A method for testing the uniaxial compression mechanical properties of prefabricated fissured sandstone constrained by CFRP hoop

Abstract

This invention discloses a method for testing the uniaxial compressive mechanical properties of CFRP-constrained pre-fractured sandstone. Addressing the problem of low test data reliability caused by secondary rock damage resulting from metal anchor insertion, this invention screens the physical properties and composition of the original sandstone sample and processes it into a cylindrical specimen with pre-fabricated fractures. Carbon fiber cloth is impregnated onto the surface of the cylindrical specimen according to a predetermined scheme using an impregnating adhesive. The circumferentially constrained cylindrical specimen is positioned in the testing equipment, and a uniaxial compressive load is applied using a displacement control mode while simultaneously acquiring stress and strain data. Energy evolution parameters are calculated based on the acquired data. Microscopic statistical damage variables and macroscopic initial fracture damage variables are calculated, and a damage stress-strain constitutive model is constructed and calibrated after introducing a circumferential constraint coefficient for correction. This invention avoids secondary physical damage and improves the reliability and repeatability of test data.

Description

Technical Field

[0001] This invention relates to the field of indoor testing technology for rock mechanics, specifically a method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone. Background Technology

[0002] With the continuous advancement of underground space development and deep resource extraction projects, the stability control of deep rock masses under complex geostress environments has become a crucial aspect of underground engineering. During deep tunnel excavation, the surrounding rock often experiences large deformations and crack propagation due to excavation unloading, necessitating effective support measures to ensure project safety. Currently, pre-anchor support is the most widely used surrounding rock reinforcement method in underground tunnel construction. Pre-anchor support improves the stress state of fractured rock masses by providing constraint stress within the rock mass. To study the mechanical response and synergistic deformation and failure mechanism of anchored rock masses, corresponding rock mechanics simulation tests need to be conducted in the laboratory.

[0003] In existing indoor rock mechanics simulation tests involving anchored rock masses, researchers typically use physically scaled models to investigate the effectiveness of anchoring support. The standard experimental procedure involves selecting natural rock and processing it into standard specimens. Holes are drilled inside the rock specimens using mechanical drilling equipment, and metal materials are inserted to approximate metal anchor bolts in actual engineering applications. After the metal materials are inserted, anchoring agents or adhesives are used to solidify the metal materials to the rock wall of the hole. Once the anchoring agents or adhesives have cured, the specimens are placed on a compression testing machine for compression loading to observe and obtain the strength characteristics, deformation patterns, and failure modes of the rock mass under anchored conditions.

[0004] However, existing indoor rock mechanics testing methods using metal materials to simulate anchor bolts have testing limitations. Because the rock specimens used in indoor tests are small, the mechanical drilling and implantation of rigid metal materials inside the specimens causes secondary physical damage to the original microstructure, altering the original mechanical conduction path of the rock. Furthermore, conventional machining generates cutting disturbances, which, combined with the inherent individual differences in composition and porosity of natural rock samples, introduce uncontrollable interference factors during loading, reducing the reliability of test data in rock support simulation tests. This results in poor repeatability and makes it difficult to accurately reflect the real mechanical behavior of surrounding rock support. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone. This method solves the problem in existing indoor simulation tests for deep rock support where mechanical drilling and the implantation of rigid metal materials inside small-sized rock specimens cause secondary physical damage to the original microstructure of the rock, thereby altering the inherent force transmission path of the rock.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone, comprising: The original sandstone samples were screened for physical properties and composition, and then processed into cylindrical specimens with prefabricated cracks with set geometric dimensions and dip angles. Prepare an impregnation resin, and use the impregnation resin to wrap the carbon fiber cloth circumferentially on the surface of the cylindrical specimen according to the set circumferential constraint scheme, and cure to form a carbon fiber cloth circumferentially constrained cylindrical specimen. A circumferentially constrained cylindrical specimen made of carbon fiber cloth was positioned in the testing equipment. A uniaxial compressive load was applied using a displacement control mode, and stress and strain data were collected simultaneously until the circumferentially constrained cylindrical specimen made of carbon fiber cloth became unstable and failed. Mechanical parameters were obtained and failure modes were identified based on stress and strain data. Energy evolution parameters of a circumferentially constrained cylindrical specimen were calculated using energy theory. The micro-statistical damage variables and the initial damage variables caused by macro-cracks are calculated. A circumferential constraint coefficient is introduced to correct the inhibitory effect of carbon fiber cloth on crack opening. A damage stress-strain constitutive model is constructed by coupling the micro-statistical damage variables and the corrected initial damage variables. The circumferential constraint coefficient is calibrated using the obtained test data.

[0007] Preferably, the steps for screening the physical properties and composition of the original sandstone samples include: observing the apparent characteristics of the original sandstone samples and removing samples with joints and cracks on the surface; measuring the porosity of the original sandstone samples and conducting P-wave velocity tests, classifying and grouping the original sandstone samples with similar P-wave velocities; using compositional analysis techniques to determine the internal mineral and chemical composition of the original sandstone samples, removing samples with excessively large differences based on the composition and content ratios, and obtaining original sandstone samples that are close to isotropic.

[0008] Preferably, the step of processing the raw sandstone sample into a cylindrical specimen with a set geometric size and a set inclination angle includes: processing the screened raw sandstone sample into a cylindrical specimen of a set size, and controlling the flatness, parallelism and smoothness of the cylindrical specimen end face to meet the preset standards during the processing; and using a high-pressure water jet to process pre-fabricated cracks with a set length, a set height and different pre-fabricated crack inclination angles on different cylindrical specimens.

[0009] Preferably, the steps of preparing an impregnating adhesive and using the impregnating adhesive to circumferentially wrap carbon fiber cloth onto the surface of a cylindrical specimen according to a set circumferential constraint scheme, and curing to form a circumferentially constrained cylindrical specimen of carbon fiber cloth, include: cutting the carbon fiber cloth into strips; mixing the main agent and curing agent of the impregnating adhesive according to a set ratio and stirring evenly, and allowing it to stand at room temperature to fully react; after drying and cooling the cylindrical specimen, sequentially pasting the carbon fiber cloth coated with the impregnating adhesive onto the surface of the cylindrical specimen according to a set scheme with different gradient circumferential constraint areas, and curing it at room temperature for a preset time.

[0010] Preferably, the steps of positioning the circumferentially constrained cylindrical specimen of carbon fiber cloth in the testing equipment, applying a uniaxial compressive load using a displacement control mode, and simultaneously acquiring stress and strain data include: installing displacement sensing components and strain sensing components, including axial extensometers and radial extensometers, on the surface of the circumferentially constrained cylindrical specimen of carbon fiber cloth, and connecting each sensing component to a dynamic and static strain testing and analysis system; starting a microcomputer-controlled electro-hydraulic servo pressure testing machine, maintaining a set constant loading rate to continuously apply uniaxial compressive load to the circumferentially constrained cylindrical specimen of carbon fiber cloth until instability and failure occur; and simultaneously acquiring stress and strain data through the dynamic and static strain testing and analysis system, measuring the uniaxial compressive strength, and obtaining the elastic modulus and Poisson's ratio.

[0011] Preferably, the steps of obtaining mechanical parameters and identifying failure modes based on stress and strain data, and calculating the energy evolution parameters of the carbon fiber cloth circumferentially constrained cylindrical specimen during deformation using energy theory, include: dividing the compression deformation process of the carbon fiber cloth circumferentially constrained cylindrical specimen into a compaction stage, a linear elastic stage, a plastic stage, and a post-peak stage; extracting peak strength, initial elastic modulus, peak strain, and Poisson's ratio parameters; observing the crack initiation and propagation path on the surface of the carbon fiber cloth circumferentially constrained cylindrical specimen, identifying the failure mode as a tensile-shear hybrid failure with tensile failure as the main component and shear failure as a secondary component; calculating the total energy by integrating and accumulating the stress-strain curve using the principle of calculus; calculating the elastic strain energy based on the initial elastic modulus and the first principal stress according to the generalized Hooke's law and the condition that the second and third principal stresses are zero under uniaxial compression; and calculating the dissipated energy by the difference between the total energy and the elastic strain energy.

[0012] Preferably, the steps for calculating the micro-statistical damage variables include: assuming that the micro-defects inside the circumferentially constrained cylindrical specimen of carbon fiber cloth follow a Weibull distribution, and deriving the micro-statistical damage variables by combining the probability density function integral; establishing a constitutive model of the initial damage of pre-fabricated fractured sandstone under uniaxial compression based on the Lematre equivalent strain assumption; establishing the micro-element strength distribution variables based on the DP strength criterion, substituting the first invariant of the stress tensor and the second invariant of the stress deviatoric tensor into the expression of the micro-element strength distribution variables, and obtaining the stress-strain relationship of sandstone with micro-damage under uniaxial compression by combining the generalized Hooke's law; and solving for the Weibull distribution parameters by combining the boundary conditions at the peak stress and peak strain.

[0013] Preferably, the step of calculating the initial damage variable caused by macroscopic cracks includes: establishing an elastic strain energy model of a circumferentially constrained cylindrical specimen with pre-existing cracks; deriving the uncorrected initial damage variable caused by macroscopic cracks by utilizing the relationship that the change in strain energy is equal to the increase in additional strain energy caused by the presence of joints in the elastic body; calculating the normal stress, shear stress, and slip driving force on the surface of the pre-existing crack under uniaxial compressive load; substituting the critical value state parameter of the crack propagation length into the fracture mechanics model to calculate the initial stress intensity factor; and substituting the initial stress intensity factor into the model to obtain the uncorrected initial damage variable caused by macroscopic cracks.

[0014] Preferably, the steps of introducing a circumferential constraint coefficient to correct the inhibitory effect of carbon fiber cloth on crack opening, and coupling microscopic statistical damage variables with the corrected initial damage variables to construct a damage stress-strain constitutive model include: calculating the circumferential constraint stress of the carbon fiber cloth by combining the carbon fiber cloth thickness, elastic modulus, tensile strain, and cylindrical specimen diameter; introducing the circumferential constraint coefficient to calculate the corrected Type I stress intensity factor, substituting the corrected Type I stress intensity factor into the energy integral equation to obtain the initial damage variable corrected based on the energy method; deriving the total damage variable by coupling the microscopic statistical damage variables with the corrected initial damage variables based on the principle of equivalent strain assumption, and using the total damage variable to establish the final damage stress-strain constitutive model.

[0015] This invention provides a method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone. It has the following advantages: 1. This invention provides passive lateral confining pressure by applying circumferential constraints to sandstone specimens with pre-fabricated fractures using carbon fiber cloth, simulating the structural constraint effect of pre-installed anchors on surrounding rock in deep underground engineering. This invention avoids the secondary physical damage to the internal microstructure of the rock specimen caused by drilling and implanting metal anchors in traditional tests. Combined with original rock composition screening, high-pressure water jet non-destructive cutting, and a displacement control mode with extremely low loading rates, it eliminates the interference of individual specimen differences and mechanical processing disturbances on the test results, improving the reliability and repeatability of rock support simulation test data.

[0016] 2. This invention reveals the evolution of mechanical behavior of pre-fractured sandstone under carbon fiber cloth confinement by extracting stress and strain data throughout the entire uniaxial compression loading process. The circumferential confinement of the carbon fiber cloth inhibits the radial expansion deformation of the rock during compression, increases the internal frictional resistance of the rock, slows down the slippage and opening of microfractures, and transforms the failure mode of the specimen from simple shear failure to a mixed tensile and shear failure mode, thereby improving the rock's energy storage limit and post-peak ductile bearing capacity.

[0017] 3. This invention constructs a damage constitutive model coupling microscopic defects and macroscopic cracks. In terms of model construction principles, the Weibull probability distribution function is used to quantify the random distribution of microscopic defects within the specimen, and microscopic statistical damage variables are derived based on stress tensor invariants and the generalized Hooke's law. The slip driving force and initial stress intensity factor of the macroscopic pre-fabricated cracks under load are calculated based on energy theory and fracture mechanics principles. A circumferential constraint coefficient is introduced into the theoretical model, quantifying the circumferential constraint stress provided by the carbon fiber cloth as a correction term to suppress the propagation of type I tensile cracks, thus deriving the corrected macroscopic crack damage variables. By combining the equivalent strain assumption, the microscopic and macroscopic damage variables are coupled to establish a damage constitutive equation. The circumferential constraint coefficient is calibrated through backfitting measured stress and strain data, achieving closed-loop verification of the theoretical model and standardized testing methods. Attached Figure Description

[0018] Figure 1 This is a flowchart of the present invention; Figure 2 This is a schematic diagram of the deformation and failure of fractured surrounding rock-advanced support according to the present invention; Figure 3 This is a diagram showing the relationship between strain energy and dissipated energy in the rock according to the present invention. Figure 4 This is a schematic diagram of the crack propagation in the wing of the present invention; Figure 5 This is a diagram illustrating the damage mechanism of carbon fiber cloth circumferentially constrained fractured sandstone according to the present invention. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Please see the appendix Figure 1-5 This invention provides a flowchart of a method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone. The method relies on a uniaxial compression testing physical system comprising a loading device, a data acquisition device, and the CFRP-confined fractured sandstone specimen to be tested.

[0021] This document provides the specific implementation details of the functional components within the aforementioned system architecture. The loading device employs a microcomputer-controlled electro-hydraulic servo pressure testing machine, specifically a YAW-2000D model. The loading device is equipped with a control system suitable for force, displacement, and deformation control, used to apply continuous uniaxial compressive loads to the prepared rock specimens. The data acquisition device utilizes a dynamic and static strain testing and analysis system, specifically a DH3817F system, used to synchronously collect, process, and output the stress and strain data of the specimens during the loading process. The uniaxial compression testing physical system is equipped with strain and displacement sensing components at its front end, including an axial extensometer for measuring axial deformation, a chain-type radial extensometer for measuring radial deformation, and strain gauges fixed to the rock surface. The data transmission channels of the sensing components are physically connected to the dynamic and static strain testing and analysis system. Through the design of this hardware architecture, a testing environment suitable for simulating the coordinated deformation of surrounding rock and advanced support systems in deep tunnels under laboratory conditions has been constructed. In this invention, by using carbon fiber cloth to apply circumferential constraints to fractured rock masses, the effective structural constraints applied to the surrounding rock by advanced anchor bolts in actual engineering can be approximately simulated, solving the technical problem that directly using metal materials to simulate anchor bolts will cause damage to rock specimens and introduce other interference factors.

[0022] Under the above-mentioned testing system architecture, the present invention provides a method for testing the uniaxial compressive mechanical properties of CFRP-constrained precast fractured sandstone. The execution workflow includes the following steps: S1. Screening of Protolith and Preparation of Standard Samples. The original sandstone obtained from mining underwent preliminary screening based on its physical properties. X-ray diffraction and X-ray fluorescence spectrometry were used to determine the mineral and chemical composition of the original sandstone, ensuring isomorphism among samples from the same batch. The original sandstone meeting the standards was processed into cylindrical specimens of a predetermined size using rock processing equipment. Pre-fabricated fractures with predetermined geometric dimensions and inclination angles were then cut into the center of the cylindrical specimens using a high-pressure water jet.

[0023] S2, Standardized application of CFRP circumferential constraints. Pre-treatment of the cylindrical specimen with pre-existing cracks involves drying, dehydration, and room temperature cooling. The impregnating adhesive's base and curing agent are prepared in a specific ratio, cured at room temperature for a specified time to ensure maximum bond strength, and then evenly applied to the surface of carbon fiber cloth cut to the specified dimensions. The carbon fiber cloth is then circumferentially wrapped and adhered to the surface of the pre-existing cracked cylindrical specimen according to different constraint area schemes, and left to cure at room temperature for a specific time to ensure full adhesion between the carbon fiber cloth and the rock.

[0024] S3, Uniaxial Compression Loading and Synchronous Data Acquisition. The cured carbon fiber cloth circumferentially constrained cylindrical specimen is positioned at the center of the pressure platform of a microcomputer-controlled electro-hydraulic servo pressure testing machine. Strain gauges and displacement sensing components are installed on the surface of the carbon fiber cloth circumferentially constrained cylindrical specimen and connected to the dynamic and static strain testing and analysis system. The servo loading program is started, applying a uniaxial compressive load to the carbon fiber cloth circumferentially constrained cylindrical specimen at a constant displacement-controlled loading rate until the specimen becomes unstable and fails. Load and displacement data are recorded synchronously during the loading cycle to obtain the stress-strain curve of the entire deformation and failure process.

[0025] S4. Mechanical Performance Evaluation and Energy Evolution Analysis. Based on the acquired stress-strain curves, the compression process is divided into compaction, linear elastic, plastic, and post-peak stages. Peak strength, elastic modulus, peak strain, and Poisson's ratio mechanical parameters of the rock under different constraint states are extracted. The macroscopic principal crack propagation direction and failure mode of the circumferentially constrained cylindrical specimens with different constraint areas and crack inclination angles are observed and recorded. Based on the first law of thermodynamics and the principle of calculus under uniaxial compression, the total energy, elastic strain energy, and dissipated energy of the circumferentially constrained cylindrical specimens during deformation are calculated.

[0026] S5, Damage Constitutive Model Construction and Constraint Parameter Calibration. A statistical damage variable reflecting the random distribution of microscopic defects within the sandstone is defined using the Weibull probability distribution function. Based on fracture mechanics theory and the stress intensity factor at the tip of the pre-fabricated fracture, the initial damage variable caused by macroscopic fractures is calculated. A circumferential constraint coefficient, influenced by spatial geometric effects, is introduced to correct the inhibitory effect of the passive constraint stress of carbon fiber cloth on the opening of type I tensile cracks. Based on the equivalent strain assumption, coupling the microscopic statistical damage variable and the initial damage variable, a fractured sandstone damage constitutive model describing the circumferential constraint effect of CFRP is constructed. Using the stress-strain curve data extracted in step S4, the circumferential constraint coefficient is mathematically fitted and calibrated, completing the theoretical characterization and verification of the coordinated deformation behavior of the surrounding rock-support system.

[0027] The S1 step, which involves screening the original test rocks and preparing standard samples, specifically includes the following sub-steps: S101. Obtain raw sandstone samples and perform physical property and ultrasonic velocity screening. Raw sandstone samples were obtained from the deep underground engineering site; specifically, raw sandstone from the Gaojialiang Coal Mine in Ordos City, Inner Mongolia Autonomous Region was selected. All selected raw sandstone samples originated from the same rock sample. The apparent characteristics of the raw sandstone samples were observed, and samples with surface joints and cracks were discarded, retaining those that were naturally gray and had uniform grain size. The porosity of the raw sandstone samples was measured, and samples with porosity meeting the preset value were screened out; in this case, the porosity was recorded as 3.42%. P-wave velocity testing was performed on the raw sandstone samples that had undergone initial screening based on appearance and porosity. Based on the P-wave velocity test results, raw sandstone samples with similar P-wave velocities were classified and grouped.

[0028] S102, Mineral and Chemical Composition Analysis of the Original Sandstone Samples. For the original sandstone samples screened and grouped in step S101, X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF) were used to determine the internal mineral and chemical composition. The XRD analysis was performed using a Rigaku TTR-III X-ray diffractometer (Japan), and the XRF analysis was performed using a Panalytical Axios XRF spectrometer (Netherlands). The mass percentage of the chemical substances within the original sandstone samples was determined. Specifically, the main chemical components detected in the original sandstone samples included silica, aluminum oxide, calcium oxide, ferric oxide, magnesium oxide, and potassium oxide. The specific percentage distribution of these components was as follows: silica 63.7%, aluminum oxide 16.3%, calcium oxide 7.4%, ferric oxide 5.2%, magnesium oxide 2.5%, and potassium oxide 2.4%. By comparing the chemical composition and content ratio, original sandstone samples with excessively large compositional differences were eliminated, ensuring that the retained original sandstone samples were nearly isotropic.

[0029] After screening the physicochemical properties of the original sandstone samples, the cylindrical specimen processing and fracture prefabrication process is carried out, which includes the following sub-steps: S103, Processing cylindrical specimens. According to the "Test Procedure for Physical and Mechanical Properties of Rocks," the screened raw sandstone samples were processed into cylindrical specimens with dimensions of 50mm × 100mm. During the processing of the cylindrical specimens, the flatness, parallelism, and smoothness of the cylindrical specimen end faces were controlled to meet the relevant experimental standards.

[0030] S104, Pre-fabricated Cracks. High-pressure water jetting was used to pre-fabricate cracks on cylindrical specimens. The pre-fabricated crack length L was 20 mm, and the pre-fabricated crack height H was 1 mm. Pre-fabricated cracks with different inclination angles were fabricated on different cylindrical specimens; the inclination angles θ were set to 0°, 30°, 45°, 60°, and 90°, respectively.

[0031] Step S2 specifically includes the following sub-steps: S201, Preparation of carbon fiber cloth for constraint. The carbon fiber cloth is cut into strips 10mm wide. The length of the cut carbon fiber cloth is slightly longer than the circumference of the cylindrical specimen to facilitate adhesive bonding. The length deviation of the carbon fiber cloth is ±1.5%, the width deviation is ±0.5%, and the unit area mass deviation is ±3.0%. The technical parameters of the carbon fiber cloth are divided into high-strength Class I and high-strength Class II standards. When the carbon fiber cloth meets the high-strength Class I standard, the parameters include: unit area mass of 300g / m². 2The theoretical thickness is 0.167 mm, the standard value of tensile strength is greater than or equal to 3400 MPa, the tensile modulus of elasticity is greater than or equal to 230 GPa, the elongation is greater than or equal to 1.6%, the flexural strength is greater than or equal to 700 MPa, and the interlaminar shear strength is greater than or equal to 45 MPa. When carbon fiber cloth meets the high-strength Class II standard, the parameters of the carbon fiber cloth include: a unit area mass of 200 g / m². 2 The theoretical thickness is 0.111 mm, the standard value of tensile strength is greater than or equal to 3000 MPa, the tensile modulus of elasticity is greater than or equal to 200 GPa, the elongation is greater than or equal to 1.5%, the bending strength is greater than or equal to 600 MPa, and the interlaminar shear strength is greater than or equal to 35 MPa.

[0032] S202, Preparation of Impregnating Adhesive. Mix the base agent and curing agent of the impregnating adhesive in a 2:1 ratio. Use an electric mixer to thoroughly stir the mixed base agent and curing agent. Let the thoroughly stirred base agent and curing agent stand at room temperature for 1 hour to allow them to fully react and maximize the adhesive strength. The parameters of the impregnating adhesive are divided into colloidal performance parameters and adhesive strength parameters. The colloidal performance of the impregnating adhesive must meet the following requirements: tensile strength greater than or equal to 38 MPa, tensile modulus of elasticity greater than or equal to 2400 MPa, elongation greater than or equal to 1.5%, flexural strength greater than or equal to 50 MPa without fracture, and compressive strength of 70 MPa. The adhesive strength of the impregnating adhesive must meet the following requirement: tensile strength greater than or equal to 38 MPa.

[0033] Step S3 specifically includes the following sub-steps: S301. Position the cylindrical specimen and set up the strain acquisition system. The cured cylindrical specimen is positioned in a computer-controlled electro-hydraulic servo pressure testing machine. This machine supports force control, displacement control, and deformation control. Specifically, the YAW-2000D model is used. Sensors for acquiring strain data are placed on the cylindrical specimen and connected to a dynamic and static strain testing and analysis system. Specifically, the DH3817F dynamic and static strain testing and analysis system, customized by Jiangsu Donghua Testing Technology Co., Ltd., is used. The measured strain data is collected, processed, and output as images on the host computer via this system.

[0034] S302, Perform displacement loading and synchronous data acquisition. Start the microcomputer-controlled electro-hydraulic servo pressure testing machine to load the cylindrical specimen. In the uniaxial compression test, the microcomputer-controlled electro-hydraulic servo pressure testing machine applies axial pressure to the cylindrical specimen using displacement loading. The loading rate of the microcomputer-controlled electro-hydraulic servo pressure testing machine is set to 0.05 mm / min. The loading rate of 0.05 mm / min is maintained continuously on the cylindrical specimen until it becomes unstable and fails. During the loading process of the microcomputer-controlled electro-hydraulic servo pressure testing machine, the strain data of the cylindrical specimen is synchronously acquired through the dynamic and static strain testing and analysis system, the uniaxial compressive strength of the cylindrical specimen is measured, and the elastic modulus and Poisson's ratio mechanical parameters of the cylindrical specimen are obtained.

[0035] Step S4 specifically includes the following sub-steps: S401, stress-strain stages were divided and mechanical parameters were extracted. Based on the stress-strain curves of the circumferentially constrained cylindrical specimen obtained during uniaxial compression loading, the compression deformation process of the circumferentially constrained cylindrical specimen was divided into four stages: compaction stage, linear elastic stage, plastic stage, and post-peak stage. In the compaction stage, the stress-strain curve showed a concave trend, and the microcracks inside the circumferentially constrained cylindrical specimen closed. In the linear elastic stage, the slope of the stress-strain curve increased and became linear, and the stress and strain of the circumferentially constrained cylindrical specimen satisfied Hooke's law. In the plastic stage, the slope of the stress-strain curve decreased, and the circumferentially constrained cylindrical specimen underwent irreversible plastic deformation and new cracks. In the post-peak stage, after exceeding the bearing capacity limit, due to the circumferential constraint of the carbon fiber cloth, the stress-strain curve of the circumferentially constrained cylindrical specimen showed a step-like stress drop, indicating ductile failure. The peak strength, initial elastic modulus, peak strain, and Poisson's ratio of circumferentially constrained cylindrical specimens with different carbon fiber cloth confinement areas and different pre-fabricated crack inclination angles were extracted from the stress-strain curves.

[0036] S402, Identifying the failure mode of a circumferentially constrained cylindrical specimen with carbon fiber cloth. Observe the crack initiation and propagation path on the surface of the circumferentially constrained cylindrical specimen with carbon fiber cloth. The circumferential constraint of the carbon fiber cloth inhibits the radial deformation of the circumferentially constrained cylindrical specimen during uniaxial compression, leading to an increase in tensile cracks. Record the failure mode exhibited by the circumferentially constrained cylindrical specimen with carbon fiber cloth, which is characterized by tensile failure as the main component and shear failure as a secondary component, exhibiting a mixed tensile-shear failure. Record the initiation and penetration states of the tensioned wing crack near the tip of the pre-fabricated crack.

[0037] S403, calculate the energy evolution parameters of a circumferentially constrained cylindrical specimen made of carbon fiber cloth. Based on the first law of thermodynamics, the total work done on the circumferentially constrained cylindrical specimen by the external environment during the uniaxial compression test is converted into elastic strain energy and dissipated energy. The total energy is calculated using the stress-strain curve and the principle of calculus. The formula for calculating the total energy is: In the formula, Represents total energy. Represents the value of the first principal stress. Represents the principal strain value. The first representative stress-strain curve Stress values ​​at each data point The first representative stress-strain curve Strain values ​​at each data point The first representative stress-strain curve Stress values ​​at each data point The first representative stress-strain curve Strain values ​​at each data point Represents the data point number. This represents the total number of data points.

[0038] Calculate elastic strain energy using the generalized Hooke's law. The general formula for calculating elastic strain energy is: In the formula, Represents elastic strain energy. Represents the first principal stress. Represents the second principal stress. Represents the third principal stress. Represents the elastic modulus. It represents Poisson's ratio.

[0039] Under uniaxial compression test conditions, both the second and third principal stresses are zero, and the elastic modulus is taken as the initial elastic modulus. The formula for calculating the elastic strain energy is simplified to: In the formula, This represents the initial elastic modulus.

[0040] Dissipated energy is calculated as the difference between total energy and elastic strain energy. The formula for calculating dissipated energy is: In the formula, Representing dissipated energy. The total energy, elastic strain energy, and dissipated energy values ​​corresponding to different axial strains during the uniaxial compression test are extracted. Based on the numerical variation law, the damage evolution process of the circumferentially constrained cylindrical specimen of carbon fiber cloth is divided into the initial damage stage, the damage plateau stage, the damage mutation stage, and the damage failure stage.

[0041] In the uniaxial compressive mechanical property testing method, after completing test data acquisition and mechanical property evaluation, the next step is to construct a damage constitutive model and calibrate constraint parameters. This specifically includes the following sub-steps: S501, Calculate the damage variable of the random distribution of micro-defects. Assume that the micro-defects inside the circumferentially constrained cylindrical specimen of carbon fiber cloth follow a Weibull distribution, the probability density function of which is expressed as: In the formula, Represents the intensity distribution variable of micro-units. The Weibull distribution parameters represent the dispersion of internal defects in a circumferentially constrained cylindrical specimen made of carbon fiber cloth. The Weibull distribution parameter represents the average micro-element strength of a circumferentially constrained cylindrical specimen made of carbon fiber cloth.

[0042] Assuming the number of micro-units inside the circumferentially constrained cylindrical specimen of carbon fiber cloth is . The number of failed micro-units is By combining the integral of the probability density function, the micro-statistical damage variable is derived. Microstatistical damage variables Expressed as: Based on the Lematre equivalent strain assumption, an initial damage constitutive model for prefabricated fractured sandstone under uniaxial compression is established. The initial damage constitutive model is expressed as: In the formula, Represents the stress under uniaxial compression. Represents the strain corresponding to stress. This represents the elastic modulus of fractured sandstone.

[0043] The intensity distribution variables of micro-units are determined based on the DP intensity criterion. Micro-unit intensity distribution variable Expressed as: In the formula, Represents the internal friction coefficient of sandstone. Represents the first invariant of the stress tensor. This represents the second invariant of the stress deviatoric tensor. Substituting the stress tensor invariant into the expression for the intensity distribution variable of the micro-element, and combining it with the generalized Hooke's law, the stress-strain relationship of sandstone with microscopic damage under uniaxial compression is expressed as follows: In the formula, This represents the internal friction angle of sandstone.

[0044] Based on the stress-strain curves obtained from uniaxial compression tests, at the peak stress... With peak strain The boundary condition satisfies the condition that the derivative is zero. Solving for the boundary condition yields the Weibull distribution parameters. and The calculation formula is specifically expressed as follows: In the formula, The Weibull random distribution variable represents the peak stress of the circumferentially constrained cylindrical specimen made of carbon fiber cloth.

[0045] S502, Calculate macroscopic crack damage variables. Based on energy theory, an elastic strain energy model is established for a circumferentially constrained cylindrical specimen with pre-existing cracks, and the damage variables caused by the cracks are derived. Damage variables Expressed as: In the formula, Represents Poisson's ratio. This represents the volume of a circumferentially constrained cylindrical specimen made of carbon fiber cloth. Represents the area of ​​the precast crack. The Type I stress intensity factor representing the tip of a pre-existing crack. Type II stress intensity factor representing the tip of a pre-existing crack.

[0046] Calculate the normal stress on the pre-fabricated crack surface under uniaxial compressive loading. Shear stress and sliding drive force Replace the axial stress variable with... Normal stress on the surface of precast cracks With shear stress They are expressed as follows: Introducing pre-fabricated crack friction angle When the tangential fracture dip angle is greater than or equal to the tangential pre-fabricated fracture friction angle, the sliding driving force... Expressed as: In the formula, Represents the dip angle of the precast crack. This represents the friction angle of the precast crack.

[0047] Substituting the critical value of the crack propagation length as a state parameter into the fracture mechanics model, the initial stress intensity factor is calculated. Substituting the initial stress intensity factor into the damage variable formula yields the uncorrected macroscopic crack damage variable. Expressed as: In the formula, Represents the half-length of the precast crack. This represents the width of the cylindrical specimen.

[0048] S503: Perform circumferential constraint correction and complete constitutive model construction and parameter calibration. Quantify the passive confining pressure effect applied by the carbon fiber fabric and calculate the circumferential constraint stress of the carbon fiber fabric. Circumferential constraint stress Expressed as: In the formula, This represents the thickness of the carbon fiber cloth. This represents the elastic modulus of carbon fiber fabric. Represents the tensile strain of carbon fiber cloth. This represents the diameter of the cylindrical specimen.

[0049] The confinement effect of the carbon fiber cloth inhibited the opening of pre-existing cracks and reduced the Type I stress intensity factor. A circumferential confinement coefficient was introduced. Calculate the corrected Type I stress intensity factor , Expressed as: The corrected Type I stress intensity factor Substituting into the energy integral equation, we obtain the macroscopic crack damage variables corrected by the energy method. Expression, modified macroscopic crack damage variable Expressed as: In the formula, This represents the crack propagation angle of the wing and is set to 70.5°. This represents the thickness of the carbon fiber cloth. and Representing the same physical quantity; Represents shear stress; Represents a constant term. Equivalent to: .

[0050] Based on the principle of equivalent effect variation, coupling micro-statistical damage variables Compared with the corrected macroscopic crack damage variable The total damage variable of the circumferentially constrained cylindrical specimen with carbon fiber cloth was derived. Total damage variable Expressed as: Total damage variable obtained through derivation A damage constitutive model for fractured sandstone under circumferential constraints is established. The final damage constitutive model is expressed as follows: In the formula, Represents uniaxial compressive stress. Represents the initial elastic modulus. This represents uniaxial compressive strain.

[0051] Test data records under different constraint areas and pre-existing crack inclination angles obtained from uniaxial compression loading tests were retrieved, and the stress and strain data were substituted into the final damage constitutive model. The circumferential constraint coefficient was calculated using mathematical curve fitting methods. The specific calibration values ​​under different working conditions enable closed-loop verification of the entire process, from mechanical performance testing to the determination of theoretical model parameters.

[0052] For the uniaxial compressive mechanical property testing method of carbon fiber cloth circumferentially constrained precast fractured sandstone, the basic operation mechanism of rock sampling and processing, the surface layout of strain gauges and the test principle of bridge conversion, and the material calibration and analysis method of X-ray diffractometer involved, those skilled in the art can refer to the rock mechanics test operation manual and instrument usage specifications for implementation. The basic operation mechanism of rock sampling and processing, the surface layout of strain gauges and the test principle of bridge conversion, and the material calibration and analysis method of X-ray diffractometer are well known technologies in this field and will not be elaborated here.

[0053] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone, characterized in that, include: The original sandstone samples were screened for physical properties and composition, and then processed into cylindrical specimens with prefabricated cracks with set geometric dimensions and dip angles. Prepare an impregnation adhesive, and use the impregnation adhesive to wrap the carbon fiber cloth circumferentially on the surface of the cylindrical specimen according to the set circumferential constraint scheme, and cure to form a carbon fiber cloth circumferentially constrained cylindrical specimen. The circumferentially constrained cylindrical specimen of carbon fiber cloth is positioned in the testing equipment, and a uniaxial compressive load is applied using the displacement control mode. Stress data and strain data are collected simultaneously until the circumferentially constrained cylindrical specimen of carbon fiber cloth becomes unstable and fails. Based on the stress data and strain data, mechanical parameters are obtained and failure modes are identified. The energy evolution parameters of the carbon fiber cloth circumferentially constrained cylindrical specimen during deformation are calculated using energy theory. The micro-statistical damage variables and the initial damage variables caused by macro-cracks are calculated. A circumferential constraint coefficient is introduced to correct the inhibitory effect of the carbon fiber cloth on crack opening. The micro-statistical damage variables and the corrected initial damage variables are coupled to construct a damage stress-strain constitutive model. The circumferential constraint coefficient is calibrated using the obtained test data.

2. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps for screening the physical properties and composition of the original sandstone samples include: The apparent characteristics of the original sandstone samples were observed, and samples with joints and fissures on the surface were removed. The porosity of the original sandstone samples was determined, and the P-wave velocity was tested. Original sandstone samples with similar P-wave velocities were classified and grouped. The internal mineral and chemical composition of the original sandstone sample was determined using component analysis techniques. Based on the composition and content ratio, samples with excessively large differences were eliminated to obtain an original sandstone sample that is close to isotropic.

3. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The step of processing a cylindrical specimen with pre-fabricated cracks having a set geometric dimension and a set inclination angle includes: The screened raw sandstone samples are processed into cylindrical specimens of a set size, and the flatness, parallelism and smoothness of the cylindrical specimen end face are controlled to meet the preset standards during the processing. Pre-fabricated cracks with set lengths, set heights, and different pre-fabricated crack inclination angles are formed on different cylindrical specimens using high-pressure water jets.

4. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps of preparing the impregnation adhesive, using the impregnation adhesive to circumferentially wrap the carbon fiber cloth onto the surface of the cylindrical specimen according to a set circumferential constraint scheme, and curing to form a circumferentially constrained cylindrical specimen of carbon fiber cloth include: The carbon fiber cloth is cut into strips; The main component and curing agent of the impregnating adhesive are mixed in a set ratio and stirred evenly, and then left to stand at room temperature to allow for full reaction; After drying and cooling the cylindrical specimen, carbon fiber cloth coated with impregnating adhesive is sequentially pasted onto the surface of the cylindrical specimen according to a set scheme with different gradient circumferential constraint areas, and then cured at room temperature for a preset time.

5. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps of positioning the circumferentially constrained cylindrical specimen in the testing equipment, applying a uniaxial compressive load using a displacement control mode, and simultaneously acquiring stress and strain data include: A displacement sensing assembly, including an axial extensometer and a radial extensometer, and a strain sensing assembly are installed on the surface of the circumferentially constrained cylindrical specimen made of carbon fiber cloth, and each sensing assembly is connected to a dynamic and static strain testing and analysis system. Start the microcomputer-controlled electro-hydraulic servo pressure testing machine and maintain the set constant loading rate to continuously apply uniaxial compression loading to the circumferentially constrained cylindrical specimen of carbon fiber cloth until it becomes unstable and fails. The dynamic and static strain test and analysis system simultaneously collects stress and strain data, measures uniaxial compressive strength, and obtains elastic modulus and Poisson's ratio.

6. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps for calculating the energy evolution parameters of the circumferentially constrained cylindrical specimen during deformation using energy theory include: The compression deformation process of the circumferentially constrained cylindrical specimen of carbon fiber cloth is divided into the compaction stage, linear elastic stage, plastic stage and post-peak stage. The peak strength, initial elastic modulus, peak strain and Poisson's ratio parameters are extracted. The crack initiation and propagation path on the surface of the circumferentially constrained cylindrical specimen were observed, and the failure mode was identified as a mixed tensile-shear failure with tensile failure as the main component and shear failure as the secondary component. The total energy is calculated by integrating and accumulating the stress-strain curves using the principles of calculus. Based on the generalized Hooke's law and the condition that the second and third principal stresses are zero under uniaxial compression, the elastic strain energy is calculated based on the initial elastic modulus and the first principal stress. The dissipated energy is calculated by the difference between the total energy and the elastic strain energy.

7. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps for calculating the micro-statistical damage variable include: Assuming that the microscopic defects inside the circumferentially constrained cylindrical specimen of the carbon fiber cloth follow a Weibull distribution, the microscopic statistical damage variable is derived by combining the probability density function integral. Based on the Lematre equivalent strain assumption, an initial damage constitutive model of precast fractured sandstone under uniaxial compression is established. The intensity distribution variables of micro-element are established based on the DP strength criterion. The first invariant of the stress tensor and the second invariant of the stress deviator tensor are substituted into the expression of the intensity distribution variables of micro-element. The stress-strain relationship of sandstone with micro-damage under uniaxial compression is obtained by combining the generalized Hooke's law. The Weibull distribution parameters are obtained by combining the boundary conditions at the peak stress and peak strain.

8. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The step of calculating the initial damage variable caused by macroscopic cracks includes: An elastic strain energy model of a cylindrical specimen with pre-fabricated cracks and circumferentially constrained carbon fiber cloth was established. By utilizing the relationship that the change in strain energy is equal to the increase in additional strain energy caused by the presence of joints in the elastic body, the initial damage variables caused by the uncorrected macroscopic cracks were derived. Under uniaxial compressive load, calculate the normal stress, shear stress, and slip driving force on the precast crack surface; Substituting the critical value of the crack propagation length state parameter into the fracture mechanics model, the initial stress intensity factor is calculated. Substituting the initial stress intensity factor into the model yields the initial damage variable caused by the uncorrected macroscopic crack.

9. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The steps of introducing a circumferential constraint coefficient to correct the inhibitory effect of the carbon fiber cloth on crack opening, and coupling the microscopic statistical damage variables with the corrected initial damage variables to construct a damage stress-strain constitutive model include: The circumferential constraint stress of the carbon fiber cloth was calculated by combining the thickness, elastic modulus, tensile strain and diameter of the cylindrical specimen. Introducing the circumferential constraint coefficient, the corrected Type I stress intensity factor is calculated. Substituting the corrected Type I stress intensity factor into the energy integral equation, the initial damage variable corrected based on the energy method is obtained. Based on the principle of equivalent strain assumption, the total damage variable is derived by coupling the micro-statistical damage variable with the corrected initial damage variable, and the final damage stress-strain constitutive model is established using the total damage variable.

10. The method for testing the uniaxial compressive mechanical properties of CFRP-confined precast fractured sandstone according to claim 1, characterized in that, The cylindrical specimen has dimensions of 50mm × 100mm; the pre-fabricated crack has a length of 20mm and a height of 1mm; the carbon fiber cloth is cut into strips with a width of 10mm; the ratio of the impregnating agent to the curing agent is 2:1; and the set constant loading rate is 0.05mm / min.

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