Global characterization method of elastic-plastic parameters of variable stiffness and strength metal by high temperature thermal softening

Abstract

The present application relates to the field of material mechanical property characterization, in particular to a high-temperature thermal softening variable stiffness and strength metal elastic-plastic parameter global characterization method, comprising: obtaining strain field data and temperature field data of the test piece, and tensile load data; constructing a thermal-mechanical coupling elastic-plastic constitutive model of the metal plate, and establishing a target function of temperature-related parameters to be solved under the constraint of multiple virtual fields based on the virtual work principle; accurately identifying high-temperature thermal softening elastic parameters by using elastic stage data, and then identifying anisotropic strength thermal softening plastic parameters by using composite field data of the whole loading process under variable stiffness; the present application breaks through the condition restriction of constant stiffness when characterizing thermal-mechanical coupling properties of metal materials by using existing test methods, and can realize global accurate characterization of high-temperature thermal softening and anisotropic complex elastic-plastic thermal-mechanical coupling property parameters by only one test, thereby minimizing the number of tests and simplifying the test process.

Description

Technical Field

[0001] This invention relates to the field of material mechanical property characterization technology, specifically to a global characterization method for elastic-plastic parameters of metals that can be changed due to high-temperature thermal softening. Background Technology

[0002] In the fields of aviation, aerospace, and energy, critical equipment is often subjected to harsh, high-temperature coupled, and complex loads in extreme service environments, threatening material structural components with failure modes such as creep and fracture. Metal sheets, as important forming materials for structural components, exhibit a strong dependence on temperature for their stiffness and strength. High-temperature environments easily lead to a decrease in material stiffness and strength. Furthermore, under actual service conditions, structural components are often subjected to non-uniform heating, resulting in uneven distribution of stiffness and strength reduction. In addition, the anisotropy of plastic properties caused by grain orientation during the rolling process of metal sheets significantly exacerbates the difficulty of accurately predicting the thermo-mechanical coupling behavior of material structures in high-temperature environments. Conventional thermo-mechanical coupling property testing methods are limited by uniform conditions, requiring the uniform distribution of deformation and temperature in the target area of ​​the specimen, which is difficult to achieve in actual testing. Conventional testing methods for characterizing anisotropic thermo-mechanical coupling properties require combined experiments with multiple material directions, multiple loading types, and multiple temperature levels, resulting in a large number of experiments and a cumbersome process. Moreover, the uniform state assumption of conventional testing methods does not match actual service conditions, leading to insufficient representativeness of their property characterization results for real-world conditions. Therefore, there is an urgent need to develop new characterization technologies for the thermo-mechanical coupling properties of metal sheets that can be changed in stiffness and strength due to high-temperature thermal softening, so as to provide convenient technical support for predicting the thermo-mechanical coupling behavior of key equipment structural components and assessing the structural service safety under extreme conditions of high temperature and complex loads.

[0003] Reference 1, "Valeri G, et al., Optics and Lasers in Engineering, 2017, 91:53-61," conducted isothermal tensile tests on metal sheets at a uniform temperature from room temperature to 900℃ by providing a relatively uniform temperature distribution in the heating region. The strain field and nominal temperature of the specimens were measured using digital image correlation and infrared thermometers to obtain the thermo-mechanical coupling plastic hardening parameters of the material. This testing method subtracts the elastic strain field provided by the "0.2% offset method" from the total strain field of the target region, identifies plastic parameters using the inelastic portion, but does not involve the identification of elastic parameters of the material, nor does it consider the stiffness thermal softening effect. Furthermore, it uses single-point temperature measurement data provided by the infrared thermometer to represent the overall temperature response of the target region. During the experiment, due to differences in electromagnetic induction density, it is difficult to ensure the temperature uniformity at various points on the specimen, introducing errors into the property characterization results.

[0004] Reference 2, "Chinese Invention Patent No. ZL202010811942.3," discloses a composite field measurement method for the high-temperature thermo-mechanical coupling properties of metals based on the virtual field method. Under the assumptions of constant stiffness and thermal softening of strength, this method proposes combining temperature and deformation full-field measurement techniques to characterize the thermo-mechanical coupling properties related to the anisotropy rate of metallic materials in a single experiment using composite gradient field data. However, this method does not address the thermal softening effect of material stiffness under high-temperature conditions. In actual testing, non-uniform temperature distribution can lead to uneven local stiffness distribution in the tested component, thereby affecting its high-temperature elasto-plastic mechanical behavior and introducing errors into the property characterization results. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a global characterization method for the elastic-plastic parameters of metals exhibiting variable stiffness and strength due to high-temperature thermal softening. This method involves conducting uniaxial tensile tests on irregularly shaped specimens under non-uniform heating, resulting in a spatially non-uniform distribution of stiffness and strength. Based on the virtual field principle, it accurately identifies the elastic parameters of the metal sheet under high-temperature thermal softening using non-uniform temperature and deformation field data during the elastic phase. Furthermore, under variable stiffness conditions, it identifies the anisotropic strength-thermal softening plastic parameters of the metal sheet using composite field data from the entire loading process. This invention overcomes the limitation of constant stiffness required by existing testing methods for characterizing the thermo-mechanical coupling properties of metallic materials. It eliminates the need to ensure a uniform specimen state and requires only a single test to achieve a globally accurate characterization of high-temperature thermal softening and anisotropic complex elastic-plastic thermo-mechanical coupling property parameters, minimizing the number of tests and simplifying the testing process.

[0006] The purpose of this invention is to provide a global characterization method for the elastic-plastic parameters of metals that exhibit variable stiffness and strength due to high-temperature thermal softening, comprising:

[0007] The target area of ​​the specimen is heated to form a non-uniform temperature field, and the specimen is subjected to uniaxial tension to obtain strain field data, temperature field data, and tensile load data.

[0008] A thermo-mechanical coupled elastic-plastic constitutive model for metal sheets was constructed, including a high-temperature thermal softening elastic constitutive model and an anisotropic strength thermal softening plastic constitutive model.

[0009] A virtual field method is used to construct an algorithm for identifying thermo-mechanical coupled elastic-plastic constitutive parameters of metal sheets, and to establish an objective function for the temperature-related unsolved parameter vectors under multiple virtual field constraints.

[0010] Substitute the strain field data, temperature field data, and tensile load data of the specimen into the objective function, select different initial guess values ​​of the model parameters and multiple different virtual fields, start the objective function minimization operation, and after the operation converges, obtain the constitutive parameters in the selected constitutive model.

[0011] Specifically, based on the relationship between the increment of the stress tensor and the increment of the elastic strain during the elastic stage, the elastic constitutive parameters for high-temperature thermal softening are obtained; based on the relationship between the increment of the stress tensor and the yield function and the total strain tensor during all loading stages, the anisotropic strength thermal softening plastic constitutive parameters are obtained.

[0012] Preferably, the relationship between the increment of the elastic stage stress tensor and the increment of the elastic strain is as follows:

[0013] dσ eT =Q T :dε e

[0014] In the formula, dσ eT dε represents the increment of the stress tensor during the elastic phase. e Q represents the elastic strain increment; T The matrix represents the overall stiffness matrix of the specimen considering temperature effects; ":" represents the tensor inner product.

[0015] Preferably, the overall stiffness matrix of the specimen n represents the number of data cells in the specimen, i.e., the number of meshes into which the specimen is divided; where the stiffness matrix of each cell is related to the Young's modulus E. S The relationship between the ratio υ and Poisson's ratio is expressed as:

[0016]

[0017] In the formula, i represents the i-th cell; E represents the stiffness matrix of the i-th cell; Si υ represents the Young's modulus of the i-th cell; υ represents the Poisson's ratio.

[0018] Preferably, the Young's modulus in the high-temperature thermal softening elastic constitutive model varies with temperature as follows:

[0019] E S (T)=E1-E2·T-E3·T 2

[0020] In the formula, T is the temperature, and E1, E2, and E3 are the high-temperature thermal softening elastic parameters of the material.

[0021] Preferably, the relationship between the stress tensor increment and the yield function and total strain tensor during all loading stages is as follows:

[0022]

[0023] In the formula, dσ T dε represents the stress tensor increment during the entire loading phase; dε represents the total strain tensor; F represents the yield function; σ T The Cauchy stress tensor considering temperature effects; It is the tensor outer product.

[0024] Preferably, the yield function is as follows:

[0025] F(σ T ,ε P )=σ eq (σ T )-σ s T (ε P ) = 0

[0026] In the formula, σ eq For σ T The corresponding equivalent stress; it follows the anisotropic yield criterion; σ s T The current flow stress considering temperature effects; ε P This is the equivalent plastic strain.

[0027] Preferably, the objective function is as follows:

[0028]

[0029] In the formula, N j N represents the total number of loading steps. i b is the selected number of independent imaginary fields; S is the specimen thickness; S is the area of ​​the target region. The traction force acting on the outer boundary; u * ε is the defined virtual displacement vector; * For u * The derived virtual strain tensor; X T This is the constitutive parameter vector that needs to be represented.

[0030] Preferably, the field data of the specimen is obtained according to the following steps:

[0031] The target area of ​​the specimen is rapidly heated to form a non-uniform temperature field, and the specimen is subjected to uniaxial tension to obtain deformation field data and temperature field data. The strain field data is obtained by coordinate differentiation based on the deformation field data. At the same time, the strain field data and temperature field data are interpolated.

[0032] Preferably, the specimen treatment process before the experiment includes:

[0033] The specimen is made of rectangular metal sheet, and part of the material is symmetrically removed from both sides of the specimen to form a double-notch irregular specimen configuration. At the same time, high-temperature resistant random speckle patterns are made on the surface of the specimen.

[0034] Beneficial effects:

[0035] This invention provides a global characterization method for the elastic-plastic parameters of metals with variable stiffness and strength caused by high-temperature thermal softening. By conducting uniaxial tensile tests on irregularly shaped specimens under non-uniform heating, the stiffness and strength of the specimens are spatially non-uniformly distributed. Based on the virtual field principle, the non-uniform temperature and deformation field data during the elastic phase are used to accurately identify the elastic parameters of the metal sheet under high-temperature thermal softening. Furthermore, under variable stiffness conditions, the composite field data throughout the loading process is used to identify the anisotropic strength-thermal softening plastic parameters of the metal sheet. This invention overcomes the limitation of constant stiffness required by existing testing methods for characterizing the thermo-mechanical coupling properties of metallic materials. It does not require ensuring a uniform specimen state and only requires a single test to achieve a globally accurate characterization of high-temperature thermal softening and anisotropic complex elastic-plastic thermo-mechanical coupling property parameters, minimizing the number of tests and simplifying the testing process.

[0036] Compared with the prior art, the present invention has at least the following beneficial effects:

[0037] (1) It takes into account the thermal softening effect of material stiffness and strength caused by high temperature conditions, which is more consistent with the actual thermo-mechanical coupling behavior of metal materials under high temperature conditions, and can improve the accuracy of predicting the thermo-mechanical coupling behavior of metal plates under extreme conditions of high temperature and complex load.

[0038] (2) By designing a simple irregular specimen thermo-mechanical loading configuration, it is possible to achieve accurate global characterization of high-temperature thermal softening and anisotropic complex elastic-plastic thermo-mechanical coupling property parameters in a single test, thereby minimizing the number of tests and simplifying the testing process.

[0039] (3) The thermo-mechanical coupled elastic-plastic properties of materials can be characterized by using non-uniform temperature field and deformation field data. There is no need for the constant stiffness assumption, which eliminates the need for constant temperature control system and insulation environment, reduces the difficulty of experimental implementation, and has good practicality. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the high-temperature loading configuration of the double-notch irregular-shaped specimen designed for this invention; Figure 1 (a) High-temperature loading configuration of the double-notch specimen, (b) Specimen displacement boundary condition settings, (c) Temperature load distribution and Young's modulus distribution cloud map of different regions of the specimen.

[0041] Figure 2 Contour plots of logarithmic strain and equivalent plastic strain of the specimen under the final loading step.

[0042] Figure 3 The distribution of stress at each point of the specimen during the entire loading process is shown in the two-dimensional normalized stress yield surface, along with the corresponding probability density contour map.

[0043] Figure 4 The Young's modulus-temperature curve and the corresponding reference value curve are based on the characterization results of the elastic parameters of high-temperature thermal softening.

[0044] Figure 5 The flow stress-equivalent plastic strain curve and the corresponding reference value curve are shown at 500℃ based on the parameter characterization results. Detailed Implementation

[0045] In order to illustrate the technical means and effects adopted by the present invention in order to achieve the intended purpose of the invention, the following detailed description is provided in conjunction with the embodiments.

[0046] This invention provides a global characterization method for the elastic-plastic parameters of metals with variable stiffness and strength caused by high-temperature thermal softening. By conducting uniaxial tensile tests on irregularly shaped specimens under non-uniform heating, the stiffness and strength of the specimens are spatially non-uniformly distributed. Based on the virtual field principle, the non-uniform temperature and deformation field data during the elastic phase are used to accurately identify the elastic parameters of the metal sheet under high-temperature thermal softening. Furthermore, under variable stiffness conditions, the composite field data throughout the loading process is used to identify the anisotropic strength-thermal softening plastic parameters of the metal sheet. This invention overcomes the limitation of constant stiffness required by existing testing methods for characterizing the thermo-mechanical coupling properties of metallic materials. It does not require ensuring a uniform specimen state and only requires a single test to achieve a globally accurate characterization of high-temperature thermal softening and anisotropic complex elastic-plastic thermo-mechanical coupling property parameters, minimizing the number of tests and simplifying the testing process.

[0047] Example 1

[0048] A finite element method (FEM) numerical simulation of high-temperature tensile stress on a double-notched irregular specimen was conducted using finite element software. A thermo-mechanical coupled elastic-plastic constitutive model and model parameters for a metallic material with variable stiffness and strength due to high-temperature thermal softening were defined. The deformation behavior of the double-notched specimen under non-uniform heating and loading conditions was simulated, and the strain field, temperature field, and load data of the specimen were derived. These data were then substituted into a pre-written algorithm program to characterize the virtual field of the thermo-mechanical coupled elastic-plastic property parameters. The simulated data were used to characterize the thermo-mechanical coupled elastic-plastic parameters considering the high-temperature thermal softening effect on stiffness and strength, and the results were compared with the reference values ​​of the property parameters input into the finite element model to verify the accuracy of the proposed method.

[0049] Step 1: Using Constructing the specimen loading configuration and conducting high-temperature loading numerical simulation:

[0050] To improve the characterization accuracy of anisotropic yield parameters, symmetrical material was removed from both sides of the rectangular specimen, forming a double-notch configuration. This allows the specimen to generate rich tensile-shear coupled stress states under high-temperature unidirectional loading. Simultaneously, to achieve non-uniform heating, a gradient temperature field was defined along the loading direction of the specimen.

[0051] Step 2: Construct a thermo-mechanical coupled elastic-plastic constitutive model for the metal sheet, including a high-temperature thermal softening elastic constitutive model and an anisotropic strength thermal softening plastic constitutive model:

[0052] Based on the theory of elastic-plastic properties of metals, a thermo-mechanical coupled elastic-plastic constitutive model for metal sheets is defined: For elastic properties, Poisson's ratio υ is set to a fixed value. Considering the high-temperature thermal softening effect of the stiffness of metallic materials, in this embodiment, a quadratic function is used to simulate the decrease of Young's modulus with temperature, and a high-temperature thermal softening elastic constitutive model is constructed as follows:

[0053] E S (T)=E1-E2·T-E3·T 2 #(1)

[0054] In the formula, T is the temperature, and E1, E2, and E3 are the high-temperature thermal softening elastic parameters of the material.

[0055] In this embodiment, the Hill 1948 yield criterion and the modified Johnson-Cook model are selected as the anisotropic strength-thermal-softening-plastic constitutive model. For plane stress state, the yield function of the material is expressed as:

[0056] F(σ T ,ε P )=σ eq (σ T )-σ s T (ε P )=0#(2)

[0057] In the formula, σ T The Cauchy stress tensor considering temperature effects; σ eq For σ T The corresponding equivalent stress follows the anisotropic yield criterion; σ s T The current flow stress considering temperature effects; ε P Equivalent plastic strain.

[0058] Specifically, the Hill 1948 yield criterion is expressed as follows:

[0059]

[0060] In the formula, To account for the Cauchy stress tensor components under temperature effects; H, G, F, and N are the anisotropic yield parameters of the material, with H + G = 1. The current flow stress σ considering the temperature effect. s T For equivalent plastic strain ε p The plastic hardening behavior under high-temperature loading conditions is described using a modified Johnson-Cook model as a function of temperature T, i.e.:

[0061]

[0062] In the formula, ε0 is the correction parameter, which is taken as ε0 = 1 × 10 -5 ;T, T M and T r These represent the actual temperature, the melting temperature of the material, and the reference temperature, respectively; A, B, and n are strain hardening parameters; and m is the thermal softening coefficient.

[0063] Specifically, in this embodiment, the material properties of the finite element model are defined with reference to titanium alloy Ti6Al4V. The material property parameters of the finite element model are shown in Appendix Table 1.

[0064] Table 1 Material property parameters of the finite element model

[0065]

[0066]

[0067] The double-notch specimen loading configuration, loading settings, and Young's modulus distribution considering temperature effects designed in this embodiment are shown in the appendix. Figure 1 The load data, temperature field, and logarithmic strain field data of each element during the entire deformation process (200 frames) of the specimen were derived from the numerical simulation results with a frame interval of 0.1 s. The contour plots of each strain component and equivalent plastic strain of the specimen under the final loading step are attached. Figure 2 The distribution of stress values ​​of each element in the specimen under all loading steps in the two-dimensional normalized stress yield surface and its probability density contour map are shown in the appendix. Figure 3 This indicates that the double-notch loading configuration can provide a rich set of tension-shear coupled stress-strain states.

[0068] Step 3: Construct a thermo-mechanical coupled elastic-plastic constitutive parameter identification algorithm for metal sheets using the virtual field method, and establish the objective function for the temperature-related parameter vector under multiple virtual field constraints:

[0069] In this embodiment, a virtual field identification algorithm for thermo-mechanical coupled elastic-plastic constitutive parameters of high-temperature thermal softening induced variable stiffness and strength metal plates is constructed: based on the least squares function, a parameter vector X to be determined under multiple virtual field constraints is established. T The objective function is as follows:

[0070]

[0071] In the formula, N j N represents the total number of loading steps. i b is the selected number of independent imaginary fields; S is the specimen thickness; S is the area of ​​the target region. The traction force acting on the outer boundary; u * ε is the defined virtual displacement vector; * For u * The derived virtual strain tensor; XT This is the vector of temperature-dependent constitutive parameters to be characterized.

[0072] In this embodiment, for the characterization of the thermo-mechanical coupled elastic-plastic constitutive parameters of a variable stiffness-strength metal sheet caused by the high-temperature thermal softening effect, the following five sets of independent virtual fields are defined using power functions and trigonometric functions:

[0073] Virtual Field 1:

[0074] Virtual Field 2:

[0075] Virtual Field 3:

[0076] Virtual Field 4:

[0077] Virtual Field 5:

[0078] In the formula, and These represent the virtual displacements of the specimen along the x and y directions in the Cartesian coordinate system, respectively. and Let x and y represent the virtual strain tensors along the x, y, and shear directions, respectively; x and y are the specimen coordinates; and l is the specimen length.

[0079] Step 4: Substitute the strain field data, temperature field data, and tensile load data of the specimen into the objective function, select the initial guess values ​​and multiple different virtual fields in the model, start the objective function minimization operation, and after the operation converges, obtain the constitutive parameters in the selected constitutive model:

[0080] Specifically, based on the relationship between the increment of the stress tensor and the increment of the elastic strain during the elastic stage, the elastic constitutive parameters for high-temperature thermal softening are obtained; based on the relationship between the increment of the stress tensor and the yield function and the total strain tensor during all loading stages, the anisotropic strength thermal softening plastic constitutive parameters are obtained.

[0081] (I) In this embodiment, the identification of high-temperature thermal softening elastic parameters is as follows:

[0082] Based on Hooke's law, the relationship between the increment of the stress tensor and the increment of the elastic strain in the elastic stage is established as follows:

[0083] dσ eT =Q T :dσ eT #(11)

[0084] In the formula, dσ eT dε represents the increment of the stress tensor during the elastic phase. eRepresents the elastic strain increment; ":" represents the tensor inner product; Q T This represents the overall stiffness matrix of the specimen considering temperature effects. n represents the number of data cells in the specimen, i.e., the number of meshes into which the specimen is divided; where the stiffness matrix of each cell is related to the Young's modulus E. S The relationship between the ratio υ and Poisson's ratio is expressed as:

[0085]

[0086] In the formula, i represents the i-th cell; E represents the stiffness matrix of the i-th cell; Si Represents the Young's modulus of the i-th cell; υ represents the Poisson's ratio. Select the defined virtual fields 1, 2, and 3 and write the high-temperature thermal softening elastic parameters based on formulas (1), (5), (11), and (12). The identification program for thermal softening elastic parameters is as follows: The strain field, temperature field, and load data of the elastic stage (first 30 frames) obtained in step two are substituted into the written thermal softening elastic parameter identification program. Three sets of unknown parameter vectors are arbitrarily selected. With different initial guesses and given upper and lower limits for the solution, the elastic parameter characterization program was started until the calculation converged. The characterization results are shown in Appendix 2. The maximum identification error of each parameter under different initial values ​​does not exceed 4%. Based on the identification results (average value) and reference values ​​of the thermal softening elastic parameters, the Young's modulus-temperature curve is plotted and shown in Appendix 2. Figure 4 The curves basically overlap, further proving the accuracy of the high-temperature thermal softening elastic parameter identification results.

[0087] Table 2 Characterization results of high-temperature thermal softening elastic parameters

[0088]

[0089]

[0090] (II) In this embodiment, the identification of anisotropic strength thermal softening plasticity parameters is as follows:

[0091] The relationship between the stress tensor increments and the yield function and total strain tensor during all loading stages is established as follows:

[0092]

[0093] In the formula, dσ T dε represents the stress tensor increment during the entire loading stage; dε represents the total strain tensor; F represents the yield function. The tensor outer product is used. Then, considering both stiffness and strength under high-temperature thermal softening, the defined virtual fields 1, 4, and 5 are selected and an anisotropic strength thermal softening plasticity parameter identification program is written based on formulas (2), (5), (12), and (13). The strain field, temperature field, and load data obtained in step two during the entire loading process are substituted into the written anisotropic strength thermal softening plasticity parameter. The recognition algorithm program arbitrarily selects three sets of unknown parameter vectors. Different initial guesses were used, and given upper and lower limits for the solution, the characterization program was started until the operation converged. The parameter characterization results are shown in Appendix Table 3. The highest identification error of each parameter under different initial values ​​was only about 5%. The flow stress-equivalent plastic strain curves were plotted based on the identification results (average value) of the strength, thermal softening, and plasticity parameters and the reference values, as shown in Appendix Table 3. Figure 5 The curves basically overlap, further proving the accuracy of the anisotropic strength thermal softening plasticity parameter identification results.

[0094] Table 3. Characterization results of anisotropic strength, heat softening, and plasticity parameters.

[0095]

[0096]

[0097] In this embodiment, the proposed high-temperature thermal softening-induced variable stiffness and strength metal elastic-plastic parameters virtual field global characterization method can achieve a single-test global accurate characterization of the anisotropic complex elastic-plastic thermo-mechanical coupling property parameters of metal plates through a single experiment. Moreover, the characterization error is low under different initial guess values, and it is stable and reliable.

[0098] Example 2: Experimental characterization method for elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening. Specimens were fabricated from Ti6Al4V titanium alloy sheets, and high-temperature resistant random speckle patterns were created on the specimen surface. In this example, the specimen configuration design, virtual field selection, characterization procedure, data input, and characterization process of the thermo-mechanical coupled elastic-plastic anisotropic constitutive parameters were the same as in Example 1.

[0099] In contrast, in this embodiment, the gradient temperature field is generated by heating the specimen with a planar induction coil, the load is applied by a testing machine, and the load data is obtained by the testing machine system. The specimen deformation field and temperature field are measured by digital image correlation equipment and an infrared thermal imager, and interpolation is performed in the spatial and temporal domains. Specifically, portions of material are symmetrically removed from both sides of a rectangular metal plate to form a double-notch irregular specimen configuration, which enables it to generate a rich stress state under simple loading conditions, and high-temperature resistant random speckle patterns are created on the specimen surface.

[0100] The specimen is clamped on a universal testing machine, and a planar electromagnetic induction coil is placed on the back of the specimen to achieve rapid heating, creating a non-uniform temperature field in the target area to generate a non-uniform variable stiffness and strength distribution. Uniaxial tension is then applied to the specimen. Digital image correlation equipment and an infrared thermal imager are used to acquire the deformation field u and temperature field data T of the specimen at a set sampling frequency. Simultaneously, the tensile load data of the testing machine is obtained.

[0101] The obtained deformation field data u is subjected to coordinate differentiation to obtain the corresponding strain field data ε. Then, the strain field ε and temperature field T are interpolated to obtain the strain and temperature data at the same data location at the same time. The strain field, temperature field and load data are then stored.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A global characterization method for elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening, characterized in that, include: The target area of ​​the specimen is heated to form a non-uniform temperature field, and the specimen is subjected to uniaxial tension to obtain strain field data, temperature field data, and tensile load data. A thermo-mechanical coupled elastic-plastic constitutive model for metal sheets was constructed, including a high-temperature thermal softening elastic constitutive model and an anisotropic strength thermal softening plastic constitutive model. A virtual field method is used to construct an algorithm for identifying thermo-mechanical coupled elastic-plastic constitutive parameters of metal sheets, and an objective function is established for the temperature-related material parameter vector under multiple virtual field constraints. Substitute the strain field data, temperature field data, and tensile load data of the specimen into the objective function, select different initial guess values ​​of the model parameters and multiple different virtual fields, start the objective function minimization operation, and after the operation converges, obtain the constitutive parameters in the selected constitutive model. Specifically, based on the relationship between the increment of the stress tensor and the increment of the elastic strain in the elastic stage, the elastic constitutive parameters of high-temperature thermal softening are obtained; based on the relationship between the increment of the stress tensor and the yield function and the total strain tensor in all loading stages, the anisotropic strength thermal softening plastic constitutive parameters are obtained. The relationship between Young's modulus and temperature in the high-temperature thermal softening elastic constitutive model is as follows: In the formula, For temperature, , , These are the elastic parameters of the material during high-temperature thermal softening. The yield function is as follows: In the formula, for The corresponding equivalent stress follows the anisotropic yield criterion; The current flow stress takes into account the temperature effect; This is the equivalent plastic strain.

2. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 1, characterized in that, The relationship between the increment of the stress tensor and the increment of the elastic strain in the elastic stage is as follows: In the formula, This represents the increment of the stress tensor during the elastic phase; Indicates the increment of elastic strain; The matrix represents the overall stiffness of the specimen considering temperature effects; ":" represents the tensor inner product.

3. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 2, characterized in that, The overall stiffness matrix of the specimen is expressed as follows: , The number of data cells for the specimen represents the number of meshes into which the specimen is divided; where the stiffness matrix and Young's modulus of each cell are... Compared to Poisson The relationship is represented as: In the formula, Indicates the first One cell; Indicates the first Stiffness matrix of each cell; Indicates the first Young's modulus of each cell; It represents Poisson's ratio.

4. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 1, characterized in that, The relationship between the stress tensor increments and the yield function and total strain tensor during all loading stages is as follows: In the formula, This represents the stress tensor increment throughout the entire loading phase; Represents the total strain tensor; Represents the yield function; The Cauchy stress tensor considering temperature effects; " is the tensor outer product.

5. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 1, characterized in that, The objective function is as follows: In the formula, This represents the total number of loading steps; The number of independent imaginary fields selected; For specimen thickness; The area of ​​the target region; The traction force acting on the outer boundary; For the defined virtual displacement vector; for The derived virtual strain tensor; This is the constitutive parameter vector that needs to be represented.

6. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 1, characterized in that, The field data of the specimen were obtained according to the following steps: The target area of ​​the specimen is rapidly heated to form a non-uniform temperature field, and the specimen is subjected to uniaxial tension to obtain deformation field data and temperature field data. The strain field data is obtained by coordinate differentiation based on the deformation field data. At the same time, the strain field data and temperature field data are interpolated.

7. The method for global characterization of elastic-plastic parameters of variable stiffness and strength metals caused by high-temperature thermal softening according to claim 1, characterized in that, The specimen preparation process before the experiment includes: The specimen is made of rectangular metal sheet, and part of the material is symmetrically removed from both sides of the specimen to form a double-notch irregular specimen configuration. At the same time, high-temperature resistant random speckle patterns are made on the surface of the specimen.

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