Main Crack Research Articles

Study on Influence of Joint Locations and Hydraulic Coupling Actions on Rock Masses’ Failure Process

Distribution of joints and fissures under hydraulic coupling condition is particularly critical to the stability of surrounding rock masses in underground engineering construction. In this paper, DDARF (Discontinuous Deformation Analysis for Rock Failure) and RFPA (Rock Failure Process Analysis) are compared and analyzed firstly based on laboratory tests. Then using preferred software RFPA, the failure process, stress state, acoustic emission characteristics and energy dissipation laws of rock masses with different joint locations are analyzed under the hydraulic coupling condition. Results show that a large tensile stress region is generated on both ends of the original joint with the micro-cracks’ propagation, water pressure in cracks promotes the generation of tensile stress to a certain extent, damage effect angle increases gradually from the rock specimen with the middle joint to that with the marginal joint; the same water pressure has a certain auxiliary effect on the main crack failure when the joint is close to the middle part of the specimen, and has a dominant effect on the local crack failure when the joint is far away from the middle; the maximum water pressure shows the “U” shaped distribution. At low initial water pressure, stresses of specimens with symmetrical joint locations have similar evolution trends, while at high initial water pressure, the water pressure in cracks has significant dissipation and thus the maximum water pressure in the system does not exceed the initial value. The length of the main crack path is positively proportional to the number of acoustic emissions and the energy accumulation capacity, and evolution of the damage variable basically shows a development trend of steady growth-rapid growth-steady growth.

The Characterization of Shale Differences Based on Petrophysical Properties and Pore Structure: A Case Study of the Longmaxi Formation in Northern Guizhou Province and the Yanchang Formation in the Ordos Basin

The pore structure and the petrophysical properties are essential parameters during the marine and continental shale reservoir evaluation. In order to the reveal the mechanism of the pore parameters differences and discuss whether the controlling factors of pore structure have extensive influence in different lithologies. Based on the analysis of N2/CO2 isotherm adsorption, FE-SEM, and geomechanical test for the marine shale samples of Longmaxi Formation and continental shale samples of Yanchang Formation are undertaken. The results indicate that (1) Continental shale is a typical transversely isotropic, while marine shale can be considered a homogeneous rock on a centimeter scale. The lithological stratification is the main reason for the difference in petrophysical properties. (2) The thermal evolution degree has significant influence on the pore structure in shale samples except for TOC content, particularly for the micropores structure. (3) Since the matrix strength of shale is much greater than the strength of bedding planes, the continental shale tends to crack and fail along the weak surface of the bedding during the axial compression, showing a lower peak strength, while the marine shale presents main crack along the principal stress direction without activating the bedding crack.

Effect of Microstructure on Fatigue-Crack Propagation of 18CrNiMo7-6 High-Strength Steel

The effect of microstructure on fatigue crack propagation of 18CrNiMo7-6 high-strength steel was studied by scanning electron microscopy and electron backscatter diffraction techniques. The crack propagation path was related to the microstructure of the original austenite grain boundary, lath martensite and crystal orientation. The crack path was tortuous and many small cracks or extrusions appeared on both sides of the main crack, with most of the extrusions parallel to the martensite lath. When the crack propagation encountered a high-angle boundary or passed through different crystal orientations, the crack was deflected, and when it encountered a low-angle boundary, the crack expanded.

3D computational model of wedge penetrator-rock mass interaction dynamics

The paper describes computational model variants of penetration dynamics of a wedge-shaped tool in rock mass. The computation uses the quasi-static approximation of penetration resistance forces obtained preliminarily in 2D and 3D formulations. The scope of the analysis embraces the elastoplastic and elastic deformation events in the medium with regard to the equilibrium propagation of a main crack. The calculation results are compared with the experimental modeling data on impact-driven wedge penetration in an organic glass block. The comparison proves efficiency of the described computational models.

Digital-image-correlation observation of cyclic plastic strain field during the damage-accumulation mode of fatigue crack propagation under pure cyclic mode II loading for cold-rolled SUS430 steel

Fatigue cracks propagate intermittently by the cyclic initiation and coalescence of voids in front of the fatigue crack tip during fatigue crack propagation in the damage accumulation mode. In this study, digital image correlation is used to measure the evolution of the measured plastic strain distribution near the crack tip with loading and cyclic loading during one cycle. The evolution is estimated based on strain distribution measurements results and mechanical considerations. The strain is set at the unloading point because results indicate that the crack propagated in the region where the experimental shear plastic strain is the maximum in the first quarter cycle (at the first maximum load); the measured plastic strain is almost constant in the region where the crack propagates, and the crack is initiated by a subsequent cyclic strain localization and coalescence with the main crack. This study clarified the damage accumulation mode fatigue crack propagation mechanism with the measured strain behavior, and presents a method to use continuum plastic strain at the crack tip as a mechanical driving force for this mode.

Quasi-in-situ observations of crack propagation and microstructure evolution of high manganese austenitic steel at −196 °C

The crack propagation behaviors and microstructure characteristics in the high manganese austenitic steel were comprehensively investigated by quasi-in-situ observations at −196 °C. The steel exhibits a superior fracture toughness at −196 °C than at room temperature. It is found that a severe plastic deformation is necessary for a crack initiation. Although the main crack propagates along the {111} planes due to the plastic strain localization caused by an extensive planar slip, the steel still possesses a superior crack propagation resistance, which is attributed to the activations of multiple twinning and slipping systems, which provide two positive effects on blocking a crack propagation: (1) the plastic deformation further enhanced by multiple twinning and slipping systems as well as (2) a tortuous crack propagation path caused by numerous twin boundaries. Besides, the pronounced deflections at grain boundaries also have a large contribution to block a crack propagation.

Modeling of multiple crack initiation in polymer pipes under oxidative environment

In this study, a novel fundamental model for the multiple crack initiation behavior of polymer pipes transporting fluids with strong oxidizing agents was proposed. Because the oxidant diffuses into the polymer medium with consumption by oxidation, a volumetric sink of the diffused oxidant was considered. The degradation kinetics of the polymer pipes were modeled by diffusion using the reaction equations. To compare the released energy with the crack initiation, Green's function for the stress intensity factor of various crack numbers, sizes, and standard dimension ratios (SDRs) was developed. The number of crack initiations and durations were successfully simulated using potential energy analysis with cracking. At a relatively low internal pressure in an oxidative environment, multiple cracks were estimated with a proper scale of the crack length. Instead, it was predicted that one main crack would be initiated at a higher level of internal pressure. In addition, the knee points representing the slope change of the internal pressure-lifetime plot were shown between these different failure modes, in agreement with previous observations. The experimental results of sustained hydrostatic pressure tests of polymer pipes under chlorinated water can be accurately simulated by the proposed model.

Effect of heat treatments and loading orientation on the tensile properties and fracture toughness of AlSi7Mg alloy produced by Laser Powder Bed Fusion

The present study investigates the effect of building orientation and heat treatment routes on the mechanical behavior of the AlSi7Mg alloy (A357) produced by Laser Powder Bed Fusion. The microstructure and mechanical behavior of A357 in the as built, T5 (directly aged) and T6 (solution treated, water quenched and aged) conditions were compared. Tensile properties of the material were evaluated along two main directions (parallel and normal to the building platform), whereas fracture toughness was measured with cracks placed in three orthogonal orientations. The results indicate that the anisotropy in mechanical properties is greatly reduced after T6 temper, but the selection of the best condition between the as built, T5 and T6 temper must consider the main loading directions and possible crack growth orientations associated with the specific application.

Experimental study on uniaxial compression failure modes and acoustic emission characteristics of fissured sandstone under water saturation

Widely distributed fissures in the open-pit slope (especially those which are in water-rich conditions) determine the slope stability. In order to study the effects of water and fissures on rock’s mechanical properties and corresponding acoustic emission characteristics, red sandstones containing dual-fissure with different angle configurations were prepared for uniaxial compression tests along with acoustic emission (AE) monitoring in this study. The impacts of water content and fissure angles on rock strength, failure mode and acoustic emission characteristics were analyzed. Moreover, the kernel density estimation (KDE) was accepted to quantify the AE events spatial distribution, then distinguishing difference between dry and saturated rocks’ failure modes. This study reveals that the presence of water in the rock lowers of the rock strength and decreases the amounts of secondary cracks and acoustic emission counts. As the fissure angle increases, both the rock strength and acoustic emission counts increase and the failure mode shifts from the tension failure to the shear failure. The b value was highly sensitive to crack propagation, reaching around 0.5 when the rock was finally destroyed under different water conditions. AE events mainly gathered in the location where the main cracks occurred. The maximum KDE density points of dry sandstone changed from concentrated state to dispersed state before failure, while the maximum density points of saturated sandstone always gathered in the middle of rock. The results of this paper lay theoretical foundations for monitoring and pre-warning of engineering rock failure in water environment.

Multi-Crack Dynamic Interaction Effect on Oil and Gas Pipeline Weld Joints Based on VCCT

In pipelines for transporting oil and gas, multiple cracks often exist in weld joints. The interaction among the cracks should be considered as it directly affects the life span of the pipeline structures. In the current investigation, based on the fluid–solid magnetic coupling model, the virtual crack-closure technique (VCCT) is applied to systematically study the multi-crack dynamic interaction effect on pipeline welds during the crack propagation process. The results show that the existence of an auxiliary crack accelerates the main crack’s propagation. When the auxiliary crack is nearer to the main crack tip, the enhancement effect of the auxiliary crack on the main crack increases. Further, when the initial length of the auxiliary crack increases, the main crack becomes easier to propagate. Two important parameters, the distance between the two interacting crack tips and the initial size of the auxiliary crack, are studied in detail. Their interference effect on the main crack has been quantified, which is very user-friendly for engineers to conduct failure assessment and prevention for oil and gas pipes with multiple cracks at weld joints.

FE Modelling and Simulation of the Size Effect of RC T-Beams Strengthened in Shear with Externally Bonded FRP Fabrics

The objective of this study is to conduct a finite-element (FE) numerical study to assess the effect of size on the shear resistance of reinforced concrete (RC) beams strengthened in shear with externally bonded carbon fibre-reinforced polymer (EB-CFRP). Although a few experimental studies have been done, there is still a lack of FE studies that consider the size effect. Experimental tests are time-consuming and costly and cannot capture all the complex and interacting parameters. In recent years, advanced numerical models and constitutive laws have been developed to predict the response of laboratory tests, particularly for issues related to shear resistance of RC beams, namely, the brittle response of concrete in shear and the failure modes of the interface layer between concrete and EB-CFRP (debonding and delamination). Numerical models have progressed in recent years and can now capture the interfacial shear stress along the bond and the strain profile along the fibres and the normalized main diagonal shear cracks. This paper presents the results of a nonlinear FE numerical study on nine RC beams strengthened in shear using EB-CFRP composites that were tested in the laboratory under three series, each containing three sizes of geometrically similar RC beams (small, medium, and large). The results reveal that numerical studies can predict experimental results with good accuracy. They also confirm that the shear strength of concrete and the contribution of CFRP to shear resistance decrease as the size of beams increases.

Size and Shape Effects on Fatigue Behavior of G20Mn5QT Steel from Axle Box Bodies in High-Speed Trains

In this paper, the axial loading fatigue tests are at first conducted on specimens ofG20Mn5QT steel from axle box bodies in high-speed trains. Then, the size and shape effects on fatigue behavior are investigated. It is shown that the specimen size and shape have an influence on the fatigue performance of G20Mn5QT steel. The fatigue strength of the hourglass specimen is higher than that of the dogbone specimen due to its relatively smaller highly stressed region. Scanning electron microscope observation of the fracture surface and energy dispersive X-ray spectroscopy indicate that the specimen size and shape have no influence on the fatigue crack initiation mechanism. Fatigue cracks initiate from the surface or subsurface of the specimen, and some fracture surfaces present the characteristic of multi-site crack initiation. Most of the fatigue cracks initiate from the pore defects and alumina inclusions in the casting process, in which the pore defects are the main crack origins. The results also indicate that the probabilistic control volume method could be used for correlating the effects of specimen size and shape o the fatigue performance of G20Mn5QT steel for axle box bodies in high-speed trains.

Dynamic properties of 3D printed cement mortar based on Split Hopkinson Pressure Bar testing

Concrete produced by 3D printing has been widely studied, however, most studies focus on the static properties of 3D printed materials, but few on dynamic properties. In this study, the dynamic properties of 3D printed cement mortar were evaluated from the dynamic compressive strength, energy dissipation density, failure modes, fragment distribution, and fractal dimension. In the strain rate range of 70 s−1-140 s−1, the anisotropy of dynamic compressive strength diminishes with increasing strain rate, but remains more significant than the anisotropy of static compressive strength. Acoustic indices were used to evaluate the anisotropy of the dynamic compressive strength of 3D printed cement mortar, revealing an exponential relationship between ultrasonic pulse velocity (UPV) and dynamic compressive strength. The energy dissipation density under the same strain rate varies in three key printing directions. Finally, the number of main cracks is different in three printing directions in the failure process under the same strain rate, which leads to different degrees of damage, and thus different fragment distributions and fractal dimensions therein.

Mechanical behaviours of sandstone containing intersecting fissures under uniaxial compression

Predicting rock cracking is important for assessing the stability of underground engineering. The effects of the intersecting angle α and the distribution orientation angle β of intersecting fissures on the uniaxial compressive strength and the failure characteristics of sandstone containing intersecting fissures are investigated through laboratory experiments and two-dimensional particle flow code (PFC2D). The relationship between the mechanical properties of sandstone and the intersecting angle α and the distribution orientation angle β is analysed. Crack initiation forms and the final failure modes are then categorised and determined via empirical methods. In addition, the cracking processes of intersecting fissures with different α and β values are discussed. The results show that variations in the peak stress, peak strain, average modulus, and crack initiation stress of sandstone containing intersecting fissures show a “moth” shape in the space of the α-β-mechanical parameters. Two crack initiation forms are identified: inner tip cracking (usually accompanied by one outer tip cracking) and only outer tips cracking. Two failure modes are observed: (1) the main fracture planes are created at the inner tip and one outer tip, and (2) the main fracture planes are formed at the two outer tips. Two main crack evolution processes of sandstone containing intersecting fissures under uniaxial compression are found. Approaches for quickly determining the crack initiation form and the failure mode are proposed. The combination of the determination equations for the crack initiation form and the failure mode can be used to predict the crack evolution. The approach for determining the crack evolution processes is hence proposed with acceptable precision.

Insights on low cycle fatigue crack formation and propagation mechanism: A microstructurally-sensitive modeling

A novel model for revealing low cycle fatigue (LCF) fracture mechanism was established based on molecular dynamics, extended finite element method, and representative volume element technology. Stored strain energy criterion provided the basic relationship for the proposed model. The proposed model was the 3D model for comprehensively simulating LCF crack formation at outer surface and propagation along slip system. In addition, micro-scale stored strain energy damage equations were proposed. The capability of the proposed model was confirmed by the microstructural fractography and 3D X-ray computed tomography. The complicated LCF crack growth direction and the secondary cracks accompanying the main crack could be simulated successfully. Particularly, the LCF crack length, the LCF crack formation/fracture life and morphology, the active slip system proportion, and the fracture angle were compared with experiment quantitatively and qualitatively. Two formation mechanisms of branch-shaped and main crack-independent secondary crack were determined. In addition, the microstructure had a strong influence on the LCF growth rate changing from short crack to long crack and the fluctuating crack growth rate during the short crack period. The proposed model had great potential to reveal the correlation between microstructure and LCF fracture mechanism.

Re-evaluation of shear contribution of CFRP and GFRP sheets in concrete beams post-tensioned with unbonded tendons

Contemporary design guidelines for the shear resistance of post-tensioned concrete (PC) beams retrofitted with fibre reinforced polymer (FRP) sheets are developed from the results of reinforced concrete (RC) beams. The accuracy of these guidelines for calculating the shear resistance of PC beams wrapped with FRP sheets is still questionable due to the difference in the shear behaviour of RC vs PC beams. The effects of the essential factors (e.g. tendon profile and anchorage system) on the shear resistance of PC beams have not been sufficiently studied, particularly for unbonded post-tensioned concrete (UPC) beams. This study presents an experimental investigation on the shear behaviour of UPC T-beams and the shear contribution of CFRP/GFRP sheets. The experiment consisted of 26 large-scale UPC T-beams with various factors including the tendon profile (harped or straight), concrete strength, FRP anchors, CFRP vs GFRP sheets, and various wrapping configurations. The experimental results showed that CFRP/GFRP sheets increased the shear resistance of UPC beams by 4–26%, which was much lower than that in RC beams. In comparison with the UPC beams with straight tendons, those with harped tendons exhibited a reduction in the shear contribution of CFRP/GFRP sheets (approximately 40%) and showed a more ductile failure mode with an improvement of the beam deformation (up to 2.3 times). FRP anchorage systems significantly enhanced the FRP shear contribution and energy absorption increased by 88% and 42% regarding those without anchors, respectively. Besides, it was observed from the test data that the widely-used method to determine the experimental shear contribution of FRP sheets in PC beams based on the difference between the shear resistance of strengthened and unstrengthened beams was inappropriate because it failed to consider the interactions between FPR sheets and other shear components, i.e. concrete and stirrups. Meanwhile, the experimental shear contribution of FRP sheets is recommended to be determined from the actual strain data of the FRP sheets across the main shear crack.

Simulation of intergranular fracture behavior inside randomly aggregated LiNixCoyMn1-x-yO2 polycrystalline particle

The degradation of secondary particles caused by intergranular fracture is an important reason for the capacity fading of LiNixCoyMn1-x-yO2 (NCM) polycrystalline electrodes. In this study, a chemomechanical damage model was established to implement a simulation of fracture behavior under different fracture energies, interfacial strengths and C-rates. Cohesive elements were applied along the interfaces of randomly distributed primary particles, generated by the Voronoi algorithm to simulate their separation during lithiation. The damage dissipation energy was calculated to characterize the propagation of cracks. The simulation results showed that the fracture energy and strength between primary particles play an important role in the intergranular fracture behavior. Increasing the C-rates brings two different aggravating effects to the fracture of secondary particles in different cycle stages. The extensive propagation of the main crack suppresses the growth of secondary cracks under a high C-rate or small fracture energy. Island particles and holes that appeared in many experiments after a long-term cycling protocol were simulated for the first time. The evolution of the simulated cracks was consistent with many published experimental images. The method and results are of great significance for understanding the fracture behavior of NCM cathodes.

Experimental investigation on torsional behaviors of ultra-high-performance fiber-reinforced concrete hollow beams

Limited studies have been conducted on the torsional performance of ultra-high-performance fiber-reinforced concrete (UHPFRC) hollow beams. This paper presents experimental investigations on eight UHPFRC hollow beams with various cross-sectional dimensions, wall thicknesses, and cross-section types. Their torsional responses, including the failure modes, torque−twist curves, torque−strain curves, torsional stiffness, and ductility indexes, are presented. Theoretical methods were employed to evaluate the cracking and ultimate torques. The results indicated that the UHPFRC hollow beams with and without flange plates exhibited a similar failure model, with the main cracks penetrating spirally. The cross-sectional dimensions, wall thicknesses, and cross-section types significantly affected the ultimate torque, whereas the cracking torque was affected only by the cross-sectional dimensions. A comparison of the experimental and theoretical results indicated that the American Concrete Institute code provisions provided good predictions for the cracking torque, whereas the existing theoretical methods offered unsatisfactory predictions for the ultimate torque. Thus, the equivalent wall thickness of UHPFRC hollow beams must be evaluated with a higher accuracy.

Mechanism Analysis of Rock Failure Process under High-Voltage Electropulse: Analytical Solution and Simulation.

This work aims to investigate and analyse the mechanism of rock failure under high-voltage electropulses in order to evaluate and increase the efficiency of high-voltage pulse technology in geological well drilling, tunnel boring, and other geotechnical engineering applications. To this end, this paper discusses the equivalent circuit of electric pulse rock breaking, the model of shock wave in electro channel plasma, and, particularly, the model of rock failure in order to disclose the rock failure process when exposed to high-voltage electropulse. This article uses granite as an example to present an analytical approach for predicting the mechanical behaviour of high-voltage electropulses and to analyse the damage that occurs. A numerical model based on equivalent circuit, shock wave model, and elasto-brittle failure criterion is developed for granite under electropulse to further examine the granite failure process. Under the conditions described in this study, and using granite as an example, the granite is impacted by a discharge device (Marx generator) with an initial voltage U0 that is 10 kV and a capacitance F that is 5 µF before it begins to degrade at about 40 µs after discharge, with the current reaching its peak at approximately 50 µs. The shock wave pressure then attains a peak at about 70 µs. Dense short cracks form around granite and the dominant cracks grow to an average length of about 20 cm at around 200 µs. The crack width dcr is predicted to be approximately 1.6 mm. This study detects dense cracks in a few centimetres surrounding the borehole, while around seven dominant cracks expand outward. The distribution of the length of the dominating cracks can be inhomogeneous because of the spatial heterogeneity of granite’s tensile strength, however the heterogeneity has an insignificant effect on the crack growth rate, total cracked area, or the number of main cracks. The mechanism of rock failure under electropulse can be well supported by the findings of numerical simulations and analytical studies.

An experimental assessment of ultra high performance concrete beam reinforced with negative Poisson’s ratio (NPR) steel rebar

An innovative meta steel rebar with negative Poisson’s ratio effect (termed as NPR steel rebar), which possesses both high strength (ultimate tensile strength is approximately 1100 MPa) and high uniform elongation (approximately 40%), is developed. This paper experimentally investigated the implications of NPR steel rebar on flexural behavior of ultra high performance concrete (UHPC) beams. Four UHPC beams reinforced with varying reinforcement ratio and longitudinal reinforcement type were prepared and tested. Traditional monotonic four-point bending test was performed. The test results showed that UHPC beams reinforced with NPR steel rebar could effectively improve the flexural strength by 47.8%-55.2%, and upgrade the deformability and ductility by 13.6%-180.9% respectively compared with UHPC beams reinforced with HRB 400 steel rebar under same reinforcement ratio, and the crack pattern of which exhibited multiple (6–8) uniformly main cracks located in pure bending region. Discussion included crack pattern, load-midspan deflection curve, ductility and load–strain response. The crack clustering mechanism of beams reinforced with NPR steel rebar was selectively emphasized in the discussion.