True triaxial rock sample strain test device and method based on DIC

Abstract

The invention belongs to the technical field of geotechnical engineering tests, and particularly discloses a true triaxial rock sample strain test device and method based on DIC, the device comprises a true triaxial test system and a DIC system, the true triaxial test system applies a three-way load to a rock sample through six independent loading heads, and the DIC system is used for loading the rock sample; and the DIC system measures the strain of the sample in real time in a non-contact manner based on a DIC algorithm by utilizing a speckle pattern on the surface of the loading head and an image acquisition assembly. Compared with a traditional contact sensor, a sensor does not need to be arranged on the surface of the sample, the test process is simplified, the damage risk of the sensor is avoided, the test efficiency and the measurement precision are remarkably improved, and the method is suitable for efficient research of rock mechanical properties under a complex stress path.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing technology, and in particular to a true triaxial rock sample strain testing device and method based on DIC. Background Technology

[0002] In the field of laboratory testing for geotechnical engineering, true triaxial testing is an important method for studying the mechanical properties of rocks. Traditional true triaxial testing typically uses strain sensors to measure the strain response of rock samples, specifically in two ways: one is to directly utilize the strain sensor built into the loading device, and the other is to install an additional strain sensor close to the surface of the rock sample. However, both methods have significant limitations.

[0003] For sensors built into the loading device, because their installation position is far from the rock sample, the measurement results actually reflect the deformation of the loading system (such as the pad or loading head), rather than the true strain of the rock sample itself, leading to insufficient data accuracy. While additional strain sensors can measure the sample strain more directly, in true triaxial tests, at least six sensors (at least two in each principal direction) are required to fully capture the triaxial strain response. This not only occupies the already limited installation space around the sample and increases operational complexity, but also may damage the sensors during the test due to sample failure or movement of the loading mechanism. Furthermore, the installation, calibration, and replacement of a large number of sensors significantly increase test time and cost, especially in large-scale tests where efficiency issues become more prominent.

[0004] In existing technologies, the aforementioned contradictions have not been effectively resolved: reducing the number of sensors makes it difficult to comprehensively characterize the multi-dimensional strain of the sample; retaining traditional methods cannot avoid efficiency and cost issues. Therefore, there is an urgent need for a technical solution that can accurately measure the true strain of rock samples while simplifying the installation process and improving experimental efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a true triaxial rock sample strain test device and method based on DIC, so as to solve the above-mentioned technical problems existing in the prior art and to quickly and accurately test the normal stress-dependent friction coefficient of altered and fractured surrounding rock and shield.

[0006] To achieve the above objectives, in one aspect, the present invention provides a true triaxial rock sample strain testing device based on DIC, comprising a true triaxial testing system and a DIC system. The true triaxial testing system is used to apply triaxial independent loads to a rock sample. The true triaxial testing system includes a fixed platform and multiple loading heads located around the rock sample in six directions, each loading head operating independently. The DIC system is used for non-contact measurement of the displacement strain of the rock sample. The DIC system includes speckle patterns on the surface of the loading heads at the test orientation and an image acquisition component for acquiring speckle images. The image acquisition component records speckle displacement images in real time based on the DIC algorithm and calculates the strain of the rock sample through image analysis.

[0007] The above technical solution aims to propose a rock sample strain testing device that combines a true triaxial testing system with a DIC system, which can reduce the installation of strain sensors, reduce sample installation time, and significantly improve testing efficiency.

[0008] Furthermore, pads are placed between each of the six stress surfaces of the rock sample and each of the loading heads.

[0009] Furthermore, the true triaxial testing system also includes a loading head constraint device, which includes a front loading head constraint and a rear loading head constraint, used to limit the rotational degrees of freedom of the front loading head and the rear loading head, ensuring that they only translate along a preset direction.

[0010] Furthermore, the image acquisition component of the DIC system adopts a dual-camera structure, with two cameras symmetrically arranged above the loading head at the location to be measured. The lenses of the cameras are aimed at the speckle area on the surface of the loading head, and uniform illumination is provided through a supplementary lighting device.

[0011] Furthermore, the speckle pattern is a high-contrast random pattern sprayed onto the surface of the loading head.

[0012] Furthermore, symmetrical reinforcement plates are installed on both sides of the fixing platform.

[0013] On the other hand, the present invention also provides a true triaxial rock sample strain test method based on DIC, using the true triaxial rock sample strain test device based on DIC described in any of the above claims, the method comprising the following steps:

[0014] S1. Test preparation: Retract the loading head to the initial position, and install the loading head and rock sample in sequence;

[0015] S2, DIC System Setup and Calibration: Adjust the camera position and lighting device, and calibrate the DIC system;

[0016] S3, True Triaxial Stress Path Setting: Set the stress path for independent triaxial loading;

[0017] S4. Loading and Image Acquisition: Start the loading program and synchronously acquire speckle images;

[0018] S5. Post-test processing: Analyze the speckle image and calculate displacement and strain.

[0019] Furthermore, the stress path setting in step S3 includes three stages:

[0020] The first stage involves simultaneously loading the load to the first preset load value in three directions.

[0021] The second stage involves keeping the minor principal stress constant while continuing to load the intermediate and major principal stresses to the second preset load value.

[0022] The third stage involves maintaining a constant intermediate principal stress while applying the major principal stress in a displacement-controlled mode until the rock sample fails.

[0023] Furthermore, the calibration of the DIC system in step S2 includes: adjusting the camera focal length and angle of view to cover the speckle area, calibrating the conversion relationship between pixels and actual displacement, and setting the image acquisition frequency to once every 2 seconds.

[0024] Furthermore, the post-test processing in step S5 includes: comparing and analyzing the speckle images acquired by DIC, calculating the pixel changes before and after deformation, inverting the strain distribution of the rock sample based on the displacement data, and completing the non-contact high-precision measurement.

[0025] Compared with the prior art, the present invention discloses at least the following beneficial effects:

[0026] This invention combines a true triaxial testing system with a DIC system to achieve non-contact, high-precision strain measurement of rock samples under triaxial independent loads. This avoids the problems caused by traditional strain sensors, such as complex installation, limited space, or measurement errors, thereby significantly improving test efficiency and data accuracy. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments 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.

[0028] Figure 1 This is a schematic diagram of the experimental apparatus according to an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the true triaxial loading chamber in the experimental apparatus of this invention.

[0030] Figure 3This is an exploded view of the true triaxial loading chamber in the experimental apparatus of this invention.

[0031] Figure 4 This is a schematic diagram of the DIC system in the experimental apparatus of this invention.

[0032] In the figure: 1. Upper loading head; 2. Lower loading head; 3. Left loading head; 4. Right loading head; 5. Front loading head; 6. Rear loading head; 7. Front loading head constraint; 8. Rear loading head constraint; 9. Fixed platform; 10. Fixed platform reinforcement plate; 11. Upper pad block; 12. Lower pad block; 13. Left pad block; 14. Right pad block; 15. Front pad block; 16. Rear pad block; 17. Rock sample; 18. Camera; 19. Light supplement device; 20. Speckle pattern. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Reference Figure 1 As shown, this embodiment provides a true triaxial rock sample strain testing device based on DIC, including a true triaxial testing system and a DIC system. The true triaxial testing system is used to apply independent loads in three directions (X, Y, and Z) to the rock sample 17, and the DIC system is used for non-contact measurement of the displacement and strain of the rock sample 17. It should be understood that DIC (Digital Image Correlation) as referred to in this embodiment is a non-contact optical measurement technology based on image analysis, mainly used to measure the displacement and strain fields of an object's surface. Its core principle is to compare images of the speckle pattern 20 (random speckle pattern) on the surface of the object before and after deformation, track the positional changes of pixels, and then calculate the displacement and strain.

[0036] Specifically, such as Figure 2 and Figure 3As shown, the true triaxial testing system includes a fixed platform 9, a fixed platform reinforcement plate 10, multiple pads, and multiple loading heads, all of which form a loading chamber. The fixed platform 9 supports the entire loading chamber and provides a test reference platform. Front loading head constraints 7 and rear loading head constraints 8 are fixedly installed on the front and rear sides of the fixed platform 9, respectively. These constraints are used to install front loading head 5 and rear loading head 6, ensuring that they remain parallel during movement and preventing warping due to displacement in other directions. The fixed platform reinforcement plates 10 are fixedly connected to the left and right sides of the fixed platform 9, symmetrically installed on both sides to enhance overall rigidity and reduce structural deformation during loading.

[0037] In one specific embodiment, the front loading head constraint 7 and the rear loading head constraint 8 are connected to the fixed stage 9 by a rigid connector or guide groove, which restricts the rotational freedom of the front loading head 5 and the rear loading head 6, ensuring that they can only translate along the front-to-back (Y-axis) direction.

[0038] In one specific embodiment, the lower loading head 2 is fixed to the fixed platform 9, providing a vertical reaction anchor point. The upper loading head 1 is located directly above the rock sample 17, applying a vertically downward load, and its surface is sprayed with speckle 20 for DIC measurement. Four horizontal loading heads are connected to the fixed platform 9 via guide rails or sliding mechanisms to ensure their movement along a preset direction. The four horizontal loading heads are designated as the left loading head 3, right loading head 4, front loading head 5, and rear loading head 6.

[0039] In one specific embodiment, a pad is provided on the side of the rock sample 17 that contacts each loading head. The pad is used to transfer the load and protect the rock sample 17. The pad specifically includes: an upper pad 11, a lower pad 12, a left pad 13, a right pad 14, a front pad 15, and a rear pad 16. Among them, the upper pad 11 and the lower pad 12 contact the upper loading head 1 and the lower loading head 2 respectively, transferring the vertical (Z-axis) load; the left pad 13 and the right pad 14 transfer the horizontal X-axis load; and the front pad 15 and the rear pad 16 transfer the horizontal Y-axis load. During the test, the lower pad 12 is placed first, and the rock sample 17 is placed on the lower pad 12. Then, the lateral left pad 13, right pad 14, front pad 15, and rear pad 16 are installed in sequence, and finally the upper pad 11 is installed. Each of the aforementioned pads is in close contact with the rock sample 17, meaning that the contact surfaces between the pads and the rock sample 17 are precisely machined to ensure no gaps and uniform load distribution.

[0040] Specifically, such as Figure 4As shown, the DIC system includes speckle 20 sprayed on the loading head and an image acquisition component for tracking the position information of speckle 20. The speckle 20 serves as the feature tracking point of the DIC algorithm, specifically a high-contrast random pattern sprayed on the surface of the upper loading head 1. The image acquisition component adopts a dual-camera structure, including two symmetrically arranged cameras 18. The two cameras 18 are aimed at the speckle 20 area of ​​the upper loading head 1 and synchronously capture deformed images at a set frequency (e.g., once every 2 seconds). The two cameras 18 are respectively equipped with supplementary lighting devices 19, which provide uniform illumination to avoid overexposure or shadow interference of the cameras 18.

[0041] In use, the true triaxial loading chamber applies triaxial loads according to the preset stress path, and the DIC system records the displacement of the speckle 20 in real time. The strain of the rock sample 17 is calculated through image analysis. Two cameras 18 are aligned with the speckle 20 area of ​​the upper loading head 1, and the brightness is adjusted by the supplementary lighting device 19. After calibration, deformation images are captured synchronously.

[0042] It should be understood that in practical applications, the DIC system and the loading chamber are not physically connected. Non-contact strain analysis is achieved only through optical measurements, and they do not interfere with each other.

[0043] It should also be understood that this embodiment only provides an implementation scheme for DIC measurement by spraying speckle 20 on the upper loading head 1. Measurements in other directions are similar and will not be described in detail here.

[0044] In some alternative implementations, the pads can be replaced to accommodate specimens of different sizes, and the sliding mechanism between the loading head and the fixed stage 9 facilitates the adjustment of test parameters.

[0045] This invention also provides a true triaxial rock sample strain test method based on DIC, using the true triaxial rock sample strain test device based on DIC described in any of the above embodiments. The method includes the following steps:

[0046] S1. Experiment Preparation

[0047] Retract all loading heads to their initial positions to provide operating space for sample installation;

[0048] Place the lower pad 12, rock sample 17, left pad 13, upper pad 11, right pad 14, rear pad 16, and front pad 15 in the following order from bottom to top and from inside to outside.

[0049] Fine-tune each pad block and the rock sample 17 so that the rock sample 17 and each pad block axis are aligned and tightly fitted.

[0050] Adjust the position of each loading head so that it fits tightly against the corresponding pad block to form a complete load transfer path.

[0051] S2, DIC System Layout and Calibration

[0052] Two cameras 18 are symmetrically arranged above the loading head at the location to be measured. Taking the upper loading head 1 as an example, the lens of the camera 18 is aimed at the surface of the upper loading head 1, and the focal length and angle of view are adjusted to ensure that it covers the speckle 20 area on the surface of the upper loading head 1.

[0053] Uniform illumination is provided by the supplementary lighting device 19, and the brightness is adjusted to avoid image overexposure or shadow interference, thereby optimizing the quality of DIC image acquisition.

[0054] The DIC system is calibrated by setting the image acquisition frequency and calibrating the conversion relationship between pixels and actual displacement.

[0055] S3, True Triaxial Stress Path Setting

[0056] According to the experimental requirements, the stress path for true triaxial loading is set through the control system, for example:

[0057] Phase 1: Three-dimensional synchronous loading to the first preset load value at the same first preset displacement rate;

[0058] Second stage: The minor principal stress remains unchanged at the first preset load value, while the intermediate principal stress and major principal stress continue to be loaded to the second preset load value at the first preset displacement rate.

[0059] Third stage: The intermediate principal stress remains unchanged at the second preset load value, while the major principal stress is applied at the second preset displacement rate until the rock sample 17 fails.

[0060] S4, Loading and Image Acquisition

[0061] The loading program is started, and the DIC system synchronously begins acquiring speckle 20 images at a preset frequency until the experiment ends.

[0062] When rock sample 17 reaches the failure criterion or the preset load threshold, the loading is stopped and the DIC system is shut down.

[0063] S5. Post-test processing

[0064] Shut down the DIC system and unload all loads, then remove rock sample 17;

[0065] Compare and analyze the speckle 20 images acquired by DIC, calculate the pixel changes before and after deformation, and calculate the displacement of the object based on the pixel changes;

[0066] The strain distribution in the vertical direction of rock sample 17 was inverted based on displacement data to complete the measurement.

[0067] In one specific embodiment, a true triaxial loading test was conducted using the above-described experimental setup and method, and the strain in the vertical direction of the rock specimen 17 was measured using a DIC system. The specific measurement steps are as follows:

[0068] First, retract the loading head to its initial position to provide sufficient space for the installation of the rock sample 17. The rock sample 17 used in this test is 10cm*10cm*10cm in size. Install the lower pad 12, rock sample 17, left pad 13, upper pad 11, right pad 14, rear pad 16, and front pad 15 in sequence. Adjust the position of the pads and rock sample 17 so that they are directly facing the loading head. Then, adjust the left loading head 3, right loading head 4, rear loading head 6, upper loading head 1, and front loading head 5 to fit tightly against the corresponding pads in sequence.

[0069] Adjust the position of the DIC device so that the two cameras 18 are aligned with the speckle 20 on the upper loading head 1. Set the brightness of the fill light to a moderate level to reduce overexposure of the camera 18. Calibrate the DIC device and set the shooting frequency to once every 2 seconds.

[0070] The stress path for true triaxial loading is set as follows: simultaneously apply a stress loading mode of 5MPa / min in all three directions to 5MPa, keep the minor principal stress at 5MPa, continue to apply a stress loading mode of 5MPa / min for the intermediate principal stress and major principal stress to 30MPa, keep the intermediate principal stress at 30MPa, and change the major principal stress to a displacement loading mode of 2mm / min to load until failure.

[0071] Loading begins, and camera 18 starts taking pictures at the set frequency.

[0072] After loading is completed, camera 18 is turned off, the load is unloaded, rock sample 17 is taken out, data is analyzed, and the speckle 20 image acquired synchronously by dual cameras 18 is compared and analyzed to calculate the pixel changes before and after deformation. The displacement and strain of the object are calculated based on the pixel changes.

[0073] This invention significantly improves the efficiency and accuracy of rock sample strain testing by combining a true triaxial testing system with digital image correlation (DIC). First, the use of a non-contact DIC system replaces traditional strain sensors, avoiding the complex operation of installing multiple sensors in a confined space, simplifying the test preparation process, and eliminating the risk of sensor damage due to sample failure or loading mechanism movement. Second, by spraying speckle pattern 20 onto the surface of the loading head and simultaneously acquiring images using dual cameras 18, the displacement changes on the sample surface can be tracked with high precision, directly reflecting the true strain of the rock sample 17, solving the measurement error problem caused by the distance between the sensor and the sample in traditional methods. Furthermore, the structural design of the loading chamber enhances overall rigidity through a fixed platform reinforcement plate 10 and constraint devices, ensuring stable load transfer in all directions during loading, preventing warping or offset of the loading head, and further improving the reliability of test data. Overall, this technical solution not only reduces test time and cost but also achieves rapid and accurate analysis of multi-directional strain in rock samples through non-contact measurement and optimized structural design, providing a more efficient and reliable technical means for geotechnical engineering research.

[0074] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0075] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A true triaxial rock sample strain testing device based on DIC, characterized in that, include: A true triaxial testing system is used to apply triaxial independent loads to a rock specimen (17). The true triaxial testing system includes a fixed platform (9) and multiple loading heads located around the rock specimen (17) in six directions. Each loading head operates independently. The DIC system is used for non-contact measurement of the displacement and strain of a rock sample (17). The DIC system includes a speckle pattern (20) on the surface of the loading head at the test position and an image acquisition component for acquiring images of the speckle pattern (20). The image acquisition component records the displacement images of the speckle pattern (20) in real time based on the DIC algorithm and calculates the strain of the rock sample (17) through image analysis.

2. The true triaxial rock sample strain testing device based on DIC according to claim 1, characterized in that, Each of the six stress surfaces of the rock sample (17) is padded with pads between itself and each of the loading heads.

3. The true triaxial rock sample strain testing device based on DIC according to claim 1, characterized in that, The true triaxial test system also includes a loading head constraint device, which includes a front loading head constraint (7) and a rear loading head constraint (8) to limit the rotational degrees of freedom of the front loading head (5) and the rear loading head (6) and ensure that they only translate along a preset direction.

4. The true triaxial rock sample strain testing device based on DIC according to claim 1, characterized in that, The image acquisition component of the DIC system adopts a dual-camera structure. Two cameras (18) are symmetrically arranged above the loading head at the location to be measured. The lens of the camera (18) is aimed at the speckle (20) area on the surface of the loading head and provides uniform illumination through the supplementary lighting device (19).

5. The true triaxial rock sample strain testing device based on DIC according to claim 1, characterized in that, The speckle pattern (20) is a high-contrast random pattern sprayed onto the surface of the loading head.

6. The true triaxial rock sample strain testing device based on DIC according to claim 1, characterized in that, The fixed platform (9) is symmetrically equipped with fixed platform reinforcement plates (10) on both sides.

7. A true triaxial rock sample strain test method based on DIC, employing the true triaxial rock sample strain test apparatus based on DIC as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Test preparation: Retract the loading head to the initial position, and install the loading head and rock sample (17) in sequence; S2, DIC System Setup and Calibration: Adjust the position of the camera (18) and the supplementary lighting device (19), and calibrate the DIC system; S3, True Triaxial Stress Path Setting: Set the stress path for independent triaxial loading; S4. Loading and Image Acquisition: Start the loading program and synchronously acquire speckle (20) images; S5. Post-test processing: Analyze the speckle (20) image and calculate the displacement and strain.

8. The true triaxial rock sample strain test method based on DIC according to claim 7, characterized in that, The stress path setting in step S3 includes three stages: The first stage involves simultaneously loading the load to the first preset load value in three directions. The second stage involves keeping the minor principal stress constant while continuing to load the intermediate and major principal stresses to the second preset load value. The third stage involves maintaining a constant intermediate principal stress while applying the major principal stress to the rock sample (17) in a displacement-controlled mode until it fails.

9. The true triaxial rock sample strain test method based on DIC according to claim 7, characterized in that, The calibration of the DIC system in step S2 includes: adjusting the focal length and angle of the camera (18) to cover the speckle (20) area, calibrating the conversion relationship between pixels and actual displacement, and setting the image acquisition frequency to once every 2 seconds.

10. The true triaxial rock sample strain test method based on DIC according to claim 7, characterized in that, The post-test processing in step S5 includes: comparing and analyzing the speckle (20) images acquired by DIC, calculating the pixel changes before and after deformation, inverting the strain distribution of the rock sample (17) based on the displacement data, and completing the non-contact high-precision measurement.

Images