A Method for Testing Type I Fracture under Cyclic Thermodynamic Effects

Abstract

This invention discloses a method for testing Type I fracture under cyclic thermodynamic loading. It couples a three-point bending test with stress-temperature cycling, extending the three-point bending test to thermodynamic experimental conditions. By measuring the peak load S in the three-point bending test and the elastic modulus E in the final stress-temperature cycle, the fracture energy Gc of the Type I fracture of the specimen under cyclic thermodynamic loading can be effectively calculated. This helps to accurately reflect the bearing capacity and fracture propagation mode of the fractured surrounding rock in compressed gas storage caverns under cyclic thermodynamic loading. The method couples stress and temperature cycles and measures the change in the elastic modulus of the specimen under cyclic stress and temperature. This method has the advantages of simple steps, quick and convenient testing, and strong practicality. It is easy to operate, has a wide range of applications, and can be combined according to different actual working conditions to meet the needs of Type I fracture testing under different specimen materials, loading temperatures, and loading conditions.

Description

Technical Field

[0001] This invention relates to a fracture testing method, specifically a type I fracture testing method under cyclic thermodynamic action. Background Technology

[0002] Compressed air storage (CAS) is a large-scale energy storage and power generation technology with broad application prospects. It utilizes energy to drive a compressor to compress air, which is then stored in surface containers or underground gas storage facilities. When needed, the air is released to generate electricity. Compared to other energy storage methods such as pumped hydro storage, CAS has the advantages of lower capital investment and maintenance costs. Furthermore, unlike pumped hydro storage, CAS is not constrained by water resource distribution, environmental protection, or construction timelines, making it more suitable for areas lacking the natural conditions for building pumped hydro storage power stations. It is currently the best form of large-scale backup energy storage. CAS power stations have large underground caverns (hundreds of thousands of cubic meters). 3 Up to tens of millions of m 3 The compressed air storage (CAS) caverns operate under high air pressure (5-10 MPa or higher). During energy storage and power generation, compressed air repeatedly circulates through the cavern walls, accompanied by cyclical temperature changes. This results in cyclic stress and temperature characteristics in the surrounding rock. The bearing capacity and fracture propagation mode of the fractured surrounding rock under cyclic thermal action are crucial indicators for the stability of CAS caverns. The fracture evolution mechanism of fractured surrounding rock under cyclic thermal action is fundamental to the theoretical research on the stability and availability control of CAS caverns. It is one of the key scientific problems and significant challenges that urgently need to be addressed in the construction of underground CAS caverns. Therefore, it is necessary to conduct Type I fracture tests on the rock under cyclic thermal action.

[0003] Existing Type I fracture testing methods involve directly subjecting the specimen to a three-point bending test and loading, which fails to couple with the effects of cyclic temperature and cannot accurately reflect the thermal response characteristics of the surrounding rock in a compressed gas storage cavern. Therefore, this invention proposes a Type I fracture testing method under cyclic thermal action. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for a type I fracture test under cyclic thermodynamic action, which is in view of the shortcomings of the prior art. This test method has the advantages of simple steps, quick and convenient testing, strong practicality, easy operation, wide applicability, and can be combined according to different actual working conditions to meet the needs of type I fracture test under different specimen materials, loading temperatures, and loading conditions.

[0005] The technical solution adopted by this invention to solve the above-mentioned technical problems is as follows: a method for testing type I fracture under cyclic thermodynamic action, comprising the following steps:

[0006] 1) Take samples of the rock material on site and process the rock material into cuboid specimens;

[0007] 2) Pre-fabricate cracks at the leftmost, middle, and rightmost ends of the specimen's bottom along its length by cutting, resulting in a left crack, a middle crack, and a right crack, which penetrate the width direction of the specimen.

[0008] 3) Apply multiple stress-temperature cycles to the specimen according to the preset stress and temperature range, record the stress-strain change curve in each stress-temperature cycle, calculate the elastic modulus E and record it;

[0009] 4) Perform a three-point bending test on the specimen after applying multiple stress-temperature cycles. The loading positions during the three-point bending test are the center of the top end face of the specimen and the parts near the left and right cracks on both sides of the bottom end face of the specimen. Record the stress-strain change curves during the three-point bending test and the crack propagation evolution during the loading process of the three-point bending test until the specimen fails.

[0010] 5) The peak load S is obtained from the stress-strain change curve in the three-point bending test. Combined with the elastic modulus E calculated in the last stress-temperature cycle, the fracture energy Gc of the specimen under cyclic thermodynamic action is calculated.

[0011] Preferably, in step 3), the stress-temperature cycle process is as follows: apply a specified stress to the side of the specimen → unload → place in a heating furnace → set the temperature value → heating process → hold at a constant temperature for 1 hour → cool down to 25°C step by step → remove the specimen from the furnace.

[0012] Preferably, the heating rate for each heating process is <5℃ / min, and the cooling rate for each stage of cooling is <2.5℃ / min.

[0013] Preferably, during each stress-temperature cyclic loading, an LVDT displacement sensor controls a servo motor to simultaneously apply pressure to both ends of the specimen along its length. The pressure points are the portions of the end faces of both ends of the specimen along its length, excluding the left and right cracks. A shim of the same size as the pressure point is placed at the pressure point. The loading speed is <0.018 mm / min. The stress-strain change curves during each loading process are recorded by the LVDT displacement sensor, and the elastic modulus E is calculated and recorded.

[0014] Preferably, before each three-point bending test, digital speckle patterns are applied to the surface of the specimen after multiple stress-temperature cycles. During each three-point bending test, an LVDT displacement sensor controls a servo motor to load the specimen at a loading speed of <0.018 mm / min. The LVDT displacement sensor records the stress-strain change curves during each loading process, calculates and records the peak load S, and simultaneously photographs the digital speckle pattern on the specimen surface using a high-precision, high-speed camera to record the crack propagation and evolution during the loading process.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) The method of this invention breaks through the traditional three-point bending test, couples the three-point bending test with stress-temperature cycle, and extends the three-point bending test to the thermodynamic test conditions. By measuring the peak load S in the three-point bending test and the elastic modulus E in the last stress-temperature cycle, the fracture energy Gc of the specimen under the cyclic thermodynamic action can be effectively calculated, which helps to truly reflect the fracture bearing capacity and fracture propagation mode of the compressed gas storage cavern under the cyclic thermodynamic action.

[0017] (2) The method of the present invention can couple stress and temperature cyclically and measure the change of elastic modulus of the specimen under cyclic stress-temperature.

[0018] (3) The method of the present invention has the advantages of simple steps, quick and convenient testing, strong practicality, and easy operation. It is also widely applicable and can be combined according to different actual working conditions to meet the needs of Type I fracture test under different specimen materials, loading temperature, loading load and other conditions. Attached Figure Description

[0019] Figure 1 This is a front view and dimensions of the cuboid specimen processed in the example;

[0020] Figure 2 This is a side view and dimensions of the cuboid specimen processed in the example;

[0021] Figure 3 This is a schematic diagram of a stress-temperature cycle process;

[0022] Figure 4 A schematic diagram of the pressure application location during each stress-temperature cycle loading;

[0023] Figure 5 This is a schematic diagram of the loading positions during the three-point bending test. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] The method for testing type I fracture under cyclic thermodynamic action in this embodiment includes the following steps:

[0026] 1) Take samples of the rock material on site and process the rock material into 5 rectangular specimens of the same size (400mm×100mm×100mm). Perform the following steps 2) to 3) on each specimen to obtain 5 precast test specimens that have undergone stress-temperature cycles of 5, 10, 20, 40 and 60 times.

[0027] 2) Cracks were pre-fabricated at the leftmost, middle, and rightmost ends of the specimen's bottom along its length, resulting in left crack 1, middle crack 2, and right crack 3. Each crack extends through the width of the specimen, with dimensions of 2mm × 15mm × 100mm. A frontal view and dimensions of the specimen are shown below. Figure 1 As shown, the side view and dimensions are as follows. Figure 2 As shown;

[0028] 3) Apply multiple stress-temperature cycles to the specimen according to the preset stress (5MPa, 10MPa, 15MPa, 20MPa) and temperature range (60℃, 90℃, 120℃, 150℃), record the stress-strain change curve in each stress-temperature cycle, calculate the elastic modulus E and record it, where:

[0029] like Figure 3 As shown, each stress-temperature cycle process is as follows: Apply a specified stress to the side of the specimen → unload (simultaneously unload the stress at both ends of the specimen to 0 MPa) → place in a heating furnace → set the temperature value → heating process → hold at temperature for 1 hour (the purpose is to ensure uniform heating inside the specimen) → gradually cool to 25℃ → remove the specimen from the furnace. Specifically: during each stress-temperature cycle, an LVDT displacement sensor is used to control a servo motor to simultaneously apply pressure to both ends of the specimen along its length, such as... Figure 4 As shown, the pressure application area is the portion of both end faces of the specimen along its length, excluding the left crack 1 and the right crack 3. A shim of the same size as the pressure application area (not shown in the figure) is placed on the pressure application area. To avoid impact loads, the loading speed is controlled to be <0.018 mm / min. The stress-strain change curves during each loading process are recorded by an LVDT displacement sensor, and the elastic modulus E is calculated and recorded. The heating rate during each heating process is <5℃ / min, and the cooling rate during each step-by-step cooling process is <2.5℃ / min. After step-by-step cooling to 25℃, the specimen is left to stand for 2 hours before being removed from the furnace.

[0030] 4) After applying digital speckle patterns to the front of each of the five precast test specimens, a three-point bending test was performed on each of the five precast test specimens. Figure 5As shown, the loading positions during the three-point bending test were point A at the center of the top end face of the specimen, and points B near the left crack 1 and point C near the right crack 3 on both sides of the bottom end face of the specimen (the horizontal distance between point B and point A is 180 mm, and the horizontal distance between point C and point A is 180 mm). No constraints were applied to the other positions except for points A, B, and C. During each three-point bending test loading, an LVDT displacement sensor was used to control the servo motor to load the specimen. To avoid impact loads, the loading speed was controlled to be <0.018 mm / min. The stress-strain change curves during each loading process were recorded by the LVDT displacement sensor, the peak load S was calculated and recorded, and at the same time, a high-precision high-speed camera photographed the digital speckle pattern on the surface of the specimen at a sampling frequency of 0.001 s to record the crack propagation and evolution during the loading process until the specimen failed.

[0031] 5) For each precast specimen, the peak load S is obtained from the stress-strain change curve in the three-point bending test. Combined with the elastic modulus E calculated in the last stress-temperature cycle, the fracture energy Gc of the specimen under cyclic thermodynamic action is calculated.

Claims

1. A cyclic thermomechanical I -fracture test method, characterized by, The method comprises the following steps: 1) on-site sampling of rock material, processing the rock material into a cuboid test piece; 2) pre-preparing a left crack, a middle crack and a right crack at the left end, the middle and the right end of the length direction of the bottom of the test piece respectively by cutting, and the left crack, the middle crack and the right crack respectively penetrating the width direction of the test piece; 3) applying multiple temperature-stress cycles to the test piece according to a preset stress and temperature range, recording the stress-strain change curve in each temperature-stress cycle, calculating the elastic modulus E and recording, wherein the process of each temperature-stress cycle is: putting into a heating furnace → setting a temperature value → a heating process → constant temperature for 1 hour → gradually cooling to 25℃ → taking the test piece out of the furnace → loading the side surface of the test piece in the width direction to a specified stress → unloading; 4) performing a three-point bending test on the test piece after multiple temperature-stress cycles, the loading position in the three-point bending test being the central part of the top end surface of the test piece and the parts close to the left crack and the right crack on both sides of the bottom end surface of the test piece, recording the stress-strain change curve in the three-point bending test and the crack propagation evolution in the loading process of the three-point bending test until the test piece is damaged; 5) obtaining the peak load S from the stress-strain change curve in the three-point bending test, and calculating the fracture energy Gc of the I-type fracture of the test piece under the action of cyclic thermal force in combination with the elastic modulus E calculated in the last temperature-stress cycle.

2. The method of claim 1, wherein the cyclic thermal action is a thermal cycle comprising a temperature increase from 20°C to 100°C and a temperature decrease from 100°C to 20°C. The heating speed of each heating process is <5℃ / min, and the cooling speed of each gradual cooling is <2.5℃ / min.

3. A method of I- fracture testing under cyclic thermal action according to claim 1 or 2, characterised by the fact that In each temperature-stress cycle loading, the LVDT displacement sensor is used to control the servo machine to press the two ends of the test piece in the length direction at the same time, the pressing parts being the parts of the two end surfaces in the length direction of the test piece except the left crack and the right crack, a gasket with the same size as the pressing part is added at the pressing part, the loading speed is <0.018mm / min, the stress-strain change curve in each loading process is recorded by the LVDT displacement sensor, the elastic modulus E is calculated and recorded.

4. The method of claim 1 or 2, wherein the cyclic thermal action is a thermal cycle of 5 °C to 95 °C. Before each three-point bending test, digital speckle is pasted on the surface of the test piece after multiple temperature-stress cycles; in each three-point bending test loading, the LVDT displacement sensor is used to control the servo machine to load the test piece, the loading speed is <0.018mm / min, the stress-strain change curve in each loading process is recorded by the LVDT displacement sensor, the peak load S is calculated and recorded, and at the same time, the digital speckle on the surface of the test piece is photographed by a high-precision high-speed camera, and the crack propagation evolution in the loading process is recorded.

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