Device and method for measuring mechanical behaviors and fracture characteristics of soil under dry-wet cycle

Abstract

The invention discloses a device and a method for measuring mechanical behaviors and fracture characteristics of a soil body under a dry-wet cycle. The device comprises four parts, namely a tensile-compression type sample box, a data acquisition and image processing system, a high-precision balance and a water bath type temperature control system. Based on an active principle, an external load does not need to be applied, and the process of force generated by a soil sample in a dry-wet cycle process can be continuously measured only through a force sensor arranged on the side surface of a movable half box; dynamic change key parameters of a sample in the dry-wet cycle process are collected through an intelligent program, and a shot soil sample image is analyzed in combination with Image J image analysis software and a digital image correlation method (DIC), so that the change process of soil fracture network morphological characteristics can be monitored in real time in the dry-wet cycle process; and the evolution law of a displacement field in the crack development and healing process of the soil body can be researched, so that the internal relationship between the mechanical characteristics and the hydraulic characteristics of the soil body under the dry-wet cycle condition is established.

Description

Technical Field

[0001] The present invention relates to a test device for geotechnical engineering, and in particular to a test technology for describing the mechanical evolution characteristics of soil under complex hydraulic paths, especially a device and method for determining the mechanical behavior of soil and its crack characteristics under dry-wet cycles. Background Art

[0002] The mechanical properties of soil under dehydration conditions have always been one of the important topics in the field of soil mechanics. Research in this field mainly focuses on the problem of soil shrinkage cracking, which poses a serious threat to geotechnical structures (such as clay barriers, dams, landfill covers, etc.) because shrinkage cracks can significantly affect the mechanical behavior of soil. Existing research on the laws of soil shrinkage cracking is mainly based on unit-scale tests on circular or rectangular specimens under controlled indoor test conditions, and the impact of dry-wet cycles on the evolution of soil cracks still needs to be further studied.

[0003] So far, the testing technologies for simulating the mechanical behavior of soil under drying and dehydration conditions mainly include two categories:

[0004] One type is developed based on the reaction principle, where the soil reacts to the applied external tension under controlled stress or strain rate until it fails:

[0005] For example, the paper “Crack patterns in clayey soils: Experiments and modeling” published by Trabelsi et al. in the International Journal for Numerical and Analytical Methods in Geomechanics discloses a soil tensile test device under controlled stress conditions. The device consists of two parts, one of which is fixed. By applying tensile stress, the other part is displaced, and its value is measured with a dial with an accuracy of 0.01 mm. The tensile strength is then obtained through the curve of tensile stress versus tensile displacement. However, the device cannot simulate the tensile-compression evolution of soil under dry-wet cycle conditions.

[0006] For example, the paper "Tensile strength during drying of remoulded and compacted clay: The role of fabric and water retention" published by Trabelsi et al. in Applied Clay Science discloses a soil tensile test device under controlled displacement conditions, which is suitable for placement in a traditional direct shear box. The tensile displacement is controlled by an LVDT sensor at a rate of 0.1 mm / min, and the tensile stress is then calculated using the tensile displacement to obtain the tensile strength. However, this device cannot control the changes in the tensile-compression displacement of the soil during the dry-wet cycle.

[0007] Chinese patent CN 107621534 A discloses a device for measuring expansion force in a constant temperature environment. The temperature inside the saturated salt solution container and the transition container in the device is accurately controlled by a water bath temperature control system, and the temperature of the sample in the pressure chamber is controlled at the same time. The suction is controlled by controlling the saturated salt solution container in the device. The patent can measure the expansion force of soil under different temperature and suction conditions. However, the patent cannot measure the stress change process in the soil tension-compression process induced by dry-wet cycles.

[0008] The other is based on the active principle to measure the stress generated by the soil during the drying and water loss process:

[0009] For example, the paper “Novel approach to measuring tensile strength of compacted clayey soil during desiccation” published by Varsei et al. in the International Journal of Geomechanics discloses a test device for measuring the tensile strength of soil during the drying process. The constraints imposed in the device can cause the soil sample to shrink in one direction, thereby measuring uniform tensile stress and strain, and predicting and controlling the location of cracks. However, the device cannot simulate the shrinkage and stretching of soil samples during the drying and wetting cycle by applying constraints.

[0010] In addition, images taken during the drying process of soil samples have also been used to study the surface crack network morphology, displacement field and other related characteristics of the soil:

[0011] Chinese patent CN 114487012 A discloses a method for predicting the development of cracks on the soil surface. The invention uses an infrared thermal imaging camera to shoot the surface of the soil to be monitored to obtain the soil surface temperature field, and then draws its isotherm map based on the soil surface temperature field. Finally, the main crack development area on the soil surface is predicted based on the adjacent low-temperature core area determined by the soil isotherm distribution. Although this patent can effectively predict the development process of soil cracks, it cannot simulate the development-closure process of soil surface cracks induced by dry-wet cycles and the evolution law of their mechanical behavior.

[0012] For example, the paper “Tensile strength of clayey soil and the strain analysis based on image processing techniques” published by Li and Tang et al. in Engineering Geology discloses a soil surface crack image processing technology based on digital image correlation (DIC) and particle image velocimetry (PIV), which can analyze the strain field change process of soil during shrinkage cracking. However, this technology has not been used to analyze the impact of dry-wet cycles on the evolution of soil cracks. Summary of the invention

[0013] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a device and method for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles, which can not only measure the tension and pressure generated by the soil during shrinkage and swelling, but also monitor the changes in the morphology and size of cracks on the soil surface during the dry-wet cycle in combination with image processing methods.

[0014] To achieve the above object, the present invention adopts the following technical solutions:

[0015] The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles includes four parts: a tensile-compression sample box, a high-precision balance, a water bath temperature control system, and a data acquisition and image processing system;

[0016] The tensile-compression sample box is arranged on a high-precision balance, and the tensile-compression sample box and the high-precision balance are placed in a visualization test housing, a semiconductor water-cooled head heat sink is installed on the visualization test housing, and the semiconductor water-cooled head heat sink is connected to a water bath temperature control system through an inlet and outlet water pipe, and a water inlet connected to the tensile-compression sample box is also arranged on the visualization test housing, and a high-precision camera is arranged above the tensile-compression sample box;

[0017] The tensile-compression sample box comprises a fixed half box, a movable half box, a force sensor and a base, wherein the fixed half box is fixed on one end of the base, and the movable half box is arranged on a ball bearing at the other end of the base through a slide groove at the bottom; a first fastening device for connecting and separating the fixed half box and the movable half box is arranged on the corresponding contact surfaces of the fixed half box and the movable half box, a waterproof film is placed on the inner wall and the gap of the fixed half box and the movable half box, and a first screw is arranged on the edge of the base at the outer end of the movable half box away from the fixed half box, which can connect and separate the movable half box to the force sensor;

[0018] High-precision balances, high-precision cameras and force sensors are all connected to the data acquisition and image processing system.

[0019] The tensile-compression sample box comprises a fixed half box, a movable half box, a force sensor and a base. The fixed half box is fixed on the base; the movable half box is located on a ball bearing, so that it can be nearly smooth on the surface of the base. Openings are reserved in the sample box grooves of the fixed half box and the movable half box to communicate with the water-adding U-shaped groove in the base, so as to evenly add water to the sample after the dehumidification stage is over, so that the sample begins to humidify to the set moisture content. Placing a waterproof film on the inner wall and gap of the fixed half box and the movable half box can ensure that the water added to the sample in the humidification test will not be lost, and can also ensure that the mud sample will not be lost during the test. The first fastening device connects the movable half box and the fixed half box through two screws located on two opposite surfaces of the sample box, and can realize the connection or disconnection of the two half boxes at different test stages. During the preparation of the sample, tightening the screws of the first fastening device will prevent the sample box from having a gap at this time. After the sample is prepared, loosen the screws of the first fastening device, and a gap will be generated in the sample box, so that the sample will drive the moving half box to move toward or away from the fixed half box when the sample shrinks or expands during dehumidification. A force sensor with a range of 200N and an accuracy of 0.01% is selected and fixed at its bottom to the edge of the base in parallel with the ball bearing. Before the test begins, its top end is connected to the moving half box with screws to measure the tension and pressure generated by the sample during dehumidification shrinkage and humidification expansion.

[0020] When measuring the force of the sample during the dry-wet cycle, the sample box has no vertical restriction conditions, which will affect the accuracy of horizontal force measurement. To this end, the present invention designs a restriction device, which consists of a second fastening device (the fastening device consists of an L-shaped connecting plate, screws, and nuts) and an acrylic plate: the second fastening device is installed on the top of the three side walls of the fixed half box; the acrylic plate is connected to the second fastening device to limit the vertical deformation of the sample during the humidification expansion process; the lubricant is applied to the contact surface between the acrylic plate and the sample to reduce the friction between the sample and the acrylic plate. This restriction device can ensure the accurate measurement of the horizontal force during the test, and does not affect the shooting and collection of visual images during the test.

[0021] Polytetrafluoroethylene is selected as a lubricant on the contact surface between the acrylic plate and the sample. There are four main reasons: First, when polytetrafluoroethylene contacts the sample, it will not exchange water with the sample due to its hydrophobicity, thereby avoiding the influence on the determination of the moisture content of the sample; the properties of polytetrafluoroethylene are slightly affected by temperature, so the present invention will not affect the function of polytetrafluoroethylene during temperature control; third, the friction coefficient of polytetrafluoroethylene is extremely low, so it can significantly reduce the friction between the acrylic plate and the sample surface, ensuring the accurate determination of the horizontal force during the test; fourth, polytetrafluoroethylene is colorless and transparent, and does not affect the clarity of the sample surface image collected during the test.

[0022] The fixed half box and the movable half box are provided with a sample box groove on the inner bottom surface, a water-adding U-shaped groove is provided on the base, a water-permeable hole connected to the water-adding U-shaped groove is provided in the sample box groove, and a water-permeable membrane is applied on the water-permeable hole. Openings are reserved in the sample box grooves of the fixed half box and the movable half box so as to be connected to the water-adding U-shaped groove in the base, so as to evenly add water to the sample after the dehumidification stage is completed, so that the sample begins to humidify to the set moisture content.

[0023] Placing waterproof membranes on the inner walls and gaps of the fixed half box and the movable half box can ensure that the moisture added to the sample in the humidification test will not be lost, and can also ensure that the mud sample will not be lost during the test.

[0024] The first fastening device connects the movable half box and the fixed half box separately through two screws located on two opposite surfaces of the sample box, and can realize the connection or disconnection of the two half boxes at different test stages. During the preparation of the sample, tighten the screws of the first fastening device so that there is no gap in the sample box at this time. After the sample preparation is completed, loosen the screws of the first fastening device, and a gap will be generated in the sample box, so that the sample drives the movable half box to move toward or away from the fixed half box when the sample shrinks or expands during dehumidification.

[0025] A force sensor with a range of 200N and an accuracy of 0.01% is selected and its bottom end is fixed to the edge of the base in parallel with the ball bearing. Before the test begins, its top end is connected to the moving half box with screws to measure the tension and pressure generated by the sample during dehumidification shrinkage and humidification expansion.

[0026] The data of the change of tension and pressure of soil in the dry-wet cycle test over time were obtained by tracking the passage of time, and the control method of the mean square error of the measured values ​​was introduced. Combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the end time of different test stages was determined when the test data tended to be stable.

[0027] The bottom surface of the sample box is composed of rectangular sample box grooves, which not only ensure good contact between the bottom of the sample and the sample box, but also limit the relative movement between the sample and the sample box during the dry-wet cycle. In this way, the real force generated by the sample during the dry-wet cycle can be measured based on the active principle.

[0028] The water inlet on the visualization test shell is connected with the water inlets of the fixed half box and the movable half box through a hose, and the water inlets of the fixed half box and the movable half box are connected with the water addition U-shaped groove of the fixed half box and the water addition U-shaped groove of the movable half box respectively.

[0029] The sample box is placed on a high-precision balance (with a range of 10kg and an accuracy of 0.01g), and the mass of the sample is weighed at regular intervals to continuously measure the change in moisture content of the sample during the dry-wet cycle.

[0030] The water bath temperature control system includes a stainless steel container, a micro water pump, an electric constant temperature environment box and an intelligent time and temperature controller. The micro water pump is arranged in the stainless steel container, and the stainless steel container is arranged in the electric constant temperature environment box. The water outlet of the micro water pump is connected with the water inlet of the semiconductor water cooling head heat sink through the water inlet pipe, and the water outlet of the semiconductor water cooling head heat sink is connected to the stainless steel container through the water outlet pipe, forming a closed loop of water circulation; the temperature probe in the intelligent time and temperature controller is installed inside the tension-compression sample box, and the temperature control switch of the intelligent time and temperature controller is connected with the micro water pump to control the water flow rate during the water circulation process.

[0031] The electric constant temperature environment box is set to a fixed temperature to heat the stainless steel container, and then a micro water pump is used to pump the water in the stainless steel container to the heat sink of the semiconductor water cooling head and reflux it into the stainless steel container.

[0032] The high-precision camera is fixed above the tension-compression sample box through a tripod, and uses a high-resolution micro single camera (3648×2736 pixels) to take pictures of the crack development process on the sample surface.

[0033] The data acquisition and image processing system refers to an intelligent data acquisition program compiled in Python language, and connects Bluetooth to the computer, aiming to accurately collect key parameters of the sample during the dynamic change of the dry-wet cycle. When setting the test plan in the system, the measurement of the high-precision balance and force sensor readings is synchronized with the shooting of the sample surface image by the micro-single camera. The system not only has a built-in moisture content calculation formula, so that the sample quality data monitored by the high-precision balance can be accurately converted into intuitive moisture content data, but also has a built-in mean square error calculation formula and force measurement error limit for tension and pressure measurements, aiming to obtain the time when the test data tends to stabilize and automatically stop different test stages.

[0034] The method for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles using the above device includes the following steps:

[0035] Step (1), sample preparation: first mix the loose soil with distilled water to the target moisture content; then put the prepared loose soil into a plastic bag and seal it and let it stand for 24 hours to promote the moisture balance in the loose soil. For the preparation of compacted samples: first determine the number of layers, the number of blows per layer, and the compaction height required to achieve the target dry density, then use a stainless steel cylinder to compact the sample in two layers, so that the hammer blows are evenly distributed on the sample surface to achieve a controllable layer height, and by compacting the bottom layer first and then the top layer in sequence to avoid the two layers of samples from slipping during the test; for the preparation of mud samples: first slowly pour the mud into the sample box in two layers to achieve the required dry density, and then tap the top surface to remove any bubbles that may exist in the mud sample.

[0036] After the sample is prepared, the fixed half box and the movable half box are connected by a first fastening device, while the force sensor is not connected to the two sample boxes.

[0037] Step (2), place the two sample boxes on a high-precision balance directly below the micro-single camera, and then use the micro-single camera to take pictures at preset time intervals; loosen the first fastening device connecting the fixed half box and the movable half box, and then use the first screw to connect the force sensor to the movable half box.

[0038] Step (3), when t=t0, the temperature T of the environment where the sample is located is controlled to 24°C±1°C by a water bath temperature control system, and then the first dehumidification test is started, and the starting time t=t0 is recorded.

[0039] In step (4), the matrix suction of the sample gradually increases during the dehumidification process, thereby inducing the contraction of the sample and causing the moving half box to move to the left. At the same time, the force sensor prevents the movement of the moving half box, thereby measuring the tensile force generated by the sample during the dehumidification process (negative value according to the sign convention of soil mechanics).

[0040] Step (5), the soil sample then continues to dehumidify under restricted conditions. As time goes by, after measuring n time intervals (n≥10), it is found that the tension gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval, and so on until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors are also gradually decreasing. Then, combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the end time of the test when the tension test data tends to be stable is determined. At this time, the system automatically stops collecting tension data, indicating the end of the first dehumidification stage.

[0041] Step (6), after the first dehumidification test is completed, the first screw is tightened again, and the reset force sensor is reconnected to the movable half box, and the first humidification test can be started. Water is added to the water-adding U-shaped grooves of the two half boxes so that the water-permeable membrane on the water-permeable holes of the two half boxes can humidify the sample, and a high-precision balance is used to monitor the change in the moisture content of the sample.

[0042] Step (7), during the humidification process of the sample, the force sensor will prevent the moving half box from moving to the right due to the expansion of the sample, thereby measuring the pressure generated by the sample during the humidification process. As time goes by, after measuring n time intervals (n≥10), it is found that the pressure gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval... until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors are also gradually decreasing. Combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the test end time when the pressure test data tends to be stable is determined. At this time, the system automatically stops collecting pressure data, indicating the end of the first humidification stage.

[0043] Step (8), after the humidification is completed, cover the surface of the sample with plastic wrap to maintain a constant moisture content for 24 hours to promote the water balance inside the sample.

[0044] Step (9) is to first tighten the first screw to reconnect the reset force sensor to the moving half box to prepare for the second dehumidification test of the sample, and then remove the first fastening device connecting the fixed half box and the moving half box to start the second dehumidification test of the sample. The test steps of this process are similar to those described in the first dehumidification test. The subsequent dry-wet cycle process is also carried out in the same way.

[0045] During the dry-wet cycle test, the crack network morphological characteristics on the sample surface and the displacement field distribution of soil particles were image analyzed and monitored in real time using the following methods:

[0046] Image J software was used to binarize the crack images on the sample surface taken by a micro-single camera to describe the development and closure evolution characteristics of the crack network on the sample surface during the dry-wet cycle test. The surface crack ratio and average crack opening were used to quantitatively describe the crack morphology. The surface crack ratio was the ratio of the crack area to the initial total area of ​​the soil sample, and the crack opening was the average width of all cracks on the sample surface. After the sample was subjected to multiple dry-wet cycle tests, the images with significant differences in the sample at different dehumidification / humidification stages were compared and selected; then the local coordinate system xy was added to it, so that the groove direction of the sample box at the bottom of the sample box was parallel to the y direction, and the setting direction of the force sensor was parallel to the x direction; the surface crack ratio and crack opening with the number of cycles were quantitatively described.

[0047] The digital image correlation method (DIC) was used to monitor the changes in the displacement field distribution of soil particles on the sample surface during the dry-wet cycle. First, the digital speckle pattern was prepared by spraying white primer on the sample surface and then spraying black paint to prepare the speckle pattern. Secondly, during the dry-wet cycle test, a micro single-lens camera was used to take pictures of the sample surface at a preset time interval to obtain the speckle image of the sample surface. Subsequently, the DIC program was run using MATLAB software to calculate the given time t=t i The displacement vector of the soil particles on the sample surface is calculated and the moment (t = t i )The deformation image of the sample surface is associated with its reference image (t=t0), and the change of the soil particle displacement field on the sample surface is obtained by obtaining the change of the digital speckle.

[0048] Compared with the existing testing technology and crack image processing technology for simulating the mechanical behavior of soil under drying and dehydration conditions, the beneficial effects of the present invention are as follows:

[0049] 1. Based on the active principle, without applying any external load, the tension and pressure generated by the sample during shrinkage and swelling can be measured by simply setting a force sensor in the tension-compression sample box, thus simulating the evolution law of the mechanical behavior of the soil during the drying and wetting cycle;

[0050] 2. During the dry-wet cycle, a digital camera was used to take pictures at set time intervals, and then combined with Image J image analysis software and digital image correlation (DIC) method, it can not only monitor the changes in the crack rate and crack opening of the soil surface in real time, but also study the evolution law of the displacement field during the development and healing of cracks on the soil surface;

[0051] 3. The experimental results on soil boundary forces and the morphological characteristics of surface shrinkage cracks are helpful to verify the numerical model describing the boundary value problem of soil under drying and wetting cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1is an overview diagram of the device of the present invention;

[0053] Figure 2 is a top view of the tension-compression specimen box;

[0054] Figure 3 It is a structural diagram of the tension-compression specimen box;

[0055] Figure 4 It is a cross-sectional view of the bottom of the fixed half box;

[0056] Figure 5 It is the bottom cross-section of the mobile half box;

[0057] Figure 6 It is the front view of the moving half box;

[0058] Figure 7 It is the right view of the mobile half box;

[0059] Figure 8 It is the structural diagram of water bath temperature control system;

[0060] Fig. 9 It is the left view of the visual test housing;

[0061] Fig.10 is a front view of the visual test housing;

[0062] Fig.11 is a top view of the visual test housing;

[0063] Fig.12 It is the right view of the visual test housing;

[0064] Fig.13 It is a step diagram of the determination method for the change process of tension and pressure on soil under dry-wet cycles;

[0065] Fig.14 It is a flow chart of the image processing method for the morphological features of the fracture network;

[0066] Fig.15 It is a flow chart of the method for monitoring the displacement field distribution of soil during the drying and wetting cycle;

[0067] Among them, A. tensile-compression sample box, B. water bath temperature control system, C. high-precision balance, D. data acquisition and image processing system. 1. fixed half box, 2. mobile half box, 3. force sensor, 4. base, 5. ball bearing, 6. first fastening device, 7. sample box groove, 8. water-permeable membrane, 9. first screw, 10. second screw, 11. first water inlet, 12. second water inlet and outlet, 13. waterproof membrane, 14. water-adding U-shaped groove of the fixed half box, 15. water-permeable hole of the fixed half box, 16. electric heating constant temperature environment box, 17. stainless steel container, 18. micro water pump, 19. temperature setting button, 20. smart time Temperature controller, 21. Safety socket, 22. Temperature probe, 23. Water outlet, 24. Third water inlet, 25. Water outlet pipe, 26. Water inlet pipe, 27. Silicone elbow, 28. Semiconductor water cooling head heat sink, 29. Fourth water inlet, 30. Fifth water inlet, 31. Slide groove at the bottom of the mobile half box, 32. Water permeable hole of the mobile half box, 33. U-shaped groove for water addition of the mobile half box, 34. Visual test housing, 35. Micro single camera, 36. Acrylic plate, 37. Second fastening device. DETAILED DESCRIPTION

[0068] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0069] The structures, proportions, sizes, etc. illustrated in the drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no substantial technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the present invention, should still fall within the scope of the technical content disclosed by the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" quoted in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of the present invention. The change or adjustment of their relative relationship should also be regarded as the scope of the implementation of the present invention without substantially changing the technical content.

[0070] like Figure 1 As shown, an intelligent measuring device for simulating the dynamic characteristics of soil tension, compression and surface cracks under dry-wet cycles includes a tension-compression sample box A, a water bath temperature control system B, a high-precision balance C, and a data acquisition and image processing system D.

[0071] like Figure 1 , Figure 2 , Figure 3As shown, the tension-compression sample box includes a fixed half box 1, a movable half box 2, a force sensor 3 and a base 4. The fixed half box 1 is placed on the base 4, and the movable half box 2 is placed on the ball bearing 5. The movable half box moves in the direction required by the test through the slide groove 31 at the bottom. The bottom surfaces of the fixed half box 1 and the movable half box 2 are both provided with sample box grooves 7 to limit the relative movement between the soil sample and the box. The bottom end of the force sensor 3 is connected to the base 4, and the top end thereof is connected to the movable half box 2. The fixed half box 1 and the movable half box 2 are connected by the first fastening device 6. After the sample is prepared, the second screw 10 is loosened, and the first screw 9 is tightened to connect the force sensor 3 to the movable half box 2. After the first fastening device 6 is loosened, the sample box can be moved, and the first screw 9 is tightened to connect the force sensor 3 to the movable half box 2. During the humidification process of the sample, a hose is used to supply water to the fixed half box 1 and the movable half box 2 through the fourth water inlet 29 and the fifth water inlet 30, respectively, so that water flows from the first water inlet 11 and the second water inlet 12 into the water supply U-shaped groove 14 of the fixed half box and the water supply U-shaped groove 33 of the movable half box, respectively, and the sample is uniformly humidified through the water permeable holes 15 of the fixed half box and the water permeable holes 32 of the movable half box and the water permeable membrane 8 placed on the two water permeable holes. The water permeable membrane 8 prevents the soil particles of the sample from being lost during the humidification process. A waterproof membrane 13 is placed on the inner wall and gap of the two boxes to ensure the integrity of the sample and the water content is not lost.

[0072] When measuring the force of the sample during the dry-wet cycle, the sample box has no vertical restriction conditions, which will affect the accuracy of horizontal force measurement. To this end, the present invention designs a restriction device, which consists of a second fastening device and an acrylic plate: the second fastening device is installed on the top of the three side walls of the fixed half box; the acrylic plate is connected to the second fastening device to limit the vertical deformation of the sample during the humidification expansion process; and the lubricant is applied to the contact surface between the acrylic plate and the sample to reduce the friction between the sample and the acrylic plate. This restriction device can ensure the accurate measurement of the horizontal force during the test, and does not affect the shooting and collection of visual images during the test.

[0073] Polytetrafluoroethylene is selected as a lubricant on the contact surface between the acrylic plate and the sample. There are four main reasons: First, when polytetrafluoroethylene contacts the sample, it will not exchange water with the sample due to its hydrophobicity, thereby avoiding the influence on the determination of the moisture content of the sample; the properties of polytetrafluoroethylene are slightly affected by temperature, so the present invention will not affect the function of polytetrafluoroethylene during temperature control; third, the friction coefficient of polytetrafluoroethylene is extremely low, so it can significantly reduce the friction between the acrylic plate and the sample surface, ensuring the accurate determination of the horizontal force during the test; fourth, polytetrafluoroethylene is colorless and transparent, and does not affect the clarity of the sample surface image collected during the test.

[0074] like Figure 8 , Fig. 9In the water bath temperature control system shown, a semiconductor water cooling head heat sink 28 is installed on the visual test housing 34 and connected in series with a silicone bent pipe 27. The semiconductor water cooling head heat sink 28 after series connection needs to be connected to the outlet pipe 25 and the inlet pipe 26 of the micro water pump 18 through the water outlet 23 and the third water inlet 24 respectively to form a closed water circulation loop. The temperature probe 22 is placed near the fixed half box 1 and the movable half box 2, and the stainless steel container 17 and the water pump 18 are placed in the electric constant temperature environment box 16.

[0075] To control the temperature inside the visual test housing 34, it is necessary to first start the electric constant temperature environment box 16, and pre-set the test temperature to heat the water in the stainless steel container; then adjust the intelligent time and temperature controller 20, and dynamically control the water circulation temperature through the temperature setting button 19, that is: if the actual test temperature captured by the temperature probe 22 does not reach its preset value, power is turned on to heat the water in the circulation loop until the test temperature reaches its preset value and then the power is turned off.

[0076] like Fig.13 As shown in FIG. 1 , the method for determining the change process of tension and pressure on the soil under dry-wet cycles includes the following steps:

[0077] Step (1), sample preparation: first mix the loose soil with distilled water to the target moisture content; then put the prepared loose soil into a plastic bag and seal it and let it stand for 24 hours to promote the moisture balance in the loose soil. For the preparation of compacted samples: first determine the number of layers, the number of blows per layer, and the compaction height required to achieve the target dry density, then use a stainless steel cylinder to compact the sample in two layers, so that the hammer blows are evenly distributed on the sample surface to achieve a controllable layer height, and by compacting the bottom layer first and then the top layer in sequence to avoid slippage of the two layers of samples during the test. For the preparation of mud samples: first slowly pour the mud into the sample box in two layers to achieve the required dry density, and then tap the top surface to remove any bubbles that may exist in the mud sample.

[0078] After the sample is prepared, the fixed half box 1 and the movable half box 2 are connected by the second screw 10 on the first fastening device 6, while the force sensor 3 is not connected to the two sample boxes.

[0079] Step (2), place the two sample boxes on a high-precision balance directly below the micro-single camera 35, and then use the micro-single camera 35 to take pictures at preset time intervals; loosen the first fastening device 6 connecting the fixed half box 1 and the movable half box 2, and then use the first screw 9 to connect the force sensor 3 to the movable half box 2.

[0080] Step (3), when t=t0, the temperature T of the environment where the sample is located is controlled to 24°C±1°C by a water bath temperature control system, and then the first dehumidification test is started, and the starting time t=t0 is recorded.

[0081] Step (4), the matrix suction of the sample gradually increases during the dehumidification process, thereby inducing the sample to shrink and move the moving half box to the left. At the same time, the force sensor 3 prevents the movement of the moving half box 2, thereby measuring the tensile force generated by the sample during the dehumidification process (negative value according to the sign convention of soil mechanics).

[0082] Step (5), the soil sample then continues to dehumidify under restricted conditions. As time goes by, after measuring n time intervals (n≥10), it is found that the tension gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval, and so on until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors are also gradually decreasing. Then, combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the end time of the test when the tension test data tends to be stable is determined. At this time, the system automatically stops collecting tension data, indicating the end of the first dehumidification stage.

[0083] Step (6), after the first dehumidification test is finished, the first screw 9 is tightened again, and the reset force sensor 3 is reconnected to the movable half box 2, and the first humidification test can be started. In the humidification test, the acrylic plate 36 is fixed to the upper part of the sample box by the second fastening device 37, and water is added to the water-adding U-shaped grooves (14 and 33) of the two half boxes so that the water-permeable membrane 8 on the water-permeable holes (15 and 32) of the two half boxes is used to humidify the sample, and a high-precision balance C is used to monitor the change of the moisture content of the sample.

[0084] Step (7), during the humidification process of the sample, the force sensor 3 will prevent the moving half box 2 from moving to the right due to the expansion of the sample, thereby measuring the pressure generated by the sample during the humidification process. As time goes by, after measuring n time intervals (n≥10), it is found that the pressure gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval... until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors are also gradually decreasing. Combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the test end time when the pressure test data tends to be stable is determined. At this time, the system automatically stops collecting pressure data, indicating the end of the first humidification stage.

[0085] Step (8), after the humidification is completed, cover the surface of the sample with plastic wrap to maintain a constant moisture content for 24 hours to promote the water balance inside the sample.

[0086] Step (9), first tighten the first screw 9 to reconnect the reset force sensor 3 to the movable half box 2 to prepare for the second dehumidification test of the sample, then remove the first fastening device 6 connecting the fixed half box 1 and the movable half box 2, so that the sample starts the second dehumidification test. The test steps of this process are similar to those described in the first dehumidification test. The subsequent dry-wet cycle process is also carried out in the same way.

[0087] In addition, during the dry-wet cycle test, the crack network morphological characteristics on the sample surface and the displacement field distribution of soil particles were image analyzed and monitored in real time using the following methods:

[0088] like Fig.14 As shown, the image of the cracks on the sample surface taken by the micro-single camera 35 is binarized by Image J software to describe the development and closure evolution characteristics of the crack network on the sample surface during the dry-wet cycle test. The surface crack ratio and the average crack opening are used to quantitatively describe the crack morphology. The surface crack ratio is the ratio of the crack area to the initial total area of ​​the soil sample, and the crack opening is the average width of all cracks on the sample surface. After the sample has been subjected to multiple dry-wet cycle tests, the images of the sample with significant differences in different dehumidification / humidification stages are compared and selected; then the local coordinate system xy is added to it, so that the sample box groove 7 at the bottom of the sample box is parallel to the y direction, and the setting direction of the force sensor 3 is parallel to the x direction; quantitatively describe the curve of the surface crack ratio and crack opening with the number of cycles.

[0089] like Fig.15 As shown in the figure, the digital image correlation method (DIC) is used to monitor the changes in the displacement field distribution of soil particles on the sample surface during the dry-wet cycle. First, the digital speckle is prepared by the spray painting method, that is, a white primer is sprayed on the sample surface once, and then black paint is sprayed to prepare the speckle; secondly, during the dry-wet cycle test, a micro single camera 35 is used to take pictures of the sample surface at a preset time interval to obtain the speckle image of the sample surface; then, the collected image is processed into a grayscale image, and these grayscale images are numbered sequentially starting from 1; finally, a DIC calculation program is developed in MATLAB software to perform displacement tracking processing on the reference points set on the sample surface, so as to obtain the changes in the digital speckle and obtain the changes in the displacement field of soil particles on the sample surface. The specific steps of the DIC calculation program are as follows:

[0090] Step (1), in the reference state image, set reference points P (x0, y0) uniformly at given intervals along the vertical and horizontal directions, and calculate their displacement. Select two images in different states, called the reference state image and the deformed state image, and use the shape function (i.e., the key function for measuring the deformation field of the object surface) to describe the points in the image before and after deformation as follows:

[0091] xi ′=x i +u(x i ,y j ) (1-a)

[0092] y j ′=y j +v(x i ,y j ) (1-b)

[0093] Where: (x i ,y j ) and (x i ′,y′ j ) are the coordinates before and after deformation, u and v are the displacements of the center point of the reference subunit in the x and y directions, respectively. Regarding the expression of shape function, the present invention adopts a first-order shape function description:

[0094] x i ′=x i +u+u x Δx+u y Δy (2-a)

[0095] y j ′=y j +v+v x Δx+v y Δy(2-b)

[0096] Where: (x i ,y j ) and (x i ′,y′ j ) are the coordinates of the subunits before and after deformation, u and v are the displacements of the center point of the reference image in the x and y directions, Δx and Δy are the displacements of the point (x i ,y j ) to the center point (x0, y0), u x 、u y 、v x and v y are the gradients of displacements u and v in the x and y directions, respectively.

[0097] Step (2), extract corresponding points in the deformed state image. In the reference state image, select a reference subunit with P(x0, y0) as the center, which is called (2n+1) 2 The correlation window composed of pixel units is used to track its corresponding position in the deformed state image. i ,y iis the coordinate in the reference subunit. Similarly, a deformed subunit is set in the deformed state image, and its size is the same as that of the reference subunit. The similarity between the reference subunit and the deformed subunit can be expressed by the zero-mean normalized correlation coefficient R as follows:

[0098]

[0099] Where: x i ′、y i ' is the coordinate in the deformation subunit; f(x i ,y j ) and g(x i ′,y′ j ) are the grayscale values ​​of the reference subunit and the deformed subunit respectively. The R value is (E x ×E y ) pixel units, and then the point where R appears at a peak is taken as the corresponding point of the reference point.

[0100] Step (3), if the position of the R peak is detected in the deformed state image, the reference subunit center P(x0, y0) and the deformed subunit center P′(x i ′,y′ j ) to calculate the displacement vector.

[0101] Furthermore, in order to further improve the resolution of the digital image correlation method, the present invention interpolates the cross-correlation coefficient field by searching the intensity and pixel unit displacement to achieve the refinement of the sub-pixel unit. Specifically, if the displacement field and the cross-correlation coefficient field within the intensity resolution are obtained by the DIC calculation program, the cross-correlation coefficient field can be interpolated using an interpolation function. The specific steps are as follows:

[0102] Step (1), assuming that the interpolated cross-correlation coefficient R field is R max Located in the center of the field and around R max When the bicubic interpolation function is used to interpolate the R field, information of 4 Rs is required. Therefore, the R field can be further divided into 4 sub-units, each with 1 R max and 3 R as nodes, and then perform bicubic interpolation on each unit. max And around 3 R we can define the bicubic interpolation function as follows:

[0103]

[0104] Where: F(x,y) is the value of the bicubic interpolation function (where x and y are the coordinates in the subunit).

[0105] Step (2), determine the coefficient a in formula (8) by satisfying the following three conditions:ij :

[0106] ①The function values ​​of the 4 nodes are equal to R at the nodes.

[0107] ② The first-order derivative of the function of the four nodes in the x direction is equal to the R difference of the node in each direction, see formula (9). This condition also applies to the y direction:

[0108]

[0109] ③ The second-order derivative value of the function of the four nodes in the xy plane is equal to the R difference of the node in each plane, see formula (10):

[0110]

[0111] Step (3), first change x and y (the change is 0.01) to calculate F(x, y), and then search for integer pixels to obtain F(x, y)>R max , and use it as the position of the sub-pixel unit displacement.

[0112] Digital image is to find the extreme value of the mutual correlation coefficient and calculate the solution of the parametric equation. When the mutual correlation coefficient is at the maximum value, the extreme value is found by the Newton-Raphson iteration method to obtain the displacement value of each point in the calculation area. Assuming that the first-order shape function is selected, the correlation function R should obtain the extreme value, that is, the gradient of the correlation coefficient R approaches 0, so:

[0113] ▽R(p)=▽R(p0)+▽▽R(p0)(p-p0)=0 (11)

[0114] Further rearranging formula (11) yields

[0115]

[0116] Where: p0 is the initial value of the iteration, p is the approximate value after iteration, ▽R(p0) is the gradient of the mutual correlation coefficient function,

[0117] ▽▽R(p0) is the quadratic partial derivative of the cross-correlation coefficient function.

[0118] In summary, when using the DIC calculation program to obtain the displacement of soil particles on the surface of the sample, the mutual correlation coefficient should be used to calculate the integer pixel displacement value of the observation area, and then the sub-pixel calculation method should be used to determine the displacement value with higher precision, and then the deformation parameters at the sub-pixel level are obtained by the bicubic interpolation calculation method, and finally the final displacement is obtained by the Newton-Raphson iteration method. The present invention can realize the study of the evolution law of shrinkage and swelling of the soil surface by using the DIC calculation program to track and calculate the displacement field on the surface of the sample.

[0119] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles, characterized in that: It includes a stretching-compression sample box, a high-precision balance, a water bath temperature control system and a data acquisition and image processing system; the stretching-compression sample box is arranged on the high-precision balance, the stretching-compression sample box and the high-precision balance are placed in a visualization test housing, a semiconductor water cooling head heat sink is installed on the visualization test housing, the semiconductor water cooling head heat sink is connected to the water bath temperature control system through an inlet and outlet water pipe, a water inlet connected to the stretching-compression sample box is also arranged on the visualization test housing, and a high-precision camera is arranged above the stretching-compression sample box; The tensile-compression sample box comprises a fixed half box, a movable half box, a force sensor and a base, wherein the fixed half box is fixed on one end of the base, and the movable half box is arranged on a ball bearing at the other end of the base through a slide groove at the bottom; a first fastening device for connecting and separating the fixed half box and the movable half box is arranged on the corresponding contact surfaces of the fixed half box and the movable half box, a sample box groove is provided on the inner bottom surface of the fixed half box and the movable half box, a water-adding U-shaped groove is provided on the base, a water-permeable hole connected to the water-adding U-shaped groove is provided in the sample box groove, a water-permeable film is applied on the water-permeable hole, a waterproof film is placed on the inner wall and gap of the fixed half box and the movable half box, a first screw is provided at the edge of the base at the outer end of the movable half box away from the fixed half box, and a force sensor capable of connecting and separating the movable half box is provided; High-precision balances, high-precision cameras and force sensors are all connected to the data acquisition and image processing system.

2. The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles according to claim 1 is characterized in that: The water inlet on the visualization test shell is connected with the water inlets of the fixed half box and the movable half box through a hose, and the water inlets of the fixed half box and the movable half box are connected with the water addition U-shaped groove of the fixed half box and the water addition U-shaped groove of the movable half box respectively.

3. The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles according to claim 1, characterized in that: The top ends of the three side walls of the fixed half box are provided with acrylic plates through a second fastening device, and the second fastening device comprises an L-shaped connecting plate, and the L-shaped connecting plate connects the acrylic plate and the fixed half box through screws and nuts.

4. The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles according to claim 1, characterized in that: A lubricant coating is applied on the contact surface between the acrylic plate and the sample, and the lubricant is polytetrafluoroethylene.

5. The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles according to claim 1, characterized in that: The water bath temperature control system includes a stainless steel container, a micro water pump, an electric constant temperature environment box and an intelligent time and temperature controller. The micro water pump is arranged in the stainless steel container, and the stainless steel container is arranged in the electric constant temperature environment box. The water outlet of the micro water pump is connected with the water inlet of the semiconductor water cooling head heat sink through the water inlet pipe, and the water outlet of the semiconductor water cooling head heat sink is connected to the stainless steel container through the water outlet pipe, forming a closed loop of water circulation; the temperature probe in the intelligent time and temperature controller is installed inside the tension-compression sample box, and the temperature control switch of the intelligent time and temperature controller is connected with the micro water pump.

6. The device for measuring the mechanical behavior of soil and its crack characteristics under dry-wet cycles according to claim 1, characterized in that: The high-precision camera is fixed above the tension-compression sample box through a tripod.

7. A determination method using the device according to any one of claims 1 to 6, characterized in that: The following steps are involved: Step (1), sample preparation; after the sample is prepared, the fixed half box and the movable half box are connected by a first fastening device, while the force sensor is not connected to the two sample boxes; Step (2), placing the two sample boxes on a high-precision balance directly below the micro-single camera, and then using the micro-single camera to take pictures at preset time intervals; loosening the first fastening device connecting the fixed half box and the movable half box, and then connecting the force sensor to the movable half box with a first screw; Step (3), when t=t0, the temperature T of the environment where the sample is located is controlled to 24°C±1°C by a water bath temperature control system, and then the first dehumidification test is started, and the starting time t=t0 is recorded; Step (4), the matrix suction of the sample gradually increases during the dehumidification process, thereby inducing the contraction of the sample and causing the moving half box to move to the left. At the same time, the force sensor prevents the moving half box from moving, thereby measuring the tensile force generated by the sample during the dehumidification process, which takes a negative value according to the sign convention of soil mechanics; Step (5), the soil sample is then further dehumidified under restricted conditions. As time passes, after measuring n time intervals, where n is an integer greater than or equal to 10, it is found that the tension gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval, and so on until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors also gradually decrease. Then, combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating Wire Sensors for Geotechnical Engineering Instruments", the test end time when the tension test data tends to be stable is determined; at this time, the system automatically stops collecting tension data, indicating the end of the first dehumidification stage; Step (6), after the first dehumidification test is completed, the first screw is tightened again, and the force sensor that has been reset to zero is reconnected to the movable half box, and the first humidification test can be started; water is added to the water-adding U-shaped grooves of the two half boxes so that the sample is humidified through the water-permeable membranes on the water-permeable holes of the two half boxes, and a high-precision balance is used to monitor the change in the moisture content of the sample; Step (7), during the humidification process of the sample, the force sensor will prevent the moving half box from moving to the right due to the expansion of the sample, thereby measuring the pressure generated by the sample during the humidification process. As time goes by, after measuring n time intervals, where n is an integer greater than or equal to 10, it is found that the pressure gradually tends to be stable. At this time, starting from the second time interval, the mean square error of the force from the second time interval to the nth time interval, the third time interval to the nth time interval... until the n-1th time interval to the nth time interval is calculated. It can be seen that these mean square errors are also gradually decreasing. Combined with the 0.1N / m force measurement error limit specified in GB / T13606-2007 "General Technical Conditions for Vibrating String Sensors for Geotechnical Engineering Instruments", the test end time when the pressure test data tends to be stable is determined; at this time, the system automatically stops collecting pressure data, indicating the end of the first humidification stage; Step (8), after the humidification is completed, cover the surface of the sample with plastic wrap to maintain a constant moisture content for 24 hours to promote the water balance inside the sample; Step (9), first tighten the first screw to reconnect the reset force sensor to the movable half box to prepare for the second dehumidification test of the sample, then remove the first fastening device connecting the fixed half box and the movable half box to start the second dehumidification test of the sample; The test steps of this process are similar to those described in the first dehumidification test; the subsequent dry-wet cycle process is also carried out in the same way.

8. The assay method according to claim 7, wherein: During the dry-wet cycle test, the crack network morphological characteristics on the sample surface and the displacement field distribution of soil particles were image analyzed and monitored in real time using the following methods: Image J software was used to binarize the crack images on the sample surface taken by a micro-single camera to describe the development and closure evolution characteristics of the crack network on the sample surface during the dry-wet cycle test. Digital image correlation method is used to monitor the changes in the displacement field distribution of soil particles on the sample surface during the drying and wetting cycle. The digital image correlation method selects two images in different states, which are called reference state image and deformed state image. In the reference state image, reference points P(x0, y0) are evenly set at given intervals along the vertical and horizontal directions, and their displacements are calculated. The shape function, which is the key function for measuring the deformation field of the object surface, is used to describe the points in the image before and after deformation as follows: x i ′=x i +u(x i ,y j )(1-a) and j 'and j +v(x i ,and j )(1-b) Where: (x i ,y j ) and (x i ′,y′ j ) are the coordinates before and after deformation, u and v are the displacements of the reference subunit point in the x and y directions, respectively; the first-order shape function is used to describe the expression of the shape function: x i x i +u+u x Δx+u y Δy(2-a) y j ′=y j +v+v x Δx+v y Δy(2-b) Where: (x i ,y j ) and (x i ′,y′ j ) are the coordinates of the subunits before and after deformation, u and v are the displacements of the center point of the reference image in the x and y directions, Δx and Δy are the displacements of the point (x i ,y j ) to the center point (x0, y0), u x 、u y 、v x and v y are the gradients of displacements u and v in the x and y directions, respectively; Extract corresponding points in the deformed state image, and select the reference state image with P(x0,y0,) as the center in the reference state image, which is called (2n+1) 2 The correlation window composed of pixel units is used to track its corresponding position in the deformed state image; i ,y i are the coordinates in the reference subunit; Similarly, a deformed subunit is set in the deformed state image, and its size is the same as that of the reference subunit; the similarity between the reference subunit and the deformed subunit can be expressed by the zero-mean normalized correlation coefficient R as follows: Where: x i ′、y i ' is the coordinate in the deformation subunit; f(x i ,y i ) and g(x i ′,y i ′) are the grayscale values ​​of the reference subunit and the deformed subunit respectively; the R value is in (E x ×E y ) pixel units, and then the point where R appears at a peak is taken as the corresponding point of the reference point.

9. The measuring method according to claim 8, characterized in that The displacement field and the mutual correlation coefficient field within the intensity resolution are obtained by the digital image correlation method, and the mutual correlation coefficient field is interpolated by using the interpolation function. The specific steps are as follows: Assume that the interpolated cross-correlation coefficient R field is given by R max Located in the center of the field and around R max When the bicubic interpolation function is used to interpolate the R field, the information of 4 R is required; the R field is further divided into 4 sub-units, each sub-unit is composed of 1 R max and 3 R as nodes, and then perform bicubic interpolation on each unit; based on 1 R max And the bicubic interpolation function defined around 3 R is expressed as follows: Where: F(x, y) is the value of the bicubic interpolation function, where x and y are the coordinates in the subunit. The coefficient a in equation (8) is determined by satisfying the following three conditions: ij : ①The function values ​​of the four nodes are equal to R at the nodes; ② The first-order derivative of the function of the four nodes in the x direction is equal to the R difference of the node in each direction, see formula (9); this condition also applies to the y direction: ③ The second-order derivative value of the function of the four nodes in the xy plane is equal to the R difference of the node in each plane, see formula (10): First change x and y by 0.01, calculate F(x,y), and then use integer search to find F(x,y)>R max , and use it as the position of the sub-pixel unit displacement.

10. The measuring method according to claim 9, characterized in that The digital image correlation method is to find the extreme value of the correlation coefficient and calculate the solution of the parametric equation. When the correlation coefficient is at its maximum value, the extreme value is found by the sub-pixel search method Newton-Raphson iteration method to obtain the displacement value of each point in the calculation area; assuming that a first-order shape function is selected, the correlation function R should obtain an extreme value, that is, the gradient of the correlation coefficient R approaches 0, so: ▽R(p)=▽R(p0)+▽▽R(p0)(p-p0)=0 (11) Further rearranging formula (11) yields Where: p0 is the initial value of the iteration, p is the approximate value after iteration, ▽R(p0) is the gradient of the cross-correlation coefficient function, and ▽▽R(p0) is the quadratic partial derivative of the cross-correlation coefficient function.

Images