A method for calculating stress triaxiality of metal sheet during loading

Abstract

A method for calculating stress triaxiality during the loading process of a metal sheet is provided. The method obtains strain history data based on direct measurement and analysis. The stress triaxiality within the time increment step is calculated by establishing an interpolation analysis process for the strain history data. A quantitative characterization of the stress state of each region is established accordingly. A method for calculating stress triaxiality during the loading process of a metal sheet of the present invention first directly obtains strain history data during the loading process of the plate. Secondly, based on the obtained strain history data and in combination with a set mathematical processing method, a characterization of all stress states in the region is finally obtained. The stress state determined by this method improves the accuracy of the material performance characterization.

Description

Technical Field

[0001] The invention belongs to the field of metal material composition performance analysis, and in particular relates to a method for calculating stress triaxiality during a metal sheet loading process. Background Art

[0002] Stress triaxiality η is an important parameter in the metal material testing process to characterize the stress state of the material during loading. The theoretical calculation formula is as follows, where σ1, σ2, and σ3 are the principal stress values.

[0003] For metal sheets, during the loading process, it can be assumed that the stress in the thickness direction σ3 = 0, and the calculation formula for stress triaxiality η can be further simplified as follows:

[0004] The principal stress σ1 and the secondary stress σ2 need to be further calculated from the stress components, but the stress components cannot be directly measured in the engineering application process. Therefore, the stress triaxiality in the material loading process is mainly evaluated indirectly using analytical methods and finite element methods. The analytical method is only applicable to specific simple loading conditions such as uniaxial tension. The finite element method requires finite element modeling and simulation analysis of the loading process. The simulation accuracy of stress triaxiality depends on the rationality of the material constitutive model, mesh size, unit type, boundary conditions and other factors.

[0005] The invention patent application with application number: CN 201910158415.4 discloses "a method for determining the forming limit of high-strength aluminum alloy sheet", including: preparing multiple high-strength aluminum alloy sheet specimens; performing tensile tests on the specimens to obtain the tension-displacement curve, elastic modulus and power function hardening model parameters of each specimen; using finite element software to simulate the stretching process; according to the tension-displacement curve and the simulation results, determining the stress triaxiality change data and Lode parameter change data of each specimen; using the Lou-Huh ductile fracture criterion analytical expression to solve and obtain multiple groups of fracture parameter values; determining the prediction error of each group of fracture parameter values ​​for each specimen; based on the multiple groups of fracture parameter values ​​whose prediction errors are less than a set threshold, using the principal strain space function to calculate the forming limit diagram of the high-strength aluminum alloy sheet in the principal strain space.

[0006] The invention application with application number: CN 201911311254.4 discloses a "method for establishing a three-dimensional fracture model of metal materials under complex stress states", including designing a first group of specimens and a second group of specimens; using a nonlinear tensile force to perform a quasi-static standard tensile test on the first group of specimens to obtain a true stress-plastic strain curve of the first group of specimens, and inputting the true stress-plastic strain curve of the first group of specimens into a material numerical test model corresponding to various specimen types in the second group of specimens to obtain stress triaxiality η and normalized Lode angle parameters corresponding to various specimen types in the second group of specimens; using the nonlinear tensile force to perform a fracture test on various types of specimens in the second group of specimens to obtain critical fracture strain values ​​corresponding to various specimen types in the second group of specimens; and obtaining a three-dimensional fracture model of the metal material under a complex stress state according to the stress triaxiality η, normalized Lode angle parameter and critical fracture strain value of each specimen type in the second group of specimens. Summary of the invention

[0007] In order to solve the above problems, the present invention provides a method for calculating the stress triaxiality during the loading process of a metal sheet, and the technical solution thereof is as follows:

[0008] A method for calculating stress triaxiality during loading of a metal sheet, characterized by:

[0009] Based on direct measurement analysis, strain history data is obtained.

[0010] By establishing an interpolation analysis process for the strain history data, the stress triaxiality within the time increment step is calculated, and based on this, a quantitative representation of the stress state of each area is established.

[0011] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0012] The direct measurement-based analysis specifically includes the following steps:

[0013] S1: Load and push the test piece according to the set loading rate;

[0014] S2: real-time tracking and shooting of the loading and advancing process according to the set shooting frequency to form continuous frame images;

[0015] S3: Establish analysis and calculation based on the DIC principle for the continuous frame images, and determine the strain field based on the full-field displacement characterization; and obtain the corresponding strain history data.

[0016] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0017] The method of calculating the stress triaxiality within the time increment step by establishing interpolation analysis processing on the strain history data specifically includes the following steps:

[0018] SS1: Determine the calculation area based on the strain history data;

[0019] SS2: Determine each calculation time point in the calculation area and the corresponding major and minor strain increments at each calculation time point;

[0020] SS3: Calculates the stress triaxiality within the time increment based on the determined calculation time point and the corresponding major and minor strain increments.

[0021] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0022] The set loading rate and the set shooting frequency are determined according to the set thresholds of the major and minor strain increments of adjacent single-frame images.

[0023] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0024] The step SS2 of determining each calculation time point of the calculation area is specifically as follows:

[0025] SS21: Filter the primary and secondary strain history data;

[0026] SS22: Calculate the plastic strain history data based on the filtered primary and secondary strain history data;

[0027] SS23: Determine the time series based on the plastic strain increment to form a representation of each calculation time point.

[0028] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0029] The determination of the major and minor strain increments corresponding to each calculation time point described in step SS2 is specifically:

[0030] First, the primary and secondary strain history data are filtered.

[0031] Then, resampling processing is established for the filtered primary and secondary strain history data.

[0032] Finally, the major and minor strain increments at adjacent time points are determined based on the resampled major and minor strain history data.

[0033] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0034] The time series is determined according to the plastic strain increment, specifically:

[0035] The strain data points are determined based on the set strain increment and the corresponding time series is obtained by combining the interpolation method.

[0036] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0037] The setting threshold of the major and minor strain increments of adjacent single-frame images is specifically: less than or equal to 0.05%.

[0038] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0039] The set strain increment is specifically: the plastic strain increment corresponding to adjacent time points is less than or equal to 20% of the ultimate plastic strain.

[0040] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0041] The stress triaxiality in step SS3 is specifically determined as follows:

[0042] First, the stress component ratio within the time increment is calculated, and the stress triaxiality is characterized according to the stress component ratio; then the stress component ratio is converted into the major and minor strain increment ratio, thereby completing the stress triaxiality based on the major and minor strain increment characterization.

[0043] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0044] The major and minor strain increments in the “major and minor strain increment ratio” are obtained based on the assumption of a linear loading path and by interpolation calculation.

[0045] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0046] The material is either isotropic or anisotropic.

[0047] A method for calculating stress triaxiality during loading of a metal sheet according to the present invention is characterized in that:

[0048] The material is either isotropic or anisotropic.

[0049] The present invention provides a method for calculating stress triaxiality during loading of a metal sheet. First, strain history data of the sheet loading process is directly obtained. Secondly, based on the obtained strain history data, a mathematical processing method is combined to finally obtain the characterization of all stress states in the region. The stress state determined by this method improves the accuracy of material performance characterization. The direct acquisition is completed based on the DIC technology. The combination of the set mathematical processing method obtains the characterization of all stress states in the region by establishing the primary and secondary strain increments and corresponding time points in the determined calculation region, and calculates the stress triaxiality within the time increment step according to the determined primary and secondary strain increments and corresponding time points. The loading rate and shooting frequency of the DIC technology part are determined according to the set strain increment threshold. The primary and secondary strain increments are obtained by resampling the strain history data. The time points corresponding to the primary and secondary strain increments are converted into plastic strain history data according to the calculation of the strain history data, and then the strain data points are determined based on the set strain increment, and the corresponding time series is obtained in combination with the interpolation method. Before the conversion to plastic strain history data, smoothing filtering is also performed. The strain history data obtained based on direct measurement analysis, as well as resampling, filtering, loading rate and shooting frequency processing, and the processing of determining strain data points based on the set strain increment constitute data accuracy services in different dimensions and gradients, and ultimately obtain accurate characterization of material properties based on direct measurement analysis. Since it is impossible to obtain the strain increment and the corresponding time point at the same time, the above two-step process is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic diagram of the calculation method flow of the present invention;

[0051] Figure 2 It is a schematic diagram of the direct measurement and analysis steps in the present invention;

[0052] Figure 3 It is a schematic diagram of the calculation steps of stress triaxiality within the time increment step in the present invention;

[0053] Figure 4 It is a schematic diagram of the steps for determining the time point corresponding to the strain increment in the present invention;

[0054] Figure 5 Schematic diagram of principal strain DIC measurement, filtering and resampling results in an embodiment of the present invention;

[0055] Figure 6 Schematic diagram of secondary strain DIC measurement, filtering and resampling results in an embodiment of the present invention;

[0056] Figure 7 Schematic diagram of the primary and secondary strain resampling results in an embodiment of the present invention;

[0057] Figure 8 A schematic diagram of the calculation results of the stress triaxiality history in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] Next, a method for calculating stress triaxiality during loading of a metal sheet according to the present invention will be further described in detail based on the accompanying drawings and specific implementation methods of the specification.

[0059] A method for calculating the stress triaxiality of metal sheets during loading.

[0060] Based on direct measurement analysis, strain history data is obtained.

[0061] By establishing an interpolation analysis process for the strain history data, the stress triaxiality within the time increment step is calculated, and based on this, a quantitative representation of the stress state of each area is established.

[0062] Among them, Figure 2 As shown,

[0063] The direct measurement-based analysis specifically includes the following steps:

[0064] S1: Load and push the test piece according to the set loading rate;

[0065] S2: real-time tracking and shooting of the loading and advancing process according to the set shooting frequency to form continuous frame images;

[0066] S3: Establish analysis and calculation based on the DIC principle for the continuous frame images, and determine the strain field based on the full-field displacement characterization; and obtain the corresponding strain history data.

[0067] Among them, Figure 3 As shown,

[0068] The method of calculating the stress triaxiality within the time increment step by establishing interpolation analysis processing on the strain history data specifically includes the following steps:

[0069] SS1: Determine the calculation area based on the strain history data;

[0070] SS2: Determine each calculation time point in the calculation area and the corresponding major and minor strain increments at each calculation time point;

[0071] SS3: Calculates the stress triaxiality within the time increment based on the determined calculation time point and the corresponding major and minor strain increments.

[0072] in,

[0073] The set loading rate and the set shooting frequency are determined according to the set thresholds of the major and minor strain increments of adjacent single-frame images.

[0074] Among them, Figure 4 As shown,

[0075] The step SS2 of determining each calculation time point of the calculation area is specifically as follows:

[0076] SS21: Filter the primary and secondary strain history data;

[0077] SS22: Calculate the plastic strain history data based on the filtered primary and secondary strain history data;

[0078] SS23: Determine the time series based on the plastic strain increment to form a representation of each calculation time point.

[0079] in,

[0080] The determination of the major and minor strain increments corresponding to each calculation time point described in step SS2 is specifically:

[0081] First, the primary and secondary strain history data are filtered.

[0082] Then, resampling processing is established for the filtered primary and secondary strain history data.

[0083] Finally, the major and minor strain increments at adjacent time points are determined based on the resampled major and minor strain history data.

[0084] in,

[0085] The time series is determined according to the plastic strain increment, specifically:

[0086] The strain data points are determined based on the set strain increment and the corresponding time series is obtained by combining the interpolation method.

[0087] in,

[0088] The setting threshold of the major and minor strain increments of adjacent single-frame images is specifically: less than or equal to 0.05%.

[0089] in,

[0090] The set strain increment is specifically: the plastic strain increment corresponding to adjacent time points is less than or equal to 20% of the ultimate plastic strain.

[0091] in,

[0092] The stress triaxiality in step SS3 is specifically determined as follows:

[0093] First, the stress component ratio within the time increment is calculated, and the stress triaxiality is characterized according to the stress component ratio; then the stress component ratio is converted into the major and minor strain increment ratio, thereby completing the stress triaxiality based on the major and minor strain increment characterization.

[0094] in,

[0095] The major and minor strain increments in the “major and minor strain increment ratio” are obtained based on the assumption of a linear loading path and by interpolation calculation.

[0096] 12. The method for calculating stress triaxiality of a metal sheet during loading according to claim 1, characterized in that:

[0097] The material is either isotropic or anisotropic.

[0098] Working process, principle and implementation example

[0099] The relevant principles, processes and characterizations are described below based on the isotropy of the materials.

[0100] like Figure 1 As shown, the specific steps are as follows:

[0101] 1) Strain history data measurement: The DIC method was used to measure the major and minor strain distribution history data of the plate under the target loading condition. The loading rate of the target loading condition and the DIC shooting frequency were set so that the major and minor strain increments in adjacent DIC shooting states were <0.05%.

[0102] 2) Strain history data smoothing: Smoothing and filtering the primary and secondary strain history data in step 2 to eliminate the jitter phenomenon of DIC measurement data;

[0103] 3) Calculation time point selection: According to the primary and secondary strain history data in step 2, calculate the plastic strain history data; select specific strain data points on the plastic strain history data, and the plastic strain increment of adjacent data points is less than or equal to 0.2. The corresponding time series is obtained by interpolation method, and this time series is used as the stress triaxiality calculation time;

[0104] 4) Resampling of strain history data: According to the time series selected in step 3, the corresponding major and minor strain data points are obtained by interpolation. By resampling the strain history data, the oscillation problem caused by too small strain data increment or measurement error between adjacent states of DIC can be eliminated, and sufficient effective measurement data can be provided for the critical loading moment (large strain increment).

[0105] 5) Stress triaxiality history data calculation: According to the major and minor strain history data resampled in step 4, calculate the major and minor strain increments of adjacent data points and major and minor strain ratio Major and minor strain increments The calculation method is: the strain value at the current moment is interpolated with the strain value at the previous moment (the strain value at the initial moment is recorded as 0). In the time history of a small strain increment, the strain loading path can be assumed to be linear, that is, the major and minor strain ratio b is a fixed value in this increment, and the stress triaxiality η remains constant. For the plate, it can be assumed that its thickness stress σ3=0, thus obtaining the stress component ratio a=σ1 / σ2 in this time increment; thus, the stress triaxiality η in this time increment can be obtained: The final expression is:

[0106] If the material properties show obvious anisotropy, the above formula should be modified to:

[0107]

[0108] Where r is the material thickness anisotropy index in the loading direction.

[0109] Example

[0110] The following takes CP980 ultra-high strength steel as an example. The anisotropy of this material is not significant, so the Von Mises yield constitutive model is selected. It can be further expanded to other constitutive models as needed. The stress triaxiality measurement process of the R80 specimen loading condition is as follows:

[0111] 1. The DIC method is used to measure the strain components of the plate during loading. The loading rate is 0.6 mm / min, the DIC shooting frequency is 10 fps, and the maximum plastic strain position of the specimen is selected as the stress triaxiality calculation area. The major and minor strain DIC history data of the area are output. The table below shows the major and minor strain DIC measurement results.

[0112]

[0113] Second, the strain measurement results are smoothed using the Savitzky-Golay filter (this embodiment uses this filter method, and other filters may also be selected). The results are shown in the following table.

[0114]

[0115] 3. According to the filtered primary and secondary strain data, the plastic strain history data is calculated. The plastic strain history is sampled at equal intervals with an increment of 0.02 to obtain the calculated time series and its corresponding primary and secondary strain values, such as Figure 5 , 6 , 7. The corresponding major and minor strain resampling results are shown in the following table (with Figure 7 correspond).

[0116]

[0117] Fourth, calculate the stress triaxiality in each time increment using the resampled major and minor strain values. Figure 8 shown.

[0118] The present invention provides a method for calculating stress triaxiality during loading of a metal sheet. First, strain history data of the sheet loading process is directly obtained. Secondly, based on the obtained strain history data, a mathematical processing method is combined to finally obtain the characterization of all stress states in the region. The stress state determined by this method improves the accuracy of material performance characterization. The direct acquisition is completed based on the DIC technology. The combination of the set mathematical processing method obtains the characterization of all stress states in the region by establishing the primary and secondary strain increments and corresponding time points in the determined calculation region, and calculates the stress triaxiality within the time increment step according to the determined primary and secondary strain increments and corresponding time points. The loading rate and shooting frequency of the DIC technology part are determined according to the set strain increment threshold. The primary and secondary strain increments are obtained by resampling the strain history data. The time points corresponding to the primary and secondary strain increments are converted into plastic strain history data according to the calculation of the strain history data, and then the strain data points are determined based on the set strain increment, and the corresponding time series is obtained in combination with the interpolation method. Before the conversion to plastic strain history data, smoothing filtering is also performed. The strain history data obtained based on direct measurement analysis, as well as resampling, filtering, loading rate and shooting frequency processing, and the processing of determining strain data points based on the set strain increment constitute data accuracy services in different dimensions and gradients, and ultimately obtain accurate characterization of material properties based on direct measurement analysis. Since it is impossible to obtain the strain increment and the corresponding time point at the same time, the above two-step process is required.

Claims

1. A method for calculating stress triaxiality of a metal sheet during loading, characterized in that: Based on direct measurement analysis, strain history data is obtained. By establishing interpolation analysis and processing of strain history data, the stress triaxiality within the time increment step is calculated, and the quantitative representation of the stress state of each area is established accordingly; The direct measurement-based analysis specifically includes the following steps: S1: Load and push the test piece according to the set loading rate; S2: real-time tracking and shooting of the loading and advancing process according to the set shooting frequency to form continuous frame images; S3: Establish DIC principle-based analysis and calculation for continuous frame images, thereby determining the strain field characterized by the full-field displacement; and obtain the corresponding strain history data; The method of calculating the stress triaxiality within the time increment step by establishing interpolation analysis processing on the strain history data specifically includes the following steps: SS1: Determine the calculation area based on the strain history data; SS2: Determine each calculation time point in the calculation area and the corresponding major and minor strain increments at each calculation time point; SS3: Calculate the stress triaxiality within the time increment step according to the determined calculation time point and the corresponding major and minor strain increments; The step SS2 of determining each calculation time point of the calculation area is specifically as follows: SS21: Filter the primary and secondary strain history data; SS22: Calculate the plastic strain history data based on the filtered primary and secondary strain history data; SS23: Determine the time series based on the plastic strain increment to form a representation of each calculation time point; The determination of the major and minor strain increments corresponding to each calculation time point described in step SS2 is specifically: First, the primary and secondary strain history data are filtered. Then, resampling processing is established for the filtered primary and secondary strain history data. Finally, the major and minor strain increments at adjacent time points are determined based on the major and minor strain history data after resampling. The stress triaxiality in step SS3 is specifically determined as follows: First, the stress component ratio within the time increment step is calculated, and the stress triaxiality is characterized according to the stress component ratio; then the stress component ratio is converted into the major and minor strain increment ratio, thereby completing the stress triaxiality based on the major and minor strain increment characterization.

2. The method for calculating stress triaxiality of a metal sheet during loading according to claim 1, characterized in that: The set loading rate and the set shooting frequency are determined according to the set thresholds of the major and minor strain increments of adjacent single-frame images.

3. The method for calculating stress triaxiality of a metal sheet during loading according to claim 1, characterized in that: The time series is determined according to the plastic strain increment, specifically: The strain data points are determined based on the set strain increment and the corresponding time series is obtained by combining the interpolation method.

4. The method for calculating stress triaxiality of a metal sheet during loading according to claim 2, characterized in that: The setting threshold of the major and minor strain increments of adjacent single-frame images is specifically: less than or equal to 0.05%.

5. The method for calculating stress triaxiality of a metal sheet during loading according to claim 3, characterized in that: The set strain increment is specifically: the plastic strain increment corresponding to adjacent time points is less than or equal to 20% of the ultimate plastic strain.

6. The method for calculating stress triaxiality of a metal sheet during loading according to claim 1, characterized in that: The major and minor strain increments in the "major and minor strain increment ratio" are obtained based on the assumption of a linear loading path and by interpolation calculation.

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