Epoxy insulating part residual stress nondestructive measurement method based on local pyroelectric strain

Abstract

The invention discloses a lossless measurement method for residual stress of an epoxy insulating part based on local pyroelectric strain. The method comprises the following steps: 1, preparing an epoxy insulating part standard sample, and measuring thermal and mechanical parameters of the material at room temperature; 2, carrying out an experiment on the standard sample to obtain required strain data; 3, establishing the relationship between the mechanical stress and the thermal and mechanical parameters of the material; 4, performing inversion on thermal and mechanical parameters of the material at different temperatures, and verifying the established model; and step 5, testing the epoxy / alumina composite material basin-type insulator with the same formula. According to the method, a relation model between the surface residual stress of the insulator and the material modulus and the thermal expansion coefficient under the condition that the insulator is heated by the laser source is constructed and verified, and the surface residual stress of the basin-type insulator is measured based on the model.

Description

Technical Field

[0001] This invention belongs to the field of insulation condition monitoring technology for high-voltage gas-insulated metal-enclosed switchgear (GIS), and particularly relates to a non-destructive measurement method for residual stress in epoxy insulation components based on local pyrolysis strain. Background Technology

[0002] Gas-insulated metal-enclosed switchgear (GIS) is widely used in power systems due to its advantages such as high reliability, convenient maintenance, flexible configuration, and minimal susceptibility to external environmental influences. Basin-type insulators, as a core component of GIS, provide electrical insulation, mechanical support, and gas sealing. During the casting process, uneven curing of the epoxy resin composite material leads to curing shrinkage and thermal strain. Furthermore, the difference in thermal expansion coefficients between the conductor and solid insulation interface causes stress concentration at the interface. Additionally, collisions and uneven bolt tightening during transportation and assembly inevitably lead to stress concentration over long-term operation. Long-term operation under high voltage conditions, coupled with external stress and internal residual stress, causes insulator cracking, further inducing surface discharge and even breakdown, seriously threatening the safety and reliability of the power system. Currently, residual stress defect detection technology still has broad development prospects. Traditional pre-shipment withstand voltage, partial discharge, and X-ray inspections are rarely used for residual stress detection, resulting in low detection rates. Therefore, researching effective methods for insulator stress detection and achieving efficient and accurate detection of insulator stress is of great significance for preventing mechanical failure of insulators and ensuring the safe and stable operation of power systems. Summary of the Invention

[0003] Based on the characteristics of epoxy composite materials, this invention constructs a model relating the surface residual stress of an insulating component to its material modulus and coefficient of thermal expansion under laser heating conditions. This model can be used to detect the surface residual stress of epoxy insulating components.

[0004] This invention provides a non-destructive measurement method for residual stress of epoxy insulation components based on local pyrolysis strain. It constructs a model of the relationship between the surface residual stress of epoxy insulation components under laser source heating conditions and the material modulus and coefficient of thermal expansion, so as to realize real-time monitoring of residual stress on the surface of insulation components. Includes the following steps: 1) Prepare a standard specimen and determine its initial elastic modulus and coefficient of thermal expansion at room temperature; 2) Attach a high-temperature resistance strain gauge to the surface of the standard specimen to measure strain. Apply a load with a set loading rate to the standard specimen, determine the step size, and uniformly apply a uniaxial load of 0–40 MPa. Irradiate the area near the strain gauge with laser at each stress level and simultaneously collect strain-temperature-time data; 3) When the specimen under stress is irradiated with laser, the temperature of the material surface of the standard specimen changes during laser irradiation, generating thermal strain. The expression for thermal strain is: (1) In the formula, e 热 The thermal strain generated at the irradiation point; α(T) The coefficient of thermal expansion of the standard sample material; T The temperature at any point during the period when the measuring point is irradiated; T ref The initial temperature at the measuring point; When the temperature at the measuring point changes, the stress distribution at the irradiation point will change due to the change in elastic modulus. The surface will generate release strain due to stress relaxation. For the micro-element, the release strain caused by laser irradiation will be transferred to the unirradiated part, but the load on the standard sample as a whole remains unchanged. Equation (1) is used to describe the relationship between the strain caused by the applied stress and the elastic modulus and thermal expansion coefficient of the material at this time: (2) In the formula, e 外 The strain generated in the specimen due to the applied load; E(T) It is the elastic modulus; e 总 The total strain generated at the irradiation point; E 0 represents the elastic modulus at room temperature; According to Hooke's Law, the relationship between the applied stress on a standard specimen and the strain at the irradiated point is as follows: (3) In the formula, s 外 The applied stress on the standard specimen; 4) Perform parameter inversion on the material based on the relationship between the strain data obtained in step 2) and the formula in step 3): (1) Discretize the elastic modulus and thermal expansion coefficient values ​​within the critical temperature range of 25℃ - 125℃, and construct continuous interpolation functions for the elastic modulus and thermal expansion coefficient respectively; (2) Based on the strain data obtained in step 2), construct a dual objective function to achieve joint inversion of multiple sets of experimental data, that is, simultaneously minimize the difference between the calculated stress and the actual applied stress in each set, as well as the variance between each set. Combine the two objectives with certain weights and normalize them according to the initial solution. 5) Test the insulators with epoxy insulation components of the same formulation as the standard sample. Select several measuring points on the insulators, attach high-temperature resistance strain gauges, and operate according to the method in step 2), i.e., irradiate, record the real-time strain, and record the temperature change of the measuring points during the irradiation process. Then process the strain data recorded by the strain gauges and calculate the residual stress on the surface of the insulation component at the measuring point.

[0005] Further, in step 2), at each stress level, the strain gauge records the initial strain when each stress is applied at room temperature. Then, the area near the strain gauge is irradiated with laser, and the real-time strain is recorded during the irradiation. An infrared thermometer is used to record the temperature change of the measuring point during the irradiation. Temperature data is collected every 30-50 seconds. After the laser irradiation ends, the above operation is repeated when the temperature of the measuring point returns to room temperature.

[0006] Furthermore, in step 4), explicit constraints are introduced when performing parameter inversion: Boundary constraints: Based on material empirical data or prior knowledge, reasonable upper and lower bounds are set for the elastic modulus and thermal expansion coefficient at each key temperature point; Monotonicity constraints: Considering the typical behavior of the material within the considered temperature range, linear inequality constraints are applied to the discrete parameters.

[0007] Further, in step 4), a smoothing term and a regularization term are added, and an optimization algorithm is used to calculate the material parameters at all key temperature points, ultimately obtaining the curves showing the relationship between the material's elastic modulus and coefficient of thermal expansion and temperature.

[0008] Furthermore, the epoxy insulation component is composed of an epoxy resin / alumina composite material.

[0009] Furthermore, the epoxy insulation components include pot insulators, three-post insulators, post insulators, or shaft insulators.

[0010] Furthermore, uniaxial loads include uniaxial compressive stress and uniaxial tensile stress.

[0011] Furthermore, the laser power density is 50–200 mW / mm², the spot diameter is 1–5 mm, and the irradiation time is 3–10 min.

[0012] Furthermore, the single-point measurement error is ≤5%, and the spatial resolution is ≤2 mm.

[0013] Beneficial effects: Constructing a model of the relationship between the residual stress on the surface of the insulating component and the material modulus and thermal expansion coefficient under the condition of being heated by a laser source, enabling efficient and accurate real-time monitoring of the residual stress on the surface of the insulating component. (1) Non-destructive: No drilling or peeling on the sample surface, and the laser power is lower than the material ablation threshold; (2) Absolute quantitative: Directly gives the stress value without the need for comparison test blocks; (3) Site-friendly: The equipment consists only of a portable laser, strain acquisition instrument and laptop, and the test can be completed at the GIS maintenance port; (4) Wide applicability: Effective for basin-type, three-post, post and rotating shaft insulators; (5) Error ≤5%, spatial resolution ≤2 mm, single-point measurement cycle ≤7 min. Attached Figure Description

[0014] Figure 1 Schematic diagram of the experimental setup.

[0015] Figure 2 The E(T) and α(T) curves obtained from the inversion.

[0016] Figure 3 Verification results under an applied stress of 10 MPa (error 4.2%).

[0017] Figure 4 Residual stress distribution at 6 measuring points of the basin-type insulator. Detailed Implementation

[0018] The present invention will be further described below through specific implementation examples and accompanying drawings.

[0019] The technical solution of this invention is a method for detecting the residual stress distribution on the surface of an epoxy composite insulation component, comprising the following steps: Step 1: Prepare standard samples of epoxy resin / alumina composite material for epoxy insulation parts to be tested, and test them at room temperature to obtain the required material parameters, including the following parameters: (1) elastic modulus; (2) coefficient of thermal expansion.

[0020] Step 2: A high-temperature resistance strain gauge is attached to the surface of the standard epoxy / alumina composite material sample to be tested. The strain gauge leads are connected to a strain gauge, and the data measured by the strain gauge is transmitted to a host computer. The sample is then clamped onto a universal testing machine, and the load and loading rate of the universal testing machine are controlled by the host computer. Figure 1 This is a schematic diagram of the experimental setup of the present invention. A uniaxial load ranging from 0 to 40 MPa is uniformly applied to the sample in 5 MPa increments. At each stress level, the strain gauge records the initial strain at room temperature when each stress application is completed. Then, the area near the strain gauge is irradiated with laser for 5 minutes, and the real-time strain during irradiation is recorded. An infrared thermometer is used to record the temperature change at the measuring point during irradiation, and temperature data is collected every 30 seconds. After laser irradiation ends, the above operation is repeated once the temperature at the measuring point returns to room temperature.

[0021] Step 3: When the stress-loaded specimen is irradiated by a laser, the temperature of the material surface changes during the laser irradiation, resulting in thermal strain. The expression for thermal strain is: (1) In the formula, e 热 The thermal strain generated at the irradiation point; α(T) The coefficient of thermal expansion of epoxy composite materials; T The temperature at any point during the period when the measuring point is irradiated; T ref The initial temperature of the measuring point is given. When the temperature of the measuring point changes, the stress distribution at the irradiation point will change due to the change in elastic modulus, and the surface will generate release strain due to stress relaxation. For the micro-element, the release strain caused by laser irradiation will be transferred to the unirradiated part, but the overall load on the sample remains unchanged. Equation (1) can be used to describe the relationship between the strain caused by the applied stress and the elastic modulus and thermal expansion coefficient of the material at this time: (2) In the formula, e 外 The strain generated in the specimen due to the applied load; E(T) The elastic modulus of the epoxy composite material; e 总 The total strain generated at the irradiation point; E 0 represents the elastic modulus of the epoxy composite material at room temperature. According to Hooke's Law, the relationship between the applied stress on the specimen and the strain at the irradiated point is: (3) In the formula s 外 The stress applied to the specimen is the external stress.

[0022] Step 4: Based on the strain data obtained in Step 2 and the relationship established in Step 3, perform parametric inversion on the material's elastic modulus and coefficient of thermal expansion.

[0023] First, within the typical service temperature range of the material (25℃-125℃), several key temperature points are selected at certain temperature intervals. At each key temperature point, the elastic modulus and coefficient of thermal expansion to be inverted are defined. Based on the parameter values ​​at these key temperature points, piecewise cubic spline interpolation or other interpolation methods with good smoothness are used to construct continuous interpolation functions for the elastic modulus and coefficient of thermal expansion. Through this step, continuous and differentiable elastic modulus and coefficient of thermal expansion functions are obtained within the 25℃-125℃ temperature range, which are used for subsequent parameter inversion and stress calculation.

[0024] To ensure the physical rationality of the material parameters obtained by inversion, explicit constraints need to be introduced during parameter inversion: 1. Boundary constraints: Based on material empirical data or prior knowledge, reasonable upper and lower bounds are set for the elastic modulus and coefficient of thermal expansion at each key temperature point to avoid non-physical extreme values ​​in the inversion results; 2. Monotonicity constraints: Considering the typical behavior of epoxy / alumina composite materials within the considered temperature range, linear inequality constraints are applied to the discrete parameters to ensure that the parameter inversion results conform to the physical law that the elastic modulus decreases as the temperature increases while the coefficient of thermal expansion increases.

[0025] Based on the strain data obtained in step two, a dual objective function is constructed to achieve the joint inversion of multiple sets of experimental data: Objective 1. Minimize the difference between the stress calculated by the inversion parameters and equation (2) and the target stress in each group: The root mean square error is used as an index to characterize the deviation between the calculated value and the actual value. For the k-th group of experimental data, the stress value calculated based on the parameter inversion results is The actual stress is Root mean square error within each group for: (4) Where n is the number of data points in the k-th group.

[0026] The total index is obtained by summing the root mean square errors under each stress group.

[0027] (5) Objective 2. Minimize the variance of stresses calculated within each group: The stress values ​​calculated using inversion parameters under the same target stress should tend to be consistent. Therefore, to reduce the influence of outliers on the results, an intra-group variance objective is introduced. For the k-th group of experimental data, the stress value calculated based on the parameter inversion results is... The average stress value calculated within the k-th group is The variance within each group is : (6) Where n is the number of data points in the k-th group.

[0028] The total index is obtained by summing the variances of each stress group.

[0029] (7) The two objectives are combined with certain weights and normalized based on the initial solution. Furthermore, to further improve the physical rationality and numerical stability of the solution, the overall objective function... Both smoothing and regularization terms are added.

[0030] The complete objective function is shown in equation (8): (8) in, , Weights for different objectives, , The normalization coefficient is... These are the regularization weight coefficients. For regularization terms, This is a smoothness penalty term.

[0031] Based on the aforementioned objective function and constraints, a nonlinear optimization algorithm with inequality constraints is employed, using the normalized parameter vector as variables for iterative solution. By setting appropriate initial values, convergence accuracy, and maximum number of iterations, the optimal material parameters at all key temperature points are obtained, such as... Figure 2 As shown. Based on the experimental results of step two, the applied stress at different temperatures was calculated. The arithmetic mean of the measurement results at each temperature was taken as the final stress test result. For ease of comparison, Figure 3 The stress results obtained at 10 MPa have an error of less than 5%, verifying the effectiveness of the proposed method.

[0032] Step 5: Test the same epoxy / alumina composite pot insulator. Select six measuring points near the metal insert area and the center of the insulator, and attach high-temperature resistance strain gauges to each point. Follow the procedure in Step 2, using a laser with a power of 100 mW / mm² and a spot diameter of ϕ3 mm for five minutes. Record the real-time strain and temperature change at the measuring points during the irradiation process. Then process the strain data recorded by the strain gauges to calculate the residual stress on the surface of the insulator at the six measuring points. The measurement results are as follows. Figure 4As shown. The results show that all six measuring points exhibit tensile stress, indicating that the basin-type insulator has an overall in-plane tensile stress field distribution during the curing and cooling process. This demonstrates that the method can identify the residual stress type of epoxy composite insulators during the molding process and achieve multi-point quantitative detection of complex curved surface structures of composite materials. The surface residual stress value is the largest at measuring point f, indicating that this area is a stress concentration location. This shows that the method can effectively locate the hidden danger area of ​​the insulator, and can predict the crack risk in advance during the product manufacturing stage, thereby achieving process optimization and quality control. Example 1: The surface residual stress of a 550 kV GIS ϕ560 mm epoxy / alumina composite basin insulator was detected using this method. Standard samples were cast according to the same formula. The thermal and mechanical parameters of the material and strain data under different stresses were obtained according to the methods in steps one and two of this invention. Then, the parameters of the epoxy / alumina composite material with different ratios were inverted according to the method in step four to obtain the elastic modulus and thermal expansion coefficient at different temperatures. Two points were taken at the flange root, the awning root, and the periphery of the insert for testing. High-temperature resistant resistance strain gauges were attached to the measuring points. The locations of the attached strain gauges were then irradiated with a laser at a power of 100 mW / mm², a spot diameter of ϕ3 mm, and an irradiation time of 5 min. Real-time strain data and temperature changes were recorded during the irradiation process. The highest temperature at the measuring point during irradiation reached 42 ℃. Based on the expression in step three, the obtained temperature and strain data were combined with the inversion results of the elastic modulus and thermal expansion coefficient to calculate the maximum residual tensile stress of 18.4 MPa, located at the chamfer of the insert, consistent with the crack initiation location observed during later disassembly.

[0033] Example 2: Surface residual stress was detected on the support section of a 550 kV epoxy / alumina composite three-post insulator using this method. Standard samples were cast according to the same formula. The thermal and mechanical parameters of the material and strain data under different stresses were obtained according to steps one and two of this invention. Then, parameter inversion was performed on the epoxy / alumina composite material according to step four to obtain the elastic modulus and coefficient of thermal expansion at different temperatures. Ten measuring points were arranged axially at 10 mm intervals along the insulator. High-temperature resistance strain gauges were attached to the measuring points. Laser irradiation was used at the attached measuring points with a laser power of 100 mW / mm², a spot diameter of ϕ3 mm, and an irradiation time of 5 min. Real-time strain data and temperature changes were recorded during the irradiation process. Based on the expression in step three, the obtained temperature and strain data were combined with the inversion results of the elastic modulus and coefficient of thermal expansion for calculation. Finally, the stress distribution curves at the ten measuring points were obtained to guide the optimization of the metal insert structure at the base of the support post. The optimization results showed that the maximum tensile stress decreased from 21 MPa to 9 MPa after secondary casting.

Claims

1. A non-destructive measurement method for residual stress in epoxy insulation components based on local pyrolysis strain, characterized in that, A model was constructed to demonstrate the relationship between the surface residual stress of epoxy insulation components under laser source heating conditions and the material modulus and coefficient of thermal expansion, so as to realize real-time monitoring of surface residual stress of insulation components. Includes the following steps: 1) Prepare a standard specimen and determine its initial elastic modulus and coefficient of thermal expansion at room temperature; 2) Attach a high-temperature resistance strain gauge to the surface of the standard specimen to measure strain. Apply a load with a set loading rate to the standard specimen, determine the step size, and uniformly apply a uniaxial load of 0–40 MPa. Irradiate the area near the strain gauge with laser at each stress level and simultaneously collect strain-temperature-time data; 3) When the specimen under stress is irradiated with laser, the temperature of the material surface of the standard specimen changes during laser irradiation, generating thermal strain. The expression for thermal strain is: (1) In the formula, ε 热 The thermal strain generated at the irradiation point; α(T) The coefficient of thermal expansion of the standard sample material; T The temperature at any point during the period when the measuring point is irradiated; T ref The initial temperature at the measuring point; When the temperature at the measuring point changes, the stress distribution at the irradiation point will change due to the change in elastic modulus. The surface will generate release strain due to stress relaxation. For the micro-element, the release strain caused by laser irradiation will be transferred to the unirradiated part, but the load on the standard sample as a whole remains unchanged. Equation (1) is used to describe the relationship between the strain caused by the applied stress and the elastic modulus and thermal expansion coefficient of the material at this time: (2) In the formula, ε 外 The strain generated in the specimen due to the applied load; E(T) It is the elastic modulus; ε 总 The total strain generated at the irradiation point; E 0 represents the elastic modulus at room temperature; According to Hooke's Law, the relationship between the applied stress on a standard specimen and the strain at the irradiated point is as follows: (3) In the formula, σ 外 The applied stress on the standard specimen; 4) Perform parameter inversion on the material based on the relationship between the strain data obtained in step 2) and the formula in step 3): (1) Discretize the elastic modulus and thermal expansion coefficient values ​​within the critical temperature range of 25℃ - 125℃, and construct continuous interpolation functions for the elastic modulus and thermal expansion coefficient respectively; (2) Based on the strain data obtained in step 2), construct a dual objective function to achieve joint inversion of multiple sets of experimental data, that is, simultaneously minimize the difference between the calculated stress and the actual applied stress in each set, as well as the variance between each set. Combine the two objectives with certain weights and normalize them according to the initial solution. 5) Test the insulators with epoxy insulation components of the same formulation as the standard sample. Select several measuring points on the insulators, attach high-temperature resistance strain gauges, and operate according to the method in step 2), i.e., irradiate, record the real-time strain, and record the temperature change of the measuring points during the irradiation process. Then process the strain data recorded by the strain gauges and calculate the residual stress on the surface of the insulation component at the measuring point.

2. The method according to claim 1, characterized in that, Step 2) At each stress level, the strain gauge records the initial strain when each stress is applied at room temperature. Then, the area near the strain gauge is irradiated with laser, and the real-time strain is recorded during the irradiation. An infrared thermometer is used to record the temperature change of the measuring point during the irradiation. Temperature data is collected every 30-50 seconds. After the laser irradiation ends, the above operation is repeated when the temperature of the measuring point returns to room temperature.

3. The method according to claim 1, characterized in that, Step 4) Introduce explicit constraints when performing parameter inversion: Boundary constraints: Based on material experience data or prior knowledge, set reasonable upper and lower bounds for the elastic modulus and thermal expansion coefficient at each key temperature point; Monotonicity constraints: Considering the typical behavior of the material within the considered temperature range, apply linear inequality constraints to the discrete parameters.

4. The method according to claim 1, characterized in that, Step 4) Add smoothing and regularization terms, and use an optimization algorithm to calculate the material parameters at all key temperature points, finally obtaining the material elastic modulus and thermal expansion coefficient versus temperature curves.

5. The method according to claim 1, characterized in that, The epoxy insulation component is made of epoxy resin / alumina composite material.

6. The method according to claim 1, characterized in that, The epoxy insulation components include pot insulators, three-post insulators, post insulators, or shaft insulators.

7. The method according to claim 1, characterized in that, Uniaxial loads include uniaxial compressive stress and uniaxial tensile stress.

8. The method according to claim 1, characterized in that, The laser power density is 50–200 mW / mm², the spot diameter is 1–5 mm, and the irradiation time is 3–10 min.

9. The method according to any one of claims 1–8, characterized in that, Single-point measurement error ≤5%, spatial resolution ≤2 mm.

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