A method for establishing a forming limit diagram model for thin sheets under complex stress states

Abstract

The present invention discloses a method for establishing a thin plate forming limit diagram model under complex stress state, comprising the following steps: 1) designing and processing a material sample according to the thin plate forming shape; 2) completing the tensile test of the material sample on a tensile testing machine; 3) performing strain calculation on the material sample during the tensile test; 4) establishing a predictive mathematical model for the forming limit diagram of the material sample; 5) calibrating and solving the parameters of the predictive mathematical model; 6) establishing the material forming limit diagram of the material sample. The present invention designs a set of sample shape and size based on the uniaxial tensile machine test, and formulates a set of simple test procedures and data processing methods, so as to establish a method for establishing a thin plate forming limit diagram under complex stress state.

Description

Technical Field

[0001] The present invention relates to metal plastic forming technology, and more specifically, to a method for establishing a thin plate forming limit diagram model under complex stress state, and in particular to a mathematical model and experimental method for establishing a thin plate forming limit diagram (curve). Background Art

[0002] With the introduction of the "low-carbon and intelligent" ecological development concept of the automotive industry, the lightweighting of vehicle bodies will be more valued by upstream and downstream industries. To this end, the steel industry continues to develop new products for vehicle body manufacturing, such as continuously expanding the advanced high-strength steel (AHSS) series to simultaneously meet the goals of weight reduction and safety and reasonable prices. The manufacturing processes of these new materials mainly include stamping, rolling, hot forming, etc., involving complex sheet metal forming process design.

[0003] Sheet metal forming is a very important part of the field of metal plastic forming. It is widely used in aerospace, automobile manufacturing, and electronic industrial-grade home appliance manufacturing. Among them, sheet metal stamping is one of the four major processes in automobile body manufacturing. In the early stages of automobile body part design, mold design, and process planning, designers hope to obtain accurate sheet metal forming limit curve values ​​as a benchmark for analysis data for CAE simulation and process formulation. In the actual production process of the stamping workshop of an automobile OEM or parts factory, on-site technicians also urgently need to know the limit deformation that the sheet metal can withstand under different strain paths or strain states, so as to take measures to prevent and avoid it.

[0004] Normally, the material warranty provided by steel mills to users only lists the basic mechanical properties of sheet materials, such as strength, elongation, n value, etc. These performance data from simple physical experiments can only make a general evaluation of the material's forming performance, and are not enough to provide a direct reference for users to judge the failure problems encountered in the stamping process under complex stress paths. Although the Forming Limit Diagram (FLD) can effectively evaluate the stamping forming performance of materials and is an important tool for solving on-site stamping problems, both the laboratory forming curve test and the stamping workshop forming limit curve acquisition usually require the use of complex experimental equipment and a variety of sample shape combinations, and even mathematical optical measurement and analysis technology to obtain the limit strain data. Based on this complex experimental test equipment and experimental process, it is impossible for steel mills to test the forming limit of each steel coil and provide it to users along with the material warranty.

[0005] In recent years, with the advancement and application of DIC digital image testing technology, the thin plate forming limit test method has been greatly improved, but its specimen preparation and testing process still have shortcomings such as long testing cycle, high experimental cost and complex data processing. Summary of the invention

[0006] In view of the above-mentioned defects existing in the prior art, the purpose of the present invention is to provide a method for establishing a forming limit diagram model of thin plate under complex stress states, design a set of specimen shapes and dimensions based on uniaxial tensile machine tests, and formulate a set of simple test procedures and data processing methods, so as to establish a forming limit diagram of thin plate under complex stress states.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for establishing a forming limit diagram model of a thin plate under a complex stress state comprises the following steps:

[0009] 1) Design and process material samples according to the sheet forming shape;

[0010] 2) completing a tensile test of the material sample on a tensile testing machine;

[0011] 3) performing strain calculation on the material sample during the tensile test;

[0012] 4) establishing a prediction mathematical model for the forming limit diagram of the material sample;

[0013] 5) calibrating and solving the parameters of the prediction mathematical model;

[0014] 6) Establishing a material forming limit diagram of the material sample.

[0015] Preferably, in step 1), the material sample is provided with three shapes, which are numbered 1#, 2#, and 3# respectively, and three of each shape of the material sample are processed by milling or wire cutting.

[0016] Preferably, the parallel parts or deformed parts on the material sample are sprayed with matte paint into a black and white speckled shape.

[0017] Preferably, in the step 2), the tensile test adopts a velocity loading method, and the loading speed is 0.5 to 2 mm / min.

[0018] Preferably, in step 3), the strain calculation adopts DIC (Digital Imagine Correlation) non-contact three-dimensional strain method, the sampling frequency of the camera is between 5 and 10 Hz, the step size of the virtual strain gauge is between 8 and 10, and the equivalent strain is marked as The equivalent principal strain is denoted by in, They represent the principal strains in the length, width and thickness directions of the material specimen, respectively. The equivalent plastic strain at the material forming limit moment (i.e., fracture moment) is marked as

[0019] The local maximum equivalent plastic strain of the material samples numbered 1#, 2#, and 3# at the material forming limit moment is calculated by the ARAMIS strain measurement system. The three calculation results of the material samples numbered 1#, 2#, and 3# are marked as The equivalent plastic strain at the material forming limit moment of the material samples numbered 1#, 2#, and 3# is obtained by the average method. as follows:

[0020]

[0021]

[0022]

[0023] Preferably, in step 4), the prediction mathematical model includes a relationship model between the material equivalent plastic strain limit and its stress state (stress triaxiality, which is a Rhodes angle parameter), which is calculated as follows:

[0024]

[0025] Among them, C1, C2, C3 represent the material constants of the material samples numbered 1#, 2#, and 3#, and the material constants are calibrated and solved by the test results of the material samples numbered 1#, 2#, and 3# in step 5).

[0026] Preferably, in step 5), the parameter calibration and solution specifically include:

[0027] The stress state of the material sample numbered 1# during the tensile loading process is the shear state, at which time its stress triaxiality η value is 0 and the Rhodes angle parameter The value is 0, and it is substituted into formula (1):

[0028]

[0029] The stress state of the material sample No. 2 during the tensile loading process is a shear state, at which the stress triaxiality η value is 1 / 3 and the Rhodes angle parameter The value is 1, and it is substituted into formula (1):

[0030]

[0031] The stress state of the material sample numbered 3# during the tensile loading process is the shear state, and its stress triaxiality η value is Rhodes angle parameters The value is 0, and it is substituted into formula (1):

[0032]

[0033] Formulas (2) to (4) form a set of three-variable linear equations, whose solution set is the calibration value of material constants C1, C2, and C3, namely:

[0034]

[0035] In summary, the equivalent plastic strain limit of the material can be written as a model describing the stress state during loading (stress triaxiality, which is the Rhodes angle parameter), that is:

[0036]

[0037] Preferably, in step 6), establishing the material forming limit diagram (curve) of the material sample further includes calculating the coordinates of the curve points on the material forming limit diagram according to the equivalent plastic strain limit. The calculation method is as follows:

[0038] 6.1) Calculate the equivalent plastic strain: According to the definition of equivalent plastic strain, the equivalent plastic strain of any point in three-dimensional space can be expressed as the expression of three principal strains:

[0039]

[0040] Assume β is the loading path of the material, β=ε2:ε1, according to the volume invariance principle, the principal strain in the thickness direction ε3=-(ε1+ε2), then the equivalent plastic strain is written as:

[0041]

[0042] 6.2) Calculate the principal strain: The expressions of principal strain and secondary strain are obtained by solving the following formula:

[0043]

[0044] 6.3) Relationship between strain path and stress state: In three-dimensional stress space, the material's stress triaxiality η, Rhodes angle θ and Rhodes angle parameter There are the following relationships between them:

[0045]

[0046] Under plane stress state, stress triaxiality η has the following relationship with the loading path of the material:

[0047]

[0048] Therefore, as long as the stress state at a certain position during the deformation process of the material is calculated, the equivalent strain limit value of the material can be deduced through formula (6) and formulas (9) to (11).

[0049] The method for establishing a thin plate forming limit diagram model under complex stress state provided by the present invention has the following beneficial effects:

[0050] 1) The present invention establishes a prediction model for judging unstable necking or cracking during the forming process of a high-strength thin plate, which can accurately predict the forming limit under complex stress states;

[0051] 2) The present invention establishes a simple and effective sheet metal forming performance evaluation tool method, which greatly simplifies the forming limit diagram (curve) testing workload and data processing time;

[0052] 3) The present invention establishes a simple limit prediction model and method, which can be used in steel mills to quickly establish an accurate sheet metal forming limit database;

[0053] 4) The present invention establishes a limit prediction model and method, which can be used to predict the forming limit in the stamping process of automobile plates and quickly guide users to design and optimize the stamping process. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a schematic flow diagram of the method of the present invention;

[0055] Figure 2 It is a schematic diagram of the shape of the material sample numbered 1# in the method of the present invention;

[0056] Figure 3 yes Figure 2 A magnified schematic diagram of part A;

[0057] Figure 4 It is a schematic diagram of the shape of the material sample numbered 2# in the method of the present invention;

[0058] Figure 5 It is a schematic diagram of the shape of the material sample numbered 3# in the method of the present invention;

[0059] Figure 6 is a comparison diagram of the model calculation results and the experimental results of the forming limit diagram of the advanced high-strength steel DP780 in Example 1 of the method of the present invention;

[0060] Figure 7It is a comparison chart between the model calculation results and the experimental results of the forming limit diagram of advanced high-strength steel QP980 in Example 2 of the method of the present invention. DETAILED DESCRIPTION

[0061] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0062] Please combine Figure 1 As shown, the present invention provides a method for establishing a thin plate forming limit diagram model under complex stress state, comprising the following steps:

[0063] 1) Preparation of test material samples:

[0064] 1.1) According to Figures 2 to 5 The shapes of the samples numbered 1#, 2#, and 3# shown in the figure are prepared for the samples respectively. Three samples of each shape are processed, for a total of 9 samples. The samples are first cut by a shearing machine to obtain small rectangular samples, and then the sample shapes are processed by milling or wire cutting. The sample section processing accuracy is 0.02mm.

[0065] 1.2) Carefully check the cross section of the processed sample, grind and polish the burrs with sandpaper to ensure that the cross section of the deformed area of ​​the sample is smooth and free of burrs and other defects;

[0066] 1.3) Spray the parallel parts or deformed parts of the sample with matte paint to form black and white speckles.

[0067] 2) Tensile test:

[0068] The processed samples numbered 1#, 2#, and 3# were loaded on the uniaxial tensile testing machine according to the speed-controlled loading method, with a loading speed of 0.5-2mm / min. While loading, a DIC non-contact optical camera was used to collect patterns on the deformation area, with a camera sampling frequency of 5-10Hz; the virtual strain gauge step size was 8-10; the entire test was loaded until the sample broke.

[0069] 3) Obtaining the ultimate strain of the sample:

[0070] The photos collected by the camera are processed by the ARAMIS system, and the last photo before the sample fracture is marked. The local maximum equivalent strain of the sample numbered 1#, 2#, and 3# at the moment of fracture is calculated. Three results are measured for each sample, and the fracture strain of the samples numbered 1#, 2#, and 3# is obtained by the average method.

[0071] 4) Calibration of model parameters:

[0072] According to the three sets of experimental data in step 3), a set of three-variable linear equations such as formulas (2) to (4) are established based on the prediction mathematical model of the forming limit diagram (curve) proposed in formula (1), and three material model parameters such as formula (5) are obtained by solving them.

[0073] 5) Drawing of forming limit diagram (curve):

[0074] According to the material parameters calibrated in step 4), the equivalent strain limit value of the material under any stress state is calculated by formula (6), and the principal strain and secondary strain under any strain path when the material is in a plane strain state are calculated by formula (9). The forming limit diagram (curve) is drawn through the coordinate points composed of the principal strain and the secondary strain.

[0075] Example 1

[0076] In this embodiment, the material is dual-phase steel HC780DP. The sample preparation and experimental test are completed according to the requirements of step 1) and step 2). The sample strain test is completed according to step 3). The maximum local strain values ​​of the samples numbered 1# to 3# are measured as follows:

[0077]

[0078]

[0079]

[0080] The model parameters were calibrated based on the three sets of experimental results combined with the instructions in step 4), and the results are:

[0081]

[0082] The predictive mathematical model for determining the forming limit diagram (curve) of dual-phase steel HC780DP is as follows:

[0083]

[0084] The strain path range of the material in the loading process under the plane stress state is: -1 / 2≤β≤1, the corresponding stress state range is: 1 / 3≤η≤2 / 3, and the Rhodes angle parameter range is: The forming limit diagram of the material under plane stress state is drawn from the above formula as follows: Figure 6 shown.

[0085] It should be noted that the forming strain can be either true strain or engineering strain. Figure 6It can be seen that the curve calculated by the model is consistent with the experimental data. The data points above the curve are the areas where HC780DP will fail due to fracture, and the area below the curve belongs to the safe forming area of ​​HC780DP. Therefore, the forming limit diagram model proposed in the present invention can better predict the forming limit of the material.

[0086] Example 2

[0087] In this embodiment, the material is advanced high-strength steel Q&P steel HC980QP. The sample preparation and experimental test are completed according to the requirements in step 1) and step 2). The sample strain test is completed according to step 3). The maximum local strain values ​​of the samples numbered 1# to 3# are measured as follows:

[0088]

[0089]

[0090]

[0091] The model parameters were calibrated based on the three sets of experimental results combined with the instructions in step 4), and the results are:

[0092]

[0093] The prediction mathematical model for determining the forming limit diagram (curve) of HC980QP is as follows:

[0094]

[0095] The strain path range of the material during loading under plane stress state is: -1 / 2≤β≤1, the corresponding stress state range is: 1 / 3≤η≤2 / 3, and the Rhodes angle parameter range is: The forming limit diagram of the material under plane stress state is drawn from the above formula as follows: Figure 7 shown.

[0096] It should be noted that the forming strain can be either true strain or engineering strain. Figure 7 It can be seen that the curve calculated by the model is consistent with the experimental data. The data points above the curve are the areas where HC980QP will fail due to fracture, and the area below the curve belongs to the safe forming area of ​​HC980QP. Therefore, the forming limit diagram model proposed in the present invention can better predict the forming limit of the material.

[0097] In summary, the method of the present invention is an effective method to solve the problem of material failure prediction in the stamping process of high-strength automobile plates for body parts. The model method can accurately predict defects such as cracking and wrinkling in the stamping process of automobile plates. It is a simple and effective sheet metal forming performance evaluation tool, which greatly simplifies the existing forming limit diagram (curve) testing workload and data processing time.

[0098] The method of the present invention relates to the establishment and use of instability and failure criteria in the forming process of high-strength steel plates. The present invention establishes a prediction model for judging unstable necking or cracking in the forming process of high-strength thin plates. Through the design and testing of tensile specimens of special shapes, a forming limit diagram (curve) of the material under complex stress states is established.

[0099] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A method for establishing a forming limit diagram model of a thin plate under complex stress state, characterized in that: The following steps are involved: 1) Design and process material samples according to the sheet forming shape; 2) completing a tensile test of the material sample on a tensile testing machine; 3) performing strain calculation on the material sample during the tensile test; 4) establishing a prediction mathematical model for the forming limit diagram of the material sample; 5) calibrating and solving the parameters of the prediction mathematical model; 6) establishing a material forming limit diagram of the material sample, In the step 1), the material sample is set in three shapes, numbered 1#, 2#, and 3#, and three of each shape of the material sample are processed by milling or wire cutting. In step 3), the strain calculation adopts the DIC non-contact three-dimensional strain method, the sampling frequency of the camera is between 5 and 10 Hz, the step size of the virtual strain gauge is between 8 and 10, and the equivalent strain is marked as The equivalent principal strain is denoted by in, They represent the principal strains in the length, width and thickness directions of the material specimen, respectively. The equivalent strain at the material forming limit is marked as The local maximum equivalent strain at the material forming limit moment of the material samples numbered 1#, 2#, and 3# is calculated by the ARAMIS strain measurement system. The three calculation results of the material samples numbered 1#, 2#, and 3# are marked as The equivalent strain of the material forming limit moment of the material samples numbered 1#, 2#, and 3# is obtained by the average method as follows: In the step 4), the prediction mathematical model includes a relationship model between the material equivalent plastic strain limit and its stress state, which is calculated as follows: Wherein, C1, C2, C3 represent the material constants of the material samples numbered 1#, 2#, and 3#, and the material constants are calibrated and solved by the test results of the material samples numbered 1#, 2#, and 3# in step 5). In the step 5), the parameter calibration and solution specifically include: The stress state of the material sample numbered 1# during the tensile loading process is the shear state, at which time its stress triaxiality η value is 0 and the Rhodes angle parameter The value is 0, and it is substituted into formula (1): The stress state of the material sample numbered 2# during the tensile loading process is a shear state. At this time, its stress triaxiality η value is 1 / 3, and the Rhodes angle parameter θ value is 1. Substituting into formula (1) is: The stress state of the material sample numbered 3# during the tensile loading process is the shear state, and its stress triaxiality η value is Rhodes angle parameters The value is 0, and it is substituted into formula (1): Formulas (2) to (4) form a set of three-variable linear equations, whose solution set is the calibration value of material constants C1, C2, and C3, namely: In summary, the equivalent plastic strain limit of the material can be written as a model describing the stress state during loading, namely: In the step 6), establishing the material forming limit diagram of the material sample further comprises: Calculate the coordinates of the curve points on the forming limit diagram of the material according to the equivalent plastic strain limit The calculation method is as follows: 6.1) Calculate the equivalent strain: According to the definition of equivalent plastic strain, the equivalent plastic strain of any point in three-dimensional space can be expressed as the expression of three principal strains: Assume β is the loading path of the material, β=ε2:ε1, according to the volume invariance principle, the principal strain in the thickness direction ε3=-(ε1+ε2), then the equivalent plastic strain is written as: 6.2) Calculate the principal strain: The expressions of principal strain and secondary strain are obtained by solving the following formula: 6.3) Relationship between strain path and stress state: In three-dimensional stress space, the material's stress triaxiality η, Rhodes angle θ and Rhodes angle parameter There are the following relationships between them: Under plane stress state, the stress triaxiality η has the following relationship with the material loading path: As long as the stress state at a certain position during the deformation of the material is calculated, the equivalent strain limit value of the material can be deduced through formula (6) and formulas (9) to (11).

2. The method for establishing a forming limit diagram model of a thin plate under a complex stress state according to claim 1, characterized in that: The parallel parts or deformed parts on the material sample are sprayed with matte paint to form a black and white speckled shape.

3. The method for establishing a forming limit diagram model of a thin plate under a complex stress state according to claim 1, characterized in that: In the step 2), the tensile test adopts a velocity loading method, and the loading speed is 0.5 to 2 mm / min.

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