CN112926173A - Method for calculating forming limit diagram of hot-rolled high-strength steel plate - Google Patents
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Abstract
The invention relates to a method for calculating a forming limit diagram of a hot-rolled high-strength steel plate, which comprises the following steps of: (1) adopting a unidirectional tensile test to obtain yield strength, tensile strength, uniform elongation and stress-strain curves after yielding in at least three directions of 0 degrees, 45 degrees and 90 degrees along the rolling direction of the hot-rolled high-strength steel plate; (2) steel plate FLD with a series of typical strength grades and thicknesses measured by plate forming tester0(ii) a (3) By uniaxial tension data and actually measured FLD0Carry out regression between to establish FLD0The calculation formula of (2); (4) determining an anisotropic yield criterion applicable to the material according to the obtained unidirectional tensile data in at least three directions; (5) fitting the hardening of the measured Material with the Swift hardening equationA curve; (6) calculating initial limit primary and secondary strain values of the material by adopting an MK model and combining the calculated FLD0And correcting the limit strain value to obtain a final forming limit diagram of the hot-rolled high-strength steel plate.
Description
Technical Field
The invention relates to a calculation method, in particular to a calculation method of a hot-rolled high-strength steel plate forming limit diagram, and belongs to the technical field of steel plate forming limit diagram construction methods.
Background
The main measures of the automobile are material lightweight and structure lightweight. The material is inevitably prone to hot band cold and high strength thinning. The hot-rolled high-strength steel is the main raw material of the automobile chassis part, and bears safety and bearing performance, along with the improvement of strength, the plasticity of corresponding materials is reduced, and the complexity of parts is not reduced, so that the problem of material forming is more and more prominent. The Forming Limit Diagram (FLD) is widely applied to the field of plate forming and processing, and can provide an efficient evaluation basis for part formability analysis and die design by matching with finite element analysis software, so that the FLD is widely applied to the field of sheet forming at present. The proposal of the forming limit diagram promotes the coordinated development of the performance, the forming process and the quality control of the sheet material, and is an important step in the forming science.
One method for obtaining the FLD is to carry out experimental tests according to the forming test standard GBT 15825.8 and carry out material forming experiments by printing small circular grids on the plate, however, the FLD has the problems of large experimental amount and low efficiency, the test result can only represent the tested sample, and the hot rolled steel plate is characterized by large material performance fluctuation; one is to adopt an empirical or theoretical calculation model, wherein a plurality of mathematical models are developed in the field of cold-rolled sheets, but due to the structural property difference between the hot-rolled steel sheets and the cold-rolled sheets, the theoretical formula of the sheets is not suitable for the hot-rolled steel sheets, the most used hot-rolled steel sheets at present are Keeler empirical formulas, however, experiments prove that the deviation ratio of the Keeler formulas is large, and particularly, the material forming capability is overestimated in a double-pulling area; in addition, the forming limit calculation is performed by adopting a finite element simulation mode, but the finite element analysis is greatly influenced by a material model, the assumption of the finite element model is relatively ideal, the surface state, damage and the like of the material are hardly considered, the calculation accuracy of the finite element analysis is limited, and the finite element analysis is also an urgent part to be improved in the current numerical simulation.
Due to the characteristics of the component design and the microstructure, the strain level of the forming limit diagram FLD of the hot-rolled high-strength steel is generally gradually reduced along with the improvement of the material strength and the reduction of the plasticity. The accurate hot-rolled high-strength steel FLD can provide evaluation basis for steel plate composition structure design and metallurgy manufacturing process optimization in a steel plate development stage, provide basis for reasonable material selection, stamping process design and mold design optimization in a part manufacturability stage, provide part process tolerance range for manufacturing standard preparation in a part batch production stage, and effectively improve design, manufacturability and machinability of the whole process from a steel plate to parts.
The patent document of Chinese patent publication No. CN 101599094A discloses a model method for establishing a forming limit diagram of a transformation induced plasticity steel plate, which comprises the following specific steps: (1) measuring the thickness and strain hardening index of the TRIP steel plate; (2) calculating a limit strain value of the transformation induced plasticity steel plate in a plane strain state; (3) and establishing a strain coordinate system, synthesizing data on the coordinate system, and establishing a forming limit diagram of the transformation induced plasticity steel plate. FLD of the invention0Depending on the actual plate forming experimental determination, FLD is required to be carried out each time under different specifications and states0The test of (1). The left side of the FLD is regarded as a straight line along 45 degrees, the right side of the FLD establishes a primary and secondary strain relational expression based on the constant ratio of the strain in the width direction to the strain in the thickness direction of the bulging region, and an approximately linear equation is regressed by adopting experimental data. It can be seen from the examples of the invention that the data of the double-drawing zone experiments for forming the cold-rolled TRIP steel sheet are compared to match the prediction curve. However, it is apparent that the characteristics of the non-linearity of the ultimate strain value and the inclination downward to the right of the double-drawn zone of the hot-rolled high-strength steel are not satisfied.
Chinese patent publication No. CN 102620980a discloses a method for predicting sheet formability using a neural network, which mainly performs a lot of sheet formability tests, trains the data through the neural network, establishes a fuzzy neural network model, and then predicts sheet formability. The adopted test data comprise yield strength, tensile strength, work hardening index and plastic strain ratio, cup IE value and primary and secondary strain value of steel die bulging FLD. The problem that this scheme exists is, neural network belongs to pure mathematical model, relies on big data, and especially actual measurement FLD's data is very important, has sufficient database just can train good model, and the experiment cost is very high, and to the steel grade of different grades, its FLD characteristics are different moreover, and the degree of difficulty is very big for accurate neural network model establishment.
Chinese patent publication No. CN 103424318A discloses a simple method using drawing experiment and finite element simulation, i.e. using DYNAFORM finite element software to simulate the actual drawing experiment and manually setting the initial FLD0And adjusting until the numerical simulation result is matched with the experimental result, and determining the FLD0Is a precise FLD of the material0And calculating two-side curves by a Keeler formula. The method has the problems that the precision of finite element simulation is greatly influenced by a material model, the invention can not obtain a reliable simulation result only by depending on a stress-strain curve obtained by a material tensile experiment, and the calculation of curves at two sides of the FLD by a Keeler formula is proved to have low precision, so that a new scheme is urgently needed to solve the technical problems.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a method for calculating a forming limit diagram of a hot-rolled high-strength steel plate, and the technical scheme is characterized in that a small amount of FLD (flash laser diode) is used0Test determination of FLD suitable for hot-rolled high-strength steel0And calculating the model, calculating curves at two sides of the FLD based on a groove theory, and comprehensively obtaining a forming limit diagram FLD suitable for the hot-rolled high-strength steel. The scheme remarkably reduces the experimental amount, ensures the accuracy of theoretical calculation results through a high-precision material model, can efficiently and accurately calculate the forming limit diagram of the hot-rolled high-strength steel, can calculate the measured material, can analyze the influence of material performance fluctuation on a forming window, and provides an effective basis for analysis of the punching property of parts and improvement of the performance of steel plates.
In order to achieve the above object, according to the technical solution of the present invention, a method for calculating a forming limit diagram of a hot-rolled high-strength steel sheet is characterized by comprising the steps of:
step 1: adopting a unidirectional tensile test to obtain yield strength, tensile strength, uniform elongation and engineering stress-strain curves after yielding in at least three directions of 0 degrees, 45 degrees and 90 degrees along the rolling direction of the steel plate, and converting the engineering stress-strain curves obtained by the test into real stress-strain curves;
step 2: according to the forming experiment test standard GBT 15825.8, a plate forming tester is adopted to actually measure rectangular or round samples with the width of 90mm to obtain typical high-strength steel plates FLD with different strength grades and different thicknesses0；
And step 3: by uniaxial tension data and actually measured FLD0Regression is carried out between the hot-rolled high-strength steel and the hot-rolled high-strength steel0The computational model of (2), performing regression according to the form of formula (1):
wherein a and b are constants, according to the FLD of the hot-rolled high-strength steel in the scheme0And AgFitting multiple sets of data to obtain formula (2):
and 4, step 4: according to the obtained unidirectional tensile data in the three directions, parameter identification is carried out by respectively adopting a main flow anisotropic yield criterion model, and when the yield stress and the anisotropic coefficient predicted by the yield criterion model are both consistent with the experimental values, the anisotropic yield criterion applicable to the material can be determined;
and 5: taking data between a yield point and a maximum force point of an engineering stress-strain curve, converting the data into a real strain curve, and fitting the real stress-strain curve behind the maximum force point by adopting a hardening curve formula;
step 6: establishing, using an MK groove model, from respective yield criteria and hardening curves based on plane stress assumptionsForming limit calculation equation, measuring the initial limit primary and secondary strain values of the material, and combining the calculated FLD0And correcting the limit strain value to obtain a final forming limit diagram of the hot-rolled high-strength steel plate.
As an improvement of the invention, in the step 5, data from a yield point to a maximum force point of an engineering stress-strain curve is taken and converted into a curve of a real stress sigma and a real strain epsilon, then a hardening equation is adopted to fit the real stress-strain curve after the maximum force point, and for hot-rolled high-strength steel, a complete stress-strain curve obtained by adopting a Swift hardening equation epitaxy is most approximate, as shown in formula (3):
σ＝k*(ε0+ε)n (3)
wherein epsilon0N is a work hardening index, k is a hardening coefficient for the prestrain, and the strain is obtained by fitting the data
As an improvement of the invention, in the step 4, parameter identification is performed by respectively adopting a mainstream anisotropic yield criterion model according to the obtained unidirectional tensile data of at least three directions, and a material yield criterion expression is defined as
The equivalent stress at which it yields
The following relationship is given to the yield plane Y (θ) at a certain angle:
at the same time, a yield function F is defined with respect to the angle thetaθYield stress Y of a certain degree of uniaxial tensionθThe following relationships apply:
Yθ＝Y(θ)/Fθ(5) according to the formula, the expression of the specific yield criterion is combined, a plurality of equations about the yield criterion parameters can be established, the yield stress and the anisotropy coefficient obtained by unidirectional stretching are respectively substituted, and when the yield criterion is adopted, the yield criterion is expressedWhen the yield stress and the anisotropy coefficient predicted by the model are both relatively consistent with the experimental values, the anisotropic yield criterion applicable to the material can be determined.
As an improvement of the invention, the forming limit diagram calculation method is suitable for hot-rolled high-strength steel with the yield strength of 350MPa-600MPa, and the thickness interval of the hot-rolled high-strength steel is 1.8-5.0 mm.
As a modification of the invention, in the step 6, an MK groove model is adopted, a forming limit calculation equation is established according to the corresponding yield criterion and the hardening curve based on the plane stress assumption, the initial limit primary and secondary strain values of the material are measured, and then the calculated FLD is combined0Correcting the limit strain value to obtain a final forming limit diagram of the hot-rolled high-strength steel plate, wherein the specific calculation process is as follows:
assuming that the hot-rolled high-strength steel is still in a plane stress state:
1) according to the coordinated deformation, the secondary strain increments in the groove direction in zones A and B are equal, i.e.
dε2A＝dε2g (6)
In the formula d epsilon2AIs the increase of the sub-strain of the A-zone, d epsilon2BIs the secondary strain increment of the B region;
2) assuming that the grooves are perpendicular to the principal stress, the instantaneous cross-sectional forces inside and outside the grooves are equal, i.e. sigma, according to the force balance condition1AtA＝σ1BtB (7)；
In the formula sigma1A、σ1BRespectively A, B region transient principal stress, tA、tBA, B zone instantaneous thicknesses, respectively;
3) the sheets being in plane stress, i.e. sigma3A＝σ3B＝0 (8)；
In the formula sigma3A、σ3BMain stress in the thickness direction of regions A, B respectively
4) Principle of constant volume, i.e.
dε1+dε2+dε3＝0 (9)；
The thickness variation of the groove is described by f, wherein the initial thickness variation can be calculated by the following formula:
obtaining the equivalent stress according to the determined yield criterion
The relation with the primary and secondary stress is as follows:
setting stress ratio
Process parameter
By giving an increase in strain d epsilon1AAt this time, the equivalent stress values in the A, B regions are respectively set as
Solving d epsilon by substituting formula (7)1BThe calculation process is as follows:
wherein
And
performing iterative calculation of the above equation (13) by using MATLAB by substituting the previously determined hardening equation (3) into the calculation
When the limit strain is considered to have been reached, e1AAnd ε2ARespectively is the ultimate primary and secondary strain values of the material under the stress ratio, and then a series of ultimate strain value points (epsilon) are obtained by traversing calculation within the range that the stress ratio is more than or equal to 0 and less than or equal to 11i，ε2j)。
FLD obtained by previous calculation0Value pair calculation (ε)1i，ε2j) The correction is made assuming that the main strain value when the secondary strain becomes 0 is ε0And then the final principal strain ε'1iThe calculation can be performed according to equations (14), (15):
Δε＝FLD0-ε0 (14)
ε′1i＝ε1i+Δε (15)
finally, the set of limit primary and secondary strain values (epsilon'1i，ε2j) And (5) drawing to a coordinate system, namely obtaining a complete forming limit curve.
Compared with the prior art, the method has the advantages that 1) the technical scheme is based on the MK groove theory under the assumption of plane stress, the yield criterion and the hardening equation are determined through the applicable material model, and the FLD is established0Model calculation and use of Experimental FLD0Model fitting is performed by accurate FLD0The calculation result of the MK theory is corrected, so that the calculation precision is obviously improved, and the calculation workload is obviously reduced; 2) according to the method, a high-precision forming limit diagram can be obtained only through a one-way stretching experiment, the problem that the forming limit diagram of the hot-rolled high-strength steel is difficult to calculate is effectively solved, and the method is high in efficiency and low in cost; 3) the calculation method provided by the invention not only has a direct evaluation effect on the tested sample, but also can evaluate the change amplitude of the material forming limit after the corresponding hot-rolled high-strength steel performance fluctuation through the change of parameters such as a stress-strain curve and the like. The calculation method has high accuracy, is simple and convenient, and has strong performability; 4) the method can be effectively applied to failure judgment of machining cracking in part forming simulation calculation, and improves numerical simulation precision.
Drawings
FIG. 1 shows the steps of the method for calculating the forming limit diagram of hot-rolled high-strength steel according to the present invention;
FIG. 2 is a schematic diagram of an MK groove model;
predicted yield stress values for the different yield criteria of the embodiment of FIG. 3;
the values of the anisotropy coefficients r predicted for the different yield criteria of the embodiment of FIG. 4;
the hardening equation fitting results for the embodiment of FIG. 5;
the calculated FLD of the example of fig. 6 is compared to experimental values.
The specific implementation mode is as follows:
for the purpose of enhancing an understanding of the present invention, the present embodiment will be described in detail below with reference to the accompanying drawings.
The application example is as follows: referring to fig. 1 to 6, a method for calculating a forming limit diagram of a hot-rolled high-strength steel plate, the method comprising the steps of:
step 1: adopting a unidirectional tensile test to obtain yield strength, tensile strength, uniform elongation and engineering stress-strain curves after yielding in at least three directions of 0 degrees, 45 degrees and 90 degrees along the rolling direction of the steel plate, and converting the engineering stress-strain curves obtained by the test into real stress-strain curves;
in this embodiment, hot-rolled high-strength steel S550MC is taken as an example, and the details are as follows: for hot-rolled high-strength steel S550MC with a thickness of 2.0mm, one drawing sample was taken every 15 degrees along the rolling direction (2 repetitions of taking an average), and uniaxial drawing data in 7 directions were obtained:
table 1S 550MC 2.0mm specification uniaxial tension data in seven directions:
step 2: the forming limit test of 90mm width was performed for the S550MC 2.0mm specification, and was repeated three times, and the test results were as follows:
table 23 test data
And step 3: according to FLD0Formula for calculation
(calculated in the direction of 90 degrees along the rolling direction) and basically accords with the experimental test value.
And 4, step 4: according to the experimental data of the uniaxial tension in seven directions, three different anisotropic yield criteria, namely Hill48, Hill90, Barlat89 and the like are respectively adopted for parameter identification, as shown in the table, and then the uniaxial tensile yield stress and the anisotropic coefficient in the 0-90-degree direction are simultaneously predicted by the different yield criteria and are compared with the experimental values. The comparison results are shown in FIGS. 3 and 4, and it can be seen that the Hill48 yield criterion is applied to the S550MC material.
TABLE 3 parameter determination for Hill48 yield criteria
TABLE 4 parameter determination for Hill90 yield criteria
TABLE 5 parameter determination of Barlat89 yield criterion
And 5: according to the engineering stress-strain curve, carrying out logarithmic conversion on data from a yield point to a maximum force point to obtain a section of real stress-strain curve, fitting the real stress-strain curve by using a Swift hardening equation, and carrying out epitaxy to obtain a real stress-strain curve of the material after a necking point: sigma 1010 (0.00485+ epsilon)0.113
Step 6: adopting an MK groove model, and based on Hill48 anisotropic yield criterion, assuming that the plane stress state is still satisfied, the yield criterion expression is as follows:
setting stress ratio
Then formula (16) can be rewritten as
Setting process parameters
Then there is
Set strain ratio
Process parameter
According to the flow rule, there are
By giving an increase in strain d epsilon1AAt this time, the equivalent stress values in the A, B regions are respectively set as
Solving d epsilon by substituting formula (7)1BThat is to say have
Wherein the hardening curve adopts sigma 1010 (0.00485+ epsilon)0.113
Performing iterative calculation by using MATLAB for the formula (21)
When the limit strain is considered to have been reached, e1AAnd ε2ARespectively is the ultimate primary and secondary strain values of the material under the stress ratio, and then a series of ultimate strain value points (epsilon) are obtained by traversing calculation within the range that the stress ratio is more than or equal to 0 and less than or equal to 11i，ε2j) See table 6.
TABLE 6 initial ultimate strain values calculated by MK theory
According to FLD0The result obtained by the calculation model was 0.257, and (. epsilon.) was calculated from the pair of equations (14) and (15)1i，ε2j) Corrections were made to obtain the final forming limit points of table 7:
TABLE 7 Final Forming Limit points of the Material
Finally, the set of limit primary and secondary strain values (epsilon'1i，ε2j) And (5) drawing to a coordinate system, namely obtaining a complete forming limit diagram. As shown in FIG. 6, the forming limit curve calculated according to the scheme has good matching degree with the experimental value.
It should be noted that the above-mentioned embodiments are not intended to limit the scope of the present invention, and all equivalent modifications and substitutions based on the above-mentioned technical solutions are within the scope of the present invention as defined in the claims.
Claims (5)
1. A method for calculating a forming limit diagram of a hot-rolled high-strength steel plate is characterized by comprising the following steps of:
step 1: adopting a unidirectional tensile test to obtain yield strength, tensile strength, uniform elongation and engineering stress-strain curves after yielding in at least three directions of 0 degrees, 45 degrees and 90 degrees along the rolling direction of the steel plate, and converting the engineering stress-strain curves obtained by the test into real stress-strain curves;
step 2: according to the forming experiment test standard GBT 15825.8, a plate forming tester is adopted to actually measure rectangular or round samples with the width of 90mm to obtain typical high-strength steel plates FLD with different strength grades and different thicknesses0；
And step 3: by uniaxial tension data and actually measured FLD0Regression is carried out between the hot-rolled high-strength steel and the hot-rolled high-strength steel0The computational model of (2), performing regression according to the form of formula (1):
FLD0＝a*Ag*eb*t (1)
wherein a and b are constants, according to the FLD of the hot-rolled high-strength steel in the scheme0And AgFitting multiple sets of data to obtain formula (2):
FLD0＝2.71*Ag*e0.056*t (2)
and 4, step 4: according to the obtained unidirectional tensile data in the three directions, parameter identification is carried out by respectively adopting a main flow anisotropic yield criterion model, and when the yield stress and the anisotropic coefficient predicted by the yield criterion model are both consistent with the experimental values, the anisotropic yield criterion applicable to the material can be determined;
and 5: taking data between a yield point and a maximum force point of an engineering stress-strain curve, converting the data into a real strain curve, and fitting the real stress-strain curve behind the maximum force point by adopting a hardening curve formula;
step 6: establishing a forming limit calculation equation according to corresponding yield criterion and hardening curve based on plane stress assumption by adopting an MK groove model, measuring initial limit primary and secondary strain values of the material, and combining the measured initial limit primary and secondary strain values with the calculated valuesFLD0And correcting the limit strain value to obtain a final forming limit diagram of the hot-rolled high-strength steel plate.
2. The method for calculating the forming limit diagram of the hot-rolled high-strength steel plate according to claim 1, wherein the step 5 is to take data from the yield point to the maximum force point of the engineering stress-strain curve, convert the data into a curve of the real stress sigma and the real strain epsilon, then fit the real stress-strain curve after the maximum force point by using a hardening equation, and aim at the hot-rolled high-strength steel, the complete stress-strain curve obtained by adopting the Swift hardening equation extension is the closest, as shown in formula (3):
σ＝k*(ε0+ε)n (3)
wherein epsilon0For pre-strain, n is the work hardening index and k is the hardening coefficient, obtained by fitting the above data.
3. The method for calculating the forming limit diagram of the hot-rolled high-strength steel plate according to claim 2, wherein the step 4 is to perform parameter identification by respectively adopting a main flow anisotropic yield criterion model according to the obtained unidirectional tensile data of at least three directions, and the material yield criterion expression is defined as
The equivalent stress at which it yields
The following relationship is given to the yield plane Y (θ) at a certain angle:
at the same time, a yield function F is defined with respect to the angle thetaθYield stress Y of a certain degree of uniaxial tensionθThe following relationships apply:
Yθ＝Y(θ)/Fθ (5)
according to the formula, a plurality of equations about the yield criterion parameters can be established by combining the expressions of the specific yield criterion, the yield stress and the anisotropy coefficient obtained by unidirectional stretching are respectively substituted, and when the yield stress and the anisotropy coefficient predicted by the yield criterion model are both in accordance with the experimental values, the anisotropic yield criterion applicable to the material can be determined.
4. The method for calculating the forming limit diagram of a hot-rolled high-strength steel plate according to claim 3, wherein the forming limit diagram calculation method is applied to hot-rolled high-strength steel with a yield strength of 350MPa to 600MPa, and the thickness range of the hot-rolled high-strength steel plate is 1.8 mm to 5.0 mm.
5. The method for calculating the forming limit diagram of the hot-rolled high-strength steel plate according to claim 3, wherein in the step 6, an MK groove model is adopted, a forming limit calculation equation is established according to corresponding yield criterion and a hardening curve based on plane stress assumption, the initial limit primary and secondary strain values of the material are measured, and then the calculated FLD is combined0Correcting the limit strain value to obtain a final forming limit diagram of the hot-rolled high-strength steel plate, wherein the specific calculation process is as follows:
assuming that the hot-rolled high-strength steel is still in a plane stress state:
1) according to the coordinated deformation, the secondary strain increments in the groove direction in zones A and B are equal, i.e.
dε2A＝dε2B (6)
In the formula d epsilon2AIs the increase of the sub-strain of the A-zone, d epsilon2BIs the secondary strain increment of the B region;
2) assuming that the grooves are perpendicular to the principal stress, the instantaneous cross-sectional forces inside and outside the grooves are equal, i.e. sigma, according to the force balance condition1AtA＝σ1BtB (7)；
In the formula sigma1A、σ1BRespectively A, B region transient principal stress, tA、tBA, B zone instantaneous thicknesses, respectively;
3) the sheets being in plane stress, i.e. sigma3A＝σ3B＝0 (8)；
In the formula sigma3A、σ3BMain stress in the thickness direction of regions A, B respectively
4) Principle of constant volume, i.e.
dε1+dε2+dε3＝0 (9)；
The thickness variation of the groove is described by f, wherein the initial thickness variation can be calculated by the following formula:
obtaining the equivalent stress according to the determined yield criterion
The relation with the primary and secondary stress is as follows:
setting stress ratio
Process parameter
By giving an increase in strain d epsilon1AAt this time, the equivalent stress values in the A, B regions are respectively set as
Solving d epsilon by substituting formula (7)1BThe calculation process is as follows:
wherein
And
performing iterative calculation of the above equation (13) by using MATLAB by substituting the previously determined hardening equation (3) into the calculation
When the limit strain is considered to have been reached, e1AAnd ε2ARespectively is the ultimate primary and secondary strain values of the material under the stress ratio, and then a series of ultimate strain value points (epsilon) are obtained by traversing calculation within the range that the stress ratio is more than or equal to 0 and less than or equal to 11i，ε2j)。
FLD obtained by previous calculation0Value pair calculation (ε)1i，ε2j) The correction is made assuming that the main strain value when the secondary strain becomes 0 is ε0And then the final principal strain ε'1iThe calculation can be performed according to equations (14), (15):
Δε＝FLD0-ε0 (14)
ε′1i＝ε1i+Δε (15)
finally, the set of limit primary and secondary strain values (epsilon'1i，ε2j) And (5) drawing to a coordinate system, namely obtaining a complete forming limit curve.
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| CN115547436B (en) * | 2022-11-25 | 2023-03-10 | 哈尔滨工业大学 | Method and device for determining bulging limit strain of plate viscous medium |
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