Strain Measurement Method Based on Concrete Ferromagneticity

Abstract

This invention relates to a strain measurement method based on the ferromagnetism of concrete, belonging to the field of performance testing of concrete structures in civil engineering. The method is based on the principle that the density distribution of ferromagnetic material within a concrete component changes when strain occurs. By measuring the magnitude of the magnetic force within the effective measurement range of the concrete component, the strain of the concrete component is determined, thereby calculating the stress. Specifically, the method involves: first, establishing a relationship between the density of the ferromagnetic material and the magnitude of the generated magnetic force; second, establishing a correspondence between the strain generated by the concrete component and the density of the ferromagnetic material; then, using a Cavendish torsion balance to measure the concrete component, calculating the magnetic force generated by the concrete component based on the displacement distance of the light spot on the receiving plate, and thus calculating the strain of the concrete component, and consequently, the stress. The testing device of this invention is simple, convenient, and can quickly measure the strain state of concrete structures.

Description

Technical Field

[0001] This invention belongs to the field of performance testing of concrete structures in civil engineering, and relates to a strain measurement method based on the ferromagnetism of concrete. Background Technology

[0002] Currently, the concrete used in all engineering projects can be broadly divided into two types: self-mixed concrete, which is produced by mixing gravel, sand, cement, and tap water on-site using a mixer according to the required strength ratio; and commercial concrete, which is produced by measuring and mixing cement, aggregates, water, and other admixtures and mineral additives at a batching plant according to a certain ratio, and then transported by truck to the site of use within a specified time. However, regardless of whether it is commercial concrete or self-mixed concrete, the components or structures cast from it all come from the same batch of materials and have the same characteristics.

[0003] In practical concrete structures, it is necessary to monitor the stress state of important components. Taking bridge engineering as an example, since the types and proportions of its constituent materials are the same, and the pouring process involves thorough mixing and vibration, it can be assumed that ferromagnetic materials are uniformly distributed inside the beam. Under different loads, the beam will generate internal forces such as axial force, shear force, and bending moment, causing changes in its internal state. These changes in the beam's internal state will cause changes in the density of ferromagnetic materials within a certain range. By measuring the ferromagnetic force in each region and then combining the measurement results for analysis and calculation, the stress state inside the entire beam can be characterized. For example, under vertical loads, the beam will flex. The upper part of the beam is compressed by compressive stress, while the lower part is stretched by tensile stress. At this time, the density of ferromagnetic materials in a certain range in the upper part of the beam increases while the density in the lower part decreases. The measured ferromagnetic force in each region will also differ, and the magnitude of the ferromagnetic force will respond to the deformation in each region. Finally, by considering the strain in each region and calculating the stress value using a constitutive model of material mechanics, the stress of the entire beam can be characterized. For the entire bridge, the density of ferromagnetic materials in its various components, such as beams, columns, and piers, will vary due to differences in material composition, construction conditions, or mechanical environment. Therefore, the strain of each component can be measured first, and then the internal stress of the entire bridge can be monitored by connecting the components.

[0004] Therefore, designing a strain measurement method based on the ferromagnetism of concrete based on the above characteristics is of great significance for the condition monitoring and analysis of concrete structures. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a strain measurement method based on the ferromagnetism of concrete, so as to realize the rapid measurement of the stress state of concrete structures.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The strain measurement method based on the ferromagnetism of concrete is based on the principle that the density distribution of ferromagnetic material inside a concrete component changes when strain occurs. The strain of the concrete component is obtained by measuring the magnitude of the magnetic force within the effective measurement range of the concrete component, and the stress is then calculated.

[0008] The method is as follows: First, a relationship between the density of ferromagnetic materials and the magnitude of the magnetic force they generate is established. Second, a corresponding relationship between the strain generated by the concrete component and the density of the ferromagnetic material is established. Then, a Cavendish torsion balance is used to measure the concrete component. The magnetic force generated by the concrete component is calculated based on the displacement distance of the light spot on the receiving plate. The strain generated by the concrete component can then be calculated, and thus the magnitude of the stress can be determined.

[0009] Furthermore, the relationship between the density of a ferromagnetic material and the magnitude of the magnetic force it generates is constructed as follows:

[0010] Take any cross-section of a concrete member, and within a certain measurement range of the cross-section, the mass m of the ferromagnetic material is:

[0011] m = ρ i ΔS

[0012] In the formula, ρ i Let S represent the density of the ferromagnetic material within the cross-section, and S represent the cross-sectional area; then, within the effective measurement range, the density ρ of the ferromagnetic material is:

[0013]

[0014] In the formula, S1 represents the projected area of ​​the magnetic measurement range on the cross section;

[0015] According to Maxwell's electromagnetic theory, within the effective measurement range, the relationship between the density and the magnitude of the magnetic force of a ferromagnetic material is as follows:

[0016]

[0017] In the formula, μ0 represents the vacuum permeability.

[0018] Furthermore, the relationship between the strain generated in a concrete component and the density of the ferromagnetic material is constructed as follows:

[0019] When a concrete member undergoes strain, the minute area change of the concrete member's cross-section is equivalent to the area of ​​the ferromagnetic material in the strain zone. Therefore, the mass m2 of the ferromagnetic material in the strain zone within the effective measurement range is:

[0020] m2=Δl×b×ρ i

[0021] In the formula, Δl represents the cross-sectional deformation of the concrete member within the effective measurement range under load, b represents the width of the effective measurement range, and ρ i This indicates the density of the ferromagnetic material within the cross-section;

[0022] Under load, the density of the ferromagnetic material within the effective measurement range of the cross-section becomes:

[0023]

[0024] In the formula, m1 represents the mass of the original ferromagnetic material within the effective measurement range, and l represents the length of the effective measurement range;

[0025] According to the strain formula ε=Δl / l, the relationship between strain ε and the density of ferromagnetic materials can be obtained as follows:

[0026]

[0027] Furthermore, when using a Cavendish torsion balance to measure concrete members, the known relationship between the torsion bar torque T and the spot displacement c is as follows:

[0028]

[0029] In the formula, GI p The value represents the torsional stiffness, l1 represents the length of the wire rope, and r represents the distance between the receiving plate and the plane mirror.

[0030] Since the torque is the same as the magnetic force generated by the concrete component, the relationship between the strain and the displacement of the light spot is established as follows:

[0031]

[0032] In the formula, B represents the magnetic induction intensity at the interface between the applied magnetic field and the magnetically conductive material;

[0033] The stress can be calculated using the stress-strain relationship σ=E×ε in the elastic stage of concrete, where E represents the elastic modulus of the concrete member.

[0034] The beneficial effects of this invention are as follows: This invention uses a Cavendish torsion balance to measure the stress of a concrete structure. The measuring device is simple, the measuring method is convenient, and it can quickly measure the strain of the concrete structure and calculate the stress at the same time.

[0035] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0036] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0037] Figure 1 This is a schematic diagram showing the density change of a ferromagnetic material under load.

[0038] Figure 2 A schematic diagram of a Cavendish torsion balance;

[0039] Figure 3 This is a schematic diagram illustrating the measurement principle.

[0040] Figure 4 A schematic diagram is constructed to illustrate the relationship between stress and light spot displacement. Detailed Implementation

[0041] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0042] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0043] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0044] During the concrete production process, the concrete is thoroughly mixed and vibrated, allowing ferromagnetic materials to be fully distributed throughout the concrete. Taking a cross-section of ordinary plain concrete, under normal circumstances, the ferromagnetic material is uniformly distributed within it. However, when external loads are applied to the concrete, it deforms, and within a certain measurement range, the density of the ferromagnetic material changes. Figure 1 As shown. Therefore, by measuring the change in the density of ferromagnetic material within a certain range inside concrete, the internal state of concrete can be reflected, and its micro-strain can be quantified. Thus, the internal stress value of concrete can be calculated through mechanical constitutive relations, thereby realizing the characterization of the internal stress state of concrete.

[0045] This invention measures the magnetic force generated in concrete components using a Cavendish torsion balance with a suspended strong magnet, and indirectly obtains the density of ferromagnetic materials by establishing a correlation between the magnetic force and the density of the material. The Cavendish torsion balance used in this invention includes a wire clip, an upper rod, a wire rope, a plane mirror, a lower rod, a neodymium magnet, and a movable nut, as shown below. Figure 2 As shown, in addition to the torsion balance, a laser lamp, a receiving plate, and a ruler were also used. The measurement process is as follows: First, the torsion balance is placed stationary. After the torsion bar comes to rest (the light spot on the receiving plate is relatively stationary), concrete is brought close to the neodymium magnet at one end of the torsion bar. The ferromagnetic material in the concrete is magnetized by the magnetic field, generating a magnetic force that causes the lower rod to deflect towards the concrete. The twisting of the lower rod causes the plane mirror to rotate, thus shifting the laser beam emitted by the laser lamp, ultimately changing the position of the light spot on the receiving plate. By recording the displacement of the light spot, the rotation angle of the torsion bar can be calculated. Then, the torsion force can be calculated using the torsional deformation formula. The torsion force is equal to the magnetic force generated between the ferromagnetic material and the neodymium magnet. The magnitude of the magnetic force reveals the relationship between the strain of the concrete under load and the density of the internal ferromagnetic material. Figure 3 As shown.

[0046] In the torsional deformation of a circular shaft, the formula for the torsion angle is:

[0047]

[0048] In the formula, Indicates the angle of twist, GI p Let l represent the torsional stiffness, l1 represent the length of the wire rope, and T represent the torque (i.e., magnetic force). The torsion angle is amplified using optical principles.

[0049]

[0050] In the formula, c represents the light spot displacement, and r represents the distance from the receiving plate to the plane mirror. Combining equations (1) and (2), the relationship between torque and light spot displacement can be obtained:

[0051]

[0052] In the formula, the torque T is equal to the magnetic force F between the ferromagnetic material and the rubidium magnet.

[0053] In classical electromagnetic theory, magnetic materials and current-carrying conductors experience a force in a magnetic field, known as the magnetic force. This force on a magnetic material can be considered as the force exerted by the molecular current. When a magnetic material is magnetized by an external magnetic field, an internal magnetizing current is generated, and a surface magnetizing current is also formed on the material's surface, as shown in the following equation:

[0054]

[0055] In the formula, δ v δ represents the magnetization current density. s Let M represent the surface magnetization current density, M represent the magnetization intensity of the medium, and n represent the external normal vector of the magnetic medium surface. Then, the formula for the force between the applied magnetic field and the magnetically conductive material is:

[0056]

[0057] In the formula, B represents the magnetic flux density. For isotropic media, we have:

[0058]

[0059] In the formula, μ0 represents the free magnetic permeability, and μ represents the relative magnetic permeability of the medium. This is achieved through the vector gradient integral formula. The formula for magnetic force can be obtained as follows:

[0060]

[0061] Since the external magnetic field is distributed very complexly in the region where the magnetic material is located, it is difficult to solve the magnetic force using this integral formula. Therefore, it is assumed that the magnetic field is uniformly distributed around the magnetic material. Since the permeability of the magnetic material is much greater than 1, equation (7) can be simplified to:

[0062]

[0063] In the formula, B represents the magnetic induction intensity at the interface between the applied magnetic field and the magnetically conductive material, H represents the magnetic field intensity at the interface between the applied magnetic field and the magnetically conductive material, S represents the interaction area between the applied magnetic field and the magnetically conductive material, and μ r It represents the relative permeability of the medium.

[0064] Because the analysis and calculation of the ferromagnetic force and density distribution of ferromagnetic materials in three-dimensional concrete are quite complex, they are simplified to a two-dimensional planar analysis. Taking any cross-section of the concrete, the mass m of the ferromagnetic material within a certain measurement range of the cross-section is:

[0065] m = ρ i ×S (9)

[0066] In the formula, ρ i Let S represent the density of the ferromagnetic material within the cross-section, and S represent the cross-sectional area (i.e., the area of ​​interaction between the applied magnetic field and the magnetically conductive material); then, within the effective measurement range, the density ρ of the ferromagnetic material is:

[0067]

[0068] In the formula, S1 represents the projected area of ​​the magnetic measurement range on the cross section (i.e., the effective measurement range). Combining equations (8), (9), and (10), the relationship between the magnetic force and the density of the ferromagnetic material within the effective measurement range can be obtained as follows:

[0069]

[0070] In mechanics of materials, there is a strain formula:

[0071]

[0072] Under load, within the effective measurement range of the cross-section, the total mass of the ferromagnetic material is the sum of the original ferromagnetic material mass m1 and the ferromagnetic material mass m2 in the strain zone within the effective measurement range. That is, the density of the ferromagnetic material in the cross-section is:

[0073]

[0074] In the formula, b represents the width of the effective measurement range, and l represents the length of the effective measurement range. Since the concrete deformation within the effective measurement range caused by the load is extremely small, and the ferromagnetic material can be considered uniformly distributed within the concrete, the minute area change in the concrete can be equated to the area of ​​the ferromagnetic material in the strain zone. Therefore, the mass m2 of the ferromagnetic material in the strain zone entering the effective measurement range is:

[0075] m2=Δl×b×ρi (14)

[0076] In the formula, Δl represents the cross-sectional deformation of the concrete member within the effective measurement range under load. The relationship between the density of the ferromagnetic material and the strain ε within the effective measurement range of equations (12), (13), and (14) is:

[0077]

[0078] Based on the known relationship between torque and light spot displacement from equation (3), by combining equations (3), (11), and (15), the relationship between strain and light spot displacement can be established:

[0079]

[0080] The stress in a concrete structure can be calculated using the stress-strain relationship σ = E × ε in the elastic stage of concrete, where E represents the elastic modulus of the concrete member. Simultaneously, the load on the concrete member can also be derived from the stress state.

[0081] F c =σ·A

[0082] In the formula, F c The symbol represents the load, and A represents the surface area of ​​the concrete member under load.

[0083] In summary, this invention establishes a mapping relationship between the displacement of a light spot and the stress in a concrete structure, such as... Figure 4 As shown, the stress of the concrete structure can be calculated by measuring the displacement distance of the light spot using a Cavendish torsion balance.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method of strain measurement based on the ferromagnetic properties of concrete, characterized in that: The method is based on the principle that the density distribution of the ferromagnetic material in the concrete member changes when the concrete member generates strain, and the strain of the concrete member is obtained by measuring the magnetic force in the effective measurement range of the concrete member, so as to calculate the stress size; The method is as follows: first, a relationship between the density of the ferromagnetic material and the magnetic force generated thereby is established, and then a corresponding relationship between the strain generated by the concrete member and the density of the ferromagnetic material is established; subsequently, the concrete member is measured by using a Cavendish torsion balance, the magnetic force generated by the concrete member is calculated according to the displacement distance of the light spot on the receiving plate, and then the strain generated by the concrete member is calculated, that is, the stress size is calculated; The relationship between the density of the ferromagnetic material and the magnetic force generated thereby is established as follows: The mass of ferromagnetic material in a certain measurement range of any cross section of a concrete member is m : wherein represents the density of the ferromagnetic material in the cross section, S represents the cross-sectional area; the density of the ferromagnetic material in the effective measurement range is then : In the formula, denotes the projected area of the magnetic measurement range on the cross section; According to Maxwell's electromagnetic theory, the relationship between the density of the ferromagnetic material and the magnetic force in the effective measurement range is: In the formula, denotes the vacuum permeability.

2. The strain measurement method of claim 1, wherein: The corresponding relationship between the strain generated by the concrete member and the density of the ferromagnetic material is established as follows: When the concrete member generates strain, the micro-area change of the concrete member section is equivalent to the area of the strain area ferromagnetic material, and then the mass of the strain area ferromagnetic material in the effective measurement range is: In the formula, represents the deformation of the cross section of the concrete member within the effective measurement range under the load action, b represents the width of the effective measurement range, represents the density of the ferromagnetic substance within the cross section; Under the action of the load, the density of the ferromagnetic material in the effective measurement range of the cross section becomes: wherein, represents the mass of the original ferromagnetic substance in the effective measurement range, l represents the length of the effective measurement range; According to the strain formula , the strain The relationship between the density of ferromagnetic substances is: 。 3. The strain measurement method according to claim 1 or 2, characterized in that: In measuring a concrete member using a Cavendish torsion balance, the torsion of the torsion bar of the torsion balance is known T and the relationship between the displacement of the light spot c is given by wherein represents torsional stiffness, represents the length of the wire rope, represents the distance between the receiving plate and the mirror. Since the torsion force is the magnetic force generated by the concrete member, the relationship between the strain and the displacement of the light spot is established as: In the formula, B B0represents the magnetic induction intensity at the surface of the magnetic conductive material acted by the applied magnetic field. According to the stress-strain relationship of the elastic stage of concrete i.e. the stress is calculated, wherein E denotes the modulus of elasticity of the concrete component.

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