Rock tensile fracture testing apparatus and method

Abstract

The rock tensile fracture testing device relates to the rock mechanics testing technical field, and the expansion shaft comprises a guide cylinder, a center expander and a batten top pressing component. The guide cylinder is a hollow shaft structure, and a plurality of guide grooves are formed on the surface of the shaft body in the radial direction. The center expander is installed in the hollow inner cavity of the guide cylinder. A plurality of batten top pressing components are slidably assembled in the guide grooves in the radial direction through guide rods. The inner surface of the ring-shaped rock test piece to be measured is provided with at least one prefabricated cutting joint, and the expansion shaft is installed in the inner cavity of the ring-shaped rock test piece. The center expander is pressurized and expanded to push the batten top pressing component to move outward in the radial direction, so that the plurality of arc-shaped battens apply radial pressure to the ring-shaped rock test piece in different directions of the circumference. The device can uniformly apply radial pressure to the whole circumference of 360 degrees, the stress state is clear, the complex stress correction of indirect loading is not needed, a lever mechanism is not needed, and the integration degree of the device is high.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics testing technology, specifically to a rock tensile fracture testing device and method. Background Technology

[0002] Rock fracture toughness (KIC) is an important mechanical parameter in rock mechanics, characterizing a material's ability to resist crack propagation. It has wide applications in engineering fields such as surrounding rock stability analysis in underground engineering, hydraulic fracturing design in deep rock masses, oil and gas reservoir stimulation, and rock cutting and fracturing. Accurate determination of rock fracture toughness is crucial for assessing the instability risk of engineering rock masses, optimizing fracturing parameters, and predicting crack propagation paths. Therefore, rock fracture toughness testing methods have always been a research focus in rock mechanics and engineering geology.

[0003] Currently, the standards and commonly used methods for determining the fracture toughness of rocks mainly include the Cracked Chevron Notched Brazilian Disc (CCNBD), Semi-Circular Bend (SCB), and Rectangular Three-Point Bend methods recommended by the International Society for Rock Mechanics (ISRM). All of these methods require mounting the specimen on a dedicated testing machine or a dedicated rock mechanics testing system. Tensile stress is generated at the tip of the notch by applying indirect loading methods such as radial compression or three-point bending, and then the fracture toughness is indirectly calculated through numerical calibration or empirical formulas.

[0004] However, the above-mentioned indirect loading methods have the following shortcomings in practical applications: First, the stress field generated inside the specimen by indirect loading is relatively complex, and the stress state at the slit tip needs to be obtained indirectly by relying on three-dimensional numerical calibration or empirical calibration formulas, resulting in large indirectness of test results and multiple sources of error; Second, radial compression loading will generate significant stress concentration in the contact area between the specimen and the indenter, which is prone to inducing parasitic failure in non-slit areas and interfering with the accurate assessment of fracture toughness; Third, such methods have high requirements for specimen preparation, especially the V-slit Brazilian disc method, which is very sensitive to the position, angle and dimensional tolerance of the slit, making specimen processing difficult and consistency hard to guarantee; Fourth, the above methods all rely on universal testing machines or large-scale dedicated rock mechanics testing systems, which are large in size, costly, and have limited versatility, making it difficult to meet the needs of rapid on-site or in-situ testing.

[0005] Therefore, how to apply tensile stress to rock specimens and achieve rock fracture toughness testing with minimal reliance on large-scale testing equipment is a technical problem that needs to be solved. Summary of the Invention

[0006] The core of this invention is to provide a rock tensile fracture testing device. By installing a miniaturized expansion shaft inside the rock specimen and applying 360° radial pressure, loading can be achieved, enabling the testing of rock fracture toughness. The specific scheme is as follows: A rock tensile fracture testing apparatus includes an expansion shaft, the expansion shaft comprising: The guide cylinder is a hollow shaft-shaped structure with several guide grooves radially formed on the surface of the shaft. A central expander is installed in the hollow inner cavity of the guide cylinder; the inner cavity of the central expander is used to fill hydraulic oil. The slat pressing component is provided in multiple forms, each slat pressing component including an arc-shaped slat, a guide rod, and a pressure-bearing block; the two ends of the guide rod are respectively fixed to the arc-shaped slat and the pressure-bearing block, and the guide rod is slidably assembled in the guide groove along the radial direction; The inner surface of the annular rock specimen to be tested is provided with at least one pre-made slit, and the expansion shaft is located in the inner cavity of the annular rock specimen; the central expander pressurizes and expands to push the slat pressing member to move radially outward, so that the multiple arc-shaped slats apply radial pressure to the annular rock specimen in different directions toward the circumference.

[0007] Optionally, the pre-cut slit is aligned with the gap between two adjacent arc-shaped strips.

[0008] Optionally, the number of the slat pressing members is even, and they are axially symmetrical about the precast cut.

[0009] Optionally, the number of the slat pressing members is four, the number of the prefabricated cuts is two, and the prefabricated cuts are distributed on the same straight line.

[0010] Optionally, the outer side of the arc-shaped strip is covered with an elastic buffer layer, which contacts the inner surface of the annular rock specimen.

[0011] Optionally, along the axial direction, the length of the arc-shaped strip is equal to or greater than the length of the annular rock specimen.

[0012] Optionally, it also includes: The oil supply system includes a hydraulic pump, a hydraulic pressure regulating valve, and an unloading valve. The hydraulic pump is connected to the inner cavity of the central expander via a high-pressure oil pipe and is used to inject high-pressure hydraulic oil into the central expander to drive the slat pressing member to move radially. The hydraulic pressure regulating valve is installed on the high-pressure oil pipe and is used to control the loading rate of the hydraulic oil injected into the central expander. The unloading valve is used to release pressure after the test to allow the central expander to retract. The pressure measurement system includes a pressure sensor installed in the oil inlet pipeline of the central expander and a digital pressure recorder connected to the pressure sensor. The pressure sensor is used to acquire in real time and the digital pressure recorder is used to record the peak breaking oil pressure in the inner cavity of the central expander. The calculation unit is used to calculate the Type I fracture toughness (KIC) of the annular rock specimen based on the peak failure oil pressure and the geometric parameters of the pre-cut slot of the annular rock specimen, and on the fracture mechanics relationship of uniform pressure on the inner wall of the thick-walled cylinder.

[0013] Optionally, it also includes:

[0014] Extensometers are installed on both sides of the precast slit to monitor the crack aperture and crack opening rate;

[0015] Acoustic emission sensors are used to monitor the crack initiation and propagation process; A high-speed camera system is used to record the morphology of the fracture process.

[0016] The present invention also provides a method for testing rock tensile fracture, applied to the above-mentioned rock tensile fracture testing apparatus, comprising: S1. Prepare an annular rock specimen with an inner hole, and process a pre-made slit at a predetermined position on its inner wall, the slit depth being a, and the wall thickness of the annular rock specimen being W; S2. Insert the guide tube into the inner hole of the annular rock specimen so that the prefabricated cut is aligned with the gap between the two arc-shaped strips; S3. Inject hydraulic oil into the inner cavity of the central expander, causing the slat pressing member to move radially outward along the guide groove and fit against the inner wall of the annular rock specimen, applying a uniform radial pressure of 360° to the inner surface of the specimen, and forming a type I stress concentration at the tip of the prefabricated cut. S4. As the oil pressure increases, when the rock fracture toughness is reached, instability and crack initiation occur and propagate to the fracture of the specimen. The peak failure oil pressure P is collected and recorded in real time. S5. Based on the peak failure oil pressure P, the pre-cut slit depth a, and the geometric factor Y, calculate the Type I fracture toughness KIC of the annular rock specimen according to the fracture mechanics relationship of uniform compression on the inner wall of the thick-walled cylinder: The geometric factor Y is the kerf depth a, wall thickness W, and inner diameter D. i Outer diameter D o The function is determined by finite element numerical analysis or standard solution of fracture mechanics.

[0017] Optionally, in step S3, hydraulic oil is injected into the inner cavity of the central expander at a constant rate, and the constant loading rate is controlled by a hydraulic pressure regulating valve. During steps S3 to S4, an extensometer is used to monitor the crack opening and opening rate on both sides of the prefabricated slit, and an acoustic emission sensor is used to monitor the crack initiation and propagation process. During the crack initiation and fracture process in step S4, a high-speed camera system is used to record the fracture morphology, and the reliability of the geometric factor calibration is verified based on the consistency of test results of multiple sets of specimens from the same batch with different cut depths.

[0018] This invention provides a rock tensile fracture testing device. By setting up a guide cylinder, a central expander, and multiple slat pressing components, the central expander expands hydraulically, pushing multiple arc-shaped slats to synchronously translate radially outward along the guide groove. This applies uniform radial pressure in different directions towards the circumference to the inner surface of the annular rock specimen, thereby forming a pure Type I tensile stress field at the tip of the pre-fabricated slit. This enables the testing of the fracture toughness of brittle materials like rocks under pure tensile mode, and the test results are closer to the true Type I fracture toughness of the rock. The device has a simple structure, requiring no lever mechanism or large testing frame, and has a high degree of integration. The annular specimen, combined with a single slit, simplifies specimen preparation. By changing the expansion shaft with different inner diameters, it can accommodate specimens of different sizes, or the test range can be adjusted by the slit parameters, demonstrating good applicability.

[0019] By installing extensometers on both sides of the precast cut, the crack opening and crack opening rate can be accurately monitored, providing quantitative basis for determining the crack initiation point and stable propagation stage; the acoustic emission sensor can capture elastic wave signals in real time during the crack initiation and propagation process, enabling accurate determination of the initiation time; the high-speed camera system can record the fracture process morphology, providing intuitive image data for studying the dynamic fracture behavior of rocks, and obtaining information on the entire fracture process from multiple dimensions of mechanics, acoustics, and optics. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a frontal view of the expansion axis in the rock tensile fracture test apparatus of the present invention; Figure 2 This is a frontal view of the extensometer mounted on the expansion shaft in the rock tensile fracture testing device of the present invention. Figure 3 This is a top view of the expansion shaft in the rock tensile fracture test apparatus of the present invention. Figure 4This is an isometric schematic diagram of the expansion shaft in the rock tensile fracture testing device of the present invention; Figure 5 This is a schematic diagram of the overall structure of the rock tensile fracture testing device of the present invention.

[0022] The image includes: 100 - Expansion axis; 110-Guide cylinder; 120-Central expander; 130-Slat pressing component; 131-Arc-shaped slat; 132-Guide rod; 133-Pressure block; 140-Extensometer; 200 - Oil supply system; 201 - Oil pump; 202 - Hydraulic pressure regulating valve; 203 - Unloading valve; 300 - Pressure measurement system; 301 - Pressure sensor; 302 - Digital pressure recorder; 400 - Ring-shaped rock specimen; 401 - Precast cut. Detailed Implementation

[0023] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. It should be noted that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Equivalent substitutions made by those skilled in the art to the structural form, connection method, or test parameters without departing from the technical concept of the present invention should all be included within the scope of protection of the present invention.

[0024] like Figures 1 to 4 As shown, the present invention provides a rock tensile fracture testing device for directly tensile fracture testing of annular rock specimens 400 with pre-existing cracks. By applying uniform radial pressure to the inner wall of the annular rock specimen 400, circumferential tensile stress is generated inside the annular rock specimen 400, and a stress concentration area is formed at the tip of the pre-existing cut 401, thereby inducing the crack to propagate along a preset direction, so as to achieve the testing of the fracture performance of rock materials.

[0025] The rock tensile fracture testing apparatus includes an expansion shaft 100, such as Figure 1 , Figure 2 , Figure 3 , Figure 4 As shown, the expansion shaft 100 includes a guide cylinder 110, a central expander 120, and a slat pressing member 130. The guide cylinder 110 is a hollow cylindrical structure, extending through both ends axially, forming an installation space inside for mounting the central expander 120. Several guide grooves are radially formed on the surface of the guide cylinder 110. These guide grooves are evenly distributed circumferentially along the guide cylinder 110, and each guide groove guides the corresponding slat pressing member 130, enabling the slat pressing member 130 to reciprocate along the radial direction of the guide cylinder 110.

[0026] The number of lath top-compression members 130 can be adjusted according to the size of the annular rock specimen 400, loading uniformity requirements, etc. It can be set to two, four, six, or more to improve the uniformity of radial loading. Figure 1 The example shown is a four-slat top-compression member 130.

[0027] The central expander 120 is installed in the hollow inner cavity of the guide cylinder 110, forming a sealed oil reservoir inside for filling with hydraulic oil. When the test begins, high-pressure hydraulic oil is introduced into the central expander 120, causing it to expand radially under internal pressure. As the central expander 120 expands, its expansion force acts on the lath pressing member 130 located in the guide groove, causing the lath pressing member 130 to move outward along the direction defined by the guide groove. The lath pressing member 130 applies pressure to the inner surface of the annular rock specimen 400.

[0028] The central expander 120 can adopt a structure such as an oil bladder or a hydraulic piston to ensure that the force applied to each slat pressing component 130 is equal.

[0029] Each lath pressing member 130 includes an arc-shaped lath 131, a guide rod 132, and a bearing block 133. The guide rod 132 extends radially along the annular rock specimen 400. The two ends of the guide rod 132 are fixedly connected to the arc-shaped lath 131 and the bearing block 133, respectively. The guide rod 132 passes through the guide groove on the guide cylinder 110 and can slide radially along the guide groove. The guide rod 132 cooperates with the guide groove to ensure that the lath pressing member 130 can only translate radially.

[0030] The pressure-bearing block 133 is located inside the guide cylinder 110 and is used to withstand the expansion force generated by the central expander 120 and transmit this force to the guide rod 132 and the arc-shaped slat 131. The arc-shaped slat 131 is located outside the guide cylinder 110 and transmits the radial force generated by the expansion shaft 100 to the annular rock specimen 400. The cross-sectional areas of both the pressure-bearing block 133 and the arc-shaped slat 131 are larger than the cross-sectional area of ​​the guide rod 132, ensuring that the pressure-bearing block 133 and the arc-shaped slat 131 cannot pass through the guide groove.

[0031] As the hydraulic oil pressure inside the central expander 120 gradually increases, the central expander 120 pushes multiple pressure blocks 133 to move outward synchronously. The pressure blocks 133 drive the guide rod 132 to slide along the guide groove, causing multiple arc-shaped strips 131 to expand outward synchronously. The multiple arc-shaped strips 131 can act on the inner wall of the annular rock specimen 400 at the same time and apply a circumferentially uniform radial pressure to the annular rock specimen 400.

[0032] Through the above structure, the expansion shaft 100 can convert hydraulic pressure into mechanical radial loading force, so that the annular rock specimen 400 is uniformly loaded from the inside. This invention employs a direct loading method on the inner wall of the annular rock specimen 400, making the loading path clearer and reducing the impact of additional stress generated in the external contact area. Furthermore, the arc-shaped strip 131 forms a large contact surface with the annular rock specimen 400, making stress concentration less likely.

[0033] A pre-fabricated notch 401 is provided on the inner surface of the annular rock specimen 400. Because the pre-fabricated notch 401 alters the local structural continuity, a stress concentration region forms at the tip of the notch 401. As the loading pressure continues to increase, the stress intensity factor at the tip of the notch 401 gradually increases. When it reaches the Mode I Fracture Toughness (KIC) of the rock material itself, instability and crack initiation occur at the tip of the notch 401, and the crack propagates along a predetermined direction, ultimately leading to the fracture of the annular rock specimen 400. Mode I fracture is a fracture mode in which the crack surface is subjected to an opening force perpendicular to the crack surface, causing the crack to propagate along the opening direction.

[0034] By recording the peak failure oil pressure at the moment the annular rock specimen 400 fractures, and combining the dimensional parameters of the precast cut 401 and the geometric parameters of the annular rock specimen 400, the type I fracture toughness KIC of the rock material can be calculated.

[0035] It should be noted that fracture toughness (KIC) is a material's ability to resist crack propagation, and its unit is 1. Stress intensity factor K is a key mechanical parameter in fracture criterion. It is a parameter characterizing the intensity of the stress field at the crack tip; when K reaches the fracture toughness of the material... At that time, the crack began to propagate unstably.

[0036] The rock tensile fracture testing device provided by this invention employs an internal hydraulic expansion loading method, applying radial pressure directly to the annular rock specimen 400 via the expansion shaft 100. This avoids the complex stress transformation caused by traditional indirect loading methods and improves the clarity of the stress state at the crack tip. Multiple lath-type pressing components 130 move synchronously, ensuring uniform circumferential loading on the annular rock specimen 400. The annular rock specimen 400 has multiple stress points, all with radial force distribution, improving the consistency of load distribution during the test. The device has a simple structure, eliminating the need for complex loading conversion mechanisms; direct tensile fracture testing of rock can be achieved solely through hydraulic pressure, reducing the complexity of the testing equipment. The Type I fracture toughness (KIC) of the rock material is obtained by monitoring the oil pressure at fracture.

[0037] Based on the above embodiments, the present invention provides at least one pre-fabricated slit 401 on the inner surface of the annular rock specimen 400. Typically, one or two pre-fabricated slits 401 are formed by radial cutting on the inner surface of the annular rock specimen 400. The pre-fabricated slits 401 are used to form a predetermined crack initiation position inside the annular rock specimen 400, so that cracks preferentially originate from the tip of the pre-fabricated slits 401 during the test.

[0038] The pre-cut slot 401 can be formed on the inner surface of the annular rock specimen 400 by mechanical cutting, laser processing or other processing methods that can form regular slots.

[0039] When the expansion shaft 100 is installed in the inner cavity of the annular rock specimen 400, the pre-fabricated cut 401 is aligned with the gap between two adjacent arc-shaped strips 131. When the arc-shaped strips 131 apply radial pressure to the inner wall of the annular rock specimen 400, the pre-fabricated cut 401 is not obstructed by the arc-shaped strips 131, ensuring the formation of a continuous circumferential tensile stress field near the pre-fabricated cut 401.

[0040] The precast slit 401 can adopt a rectangular groove structure, or it can be set to different shapes according to different fracture mode requirements, such as: V-shaped groove structure, oblique slit structure with different inclination angles.

[0041] To ensure uniform stress distribution on the annular rock specimen 400, the number of lath compression members 130 is set to an even number, and the multiple lath compression members 130 are axially symmetrically distributed about the precast cut 401. The multiple lath compression members 130 are evenly arranged circumferentially along the expansion axis 100. When four lath compression members 130 are set, they are evenly distributed at 90°; when six lath compression members 130 are set, they are evenly distributed at 60°; and when eight lath compression members 130 are set, they are evenly distributed at 45°.

[0042] Because the lath top pressure member 130 is arranged symmetrically about the precast cut 401, the radial pressure on the annular rock specimen 400 can remain symmetrical about the crack initiation position, which can reduce the off-center loading phenomenon caused by uneven loading and make the tip of the precast cut 401 form a more stable I-type opening stress state.

[0043] like Figure 2 As shown, in a specific embodiment, the number of slat pressing members 130 is preferably set to four, and the four slat pressing members 130 are evenly distributed circumferentially along the expansion axis 100. Correspondingly, there are two prefabricated slits 401, and the two prefabricated slits 401 are distributed on the same straight line, with the straight line of the two prefabricated slits 401 being the axis of symmetry, and two slat pressing members 130 are respectively provided on both sides.

[0044] To prevent localized stress in the annular rock specimen 400, the circumferential gap between adjacent arc-shaped strips 131 is minimized, for example, the central angle of the circumferential gap is less than 10°, ensuring a sufficiently large contact area between the arc-shaped strips 131 and the annular rock specimen 400. The use of four strip-supporting members 130 ensures adequate stress distribution while avoiding excessive gaps.

[0045] In one specific embodiment, to reduce the impact of rigid contact between the arc-shaped slat 131 and the annular rock specimen 400, an elastic buffer layer is wrapped around the outside of the arc-shaped slat 131. The elastic buffer layer is disposed in the contact area between the arc-shaped slat 131 and the inner surface of the annular rock specimen 400. When the arc-shaped slat 131 moves outward, the elastic buffer layer first contacts the inner wall of the annular rock specimen 400.

[0046] The elastic buffer layer has a certain elastic deformation capacity, which can improve the fit between the arc-shaped strip 131 and the inner wall of the annular rock specimen 400, reduce the local crushing phenomenon caused by the direct contact of the metal arc-shaped strip 131 with the rock, and improve the uniformity of radial pressure transmission.

[0047] The elastic buffer layer can be made of a variety of materials: rubber, polyurethane elastomer, silicone rubber, or composite elastic materials.

[0048] To ensure that the annular rock specimen 400 can be fully loaded along the axial direction, in this embodiment, the length of the arc-shaped strip 131 along the axial direction of the expansion shaft 100 is set to be equal to or greater than the length of the annular rock specimen 400. For example... Figure 3 As shown, the axial length of the arc-shaped strip 131 is greater than the axial length of the annular rock specimen 400. The arc-shaped strip 131 can cover the entire axially loaded area of ​​the annular rock specimen 400.

[0049] Furthermore, the rock tensile fracture testing apparatus of the present invention also includes an oil supply system 200, a pressure measurement system 300, and a calculation unit. The expansion shaft 100 serves as the main loading execution component, used to convert hydraulic pressure into a radial loading force acting on the inner wall of the annular rock specimen 400. The oil supply system 200 is used to supply high-pressure hydraulic oil to the expansion shaft 100 to drive the expansion shaft 100 to generate radial expansion action. The pressure measurement system 300 is used to acquire the changes in hydraulic pressure inside the expansion shaft 100 in real time and record the peak failure oil pressure corresponding to the fracture of the annular rock specimen 400.

[0050] like Figure 5 As shown, the oil supply system 200 includes a hydraulic pump 201, a hydraulic pressure regulating valve 202, and an unloading valve 203. The hydraulic pump 201 is connected to the inner cavity of the central expander 120 through a high-pressure oil pipe and is used to inject high-pressure hydraulic oil into the central expander 120.

[0051] A hydraulic pressure regulating valve 202 is installed on the high-pressure oil pipe connecting the hydraulic pump 201 and the central expander 120. It is used to regulate the flow rate of hydraulic oil into the central expander 120, thereby controlling the rate of pressure change inside the central expander 120. Through the hydraulic pressure regulating valve 202, the loading speed can be adjusted according to the mechanical properties of different types of rock materials, so that the test process meets the quasi-static loading requirements.

[0052] The unloading valve 203 is used to release pressure after the test is completed. After the annular rock specimen 400 completes the fracture test, the unloading valve 203 is opened, allowing the hydraulic oil inside the central expander 120 to return to the oil supply system 200, thereby reducing the internal pressure of the central expander 120. As the pressure in the central expander 120 decreases, the lath pressing member 130 retracts inward under the action of elastic restoring force, separating from the annular rock specimen 400.

[0053] The pressure measurement system 300 is used to detect real-time pressure changes inside the central expander 120 and acquire the peak breaking oil pressure corresponding to the fracture of the annular rock specimen 400. The pressure measurement system 300 includes a pressure sensor 301 and a digital pressure recorder 302. The pressure sensor 301 is installed in the oil inlet pipe of the central expander 120 to collect hydraulic oil pressure in real time. The digital pressure recorder 302 is connected to the pressure sensor 301 to receive and store pressure data. When the annular rock specimen 400 fractures, the internal pressure of the central expander 120 reaches its peak value, and the digital pressure recorder 302 records this peak pressure as the peak breaking oil pressure.

[0054] The calculation unit is used to calculate the Type I fracture toughness KIC of the annular rock specimen 400 based on the peak failure oil pressure, the dimensional parameters of the annular rock specimen 400, and the geometric parameters of the precast slot 401. Based on the fracture mechanics relationship under uniform compression conditions on the inner wall of a thick-walled cylinder, the peak failure oil pressure and the Type I fracture toughness KIC of the precast slot 401 are calculated.

[0055] The stress intensity factor K is used to characterize the strength of the stress field in the crack tip region. When the stress intensity factor K reaches the fracture toughness KIC of the rock material, the tip of the precast cut 401 undergoes unstable propagation.

[0056] like Figure 5 As shown, to accurately obtain crack propagation information during the fracture process of the annular rock specimen 400, this embodiment also includes an extensometer 140, an acoustic emission sensor, and a high-speed camera system. Figure 2As shown, extensometers 140 are installed on both sides of the precast cut 401 to detect changes in crack aperture and crack opening rate. During loading, as the pressure of the central expander 120 increases, tensile deformation occurs near the tip of the precast cut 401. The extensometers 140 can collect the relative displacement changes on both sides of the precast cut 401 in real time, thereby obtaining the crack opening process. By analyzing the crack aperture change pattern, the transition process from the stable propagation stage to the unstable propagation stage of the crack can be determined.

[0057] Acoustic emission sensors are used to monitor crack initiation and propagation. Within rock materials, the initiation, propagation, and penetration of cracks release elastic energy and generate acoustic emission signals. Acoustic emission sensors capture the activity of microcracks within the rock. Combined with pressure change data collected by the pressure measurement system 300, the crack evolution patterns corresponding to different loading stages can be analyzed.

[0058] The high-speed camera system is used to record the changes in the external morphology of the annular rock specimen 400 during the fracture process. The high-speed camera system can continuously acquire image information during the crack propagation process, and can be used to observe information such as the initial crack initiation location, crack propagation direction, and final fracture morphology of the precast cut 401.

[0059] The present invention also provides a method for testing rock tensile fracture, which is applied to the above-mentioned rock tensile fracture testing apparatus and includes the following steps: S1. Prepare annular rock specimen 400 and machine pre-cut slits 401. First, select the rock material to be tested according to the experimental requirements, and machine the rock into annular rock specimen 400 with an internal hole structure, forming an installation space for mounting the expansion shaft 100. The annular rock specimen 400 is machined to have an outer diameter D. o , inner diameter D i A ring-shaped specimen with a thickness of T.

[0060] After the ring rock specimen 400 is processed, the end face and inner surface of the ring rock specimen 400 are treated. The end face is ground flat and the inner wall is smooth so that the ring rock specimen 400 meets the test size requirements.

[0061] Subsequently, a pre-fabricated notch 401 is formed at a predetermined position on the inner wall of the annular rock specimen 400. The pre-fabricated notch 401 is used to control the crack initiation location and crack propagation direction. In this embodiment, a pre-fabricated notch (straight groove) is formed at a predetermined position on the inner wall, with a notch depth a and a wall thickness W. By controlling the proportional relationship between the pre-fabricated notch depth a and the wall thickness W, the stress concentration at the crack tip can be adjusted, thus making it suitable for fracture performance testing of different types of rock materials.

[0062] In practical applications, the parameters of the precast slot 401 can be adjusted according to the rock type, test purpose, and target fracture mode. For example, the depth a of the precast slot 401 can be adjusted to change the ease of crack propagation; the width of the precast slot 401 can be adjusted to change the local stress state at the crack tip; and multiple precast slots 401 can be set to study the fracture behavior under the condition of multiple crack interaction.

[0063] S2. Install the expansion shaft 100 and adjust the position of the prefabricated cut 401. After the annular rock specimen 400 is prepared, insert the expansion shaft 100 into the inner hole of the annular rock specimen 400. During installation, ensure that the central axis of the guide cylinder 110 coincides with the central axis of the annular rock specimen 400. Adjust the circumferential position of the expansion shaft 100 so that the prefabricated cut 401 is directly opposite the gap between two adjacent arc-shaped strips 131.

[0064] With the above installation method, the arc-shaped strip 131 can avoid the position of the precast cut 401 during the subsequent loading process, and apply uniform radial pressure to the annular rock specimen 400 from both sides of the precast cut 401.

[0065] After installation, check the relative position between the lath pressing member 130 and the inner wall of the annular rock specimen 400 to ensure that each arc-shaped lath 131 can move outward normally; initial pressure can be applied to the central expander 120 to make the arc-shaped lath 131 contact the annular rock specimen 400 to prevent displacement.

[0066] S3. Hydraulic loading and stress concentration at the crack tip. The hydraulic pump 201 is connected to the central expander 120 via a pipe and quick-connect fitting. When the hydraulic pump 201 is started, hydraulic oil continuously enters the central expander 120, gradually increasing the internal pressure. Under hydraulic pressure, the central expander 120 expands radially, pushing multiple pressure blocks 133 outwards. The pressure blocks 133 drive the guide rod 132 to slide radially along the guide groove on the guide cylinder 110, causing the arc-shaped strip 131 to expand outwards synchronously.

[0067] As the arc-shaped strips 131 gradually come into contact with the inner wall of the annular rock specimen 400, the hydraulic pressure is further increased, causing the arc-shaped strips 131 to apply radial pressure to the inner wall of the annular rock specimen 400. Since multiple arc-shaped strips 131 are evenly distributed circumferentially, the annular rock specimen 400 is subjected to a uniform radial pressure of 360°, and the annular rock specimen 400 generates circumferential tensile stress.

[0068] Since the pre-cut slit 401 alters the local continuity of the annular rock specimen 400, a type I stress concentration region is formed at the tip of the pre-cut slit 401. As the hydraulic pressure continues to increase, the stress intensity factor in the tip region of the pre-cut slit 401 continuously increases.

[0069] S4. Reach the fracture condition and record the peak failure oil pressure. When the stress intensity factor at the tip of the precast cut 401 reaches the fracture critical value corresponding to the rock material, the tip of the precast cut 401 becomes unstable and cracks. The crack starts to propagate from the tip of the precast cut 401 and extends towards the annular rock specimen 400 in a predetermined direction, eventually leading to fracture.

[0070] During the fracture process, the pressure sensor 301 collects the pressure changes inside the central expander 120 in real time, and the digital pressure recorder 302 records the pressure curve throughout the loading process and obtains the peak failure oil pressure P corresponding to the moment of fracture. This peak failure oil pressure P is used as a key test parameter for subsequent calculation of rock fracture toughness KIC.

[0071] S5. Calculate the Type I fracture toughness KIC. Based on the peak failure oil pressure P obtained in step S4, and the geometric parameters such as the depth a of the prefabricated slit 401 and the wall thickness W of the annular rock specimen 400 obtained in step S1, calculate the Type I fracture toughness KIC of the annular rock specimen 400 through fracture mechanics relationships.

[0072] Specifically, based on the fracture mechanics model of a thick-walled cylinder under uniform compression, the following relationship is established:

[0073] Where: KIC represents the type I fracture toughness of the rock material; P represents the peak failure oil pressure corresponding to the fracture of the annular rock specimen at 400°; Y represents the dimensionless geometric factor.

[0074] The geometric factor Y is a dimensionless coefficient related to the specimen and kerf geometry, used to establish the relationship between the critical load and KIC. The geometric factor Y is a / W and D. o / D i The function can be determined by finite element numerical analysis or standard solution of fracture mechanics.

[0075] To improve the stability of the test loading process and the ability to observe the crack propagation process, in step S3, the hydraulic oil input rate is controlled by the hydraulic pressure regulating valve 202 so that the central expander 120 performs pressure loading at a constant loading rate.

[0076] Specifically, after starting the hydraulic pump 201, the hydraulic oil flow rate entering the central expander 120 is kept stable by adjusting the hydraulic pressure regulating valve 202. Since the hydraulic oil input speed remains constant, the internal pressure of the central expander 120 can gradually increase according to the preset loading law, reducing the impact of loading speed changes on the rock fracture process and improving the comparability of test results between different specimens.

[0077] During steps S3 to S4, the extensometer 140 and the acoustic emission sensor are activated simultaneously. The extensometer 140 is installed on both sides of the precast cut 401 to detect changes in crack aperture in real time, thereby obtaining deformation characteristics during both the stable and unstable crack propagation stages. The acoustic emission sensor is used to collect acoustic emission signals generated during the formation of microcracks inside the rock, which can determine the crack initiation time and crack propagation process.

[0078] In step S4, during the crack initiation and fracture of the annular rock specimen 400, a high-speed camera system is activated to continuously capture images of the fracture process, recording the crack formation process and final fracture morphology in the area near the precast cut 401. The image data acquired by the high-speed camera system allows for analysis of information such as the initial crack initiation location, crack propagation direction, and final fracture morphology.

[0079] Here are some specific parameter examples: Example 1: Sandstone Fracture Toughness Test Medium-grained quartz sandstone ring specimen, outer diameter D o =75mm, inner diameter D i =30mm, thickness T=20mm, wall thickness W=22.5mm. A straight kerf was machined on the inner wall of the specimen, with a pre-cut kerf depth a=6mm (a / W≈0.27). An expansion shaft was inserted into the inner hole of the specimen and loaded at a rate of 1.0MPa / s. The specimen fractured at an oil pressure P=4.2MPa. According to the finite element calibration, for D i / D o =0.4, the geometric factor Y corresponding to a / W=0.27 is approximately 1.52, and the fracture toughness KIC is calculated to be 4.2 ×√πa × 1.52 ≈0.876 MPa·m^(1 / 2).

[0080] Example 2: Comparison Test of Different Cut Depths

[0081] Three sets of granite ring specimens from the same batch, with an outer diameter D o =60mm, inner diameter D i =25mm, and three kerf depths of a=3mm, 5mm, and 7mm were machined respectively. Using the same expansion shaft, tests were conducted sequentially, and the failure oil pressure P was recorded for each group. The fracture toughness KIC was calculated according to the corresponding geometric factors. The three sets of calculation results showed good consistency (coefficient of variation <8%), verifying the effectiveness of the method and the reliability of the geometric factor calibration.

[0082] In summary, the present invention provides a rock tensile fracture test device and method. By setting an expansion shaft 100 as the core loading component, the hydraulic pressure inside the central expander 120 drives multiple strip pressing components 130 to move synchronously in the radial direction, so that multiple arc-shaped strips 131 apply uniform radial pressure to the inner wall of the annular rock specimen 400. This changes the loading method of relying on external compression to generate indirect tensile stress in traditional rock fracture testing, and realizes direct tensile fracture testing based on internal pressure loading.

[0083] By arranging multiple lath top pressure members 130 along the circumference, the annular rock specimen 400 is subjected to a uniform radial load of 360°, which reduces the local stress disturbance caused by the concentration of loading points in the traditional loading method, and allows the crack to preferentially start from the precast cut 401 position.

[0084] Compared with the prior art, the advantages of the present invention are as follows: (1) Direct tensile fracture: The uniform radial pressure of 360° around the circumference directly generates different types of fractures at the tip of the precast cut, the stress state is clear, and there is no need for complex stress correction by indirect loading.

[0085] (2) Simple structure: No lever mechanism or large test frame is required, and the device has a high degree of integration.

[0086] (3) High efficiency: It takes about 3-5 minutes from installation to completion of testing.

[0087] (4) Simple specimen preparation: The ring specimen is combined with a single slit, and the processing difficulty is lower than that of the CCNBD and SCB methods.

[0088] (5) Good applicability: It can adapt to different sizes of specimens by changing the expansion shaft with different inner diameters, or adjust the test range by cutting parameters.

[0089] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A rock tensile fracture testing device, characterized in that, Includes an expansion shaft (100), the expansion shaft (100) comprising: The guide cylinder (110) is a hollow shaft-shaped structure with several guide grooves radially opened on the surface of the shaft. A central expander (120) is installed in the hollow inner cavity of the guide cylinder (110); the inner cavity of the central expander (120) is used to fill hydraulic oil; Multiple slat pressing members (130) are provided. Each slat pressing member (130) includes an arc-shaped slat (131), a guide rod (132), and a pressure block (133). The two ends of the guide rod (132) are respectively fixed to the arc-shaped slat (131) and the pressure block (133), and the guide rod (132) is slidably assembled in the guide groove in the radial direction. The inner surface of the annular rock specimen (400) to be tested is provided with at least one pre-made slit (401), and the expansion shaft (100) is located in the inner cavity of the annular rock specimen (400). The central expander (120) is pressurized and expanded to push the slat pressing member (130) to move radially outward, so that the multiple arc-shaped slats (131) apply radial pressure to the annular rock specimen (400) in different directions toward the circumference.

2. The rock tensile fracture testing apparatus according to claim 1, characterized in that, The prefabricated slit (401) is directly opposite the gap between two adjacent arc-shaped strips (131).

3. The rock tensile fracture testing apparatus according to claim 2, characterized in that, The number of the slat top pressing members (130) is even, and they are axially symmetrical about the prefabricated cut (401).

4. The rock tensile fracture testing apparatus according to claim 3, characterized in that, The number of the slat top pressing members (130) is four, the number of the prefabricated cuts (401) is two, and the prefabricated cuts (401) are distributed on the same straight line.

5. The rock tensile fracture testing apparatus according to claim 3, characterized in that, The outer side of the arc-shaped strip (131) is covered with an elastic buffer layer, which contacts the inner surface of the annular rock specimen (400).

6. The rock tensile fracture testing apparatus according to claim 3, characterized in that, Along the axial direction, the length of the arc-shaped strip (131) is equal to or greater than the length of the annular rock specimen (400).

7. The rock tensile fracture testing apparatus according to any one of claims 2-6, characterized in that, Also includes: The oil supply system (200) includes an oil pump (201), a hydraulic pressure regulating valve (202), and an unloading valve (203). The oil pump (201) is connected to the inner cavity of the central expander (120) through a high-pressure oil pipe and is used to inject high-pressure hydraulic oil into the central expander (120) to drive the slat pressing member (130) to move radially. The hydraulic pressure regulating valve (202) is installed on the high-pressure oil pipe and is used to control the loading rate of injecting hydraulic oil into the central expander (120). The unloading valve (203) is used to release pressure after the test to allow the central expander (120) to retract. The pressure measurement system (300) includes a pressure sensor (301) installed in the oil inlet pipeline of the central expander (120) and a digital pressure recorder (302) connected to the pressure sensor (301). The pressure sensor (301) is used to collect in real time and the digital pressure recorder (302) is used to record the peak breaking oil pressure inside the central expander (120). The calculation unit is used to calculate the Type I fracture toughness KIC of the annular rock specimen (400) based on the peak failure oil pressure and the geometric parameters of the prefabricated cut (401) of the annular rock specimen (400) and the fracture mechanics relationship of uniform pressure on the inner wall of the thick-walled cylinder.

8. The rock tensile fracture testing apparatus according to claim 7, characterized in that, Also includes: An extensometer (140) is installed on both sides of the precast slit (401) to monitor the crack aperture and crack opening rate; Acoustic emission sensors are used to monitor the crack initiation and propagation process; A high-speed camera system is used to record the morphology of the fracture process.

9. A method for testing the tensile fracture of rock, characterized in that, The rock tensile fracture testing apparatus according to any one of claims 1-8 comprises: S1. Prepare an annular rock specimen (400) with an inner hole, and process a pre-made slit (401) at a predetermined position on its inner wall, the slit depth being a, and the wall thickness of the annular rock specimen (400) being W; S2. Insert the guide tube (110) into the inner hole of the annular rock specimen (400) so that the prefabricated slit (401) is aligned with the gap between the two arc-shaped strips (131); S3. Inject hydraulic oil into the inner cavity of the central expander (120) to make the slat pressing member (130) move radially outward along the guide groove and fit against the inner wall of the annular rock specimen (400), applying a uniform radial pressure of 360° to the inner surface of the specimen, and forming a type I stress concentration at the tip of the prefabricated cut (401); S4. As the oil pressure increases, when the rock fracture toughness is reached, instability and crack initiation occur and propagate to the fracture of the specimen. The peak failure oil pressure P is collected and recorded in real time. S5. Based on the peak failure oil pressure P, the pre-cut groove depth a, and the geometric factor Y, calculate the Type I fracture toughness KIC of the annular rock specimen (400) according to the fracture mechanics relationship of uniform compression on the inner wall of the thick-walled cylinder: The geometric factor Y is the kerf depth a, wall thickness W, and inner diameter D. i Outer diameter D o The function is determined by finite element numerical analysis or standard solution of fracture mechanics.

10. The rock tensile fracture test method according to claim 9, characterized in that, In step S3, hydraulic oil is injected into the cavity of the central expander (120) at a constant rate, and the constant loading rate is controlled by the hydraulic pressure regulating valve (202); During steps S3 to S4, an extensometer (140) is used to monitor the crack opening and opening rate on both sides of the prefabricated slit (401), and an acoustic emission sensor is used to monitor the crack initiation and propagation process. During the crack initiation and fracture process in step S4, a high-speed camera system is used to record the fracture morphology, and the reliability of the geometric factor calibration is verified based on the consistency of test results of multiple sets of specimens from the same batch with different cut depths.

Images