Dynamic Fracturing Research Articles

Fracturing behaviours and AE signatures of anisotropic coal in dynamic Brazilian tests

Anisotropy caused by bedding structures has a great effect on tensile properties and fracture behaviours of coal. Acoustic emission (AE) monitoring is a powerful tool in understanding the anisotropy of inherent failure mechanisms. Brazilian disc tests in a split Hopkinson pressure bar (SHPB) system were conducted on coal specimens with bedding orientations β (0°, 30°, 45°, 60°, and 90°) to investigate the damage evolution and AE characteristics. Lead Zirconate Titanate (PZT) sensors were attached on the specimens to monitor AE activities during dynamic fracturing. To obtain complete AE signals, an attenuator formed by a linear resistor was innovatively used in the circuit, which can reduce the power of a signal without distorting its waveform. Meanwhile, two high-speed cameras and the digital image correlation (DIC) technique are synchronously used with AE monitoring to record real-time coal fracturing processes and verify AE results. Results show that the jump of AE energy can be observed when the dynamic load approaches its peak, which indicates the onset of crack growth. The time to fracture tf determined by AE energy against β shows an inverted “V” shape, with the lowest and highest values at β = 0° and 60°, respectively. AE parameters like amplitude, counts and duration are heavily affected by anisotropy, among which AE count-duration relationship can be used to classify different failure modes. Moreover, the influence of anisotropic bedding planes on the evolutions of AE energy with crack growth is quantitatively presented.

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Numerical investigation of the effect of holes on dynamic fracturing in multi‐flawed granite

AbstractA numerical study is conducted using the improved continuum‐based discrete element method (CDEM) to investigate the effect of holes on the dynamic fracturing of multi‐flawed rocks. The specimen geometries contain a perpendicular crack‐like flaw, a hole‐like flaw, and an inclined crack‐like flaw. A fracture model is implemented into the improved CDEM that combines the nonlinear pressure‐dependent shear strength and tensile strength of rocks. The digital image correlation method combined with ultra‐high‐speed photography is applied in a split Hopkinson pressure bar system to verify the accuracy of the proposed model. The experimental results show that the improved CDEM accurately reproduces dynamic crack behavior in rocks. The simulation results show that hole‐like flaws significantly affect crack behavior compared with the two investigated crack‐like flaws. However, this effect gradually weakens with increasing loading stresses. This study provides important insight into the dynamic fracturing of multi‐flawed rocks.

Experimental study of the quasi-static and dynamic fracture toughness of freshwater ice using notched semi-circular bend method

The dynamic fracture toughness of ice involves its fracture resistance under impact loading, which is essential for impact or blasting parameter selection and evaluation. Temperature is an important factor influencing ice mechanical properties. To examine the evolution of ice fracture characteristics at different loading rates and temperatures, notched semi-circular bend method was applied to determine the freshwater ice quasi-static and dynamic fracture toughness. A modified low-temperature split Hopkinson pressure bar system was developed to measure dynamic ice fracturing properties. A high-speed camera captured ice specimen cracking initiation and propagation processes. The experimental results indicate that the overall initiation fracture toughness (IFT) of freshwater ice under dynamic loading exceeds that under quasi-static loading. The loading rate effect on the dynamic fracture toughness and failure modes greatly exceeds the test temperature effect. The freshwater ice dynamic IFT increases approximately linearly with the loading rate and rapidly at temperatures from –5°C to –45 °C. In all tests, cracks always initiate from the notch plane. Two symmetrical curved cracks then propagate from the contact end of two support rollers to the flat contact end, ultimately leading to different sample fragmentation distributions. These results improve the understanding of the dynamic fracturing mechanism in ice engineering applications.

Numerical simulation of forerunning fracture in saturated porous solids with hybrid FEM/Peridynamic model

In this paper, a novel hybrid FEM and Peridynamic modeling approach proposed in Ni et al. (2020) is used to predict the dynamic solution of hydro-mechanical coupled problems. A modified staggered solution algorithm is adopted to solve the coupled system. A one-dimensional dynamic consolidation problem is solved first to validate the hybrid modeling approach, and both δ-convergence and mr-convergence studies are carried out to determine appropriate discretization parameters for the hybrid model. Thereafter, dynamic fracturing in a rectangular dry/fully saturated structure with a central initial crack is simulated both under mechanical loading and fluid-driven conditions. In the mechanical loading fracture case, fixed surface pressure is applied on the upper and lower surfaces of the initial crack near the central position to force its opening. In the fluid-driven fracture case, the fluid injection is operated at the centre of the initial crack with a fixed rate. Under the action of the applied external force and fluid injection, forerunning fracture behavior is observed both in the dry and saturated conditions.

A rate-dependent peridynamic model for predicting the dynamic response of particle reinforced metal matrix composites

To predict the mechanical properties and reproduce the dynamic fracturing behaviors of the particle reinforced metal matrix composites, the mesh-free composite models are constructed using peridynamic (PD) theory. On this basis, a rate-dependent PD model is proposed and introduced into the bond-based PD formulation. In this model, the classic Johnson-Cook (JC) equation is modified to characterize the non-linear strain rate effect on the performance of the matrix. The good agreement between the predicted results and the experimental results validates the applicability of the constitutive model. Then we take the thermal mismatch strengthening mechanisms into account for better describing the strong obstacles to the dislocation movement in composite brought by the stacking inclusions. After that, the obtained constitutive model is applied in PD simulations. The unit-particle model is discussed firstly to reveal the damage and failure mechanisms of the composite during the failure process under different strain rate loadings. The results present that the strain rate will change the failure mode and consequently influence the mechanical properties including the ductility and the strength in given strains. Furthermore, the multi-particle models are taken into account, a non-negligible factor, i.e., particle volume fraction, is introduced for systematic study.

Microseismic Events Cause Significant pH Drops in Groundwater

AbstractEarthquakes cause rock fracturing, opening new flow pathways which can result in the mixing of previously isolated geofluids with differing geochemistries. Here, we present the first evidence that seismic events can significantly reduce groundwater pH without the requirement for fluid mixing, solely through the process of dynamic rock fracturing. At the Grimsel Test Site, Switzerland, we observe repeated, short‐lived groundwater pH drops of 1–3.5 units, while major and minor ion groundwater concentrations remain constant. Acidification coincides with reservoir drainage and induced microseismic events. In laboratory experiments, we demonstrate that fresh rock surfaces made by particle cracking interact with the in situ water molecules, likely through creation of surface silanols and silica radicals, increasing the H+ concentration and significantly lowering groundwater pH. Our findings are significant; pH exerts a fundamental control on the rate and outcome of most aqueous geochemical reactions and microseismic events are commonplace, even in seismically inactive regions.

Ultra-high speed X-ray imaging of dynamic fracturing in cementitious materials under impact

In this work the dynamic fracturing of an ultra-high strength cementitious material is probed with in-situ ultra-high speed X-ray phase-contrast diagnostics to investigate the phenomenology of dynamic fracture. Gas gun experiments were conducted on two characteristic samples with two different impact speeds, namely 80 and 190 m/s using the edge-on impact test configuration. The samples were placed within the intense X-ray beam providing an observation field of 12.8 mm in width and 8 mm in height. Thanks to equispaced 16 bunches of short X-ray pulses, the samples were imaged through an indirect detector arrangement using the Shimadzu HPV-X2 camera lens-coupled to a fast scintillator capturing through-thickness measurements with an interframe time of 1.06 µs. The comparison of fragmentation patterns between two samples revealed an important insight into velocity dependant spall formation as well as the effects of crack closure and bridging.

Dynamic deformation and fracturing properties of concrete under biaxial confinements

The study of deformation and fracturing properties of concrete is essential to understand failure mechanisms of concrete material, especially under extreme and complex loading conditions, e.g. high strain rates and multiple confinements. In this study, dynamic deformation and fracturing behaviour of ordinary concrete under biaxial confinements are investigated by using a triaxial Hopkinson bar system, three-dimensional digital image correlation, and synchrotron X-ray computed tomography techniques. Results show that compressive strain localisation areas appear around aggregates due to the elastic difference between aggregate and matrix, where accompanied with initiation of interfacial cracks and propagation of main cracks. Transgranular cracks can also be observed in the central of specimen in CT slices due to the effect of strain concentration. In addition, CT slices with distinct properties in various directions indicate the anisotropic fracturing behaviour of concrete due to the effect of biaxial confinements.

Full-field deformation measurements from Brazilian disc tests on anisotropic phyllite under impact loads

To explore the dynamic fracturing behaviours and failure mechanisms, transversely isotropic phyllite rocks with five bedding plane angles were investigated for full-field strain Brazilian disc tests under different strain rates using a split Hopkinson pressure bar and high-speed digital image correlation (DIC) technology. Two kinds of crack evolution processes: the cracks in specimens with low bedding angles activated near the non-loading point, and other cracks initiated at the centre of the disc prior to exceeding the peak stress were observed. Considering the four types of failure modes characterized by different mechanisms, the dynamic failure mechanism of transversely isotropic rock was discussed. The Brazilian disc strength of transversely isotropic rock subjected to impact loads was computed by introducing the patchy weakness model to reflect the dominant role of the rock matrix in the dynamic test. The DIC results demonstrated that additional random orientation defects are activated and the competition for evolving into cracks becomes intense, indicating that cracks develop less along bedding planes as the strain rate increases. Moreover, the fracture patterns of the transversely isotropic phyllite specimens changed under static and dynamic conditions. Therefore, it is necessary to study the properties of transversely isotropic rocks under dynamic impacts rather than simply using inferences from static results.

Investigation on the effect of freeze-thaw on fracture mode classification in marble subjected to multi-level cyclic loads

For rock engineering in cold regions, rock is often subjected to coupled fatigue damage caused by freeze-thaw (F-T) and stress disturbance. To investigate the cracking propagation and to quantify the degree of damage and the type of crack classification, multi-level cyclic loading experiments were carried out on marble with F-T treatments of 0, 20, 40 and 60 cycles. Real-time acoustic emission (AE) and post-test computed tomography (CT) scanning technologies were employed to reveal the fracturing evolution and to further classify different crack types to aid in understanding dynamic fracturing. An AE parameter analysis method combined with a b-value analysis is able to identify the cracking event modes and cracking propagation. Results show that the RA (ratio of the rise time to amplitude) and AF (ratio of the AE counts to the duration) pattern is strongly influenced by the previous F-T treatment. The results of the kernel density estimation (KDE) and the k-mean cluster method indicate that a tensile cracking event and shear/mixed cracking event can be classified from an RA and AF data set. The proportion of tensile cracks decreases and shear/mixed cracks increases with the increase of F-T cycles. In addition, b-value analysis reveals the damage propagation at different cyclic levels and a minimum b-value was observed at the last cyclic level. Moreover, post-test CT scanning shows that complex crack network can form for a sample subjected to low freeze-thaw treatment, and the scale of tensile cracks decreases and shear/mixed cracks increases as F-T cycle grows. It is suggested that the deterioration of rock meso-structures impacts the fracturing mode and the associated rock stability in cold regions. The decreasing of the b-value and an increasing of the RA values provide an early warning for rock fracturing and implementation of rock mass stability prediction.

Dynamic mechanical characteristic and fracture evolution mechanism of deep roadway sandstone containing weakly filled joints with various angles

To investigate the influence of weakly filled joint angles on the dynamic mechanical properties and fracture evolution mechanism of sandstone, dynamic uniaxial compression tests were performed on sandstone specimens with joint angles of 0°–90° under strain rates of 35–125 s−1 and an interval of 15° using a modified split Hopkinson pressure bar (SHPB) apparatus. The dynamic stress–strain relationship, dynamic strength, dynamic peak strain, energy characteristic, failure mode, typical failure progress, crack coalescence category, fracture micro-mechanism, and theoretical analysis were examined in this research. The test results indicated that both the dynamic compressive strength and peak strain of jointed sandstone first decreased and then increased with the increase in the joint angle. To maximise the energy reflection coefficient and minimise the absorbed energy density, a critical joint angle α(approximately 45°), was employed. The specimens with joint angles of 0°, 15°, and 30° exhibited considerable plastic deformation capacity, whereas the brittle failure feature of specimens with joint angles of 60°, 75°, and 90° was more typical than that of intact specimens. The stress bimodality phenomena of the specimen with a joint angle of 45° were more distinct than those of the specimen with a joint angle of 60°. Four types of crack coalescence behaviours were identified during the dynamic fracturing processes of the jointed specimens using a high-speed camera; subsequently, these were compared with the crack coalescence categories of jointed specimens under static loading. Moreover, the micro-fracture morphology well-revealed the macroscopic fracture behaviours of jointed sandstone specimens. It was further confirmed that the variable tendency of the dynamic compressive strength of jointed specimens based on experimental results agreed with the theoretical analysis.

Experimental investigation on the characteristics of transient electromagnetic radiation during the dynamic fracturing progress of gas-bearing coal

AbstractThere are extensive studies on electromagnetic radiation (EMR) effects during coal and rock deformation and fracturing processes, but few systematic studies on the EMR features of gas-bearing coal under impact failure circumstances. In order to investigate whether the EMR is affected by the gas (CO2,CH4,N2) sorption, coal type and impact energy, we performed a series of impact loading tests on both briquette coal specimens (BCSs) and raw coal specimens (RCSs) at various pore pressures (0–1.5 MPa). We developed a drop hammer test apparatus to allow impact loading tests on gas-bearing coal, and recorded the EMR signal of the damage of coal simultaneously. The result showed that (i) the amplitude range of the EMR signal generated by the impact damage of the gas-bearing coal is approximately 10–600 mV, with an effective duration time of 3–1500 ms and accumulated energy of 0.1–1000 μJ; (ii) when pore pressure is increased, the maximum amplitude, duration and pulse counts of the EMR decrease accordingly; (iii) the coal powder size and impact energy affect damage features and EMR characteristics, where a higher impact impulse causes more severe damage to coal specimens and (iv) the influence of the presence of adsorptive gas on the EMR signal caused by the destruction of coal bodies has both enhancement and reduction effects. During the early warning assessment of coal and gas outbursts using EMR parameters, not all the waveform parameters are sensitive to coal mine methane, but the significant effect of methane on signal volatility should be considered.

Parallel implementation of the four-dimensional lattice spring model on heterogeneous CPU-GPU systems

As a newly developed computational method, the four-dimensional lattice spring model (4D-LSM) is computationally intensive due to the introduction of extra-dimensional interactions. In this work, the 4D-LSM is parallelized to fully utilize the available computational resources of modern computers, namely, the multi-core CPU and the GPU. To utilize computing power of the multi-core CPU, OpenMP with a fork-join scheme is used to assign computational tasks to different CPU threads, whereas CUDA, with a granular computing scheme, is adopted to assign computations to thousands of GPU threads. A domain decomposition with a data communication scheme is proposed to utilize both the multi-core CPU and the GPU. The influence of digital precision and hardware on the parallel computing performance of the 4D-LSM are investigated through a number of numerical examples including elastic deformation, elastic bulking and dynamic fracturing. Finally, the multi-core CPU 4D-LSM is used to solve a crack propagation problem and is compared with existing experimental and numerical results.

Experimental and theoretical analysis of hydraulic fracturing and gas fracturing of rock under true triaxial compressions

A test system was developed for true triaxial hydraulic fracturing (HF) and gas fracturing (GF) combined with acoustic emission (AE) monitoring and 3D computerized tomography (CT) scanning reconstruction to carry out a series of HF and GF tests under different true triaxial loading conditions. The critical breakdown pressure characteristics, the AE activities, and the fracture propagation and spatial morphologies were obtained during the fracturing process. The test results showed that the GF breakdown pressure was approximately 29.0–31.3% less than the HF breakdown pressure. Moreover, for the GF specimen, the gas pore pressure induced a large number of new microfractures before critical fracture. However, both HF and GF under true axial compression can only produce simple main longitudinal fractures (LFs, along the borehole) and transverse fractures (TFs, across the borehole) that are strictly dependent on the initial true triaxial stress field. Based on the experimental observations, a hypothesis for the initiation of the HF and GF under true triaxial compression was proposed by using the maximum tangential stress on the borehole wall and the maximum axial stress along the borehole. Accordingly, analytical solutions of the critical breakdown pressure for the HF (impermeable) and GF (permeable) specimens were derived. The theoretical predictions agree well with the observations from the 3D numerical simulations and tests. Lastly, a novel pulsed GF method (dynamic loading fracturing) was proposed to overcome the dependence of the in-situ crustal stress field associated with traditional HF and GF methods (static loading fracturing). Laboratory test results showed that the dynamic pulsed GF at moderate pressure can generate a complex and random fracture network independent of the in-situ stress field in rock.

Variation of Hydraulic Properties Due to Dynamic Fracture Damage: Implications for Fault Zones

AbstractHigh strain rate loading causes pervasive dynamic microfracturing in crystalline materials, with dynamic pulverization being the extreme end‐member. Hydraulic properties (permeability, porosity, and storage capacity) are primarily controlled by fracture damage and will therefore change significantly by intense dynamic fracturing—by how much is currently unknown. Dynamic fracture damage observed in the damage zones of seismic faults is thought to originate from dynamic stresses near the earthquake rupture tip. This implies that during an earthquake, hydraulic properties in the damage zone change early. The immediate effect this has on fluid‐driven coseismic slip processes following the rupture, and on postseismic and interseismic fault zone processes, is not yet clear. Here, we present hydraulic properties measured on the full range of dynamic fracture damage up to dynamic pulverization. Dynamic damage was induced in quartz‐monzonite samples by performing uniaxial high strain rate (>100 s−1) experiments in compression using a split‐Hopkinson pressure bar. Hydraulic properties were measured on samples subjected to single and successive loadings, the latter to simulate cumulative damage from repeated rupture events. We show that permeability increases by 6 orders of magnitude and porosity by 15% with dissipated energy up to dynamic pulverization, for both single and successive loadings. We present damage zone permeability profiles induced by earthquake rupture and how it evolves with repeated ruptures. We propose that the enhanced hydraulic properties measured for pulverized rock decrease the efficiency of thermal pressurization, when emplaced adjacent to the principal slip zone.

Dynamic fracturing process of fissured rock under abrupt unloading condition: A numerical study

Unloading can cause many geotechnical disasters, e.g., the rock burst. The underlying mechanism of rock failure is the propagation and coalescence of cracks. To get insight into rock unloading failure, the discretized virtual internal bond (DVIB) model in conjunction with element partition method (EPM) is used to simulate the dynamic fracturing process under unloading conditions. At first, the samples with the single flaw are simulated under unloading conditions. It is found that the fracturing pattern exhibits the tensile-shear transition with the initial stress level increasing. When the flaw inclination is closer to 45 degree or the flaw is longer, shear cracks are more likely to occur. Then, the unloading failure process of rock with multiple flaws are simulated. It is found that the inclination of the parallel flaws has remarkable influence on the unloading failure of rock. When the inclination angle is about 45 degree, the rock is easier to fail. For the rock with multiple randomized flaws, the amount of flaws has slight influence on acoustic emission counts. However, the configuration of randomized flaws can significantly affect the fracture process and failure patterns. These findings can improve understanding of rock burst and provide valuable references for the prediction of unloading failure of rock.

Dynamic fracturing properties of marble after being subjected to multiple impact loadings

A multiple impact loading test was conducted on notched semi-circular bend (NSCB) marble specimens by using a split Hopkinson pressure bar (SHPB) apparatus. Then, specimens with various dynamic cumulative damages were subjected to dynamic three-point bending loading. During the multiple impact loading test, the dynamic incident energy was kept constant in each impact. The dynamic peak stress and the elastic modulus degrade gradually with an increasing impact number. In the dynamic three-point bending test, the dynamic fracture toughness and the fracture energy of the specimens decrease gradually with increasing dynamic cumulative damage, and the maximum declines are 65.27% and 76.23%, respectively. In terms of the failure modes, the fractal dimension of the crack propagation path increases approximately linearly with the dynamic cumulative damage, namely, the crack propagation path becomes increasingly tortuous. According to the quantitative analysis of the fracture surface roughness, the larger the dynamic cumulative damage of the specimen is, the rougher the fracture surface, and the larger the area of the fracture surface. These results can improve the understanding of the dynamic fracturing properties of rock structures after multiple impact loadings.

Reach and geometry of dynamic gas-driven fractures

Dynamic gas fracturing is a well stimulation technique that is able to create multiple fractures emanating from a wellbore. It operates by pressurizing at rise-times and peak pressures intermediate between conventional hydraulic fracturing and explosive fracturing. Two consecutive processes operate during this fracturing process: (i) generation and propagation of a dynamic stress wave that overpowers the static stress field and creates multiple radial fractures around borehole, followed by (ii) quasi-static pressurization and further extension of those starter-fractures by the expanding gas. Dynamic analysis is first performed to follow the evolution of the stress wave propagating from the borehole. The radial (r) distribution of peak tensile hoop-stress diminishes as 1/rα with the power exponent (α) asymptoting to 2 as the loading rate decreases. This rapid attenuation generally limits the length of the body-wave-generated radial fractures to several borehole radii. The gas-loading of the borehole wall is followed by permeation of the gas pressure into the newly created radial fractures. Linear elastic fracture mechanics (LEFM) perturbation analysis shows that a regular distribution of multiple radial starter-cracks will preferentially propagate the longer cracks at the expense of the shorter cracks – that will arrest and snap-shut. This system naturally selects of the order of six dominant fractures that may grow in unison until the in situ stress field reasserts control as the fractures lengthen. This restricts the maximum number of the dominant fractures to of the order of six at the conclusion of treatment - a common observation in both in situ and laboratory experiments.

Fracturing behavior around a blasthole in a brittle material under blasting loading

Dynamic fracturing behavior in brittle materials under blasting loading occurs in a very short time. It is usually challenging to observe the behavior experimentally. Therefore, visual observation of the fracturing behavior can be useful information for understanding dynamic fracturing and crack propagation process under blasting loading. In this study, dynamic fracturing behavior under blasting loading was investigated through experimental and numerical methods. Transparent and homogeneous PMMA was used for specimen, while DDNP(diazodinitrophenol) detonators were used to apply blasting loadings. During the blasting experiment, fracturing and crack propagation process were observed using a high-speed camera, then the crack propagation behavior was analyzed in both quantitative and qualitative manners. The results showed that gas-driven fracture in an ear-shape was more predominant than shock-wave-driven fractures. Further, the extent of crack propagation significantly depended on the amount of explosives charge. The experimental results matched well with the analytical solution based on the fracture mechanics theories. A numerical simulation was carried out using a commercial FEM software, which is capable of solving non-linear dynamic problems. The numerical simulation reasonably reproduced the cracking behavior in terms of crack length and orientation which were observed in the experiment.