Method for judging two-way tensile stress instability of plate based on load control

Abstract

The invention provides a plate two-way stretching stress instability judgment method based on load control, and relates to the technical field of two-way stretching. Comprising the following steps: determining a biaxial tensile test proportion of a material and preparing a sample; determining that the contour shape of the overlapping area of the two stretching axis directions of the sample is a rectangle or a regularized shape, and forming an inner rectangular contour shape and an outer rectangular contour shape in the overlapping area of the sample; sticking mark points on inner and outer contour side lines and a central point; starting a two-way tensile test in a load or stress control mode, and outputting a strain-time curve of outer and inner contour side lines in a tensile direction; calculating the strain difference value of the inner contour side line and the outer contour side line, and judging an instability moment when the strain difference value reaches 0.8-1.2% of the maximum strain of the inner contour side line and the strain of the inner contour side line reaches 9.5-10.5% of the maximum strain of the inner contour side line; comparing the instability moments in all the stretching directions, and taking the stress corresponding to the stretching axis which is unstable at the earliest as the instability stress of the material in the current stretching proportion; and inputting the instability stress into finite element software, and judging the forming safety of the plate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of bidirectional stretching, in particular, especially relates to a sheet metal bidirectional stretching stress instability judgment method based on load control. BACKGROUND

[0002] When carrying out manufacturing process design of a vehicle body part, a finite element analysis software is generally used to perform forming simulation design on the part, potential various safety risks in the part forming process are analyzed and verified according to the mechanical property attributes of the material, if the stress or strain generated by the part during simulation analysis exceeds the mechanical property range of the material, the part has safety risks. However, the material mechanical property parameters input during the finite element forming analysis are obtained under unidirectional tensile test or bulging test, and these material parameters are quite different from the stress state exhibited during sheet metal forming, so it is bound to have a large error to use them as the risk judgment basis during sheet metal forming analysis, which will lead to underestimation or overestimation of the forming performance of the material. Therefore, it is very important to develop a sheet metal bidirectional stretching stress instability judgment method based on load control, which can construct the complex mechanical response and two-dimensional stress space of the sheet metal under forming conditions.

[0003] At present, when analyzing the forming of a part, the unidirectional tensile curve and bulging limit curve of the material are imported into the finite element software, and the fracture strain of the material generated in the unidirectional tensile test and bulging limit test is used as the judgment condition, if the strain in the finite element analysis exceeds the fracture strain in the material test, the sheet metal has a risk of cracking, otherwise, the sheet metal forming is safe. However, during the forming of a part, the unidirectional tensile fracture strain and bulging fracture strain obtained by the traditional test method often do not reach the fracture strain, and the material cracks prematurely, and then the material is repeatedly adjusted or replaced, so it can be seen that if the results of the traditional test are used to analyze the forming results of the sheet metal, there will be a misjudgment, which seriously affects the design and manufacturing cycle of the part, and cannot meet the application requirements of engineering practice and scientific research.

[0004] In summary, the purpose of the present application is to accurately judge the potential safety risks such as cracking defects that may occur during the forming analysis of a vehicle body part, and to propose a sheet metal bidirectional stretching stress instability judgment method based on load control, which can conform to the stress state during sheet metal forming, and use the evolution law of the sheet metal in bidirectional stretching test to predict the instability of the sheet metal, so as to improve the judgment accuracy of part forming analysis and widely meet the engineering practical needs. SUMMARY

[0005] According to the above, the result of adopting the traditional test to analyze the forming result of the sheet metal will inevitably misjudge, seriously affect the design and manufacturing cycle of the parts, and cannot meet the application requirements of the engineering practice and scientific research work, and a sheet metal biaxial tension stress instability judgment method based on load control is provided, the biaxial tension test driven by the load or stress control mode is used to analyze the complex deformation law of the material forming, the stress and strain level of the sheet metal in biaxial tension is used to characterize the forming characteristics of the material, the problems of high labor cost and time cost caused by the fact that the traditional test method does not conform to the actual working condition are avoided, the judgment accuracy and analysis efficiency of the sheet metal forming are greatly improved, and the application requirements of the engineering practice and scientific research work are widely met.

[0006] The technical means adopted by the present application are as follows: A sheet metal biaxial tension stress instability judgment method based on load control, comprising the following steps: According to the test requirements, the biaxial tension test proportion of the material under the load or stress control mode is determined, and the biaxial tension sample is prepared, and the number of the biaxial tension sample is not less than the tension proportion; the contour shape of the overlapping area of the two tension axes of the biaxial tension sample is determined as a rectangle or a shape regularized by an inscribed rectangle, and then the length and width of the rectangle are taken as the basis to continue to inscribe a rectangle to the center of the sample, and the contour shape of the inner and outer two rectangles is formed in the overlapping area of the sample; Marking points are pasted on the inner and outer contour lines and the center point; The biaxial tension test is started in the load or stress control mode, and the strain-time curves of the outer contour line and the inner contour line in the tension direction are output respectively; According to the strain-time curve, the difference between the inner and outer contour line strains is calculated, when the strain difference reaches 0.8-1.2% of the maximum strain of the inner contour line and the inner contour line strain reaches 9.5-10.5% of its maximum strain, the instability time is determined; The instability time of each tension direction is compared, and the stress corresponding to the tension axis of the earliest instability is taken as the instability stress of the material at the current tension proportion; The instability stress is input into the finite element software to determine the safety of sheet metal forming.

[0007] Further, the forming method of the inner and outer contours is: If the contour of the sample overlapping area is irregular, then an inscribed rectangle is made to the center of the sample, and the length and width of the rectangle are taken as the basis to continue to inscribe, and the inner and outer two rectangular contours are formed.

[0008] Further, the method of pasting the marking points is: The outer contour marking points 1-4 are pasted on the outer contour rectangular line, and they are ensured to be located on the axis lines intersecting the four tension arms of the sample. Paste the inner contour mark points 5-8 on the inner contour rectangular edge line, and ensure that they are located on the axis intersecting the four stretching arms of the sample; Paste the center mark point 9 on the center point of the sample overlap area, and ensure that it is located at the intersection of the two stretching axes.

[0009] Further, the determination condition of the instability moment is: If the difference between the inner contour edge line strain and the outer contour edge line strain reaches 0.8-1.2% of the maximum strain of the inner contour edge line, and the current inner contour edge line strain reaches 9.5-10.5% of its maximum strain, the material is determined to be unstable; If the current inner contour edge line strain is less than 9.5-10.5% of its maximum strain, when the difference between the inner contour edge line strain and the outer contour edge line strain reaches 0.8-1.2% after the inner contour edge line strain reaches 9.5-10.5% of its maximum strain, the material is determined to be unstable.

[0010] Further, the determination method of the instability stress is: comparing the instability moments of the material in four stretching directions, and taking the stress corresponding to the stretching axis pair with the earliest instability as the instability stress.

[0011] Further, the method further comprises: obtaining the instability stresses of the material under different proportions through a biaxial stretching test with different stretching proportions.

[0012] Further, in the finite element software, the determination condition of the sheet forming safety is: When the biaxial stress in the simulation analysis is lower than the instability stress, it is determined to be safe, otherwise it is determined to exist failure risk.

[0013] Compared with the prior art, the present application has the following advantages: The present application discloses a sheet biaxial stretching stress instability determination method based on load control, which uses a biaxial stretching test driven by load or stress control mode to analyze the complex deformation law of material forming, uses the stress and strain level of sheet in biaxial stretching to represent the forming characteristics of the material, avoids the problems of the traditional test method that does not conform to the actual working condition and needs to carry out repeated experiments, greatly improves the determination accuracy and analysis efficiency of sheet forming, and widely meets the application requirements of engineering practice and scientific research work. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those of ordinary skill in the art without any creative effort based on these drawings.

[0015] Figure 1 The figure is a schematic diagram of the inner and outer contours and the mark points in the irregular shape in the embodiment.

[0016] Figure 2 The figure is a curve diagram of the inner and outer contour mark points in the embodiment.

[0017] Figure 3 The figure is a schematic diagram of the inner and outer contours and the mark points in the regular shape in the embodiment. DETAILED DESCRIPTION

[0018] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0019] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. The description of the at least one exemplary embodiment is actually only illustrative, but not as any limitation on the present application and its application or use. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort belong to the scope of protection of the present application.

[0020] It should be noted that the terms used herein are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present application. As used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and it should also be understood that when the terms "comprise" and / or "include" are used in the specification, there is a feature, step, operation, device, component and / or combination thereof.

[0021] The relative arrangement of parts and steps, numerical expressions, and numerical values set forth in the examples herein are not meant to limit the scope of the present application unless otherwise specifically indicated. It is to be understood that the dimensions of the various parts shown in the drawings are not drawn to scale for ease of illustration. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art can not be discussed in detail, but should be considered within the scope of the disclosure where appropriate. In all examples shown and discussed herein, any specific values are to be interpreted as merely exemplary and not limiting. Thus, other examples of the exemplary embodiments can have different values. It is noted that like numbers and letters refer to like elements throughout the several views of the drawings, and thus, once an element is defined in one drawing that it is not necessary to discuss it further in subsequent drawings.

[0022] The present application provides a kind of based on load control sheet bidirectional tensile stress instability determination method, based on the evolution law of sheet bidirectional tensile, the cross-regional deformation characteristics of bidirectional tensile test is associated with the complex stress instability characteristics of material, the failure characteristics of material in forming process are analyzed based on the part forming condition, accurately determine the analysis result of sheet in part forming, greatly improve the efficiency of component design and manufacture, significantly reduce the cost of manpower and material resources, its specific method and step are: 1, according to the test demand, determine the bidirectional tensile test proportion of material under load or stress control mode, prepare not less than the tensile proportion quantity of bidirectional tensile sample, determine the contour shape of the overlapping area of the two tensile axis directions of bidirectional tensile sample as rectangle, if the contour of the overlapping area is irregular shape, then do the inscribed rectangle to the sample center with the irregular shape contour, then continue to do the inscribed rectangle to the sample center with 45-55% of the length and width of rectangle as the benchmark, finally form the contour shape of the inner and outer two rectangles in the overlapping area of sample; 2, paste mark point in the overlapping area of sample determined in step 1, paste outer contour mark point 1-4 on the outer contour rectangle edge line of sample overlapping area respectively, while ensuring that outer contour mark point 1-4 is on the axis of four tensile arm intersection of sample, then paste inner contour mark point 5-8 on the inner contour rectangle edge line of sample overlapping area respectively, while ensuring that inner contour mark point 5-8 is on the axis of four tensile arm intersection of sample, finally paste center mark point 9 on the center point of sample overlapping area, while ensuring that center mark point 9 is on the intersection point of two tensile axis of sample; 3, according to the tensile proportion determined in step 1, start bidirectional tensile test in load or stress control mode, respectively output four tensile direction outer contour edge line strain-time curve, inner contour edge line strain-time curve; 4. Calculate the difference between the strain of the inner contour edge and the strain of the outer contour edge obtained in step 3 in the four tensile directions. When the strain difference between the two reaches 0.8-1.2% of the maximum strain of the inner contour edge, and the strain of the inner contour edge generated at the current moment has reached 9.5-10.5% of its maximum strain, it is considered that the material has become unstable at the current moment.

[0023] 5. If the strain difference between the inner and outer contour line strains reaches 0.8-1.2% of the maximum strain of the inner contour line, and the current strain of the inner contour line is less than 9.5-10.5% of the maximum strain of the inner contour line, then after the inner contour line strain reaches 9.5-10.5% of its maximum strain, when the strain difference between the inner and outer contour line strains reaches 0.8-1.2% of the maximum strain of the inner contour line, the material becomes unstable at the current moment. This process is repeated to obtain the instability time of the material in each tensile direction. 6. Compare the material obtained in step 4 at the instability time in the four tensile directions. Based on the inner contour edge tension axis where the earliest instability occurs, determine the stress corresponding to the two tension axes in the four tensile directions of the inner contour edge at the current moment as the instability stress of the material at the current tension ratio. 7. Following the steps above, continue to conduct biaxial tensile tests at other ratios using load or stress control methods to obtain the instability stress of the material under different tensile ratios. Input the obtained instability stress into the finite element software. When the biaxial stress during finite element analysis is lower than the instability stress, the sheet metal forming is safe; otherwise, the sheet metal will be at risk of failure.

[0024] Example 1: 1. Based on the test requirements, the tensile ratio of DP780 steel under load control mode is determined to be 4:1. At least one biaxial tensile specimen is prepared according to the tensile ratio. The outline shape of the overlapping area of ​​the two tensile axes of the biaxial tensile specimen is determined to be an irregular shape. An inscribed rectangle is made towards the center of the specimen with the irregular shape outline. Then, based on 50% of the length and width of the rectangle, an inscribed rectangle is made towards the center of the specimen. Finally, the outline shapes of the inner and outer rectangles are formed in the overlapping area of ​​the specimen. 2. Marking points are affixed inside the overlapping area of ​​the specimen as determined in step 1. Outer contour marking points 1-4 are affixed to the outer rectangular edge of the overlapping area, ensuring that these points are on the intersecting axes of the four tensile arms of the specimen. Then, inner contour marking points 5-8 are affixed to the inner rectangular edge of the overlapping area, again ensuring that these points are on the intersecting axes of the four tensile arms. Finally, center marking point 9 is affixed to the center point of the overlapping area, ensuring that it is at the intersection of the two tensile axes of the specimen. Figure 1 As shown; 3. Start biaxial tensile test in load control mode according to the stretch ratio determined in step 1, and output the outer contour line strain-time curve and the inner contour line strain-time curve of the four stretching directions respectively, as shown in FIG. 8; Figure 2 4. Calculate the difference between the inner contour line strain and the outer contour line strain in the four stretching directions obtained in step 3. When the strain difference of 9-5 and 9-1 reaches 1.2% of the maximum strain of the inner contour line, and the inner contour line strain generated at the current time reaches 10.5% of the maximum strain, it is considered that the material loses stability at the current time 363.3 seconds.

[0025] 5. When the strain difference between the inner contour line strain and the outer contour line strain of 9-7 and 9-3 reaches 1.2% of the maximum strain of the inner contour line, and the inner contour line strain generated at the current time is less than 10.5% of the maximum strain of the inner contour line, when the strain difference between the inner contour line strain and the outer contour line strain reaches 1.2% of the maximum strain of the inner contour line after the inner contour line strain reaches 10.5% of the maximum strain, the material loses stability at the current time 319.7 seconds. In this way, the instability time of the material in each stretching direction 9-6, 9-2 and 9-8, 9-4 is obtained, as shown in Table 1; Table 1 Instability time acquisition (DP780)

[0026] 6. Compare the instability time of the material in the four stretching directions obtained in step 4, and determine the stress corresponding to the two stretching axes of the inner contour line in the four stretching directions at the current time 319.7 seconds as the instability stress of the material at the current stretching ratio according to the inner contour line stretching axis 9-7 which loses stability earliest, which are 632.37 MPa and 159.08 MPa respectively; 7. Continue to carry out biaxial tensile test in load control mode at other ratios according to the above steps, and the instability stress of the material under different stretching ratios can be obtained. When the biaxial stress in the finite element analysis is lower than the instability stress, the sheet forming is safe, otherwise, the sheet will have a risk of failure.

[0027] Example 2: 1. According to the test requirements, the biaxial tensile test stretch ratio of DP590 steel under stress control mode is 8:1. At least one biaxial tensile sample is prepared according to the number of stretch ratios, and the contour shape of the overlapping area of the two stretching axes of the biaxial tensile sample is determined as a rectangle. Take the inner tangent rectangle to the center of the sample based on 50% of the length and width of the rectangle, and finally form the contour shape of the inner and outer two rectangles in the overlapping area of the sample; ​2. Paste the mark points in the sample overlap area determined in step 1, paste the outer contour mark points 1-4 on the outer contour rectangular edge line of the sample overlap area respectively, while ensuring that the outer contour mark points 1-4 are on the axis where the four stretching arms of the sample intersect, then paste the inner contour mark points 5-8 on the inner contour rectangular edge line of the sample overlap area respectively, while ensuring that the inner contour mark points 5-8 are on the axis where the four stretching arms of the sample intersect, and finally paste the center mark point 9 on the center point of the sample overlap area, while ensuring that the center mark point 9 is on the intersection of the two stretching axes of the sample, as shown in FIG. 3. Figure 3 3. Start the biaxial stretching test in a stress control mode according to the stretching ratio determined in step 1, and output the outer contour edge line strain-time curve and the inner contour edge line strain-time curve in four stretching directions respectively. 4. Calculate the difference between the inner contour edge line strain and the outer contour edge line strain in four stretching directions obtained in step 3, when the strain difference of 9-5 and 9-1 reaches 0.8% of the maximum strain of the inner contour edge line, and the inner contour edge line strain generated at the current time reaches 9.5% of the maximum strain, it is considered that the material loses stability at the current time 479.6 seconds.

[0028] 5. When the strain difference between the inner contour edge line strain and the outer contour edge line strain of 9-7 and 9-3 reaches 0.8% of the maximum strain of the inner contour edge line, the inner contour edge line strain generated at the current time is less than 9.5% of the maximum strain of the inner contour edge line, then after the inner contour edge line strain reaches 9.5% of the maximum strain, when the strain difference between the inner contour edge line strain and the outer contour edge line strain reaches 0.8% of the maximum strain of the inner contour edge line, the material loses stability at the current time 468.1 seconds. In this way, the instability time of the material in each stretching direction 9-6, 9-2 and 9-8, 9-4 is obtained, as shown in Table 2. Table 2 Instability time acquisition (DP590)

[0029] 6. Compare the instability time of the material in four stretching directions obtained in step 4, and determine the stress corresponding to the two stretching directions of the inner contour edge line four axes at the current time 414.2 seconds as the instability stress of the material at the current stretching ratio according to the inner contour edge line stretching axis 9-8 where instability occurs earliest, which are 551.66 MPa and 182.12 MPa respectively. 7. Continue to carry out biaxial stretching tests in a stress control mode at other ratios according to the above steps, and the instability stress of the material under different stretching ratios can be obtained, and when the biaxial stress in the finite element analysis is lower than the instability stress, the sheet forming is safe, otherwise, the sheet will have a risk of failure.

[0030] ​It should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions recorded in the above embodiments can be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for determining biaxial tensile stress instability of sheet metal based on load control, characterized in that, Includes the following steps: Determine the proportion of biaxial tensile tests of the material under load or stress control according to the test requirements, and prepare no less than the number of biaxial tensile specimens of the tensile proportion; determine the outline shape of the overlapping area of ​​the two tensile axes of the biaxial tensile specimen as a rectangle or a shape regularized by inscribed rectangle, and then continue to make inscribed rectangles towards the center of the specimen with 45-55% of the length and width of the rectangle as the reference, so as to form the outline shape of two rectangles, inner and outer, in the overlapping area of ​​the specimen. Paste markers on the inner and outer contour lines and at the center point; Start the biaxial tensile test using load or stress control, and output the strain-time curves of the outer and inner contour lines in the tensile direction, respectively. The difference in strain between the inner and outer contour edges is calculated based on the strain-time curve. When the strain difference reaches 0.8-1.2% of the maximum strain of the inner contour edge and the strain of the inner contour edge reaches 9.5-10.5% of its maximum strain, the instability moment is determined. By comparing the instability determination time in each tensile direction, the stress corresponding to the tensile axis where instability occurs earliest is taken as the instability stress of the material at the current tensile ratio. The instability stress is input into the finite element software to determine the safety of sheet metal forming.

2. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, The method for forming the inner and outer contours is as follows: If the outline of the overlapping area of ​​the sample is irregular, then an inscribed rectangle is made towards the center of the sample, and the inscribed rectangle is continued with 45-55% of the length and width of the rectangle as a reference, forming two rectangular outlines, one inside and one outside.

3. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, The method for pasting the marker points is as follows: Paste outer contour markers 1-4 on the outer contour rectangle edge line, ensuring that they are located on the axis where the four tensile arms of the specimen intersect; Paste inner contour markers 5-8 on the inner contour rectangle edge line, ensuring that they are located on the axis where the four tensile arms of the specimen intersect; Affix center marker 9 to the center of the overlapping area of ​​the specimen, ensuring that it is located at the intersection of the two tensile axes.

4. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, The criteria for determining the moment of instability are as follows: If the difference between the strain of the inner contour edge and the strain of the outer contour edge reaches 0.8-1.2% of the maximum strain of the inner contour edge, and the current strain of the inner contour edge reaches 9.5-10.5% of its maximum strain, then the material is determined to be unstable. If the current strain of the inner contour edge is less than 9.5-10.5% of its maximum strain, then when the difference between the strain of the inner contour edge and the strain of the outer contour edge reaches 0.8-1.2% after the strain of the inner contour edge reaches 9.5-10.5% of its maximum strain, the material is judged to be unstable.

5. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, The method for determining the instability stress is as follows: compare the instability times of the material in four tensile directions, and take the stress corresponding to the tensile axis where instability occurs earliest as the instability stress.

6. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, The method further includes obtaining the instability stress of the material under different ratios through biaxial tensile tests with different tensile ratios.

7. The method for determining biaxial tensile stress instability of sheet metal based on load control according to claim 1, characterized in that, In the aforementioned finite element software, the criteria for determining the safety of sheet metal forming are as follows: When the bidirectional stress in the simulation analysis is lower than the instability stress, it is considered safe; otherwise, it is considered to have a risk of failure.

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