based on stress wave theory. The Riedel–Hiermaier–Thoma (RHT) material model was employed to perform the numerical simulations on ECT models. Stress distribution around the tunnels and final failure patterns of the tunnels were characterised. The results show that, under blasting loads, the pre-crack propagates and then new cracks initiates on the incident side of the tunnel. These cracks extend towards each other and eventually coalesce. Blasting load directions significantly influence the ultimate failure mode of the tunnel in the fractured rock masses. The new cracks on the shadow side of the tunnel appear at different positions when the blasting stress waves come from various directions. The results are meaningful to the analysis of tunnel stability and optimisation of the tunnel support scheme.
The inherent anisotropy of shale makes it exhibit different mechanical properties when measured in different directions. Generally, the measurements of the anisotropic mechanical properties of shale ...are generally conducted on cylindrical samples, and the computation of elastic constants is usually an empirical approximation. To accurately estimate the elastic parameters of shale rocks, an iterative method for approaching the crack initiation stress threshold was proposed in this work. Several uniaxial compression experiments were performed on cuboid Longmaxi shale samples with different bedding plane orientations. With a newly presented method, the apparent elastic modulus and Poisson’s ratios and five elastic constants were simultaneously determined. By examining the volumetric strain reversal point, the crack damage stress was obtained for each sample. The results demonstrate that the elastic regime, which was the range in which the elastic parameters were determined, significantly varies with bedding plane orientation. A transverse anisotropy ratio of 1.28 between in-plane modulus E1 and out-of-plane modulus E3 was determined for Longmaxi shale. Additionally, with the obtained elastic parameters, the energy releasing and dissipating behaviors of the samples during uniaxial loading were quantitatively characterized. The energy dissipation was found to rapidly increase once the crack damage stress was reached. However, before rock failure occurred, the total dissipated energy in each sample was very limited, indicating that microcrack propagation and rock damage were quite limited before rock rupture and that the shale samples were essentially destroyed by rapid energy release.
•A new method to accurately estimate the elastic constants of rocks was proposed.•The stress regime for elastic constants assessment varies with vein angle.•Anisotropies in elasticity, failure process and energy flow were highlighted.•The damage of each sample was found to be quite limited before rock failure.•The shale samples were found to be essentially destroyed by rapid energy releasing.
Cracked straight-through Brazilian disc (CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700 °C. Uniaxial splitting experiments were ...performed using an automatic electro-hydraulic servo press to study the evolution laws of physical and fracture properties of different deep rock materials under high-temperature geological conditions. The fracture characteristics were measured using an industrial camera and digital image correlation technology to analyze the effect of high temperature on fracture properties and failure modes of the CSTBD samples after different thermal treatments. The micro-damage properties of green sandstone and granite materials were obtained using a scanning electron microscope (SEM). The following conclusions were drawn from the test results: (1) With the increasing temperature, the fracture characteristics of green sandstone and granite change from brittle fracture to plasticity fracture, the longitudinal wave velocity of granite decreases sharply at 600 °C, and the damage factor reaches 0.8748 at 700 °C. (2) The fracture toughness of green sandstone and granite decreases with increasing temperature; however, the decreasing range of granite is larger than that of green sandstone. (3) As the temperature increases, the fracture morphologies of green sandstone and granite materials become rougher, whereas thermal damage cracks of granite and intergranular fractures inside sandstone as well as pores of sandstone increase. (4) The crack tip opening displacement and peak strain corresponding to peak load increase with the temperature.
•The effect of thermal treatment on the fracture characteristics of granite and green sandstone materials was studied.•The evolution law of fracture toughness of rock with temperature was studied.•The fracture morphology of granite and green sandstone after thermal treatment was analyzed using SEM.•The CTOD and strain field curves of CSTBD specimens after thermal treatment were calculated using DIC technology.
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