Device and method for testing fracture performance and failure characteristics of phase-change clay

Abstract

The invention relates to a phase change clay fracture performance and failure characteristic test device, a prefabricated crack soil beam test piece, a servo motor, a sensor and a camera unit are arranged on a three-point / four-point bending moment loading frame, and the servo motor and the sensor are connected with a data acquisition unit. The invention also relates to a phase change clay fracture performance and failure characteristic test method. According to the invention, the compressive fracture process of the phase-change clay can be truly simulated, the fracture performance and damage characteristics of the phase-change clay in various complex fracture modes (including I-type, II-type and I-II-type fracture modes) can be accurately and reliably measured, and a premise can be provided for research of the fracture characteristics of the phase-change clay and analysis of core wall fracture damage and hydraulic fracture problems.

Description

Technical Field

[0001] The invention belongs to the technical field of water conservancy project construction, and particularly relates to a device and method for testing the fracture performance and failure characteristics of phase-change clay. Background Art

[0002] Cracks within the core wall of an earth-rock dam can lead to hydraulic fracturing, threatening the dam's stability. Therefore, the fracture properties of the core wall material have a significant impact on dam safety. Earth-rock dams constructed in high-altitude cold regions often face freezing and thawing issues during winter. Phase-change clays, incorporating phase-change materials (PCMs), offer a new approach to addressing this issue of core wall freezing and temperature control in low-temperature environments.

[0003] However, the incorporation of PCM can lead to a decrease in the soil's crack resistance and an increase in the risk of core wall fracture failure. In order to analyze the feasibility of dam construction using phase-change clay, it is necessary to conduct in-depth research on its fracture performance and failure characteristics. Currently, experimental research on the fracture performance and failure characteristics of phase-change clay still has the following deficiencies:

[0004] (1) Lack of appropriate test methods. Currently, there are many test methods for the fracture parameters of conventional clay materials. [1-3] However, no test method suitable for the fracture characteristics of phase change clay has been proposed. In addition, considering that the core wall phase change clay will face different fracture failure modes under complex operating environments, the existing test equipment can only study one specific fracture failure mode. [4-5] , it is urgent to develop a test device that can take into account the soil fracture modes of type I, type II and type I-II composite.

[0005] (2) Lack of appropriate sample preparation methods. The main difficulty of the phase change clay fracture test is that the intergranular bonding of clay is weak and the strength is low. The test results are greatly affected by the deadweight, and the specimens are easily damaged during the sample preparation, transportation, placement and demoulding process, resulting in a large number of invalid specimens. In addition, there are also problems in the production of the initial cracks in the soil beam. Cutting and seaming after demoulding can easily lead to direct fracture of the soil beam, while pre-grooving and cutting seams in the mold will face the problem of difficulty in cutting due to sand and gravel jamming and difficulty in controlling the crack depth. [4] .

[0006] To overcome these shortcomings, it is necessary to develop a test device and method for the fracture properties and failure characteristics of phase-change clay. This will provide a basis for determining the fracture criteria of phase-change clay and analyzing the fracture and failure of core walls and hydraulic fracturing. This is of great significance for reducing the fracture and failure risk of phase-change clay core walls and ensuring the safe operation of dams.

[0007] References:

[0008] [1] Hong Yi, Wang Lizhong, Sun Fan, et al. A device and method for measuring the fracture toughness of soft clay under arbitrary confining pressure [P]. Zhejiang Province: CN201610657916.3, 2019-02-12.

[0009] [2] Wang Junjie. Research on hydraulic splitting of earth-rock dam core wall based on fracture mechanics[D]. Hohai University, 2005.

[0010] [3]Yamamoto Y, Springman S M.Three-and four-point bending tests onartificial frozen soil samples at temperatures close to 0℃[J]. Cold RegionsScience andTechnology, 2017,134:20-32.

[0011] [4] Hu Junfeng. Study on fracture failure characteristics and judgment criteria of compacted clay[D]. Chongqing Jiaotong University, 2016.

[0012] [5] Zhang Yun, Ma Weijun, Li Zehua. A strain-controlled type I fracture toughness tester and test method for clay soil [P]. Jiangsu: CN201610412961.2, 2016-08-17. Summary of the Invention

[0013] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a phase change clay fracture performance and failure characteristics test device and method, which can truly simulate the compressive fracture process of phase change clay and realize the accurate and reliable measurement of the fracture performance and failure characteristics of phase change clay under various complex fracture modes (including type I, type II and type I-II fracture modes). It can provide a premise for the study of the fracture characteristics of phase change clay and the analysis of core wall fracture failure and hydraulic fracturing problems.

[0014] The present invention solves the technical problem by the following technical solutions:

[0015] A phase change clay fracture performance and failure characteristics test device includes a prefabricated cracked soil beam specimen, a three-point / four-point bending moment loading frame, a servo motor, a sensor, a data acquisition unit, and a camera unit. The prefabricated cracked soil beam specimen, the servo motor, the sensor, and the camera unit are arranged on the three-point / four-point bending moment loading frame, and the servo motor and the sensor are connected to the data acquisition unit.

[0016] The three-point / four-point bending moment loading frame includes a pressure-bearing base plate, a first slide rail, a second slide rail, a ball screw, an adjustable pad, a fixing clamp, a pressure head, a slider and a baffle;

[0017] The first slide rail and the second slide rail are sequentially installed above the pressure base plate, and the first slide rail and the second slide rail are connected by a ball screw and are respectively connected to the servo motor and the pressure head to realize the control of the servo motor on the pressure head;

[0018] The adjustable pad and the fixing clamp are installed in the middle of the ball screw to fix the sensor on the ball screw, thereby assisting in measuring the displacement change during the loading process;

[0019] The pressurizing heads are divided into two groups, an upper group and a lower group. The upper group of pressurizing heads is connected to the second slide rail and the number of the pressurizing heads is one or two. The lower group of pressurizing heads is installed on the baffle. The pressurizing heads are provided with adjustable locking screws so that the pressurizing heads can move freely between the second slide rail and the baffle, thereby changing the fracture failure mode of the specimen and realizing the measurement of the fracture parameters of the specimen under the above-mentioned fracture conditions.

[0020] The baffle is vertically installed at the bottom of the pressure base plate. A scale is provided on the baffle and the second slide rail for measuring the distance between the pressure heads and the distance between the pressure heads and the cracks, so as to achieve precise movement and control of the loading point.

[0021] The servo motor includes a motor and a servo controller. The motor is mounted on the first slide rail to control the loading of the pressurizing head. The servo controller is connected to the motor through a motor electrical signal conversion device and a signal transmission line and transmits a control signal to the motor.

[0022] The sensor includes a pressure sensor and a displacement sensor, the pressure sensor is mounted on the pressure head, and the displacement sensor is fixed to the ball screw through the adjustable pad and the fixing clamp;

[0023] The data acquisition unit includes a data acquisition instrument, a motor-electrical signal conversion device, a voltage-regulating power supply, and a computer terminal. The voltage-regulating power supply supplies power to the data acquisition instrument and connects the data acquisition instrument to the pressure sensor, the displacement sensor, and the computer terminal via a signal transmission line. The data acquisition instrument acquires data from the pressure sensor and the displacement sensor in real time, and transmits the data to the computer terminal for display and storage, and performs calculation and analysis of fracture parameters.

[0024] The camera unit includes a high-definition camera and an external light source. The high-definition camera is set directly above the prefabricated cracked soil beam specimen. The external light sources are no less than 3, and their height is set to be flush with the high-definition camera and evenly arranged around the prefabricated cracked soil beam specimen.

[0025] Moreover, the pressure-bearing base plate, the first slide rail, the second slide rail, the ball screw and the baffle are made of a metal material with high rigidity and light weight, preferably an aluminum alloy.

[0026] Moreover, the pressure head is a rollable cylinder, and the slider has universal wheels and is placed below the second slide rail and at the bottom of the prefabricated cracked soil beam specimen to balance the weight of the specimen and reduce the influence of friction during loading.

[0027] A method for testing the fracture properties and failure characteristics of phase change clay, using the phase change clay fracture properties and failure characteristics testing device, the method comprises the following steps:

[0028] S1. Use a prefabricated cracked soil beam specimen sample preparation mold to complete the preparation of the prefabricated cracked soil beam specimen. The prefabricated cracked soil beam specimen sample preparation mold includes a base, a fill mold, a cushion sleeve, a fill pressure head, a demoulding sleeve and a crack prefabricated piece; the base is used to support the entire sample preparation mold and the soil material to be sampled; the fill mold is connected to the base to accommodate the soil material and control the size of the prefabricated cracked soil beam specimen; the cushion sleeve is connected to the fill mold to accommodate the soil material that has not been compacted; the fill pressure head is used to compact the soil material in layers, and the pressure head size is 20mm. The size is consistent with the internal size of the fill mold; the demoulding sleeve is used to demould the prepared prefabricated cracked soil beam specimen; the crack prefabricated sheet is used to make the initial crack of the prefabricated cracked soil beam specimen, and a steel sheet with a thickness of 1.0 mm is pre-placed on the top of the fill mold and pulled out after the soil is compacted; the thickness B of the prepared prefabricated cracked soil beam specimen is not less than 10 times the maximum particle size of the soil, the width W of the prefabricated cracked soil beam specimen is not less than 2 times the thickness B of the prefabricated cracked soil beam specimen, and the length L of the prefabricated cracked soil beam specimen is not less than 5 times the width W of the prefabricated cracked soil beam specimen;

[0029] S2. Place the prefabricated cracked soil beam specimen horizontally on a slider with universal wheels, and change the number and position of the loading points by replacing the number and position of the pressure heads to measure the fracture parameters of the prefabricated cracked soil beam specimen under type I, type II, and type I-II fracture conditions;

[0030] Specifically, the arrangement of the pressure heads corresponding to the type I fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on the same straight line, with one upper pressure head and two lower pressure heads, and they are symmetrical with the prefabricated crack as the axis, and the specimen is subjected to symmetrical three-point loads; the arrangement of the pressure heads corresponding to the type II fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on the same straight line, with two upper pressure heads and two lower pressure heads, and under the action of asymmetric four-point loads, the crack surface is subjected to shear force; the arrangement of the pressure heads corresponding to the type I-II fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on different straight lines, with two upper pressure heads and two lower pressure heads, and under the action of asymmetric four-point loads, the crack surface is subjected to the combined action of shear force and bending moment;

[0031] S3, starting the servo motor, and controlling the motor installed on the three-point / four-point moment loading frame through the servo controller to perform horizontal loading on the prefabricated cracked soil beam specimen;

[0032] Specifically, the motor rotates at the speed set by the servo controller, converts the rotational motion into linear displacement through the transmission mechanism, and sequentially drives the first slide rail, ball screw, second slide rail, and pressure head to load the specimen at a constant speed;

[0033] S4, continue loading until the prefabricated cracked soil beam specimen is completely broken, and the computer terminal simultaneously displays and saves the data collected by the pressure sensor and the displacement sensor during the process from the start of loading to the failure of the prefabricated cracked soil beam specimen;

[0034] S5. The fracture toughness of the prefabricated cracked soil beam specimens under different fracture conditions is measured and calculated. At the same time, the failure characteristics of the prefabricated cracked soil beam specimens under different fracture conditions, including the crack propagation process and fracture angle, are observed and calculated.

[0035] 5. The method for testing the fracture properties and failure characteristics of phase change clay according to claim 4, wherein the step S1 is:

[0036] (1) preparing a phase change clay material and dividing it into 2 to 4 equal parts for subsequent layered compaction, wherein the number of layered compactions is determined according to the length of the prefabricated crack and the height of the fill mold;

[0037] (2) Place the cushion sleeve on the fill mold, add phase change clay material into the fill mold, compact the soil material with a fill pressure head, and fully scrape the surface of the compacted soil material and evenly flatten it;

[0038] (3) Repeat the steps until the surface of the compacted soil material is flush with the bottom surface of the prefabricated crack;

[0039] (4) Place the crack prefabricated sheet vertically above the compacted soil;

[0040] (5) Continue to add phase change clay material into the filling mold, compact the soil material with a filling pressure head, and fully scrape the surface of the compacted soil material and evenly flatten it;

[0041] (6) Repeat the steps until the surface of the compacted soil material is level with the top surface of the fill mold;

[0042] (7) Pull out the crack prefabricated sheet and use the demoulding sleeve to slowly push the specimen out from the bottom to complete the demoulding of the specimen, thereby obtaining a prefabricated crack soil beam specimen with initial cracks.

[0043] Moreover, the S5 is specifically:

[0044] Mode I fracture toughness K of prefabricated cracked soil beam specimen under mode I fracture failure ICThe test was carried out in the form of a three-point bending test. The I-type stress intensity factor K of the three-point bending specimen was I Calculate as follows:

[0045]

[0046] Where: a is the crack length, P is the pressure applied to the top of the specimen, B is the specimen thickness, W is the specimen width, and S is the span;

[0047] Mode II fracture toughness K of prefabricated cracked soil beam specimen under mode II fracture failure IIC , and K under I-II composite fracture failure IC and K IIC The test was carried out in the form of a four-point bending test. The I-type stress intensity factor K of the four-point bending specimen was I and type II stress intensity factor K II Calculate as follows:

[0048]

[0049] Where: M is the bending moment on the crack surface, F τ is the shear force on the crack surface, L1 and L2 are the horizontal distances from the two upper pressure heads to the prefabricated crack, and c is the horizontal distance from the pressure P application point to the prefabricated crack.

[0050] Moreover, the S5 is specifically:

[0051] (1) During the bending test, a high-definition camera was used to shoot a video of the fracture process of the prefabricated cracked soil beam specimen to observe the crack propagation process of the prefabricated cracked soil beam specimen;

[0052] (2) The angle between the line between the initial crack initiation point and the end point of the crack extension and the initial crack is used as the fracture angle θ of the specimen. c Considering that the I-II composite fracture path of the specimen sometimes undergoes a large-scale turn, the selection of the crack extension end point will have an impact on θ c The determination results have a great influence on the force-displacement curve, and it is further stipulated that the crack tip corresponding to the point where the force-displacement curve drops to 70% of the peak strength after the peak is used as the end point of the crack extension to calculate θ. c ;

[0053] (3) After the test, the loading curve drawn by the data logger and the time axis of the fracture process video shot by the high-definition camera are compared to extract the key frame of the crack extension end point and calculate the θ of the specimen based on it. c .

[0054] The advantages and beneficial effects of the present invention are:

[0055] 1. The present invention can take into account the determination of the fracture performance and failure characteristics of phase change clay under different fracture modes, thereby providing a basis for the analysis of the fracture failure and hydraulic fracturing problems of core wall phase change clay.

[0056] 2. The present invention can measure the fracture performance and failure characteristics of phase change clay in a variety of complex fracture modes (including type I, type II and type I-II fracture modes), and has a wide range of applications.

[0057] 3. The present invention can eliminate the influence of the specimen's own weight and reduce the influence of friction during loading through the innovative design of the test device, while ensuring the stability of specimen loading, thereby improving the validity and reliability of the test results.

[0058] 4. The test results of the test device of the present invention can provide a premise for the study of the fracture characteristics of phase change clay and the analysis of core wall fracture damage and hydraulic fracturing problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a diagram of the mold for preparing the sample of the prefabricated cracked soil beam specimen of the present invention;

[0060] Figure 2 Schematic diagram of the three-point / four-point bending test device of the present invention (the solid line in the figure represents the power supply line, and the dotted line represents the signal transmission line);

[0061] Figure 3 Schematic diagram of the mode I fracture toughness test under the three-point bending test of the present invention;

[0062] Figure 4 Schematic diagram of type II and type I-II composite fracture toughness testing under the four-point bending test of the present invention;

[0063] Figure 5 The force-displacement curves of the three-point bending test and the four-point bending test of the test device of the present invention are shown;

[0064] Figure 6 The failure paths of the specimens under different fracture modes observed by the test device of the present invention are shown in Figure 1, where (a) is the failure path under type I fracture mode, (b) is the failure path under type II fracture mode, and (c) is the typical failure path under type I-II composite fracture mode.

[0065] Description of Reference Numerals

[0066] 101. Base; 102. Filling mold; 103. Cushion sleeve; 104. Filling pressure head; 105. Demolding sleeve; 106. Prefabricated steel sheet;

[0067] 201, pressure base; 202, first slide rail; 203, second slide rail; 204, ball screw; 205, adjustable pad; 206, fixing clamp; 207, pressure head; 208, slider; 209, baffle;

[0068] 301, motor; 302, servo controller;

[0069] 401. Pressure sensor; 402. Displacement sensor;

[0070] 501. Data logger; 502. Motor-to-electrical signal conversion device; 503. Voltage-regulating power supply; 504. Computer terminal; 601. High-definition camera; 602. External light source. DETAILED DESCRIPTION

[0071] The present invention will be further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the scope of protection of the present invention cannot be limited thereto.

[0072] The embodiment of the present invention provides a phase change soil fracture performance and failure characteristics test device, including a prefabricated cracked soil beam specimen sample preparation mold, a three-point / four-point bending moment loading frame, a servo motor, a pressure and displacement sensor, a data acquisition instrument, and a high-definition camera. Figure 1 、 Figure 2 shown.

[0073] (1) Prefabricated cracked soil beam specimen sample mold

[0074] The embodiment of the present invention uses a prefabricated cracked soil beam specimen sample preparation mold 1 to prepare a phase change soil beam specimen, including a base 101, a fill mold 102, a cushion sleeve 103, a fill pressure head 104, a demoulding sleeve 105 and a crack prefabricated sheet 106. Figure 1 As shown;

[0075] The base 101 is used to support the entire sample preparation mold and the soil material to be sampled;

[0076] The filling mold 102 is connected to the base 101 and is used to contain soil and control the size of the soil beam specimen;

[0077] The cushion sleeve 103 is connected to the soil filling mold 102 and is used to accommodate the uncompacted soil material;

[0078] The filling head 104 is used to compact the soil material in layers, and the size of the head is consistent with the internal size of the filling mold 102;

[0079] The demoulding sleeve 105 is used to demould the prepared soil beam specimen;

[0080] The crack prefabricated sheet 106 is used to make the initial cracks of the specimen. A 1.0 mm thick steel sheet is pre-placed on top of the soil filling mold 102 and is pulled out after the soil is compacted to improve the success rate of soil beam specimen preparation and ensure the stability of the sample.

[0081] Specifically, the method for determining the internal dimensions of the mold for preparing the prefabricated cracked soil beam specimen is as follows:

[0082] According to the maximum particle size of the phase change soil material (2.0 mm), the thickness of the specimen (the shortest side length) is determined to be B = 25 mm;

[0083] Considering that the requirements of the three-point and four-point bending tests must be met simultaneously by moving the loading point, and in combination with the relevant specifications of the three-point bending test and the four-point bending test, the specimen width W = 2B = 50 mm and the specimen length L = 5W = 250 mm are determined;

[0084] Considering the low strength of phase change soil, the ratio of initial crack length a to specimen width W, a / W, should not be set too large. Therefore, a / W = 0.4 and crack length a = 20 mm are taken to reduce the test error caused by other interference factors.

[0085] Specifically, according to the determined specimen geometric parameters, the embodiment of the present invention makes a prefabricated cracked phase change soil beam specimen sample preparation mold of corresponding size. The prepared mold is used to prepare the soil beam specimen. The sample preparation process is as follows:

[0086] (1) Prepare a phase change soil material with a PCM content of 8% and divide it into two equal parts for subsequent layered compaction;

[0087] (2) Place the cushion sleeve 103 on the filling mold 102, add the first portion of phase change soil material into it, use the filling press head 104 and adopt the static slow loading method to compact the soil material, fully scrape the surface of the compacted soil material and evenly flatten it so that the surface of the compacted soil material is flush with the bottom surface of the prefabricated crack;

[0088] (3) vertically placing the crack prefabricated sheet 106 above the compacted soil material;

[0089] (4) Continue to add the second portion of phase change soil material into the fill mold 102, use the fill press 104 and adopt a static slow loading method to compact the soil material, fully scrape the surface of the compacted soil material and evenly flatten it so that the surface of the compacted soil material is flush with the top surface of the fill mold 102;

[0090] (5) Pull out the crack prefabricated sheet 106 and use the demoulding sleeve 105 to slowly push the specimen out from the bottom to complete the demoulding of the specimen and obtain the soil beam specimen with initial cracks.

[0091] (2) Three-point / four-point moment loading frame

[0092] The embodiment of the present invention uses a three-point / four-point bending moment loading frame 2 to perform a three-point / four-point bending test on the prepared soil beam specimen, including a pressure base 201, a first slide rail 202, a second slide rail 203, a ball screw 204, an adjustable pad 205, a fixing clamp 206, a pressure head 207, a slider 208 and a baffle 209, as shown in FIG. Figure 2 As shown;

[0093] The pressure base plate 201, the first slide rail 202, the second slide rail 203, the ball screw 204, the baffle 209 and other components are made of a metal material with high rigidity and light weight to ensure the rigidity and stability of the loading frame. In the embodiment of the present invention, aluminum alloy is used;

[0094] The second slide rail 203 is used to fix the pressure head on the upper part of the test piece, and its position can be adjusted through multiple fixing holes provided on the device.

[0095] The pressure head 207 is designed as a rolling cylinder and is used to apply a concentrated load to the specimen. An adjustable locking screw is provided on the pressure head, allowing the upper and lower pressure heads to move freely on the second slide rail 203 and the baffle 209. The fracture parameters of the specimen under Type I, Type II, and Type I-II fracture conditions are measured through three-point bending tests and four-point bending tests.

[0096] The ball screw 204 connects the motor 301 to the pressurizing head 207 via the first slide rail 202 and the second slide rail 203, so as to realize the control of the pressurizing head by the servo motor;

[0097] The adjustable pad 205 and the fixing clamp 206 are used to fix the displacement sensor 402 on the ball screw, thereby assisting in measuring the displacement change during the loading process;

[0098] The slider 208 has universal wheels and is placed under the specimen to balance the weight of the specimen and reduce the influence of friction during loading;

[0099] The second slide rail 203 and the baffle 209 are provided with scales for measuring the distance between the pressurizing heads and the distance between the pressurizing heads and the cracks, so as to achieve precise movement and control of the loading point;

[0100] (3) Servo motor

[0101] The embodiment of the present invention uses a high-precision programmable servo motor 3, which includes a motor 301 and a servo controller 302. The two are connected through a motor electrical signal conversion device 502 and a signal transmission line. The servo controller transmits signals to the motor installed on the three-point / four-point bending moment loading frame 2 to further control the loading rate of the soil beam specimen during the test to ensure the stability of the loading control.

[0102] Specifically, during the bending test, a voltage-regulated power supply 503 is used to power the servo motor 3 (Note: the embodiment of the present invention does not make specific requirements on the power supply, and different brands can be selected), and the loading rate is stably controlled to 0.8 mm / min through the servo controller 302.

[0103] (4) Pressure and displacement sensors

[0104] In the embodiment of the present invention, the pressure sensor 401 and the displacement sensor 402 are installed on the three-point / four-point bending moment loading frame to measure the pressure applied to the specimen and the deformation of the specimen during the bending test;

[0105] In order to ensure the accuracy of the data collected by the sensor, two spare pressure sensors 401 are set, with maximum ranges of 50N and 500N respectively, and an accuracy of 0.03% FS; the maximum range of the displacement sensor 402 is 50mm, and the accuracy is 0.05% FS.

[0106] (5) Data acquisition instrument

[0107] The embodiment of the present invention uses a voltage-regulated power supply 503 to supply power to the data acquisition instrument 501 (Note: The embodiment of the present invention does not make specific requirements on the power supply, and different brands can be selected), and connects the data acquisition instrument 501 to the pressure sensor 401, the displacement sensor 402, and the computer terminal 504 via a signal transmission line;

[0108] Specifically, the data acquisition instrument 501 collects data sensed by the pressure sensor 401 and the displacement sensor 402 in real time, and transmits it to the computer terminal 504 for display and storage. Subsequently, the saved data can be used to draw a loading curve and calculate and analyze fracture parameters.

[0109] (6) High-definition camera

[0110] The embodiment of the present invention uses a high-definition camera 601 with a frame rate of 120FPS, which is placed directly above the specimen to capture the crack expansion process of the specimen; and three external light sources 602 are set during the test to avoid shadows and ensure clear crack imaging.

[0111] The process of performing a bending test using the bending test apparatus in this embodiment is as follows:

[0112] (1) using the prefabricated cracked soil beam specimen sample preparation mold 1 to prepare the soil beam specimen;

[0113] (2) The soil beam specimen is placed horizontally on a slider 208 with universal wheels, and the loading point is moved by changing the position of the pressure head 207 to achieve the measurement of the fracture parameters of the soil beam specimen under type I, type II, and type I-II fracture conditions;

[0114] (3) Start the servo motor 3 and control the motor 301 installed on the three-point / four-point moment loading frame 2 through the servo controller 302 to load the soil beam specimen horizontally at a rate of 0.8 mm / min to eliminate the influence of the soil beam specimen's own weight;

[0115] (4) The loading is continued until the soil beam specimen is completely broken. The computer terminal 504 simultaneously displays and saves the data collected by the pressure sensor 401 and the displacement sensor 402 from the start of loading to the failure of the specimen. The obtained force-displacement curve is as follows: Figure 5 As shown;

[0116] (5) During the test, a high-definition camera 601 is used to shoot a video of the specimen's fracture process. After the test, the time axis of the loading curve and the fracture process video is compared to extract the key frames ( Figure 6 ) and calculate the θ of the soil sample based on this c ;

[0117] Furthermore, the method for measuring the fracture angle of the phase change soil beam specimen in this embodiment is as follows:

[0118] The angle between the line between the initial crack initiation point and the end point of the crack extension and the initial crack is used as the fracture angle θ of the specimen. c Considering that the I-II composite fracture path of the specimen sometimes undergoes a large-scale turn, the selection of the crack extension end point will affect θ c The determination results have a great influence on the results. It is further stipulated that the crack tip corresponding to the point where the force-displacement curve drops to 70% of the peak strength after the peak is used as the endpoint to calculate θ. c ;

[0119] Furthermore, the method for measuring and calculating the fracture toughness of the soil beam specimen under different fracture modes in this embodiment is as follows:

[0120] (1) Mode I fracture toughness K of the soil beam specimen under mode I fracture failure IC The test is carried out in the form of a three-point bending test, such as Figure 3 As shown;

[0121] Mode I stress intensity factor K of three-point bending specimen I Calculate as follows:

[0122]

[0123] Where a is the crack length, P is the pressure applied to the top of the specimen, B is the specimen thickness, W is the specimen width, and S is the span.

[0124] (2) Mode II fracture toughness K of the soil beam specimen under mode II fracture failure IIC, and K under I-II composite fracture failure IC and K IIC The test is carried out in the form of a four-point bending test, such as Figure 4 When the horizontal distance c from the resultant force point to the prefabricated crack is 0, the crack surface is only subjected to shear force, and the fracture failure mode is pure type II; when c≠0, the crack surface is subjected to the combined action of shear force and bending moment, and the fracture failure mode is I-II composite type;

[0125] Mode I stress intensity factor K of four-point bending specimen I and type II stress intensity factor K II Calculate as follows:

[0126]

[0127] in,

[0128]

[0129] Where: M is the bending moment on the crack surface, F τ is the shear force on the crack surface, L1 and L2 are the horizontal distances from the two upper pressure heads to the prefabricated crack, and c is the horizontal distance from the pressure P application point to the prefabricated crack.

[0130] Furthermore, in addition to phase change soil, the device in this paper is also suitable for fracture performance testing of other soil materials.

[0131] Furthermore, the force-displacement curves of the three-point bending test and the four-point bending test in this embodiment are as follows: Figure 5 As shown by Figure 5 It can be seen that the peak load of the three-point bending test is much lower than that of the four-point bending test, that is, different loading methods have a great influence on the bending strength of the specimen;

[0132] During the bending test, the failure characteristics of the specimen were observed, and the fracture failure paths in type I, type II, and type I-II fracture modes were obtained as follows: Figure 6 As shown by Figure 6 It can be seen that under the action of three-point and four-point bending, cracks will begin to develop and initiate at the top of the prefabricated cracks in the soil sample, and then extend along a specific direction, eventually forming a macro crack visible to the naked eye, causing the sample to become unstable and damaged; among them, the cracks of type I fracture damage continue to extend in the vertical direction along the original crack direction; the crack extension direction of type II fracture damage is obviously different from the original crack, and generally develops in an oblique direction of about 55°; the crack extension direction of type I-II composite fracture damage is related to the horizontal distance c from the pressure point to the prefabricated crack, and its fracture angle θ c It decreases as c increases.

[0133] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A phase change clay fracture performance and failure characteristics test device, characterized by: The invention comprises a prefabricated cracked soil beam specimen, a three-point / four-point bending moment loading frame (2), a servo motor (3), a sensor (4), a data acquisition unit (5) and a camera unit (6); the prefabricated cracked soil beam specimen, the servo motor (3), the sensor (4) and the camera unit (6) are arranged on the three-point / four-point bending moment loading frame (2); and the servo motor (3) and the sensor (4) are connected to the data acquisition unit (5); The three-point / four-point bending moment loading frame (2) comprises a pressure-bearing base plate (201), a first slide rail (202), a second slide rail (203), a ball screw (204), an adjustable pad (205), a fixing clamp (206), a pressure head (207), a slider (208) and a baffle (209); The first slide rail (202) and the second slide rail (203) are sequentially installed above the pressure-bearing bottom plate (201); the first slide rail (202) and the second slide rail (203) are connected via a ball screw (204) and are respectively connected to a servo motor (3) and a pressure head (207) to enable the servo motor to control the pressure head; The adjustable pad (205) and the fixing clamp (206) are installed in the middle of the ball screw (204) and are used to fix the sensor (4) on the ball screw (204), thereby assisting in measuring the displacement change during the loading process; The pressurizing head (207) is divided into two groups, an upper group and a lower group. The upper group of pressurizing heads is connected to the second slide rail (203) and the number of the pressurizing heads is one or two. The lower group of pressurizing heads is installed on the baffle (209). The pressurizing head (207) is provided with an adjustable locking screw so that the pressurizing head (207) can move freely on the second slide rail (203) and the baffle (209), thereby changing the fracture failure mode of the specimen and realizing the measurement of the fracture parameters of the specimen under the above-mentioned fracture conditions. The baffle (209) is vertically mounted at the bottom of the pressure-bearing base plate (201). Scales are provided on the baffle (209) and the second slide rail (203) for measuring the distance between the pressure heads and the distance between the pressure heads and the cracks, so as to achieve precise movement and control of the loading point. The servo motor (3) comprises a motor (301) and a servo controller (302); the motor (301) is mounted on the first slide rail (202) to control the loading of the pressurizing head (207); the servo controller (302) is connected to the motor (301) via a motor-electrical signal conversion device (502) and a signal transmission line, and transmits a control signal to the motor (301); The sensor (4) comprises a pressure sensor (401) and a displacement sensor (402), wherein the pressure sensor (401) is mounted on the pressure head (207), and the displacement sensor (402) is fixed to the ball screw (204) via the adjustable pad (205) and the fixing clamp (206); The data acquisition unit (5) includes a data acquisition instrument (501), a motor-electrical signal conversion device (502), a voltage-regulating power supply (503) and a computer terminal (504). The voltage-regulating power supply (503) supplies power to the data acquisition instrument (501) and connects the data acquisition instrument (501) with the pressure sensor (401), the displacement sensor (402) and the computer terminal (504) via a signal transmission line. The data acquisition instrument (501) acquires data from the pressure sensor (401) and the displacement sensor (402) in real time, and transmits the data to the computer terminal (504) for display and storage, and performs calculation and analysis of fracture parameters. The camera unit (6) comprises a high-definition camera (601) and an external light source (602). The high-definition camera (601) is arranged directly above the prefabricated cracked soil beam specimen. The external light sources (602) are at least three locations, and their heights are set to be flush with the high-definition camera (601) and are evenly arranged around the prefabricated cracked soil beam specimen.

2. The phase change clay fracture performance and failure characteristics testing device according to claim 1, characterized in that: The pressure-bearing base plate (201), the first slide rail (202), the second slide rail (203), the ball screw (204) and the baffle (209) are made of a metal material with high rigidity and light weight, preferably an aluminum alloy.

3. The phase change clay fracture performance and failure characteristics testing device according to claim 1, characterized in that: The pressure head (207) is a rollable cylinder, and the slider (208) has universal wheels and is placed below the second slide rail (203) and at the bottom of the prefabricated cracked soil beam specimen to balance the deadweight of the specimen and reduce the influence of friction during loading.

4. A method for testing the fracture properties and failure characteristics of phase change clay, characterized by: Using the phase change clay fracture performance and failure characteristics testing device according to any one of claims 1 to 3, the method comprises the following steps: S1. The prefabricated cracked soil beam specimen sample preparation mold (1) is used to complete the preparation of the prefabricated cracked soil beam specimen. The prefabricated cracked soil beam specimen sample preparation mold (1) comprises a base (101), a filling mold (102), a cushion sleeve (103), a filling pressure head (104), a demoulding sleeve (105) and a crack prefabricated sheet (106); the base (101) is used to support the entire sample preparation mold and the soil material to be sampled; the filling mold (102) is connected to the base (101) and is used to accommodate the soil material and control the forming of the prefabricated cracked soil beam specimen. size; the cushion sleeve (103) is connected to the fill mold (102) and is used to accommodate the uncompacted soil material; the fill pressure head (104) is used to compact the soil material in layers, and the size of the pressure head is consistent with the internal size of the fill mold (102); the demoulding sleeve (105) is used to demould the prepared prefabricated cracked soil beam specimen; the crack prefabricated sheet (106) is used to make the initial crack of the prefabricated cracked soil beam specimen, and a steel sheet with a thickness of 1.0 mm is pre-placed on the top of the fill mold (102) and is pulled out after the soil material is compacted; The thickness B of the prefabricated cracked soil beam specimen shall not be less than 10 times the maximum particle size of the soil material, the width W of the prefabricated cracked soil beam specimen shall not be less than 2 times the thickness B of the prefabricated cracked soil beam specimen, and the length L of the prefabricated cracked soil beam specimen shall not be less than 5 times the width W of the prefabricated cracked soil beam specimen; S2, placing the prefabricated cracked soil beam specimen horizontally on a slider (208) with universal wheels, and changing the number and position of the loading points by replacing the number and position of the pressure heads (207), so as to achieve the measurement of the fracture parameters of the prefabricated cracked soil beam specimen under type I, type II and type I-II fracture conditions; Specifically, the arrangement of the pressure heads corresponding to the type I fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on the same straight line, with one upper pressure head and two lower pressure heads, and they are symmetrical with the prefabricated crack as the axis, and the specimen is subjected to symmetrical three-point loads; the arrangement of the pressure heads corresponding to the type II fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on the same straight line, with two upper pressure heads and two lower pressure heads, and under the action of asymmetric four-point loads, the crack surface is subjected to shear force; the arrangement of the pressure heads corresponding to the type I-II fracture condition is as follows: the applied pressure and the prefabricated crack of the specimen are located on different straight lines, with two upper pressure heads and two lower pressure heads, and under the action of asymmetric four-point loads, the crack surface is subjected to the combined action of shear force and bending moment; S3, starting the servo motor (3), and controlling the motor (301) installed on the three-point / four-point moment loading frame (2) through the servo controller (302) to perform horizontal loading on the prefabricated cracked soil beam specimen; Specifically, the motor (301) rotates at a speed set by the servo controller (302), converts the rotational motion into linear displacement through the transmission mechanism, and sequentially drives the first slide rail (202), the ball screw (204), the second slide rail (203), and the pressure head (207) to load the specimen at a constant speed; S4, continuously loading until the prefabricated cracked soil beam specimen is completely broken, and the computer terminal (504) simultaneously displays and saves data collected by the pressure sensor (401) and the displacement sensor (402) during the process from the start of loading to the failure of the prefabricated cracked soil beam specimen; S5. The fracture toughness of the prefabricated cracked soil beam specimens under different fracture conditions is measured and calculated. At the same time, the failure characteristics of the prefabricated cracked soil beam specimens under different fracture conditions, including the crack propagation process and fracture angle, are observed and calculated.

5. The method for testing the fracture properties and failure characteristics of phase change clay according to claim 4, characterized in that: The S1 is specifically: (1) preparing a phase change clay material and dividing it into 2 to 4 equal parts for subsequent layered compaction, wherein the number of layered compactions is determined according to the length of the prefabricated crack and the height of the fill mold; (2) Place the cushion sleeve on the fill mold, add phase change clay material into the fill mold, compact the soil material with a fill pressure head, and fully scrape the surface of the compacted soil material and evenly flatten it; (3) Repeat step (2) until the surface of the compacted soil material is flush with the bottom surface of the prefabricated crack; (4) Place the crack prefabricated sheet vertically above the compacted soil; (5) Continue to add phase change clay material into the filling mold, compact the soil material with a filling pressure head, and fully scrape the surface of the compacted soil material and evenly flatten it; (6) Repeat step (5) until the surface of the compacted soil material is flush with the top surface of the fill mold; (7) Pull out the crack prefabricated sheet and use the demoulding sleeve to slowly push the specimen out from the bottom to complete the demoulding of the specimen, thereby obtaining a prefabricated crack soil beam specimen with initial cracks.

6. The method for testing the fracture properties and failure characteristics of phase change clay according to claim 4, characterized in that: The S5 is specifically: Mode I fracture toughness K of prefabricated cracked soil beam specimen under mode I fracture failure IC The test was carried out in the form of a three-point bending test. The I-type stress intensity factor K of the three-point bending specimen was I Calculate as follows: Where: a is the crack length, P is the pressure applied to the top of the specimen, B is the specimen thickness, W is the specimen width, and S is the span; Mode II fracture toughness K of prefabricated cracked soil beam specimen under mode II fracture failure IIC , and K under I-II composite fracture failure IC and K IIC The test was carried out in the form of a four-point bending test. The I-type stress intensity factor K of the four-point bending specimen was I and type II stress intensity factor K II Calculate as follows: Where: M is the bending moment on the crack surface, F τ is the shear force on the crack surface, L1 and L2 are the horizontal distances from the two upper pressure heads to the prefabricated crack, and c is the horizontal distance from the pressure P application point to the prefabricated crack.

7. The method for testing the fracture properties and failure characteristics of phase change clay according to claim 4, characterized in that: The S5 is specifically: (1) During the bending test, a high-definition camera (601) is used to shoot a video of the fracture process of the prefabricated cracked soil beam specimen to observe the crack propagation process of the prefabricated cracked soil beam specimen; (2) The angle between the line between the initial crack initiation point and the end point of the crack extension and the initial crack is used as the fracture angle θ of the specimen. c Considering that the I-II composite fracture path of the specimen sometimes undergoes a large-scale turn, the selection of the crack extension end point will have an impact on θ c The determination results have a great influence on the force-displacement curve, and it is further stipulated that the crack tip corresponding to the point where the force-displacement curve drops to 70% of the peak strength after the peak is used as the end point of the crack extension to calculate θ. c ; (3) After the test, the loading curve drawn by the data acquisition instrument and the time axis of the fracture process video shot by the high-definition camera (601) are compared to extract the key frame of the crack extension end point and calculate the θ of the specimen based on it. c .

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