A smart FRP-concrete composite structure deflection identification method based on resistance change

Abstract

The application discloses a kind of based on resistance change intelligent FRP-concrete composite structure deflection identification method, including determining composite structure and intelligent FRP material information;Along the length direction of FRP, transverse electrode is arranged, and resistance change data of each electrode is collected using resistance acquisition system;Resistance change rate is carried out multi-point smooth fitting, and polynomial mathematical model is established;Input core function based on resistance change, and the real-time deflection of any position point of composite structure is calculated.The application can nondestructively monitor and reflect the real-time deflection of any position point of structure without affecting the normal operation of structure, with real-time, comprehensiveness, high accuracy, strong adaptability, nondestructive monitoring characteristics, and can be applied to deflection monitoring of bridge, tunnel and other structures.

Description

Technical Field

[0001] The present invention relates to the field of intelligent materials and structural health monitoring in civil engineering, and in particular to a method for identifying the deflection of an intelligent FRP-concrete composite structure based on resistance change. Background Art

[0002] Structural health monitoring is a key technology to ensure the long-term safe operation of buildings and infrastructure. With the rapid development of urbanization and industrialization, the stability and durability of structures such as bridges and tunnels have become an important part of public safety. Among the many structural performance parameters, deflection is an important indicator to measure structural deformation and is crucial for evaluating the safety and functionality of structures. Traditional deflection measurement methods, such as contact displacement meters and optical measurement equipment, are usually costly, complex to install, and in some cases may cause damage to the structure. In addition, these methods often have difficulty in achieving continuous monitoring, which limits real-time understanding and rapid response to changes in structural performance.

[0003] Fiber-reinforced plastic (FRP) materials are widely used in civil engineering to reinforce concrete structures due to their lightweight, high strength, and corrosion resistance. FRP reinforcement is usually applied to the concrete surface by gluing or wrapping to improve its bending, shear, and seismic properties. For example, FRP reinforcement can significantly increase the load-bearing capacity of beams, slabs, and columns, extend their service life, and in some cases achieve rapid reinforcement of structures. In particular, carbon fiber reinforced plastic (CFRP) can be used as a self-sensing material due to its piezoresistive effect, showing the potential for self-monitoring of structures.

[0004] The piezoresistive effect refers to the change in a material's electrical resistance when subjected to pressure. CFRP exhibits a pronounced piezoresistive effect due to its microstructural properties. When CFRP is applied to concrete structures and subjected to load, the contact resistance between the fibers and the matrix changes in direct proportion to the applied stress. By accurately measuring this change in CFRP's electrical resistance, information on the stress and deformation of the structure can be indirectly obtained. This method offers advantages such as simple installation, low cost, and continuous data acquisition, providing a new technical approach for structural health monitoring.

[0005] Although a number of patented technologies have been dedicated to monitoring structural deflection, they still have some limitations in practical applications. For example, some patents rely on external sensors and complex data acquisition systems, such as the utility model patents with announcement numbers CN205426061U and CN204007499U, which not only increase costs but may also be affected by environmental factors. In addition, some technologies may require invasive modifications to the structure, such as the utility model patent with announcement number CN202255273U, which discloses a strain sensor for measuring structural deflection. One end of the sensor needs to be welded to the tested structure, and the other end passes through the mounting hole and is fixed to the mounting hole by a self-locking nut, affecting the integrity of the structure. In addition, the existing technology also has challenges in data analysis and long-term stability, which limits its widespread application in complex engineering environments. Therefore, based on the self-sensing characteristics of CFRP, the present invention proposes an intelligent FRP-concrete composite structure deflection identification method that does not require additional sensors, and highly integrated, non-destructive, real-time monitoring of structural deflection. Summary of the Invention

[0006] The purpose of the present invention is to address the limitations of structural deflection monitoring methods in terms of real-time performance, cost-effectiveness, and possible damage to structural integrity. This method provides an intelligent FRP-concrete composite structure deflection identification method based on resistance change, enabling continuous and non-destructive monitoring of structural deflection to enhance the accuracy and timeliness of structural performance assessment, thereby ensuring structural safety and extending its service life.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change includes determining the composite structure and intelligent FRP material information; arranging transverse electrodes along the length of the FRP and using a resistance acquisition system to collect resistance change data for each electrode; performing multi-point smoothing fitting of the resistance change rate to establish a polynomial mathematical model; and inputting a core function based on resistance change to calculate the real-time deflection of any position of the composite structure.

[0009] The FRP is CFRP or a mixture of CFRP and other non-conductive FRP. The non-conductive FRP may be GFRP (glass fiber reinforced composite material), BFRP (basalt fiber reinforced composite material), or AFRP (aramid fiber reinforced composite material).

[0010] Furthermore, the number of FRP layers is single or multiple.

[0011] Furthermore, the hybrid method is that CFRP is in the middle layer, other non-conductive FRP is in the outer layer, and the FRP layers are impregnated and cured with epoxy resin.

[0012] The electrode is a conductive copper sheet or a conductive copper wire, wherein the width of the conductive copper sheet is 3 to 5 mm. The electrode is fixed between CFRP layers or between CFRP and non-conductive FRP layers by silver paste or conductive glue, and is connected to an external resistance collection system to achieve an end-to-end connection.

[0013] The resistance acquisition system is a sophisticated electronic device that can record and store the resistance value of each electrode in the FRP in real time, with an accuracy requirement of at least four decimal places to meet the standards of high-precision measurement.

[0014] The function based on the resistance change is:

[0015]

[0016] Where: a is the parameter of the FRP resistance change rate; y is the distance from the outer surface of the FRP in the structural cross section to the neutral axis; is a polynomial based on the resistance change rate, and the value of x is within the effective length L of the structure, that is, 0≤x≤L.

[0017] Furthermore, the parameter a of the FRP resistance change rate is the inherent property of the material's conductivity, and the relationship satisfies:

[0018]

[0019] Where: ε is the FRP tensile strain.

[0020] The polynomial function mathematical model is in the form of:

[0021]

[0022] Where: ∑ represents summation; subscript k=1 represents the starting point of summation, and superscript n represents the end point of summation; α k is the coefficient of the kth term, x k is x raised to the power of k.

[0023] The smooth fitting adopts the least square method.

[0024] Furthermore, the theoretical formula of the core function based on resistance change is:

[0025] The calculation formulas for the deflection f(x) and bending moment M(x) at any section x are:

[0026]

[0027] Where: E is the elastic modulus of the structural material, and I is the moment of inertia of the structural section.

[0028] The strain ε(x) at any point x at the bottom of the structure is:

[0029]

[0030] Furthermore, the resistance change rate of FRP at any point on the bottom of the structure is The relationship between the bending moment M(x) is:

[0031]

[0032] Where: is the resistance change rate at different positions x of the FRP, R0 is the initial resistance of each FRP electrode, and ΔR is the resistance R at any time at the FRP length x. t The difference from the initial resistance R0, that is, ΔR=R t -R0;

[0033] Furthermore, the deflection f(x) at any point on the bottom of the structure and the resistance change rate of the FRP The relationship is:

[0034]

[0035] The beneficial effects of the present invention are:

[0036] (1) Through the multi-point smoothing fitting technology, the noise and random fluctuations in the resistance change data are effectively reduced, and the accuracy and reliability of deflection identification are improved.

[0037] (2) By monitoring the structural deflection through resistance changes, the present invention does not require additional sensors or equipment, greatly reducing the monitoring cost and avoiding intrusive detection methods that cause additional damage to the structure, such as drilling or adding weight, thereby protecting the integrity and safety of the structure.

[0038] (3) The method of the present invention is applicable to FRP-concrete composite structures in different environments and working conditions, including but not limited to high temperature, low temperature, humid, dry and chemically corrosive environments; and has good adaptability and versatility under both static and dynamic load conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be further described below with reference to the accompanying drawings. However, the embodiments in the accompanying drawings do not constitute any limitation to the present invention.

[0040] Figure 1 Schematic diagram of the structural deflection identification method of the present invention.

[0041] Figure 2 Schematic diagram of the electrode arrangement of the present invention.

[0042] Figure 3 Schematic diagram of the FRP hybrid method of the present invention.

[0043] Figure 4 4 is a calculation diagram of an embodiment of the present invention.

[0044] Figure 5 Schematic diagram of the cross section of the FRP-concrete composite structure according to an embodiment of the present invention.

[0045] exist Figures 1 to 5 These include:

[0046] 1. Concrete structure, 2. Load, 3. Test point, 4. FRP, 5. Deflection curve, 6. Resistance test system, 7. Electrode, 8. CFRP, 9. Non-conductive FRP. DETAILED DESCRIPTION

[0047] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0048] like Figures 1 to 5 As shown, a method for identifying the deflection of an intelligent FRP-concrete composite structure based on resistance change includes the following steps:

[0049] Step 1: The composite structure is a simply supported beam or an outrigger beam at the bottom of the intelligent FRP reinforced concrete structure. Determine the effective length L and cross-sectional form of the composite structure, which can determine the required structural length and the distance between the outer surface of the cross-sectional FRP and the neutral axis in the solution process.

[0050] Specifically, FRP is CFRP or a mixture of CFRP and other non-conductive FRP. The non-conductive FRP can be GFRP (glass fiber reinforced composite material), BFRP (basalt fiber reinforced composite material), AFRP (aramid fiber reinforced composite material), and the number of FRP layers is single or multi-layer. The mixing method is that CFRP is in the middle layer and other non-conductive FRP is in the outer layer. The FRP layers are impregnated and cured with epoxy resin. The effective length L is the distance between the supports at both ends of the structure. In the case of a rectangular cross-section, the distance from the outer surface of the FRP to the neutral axis is half the height of the rectangle; for complex cross-sections, a graphical method is used to calculate the position of the neutral axis. The formula is:

[0051]

[0052] In formula (1), A i is the area of ​​the ith part, y i is the y-coordinate of the centroid of the ith part.

[0053] Step 2: Test the tensile strain and resistance change rate of the smart FRP sample. The specific operations are as follows:

[0054] S1. Cut the smart FRP specimen, ensuring the surface is clean and the conductive fibers are exposed. Install the electrodes and connect the wires to the specimen's conductive fibers using conductive glue or clamps. Secure the specimen to the fixture of the tensile testing machine, connect the resistance acquisition system and strain gauge, and ensure all connections are secure.

[0055] S2. Calibrate the tensile testing machine, strain gauge, and resistance acquisition system to test the initial resistance R0 of the FRP. Start the tensile testing machine and gradually apply the tensile load. At different stretching stages, record the corresponding strain values ​​ε t And the resistance value of the sample R t .

[0056] S3. Use the data acquisition system to record the strain and resistance changes in real time. After the test, analyze the recorded data, draw a curve of the relationship between strain and resistance change rate, and calculate the resistance change rate. The relationship is:

[0057]

[0058] In formula (2): ΔR=R t -R0, a is the resistance change rate parameter, and ε is the FRP tensile strain.

[0059] Step 3: Arrange transverse electrodes along the length of the existing smart FRP, connect each electrode to the outermost electrode at one end, and use the resistance acquisition system to test the initial resistance and resistance change data of each electrode. The specific operation is as follows:

[0060] S1. Cut conductive copper sheets or wires to serve as electrodes, with a width of 3-5 mm. Secure the electrodes between CFRP layers or between CFRP and non-conductive FRP layers using silver paste or conductive adhesive. Ensure that each electrode is evenly spaced along the length of the smart FRP sample and connect to the outermost electrode at one end.

[0061] S2. Connect a wire to each electrode, ensuring the connection is secure. Connect the other end of the wire to the resistance acquisition system. Ensure all connections are secure and calibrate the resistance acquisition system to ensure the accuracy of the initial resistance measurement.

[0062] S3. Use the resistance acquisition system to test the initial resistance value of each electrode During load application or environmental changes, the resistance change data of each electrode is recorded in real time The resistance acquisition system should have an accuracy of at least four decimal places to meet high-precision measurement standards. Real-time recording and storage of the resistance value of each electrode in the FRP, and calculation of the resistance change rate of each distributed electrode The relationship is:

[0063]

[0064] Step 4: Use the least squares method to smoothly fit the resistance change rate of the electrodes at different positions at different times, and the resistance change rate of the electrodes at different positions x The polynomial is:

[0065]

[0066] In formula (4), R0 is the initial resistance of each FRP electrode, ΔR is the resistance R at any time at the FRP length x t The difference from the initial resistance R0, that is, ΔR=R t -R0; ∑ represents summation; subscript k=1 represents the starting point of the summation, and superscript n represents the end point of the summation; α k is the coefficient of the kth term, x k is x raised to the power of k.

[0067] Step 5: Based on the theoretical formulas of material mechanics and boundary conditions, a core function based on resistance change is established. The structural information in step 1, the material information in step 2, and the polynomial mathematical model in step 4 are input to calculate the real-time deflection of any position of the composite structure. The core function based on resistance change is:

[0068]

[0069] In formula (5), a is the parameter of the FRP resistance change rate; y is the distance from the outer surface of the FRP in the structural cross section to the neutral axis; is a polynomial based on the resistance change rate, and the value of x is within the effective length L of the structure, that is, 0≤x≤L.

[0070] Specifically, the theoretical formula based on material mechanics is:

[0071] The calculation formulas for the deflection f(x) and bending moment M(x) at any section x are:

[0072]

[0073] In formula (6), E is the elastic modulus of the structural material, and I is the moment of inertia of the structural section.

[0074] The strain ε(x) at any point x at the bottom of the structure is:

[0075]

[0076] Combining formula (2) and formula (4), the resistance change rate of FRP at any point on the bottom of the structure can be obtained: The relationship between the bending moment M(x) is:

[0077]

[0078] Combining formula (6) and formula (8), the deflection f(x) at any point on the bottom of the structure and the resistance change rate of FRP are The relationship is:

[0079]

[0080] Formula (9) is integrated twice, and the boundary condition is introduced: f(0) = f(L) = 0, and the core function based on the resistance change is obtained as:

[0081]

[0082] Example 1

[0083] like Figure 4 As shown in the figure, a reinforced concrete simply supported beam with a cross-section of 0.15m×0.3m and an effective span of 3.6m is applied with a vertical concentrated load at the three equal points, and displacement meters are arranged at the supports, loading points, and mid-span. At the same time, transverse electrodes are arranged at the supports, loading points, and mid-span in the FRP within the effective span. The resistance change rate of the FRP is Measure the resistance between the electrode and the leftmost support. Define the initial resistance between each electrode and the leftmost support electrode before the test starts as i takes 0, 1, 2, 3, 4; when the test is completed at time t, the resistance between the electrodes is Then at time t, the rate of change of the distributed resistance is At the beginning of the test, at time t1, the FRP distributed resistance change rate is x∈[0,3.6]; at t2, the FRP distributed resistance change rate is x∈[0,3.6].

[0084] According to the above theoretical derivation, the results of the theoretical and actual measurement of the deflection changes of the simply supported beam are compared and analyzed, as shown in Table 1 and Table 2

[0085] Table 1 Comparison of calculated data on deflection changes of simply supported beams at time t1

[0086]

[0087] Table 2 Comparison of calculated data on deflection changes of simply supported beams at time t2

[0088]

[0089]

[0090] Tables 1 and 2 show that the calculated results based on resistance change have a relatively small deviation from the actual measured values, with a relative error of less than 2%. This deviation decreases as the power of the fitting function increases. This indicates that the resistance change-based method for calculating the deflection of intelligent FRP-concrete composite structures has high accuracy and holds great application value in the field of nondestructive monitoring.

[0091] The above contents are only preferred embodiments of the present invention. For ordinary technicians in this field, according to the concept of the present invention, there may be changes in the specific implementation methods and application scopes. The contents of this specification should not be understood as limiting the present invention.

Claims

1. A method for identifying the deflection of an intelligent FRP-concrete composite structure based on resistance change, characterized in that: The steps include: Step 1: The composite structure is a simply supported beam or an outrigger beam at the bottom of the intelligent FRP reinforced concrete structure. The effective length and cross-sectional form of the composite structure are determined. The FRP is CFRP or a mixture of CFRP and other non-conductive FRP. Step 2: Test the tensile strain and resistance change rate of the smart FRP sample; Step 3: Arrange transverse electrodes along the length of the existing smart FRP, connect each electrode to the outermost electrode at one end, and use a resistance acquisition system to test the initial resistance and resistance change data of each electrode; Step 4: Polynomial smooth fitting of the resistance change rate of the electrodes at different positions at different times, the smooth fitting adopts the least squares method; the resistance change rate of the electrodes at different positions x The polynomial is: Where: R0 is the initial resistance of each FRP electrode, ΔR is the resistance R at any time at different positions x of the FRP t The difference from the initial resistance R0, that is, ΔR=R t -R0; ∑ represents summation; subscript k=1 represents the starting point of the summation, and superscript n represents the end point of the summation; α k is the coefficient of the kth term, x k is x raised to the power of k; Step 5: Establish a core function based on resistance change, and input the structural information in step 1, the material information in step 2, and the polynomial mathematical model in step 4 to calculate the real-time deflection of any position point of the composite structure; the core function based on resistance change is: Where: a is the parameter of the FRP resistance change rate; y is the distance from the outer surface of the FRP in the structural cross section to the neutral axis; is a polynomial based on the resistance change rate, and the value of x is within the effective length L of the structure, that is, 0≤x≤L.

2. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 1, characterized in that: The FRP is CFRP (carbon fiber reinforced composite material) or a mixture of CFRP and other non-conductive FRP. The non-conductive FRP is GFRP (glass fiber reinforced composite material), BFRP (basalt fiber reinforced composite material), and AFRP (aramid fiber reinforced composite material).

3. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 2, characterized in that: The number of FRP layers is single or multi-layer.

4. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 2, characterized in that: The hybrid method is that CFRP is in the middle layer and other non-conductive FRP is in the outer layer. The FRP layers are impregnated and cured with epoxy resin.

5. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 1, characterized in that: The parameter a of the FRP resistance change rate is the inherent property of the material's conductivity, and the relationship satisfies: Where: ε is the FRP tensile strain.

6. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 1, characterized in that: The electrode is a conductive copper sheet or a conductive copper wire, wherein the width of the conductive copper sheet is 3 to 5 mm. The electrode is fixed between CFRP layers or between CFRP and non-conductive FRP layers by silver paste or conductive glue, and is connected to an external resistance collection system to achieve an end-to-end connection.

7. The method for identifying deflection of an intelligent FRP-concrete composite structure based on resistance change according to claim 1, characterized in that: The resistance acquisition system is a sophisticated electronic device that can record and store the resistance value of each electrode in the FRP in real time, with an accuracy requirement of at least four decimal places to meet the standards of high-precision measurement.

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