Failure Process Of Specimens Research Articles

Study on the dynamic fracture behavior of anisotropic CCNBD shale specimens under different impact angles

Drilling and fracturing are the key technologies for shale gas extraction, while most of the loads generated during rock drilling or fracturing are dynamic. In this study, dynamic impact experiments were conducted on the cracked chevron-notched Brazilian disc shale specimens with different chevron-notched crack (CNC) inclinations (β) and layer inclinations (α) by the split Hopkinson pressure bar. The results show that the dynamic peak load and dissipated energy increases with the increase of β, α, and strain rate, but their changing trends are different. The generation and expansion of cracks during the specimen failure process are mainly affected by β. As β increases, the failure type of the specimen can be divided into pure mode I fracture, mode I-II mixed fracture, pure mode II fracture, mixed tension-shear failure, and Brazilian splitting failure. The increase in strain rate will lead to a decrease in the time for crack initiation and propagation as well as an increase in secondary cracks. In addition, the mode I fracture toughness (KⅠC) and mode II fracture toughness (KⅡC) both grow with the increase of α and strain rate. The KⅡC predicted by the generalized maximum tangential stress criterion (GMST) exhibits more accurate.

Numerical study on mechanical properties and instability characteristics of sandy reservoir containing hydrate interlayer

A sound understanding of the geomechanical behavior of sandy reservoir containing hydrate interlayer is crucial not only for safe exploitation of natural gas but also for assessing the continental slope stability. To date, there are extremely limited reports on synergistic consideration of hydrate interlayer together with overburden and underburden. In order to study the mechanical properties and instability characteristics of sandy reservoir containing hydrate interlayer, in this study, sandy reservoir specimens were prepared using the discrete element method firstly. Subsequently, triaxial undrained tests were carried out under different thicknesses and saturation of hydrate interlayer. Results show that formation of hydrate can enhance the strength of the sediments, but not necessarily the reservoir. Conversely, lower intercalation saturation and thickness can reduce reservoir strength. The critical saturation and thickness that significantly alter the geomechanical properties of reservoirs are further discussed. Additionally, there is a dissipation of energy respectively before and after the peak stress, which may be a unique feature of the reservoir. By analyzing the crack propagation and evolution of the shear band during shear, the damage and failure process of specimen is clarified. And a prediction method for the mechanical response of sandy reservoir containing hydrate interlayer was suggested.

Variable angle shear test and finite element simulation of polyurethane – bentonite composite structure

Polyurethane grouting is more and more widely used in projects. In this paper, the variable angle shear test (VAST) was carried out on polyurethane – bentonite (PU-B) composite specimens using a set of angle adjustable shear test devices. The failure process of specimens was studied by digital image correlation (DIC) technology. The finite element model was established to simulate the shear failure process of PU-B specimens. The results show that, within the range of angles set in the test, the greater the shear angle, the greater the shear strength of the specimen. The normal component can improve the shear strength of the PU-B specimen to a certain extent. The relationship between the normal stress and the tangential stress in cases of specimen failure conforms to the linear law, from which the cohesion and internal friction angle of the PU-B composite specimen can be obtained. The finite element simulation results are basically consistent with the VAST results. The conclusions may play a role in the study of the shear properties of the interface between polyurethane and bentonite to some extent.

Experimental study on mechanical properties and failure characteristics of sandstone containing a pre-existing surface flaw under dynamic loading

Dynamic loading tests were conducted on sandstone specimens containing a three-dimensional surface flaw with different angles and depths using a modified split Hopkinson pressure bar (SHPB) device. A high-speed camera with digital image correlation method was used to record the crack initiation, propagation and failure process of specimens and to monitor the surface strain field. The test results show that both the angle and depth of the surface flaw have an important influence on the dynamic mechanical properties and energy dissipation of the specimens. The specimen is least weakened at a surface flaw angle of 90°, with the angle having a greater effect on dynamic strength and energy absorption at greater depths. Analysis of the x-axis direction strain, shear strain and y-axis direction strain fields by DIC shows that the crack initiation is due to a combination of tensile and shear stresses. Both the angle and depth of the surface flaw have a significant effect on the failure patterns. Specimens show shear failure at surface flaw angles less than or equal to 60° and tensile failure at surface flaw angles of 75° and 90°. When the angle of surface flaw is less than or equal to 60°, there are more shear cracks at greater depths. The complexity of three-dimensional crack propagation under dynamic loading was confirmed by the observation of traces of internal shell-like crack growth in a partial fragmentation of specimen after failure.

Numerical simulation of stress wave propagation in joint rock specimens with cavity defects

The process of crack initiation, propagation, and coalescence is the essential cause of rock failure. A three-dimensional numerical model based on microscopic damage mechanics is adopted to simulate the failure process and acoustic emissions (AEs) of a jointed rock mass containing a pre-existing hole subjected to stress waves. The numerically simulated results demonstrate that transmission energy plays an important role in the failure process of specimens. The greater the energy of joint transmission is, the greater the damage to the joint transmission area of the rock mass is. Furthermore, the joint width could significantly influence crack propagation patterns and the damage of the joint transmission area of rock specimens. Moreover, the degree of damage to the local joint transmission area of the rock mass is small but then becomes more obvious when the joint angle grows larger. In addition, the wavelength of the stress wave can also affect the failure modes of the rock when stress waves are applied. As the wavelength of the stress wave reduces, the larger the damage of the rock mass is and the smaller the effect of the joint on crack propagation is. Finally, the numerical results demonstrate that the width of the specimen has a significant effect on its dynamic failure mode and degree, showing an obvious size effect. This finding could explain the lateral growth of an existing flaw in its own plane, which is a phenomenon that has not been observed in laboratory experiments.

Experimental Study on Dynamic Response and Damage Evolution of Coal under Shocks by Multiple High-Pressure Air Blasting

To explore the dynamic response and damage evolution of coal under multiple high-pressure air blasting (HPAB), simulated coal specimens were used in the HPAB experiments, and the variation laws of stress field, vibration field, damage field, and cumulative fracture failure process in specimens were analyzed from a macro point of view. A scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) were used to observe the pore distribution near the blasthole of the specimen and analyze the variation law of pore structure parameters under multiple HPAB from the microscopic point of view. The test results show that (1) the stress wave generated by HPAB has a great impact on the near zone. After multiple HPAB, the damage value at the place 50 mm away from the blasthole increases by 3.91 times compared with the one shock from HPAB, and the strain peak and vibration velocity are reduced by 17.86% and 63.05%, respectively. With the increase of distance, the internal damages of the specimens in the middle and far zones are mainly driven by the stress wave and the high-pressure air, and the strain peak, vibration velocity, and damage degree gradually decrease. (2) With the increase of shock times in HPAB tests, the stress wave attenuation index decreases at first and then increases, and the damages degree of the middle and far zones increase slowly in the first few shocks and then increase rapidly. The site coefficient ( k ) shows an overall decreasing trend, whereas the attenuation coefficient ( α ) tends to increase. (3) The multiple HPABs have a great impact on the pore structure of the specimens. Compared with unshocked specimen, the cumulative mercury injection and pore volume increased by 152.04% and 135.05%, respectively. The number of connecting pores with large pore diameter is significantly increased. The multiple HPAB can effectively improve the pore and fracture structure in the specimens and form a relatively developed fracture network channel. The study results have certain guiding value for solving practical engineering problems of low extraction efficiency in low permeability coal seams.

Cracking and deformation of cuboidal sandstone with a single nonpenetrating flaw under uniaxial compression

To get a good understanding of the cracking mechanism of a nonpenetrating flaw, digital carving technology was employed to pre-cut a nonpenetrating flaw within a rock block. Uniaxial compression tests were conducted on the specimens containing a single nonpenetrating flaw. A strain measuring system was employed to monitor the local microstrain near the flaw tips. Meanwhile, the failure process of specimens was recorded simultaneously using a video camera and the noncontact video gauge. Based on the experimental results, the crack propagation behaviors and the fracture characteristics of the nonpenetrating-flaw sandstone specimens during uniaxial loading were presented. Also, the relationship between peak strength and pre-existing flaw angle was shown. Furthermore, numerical models containing a penetrating or nonpenetrating flaw were developed using 3D particle flow code to investigate the crack propagation mechanisms under uniaxial compression. By comparing the contact force distribution and the principal stress distributions around pre-existing nonpenetrating or penetrating flaw just before crack initiation, the crack initiation and propagation of sandstone specimens containing a single nonpenetrating flaw was explained from the microscopic point. It is found that (a) the average peak strength increases with the growth of nonpenetrating flaw angle and reduced by 21.8% for 0°, 16.0% for 30°, 13.0% for 45°, 4.7% for 60°, and 0.5% for 90° comparing with the peak strength of the intact specimen; (b) the crack first appears at the flaw tip on the front surface and then a new crack appears in the middle of the flaw on the back surface, the tensile stress gradually decreases from the front to the back; (c) the local strain at the front surfaces of the pre-existing flaw tip is greater than that at the same position on the back, the flaw tip of the model front is prone to stress concentration; (d) the final failure pattern relies on the pre-existing flaw angle and as the pre-existing flaw angle increases, the failure mode changes from mixed tension and shear failure to shear failure.

The seismic responses of RC frames infilled with full and partial masonry walls under cyclic lateral load

This paper presents the seismic responses of full and partial brick masonry infilled frames under lateral load. Four test specimens of ¼ scale-down single-story single-bay, including one RC bare frame, one RC frame with a full brick masonry wall, and two RC frames with partial brick masonry walls were tested under quasi-static cyclic loading. During the tests, initial crack, cracks propagation, and failure processes of specimens were monitored. Lateral forces and displacements in-plane direction at several points on specimens were recorded. Consequently, it was found the differences of seismic responses between a bare frame, fully masonry infilled frame, and partially masonry infilled frames. Installing full or partial masonry infills in the RC frames structure can change the failure mechanism, increase the lateral strength and stiffness, but decrease the ductility performance of the whole structure.

Effect of stress pulse shape on the dynamic fracture of soda-lime glass

The goal of this paper is to evaluate the tensile strength and fracture behavior of the soda lime glass under static and dynamic loading using Brazilian tests. The evaluation of the static tensile strength was performed using the universal testing machine; the fracture behavior under dynamic loading was studied using the Hopkinson Split Pressure Bar (HSPB) Technique. The dynamic loading is realized using the stress pulses of the different shape. In order to obtain the different loading stress pulses impact strikes made from four different materials (Steel, Teflon, Beech, and Polymer) were used. The high-speed camera in both static and dynamic tests was used to obtain some detail view on the failure process of specimens. Experimental results showed that the dynamic tensile strength was at least three times higher than the static one. The initiation of the dynamic fracture occurs when some parameters of the loading stress pulse reaches critical values. These parameters are independent on the stress pulse shape.

Mechanical behavior and bearing capacity calculation of self-stressing steel slag aggregate reinforced concrete filled circular steel tube columns

To study the mechanical mechanism of self-stressing steel slag aggregate reinforced concrete filled circular steel tube (steel slag aggregate/concrete@circular steel tube) columns, eight specimens including six short columns and two intermediate length columns were designed for axial compression test, the variable parameters, such as the diameter-thickness ratio, expansion rate of steel slag aggregate concrete and length-diameter ratio were considered. The whole failure process of specimens was observed, and then the strain-stress curves as well as the peak stress was obtained. The influence of variable parameters on the mechanical behavior of self-stressing steel slag aggregate/concrete@circular steel tube columns was analyzed. Test results indicate that the short columns under axial load exhibit shear failure while the intermediate length columns experience global flexural buckling failure mode. The stress-strain curves of all specimens are basically similar, which undergo the peak point, descending section, slow rise section and so on. The peak strain and peak stress of all specimens are significantly increased compared with those of common steel slag aggregate concrete, and the improvement effects are more obvious in short columns than those in intermediate length columns. According to the limit equilibrium condition and entire process analysis, the calculation formula of bearing capacity of self-stressing steel slag aggregate/concrete@circular steel tube columns was proposed, and then the stress-strain model of self-stressing steel slag aggregate/concrete@circular steel tube columns was established depending on the experimental data. The theoretical calculation results are in good agreement with test data. The research results can provide reference for further research and engineering application of self-stressing steel slag aggregate/concrete@circular steel tube columns.

Confinement Shear Effect in Fiber Composites

Composites have been playing an increasingly important role in various engineering applications. We examine the confinement shear effect arising in the neighborhood of the fiber-matrix interface in fiber-reinforced materials, which is a problem calling for further investigations. It is well known that the progressive failure process strongly depends on the interfacial transition zone (ITZ). Thus, we take the studied material as three-phased, i.e. fiber, matrix and interface in between. The generalized beam (GB) lattice model is adopted to simulate the compressive failure behavior of a representative volume element with one fiber included. Numerical results are provided to explain the confinement shear effect during the progressive failure process in specimens with various fiber-matrix properties, fiber orientations and distributions.

Experimental and numerical study on structural performance of reinforced concrete box sewer with localized extreme defect

An experimental and numerical investigation into the structural performance of reinforced concrete box sewers with typical corrosion-related extreme defects localized at the ceiling was conducted. Firstly, during the large-scale laboratory test, some key structural responses were captured and evaluated, including the crack width development process (via digital image correlation measurement), ceiling deflection, and material strains of both complete and typical defective boxes. The failure modes and load-carrying mechanism throughout the specimen loading phases were analyzed. Furthermore, the specimen failure process was simulated using a damage-based finite element method, and a related parameter sensitivity analysis was performed. The results indicate that the defective ceiling cracked at mid-span under a low load value, but the bending capacity loss can be substituted by two shoulders and carry three to five times more load before completely collapsing. The simulation matched the lab test qualitatively, and with the suggested set strategy of material parameters, the load-deflection feature curve could provide a practical prediction of the ultimate bearing capacity of the defective sewers, with a 10–15% error on the safe side.

Failure characteristics of jointed rock-like material containing multi-joints under a compressive-shear test: Experimental and numerical analyses

Extensive efforts have been made to gain a better understanding of the failure behaviour of rocks and rock-like materials, but crack propagation and failure processes under compressive-shear loading have not yet been comprehensively investigated. To address this area of research, the peak shear strengths (τ) and failure processes of specimens with multiple joints are studied by lab testing and particle flow code (PFC2D). Four types of failure modes are observed: (a) shear failure through a plane (Mode-I), (b) intact shear failure (Mode-II), (c) oblique shear crack connection failure (Mode-III), and (d) stepped path failure (Mode-IV). The failure mode gradually transformed to Mode-III as α (joint inclination angle) increases from 0° to 90° in the specimens. In addition, with increasing joint distance (d) in the specimens, the failure mode changes to Mode-II. As the non-overlapping length between joints (c) in the specimens increases, the failure mode changes to Mode-IV. The joint geometry has a major influence on the shear strength of the jointed specimens. The peak shear strength of specimens with different joint inclination angles is obtained when α=45°. Additionally, the peak shear strength increases as the joint distance (d) and non-overlapping length (c) increase.

Experimental and Numerical Studies on the Dynamic Behaviors of Concrete Material Based on the Waveform Features in SHPB Test

The tendency of the waveform curve can directly reflect the deformation and failure process of specimen in the SHPB (Split Hopkinson Pressure Bar) test of concrete. Different loading rates will result in the different ultimate failure modes, waveform curves. Furthermore, these differences are obviously characterized by some feature points of waveform or stress-strain curves. It is to say for concrete-like damage softening materials, the waveform features contains lots of information of material response. In this study, large dimension (Ф120mm) SHPB tests of concrete specimens have been conducted. Four typical failure patterns of concrete specimens are classified, as well as some typical waveform features, e.g. the “double-peak” and“compression wave” phenomena of reflection wave, etc. On the other hand, the numerical simulations corresponding to the experimental tests are performed by means of the 3D meso-scale model of concrete material. In the numerical results, waveform features observed in experiment are reliably reproduced and predicted. Associating with waveform features, the violation indicator of the specimen stress equilibrium in the SHPB test is first identified for concrete-like damage softening materials. The concrete material behaviors forstress non-equilibrium are further analyzed, e.g. DIF and damage development, etc.

Crack propagation simulation of rock-like specimen using strain criterion

A digital image correlation (DIC) method was used to obtain the full-field strain and crack propagation path for a rock-like specimen with a single prefabricated crack under uniaxial compression. Additionally, scanning electron microscopy (SEM) was used to scan the fracture surface of the specimen after compression. The macro-mechanical response of the loading process was found to be in accordance with the micro-mechanism obtained by SEM, which demonstrates that the DIC system is accurate and that the full-field strain is accordant with crack propagation. Thereafter, a strain criterion was proposed and was applied to the extending discrete element method to simulate crack propagation. The strain criterion was found to be appropriate for simulating the crack initiation stress, peak strength and crack growth morphology of the specimen. Moreover, both the numerical simulation and experimental results show that the specimen failure process had four typical stages – initiation, unstable propagation, stable propagation and failure.

Development of reinforced ultra-high toughness cementitious composite permanent formwork: Experimental study and Digital Image Correlation analysis

This paper presents a development of reinforced ultra-high toughness cementitious composite (UHTCC) participating permanent formwork, which acts as a self-supporting formwork during construction and contributes to the load capacity and durability of the concrete member through the service life of the structure. The flexural behavior of the specimens made with the formwork is analyzed with the help of Digital Image Correlation (DIC) technology. Compared to reinforced concrete beam, the specimens made with UHTCC formwork own a larger deflection, when their maximum crack width reached 0.45mm. The cracks caused by the delamination of the formwork have little effect on the longitudinal bars, but they would reduce the load capacity in some circumstance. Based on the failure process of specimens, several design details, including the load capacity of various sections and the surface treatment methods, were given to improve the performance of the formwork.

Analysis on the waveform features of the split Hopkinson pressure bar tests of plain concrete specimen

The tendency of the waveform curve can directly reflect the deformation and failure process of specimen in the SHPB (Split Hopkinson Pressure Bar) test of concrete. Different loading rates will result in the different ultimate failure modes, waveform curves and the stress–strain curves of the concrete specimens. Furthermore, these differences are obviously characterized by some feature points of waveform or stress–strain curves. In the present study, with the pulse shaping technique and high-speed photograph, large dimension (Ф120×100mm) SHPB tests of concrete specimen have been conducted. The typical features of the waveform curves under different loading rates have been further classified. According to the experimental observation, the “double-peak” phenomenon of reflection wave in the SHPB tests of concrete-like materials is revealed and interpreted, as well as the “compression wave” phenomenon occurring in the tail of reflection wave under relatively lower strain rate and the obvious “post-peak inward concave descending” of the transmission wave under high strain rate. What's more, the plastic carrying capacity and the effect of the strain rate are also studied. Some empirical understanding on the correspondence between the waveform features and the ultimate failure status of the concrete specimen under various loading rates are also presented. The above research shows that “compression wave” phenomenon of the reflection wave and sustainable plastic carrying capacity will vanish with the increasing loading rate. On the contrary, the “double-peak” phenomenon of the reflection wave, which is considered to be the specific property possessed by concrete-like materials, will become more apparent with the increasing loading rate.

Cracking Process and Stress Field Evolution in Specimen Containing Combined Flaw Under Uniaxial Compression

Hydraulic slotting, an efficient technique for underground enhanced coal bed methane (ECBM) recovery, has been widely used in China. However, its pressure relief mechanism is unclear. Thus far, only limited research has been conducted on the relationships among the mechanical properties, flaw parameters, and crack propagation patterns of coal after hydraulic slotting. In addition, because of the limitations of test methods, an in-depth information is not available for this purpose. In this work, numerical models of specimens containing combined flaws are established based on particle flow code method. Our results provide insights into the effects of flaw inclination angle on the mechanical properties, crack propagation patterns, and temporal and spatial evolution rules of stress field in specimens containing combined flaws during the loading process. Besides, based on the initiation position and underlying mechanism, three types of crack initiation modes are identified from the failure processes of specimens. Finally, the crack propagation pattern is quantitatively described by the fractal dimension, which is found to be inversely proportional to the uniaxial compressive strength and elastic modulus of the specimen. To verify the rationality of the numerical simulation results, laboratory tests were conducted and their results match well with those obtained from the numerical simulation.

Experimental Study on Mechanical Behaviors of Precast Concrete Shear Wall with Vertical Joint

Precast two-way hollow slab concrete shear wall is a new structure adapted to housing industrialization. To study the effect of the vertical joint on mechanical behaviors of shear walls, one reinforced concrete shear wall and two precast concrete shear walls built with hollow slabs were quasi-statically tested under low cyclic loading. The study of failure mode and failure process of specimens shows that vertical macro-cracks occurred in precast walls under loading, which made failure behavior of walls evolve from integral wall into split wall. It also shows that relative deformation formed along the vertical joint before peak load, so the ductility of walls is increased. New type shear walls exhibit good ductility and brittle shear failure can be avoided effectively.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample’s uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.