Length Of Fracture Process Zone Research Articles

The length of fracture process zone deciphers variations of rock tensile strength

Tensile strength is one of the most critical design factors in many rock engineering projects. However, despite many available testing techniques, an accurate estimation of the true tensile strength of quasi-brittle rock-like materials is yet a controversial problem since it can vary by the shape and size of a test specimen, the adopted test method, and applied loading conditions. Different studies have tried to address this issue by providing (mainly empirical) laws for determining variations of rock tensile strength as a function of a particular test parameter such as specimen size. In this study, however, a new general approach is presented that can decipher the tensile strength variations of rock under various testing conditions. Using coupled Finite Fracture Mechanics (FFM), it is first proved that the length of the Fracture Process Zone (FPZ) can be determined with accuracy and ease using the energy criterion of coupled FFM. Then, the length of FPZ is used in the stress criterion of coupled FFM to determine rock tensile strength. The failure stress of a material is then proved to be mainly a function of the FPZ length following a power law originated from the Linear Elastic Fracture Mechanics (LEFM). The results assist in deciphering variations of rock tensile strength related to the sample size and test method.

Fracture properties and acoustic emission characteristics of manufactured sand recycled fine aggregate concrete

To promote the application of recycled fine aggregates, fracture characteristics of concrete beams containing manufactured sand (MS) and recycled fine aggregate (RFA) were investigated. The fracture properties and acoustic emission (AE) characteristics of manufactured sand recycled fine aggregate concrete (MSRFAC) with 0 %, 25 %, 50 %, 75 % and 100 % RFA replacement rates were evaluated by three-point bending test of single-sided notched beams. The fracture mechanisms of MSRFAC beams were explored using AE and digital image correlation (DIC) techniques, and fracture process zone (FPZ) was qualitatively and quantitatively analyzed. The results indicated that the critical nominal stiffness, initial fracture toughness, fracture energy and characteristic length of MSRFAC decreased significantly with the increase of RFA replacement rate, up to 20.2 %, 37.7 %, 22.8 %, and 26.6 %, respectively, while the unstable fracture toughness was insensitive to the variation of RFA. Moreover, the AE activity, the tensile crack, large-scale crack, high amplitude, and high peak frequency in MSRFAC increased with increasing RFA replacement rate. According to the FPZ characteristics of MSRFAC, its FPZ length increased with the increase of RFA replacement rate, while the FPZ width decreased with the increase of RFA replacement rate. The critical FPZ lengths of MSRFAC with different RFA replacement rates were about 20.2–30.0 mm, and the maximum FPZ widths were about 26.0–36.7 mm. The RFA weakened the friction mechanism and interlocking effect between aggregates of MSRFAC, resulting in a faster FPZ propagation rate and smaller damage width of MSRFAC than normal manufactured sand concrete.

Study on crack propagation behaviors of three-point bending concrete beams with small span-depth ratios

Three-point bending (TPB) beams with small span-depth ratios, i.e. TPB beams with the same span of 400 mm but various depths (100 mm, 150 mm and 200 mm), were tested to study the crack propagation behaviors of concrete. The evaluations of crack extension resistance curve (KR-curve), fracture process zone (FPZ) length, lFPZ, and crack tip opening displacement (CTOD) were studied. The digital image correlation (DIC) technique was utilized to monitor the entire loading process. Moreover, the effects of strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s) on the fracture process of TPB beams with small span-depth ratios (2, 2.5 and 3) were discussed based on findings in literature. The results shown that (1) with the increase of specimen depth from 100 mm to 200 mm, the obtained KR-curves and initial fracture toughness were not influenced by the small span-depth ratios, while different increase trends were observed for the unstable fracture toughness, the average maximum value of lFPZ and the value of CTOD. (2) With the loading rate increasing from 10−5/s to 10−2/s, the obtained KR-curve and the maximum length of FPZ increased slightly, while the evolution of CTOD was not affected by the loading rate.

The influence of temperatures on the mechanical properties and fracture behavior of rocks under mixed mode I/II loading

A comprehensive understanding of the fracture behavior of pre-heated rocks under to mixed mode I/II loading is crucial for deep underground engineering. The effect of high temperature on the fracture process zone (FPZ) and fracture toughness of rocks was experimentally investigated using semi-circular bend (SCB) samples. Surface displacement and associated strain field under loading were captured using digital image correlation. The results showed that the mechanical properties of SCB samples gradually decreased with increasing temperature. Rocks exposed at temperature exceeding 600 °C demonstrated a significant reduction in peak load, generalized stiffness, fracture toughness, and absorbed energy. Moreover, the load–displacement curves and macroscopic fracture characteristics of pre-heated SCB samples revealed a transition from brittleness to ductility with increasing temperature under mixed mode I/II loading. Both the length and width, as well as the crack tip opening displacement of the FPZ, increased with rising temperature. At lower temperatures (25 ∼ 400 °C), the ratio of FPZ length to width typically ranged between 2 and 3. While, at elevated temperature (600 ∼ 1000 °C), this ratio expanded to between 3 and 4. The initiation of thermally induced cracks in rocks will accelerate damage evolution under mixed mode I/II loading, as indicated by the development of the maximum principal strain with displacement. Scanning electron microscopy and polarization microscopy images of the microscopic fracture morphology in pre-heated rocks illustrated isolated and dispersed microcracks appeared at lower temperatures, whereas at elevated temperatures, intricate microcrack networks and broken grains were observed.

Determining the Cohesive Length of Rock Materials by Roughness Analysis

In this research, the cohesive length of various rock types is measured using quantitative fractography alongside a recently developed multifractal analysis. This length is then utilized to gauge material cohesive stress through the theory of critical distances. Furthermore, the fracture process zone length of different rings sourced from identical rocks is assessed as a function of ring dimensions and experimental measurements of fracture toughness, in accordance with the energy criterion of the finite fracture mechanics theory. Subsequently, employing the stress criterion within coupled finite fracture mechanics, the failure stress corresponding to the fracture process zone is determined for various rings. Ultimately, through interpolation, the critical stress corresponding to the cohesive length, quantified via quantitative fractography, is approximated. Remarkably, the cohesive stress values derived from both methodologies exhibit perfect alignment, indicating the successful determination of cohesive length for the analyzed rock materials. The study also delves into the significant implications of these findings, including the quantification of intrinsic tensile strength in quasi-brittle materials and the understanding of tensile strength variations under diverse stress concentrations and loading conditions.

Effect of size and anisotropy on fracture process zone of coal: An experiment and numerical simulation

The size and properties of the fracture process zone (FPZ) have a significant impact on the fracture toughness of materials. Researching and understanding the characteristics of FPZ contribute to optimizing material performance and enhancing the reliability of engineering structures. In this study, the newly proposed modified semi circular bending (SCB) test method was used to carry out the pure mode I fracture test of coal samples with four sizes and four bedding angles, the development process of FPZ was obtained by using digital image correlation method, and the cohesive zone model considering the bedding plane was established. The influence of size and anisotropy on the development and size of FPZ was studied, and the influence mechanism of FPZ on the size effect and anisotropy of fracture toughness was revealed. The results show that the FPZ length of coal has obvious size effect and anisotropy. The FPZ length increases with the increasing specimen size, and increases first and then decreases with the increasing bedding angle. The FPZ relative length decreases with increasing specimen size, indicating that as the specimen size increases, the influence of specimen boundaries on PFZ gradually decreases, and the size effect of fracture toughness also gradually weakens. The competition between microcrack development driven by principal stress and microcrack development driven by bedding plane results in anisotropy in the development process of FPZ. The anisotropy of FPZ development process results in the anisotropy of FPZ length, fracture toughness and crack propagation path. The research findings can provide valuable guidance for the design of hydraulic fracturing in coal seams.

Method for identifying the position of crack tip by displacement distribution based on digital image correlation

Determining the position of crack tip is crucial for rock fracture characteristic analysis. This study aims to establish a quantitative standard for determining crack tip positions in crystalline rocks based on digital image correlation (DIC). Semi-circular bending (SCB) tests were conducted on four types of rocks under three-point bending. Optical microscopy and DIC were used to investigate characteristics of the horizontal displacement fields around the crack tips. A significant disparity in the displacement gradient was observed across the crack tips, with the internal gradient being approximately 3–5.1 times higher than the external gradient. A critical value ranging from 0.41%–0.64% was determined for the high internal displacement gradient (∂u/∂x) at a 20 pixel resolution equivalent to 1 mm. Furthermore, a novel DIC-based method was proposed that used three patterns of opening displacement gradients to classify SCB specimens into non-cohesive, cohesive, and elastic zones, providing a quantitative determination of Type I crack tip positions. This study offers valuable insights and a crack tip determination method based on DIC gradients, applicable for assessing parameters such as the crack extension length and fracture process zone length.

Mode-I failure and mechanical behavior of sandstone under cyclic loading: Laboratory testing and DEM simulation

To study the mode-I failure and mechanical behavior of rock under cyclic loading, the cracked chevron notched Brazilian disc (CCNBD) of sandstone was employed in this paper, and thus three different upper-limit loads cyclic tests were carried out, including 95 %SUL(static ultimate load) ∼ 35 %SUL, 85 %SUL ∼ 25 %SUL, 75 %SUL ∼ 15 %SUL. The 3D discrete element model of CCNBD specimen was built to study mechanical properties and cracking behavior at a meso-scale. Meanwhile, the effect of cyclic loading on microstructure characteristics and fracture process zone (FPZ) was discussed. The results show that cyclic loading has a weakening effect on fracture toughness, and a modified fracture toughness calculation method was proposed considering the effect of irreversible plastic deformation under cyclic loading. There are obvious stages of microcracking behavior under cyclic loading, specific for the condition of 85 %SUL ∼ 25 %SUL, and the microcrack density is positively correlated with the number of cycles. Meanwhile, the effect of cyclic load on sandstone microstructure contains weak-structure fracture effect and compaction effect, and they exist simultaneously and work together. In addition, cyclic loading has a great influence on the length of FPZ, and the length of FPZ under cyclic loading is greater than that under monotonic loading.

Mode I fracture propagation and post-peak behavior of sandstone: Insight from AE and DIC observation

Microcracks generated at the crack tip, particularly in quasi-brittle materials such as sandstone, give rise to a non-linear region known as the fracture process zone (FPZ), which cannot be analyzed using linear elastic fracture mechanics (LEFM). To gain insights into the nonlinear fracturing behavior of rock, this study conducted three-point bending experiments on sandstone beams with prefabricated notch. Stable mode-Ⅰ fracture propagation during the post-peak stage was achieved by the monotonic increment of crack mouth opening displacement (CMOD) measured by a clip gauge. The acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to visually track the detailed crack propagation process from both micro and macro perspectives. Nonlinear fracture parameters, including FPZ length, were identified by AE and DIC measurements and compared with the theoretical calculation. A nonlinear fracture criterion relating applied load, equivalent crack length, and stress intensity factor were established, from which the theoretical CMOD-P curve at the post-peak stage were obtained and validated with experimental measurement from MTS machine. The results reveal that microcracks formulating the FPZ materialize in the pre-peak stage, with the majority emerging post-peak to facilitate the propagation of the traction-free crack and FPZ in the loading direction, as evidenced by AE locations and DIC displacement gradient. Throughout the crack propagation process, over 65 % of microcracks are estimated to be tensile mode based on both AF/RA and polarity analysis, aligning with anticipated outcomes from macroscopic three-point bending fracture tests. Energy dissipation primarily occurs within the FPZ to facilitate separation of crack faces, with the dissipated energy region advancing towards the beam boundary. By integrating AE source locations and DIC displacement gradient, the FPZ length of the 3-point-bending sandstone specimens is determined to be lp = 8.1 mm, slightly exceeding the theoretical FPZ lengths calculated by the Irwin model (lp = 7.0 mm) and the Dugdale-Barenblatt model (lp = 7.6 mm). Using a non-linear fracture criterion that incorporates the FPZ and employing the in-time equivalent crack length measured by AE and DIC as an intermediate variable, the theoretical applied load-crack length curve during the post-peak stage is calculated, showing close agreement with load cell measurements with a negligible error of approximately 2 %. Furthermore, the theoretical post-peak CMOD vs. load curve aligns with experimentally obtained results, demonstrating a non-linear crack propagation speed at a constant CMOD rate, wherein crack propagation speed decreases as crack length increases.

Prediction of crack initiation for notched concrete beam from FPZ at maximum load

The initial cracking load (Pi) of a notched concrete beam, difficult to measure, has been predicted from the maximum fracture load (Pmax) together with the notch-tip fracture process zone (FPZ) at Pmax. A simple analytical model is used to link Pi and Pmax together through the FPZ. The corresponding initiation and unstable fracture toughness, Ki and Kun, have also been determined. The model assumes that the lower limit of Pmax from a group of identical notched concrete beams corresponds to the lower limit of the FPZ length FPZL, so that the lowest limit Pmax = Pi is established for FPZL = 0. Detailed FPZ measurements before and at Pmax using a digital image correlation technique were analysed and used to predict the initial cracking load Pi. Six different specimens with various sizes (W = 40, 60 and 80 mm), initial notches (a0 = 12 to 48 mm) and FPZL (7 to 13 mm) at Pmax showed the average ratio of FPZL/(W – a0) was around 0.25, indicating the Pi/Pmax ratio was around 0.67 based on the analytical model. The crack growth resistance KR-curve between the crack initiation toughness Ki at Pi and the unstable fracture toughness Kun at Pmax was also established approximately by the simple model. Estimated intrinsic fracture toughness KIC was compared with Ki and Kun. The influence of average aggregate size dav on Pi and FPZL at Pmax was also discussed.

The effect of fracture growth rate on the fracture process zone of salt rock after heat treatment

The importance of rate on the mechanical properties of rocks has been widely acknowledged by numerous scholars. Currently, most researchers mainly focus on investigating the influence of loading rate, while there is limited research on the effects of fracture growth rate on mechanical properties such as fracture process zone (FPZ) and fracture toughness of rocks. In order to explore the impact of fracture growth rate on the mechanical properties of rocks, this study conducted three-point bending pure I-mode fracture experiments on salt rocks at temperatures ranging from ambient to 700 °C using different quasi-static loading rates (0.02–2 mm/min). Meanwhile, digital image correlation (DIC) technology was employed to monitor displacement and strain variations, identify and calculate the fracture growth rate and FPZ size, and subsequently compute the corrected fracture toughness. The study discussed the effects of temperature and fracture growth rate on the FPZ and fracture toughness of rocks. The following main conclusions are drawn based on the analyzed results: (1) Temperature and loading rate have a great influence on the onset of FPZ in salt rocks. The higher the temperature and loading rate, the earlier the onset of FPZ. (2) With the increase of the average fracture growth rate, the FPZ length at the peak load of rock salt and the modified fracture toughness increased continuously, and was divided into two stages, in the stage of the crack extension rate generated by the loading rate of 0.02 mm/min-1 mm/min grew faster, and the growth of the crack extension rate generated by the loading rate of more than 1 mm/min became slower. (3) The effect of temperature on the fracture parameters of rock salt is significant, the peak load moment FPZ length and fracture toughness first increase and then slowly decrease with increasing temperature from room temperature to 700℃. The inflection point is at 500℃, where the fracture toughness and FPZ length reach the maximum value. (4) As the loading rate increases the proportion of transgranular fracture in the rock salt fracture surface gradually rises, and the crack extension path becomes smoother and smoother.

Investigation on the size effect of compacted clay mode I fracture based on NDB specimens

The size effect is closely related to the cracking resistance of compacted clay, which is crucial in engineering applications, but little attention has been given to it in previous studies. This study investigated the size effect on mode I fracture behaviors of compacted clay using single-edge notched deep beam (NDB) specimens. Through the digital image correlation (DIC) method, the crack initiation mechanism of compacted clay was analyzed. The results showed that the specimen size and dimensionless crack length did not change the failure pattern, while peak load, peak displacement, and fracture energy gradually increased with the increase of specimen size. Based on the load–displacement curves and the horizontal strain evolution laws, the three horizontal strain stages at the crack tip crack initiation point were defined, i.e., the steady strain growth stage, the nonlinear strain growth stage, and the nonlinear strain intensification stage. Through the Bažant size effect model (SEM), the fracture process zone length Cf and mode I fracture toughness KIC of compacted clay without the size effect were inferred. The separated form of SEM was applied, and the contribution ratios of the strength criterion and linear elastic fracture mechanics were quantitatively analyzed. Combing the characteristics of the NDB specimens, a prediction model considering the support spacing, specimen structure parameters, and fracture toughness was established.

Experimental study on mechanical properties and evolution of fracture failure of granite considering initial temperature damage and prefabricated fissure length

With the increasing mining depth of deep mineral resources, the influence of high temperature on the fracture failure of surrounding rock with fissures becomes increasingly serious. To investigate the fracture mechanical properties and the evolutionary characteristics of fracture process of fissured rock experiencing initial temperature damage, the three-point bending experiment was conducted on the small sized granite samples with different initial damaging temperatures and prefabricated fissure lengths by applying a μTS mesoscopic testing system. Based on the digital image correlation methods, the evolution of fracture process zone of sample was revealed. Moreover, the macroscopic fracture failure of sample was predicted by the evolution of time-varying acoustic emission b-value. The results show that with the increase of initial damaging temperature and prefabricated fissure length, the peak load of granite sample has a decreasing trend and the fracture characteristics of sample experiencing initial temperature damage gradually transform from brittleness fracture to ductility fracture, resulting in a decrease in the fracture toughness of sample. The crack tip opening displacement, the critical opening displacement and the length of fracture process zone of sample all show an increasing trend. The time interval from the time when time-varying acoustic emission b-value continues to decline to the appearance moment of macroscopic cracks is selected as the warning time of fracture failure of sample, which can conclude that the warning time increases with increasing the initial damaging temperature and prefabricated fissure length, indicating an increase in ductility and a reduction in failure suddenness of sample.

Effect of loading rate on mode I fracture behavior of red sandstone: Insights from AE and DIC techniques

The loading rate is an essential factor that influences the fracture characteristics and behavior of rock materials. In the study, a series of mode I fracture tests were performed on semi-circular bending (SCB) specimens of red sandstone under a range of static loading rates (0.005 mm/min–2 mm/min). The results showed that fracture toughness increases linearly with the logarithm of loading rates but the critical crack mouth opening displacement (CMODc) almost keeps constant. The fracture processes of rock were monitored using the acoustic emission (AE) and digital image correlation (DIC) techniques. The accumulated ringing counts increases with the loading rates at a natural logarithm relationship. The quantitative analysis for RA-AF data reveals that the contribution of shear cracks to mode I fracture increases with the loading rates. Prior to the DIC analysis, the DIC parameters were firstly calibrated by comparing the CMOD obtained from the clip gauge with that from the DIC data. By virtue of the strain and displacement fields, the fracture process, including post-peak fracture behavior, was analyzed using the cohesion zone model (CZM) in detail. Some fracture parameters, such as fracture process zone (FPZ) length and width, critical crack tip opening displacement (CTODc) were identified. The results indicate that FPZ length decreases with the loading rates but FPZ width and CTODc are nearly unaffected. The FPZ length at post-peak stage is lower than that at peak, regardless of the loading rate. Finally, the FPZ length measured by DIC is compared to that estimated by five theoretical models. The results show that the FPZ length calculated by the modified Irwin model is very closed to that from DIC.

Study on fracture characteristics and fracture mechanism of fully recycled aggregate concrete using AE and DIC techniques

Fracture behaviors of manufactured sand-recycled coarse aggregate concrete (MSRCAC), recycled fine aggregate concrete (RFAC), and fully recycled aggregate concrete (FRAC) were investigated by performing three-point bending tests on single-edge notched beams using acoustic emission (AE) and digital image correlation (DIC) techniques. The effects of recycled fine aggregate (RFA), recycled coarse aggregate (RCA), and their interaction on fracture parameters, AE characteristics and fracture process zone (FPZ) of concrete were comparatively analyzed. The results indicated that it was mainly transgranular failure and intergranular failure for RFAC, while transgranular failure for MSRCAC and FRAC. The RFA and RCA reduced the initial fracture toughness, unstable fracture toughness and fracture energy of concrete, and their interaction resulted in poor fracture properties of FRAC. In addition, the RFA and RCA significantly increased the AE activities, the proportion of tensile cracks, and the high-frequency amplitude and peak frequency of concrete. The toughening effect of FPZ was weakened by the RFA and RCA, resulting in an obvious increase in FPZ length and damage propagation of FRAC. Meanwhile, the width of FPZ was reduced, the maximum FPZ width of concrete with different combinations of recycled aggregates was approximately 1.8–3.1 times the maximum aggregate size, which could be explained as the RFA and RCA reduced the aggregate interlocking effect of concrete, while the RCA had a greater effect on the damage mechanism and FPZ of concrete than the RFA.

Evolution of the mixed mode FPZ and fracture roughness of Beishan granite after coupled thermo-hydro-mechanical treatment

Deep surrounding rocks contain a network of longitudinally intersecting fissures. Exploration of their fracture process zone (FPZ) size evolution during mixed loading and fracture surface roughness after failure has always been a hot topic, especially for coupled thermo–hydro–mechanical (THM) treatment. This study takes Beishan granite as the research object. The fracture surface roughness and FPZ length were obtained using Grasselli statistical and mixed strain–displacement methods. Finally, an electron microscope was employed to observe the fracture surface after failure at different temperatures and loading angles. Results reveal a notable temperature and load dependence of the fracture surface roughness and FPZ length evolution. In the low-temperature stage (25°C–300°C), the mineral primarily exhibited transgranular fractures and the average fracture surface roughness showed a slightly decrease with increasing temperature. In the high-temperature stage (500℃–650℃), the degree of internal damage to the prepared specimens substantially increased, with intergranular fractures being the dominant fracture mode, and rapid roughness growth occurred. Under the same temperature condition, the roughness value was in the order of mode I > mixed mode > mode II. The FPZ length increased, stabilized, and decreased as the load increased. In the low-temperature stage, the FPZ length reached its maximum value at the maximum load, whereas in the high-temperature stage, the FPZ length was fully developed before the peak load. Under the same loading angle condition, the FPZ lengths corresponding to the three loading modes followed the order of mode I < mixed mode < mode II.

Mechanical and fracture characteristics in center cracked circular discs of coal specimens with different sizes: An experimental investigation

The propagation of cracks on a small scale can result in dynamic disasters, such as rock bursts induced by large-scale fractures in coal seams. To gain insights into the mechanism of fracture-induced disasters in deep coal seams, this study investigates the micro-process and size effect of crack propagation in coal specimens featuring different sizes of center-crack circular discs (CCCD). We conducted Brazilian splitting tests on coal samples with varying dimensions and employed acoustic emission (AE) and digital image correlation (DIC) techniques to quantitatively describe the crack propagation process. Additionally, coal specimens of different diameters were compared for the size effect characteristics of different fracture parameters during the tensile failure process. The results show that as the specimen size increases, quasi-brittle failure characteristics become pronounced, tensile strength gradually decreases and stabilizes, while fracture toughness and fracture process zone length gradually increase and tend to stabilize. The macroscopic cracks in the coal specimens generally extend from the centre of the specimen to both ends along the precracks. Large energy events are mainly concentrated at the lower end of the precracks. As coal specimen size increases, the number of AE events decreases, while the proportion of large energy events gradually increases, and the critical crack opening quantity exhibits a linear increase with specimen size. By conducting the fracture size effect experiment on coal specimens, we extend the relevant mechanical parameters from laboratory-scale tests to field engineering scales. This provides fundamental parameters for the analysis of large-scale fracture process of in-situ coal based on fracture mechanics.

Shear response and fracture processes of hybrid steel fiber reinforced concrete

Ten hybrid steel fiber reinforced concrete mixtures are designed to analyze the influence of the hybridization of three different steel fibers (long hooked-end, medium hooked-end, short wave) on the shear performance by Z push-off shear tests. The presence of steel fibers improves the shear strength and toughness of the concrete, which increase with the volume fraction of hybrid fibers. The hybrid steel fiber reinforcement affects the fracture processes. Micro cracks propagation, cracks onset, cracks propagation and cracks post-peak propagation are the four main fracture stages. The volume fraction above 1.0% changes the evolution trend of fracture process zone length. The hybridization of the long hooked-end steel fiber with the other steel fibers produces a synergistic effect on inhibiting the propagation of the fracture process zone (FPZ) compared with the long hooked-end steel fiber reinforced concrete. The hooked-end steel fiber produces a better effect on slowing down the damage degree factor than short wave shaped steel fiber. The damage degree factor at the same loading level is reduced with the increase of the volume fraction of hybrid steel fiber.

Research on the mixed-mode fracture damage characteristics of shale soaked in different drilling fluids

Drilling fluid is a vital component in drilling engineering and can be considered as the lifeblood of the drilling. It serves multiple functions such as cooling the drill bit, cleaning the wellbore, and maintaining wellbore stability. However, when the drilling fluid penetrate into the formation, it significantly reduces the mechanical properties of the rocks thus making the wellbore prone to instability. This paper focuses on the studying the fracture characteristics of shale samples with different pre-crack angles (15°, 30°, 45°) when immersion in different types of drilling fluids (water-based drilling fluid, alcohol-based drilling fluid, oil-based drilling fluid) using the SCB (Semi-circular bending) test. During the experiment, digital image correlation and acoustic emission techniques were employed to monitor and analyze the fracture characteristics of shale. The research findings showed that as the pre-crack angle increases, the fracture toughness and effective fracture toughness of Mode I fractures decrease continuously, while the fracture toughness of Mode II fractures gradually increases. The impact of drilling fluids on the fracture toughness of shale samples is in the following order: water-based drilling fluid > alcohol-based drilling fluid > oil-based drilling fluid. The length of the fracture process zone (FPZ) decreased with the increase of the pre-crack angle, and the FPZ length under different drilling fluid immersion shows varying degrees of enlargement, with the following effect: water-based drilling fluid > alcohol-based drilling fluid > oil-based drilling fluid. The horizontal strain and horizontal displacement generated before the rock sample fractures increase with the increasing angle of the pre-crack. After immersion in drilling fluid, the layering of horizontal displacement field becomes more evident, with the degree of straightness of horizontal displacement affecting it in the following order: oil-based drilling fluid > alcohol-based drilling fluid > water-based drilling fluid. As the pre-crack angle of the SCB sample gradually increases, the relative decrease in acoustic emission energy becomes more pronounced, and the decrease in b-value becomes more significant. After immersion in drilling fluid, the acoustic emission becomes more pronounced, and the b-value becomes more disorderly and decreases further. Finally, by analyzing the fracture patterns of SCB samples immersed in different drilling fluids, relevant fracture mode diagrams were established. The findings could be useful in developing strategies for optimizing drilling fluid selection and fracture stimulation design in shale formations.

Research on mode-I fracture characteristics of basalt fiber reactive powder concrete

Basalt fiber reactive powder concrete (BFRPC) has strong potential application value because of its green environmental protection and good comprehensive properties. However, basalt fiber content is the important factor affecting the mode-I fracture characteristics of reactive powder concrete (RPC). Through the three-point bending fracture experiments, the effect of different basalt fiber content (0 %∼2.0 %) on the fracture parameters of RPC was studied. Based on DIC technology, the crack propagation and fracture process zone (FPZ) evolution of BFRPC were deeply studied, and the deformation and failure ability of BFRPC was evaluated by horizontal displacement standard deviation. The results showed that the double-K fracture toughness of RPC reaches the maximum (0.71 and 1.78 MPa mm1/2) under the action of 1.5 % basalt fiber content. However, the interfacial fracture energy is positively correlated with the basalt fiber content. Due to the bridging and restraint effect of basalt fiber on RPC, an appropriate amount of basalt fiber (0.5%–1.5 %) can effectively reduce the length of FPZ. The basalt fiber content has a positive effect on the growth of horizontal displacement standard deviation of BFRPC, which further highlights the excellent ability of BFRPC to resist deformation and failure.