Microcracking Behavior Research Articles

Microcracking behaviors and acoustic emission characteristics of granite subjected to direct shear based on a novel grain-based model

In this study, we numerically examine the microcracking behaviors of Alxa porphyritic granite under direct shear using a novel grain-based model considering particle-scale softening mechanism. Associated acoustic emission (AE) activities were monitored using a moment tensor algorithm, and then the effects of normal stress on the microcracking behaviors and AE characteristics of granite were investigated. The shear fracture systems primarily comprised linked tensile crack arrays, of which the proportion of shear cracks increases with an increase in normal stress. Furthermore, additional intra-grain cracks formed because of the strengthening grain interlocking effect, and the thickness of shear band gradually increased. Moreover, with the normal stress increasing, the decreasing b-value of AE events indicated that the proportion of large-magnitude AE events increased. We reported that the maximum-magnitude AE event in the prepeak stage could be used to identify the crack damage point, which could be considered as a potential failure precursor of granites. The displacement ratio of the peak strength point to crack damage point slightly increased with an increase in normal stress, providing a potential warning indicator for the granite failure. Overall, this study provides certain novel insights into the damage behaviors and failure prediction of granites under direct shear.

Dynamical In Situ Study of Morphological Variations on Stainless Steel in a Sour Corrosive Environment

It is still unclear how environmentally assisted cracking, especially sulfide stress cracking (SSC), is caused by corrosion and fracture behavior. In this study, a new in situ technique for an SSC visualization challenge was established and applied for the visualization of microcrack propagation resulting from a single pitting on the steel surface of 13% Cr martensitic stainless steel during four-point bending SSC tests and discussed by combining ex situ x-ray computed tomography (XCT) and electron backscatter diffraction (EBSD). The dynamic behavior of microcracks was visualized successfully, and the microbubbles behavior with the corrosion reaction was also captured. Regarding the relationship between internal structure and microstructure for cracking, it was found that a cavity was formed in the interior of prior to the cracks seen on the surface by XCT, and cracks by EBSD were propagated along prior austenite grain boundaries. On the basis of in situ observations and complementary ex situ analyses, the actual situation of crack on the steel surface was revealed and discussed with both corrosion and fracture over time. This approach provides an opportunity to clarify the SSC occurrence process based on actual SSC tests and contributes to the determination of phenomenological criteria. Furthermore, by combining this in situ technique with commonly known electrochemical measurements such as polarization curves, open-circuit potentials, and electrochemical impedance spectroscopy, it is possible to directly discuss various electrochemical data corresponding to corrosion and fracture behavior, including dissolution and gas evolution.

Micromechanics-based simulation of B4C-TiB2 composite fracture under tensile load

Micromechanics modeling was performed to study the effects of thermal residual stress, weak interphases, TiB2 volume fraction and particle size on the mechanical responses and fracture behaviors of B4C-TiB2 composites. Experimentally observed fracture behaviors including micro-cracking and crack deflection were successfully captured. The weak interphases at B4C-TiB2 boundaries and the thermal residual stress induced during cooling by the large CTE mismatch between B4C and TiB2 were identified as two major factors to promote micro-cracking that caused the enhanced progressive failure behavior. Micro-cracking was enhanced with higher TiB2 volume fraction due to higher fraction of weak interphase and material affected by thermal residual stress. Meanwhile, micro-cracking behaviors exhibited limited change with varying TiB2 particle sizes. This modeling study successfully captured the main fracture behaviors and their trends by varying micro-structures of B4C-TiB2 composites and can potentially aid microstructure design of tougher B4C-TiB2 composites in the future.

Energy characteristics and micro-cracking behaviors of deep slate rock under triaxial loadings

Energy characteristics and micro-cracking behaviors of deep slate rock under triaxial loadings

Acoustic Emission Monitoring of Progressive Damage of Reinforced Concrete T-Beams under Four-Point Bending.

Acoustic Emission (AE) is revealed to be highly adapted to monitor materials and structures in materials research and for site monitoring. AE-features can be either analyzed by means of physical considerations (geophysics/seismology) or through their time/frequency waveform characteristics. However, the multitude of definitions related to the different parameters as well as the processing methods makes it necessary to develop a comparative analysis in the case of a heterogeneous material such as civil engineering concrete. This paper aimed to study the micro-cracking behavior of steel fiber-reinforced reinforced concrete T-beams subjected to mechanical tests. For this purpose, four-points bending tests, carried out at different displacement velocities, were performed in the presence of an acoustic emission sensors network. Besides, a comparison between the sensitivity to damage of three definitions corresponding to the b-value parameter was performed and completed by the evolution of the RA-value and average frequency (AF) as a function of loading time. This work also discussed the use of the support-vector machine (SVM) approach to define different damage zones in the load-displacement curve. This work shows the limits of this approach and proposes the use of an unsupervised learning approach to cluster AE data according to physical and time/frequency parameters. The paper ends with a conclusion on the advantages and limitations of the different methods and parameters used in connection with the micro/macro tensile and shear mechanisms involved in concrete cracking for the purpose of in situ monitoring of concrete structures.

Microcracking mechanisms of cyclic freeze–thaw treated red sandstone: Insights from acoustic emission and thin-section analysis

We experimentally investigate the effects of cyclic freeze–thaw (F-T) on the microcracking behaviour of sandstone under mode I loading. The centre-cracked Brazilian disc tests were conducted on specimens after various numbers (0, 30, 60, 90, and 120) of cyclic F-T treatments with acoustic emission (AE) monitored. The micro-damage caused by the F-T treatments to the sandstone is examined via thin-section analysis. The results show that the damage degree does not significantly increase until the F-T cycle number exceeds a threshold between 60 and 90. The AE results indicate that the cyclic F-T treatments inhibit the microcracking activities of the sandstone.

Feasibility of Using Shear Wave Ultrasonic Probes as Pump-Wave Sources in Concrete Microcrack Detection and Monitoring by Nonlinear Ultrasonic Coda Wave Interferometry.

This paper represents a first attempt to study the feasibility of using shear wave (SW) ultrasonic probes as pump-wave sources in concrete microcrack detection and monitoring by Nonlinear Ultrasonic Coda Wave Interferometry (NCWI). The premise behind our study is that the nonlinear elastic hysteretic behavior at microcracks may depend on their orientation with respect to the stationary wave-field induced by the pump-wave source. In this context, the use of a SW probe as a pump-wave source may induce the nonlinear elastic behavior of microcracks oriented in directions not typically detected by a conventional longitudinal pump-wave source. To date, this premise is hard to address by current experimental and numerical methods, however, the feasibility of using SW probes as a pump-wave source can be experimentally tested. This idea is the main focus of the present work. Under laboratory conditions, we exploit the high sensitivity of the CWI technique to capture the transient weakening behaviour induced by the SW pump-wave source in concrete samples subjected to loading and unloading cycles. Our results show that after reaching a load level of of the ultimate stress, the material weakening increases as a consequence of microcrack proliferation, which is consistent with previous studies. Despite the lack of exhaustive experimental studies, we believe that our work is the first step in the formulation of strategies that involve an appropriate selection and placement of pump-wave sources to improve the NCWI technique. These improvements may be relevant to convert the NCWI technique into a more suitable non-destructive testing technique for the inspection of microcracking evolution in concrete structures and the assessment of their structural integrity.

Mechanical responses of grain-based models considering different crystallographic spatial distributions to simulate heterogeneous rocks under loading

Grain-based models (GBM) are widely used to reproduce the heterogeneous characteristics of natural rocks at the grain size level. To incorporate the spatial distribution of the constituent minerals into the GBM, the digital image processing (DIP) technique and the random distribution of property (RDP) method are frequently adopted. However, an analysis of the differences, advantages and disadvantages of the named two popular methods in simulating the mechanical response of rocks under loading has so far not been undertaken. This study considers Brazilian and uniaxial compression tests using DIP and RDP models, showing distinct differences in stress-deformation behavior (especially tensile strength) and final failure patterns. Fracture initiation , accumulation, propagation, and the corresponding stress-deformation behavior of polycrystalline rocks are sensitive to the crystallographic spatial distribution and the micro-strength parameters of mineral contacts. The clustering of mineral grains tends to induce more significant local stresses while the stress fluctuations in RDP models are more widely distributed and isolated, leading to different microcracking behavior furtherly affected by the superimposed effect of heterogeneous contact properties. Therefore, it is essential to use DIP methods to model heterogeneous rocks if the grain-scale mechanical behavior is of interest. If a heterogeneous model is used to study the overall macro-mechanical behavior of rocks, using the RDP method may be appropriate with the advantage of reduced effort for model set-up.

Numerical investigation of the effects of the micro-parameters of the transgranular contact on the mechanical response of granite

In this work, a novel three-dimensional grain-based model based on the Discrete Element Method (GBM3D-DEM) was proposed, and then the granite’s internal structure was reproduced in a realistic way. The effects of the micro-strength and micro-elastic modulus of the transgranular contact on the macroscopic mechanical characteristics and micro-cracking behavior of the sample in the Brazilian splitting test were investigated. Moreover, the variation of the fracture energy required for a single intergranular/transgranular contact was discussed. The consistency between the experimental and numerical results indicates that the proposed GBM3D-DEM is reliable. Results indicate that as the micro-strength of transgranular contact increases, the slope of the load–displacement curves in the linear elastic stage increases. The external load value required the macro-scale fracture of sample increases. The energy required for the transgranular contact fracture increases. In the propagation process of the main crack, the grain boundary structure is more easily destroyed, increasing the intergranular crack number. When the micro-elastic modulus of the transgranular contact rises, the stress region in the disk sample decreases gradually. In this case, the maximum external load value that the sample can withstand decreases. The transgranular contact is more likely to be destroyed under loading. The numerical results based on the GBM3D-DEM can predict the energy required for macro-scale fracture of crystalline granite, which is of great significance to rock mechanics.

Micro-cracking mechanism of RENÉ 108 thin-wall components built by laser powder bed fusion additive manufacturing

This study investigates the effect of wall thickness on the micro-cracking mechanism in thin-wall components produced by laser powder bed fusion (LPBF) using a hard-to-weld nickel-based superalloy, RENÉ 108. Microstructure analysis shows higher fraction of micro-cracking for thicker parts and a discontinuity at the 1 mm wall thickness. All micro-cracks exhibit interdendritic behavior, suggesting micro-crack formation in the final stages of solidification. Two finite element modeling (FEM) approaches are employed to evaluate the stress states at the beam and layer scales during laser processing. The beam-scale model shows positive stress triaxiality within the melt pool above the solidus temperature, supporting the view that solidification cracking is the dominant micro-cracking mechanism. The layer-scale model predicts higher stress triaxiality with increasing part thickness, favoring the finding for longer and larger number of micro-cracks in thicker parts. Hence, the anomalous micro-cracking behavior observed in the 1 mm part cannot be explained using the current models. Detailed microstructure analysis reveals larger variation in the primary dendrite arm spacing (PDAS) and cooling rate for the 1 mm part during LPBF. A higher thermal gradient is expected under these conditions, thereby explaining the anomalous effect observed between 0.25 mm and 5 mm wall thicknesses.

Notch strength and notch fracture mechanisms of a cast Mg-Gd-Y alloy

This study aims to elucidate the mechanical properties and fracture mechanisms of cast Mg-6Gd-3Y-0.4Zr (wt.%) (GW63) alloy in the presence of geometrical discontinuities induced by notches. Notch strength is evaluated by room-temperature tensile tests using circumferentially notched bars of different notch acuity. Finite element calculations suggest that notch fracture is controlled by the von Mises stress and effective plastic strain at the notch tip. Through fractographic observations and EBSD analysis on samples subjected to interrupted tests, a cleavage micro-cracking behavior uncommon for conventional Mg alloys is found to dominate notch fracture. Grain-sized cleavage microcracks tend to occur along the basal plane of grains favorably oriented for basal slip, which is explained by crack nucleation at co-planar slip bands blocked by the grain boundary according to Smith and Barnby's theory. Failure is initiated by the accumulation and coalescence of such microcracks in the notch vicinity. The unexpected cleavage micro-cracking behavior is mainly ascribed to the RE solutes that suppress {10–12} twinning, and is responsible for the limited deformation capacity and notch strength of the cast-T6 GW63 alloy with coarse grains and significant strengthening due to precipitation.

Dislocation dynamics analysis of fracture behavior of microcracks near a macrocrack

In this study, the fracture behavior of a microcrack near a macrocrack tip was analyzed by dislocation dynamics using both the uncoupled model, in which the microcrack and macrocrack are considered separately, and the coupled model, in which they are considered within the same model, to evaluate the effect of the macrocrack on the fracture toughness value. The coupled model allows analysis in which microcrack, and dislocations emitted from microcrack tips are directly affected by stresses from macrocracks and dislocations emitted from macrocrack tips. The validity of each model was examined by comparing the results obtained from the uncoupled and coupled analyses.

Influence of grain-to-particle size ratio on the tensile mechanical response of granite based on a novel three-dimensional grain-based model

In this paper, we developed a novel three-dimensional grain-based model based on the discrete element method (DEM) and reproduced the heterogeneous structure of crystalline granite. We carried out static Brazilian splitting tests to analyze the mechanical properties and micro-cracking behavior of granite samples. Furthermore, we investigated the effect of grain-to-particle size ratio on the tensile strength and discussed the difference in mechanical response of granite samples with different grain sizes and particle sizes under static loading. Our results show that the load–displacement curve of the sample can be divided into initial compaction stage, linear elasticity stage, damage stage, and post-peak stage. Tensile failure is the main failure mode leading to the macroscopic fracture of the sample, and intergranular failure is dominant. As the grain-to-particle size ratio increases, the proportion of the number of transgranular contacts to the number of total contacts increases and stabilizes. Compared with intergranular contact, transgranular contact has a higher bonding strength, and its fracture requires a larger stress concentration magnitude, which leads to an increase in the external load required for the macroscopic fracture of the sample.

Interdiffusion behavior between Cr and Zr and its effect on the microcracking behavior in the Cr-coated Zr-4 alloy

Interdiffusion behavior between Cr and Zr and its effect on the microcracking behavior in the Cr-coated Zr-4 alloy

Visualization and quantification of self-healing behaviors of microcracks in cement-based materials incorporating fluorescence-labeled self-healing microcapsules

Fluorescence-labeled self-healing microcapsules demonstrate the excellent ability to heal microcracks, and can also visualize the healing process of microcracks. The healing process of internal and external microcracks was visualized and quantified with laser scanning confocal microscope (LSCM) and step profiler. When 3.0 wt% microcapsules were incorporated into cement-based materials, the strength recovery rate was the highest, reaching 35.8%, and the healing effect of external microcracks reached 19.2%. When microcracks appear inside the resin matrix, fluorescence-labeled self-healing microcapsules can not only detect the specific location of the microcracks but also repair them, and quantify the width of microcracks. The change in the pore structure of cement-based self-healing materials is mainly due to the release of core materials into the microcracks and pores, thereby achieving the healing effect. It provides a unique insight into the visualization and quantification of self-healing behaviors of microcracks.

Laboratory characterisation of anisotropic and heterogeneous damage of rock sample using acoustic emission and ultrasonic monitoring technologies

Rock damage characterisation of Gosford sandstone samples under triaxial loading was achieved using an integrated approach, combing passive acoustic emission (AE) and active ultrasonic wave monitoring. The location of AE events is firstly obtained using isotropic wave velocity field measured by continuous ultrasonic wave. Then, passive AE tomography is conducted to progressively-update the anisotropy and heterogenous P-wave velocity field of rock samples. According to the variance in P-wave velocity, damage induced anisotropy and heterogeneity can be identified. The results suggest that the inhomogeneous damage evolution highly dependent on the loading stages and can well reflect the microcrack behaviour. Anisotropic damage variable along the whole loading process is also analysed, by a second order damage tensor with a transversely anisotropy assumption. It is observed that the lateral component of damage is much higher than the axial component. This is mainly due to the dilation behaviour of rock sample. This paper will cast more light on anisotropic and heterogenous damage characterisation using acoustic/ultrasonic lab experiments.

Influence of the microstructure on stress-dependent P-wave anisotropy in sandstone

SUMMARY To understand the factors that affect the stress-dependent P-wave velocity anisotropy, a method is proposed to simulate anisotropic microcracks and minerals based on the discrete element method (DEM). Laboratory triaxial tests and numerical simulations were performed on sandstone samples with bedding orientations parallel and perpendicular to the maximum principal stress. The ellipse fitting method was applied to analyse the variation in P-wave anisotropy. The micromechanism of stress-dependent P-wave anisotropy was revealed. The evolution of microcracks is the main reason for the change in P-wave anisotropy under compression. As the confining pressure increases, the magnitude of the P-wave anisotropy is reduced. The weakening of the P-wave anisotropy results from the decrease in the number of open microcracks. Under deviatoric stress loading, the P-wave anisotropy of the bedding-parallel sample in the axial direction is strengthened. Anisotropy reversal occurs in the bedding-normal sample. The microcrack behaviour depends on the direction of maximum principal stress. The variation in microcrack anisotropy induced by stress controls the evolution of P-wave velocity anisotropy. The stress at which anisotropic reversal occurs depends on the preferred orientation mineral. The DEM model offers the unique ability to directly examine the variation in microstructure anisotropy that causes the change in P-wave anisotropy.

Fluid-driven micro-cracking behaviour of crystalline rock using a coupled hydro-grain-based discrete element method

The grain-scale heterogeneity of rock has been found to greatly affect the cracking behaviour in mechanical tests. However, the heterogeneity has been insufficiently considered in hydraulic fracturing. To fill the gap, this paper proposes a coupled hydro-grain-based discrete element model and explores the fluid-driven micro-cracking of crystalline rock. The micro-heterogeneity of the rock is represented by a grain-based model. Three laboratory-scale fracturing cases over granite under different in-situ stresses are simulated. First, typical micro behaviour and hydro/hydro-mechanical responses are presented and interpreted. Next, sensitivity analysis of in-situ stress conditions is carried out. It is found that hydraulic fracturing of crystalline rock involves many unique behaviours on grain-scale such as non-symmetrical propagation, “zipper mode” initiation time order, generations of isolated and branching cracks, and re-closure of the cracks earlier initiated. Cracks can be induced in mineral grains and along the grain interfaces. The cracking mode is dominated by tension failure, with a small proportion in shear (between 3.5% and 1.5%). The interplays among orientation, aperture and number of different types of crack show that: 1. The inclination of cracks to the direction of maximum confining stress may occupy the entire range from 0° to 180°; 2. The aperture of cracks decreases statistically with their orientations turning to the direction of the minimum confining stress (σ3); 3. Increases in σ3 suppress the generation of total cracks and tend to force cracks to propagate in mineral grains; 4. A small in-situ stress difference results in fewer branching and isolated cracks. In this paper, evolutions of injection pressure and rock deformation during hydraulic fracturing are also discussed.

DEM simulation of crack evolution in cement-based materials under inclined shear test

The study aimed to investigate the initiation and propagation of cracks in cement-based materials subjected to shear stresses via discrete element method (DEM) simulations. It carried out numerical simulations and calibration of confinement stress-dependent bond parameters in inclined shear tests (IST), coupled with acoustic emission (AE) technique, on cement-based samples. Without modification, the default bonded models (DBM) in existing DEM software cannot consider the effect of confinement, which is critical for the strength and crack behavior of cemented materials. The study put forth a set of linear equations for determining confinement-dependent bond parameters, which are related to shear angle and confinement stress. The DEM simulation showed that the peak shear stresses were 6.26, 17.78 and 24.43 MPa at shear angles of 80°, 70° and, 65° respectively and corresponds to those results obtained from the tests. The broken bonds initiation (crack initiation) position was located around the center of the sample and subsequent crack propagation behavior was affected by the shear angle which agrees with the experimental results of AE evolution. Broken bonds proved to be a useful tool in studying micro-crack behavior. The proposed model is simple and yields results close to the test.

Investigation of microcracking behaviors in brittle rock using polygonal grain‐based distinct method

AbstractThe failure process in brittle rocks is an important topic in rock mechanics, whose good understanding assists in predicting the strength and deformation characteristics of rocks. Because it is difficult to directly observe microcracks in laboratory tests, a numerical model is a useful tool for investigating microcracking behaviors. However, the mechanism of microcrack evolution is still unclear at the grain scale considering the microscopic heterogeneities. This paper proposes a polygonal universal distinct element code grain‐based model to solve this problem. Compared with other grain‐based models, this model is different in that mineral grains are subdivided into polygon blocks. The grain size, composition, and bond types of the contacts are incorporated, and both inter‐ and intra‐grain cracks are mimicked. Subsequently, micro‐parameters of the blocks and contacts are carefully calibrated according to the laboratory results. Following this, the calibrated model is employed to study the microcracking behaviors of granite in unconfined compression, confined compression, and Brazilian splitting tests. Both the cumulative numbers and locations of microcracks in different grains are recorded, and the types of macroscopic fractures are identified. The modeling results show that tensile cracks dominate the generated microcracks in the low confined compression tests, and the macroscopic fracture pattern is axial splitting. Shear cracks are dominant in the high confined compression tests, and the macroscopic failure pattern is X‐type shear failure. The simulated results are consistent with those observed in the laboratory tests, and thus, may assist in improving rock acoustic emission monitoring and understanding the damage process of rock.