Tempered glass insulator stress distribution measurement method

Abstract

A tempered glass insulator stress distribution measurement method comprises the steps of simulating a fluid field and a transient temperature field in tempering cooling by using CFD software based on a glass viscoelasticity theory, importing a temperature field result into FEA software, and solving a residual stress field in combination with a generalized Maxwell model. And selecting an insulator disc sheet-shaped area, acquiring stress data by using photoelastic stress measurement equipment, and comparing the stress data with a simulation result to verify the reliability of the model. And outputting the overall residual stress distribution of the insulator based on the verified model, and identifying a high-tensile stress concentration region. Cooling process parameters are adjusted according to stress distribution, simulation prediction is conducted again, photoelastic measurement verification is conducted, and process iterative optimization is achieved. According to the method, numerical simulation and experimental measurement are deeply fused, a closed-loop optimization process is formed, and the stress distribution condition of the tempered glass insulator can be more comprehensively and accurately mastered, so that the limitation of the prior art is effectively overcome, and accurate guidance and optimization of a tempering process are realized.

Description

Technical Field

[0001] The present invention relates to the manufacturing and detection technologies of toughened glass insulators in the field of power transmission and transformation, and particularly to a method for measuring the stress distribution of toughened glass insulators. Background Art

[0002] Glass insulators are widely used in transmission lines due to their "self-breaking when zero-value" characteristics, long service life, and good insulation performance. However, the self-explosion problem of toughened glass insulators is becoming increasingly prominent, especially in large-tonnage insulators, which seriously affects the safe and stable operation of the power grid. One of the main reasons for self-explosion is the stress concentration caused by internal impurities in the glass (such as nickel sulfide, aluminum sulfide, etc.). When the internal tensile stress is superimposed with the defect stress and exceeds the intrinsic strength of the glass, cracking will occur.

[0003] Traditional methods for detecting the stress of toughened glass, such as the photoelastic method, are mainly applied to the self-explosion risk detection of flat building glass. However, glass insulators have complex geometries, different wall thicknesses at different parts, and large curvatures. The thicknesses of different parts vary, and the curvatures at the corners of the head and the disc are relatively large, resulting in a much more complex internal stress distribution than that of flat glass. Moreover, existing methods often lack mature defect detection means for insulators. The factory inspections of manufacturers are mostly destructive tests and rely on manual experience, lacking standardized data support and having poor detection consistency. For the complex stress field of glass insulators, the traditional stress distribution model of flat glass cannot be simply applied.

[0004] Currently, the research on the self-explosion mechanism of glass insulators, especially the research on the residual stress distribution, is still insufficient. Although numerical simulation methods have been used to simulate the toughening process and obtain the residual stress distribution, and the simulation results can be verified by the photoelastic method, existing photoelastic measurement equipment (such as the SCALP series stress analyzers) has limitations when measuring insulators with such complex structures. Their measurement depth and measurable area are limited, and they can only measure the quasi-flat part in the shape of a disc, with a measurement depth not exceeding 5 millimeters. This means that using the photoelastic method alone cannot comprehensively obtain the stress information of the insulator in the full size and full thickness direction. Therefore, there is an urgent need for a method that can combine the advantages of simulation and measurement to more comprehensively and accurately obtain the stress distribution of toughened glass insulators and can be used to guide the optimization of the toughening process.

[0005] It should be noted that the information disclosed in the above background art section is only used for understanding the background of the present application, and thus may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0006] The main objective of the present invention is to overcome the defects existing in the above background art and provide a method for measuring the stress distribution of toughened glass insulators.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A method for measuring the stress distribution of toughened glass insulators based on the combination of simulation and measurement, comprising the following steps: S1. Based on the glass viscoelasticity theory, simulate the fluid field and the transient temperature field of the insulator during the toughening cooling process through the computational fluid dynamics (CFD) software, and import the temperature field results into the finite element analysis (FEA) software. Combine the time-varying law of material properties described by the generalized Maxwell model and the structural relaxation effect to solve the distribution of the residual stress field; S2. Select the measurable disc-shaped area in the insulator, obtain the stress data in the surface and thickness directions through the photoelastic stress measurement equipment, and compare and verify with the simulation results of the corresponding area. When the error does not exceed the set threshold, confirm the reliability of the simulation model; S3. Based on the verified simulation model, output the residual stress distribution of the entire insulator including the disc and the head, and identify the high tensile stress concentration area; S4. Adjust the cooling process parameters according to the stress distribution results, re-simulate and predict the optimization effect, and verify the actual sample through photoelastic measurement to realize the iterative optimization closed-loop of the toughening process.

[0008] Further, in step S1, the simulation of the fluid field and the transient temperature field of the insulator during the toughening cooling process includes: Set the differential pressure boundary conditions of the upper and lower air grilles; Coupledly solve the non-uniform flow field and temperature field through the fluid continuity equation, Navier-Stokes equation and energy equation; Calculate the non-uniform convective heat transfer coefficient on the surface of the insulator based on the temperature field results.

[0009] Further, step S1 specifically includes the following fluid-thermal-solid coupled numerical simulation: Establish a three-dimensional CFD model including the air fluid domain and the insulator geometry, and set the turbulence model and non-equilibrium wall function; Densify the grid along the thickness direction of the insulator in the FEA to capture the temperature gradient, and ensure the accuracy through grid independence verification; Determine the thermorheological simple properties and structural relaxation effect of the glass, calculate the fictive temperature using the Tool-Narayanaswamy model, and solve the thermo-mechanical coupling stress field based on the viscoelastic constitutive equation.

[0010] Further, the construction of the viscoelastic constitutive equation includes: Use the generalized Maxwell model to describe the time dependence of the shear modulus and the bulk modulus; Introduce a temperature shift function to convert the temperature effect into a scaled time variable; The thermal expansion strain is calculated using a fictitious temperature and substituted into the stress integral expression to solve for the transient stress.

[0011] Furthermore, the structural relaxation effect is quantified by a fictitious temperature model: The fictitious temperature is calculated by integrating the current temperature and the historical temperature; Thermal strain is determined by multiplying the difference in liquid / solid thermal expansion coefficients by the change in fictitious temperature.

[0012] Further, in step S2: The measurement area is limited to a disc-shaped quasi-flat plate structure, and the measurement depth is ≤5mm; Quantitative verification is achieved by comparing the root mean square error of stress curves at multiple points in the thickness direction.

[0013] Furthermore, in step S2, when the error is ≤10%, it is confirmed that the simulation model is reliable.

[0014] Furthermore, the stress distribution identified in step S3 is used to: guide the optimization of pressurization parameters of the water pressure test inside the head, and / or provide an anti-fracture basis for the disc structure design.

[0015] Furthermore, the process optimization in step S4 includes: For the high tensile stress concentration area at the connection, the stress peak can be reduced by adjusting the nozzle pressure distribution, local cooling intensity or cooling rate; The optimized parameters include heating temperature, cooling time and regional differentiated wind pressure.

[0016] Furthermore, the method further comprises the following steps: Online monitoring is performed based on the full-size stress distribution, and the risk of self-explosion is predicted by tracking and identifying stress changes in key areas.

[0017] The present invention has the following beneficial effects: The present invention provides a method for measuring stress distribution of tempered glass insulators, which combines numerical simulation with photoelastic nondestructive measurement technology, and can accurately obtain the internal stress distribution of tempered glass insulators in full size, thereby guiding the closed-loop control optimization of the tempering process.

[0018] Specifically, the present invention provides a method based on the coupled simulation of computational fluid dynamics (CFD) and finite element analysis (FEA) combined with local photoelastic non-destructive measurement. Compared with the prior art, which relies solely on photoelastic measurement and cannot fully obtain the stress information of insulators of complex shapes, and the simulation results lack comprehensive verification by measured data, the method of the present invention deeply integrates numerical simulation and experimental measurement to form a closed-loop optimization process, which can more comprehensively and accurately grasp the stress distribution of tempered glass insulators, thereby effectively overcoming the limitations of the prior art and achieving precise guidance and optimization of the tempering process.

[0019] The present invention not only lays a foundation for the research on the self - explosion mechanism of insulators, but also can predict and prevent self - explosion by tracking the stress changes in key areas through online monitoring. The experimental results show that the present invention is of great significance to the design and manufacture of glass insulators. For example, based on the method of the present invention, by optimizing the cooling process, the stress concentration at the connection and the self - explosion rate can be reduced, the material toughness can be improved, or local strengthening technology can be adopted to improve the mechanical properties of the insulators, and it can provide a basis for the development of an online monitoring system, thereby preventing self - explosion. In short, the present invention provides a powerful means for the research on the self - explosion mechanism of glass insulators, and also provides a reliable support for the improvement of the toughening process and the selection of the water pressure value in the internal water pressure test.

[0020] Other beneficial effects in the embodiments of the present invention will be further described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the overall flowchart of the stress distribution measurement method for the toughened glass insulator of the present invention.

[0022] Figure 2(a) is the distribution diagram of the maximum principal stress along the thickness direction in the disc area of the embodiment of the present invention.

[0023] Figure 2(b) is the distribution diagram of the maximum principal stress along the length direction in the disc area of the embodiment of the present invention.

[0024] Figure 3(a) is the distribution diagram of the maximum principal stress in the umbrella rib area of the embodiment of the present invention.

[0025] Figure 3(b) is the distribution diagram of the maximum principal stress in the head area of the embodiment of the present invention.

[0026] Figure 4(a) is the distribution diagram of the stress components of the quasi - cylindrical structure in the head area of the embodiment of the present invention.

[0027] Figure 4(b) is the distribution diagram of the stress components at the corner in the head area of the embodiment of the present invention.

[0028] Figure 5(a) is the comparison diagram of the stress measurement result and the calculation result in the left direction corresponding to Figure 2(a) in the embodiment of the present invention.

[0029] Figure 5(b) is the comparison diagram of the stress measurement result and the calculation result in the middle direction corresponding to Figure 2(a) in the embodiment of the present invention.

[0030] Figure 5(c) is the comparison diagram of the stress measurement result and the calculation result in the right direction corresponding to Figure 2(a) in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following will give a detailed description of the embodiments of the present invention. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.

[0032] It should be noted that when an element is referred to as "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element. In addition, the connection can be for fixing or for coupling or communicating.

[0033] It should be understood that the orientation or positional relationship indicated by the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention.

[0034] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the embodiments of the present invention, the meaning of "a plurality" is two or more, unless otherwise specifically defined.

[0035] The present invention provides a method for obtaining the stress distribution of a toughened glass insulator and optimizing the toughening process, which deeply integrates numerical simulation and experimental measurement to form a closed-loop optimization process, aiming to overcome the limitations in the prior art that relying solely on photoelastic measurement cannot comprehensively obtain the stress information of insulators with complex shapes, and the simulation results lack comprehensive verification by measured data, so as to achieve precise guidance and optimization of the toughening process.

[0036] Refer to Figure 1 , the embodiments of the present invention provide a method for measuring the stress distribution of a toughened glass insulator based on the combination of simulation and measurement, including the following steps: Step S1, fluid-thermal-solid coupling numerical simulation: Based on the glass viscoelasticity theory, the fluid field and the transient temperature field of the insulator during the toughening cooling process are simulated by using computational fluid dynamics (CFD) software, and the temperature field results are imported into finite element analysis (FEA) software. Combining the time-varying law of material properties described by the generalized Maxwell model and the structural relaxation effect, the distribution of the residual stress field is solved.

[0037] In some embodiments, in step S1, the simulation of the fluid field and the transient temperature field of the insulator during the simulated toughening cooling process includes: setting differential pressure boundary conditions for the upper and lower air grilles; solving the non-uniform flow field and temperature field by coupling the fluid continuity equation, the Navier-Stokes equation, and the energy equation; and calculating the non-uniform convective heat transfer coefficient on the surface of the insulator based on the temperature field results.

[0038] In some embodiments, in step S1, the fluid-thermal-solid coupling numerical simulation specifically includes: establishing a three-dimensional CFD model including the air fluid domain and the geometry of the insulator, and setting the turbulence model and the non-equilibrium wall function; densifying the grid along the thickness direction of the insulator in the FEA to capture the temperature gradient, and ensuring the accuracy through grid independence verification; determining the simple thermorheological properties and the structural relaxation effect of the glass, calculating the fictive temperature using the Tool-Narayanaswamy model, and solving the thermo-mechanical coupling stress field based on the viscoelastic constitutive equation.

[0039] The construction of the viscoelastic constitutive equation specifically includes: using the generalized Maxwell model to describe the time dependence of the shear modulus and the bulk modulus; introducing the temperature shift function to convert the temperature effect into a scaled time variable; calculating the thermal expansion strain through the fictive temperature, and substituting it into the stress integral expression to solve the transient stress.

[0040] In some embodiments, the structural relaxation effect is quantified by the fictive temperature model: the fictive temperature is calculated by the product integral of the current temperature and the historical temperature; the thermal strain is determined by the product of the difference between the liquid / solid thermal expansion coefficients and the change in the fictive temperature.

[0041] Step S2, photoelastic measurement and simulation verification of the key area: Select the measurable disk-shaped area in the insulator, obtain the stress data on the surface and in the thickness direction through the photoelastic stress measurement device, and compare and verify with the simulation results of the corresponding area. When the error does not exceed the set threshold, confirm the reliability of the simulation model.

[0042] In some embodiments, the measurement area is limited to a disk-shaped quasi-flat plate structure, and the measurement depth ≤ 5 mm; quantitative verification is achieved by comparing the root mean square error of the stress curves at multiple points in the thickness direction.

[0043] In some embodiments, in step S2, when the error ≤ 10%, confirm that the simulation model is reliable.

[0044] Step S3, identification of the full-size stress distribution: Based on the verified simulation model, output the residual stress distribution of the entire insulator including the disk and the head, and identify the high tensile stress concentration areas.

[0045] In some embodiments, the identified stress distribution is used to guide the optimization of the pressurization parameters for the internal hydraulic test of the head. In addition, the identified stress distribution is also used to provide a basis for fracture resistance for the disk structure design.

[0046] Step S4, closed-loop optimization of the tempering process: adjust the cooling process parameters according to the stress distribution results, re-simulate and predict the optimization effect, and verify the actual sample through photoelastic measurement to form an iterative optimization closed loop.

[0047] In some embodiments, the process optimization in step S4 includes: for the high tensile stress concentration area at the connection, reducing the stress peak by adjusting the nozzle pressure distribution, local cooling intensity or cooling rate; the optimization parameters include heating temperature, cooling time and regional differentiated wind pressure.

[0048] In some embodiments, the stress distribution measurement method of tempered glass insulators based on the combination of simulation and measurement also includes: online monitoring based on full-size stress distribution, and predicting the risk of self-explosion by tracking and identifying stress changes in key areas.

[0049] The stress distribution measurement method of tempered glass insulators based on the combination of simulation and measurement provided by the present invention solves the problem of accurately obtaining the full-size stress distribution of tempered glass insulators with complex geometric shapes by deeply integrating CFD-FEA fluid-thermal-solid coupling simulation and local photoelastic non-destructive measurement technology. Based on the generalized Maxwell model and structural relaxation effect simulation framework constructed based on the glass viscoelasticity theory, combined with the thickness direction grid encryption strategy, the high-precision calculation of residual stress in special-shaped areas such as insulator discs and heads is realized; the photoelastic method is used to check the stress data of the disc-shaped measurable area (such as error ≤10% to verify the reliability of the model), breaking through the limitation that traditional photoelastic technology is only applicable to flat plates, and forming a simulation-measurement bidirectional verification mechanism; based on the high tensile stress concentration area identified by the full-size stress distribution (such as the 90MPa peak at the connection), the closed-loop optimization of process parameters such as nozzle pressure distribution and regional differential cooling is guided, significantly reducing the risk of self-explosion; at the same time, it provides a scientific basis for the study of the self-explosion mechanism of insulators, the optimization of internal water pressure test parameters and the development of online monitoring systems, and fundamentally improves the safety and stability of the power grid and reduces the dependence on destructive tests.

[0050] The following further describes specific embodiments of the present invention, its algorithm examples and experimental verification.

[0051] A stress distribution measurement method for tempered glass insulators based on computational fluid dynamics (CFD) and finite element analysis (FEA) coupled simulation combined with local photoelastic nondestructive measurement. This method deeply integrates numerical simulation and experimental measurement to form a closed-loop optimization process, which specifically includes the following steps: Step 1: Fluid-thermal-solid coupling numerical simulation of insulator tempering process: Based on the glass viscoelasticity theory, the CFD software is used to accurately calculate the cooling fluid field and the insulator temperature field during the toughening process. In particular, the influence of complex structures such as the insulator disc and head on the cooling air flow path is considered, including the phenomenon that the protruding parts accelerate heat dissipation and the concave parts may form low-speed areas, resulting in heat accumulation and slow cooling.

[0052] The transient temperature field results obtained from the CFD calculation are imported into the FEA software for thermo-structural coupling analysis to solve the distribution of the residual stress field generated in the insulator during the toughening and cooling process.

[0053] In the FEA modeling, special attention is paid to the mesh division density along the thickness direction to accurately capture the temperature gradient that is crucial for the formation of residual stress.

[0054] Step 2: Photoelastic measurement and simulation verification of key areas: Select key areas in the glass insulator that can be measured by the photoelastic method and are representative of the self-explosion risk, and use photoelastic stress measurement equipment for non-destructive measurement to obtain the stress distribution data of these areas.

[0055] Compare and verify the photoelastic measurement results with the simulation calculation results of the corresponding areas to check the accuracy and reliability of the simulation model. When the error between the two is within 10%, the simulation model is considered to have a high credibility for the full-size and full-thickness stress distribution of the glass insulator.

[0056] Step 3: Obtaining the full-size stress distribution and identifying key stress concentration areas: Based on the simulation model verified by photoelastic measurement, obtain the residual stress distribution map within the full size of the glass insulator, including all complex geometric areas on the disc and head.

[0057] Step 4: Optimization guidance and verification of the toughening process: According to the obtained full-size stress distribution results, especially the distribution and values of the high tensile stress areas and stress concentration points, specific optimization suggestions are put forward for the toughening process parameters (such as heating temperature, cooling rate, nozzle pressure distribution, cooling time, etc.). For example, by optimizing the cooling process, the stress concentration at the connection can be reduced.

[0058] Targetedly adjust the cooling scheme to reduce the peak value of the tensile stress in the key high tensile stress areas.

[0059] Use the optimized process parameters to perform fluid-thermal-solid coupling simulation again to predict the stress distribution under the new process, and verify the insulator samples produced by the new process through the photoelastic method to form an iterative optimization closed loop until the expected stress distribution and performance indicators are achieved.

[0060] This method provides a basis for subsequent research on the self - explosion mechanism of insulators and supports the development of an online monitoring system to predict and prevent self - explosion by tracking stress changes in key areas.

[0061] In this embodiment, the specific methods for operations such as three - dimensional modeling, simulation analysis, and process optimization are as follows.

[0062] Establishment of three - dimensional geometric model and mesh generation: According to the three - dimensional dimensions of the tempered glass insulator to be measured, an accurate geometric model is established in CAD software.

[0063] The geometric model is imported into the mesh generation software, and unstructured mesh generation is performed on the insulator model. In particular, local mesh refinement is carried out in the thickness direction and areas with drastic stress gradient changes to ensure the accuracy of stress calculation.

[0064] Fluid field and temperature field simulation (CFD): In CFD software, an air fluid domain model inside the tempered cooling furnace is established, and inlet and outlet boundary conditions are set.

[0065] The glass insulator model is imported into the fluid domain, and the coupled heat transfer interface between the glass and air is set.

[0066] Using a suitable turbulence model, the fluid field during the cooling process and the transient temperature field inside the glass insulator are simulated.

[0067] Stress field simulation (FEA): The transient temperature field data of the glass insulator obtained from the CFD simulation is imported into the FEA software.

[0068] According to the visco - elastic theory of glass, material properties are defined, including the variation laws of shear modulus G and bulk modulus K with time, which can be described using the generalized Maxwell model.

[0069] Considering the thermo - rheological simplicity (TS) characteristics and structural relaxation effects of glass, the dependence of material parameters on temperature is defined.

[0070] Thermo - mechanical coupling calculations are performed to solve the residual stress distribution in the glass insulator during cooling due to uneven internal and external temperature differences and volume shrinkage.

[0071] Photoelastic measurement and simulation verification: Prepare samples of tempered glass insulators to be measured, and select areas suitable for photoelastic method measurement on them.

[0072] Using the photoelastic stress measurement device SCALP - 05, non - destructive measurement of the surface stress or stress in the thickness direction of the selected area is carried out.

[0073] Quantitatively compare the measured stress curve or values with the FEA simulation results, calculate the root mean square error or other evaluation indicators, and evaluate the accuracy of the simulation results.

[0074] Stress distribution analysis and toughening process optimization: After the simulation results are fully verified by photoelastic measurement, use the simulation model to comprehensively analyze the residual stress distribution in each part of the insulator, and focus on the maximum tensile stress area and stress concentration points.

[0075] According to the analysis results, for example, if it is found that the tensile stress in a certain area is too high, the cooling intensity or cooling rate in this area can be adjusted accordingly (such as by changing the nozzle aperture, air flow velocity, distance, or increasing the local cooling air volume, etc.) to reduce the tensile stress.

[0076] Re-enter the optimized process parameters into the simulation model for prediction and evaluate the optimization effect. Through multiple simulation - analysis - optimization iterations until the residual stress distribution reaches the best state.

[0077] Finally, apply the optimized toughening process parameters to the actual production line and verify their effectiveness again through sampling photoelastic measurement or destructive testing.

[0078] Examples and result verification Use computational fluid dynamics (CFD) software to calculate the flow field and temperature field during the glass toughening process, and then import the temperature field results into finite element analysis (FEA) software for structural analysis to solve the stress field. The simulation model is elaborated as follows.

[0079] After on-site investigation, the pressure of the upper air grille of the toughening cooling equipment is 1.4 bar, and the pressure of the lower air grille is 0.6 bar. According to the basic equations of fluid mechanics, the continuity equation (1) and the Navier - Stokes equation (2) can be obtained.

[0080] (1) In the formula, ρ is the fluid density, u , v , w are the velocity components in the x, y, and z directions respectively. (2) In the formula, p is the pressure, μ is the dynamic viscosity, fx is the body force in the x direction. The equations in the y and z directions are in a similar form.

[0081] The Realizable k-ε model is adopted as the turbulence model. This model has advantages in simulating heat transfer phenomena related to impinging jets and has relatively low requirements for computing resources. In addition, the non-equilibrium wall function is used as the wall function.

[0082] During the toughening process, the glass component is usually rapidly cooled from an initial temperature of about 680 °C to 300 °C within 160 s. The toughening process belongs to a three-dimensional transient temperature field problem. To simplify the problem, the present invention assumes that the glass has isotropic properties and ignores the heat generated by glass deformation and thermal radiation. Equation (3) is the heat conduction equation: (3) In the formula, ρ g is the glass density, T is the temperature, which is a function of spatial coordinates ( x , y , z ) and time t , c p is the specific heat capacity, λ is the thermal conductivity of the material.

[0083] Glass is an amorphous substance with a short-range ordered and long-range disordered atomic arrangement, similar to a liquid but maintaining a specific shape like a solid. During the process of glass changing from a liquid state to a solid state, the temperature point at which it begins to exhibit solid characteristics is called the transition temperature Tg , which is related to the cooling rate. From the perspective of mechanical response, glass is also a viscoelastic material, and its mechanical properties are related to both temperature and time.

[0084] When a constant shear strain is applied to a viscoelastic material, the ratio of its stress response to the strain is called the shear modulus, and this value will slowly decrease. This phenomenon is called relaxation. If a constant stress is applied and the corresponding strain change is observed, the corresponding variable is defined as the compliance, and the change process is called creep.

[0085] If the applied stress or strain is a variable, the Boltzmann superposition principle needs to be introduced to describe the mechanical behavior of linear viscoelastic materials. Combining the definitions of relaxation and creep, the constitutive relationship of linear viscoelastic materials can be obtained: (4) In the formula, σ and P are the shear stress and hydrostatic pressure, G and K are the shear and bulk moduli, γ and ε kk are the shear and normal strains, t is the actual time,t ′ is the integration variable, representing the elapsed time. It is expressed in tensor form as in Equation (5): (5) Where, σ ij is the stress tensor, ε ij is the deviatoric strain, δ ij is the Kronecker symbol.

[0086] The variation laws of G and K with time t need to be described by the generalized Maxwell model. The generalized Maxwell model consists of multiple parallel Maxwell units. Each Maxwell unit has its own shear modulus G i and viscosity η i , as well as the corresponding relaxation time τ i . The relationship among the three is given by Equation (6): (6) These parallel units in the generalized Maxwell model simultaneously bear the externally applied stress or strain, as shown in Equation (7): (7) With the help of the Laplace transform, the overall shear modulus and bulk modulus of the generalized Maxwell model can be obtained (8): (8) Where, G0 and K0 are the values of G and K at t = 0, and g i and k i are the normalized values. In fact, g i , k i and τ i are exactly the parameters required to define viscoelastic materials.

[0087] The behavior of linear viscoelastic materials at constant temperature was described above. However, the toughening process of glass is related to both time and temperature, not just a function of time. To more accurately define the non-linear temperature-dependent viscoelastic properties of glass, glass is regarded as a thermorheologically simple (TS) material. Thus, the variation of the relaxation modulus with time at different temperatures can be obtained through the Arrhenius shift function (9) and the scaled time ξ (10): (9) (10) In the formula, T and T ref are the instantaneous temperature and the reference temperature respectively, Dis the material coefficient, which depends on the reference temperature T ref After introducing the temperature dependence, the viscoelastic constitutive equation under temperature change conditions can be obtained: (11) (12) where ε th is the thermal strain, α is the coefficient of thermal expansion.

[0088] Also expressed in tensor form, the constitutive equation is as shown in (13): (13) The structural relaxation of glass refers to the process in which the internal structure of glass approaches the thermodynamically stable state over time. This phenomenon occurs because glass, as an amorphous material, is not in a thermodynamically equilibrium state. The intuitive manifestation of structural relaxation is that the volume of glass decreases during the cooling process, indicating an increase in its density, which will undoubtedly have an impact on the formation of residual stress in glass.

[0089] The fictive temperature is a hypothetical temperature used to describe the structural state of glass relative to the thermodynamically equilibrium state. The structural relaxation model describes the structural relaxation of materials under variable temperature conditions, called the Tool-Narayanaswamy (T-N) model. This model takes the fictive temperature as the core variable and describes the behavior of glass under complex thermal histories. The expression of the fictive temperature is: (14) where M ( t ) is the response function, which is obtained through experiments. After obtaining the fictive temperature using equation (14), the liquid and solid coefficients of thermal expansion α 1 and α g can be used to calculate the thermal strain through equation (15): (15) In order to obtain the temperature distribution of the glass insulator after the forced convection heat transfer during toughening, a three-dimensional CFD model of the glass component of the glass insulator was established using computational fluid dynamics software for fluid-thermal coupling analysis. According to the information provided by the manufacturer and the results of on-site investigations, the injection wind pressure of the upper wind grid was set to 1.4 bar, and the injection wind pressure of the lower wind grid was set to 0.6 bar.

[0090] According to the calculation results of the flow field and temperature field during the toughening process, there are significant differences in the cooling rates of different parts of the glass insulator. The structure of the insulator, such as the disc and the head, changes the cooling air flow path - the protruding parts accelerate the air flow and dissipate heat quickly; the concave parts hinder the flow, which may form low-speed areas or vortices, resulting in heat accumulation and slow cooling rates.

[0091] Define the glass material properties through the finite element analysis subroutine and perform thermo-mechanical coupling calculations in the finite element analysis software. The temperature gradient along the thickness direction is the key factor for forming the toughened residual stress. Therefore, it is necessary to allocate a sufficient number of grids in the thickness direction of the insulator. By gradually refining the grid size (starting from the initial 10 mm), when the grid size is refined to 2 mm, the stress change amplitude is less than 1%, indicating that the results have reached grid independence. At the same time, by setting symmetric constraints in the x and z axis directions, only a quarter model of the insulator is calculated.

[0092] According to the structural and thermal characteristics, the disc can be further divided into a quasi-flat plate part and a rib connection part. The cooling rate of the latter is significantly slower than that of the former. When the time reaches 160 s, the temperature of the quasi-flat plate part drops to about 100 °C, while the temperature of the connection part remains at about 200 °C. The existence of this temperature gradient will affect the distribution of the residual stress. The five labeled arrows in Figures 2(a) and 2(b) represent the observation directions of the maximum principal stress, which are three directions along the plate thickness and two directions in the middle layer respectively.

[0093] The head and the rib can be regarded as a quasi-cylindrical structure together. For these regions, the numerical values of the stress components in the head will also be concerned to provide a reference for the implementation of the internal hydraulic test in the head. Figures 3(a) and 3(b) respectively show the maximum principal stress distributions of the rib and the head along the directions marked by the arrows in the figures. Figures 4(a) and 4(b) show the components of the stress tensor including σ 11 , σ 22 , σ 33 , σ 12 , σ 13 , σ 23 . It is worth mentioning that σ 12 and σ 23 are almost equal in value, so the curves are not clearly distinguishable.

[0094] Photoelasticity is one of the effective means for non-destructive testing of tempered glass. It can not only measure the surface stress but also the stress distribution in the thickness direction of the glass. However, there are certain limitations in the measurement of this method. It can only measure flat glass and the measurement depth does not exceed 5 mm. For glass insulators, only the quasi-flat part of the disc meets the measurement requirements. In this invention, three observation directions in Figure 2(a) are selected for photoelastic stress measurement and compared with the simulation results, as Figures 5(a) to 5(c) shown. By comparing the experimental and simulation data, it is found that the agreement between the two is relatively high. The root mean square errors of the three groups of comparisons are 7.916 MPa, 7.932 MPa, and 11.768 MPa respectively.

[0095] The distribution of residual stress in tempered glass insulators is crucial for understanding its self-explosion mechanism. Experiments show that the maximum tensile stress in the disc area is concentrated in the middle layer, reaching about 73 MPa, while the tensile stress at the connection is higher, about 90 MPa. According to Griffith crack theory, the critical stress for crack propagation is closely related to the fracture toughness of the material. The higher tensile stress at the connection indicates that this area is more prone to self-explosion, which is consistent with the high failure rate observed on-site.

[0096] Compared with previous studies mainly focused on flat glass, this invention comprehensively analyzes the residual stress distribution of glass insulators with complex shapes. The results show that although the geometric shapes of the head and ribs are not flat, the distribution of their maximum principal stress generally follows a parabolic law. The tensile stress range of the ribs is 85 - 95 MPa, and the surface compressive stress is 90 - 120 MPa.

[0097] The above results show that the method of this invention is of great significance for the design and manufacture of glass insulators. For example, optimizing the cooling process during tempering can reduce the stress concentration at the connection, thereby reducing the self-explosion rate. In addition, improving the material toughness or adopting local strengthening techniques can further improve the mechanical properties of the insulators. In practical applications, these results can also provide a basis for developing an online monitoring system to predict and prevent self-explosion by tracking the stress changes in key areas.

[0098] If a flat glass is regarded as a superposition of thin layers in the thickness direction, with the Z-axis representing the thickness direction, then each thin layer lies in the X-Y plane and only has a normal stress related to the z value , and shear .

[0099] The stress observation results of the quasi-cylindrical structure of the head are consistent with the above model characteristics: within the observation area, the X-axis can be regarded as the thickness direction. Of course, the stress components are not strictly zero, and The curves of are not exactly coincident. The shear stress in this area is almost zero (its magnitude is that of the normal stress one-tenth of

[0100] The stress component situation at the corner is more complex. The X-axis cannot be regarded as its thickness direction, so the value of is no longer close to 0. The value of is close to the maximum principal stress because it can be regarded as the normal stress of the differential surface. Except for, the other two shear stress values at the corner are also approximately zero.

[0101] Internal hydrostatic test is a destructive test, which eliminates inferior insulators by applying water pressure to the head. And the detailed analysis of the residual stress distribution in the head area provides valuable insights into the mechanical behavior of glass insulators under internal pressure. These findings provide a theoretical basis for optimizing test parameters (such as pressure limit and loading rate) to improve the accuracy and safety of the test.

[0102] In summary, the high self-explosion rate of glass insulators is due to the failure of the glass matrix caused by the volume expansion of impurities in the tensile stress layer. The method of the present invention obtains the tempered residual stress distribution of glass insulators through fluid-thermal-solid coupling simulation, and measures the surface stress of a certain area of the disc by the photoelastic method. Based on the viscoelastic theory, the numerical simulation of the tempering process of the glass components of glass insulators is carried out to obtain the tempered residual stress distribution of the glass components, and it is compared with the measurement results of the photoelastic stress measurement device SCALP-05 to verify the correctness of the simulation results. The results show that: in the disc area, the surface compressive stress of the quasi-flat plate part is about 170 MPa, and the maximum tensile stress in the middle layer is about 73 MPa; while the maximum tensile stress at the connection is about 90 MPa, and its surface compressive stress has no significant difference from that of the quasi-flat plate part. The distribution of the maximum principal stress in the head and umbrella rib parts generally shows a parabolic characteristic. The tensile stress of the umbrella rib is about 85-95 MPa, and the surface compressive stress is about 90-120 MPa. The stress components in different areas of the head also show differences. The simulation results are verified to be accurate by the photoelastic method.

[0103] The following conclusions are obtained through experiments: (1) In the disc area, the surface compressive stress of the quasi-flat plate part is about 170 MPa, the maximum tensile stress in the middle layer is about 73 MPa, and the maximum tensile stress at the connection is higher, about 90 MPa, and its surface compressive stress has no significant difference from that of the quasi-flat plate part.

[0104] (2) The distribution of the maximum principal stress of the insulators all conforms to the parabolic characteristic, and the ratio of the tensile stress (85-95 MPa) to the compressive stress (90-120 MPa) of the rib part is relatively large.

[0105] (3) The quasi-cylindrical structure of the head can still be regarded as a flat plate model, while the stress components at the corner of the head are more complex.

[0106] The present invention provides a technical tool for the study of the self - explosion mechanism of glass insulators, and also provides reliable support for the improvement of its toughening process and the selection of water pressure values for internal water pressure tests.

[0107] The above content is a further detailed description of the present invention in combination with specific / preferred embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several alternatives or modifications can be made to these described embodiments, and these alternative or modified forms should all be regarded as belonging to the protection scope of the present invention. In the description of this specification, the description with reference to terms such as "an embodiment", "some embodiments", "preferred embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. Without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made to the present invention without departing from the protection scope of the patent application.

Claims

1. A method for measuring the stress distribution of a toughened glass insulator, characterized in that, It includes the following steps: S1. Based on the glass viscoelasticity theory, simulate the fluid field and the transient temperature field of the insulator during the toughening cooling process through the computational fluid dynamics (CFD) software, import the temperature field results into the finite element analysis (FEA) software, and solve the residual stress field distribution by combining the time-varying law of material properties described by the generalized Maxwell model and the structural relaxation effect; S2. Select the measurable disc-shaped area in the insulator, obtain the stress data in the surface and thickness directions through the photoelastic stress measurement equipment, compare and verify with the simulation results of the corresponding area, and confirm the reliability of the simulation model when the error does not exceed the set threshold; S3. Based on the verified simulation model, output the residual stress distribution of the whole insulator including the disc and the head, and identify the high tensile stress concentration areas; S4. Adjust the cooling process parameters according to the stress distribution results, re-simulate and predict the optimization effect, and verify the actual sample through photoelastic measurement to realize the iterative optimization closed-loop of the toughening process.

2. The method according to claim 1, characterized in that In step S1, the simulation of the fluid field and the transient temperature field of the insulator during the toughening cooling process includes: Set the differential pressure boundary conditions of the upper and lower air grilles; Coupled solve the non-uniform flow field and temperature field through the fluid continuity equation, Navier-Stokes equation and energy equation; Calculate the non-uniform convective heat transfer coefficient on the surface of the insulator based on the temperature field results.

3. The method according to claim 1, wherein Step S1 specifically includes the following fluid-thermal-solid coupling numerical simulation: Establish a three-dimensional CFD model including the air fluid domain and the insulator geometry, and set the turbulence model and non-equilibrium wall function; In the FEA, encrypt the grid along the thickness direction of the insulator to capture the temperature gradient, and ensure the accuracy through grid independence verification; Determine the simple thermorheological properties and structural relaxation effect of the glass, calculate the fictive temperature using the Tool-Narayanaswamy model, and solve the thermo-mechanical coupling stress field based on the viscoelastic constitutive equation.

4. The method according to claim 3, wherein The construction of the viscoelastic constitutive equation includes: Use the generalized Maxwell model to describe the time dependence of the shear modulus and bulk modulus; Introduce a temperature shift function to convert the temperature effect into a scaled time variable; Calculate the thermal expansion strain through the fictive temperature, and substitute it into the stress integral expression to solve the transient stress.

5. The method according to claim 4, wherein The structural relaxation effect is quantified by the fictive temperature model: The fictive temperature is calculated by the integral solution of the current temperature and the historical temperature; The thermal strain is determined by the product of the difference between the liquid / solid thermal expansion coefficients and the change in the fictive temperature.

6. The method according to any one of claims 1 to 5, characterized in that In step S2: The measurement area is limited to a disc-shaped quasi-flat plate structure, and the measurement depth ≤ 5 mm; Quantitatively check by comparing the root mean square error of the stress curves at multiple points in the thickness direction.

7. The method according to any one of claims 1 to 5, characterized in that, In step S2, when the error ≤ 10%, it is determined that the simulation model is reliable.

8. The method according to any one of claims 1 to 5, characterized in that, The stress distribution identified in step S3 is used to: guide the optimization of the pressurization parameters of the internal hydraulic test of the head, and / or provide a basis for anti-fracture for the disc structure design.

9. The method according to any one of claims 1 to 5, characterized in that The process optimization in step S4 includes: For the high tensile stress concentration area at the connection, reduce the stress peak by adjusting the nozzle pressure distribution, local cooling intensity or cooling rate; The optimized parameters include the heating temperature, cooling time and regional differential wind pressure.

10. The method according to any one of claims 1 to 5, characterized in that, It also includes the following steps: Online monitoring is carried out based on the full-size stress distribution, and the self-explosion risk is predicted by tracking the stress changes in the identified key areas.

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