Preparation method of internal crack of cement sample based on magnetic powder and magnetic suspension technology

Abstract

The invention discloses a preparation method of internal fractures of a cement sample based on magnetic powder and a magnetic suspension technology, and belongs to the technical field of manufacturing of rock-like material prefabricated fractures. The method comprises the following steps: preparing a crack template with a set shape by adopting a mixture of ice blocks and iron oxide powder; accurately suspending the crack template at a preset position in the mold by utilizing a magnetic suspension technology to avoid contacting with the inner wall of the mold; then cement grout is slowly poured into the mold till half of the crack formwork is immersed; after the slurry is initially set, stopping the magnetic suspension device, and pouring until the mold is filled with the slurry; and finally, the ice blocks are soaked in water to melt and carry out the iron oxide powder, and internal cracks are formed. The crack position is controlled through magnetic suspension, the error is small, residue interference is almost avoided, and the influence of fine cotton threads or 3D printing sheets on a mechanical test is reduced. The method is simple in structure, accurate in control and suitable for preparing internal hidden cracks of different shapes and sizes.

Description

Technical Field

[0001] This invention belongs to the field of prefabricated fracture technology for manufacturing rock materials, specifically relating to a method for preparing internal fractures in cement samples based on magnetic powder and magnetic levitation technology. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Creating internal cracks in cement specimens has always been a challenge. Previous methods using 3D printing resulted in direct contact between the cracks and the external environment, making it difficult to ensure their concealment. Forcing concealment, however, would leave the printed thin plate residue within the specimen. An existing patent, "Mold and Crack Creation Method for Creating Rock Materials with Internal Pre-existing Cracks," proposes using ice to create pre-existing cracks, preventing internal residue. However, the fine cotton thread used to fix the ice still leaves residue. This method introduces errors into subsequent mechanical tests of the cement specimens. Therefore, it is necessary to provide a new method to avoid this error. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing internal cracks in cement mortar samples based on magnetic powder and magnetic levitation technology. The method includes preparing a crack template of a predetermined shape using a mixture of ice and iron oxide powder; precisely suspending the crack template at a predetermined position within a mold using magnetic levitation technology, avoiding contact with the inner wall of the mold; then slowly pouring cement slurry into the mold until it covers half of the crack template; after the slurry has initially set, stopping the magnetic levitation device, and then continuing pouring until the mold is completely filled; finally, soaking in water melts the ice and carries away the iron oxide powder, forming internal cracks. This invention controls the crack position through magnetic levitation, resulting in small errors and almost no residual interference, reducing the influence of fine cotton thread or 3D printed thin plates on mechanical testing. The method has a simple structure, precise control, and is suitable for preparing internal hidden cracks of different shapes and sizes.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: In a first aspect, the present invention provides a method for preparing internal cracks in cement mortar samples based on magnetic powder and magnetic levitation technology, comprising the following steps: S1. Prepare a crack template of a predetermined shape by freezing a mixture of water and magnetic powder; S2. Magnetic levitation technology is used to suspend the crack template at a set position in the mold; S3. Then pour cement slurry into the mold until it covers half of the crack template. After the cement slurry has initially set, stop the magnetic levitation and continue pouring until the mold is full. After the cement slurry has fully set, cure the cement sample. After curing, place the cement sample in water so that the dissolved water and magnetic powder inside the sample flow out together.

[0006] In one or more embodiments, the magnetic powder includes iron oxide powder. It is typically necessary to obtain the pore diameter data of the cement sample beforehand to ensure that the particle size of the added iron oxide powder is much smaller than the diameter of the smaller pores in the cement sample.

[0007] The freezing temperature is not specifically limited, as long as it is sufficient to freeze the mixture of water and magnetic powder into ice cubes. Preferably, the freezing temperature of the mixture is -15°C. The following method ensures that there are 200 ice cubes. The above tensile strength is to prevent damage due to excessive magnetic force during the experiment.

[0008] In one or more embodiments, the mass ratio of magnetic powder to water is 1:4 to 1:10.

[0009] For stronger magnetism, choose an iron oxide powder:water ratio of 1:4; for higher strength, choose 1:10. A ratio exceeding 1:4 will result in insufficient crack strength. A ratio below 1:10 will result in insufficient magnetism. A mass ratio of 1:6 ensures the crack template has sufficiently strong magnetism to meet experimental displacement requirements and guarantees high ice strength, preventing disintegration during operation.

[0010] The ratio has no effect on the creation of cracks, but only on the creation process.

[0011] In one or more embodiments, there are one or more slit templates. When there is one slit template, it is placed at the geometric center of the mold. When there are multiple slit templates, they are also placed at the geometric center of the mold and arranged sequentially, one above the other.

[0012] In one or more embodiments, when there are multiple crack templates, the specific steps of step S3 include: when multiple crack templates need to be placed into the same sample, they need to be fixed one by one from bottom to top (they can be spaced a certain distance apart or placed close together, as long as the bottom surface of the next crack template is not lower than half of it, for example, the bottom surface of the second one should not be lower than half of the first one), first submerge half of the first crack template, wait for the poured cement slurry to initially set, then place the second template into the mold for suspension, and then pour, until it submerges half of the second crack template, and so on.

[0013] In one or more embodiments, step S3, during the process of placing the cement sample in water to dissolve the crack template of the mixture of ice and magnetic powder, further includes applying magnetic force to the system to accelerate the dissolution and outflow of iron oxide powder.

[0014] Place the cement sample in water. The soaking time is not specifically limited, as long as the melted water and magnetic powder can flow out. However, in order to ensure that the melted water and magnetic powder flow out as completely as possible, the soaking time should not be too short, such as 2 to 10 hours. The temperature of the soaking water is not specifically limited, as long as the ice can be completely melted, such as room temperature (25°C).

[0015] The magnetic levitation technology described in this invention utilizes electromagnets to generate and control the magnetic force, causing iron-containing materials to react accordingly. This allows for the capture, movement, and stable levitation of objects in the air without physical contact. Its core principle involves using an array of electromagnets to generate magnetic force. By controlling factors such as the current flowing through the electromagnets, the magnitude of the magnetic force is controlled. By precisely controlling the strength of the electromagnets' magnetic force, iron-containing materials can be contained within a predetermined area in space, thus achieving precise levitation and positioning in the air.

[0016] In one or more embodiments, when the crack template is suspended in the mold, contact between the crack template and the inner wall of the mold is avoided. This ensures that the crack template is located inside the cement sample.

[0017] In one or more embodiments, when pouring cement grout into the mold, the rising speed of the cement grout within the mold is 1–5 mm / min. This speed range effectively balances the impact effect on the suspended formwork with the stability of the formwork. Excessive speed (>5 mm / min) leads to increased hydrodynamic impact, potentially causing the formwork to shift; excessive speed (<1 mm / min) results in prolonged contact between the lower part of the formwork and the cement grout, potentially causing localized melting of ice. Especially when contacting cracked formwork, the pouring speed of the cement grout can be 1–3 mm / min.

[0018] The proportion of cement slurry is not specifically limited and any commonly used one can be used. For example, it can be a cement slurry made by mixing ordinary Portland cement, standard sand and water in a mass ratio of (0.8~1.5):(0.8~1.5):(0.2~0.5) (preferably 1:1:0.4).

[0019] When the cement slurry is poured too quickly, it will have a large impact on the suspended crack template, causing the crack template to deviate from the set suspension position, which in turn will cause the crack formed in the sample to have displacement deviation.

[0020] When the cement grout is poured too slowly, the contact time between the part below the crack template and the cement grout is too long, which will cause some of the ice to melt, resulting in a large error in the shape of the crack.

[0021] In one or more embodiments, when using magnetic levitation technology to suspend the crack template, electromagnets are positioned at the center of the mold's sidewall and at the top corner of the mold's top surface.

[0022] Preferably, when using magnetic levitation technology to levitate the slit template, electromagnets are positioned at the centers of the four side walls and the four corners of the mold. The electromagnets at the centers of the four side walls primarily control the horizontal positioning of the slit template in the X and Y directions; adjusting the current changes the magnetic force, thus adjusting the template's position. The electromagnets at the four corners primarily provide levitation force in the Z-axis direction, preventing the template from sinking or floating. Through the coordinated operation of eight electromagnets, a stable magnetic field environment can be formed in the central region of the mold, thereby achieving precise levitation and positioning of the template.

[0023] An electromagnet installed in the center of the mold sidewall is used to control movement in the X and Y axes, while an electromagnet installed at the top corner of the mold is used to enhance the Z-axis levitation force.

[0024] Preferably, the electromagnet installed at the center of the mold sidewall can rotate 10-15° around the Z-axis, so that the direction of the magnetic force can be deflected, making it easier to adjust back to the correct position when the mold deviates from the designated area.

[0025] Preferably, the electromagnet installed at the center of the mold sidewall is 10-30mm in size, has a power of 1-15 W, and a typical suction force range of 10-200N. In practical applications, it needs to be dynamically adjusted according to factors such as the size of the template, the mold dimensions, and the properties of the cement slurry.

[0026] Preferably, the direction of the electromagnet installed at the top corner of the mold is along the body diagonal and points to the geometric center of the crack template.

[0027] More preferably, the electromagnet installed at the top corner of the mold has a diameter of 10-30mm, a power of 1-15W, and a typical suction force range of 10-200N.

[0028] In one or more embodiments, the mass of the fracture template is 0.01-500 mg, preferably 0.09-5.21 mg. This range covers fractures from microscopic (e.g., 0.1 mm) to microscopic (e.g., 0.1 mm). 3 From larger cracks (e.g., 5 mm) 3 The common size of the mixture is approximately 0.9 g / cm³, and the actual mass depends on the density of the ice-iron oxide powder mixture. 3 ) and fracture volume.

[0029] In one or more embodiments, the viscosity of the cement grout is 2500-3000 mPa·s to suppress flow impact. "Flow impact" refers to the hydrodynamic forces exerted by the cement grout on the suspended formwork during pouring, including shear forces and impact forces. Higher viscosity results in greater flow resistance and a smaller velocity gradient, thereby reducing the impact force on the formwork and preventing displacement or deformation.

[0030] When the slurry viscosity is maintained in the high range of 2500-3000 mPa·s, its internal shear stress is large and the flow resistance is strong. Under this premise, strictly controlling the pouring rise speed within the low range of 1~5 mm / min can ensure that the Reynolds number of the slurry flow in the mold remains below the critical value, thus presenting a stable laminar flow state. In the laminar flow state, the trajectory of slurry particles is regular, the impact force on the suspended template is small and symmetrical, and the magnetic levitation system can easily maintain the stability of the template through PID algorithm.

[0031] In particular, in order to create cracks of different shapes, crack templates of various other shapes can be made according to the actual situation, all of which are within the protection scope of this invention.

[0032] In one or more embodiments, step S2 further includes: precise initial position calibration of the fracture template suspended at a predetermined position using magnetic levitation technology. This process eliminates initial position errors caused by minute differences in mass, shape, and magnetization intensity of the ice and iron oxide powder mixture.

[0033] This calibration procedure is a crucial prerequisite for achieving high accuracy in this method, and it specifically includes the following steps: a. Initial estimation and coarse positioning: First, based on the preset mass and target suspension position of the crack template, a set of initial current values ​​are estimated based on geometric calculations and electromagnetic force formulas, and applied to the corresponding 8 electromagnets so that the crack template is roughly suspended in the central area of ​​the mold.

[0034] When the crack template is located at the geometric center of the mold, it is mainly suspended by the tension provided by four electromagnets at the apex. This can be achieved through simple theoretical mechanics and trigonometric formulas: The tension in the electromagnet at each vertex can be calculated, and thus the current value can be determined. Among these, Density of the fracture template (approximately 0.9) ); : Volume of the crack template; Gravitational acceleration (taken as 9.8) ); b. High-precision measurement and deviation feedback: Subsequently, the high-precision laser displacement sensor pre-integrated on the outside of the mold is activated to measure the three-dimensional coordinates (X, Y, Z) of the geometric center point of the template in real time with a sampling frequency of not less than 100Hz, and immediately compares it with the preset target coordinates (X, Y, Z) to calculate the real-time position deviation (ΔX, ΔY, ΔZ).

[0035] c. Iterative Approximation and Fine Calibration: This invention employs a PID control algorithm for fine calibration. The core of this algorithm lies in axis-by-axis, small-step, and incremental approximation. Taking the X-axis direction as an example, if the system detects ΔX>+0.1mm (i.e. the template is biased to the right), the control unit will reduce the driving current of the right electromagnet in a predetermined small step, such as 0.01mA, and increase the driving current of the left electromagnet accordingly.

[0036] After the current is adjusted, the system waits for a short stabilization period, such as 0.5 seconds, and then measures the template position again using the laser displacement sensor to obtain a new deviation value.

[0037] This process will repeat cyclically until the deviation |ΔX| in the X-axis direction is less than or equal to 0.1 mm. Subsequently, the system will perform independent calibration operations on the deviations of the Y-axis and Z-axis in the same manner.

[0038] d. Calibration completion judgment: The system will determine that the initial calibration is complete and can proceed to the next stage of cement slurry pouring if and only if the positional deviation of the template in the X, Y, and Z directions is stable within ±0.1mm and lasts for at least 5 seconds.

[0039] The aforementioned calibration process is crucial. Experiments show that if this precise calibration step is omitted or simplified, and a fixed current is applied based solely on experience, the initial suspension position error can reach as high as 2-5 mm due to the unavoidable slight differences in the physical properties of each crack template. This huge initial error will be further amplified by hydrodynamics during subsequent casting, leading to a complete loss of control over the final crack position and making it impossible to achieve the high-precision positioning required by this invention. Therefore, this calibration process is an indispensable technical step to ensure the successful implementation of the invention.

[0040] Secondly, the present invention provides a method for preparing internal cracks in a cement sample based on magnetic powder and magnetic levitation technology, which is prepared by the above-mentioned preparation method.

[0041] One or more of the above technical solutions have the following advantages or beneficial effects: In this invention, the template (a mixture of magnetic powder and water) used to create the cracks is ultimately carried away by the water during soaking. This method reduces the mechanical research errors caused by the presence of substances such as fine cotton ropes remaining inside the sample. The removal of water and magnetic powder after the ice melts also effectively solves the serious error problem caused by the residue of 3D printed thin plates in the cement sample.

[0042] The ice blocks used to create the internal cracks have no direct physical contact with other parts. After melting, the magnetic particles can be discharged with the water flow, which reduces the mechanical experimental error caused by the usual method of fixing with fine cotton thread. In addition, the prepared cracks do not directly connect with the outside of the cement specimen, making them highly concealed.

[0043] The present invention uses magnetic levitation technology to control the size, direction and spatial distribution of prefabricated cracks with high precision. Through feedback control, the error between the crack template and the set position is usually ≤0.5 mm.

[0044] This invention uses electromagnets for magnetic control, which is more direct and convenient than other methods. For example, when using ultrasonic control, the sound waves need to penetrate the slurry, and impurities may affect the control quality of the sound waves. High-frequency compensation is also required. Magnetic control is simpler and does not need to consider factors such as magnetic fields and complex media, thus reducing the workload.

[0045] The calculations in this invention are relatively simple, and compared to other control methods, this method is easier to understand and calculate. Attached Figure Description

[0046] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0047] Figure 1 This is a schematic diagram of the position of an ice block containing iron oxide powder controlled by an electromagnet in Embodiment 1 of the present invention; Among them, 1. an electromagnet at the top corner of the mold; 2. a high-precision displacement sensor; 3. a mixture of ice and iron oxide powder; 4. an electromagnet at the center of the side of the mold. Detailed Implementation

[0048] In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0049] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0050] Example 1 (1) First, mix iron oxide powder and water (mass ratio of 1:4-1:10, optimally 1:6) evenly, place the mixture in a mold with the desired crack shape, and create an ice cube containing iron oxide powder, for example... .

[0051] The mixture needs to be at -15 Next, freeze the ice cubes, ensuring they have 200 kcal. The above tensile strength is to prevent damage due to excessive magnetic force during the experiment.

[0052] (2) Then, an electromagnet was pre-embedded in a cement slurry sample mold with dimensions of 200mm×200mm×200mm. In a low-temperature environment, an ice block containing iron oxide powder made in (1) was placed in a magnetic field. The ice block was controlled to suspend at a predetermined geometric center position by the magnetic field to avoid contact with the mold wall.

[0053] (3) Then, the cement slurry (ordinary silicate cement, standard sand and water mixed in a mass ratio of 1:1:0.4) is slowly poured in a low-temperature environment. This ratio is commonly used in the study of rock-like materials and has good fluidity and controllability of initial setting time. It can be adjusted according to the experimental needs. The flow rate is 2. (Injection pump control) until half of the cracked formwork is filled. After the poured cement grout has initially set, stop the magnetic levitation device (disconnect the power supply or switch of the magnetic levitation device), and then use 5 The remaining portion was poured at the flow rate, and the entire pouring was completed in two stages.

[0054] Dynamic feedback system: An integrated high-precision laser displacement sensor monitors the position of the fracture template in real time, and the current of each electromagnet is dynamically adjusted through a PID control algorithm to change the magnitude of the magnetic force, thereby keeping the mixture in a predetermined position.

[0055] After the cement has fully set, the cement sample is cured under standard conditions.

[0056] The PID algorithm mentioned above is short for Proportional-Integral-Derivative control algorithm, which is the most classic and widely used closed-loop control algorithm in fields such as industrial control, robot control, and temperature control systems.

[0057] Based on the deviation between the actual output of the system and the desired target, the control quantity is output through weighted calculation of the proportional, integral, and derivative components, so that the system deviation quickly and stably approaches 0.

[0058] (4) After the cement sample is cured, place the cement sample in water for 4 hours. The temperature of the soaking water is room temperature (25°C). After the ice melts completely and brings out the iron oxide powder in the sample, take out the sample and dry it under standard curing conditions to obtain a cement sample with internal cracks.

[0059] In step (1), the shape of the crack template needs to be fabricated according to the required crack shape. In step (2), magnetic levitation technology is used, and the specific arrangement is as follows: Magnetic levitation device layout scheme: 1. Electromagnet array layout Number and location of electromagnets: Electromagnet array: 8 sets of electromagnets are arranged at the center of the 4 side walls and the 4 top corners of the mold.

[0060] Installation angle: The side wall electromagnet is rotated 10-15° around the Z-axis so that the direction of its main magnetic force forms a horizontal angle with the normal direction of the side wall, and the main magnetic force vector converges at the geometric center point. The magnetic force direction of the apex electromagnet points to the geometric center along the body diagonal.

[0061] Electromagnet parameters: Side-wall electromagnet: The diameter of the electromagnet is 15. The power is 7.5. Typical suction power is 100 .

[0062] Vertex electromagnet: The diameter of the electromagnet is 30. The power is 15 Typical suction power is 200 .

[0063] 2. Parameters of high-precision laser displacement sensor A sensor is placed outside the midpoint of one side of the top surface of the sample mold, with the center of its measuring beam pointing towards the geometric center of the sample mold. A single laser displacement sensor can typically only accurately measure distance in one direction. By positioning it to the side and above, its measuring beam is not parallel to the X, Y, and Z coordinate axes. Therefore, any displacement of the fracture template in any direction will cause a change in the measured distance. Through simple geometric transformations, the three-dimensional coordinate deviation of the template can then be calculated.

[0064] 3. Suspension force verification Magnetic node positioning: By adjusting the current, the magnetic forces of the eight electromagnets are balanced at a magnetic equilibrium node at the center of the sample mold (coordinates 100 mm, 100 mm, 100 mm), where the crack template is fixed. A three-dimensional Cartesian coordinate system is established within the mold's internal space, with the origin O(0,0,0) at the lower left corner of the mold's bottom surface. The X, Y, and Z axes extend along the mold's length, width, and height, respectively, and the coordinates of the mold's apex angle are (200, 200, 200) mm. The magnetic equilibrium node is the geometric center of the mold, with coordinates (100, 100, 100) mm.

[0065] Suspension force estimation: The formula for the magnetic force that a single electromagnet can produce is:

[0066] F: Suspension force (unit: Newtons / N); μ0: Vacuum permeability (approximately 4π × 10⁻⁶ N); -7 H / m); N: Number of turns of the electromagnet coil (unitless); i: Current through the coil (unit: Ampere / A); A: Cross-sectional area of ​​the electromagnet poles (unit: square meter / m²); δ: Suspension air gap between the electromagnet and the suspended object (unit: meter / m).

[0067] It can be calculated that the force generated by an electromagnet far exceeds the resistance encountered by the ice.

[0068] Before suspending the fracture template using the aforementioned magnetic levitation device, a precise initial position calibration procedure must be performed to eliminate initial position errors caused by minute differences in mass, shape, and magnetization intensity of the ice and iron oxide powder mixture. This calibration procedure is a crucial prerequisite for achieving high precision in this method and specifically includes the following steps: a. Initial estimation and coarse positioning: First, based on the preset mass and target suspension position of the crack template, a set of initial current values ​​are estimated based on geometric calculations and electromagnetic force formulas, and applied to the corresponding 8 electromagnets so that the crack template is roughly suspended in the central area of ​​the mold.

[0069] When the fracture template is located at the geometric center of the mold, it is primarily suspended by the tension provided by four electromagnets at the apex. This suspension is achieved through simple theoretical mechanics and trigonometric formulas.

[0070] Density of the fracture template (approximately 0.9) ); : Volume of the crack template; Gravitational acceleration (taken as 9.8) ); The pulling force of the electromagnet at each vertex can be calculated, and thus the current value can be determined.

[0071] b. High-precision measurement and deviation feedback: Subsequently, the high-precision laser displacement sensor pre-integrated on the outside of the mold is activated to measure the three-dimensional coordinates (X, Y, Z) of the geometric center point of the template in real time with a sampling frequency of not less than 100Hz, and immediately compares it with the preset target coordinates (X, Y, Z) to calculate the real-time position deviation (ΔX, ΔY, ΔZ).

[0072] c. Iterative Approximation and Fine Calibration: This invention employs a PID control algorithm for fine calibration. The core of this algorithm lies in axis-by-axis, small-step, and incremental approximation. Taking the X-axis direction as an example, if the system detects ΔX>+0.1mm (i.e. the template is biased to the right), the control unit will reduce the driving current of the right electromagnet in a predetermined small step, such as 0.01mA, and increase the driving current of the left electromagnet accordingly.

[0073] After the current is adjusted, the system waits for a short stabilization period, such as 0.5 seconds, and then measures the template position again using the laser displacement sensor to obtain a new deviation value.

[0074] This process will repeat cyclically until the deviation |ΔX| in the X-axis direction is less than or equal to 0.1 mm. Subsequently, the system will perform independent calibration operations on the deviations of the Y-axis and Z-axis in the same manner.

[0075] d. Calibration completion judgment: The system will determine that the initial calibration is complete and can proceed to the next stage of cement slurry pouring if and only if the positional deviation of the template in the X, Y, and Z directions is stable within ±0.1mm and lasts for at least 5 seconds.

[0076] The aforementioned calibration process is crucial. Experiments show that if this precise calibration step is omitted or simplified, and a fixed current is applied based solely on experience, the initial suspension position error can reach as high as 2-5 mm due to the unavoidable slight differences in the physical properties of each crack template. This huge initial error will be further amplified by hydrodynamics during subsequent casting, leading to a complete loss of control over the final crack position and making it impossible to achieve the high-precision positioning required by this invention. Therefore, this calibration process is an indispensable technical step to ensure the successful implementation of the invention.

[0077] In step (4), the cement sample is placed in water. Usually, it is necessary to obtain the pore diameter data of the cement sample beforehand to ensure that the particle size of the added iron oxide powder is much smaller than the diameter of the smaller pores in the cement sample.

[0078] Example 2 To create multiple cracks by placing multiple crack templates in a single specimen, some steps and methods in Example 1 need to be adapted. Specifically, the following steps are included: The fabrication of the crack template is the same as in Example 1.

[0079] Electromagnets were pre-embedded in a cement slurry sample mold with dimensions of 200mm×200mm×200mm. In a low-temperature environment, multiple crack templates were placed in the magnetic field in order from bottom to top, with the bottom crack template placed first. The magnetic field was used to control the ice block to suspend at a predetermined geometric position, avoiding contact with the mold wall.

[0080] Then, grout is poured into the mold until it reaches half of the first crack template from the bottom up. After the poured cement grout has initially set, the magnetic levitation device is reset to the predetermined position, which is the crack position of the second to last template. Then, the second template from the bottom up is placed into the magnetic field for initial position calibration and the second template is controlled to levitate at the predetermined position by the magnetic field. Then, cement grout is poured into the mold until it reaches half of the second crack template. The subsequent process is repeated until the entire sample is poured.

[0081] The maintenance and removal of iron oxide powder steps are the same as in Example 1.

[0082] In step 2 of Example 2, the pouring speed needs to be adjusted adaptively according to the situation. For example, before contacting the crack template, a speed of 5 can be used. The speed was changed to 2 just before contacting the crack template. To ensure the production process does not take too long, and to ensure that the cement slurry initially poured is still in a favorable state when the entire sample is finally poured.

[0083] In the steps of Example 2, changing the magnetic field setting center of the magnetic levitation device does not affect other crack templates, and the presence of multiple crack templates in the mold does not affect magnetic field control or initial position calibration. For example, if the magnetic field setting center is changed to the position of the second crack template, half of the first crack template is already fixed by the initially set cement mortar, thus fixing the position of the first crack template. The third crack template has not yet been placed in the magnetic field, thereby avoiding the influence of multiple crack templates on the magnetic field. Only the interaction between one crack template and the magnetic field needs to be focused on. During magnetic field control, the PID algorithm can completely ensure that the position control of the current crack template is maintained without affecting other crack templates. In Example 2, the multiple crack templates themselves do not affect each other. The iron oxide powder in the crack templates is not magnetized and has no interaction force with each other.

[0084] In Example 2, if multiple cracks are to be arranged in a regular pattern and the surfaces of two or more cracks overlap horizontally, and cannot be produced by placing them in sequence, the sample can be poured from the side or other angles instead of from the bottom. If necessary, a mold with the corresponding angle and shape can be made so that it can be produced by placing it in sequence.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing internal cracks in a cement sample based on magnetic powder and magnetic levitation technology, characterized in that, Includes the following steps: S1. Prepare a crack template of a predetermined shape by freezing a mixture of water and magnetic powder; S2. Magnetic levitation technology is used to suspend the crack template at a set position in the mold; S3. Pour cement slurry into the mold until it covers half of the cracked template. After the slurry has initially set, stop the magnetic levitation and continue pouring until the mold is full. After the cement slurry has fully set, cure the cement sample. After curing, place the cement sample in water so that the dissolved water and magnetic powder inside the sample flow out together.

2. The preparation method according to claim 1, characterized in that, The mass ratio of magnetic powder to water is 1:4 to 1:10; preferably, the magnetic powder includes iron oxide powder. Preferably, the particle size of the added magnetic powder is much smaller than the diameter of the smaller pores in the cement sample.

3. The preparation method according to claim 1, characterized in that, The crack template may be one or more; Preferably, when there is only one slit template, it is placed at the geometric center of the mold; Preferably, when there are multiple crack templates, the specific steps of step S3 include: when multiple crack templates need to be placed into the same sample, they need to be fixed one by one from bottom to top. The cement grout first covers half of the first crack template. After the cement grout has initially set, the second crack template is placed into the mold for suspension, and then the grout is poured until it covers half of the second crack template, and so on.

4. The preparation method according to claim 1, characterized in that, In step S3, the process of placing the cement sample in water also includes applying magnetic force to the system.

5. The preparation method according to claim 1, characterized in that, When pouring cement slurry into the mold, the rising speed of the cement slurry in the mold is 1~5 mm / min.

6. The preparation method according to claim 1, characterized in that, When using magnetic levitation technology to suspend the crack template, electromagnets are placed at the center of the mold's side wall and at the top corner of the mold's top surface; Preferably, when using magnetic levitation technology to suspend the crack template, electromagnets are placed at the center of the four side walls of the mold and at the four top corners of the mold; Preferably, the electromagnets arranged at the center of the four side walls of the mold are mainly used to control the horizontal positioning of the slit template in the X and Y directions. By adjusting the current, the magnetic force is changed and the position of the template is adjusted. The electromagnets arranged at the four corners are mainly used to provide levitation force in the Z-axis direction. Through the coordinated work of the eight electromagnets, a stable magnetic field environment is formed in the central area of ​​the mold, thereby achieving precise levitation and positioning of the template.

7. The preparation method according to claim 6, characterized in that, An electromagnet installed at the center of the mold sidewall can rotate 10-15° around the Z-axis; Preferably, the electromagnet installed in the center of the mold sidewall is 10-30mm in size, has a power of 1-15 W, and a typical suction force range of 10-200N; Preferably, the direction of the electromagnet installed at the top corner of the mold is along the body diagonal and points to the geometric center of the crack template; Preferably, the electromagnet installed at the top corner of the mold has a diameter of 10-30mm, a power of 1-15W, and a typical suction force range of 10-200N.

8. The preparation method according to claim 1, characterized in that, The mass of the fracture template is 0.01-500 mg, preferably 0.09-5.21 mg; The viscosity of the cement slurry is 2500-3000 mPa·s.

9. The preparation method according to claim 1, characterized in that, Step S2 also includes: performing precise initial position calibration on the fracture template that is suspended at a set position using magnetic levitation technology; The preferred method for initial position calibration specifically includes the following steps: a. Initial estimation and coarse positioning: First, based on the preset mass and target suspension position of the crack template, a set of initial current values ​​are estimated based on geometric calculations and electromagnetic force formulas, and applied to the corresponding 8 electromagnets so that the crack template is roughly suspended in the central area of ​​the mold. When the crack template is located at the geometric center of the mold, it is mainly suspended by the tension provided by the four electromagnets at the apex; through the formula The pulling force of the electromagnet at each vertex can be calculated, and thus the current value can be determined; among them, : Density of the crack template; : Volume of the crack template; Gravitational acceleration; b. High-precision measurement and deviation feedback: Subsequently, the high-precision laser displacement sensor pre-integrated on the outside of the mold is activated to measure the three-dimensional coordinates (X, Y, Z) of the geometric center point of the template in real time with a sampling frequency of not less than 100Hz, and immediately compares it with the preset target coordinates (X, Y, Z) to calculate the real-time position deviation (ΔX, ΔY, ΔZ). c. Iterative Approximation and Fine Calibration: Fine calibration is performed using a PID control algorithm; the core of this algorithm lies in axis-by-axis, small-step, and incremental approximation. Taking the X-axis direction as an example, if the system detects ΔX > +0.1mm, the control unit will reduce the driving current of the right electromagnet in a predetermined small step and increase the driving current of the left electromagnet accordingly. After the current is adjusted, the system waits for a short period of stabilization, and then measures the template position again using the laser displacement sensor to obtain a new deviation value. This process will be repeated until the deviation |ΔX| in the X-axis direction is less than or equal to 0.1mm; subsequently, the system will perform independent calibration operations on the deviations of the Y-axis and Z-axis in the same manner according to the same logic. d. Calibration completion judgment: The system will determine that the initial calibration is complete and can proceed to the next stage of cement slurry pouring if and only if the positional deviation of the template in the X, Y, and Z directions is stable within ±0.1mm and lasts for at least 5 seconds.

10. A method for identifying internal cracks in a cement sample based on magnetic powder and magnetic levitation technology, characterized in that... It is prepared by the preparation method described in any one of claims 1 to 9.

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