A sheet metal bidirectional tensile strain instability judgment method using strain control

Abstract

This invention provides a strain-controlled method for determining strain instability in biaxial tensile testing of sheet metal, relating to the field of biaxial tensile technology. The method includes: determining the tensile ratio for the biaxial tensile test of the material; preparing a biaxial tensile specimen; determining the outline shape of the overlapping area of ​​the two tensile axes of the specimen to be circular or a shape regularized by inscribed circles; marking the intersection points of the four tensile axis axes, the intersection points of the two diagonal directions, and the center point on the outline; starting the biaxial tensile test in strain-controlled mode; outputting the displacement-time curves of the marked points in the tensile and diagonal directions; calculating the average displacement increment of the marked points in the diagonal directions before 50% of the total test time, and determining the instability moment when the displacement increment exceeds the average value; comparing the instability moments in each diagonal direction, using the strain in the tensile direction corresponding to the earliest instability direction as the instability strain under the current tensile ratio; and inputting the instability strain into finite element software to determine the sheet metal forming safety.

Description

Technical Field

[0001] This invention relates to the field of biaxial tensile technology, and more particularly to a method for determining strain instability in biaxial tensile sheet metal using strain control. Background Technology

[0002] In the design of automotive parts forming processes, the mechanical properties of the materials need to be input into finite element method (FE) forming analysis software. The forming compatibility between the material and the part is analyzed based on the actual performance of the sheet metal during forming simulation. If defects such as cracking or wrinkling appear in the sheet metal during FE analysis, the forming performance of the current material is considered insufficient, requiring replacement with a material with higher forming performance. However, failure determination in forming simulation analysis depends on the input material mechanical properties. If the input mechanical properties do not match the actual forming conditions of the sheet metal, the forming simulation analysis results will inevitably contain misjudgments. While strain-controlled biaxial tensile testing can construct the same working conditions as sheet metal forming, even materials with good plasticity can still experience instantaneous fracture upon sheet metal failure. This makes it difficult to identify the process from initial loading to uniform deformation, and from deformation instability to fracture failure. Therefore, developing a strain-controlled biaxial tensile strain instability determination method for sheet metal is crucial.

[0003] Currently, the industry typically uses uniaxial tensile tests and punch bulging tests to approximate the actual forming conditions of sheet metal when analyzing the forming of parts. However, uniaxial tensile tests differ significantly from the biaxial deformation conditions of sheet metal forming. While punch bulging tests involve biaxial deformation, they apply additional bending stress and friction to the sheet metal, resulting in a significant difference from actual conditions. Although strain-controlled biaxial tensile tests can ensure consistency between the test conditions and the sheet metal forming conditions, they fail to identify the complex deformation patterns of the sheet metal during the test, leading to a "brittle fracture" maximum fracture strain that is significantly higher than the actual sheet metal performance. Therefore, using sheet metal properties obtained through these traditional testing methods for forming simulation analysis inevitably results in significant analytical errors, severely impacting the design and manufacturing processes of part forming and failing to meet the application needs of engineering practice and scientific research.

[0004] In summary, the purpose of this invention is to improve the analytical accuracy of component forming simulation, accurately identify various potential defects in the sheet metal forming process, and propose a method that can maintain a high degree of consistency with the actual sheet metal forming conditions. This method combines the strain-controlled biaxial tensile test mechanism with the coordinated deformation law of the sheet metal to identify the instability characteristics of the sheet metal under complex working conditions, thus widely meeting the actual engineering needs of a strain-controlled biaxial tensile strain instability determination method for sheet metal. Summary of the Invention

[0005] Traditional experimental methods for analyzing sheet metal properties used in forming simulations often result in significant analytical errors, severely impacting the design and manufacturing of parts and failing to meet the application requirements of engineering and research. This invention addresses this problem by providing a strain-controlled method for determining strain instability in biaxial tensile testing of sheet metal. This invention utilizes strain control to conduct biaxial tensile tests to construct complex forming conditions for sheet metal. By capturing the coordinated deformation patterns of the sheet metal in different tensile directions, it characterizes its true forming properties. This solves the technical challenges of identifying strain instability characteristics and large strain errors in "brittle fracture" during strain-controlled biaxial tensile testing, significantly improving the accuracy of analysis and judgment in part forming simulations and broadly meeting the application requirements of engineering and research.

[0006] The technical means employed in this invention are as follows: A method for determining biaxial tensile strain instability of sheet metal using strain control includes the following steps: Based on the experimental requirements, determine the tensile ratio of the material in the biaxial tensile test under strain control, prepare the biaxial tensile specimen, determine the outline shape of the overlapping area of ​​the two tensile axes of the specimen to be a circle or the shape after regularization by inscribed circle, and mark the intersection of the four tensile axis axes, the intersection of the two diagonal directions and the center point on the outline. Marking points are pasted at the intersection of the axes, the intersection of the diagonals, and the center point of the outline; Based on the stretching ratio, a biaxial tensile test is started in strain control mode, and the strain-time curves of the marked points in the stretching direction and the displacement-time curves of the marked points in the diagonal direction are output. Calculate the average displacement increment of the marked points along the diagonal direction before 50% of the total test time, and determine the moment when the displacement increment exceeds the average displacement increment as the moment of instability. By comparing the instability moments in each diagonal direction, the strain in the tensile direction corresponding to the earliest instability direction is taken as the instability strain under the current tensile ratio. The instability strain is input into the finite element software to determine the safety of sheet metal forming.

[0007] Furthermore, when the outline of the biaxial tensile specimen is circular, the intersection of the four tensile direction axes and the two 45-degree diagonals with the circular outline is directly used as the mark point position; if the outline is irregular, the mark point position is determined by regularization processing through inscribed circles.

[0008] Furthermore, the marker points include: Marker points 1-4: located at the intersections of the circular profile and the four extrusion direction axes; Marker points 5-8: located at the intersection of the circular outline and the two 45-degree diagonals; Marker 9: Located at the center of the circular profile and at the intersection of the two stretching axes.

[0009] Furthermore, the determination of the instability moment specifically includes: If the displacement increment after 50% of the total test time exceeds the average displacement increment before 50%, then the moment of the first exceedance shall be taken as the moment of instability. If the displacement increment after 50% of the total test time does not exceed the average displacement increment before 50%, then the moment when it first exceeds the average displacement increment before 50% of the total test time is determined as the instability moment.

[0010] Furthermore, the method for determining the instability strain is as follows: taking the time corresponding to the diagonal direction of the earliest instability, extract the strain values ​​of two tensile axes in the four tensile directions at that time.

[0011] Furthermore, the method also includes repeating biaxial tensile tests with different stretching ratios to obtain the instability strain values ​​of the material under multiple stretching ratios.

[0012] Furthermore, in the finite element software, the criteria for determining the safety of sheet metal forming are as follows: When the bidirectional strain in the simulation analysis is lower than the instability strain corresponding to the stretching ratio, it is considered safe; otherwise, it is considered to have a risk of failure.

[0013] Compared with the prior art, the present invention has the following advantages: This invention discloses a strain-controlled method for determining strain instability in biaxial tensile sheet metal. By conducting biaxial tensile tests using strain control, the method constructs complex forming conditions for sheet metal and characterizes its true forming properties by capturing the coordinated deformation law of the sheet metal in different tensile directions. This method solves the technical problems of difficulty in identifying strain instability characteristics and large strain errors in "brittle fracture" during biaxial tensile tests using strain control. It significantly improves the analysis and judgment accuracy of component forming simulation and widely meets the application requirements of engineering practice and scientific research. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of regular shape marker points in this embodiment.

[0016] Figure 2 This is a strain-time curve of the marked points in this embodiment.

[0017] Figure 3 This is a schematic diagram of irregularly shaped marker points in this embodiment. Detailed Implementation

[0018] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0021] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of the invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0022] This invention provides a method for determining the biaxial tensile strain instability of sheet metal using strain control. The purpose of this invention is to improve the analytical accuracy of component forming simulation, accurately identify various potential defects in the sheet metal forming process, and propose a method that can maintain a high degree of consistency with the actual forming conditions of sheet metal. This method combines the biaxial tensile test mechanism of strain control with the coordinated deformation law of sheet metal to identify the instability characteristics of sheet metal under complex working conditions, thus widely meeting the actual needs of engineering.

[0023] Therefore, the technical solution adopted by this invention is: a method for determining the biaxial tensile strain instability of sheet metal using strain control. Based on the biaxial tensile mechanism of strain control, the complex working conditions during sheet metal forming are associated with the biaxial deformation instability characteristics of the sheet metal. By constructing test conditions consistent with the actual forming of the parts, the instability characteristics of the sheet metal are identified to accurately determine the forming analysis process of the parts, which greatly improves the efficiency of part forming process design and manufacturing analysis. The specific method and steps are as follows: 1. Determine the tensile ratio of the material in the strain-controlled biaxial tensile test according to the experimental requirements. Prepare at least the required number of biaxial tensile specimens. Determine that the outline shape of the overlapping area of ​​the two tensile axes of the biaxial tensile specimen is circular. If the outline of the overlapping area is irregular, draw an inscribed circle from the irregular outline toward the center of the specimen, forming four intersection points with the two axes of the four tensile directions. Then, using the intersection points of the two axes of the four tensile directions as the center, draw two 45-degree diagonals 1 and 45-degree diagonals 2 clockwise from the two axes, intersecting with the inscribed circle to form four intersection points. Finally, determine eight intersection points on the circular outline. 2. On the circular outline of the specimen determined in step 1, mark points 1 to 4 at the intersections of the circular outline of the specimen with the four tensile axis lines. Then mark points 5 to 8 at the intersections of the circular outline of the specimen with diagonal 1 and diagonal 2. Finally, mark point 9 at the center point of the circular outline of the specimen, while ensuring that the center mark point 9 is at the intersection of the two tensile axes of the specimen. 3. Based on the tensile ratio determined in step 1, start the biaxial tensile test in strain control mode, and output the strain-time curves of the marker points in the four tensile directions and the displacement-time curves of the marker points in the four diagonal directions respectively. 4. Calculate the average displacement increment of the marker points obtained in step 3 before the total time of the biaxial tensile test reaches 50%, and then determine the moment when the displacement increment of the marker points exceeds the average displacement increment as the instability moment from the displacement increment of the marker points after the total time of the biaxial tensile test reaches 50%. 5. If the displacement increment of the marked points after 50% of the total time of the biaxial tensile test does not exceed the average displacement increment, then the moment when the displacement increment of the marked points before 50% of the total time of the biaxial tensile test exceeds the average displacement increment is determined as the instability moment; and so on, to obtain the instability moment of the material in the four diagonal directions. 6. Compare the material's instability moments in the four diagonal directions obtained in step 4. Using the diagonal direction where instability first occurs as a reference, determine the strain corresponding to the two tensile axes in the four tensile directions at the current moment as the instability strain of the material at the current tensile ratio. 7. Continue to conduct biaxial tensile tests at other ratios using the strain control method following the above steps to obtain the instability strain of the material at different tensile ratios. Input the obtained instability strain into the finite element software. When the biaxial strain in the finite element analysis is lower than the instability strain, the sheet metal forming is safe; otherwise, the sheet metal will be at risk of failure.

[0024] Example 1: 1. Based on the experimental requirements, determine the tensile ratio of TRIP780 steel for the biaxial tensile test under strain control as 5:1. Prepare at least one biaxial tensile specimen according to the tensile ratio. Determine the outline shape of the overlapping area of ​​the two tensile axes of the biaxial tensile specimen as a circle, forming four intersection points with the two axes of the four tensile directions. Then, using the intersection points of the two axes of the four tensile directions as the center, draw two 45-degree diagonals 1 and 45-degree diagonals 2 clockwise on the two axes respectively, intersecting with the inscribed circle to form four intersection points. Finally, determine eight intersection points on the circular outline. 2. Marking points are affixed to the circular outline of the specimen as determined in step 1. Marking points 1-4 are affixed at the intersections of the circular outline with the four tensile axes. Marking points 5-8 are affixed at the intersections of the circular outline with diagonals 1 and 2. Finally, center marking point 9 is affixed at the center point of the circular outline, ensuring that center marking point 9 is at the intersection of the two tensile axes of the specimen. Figure 1 As shown; 3. Based on the tensile ratio determined in step 1, start the biaxial tensile test in strain control mode, and output the strain-time curves of the marked points in the four tensile directions and the displacement-time curves of the marked points in the four diagonal directions, as follows: Figure 2 As shown; 4. The average displacement increment of the marker points in the diagonal direction 9-5 obtained in step 3 before the total time of the biaxial tensile test reaches 50% is 0.00055mm. Then, the moment when the displacement of the marker points exceeds the average displacement value of 295 seconds after the total time of the biaxial tensile test reaches 50% is determined as the instability moment. 5. If the displacement of the marked points after 50% of the total time of the biaxial tensile test has not exceeded the average displacement, then the moment when the displacement of the marked points before 50% of the total time of the biaxial tensile test exceeds the average displacement is determined as the instability moment; and so on, to obtain the instability moments of the material in the four diagonal directions 9-5, 9-6, 9-7, and 9-8, as shown in Table 1. Table 1. Instability time acquisition (TRIP780)

[0025] 6. Compare the material obtained in step 4 at the instability time in the four diagonal directions. Taking the diagonal direction 9-5 where the earliest instability occurred as a reference, determine the strains 0.137 and 0.033 corresponding to the two tensile axes in the four tensile directions at the current moment as the instability strains of TRIP780 at the current tensile ratio of 5:1. 7. Continue to conduct biaxial tensile tests at other ratios using the strain control method following the above steps to obtain the instability strain of the material at different tensile ratios. Input the obtained instability strain into the finite element software. When the biaxial strain in the finite element analysis is lower than the instability strain, the sheet metal forming is safe; otherwise, the sheet metal will be at risk of failure.

[0026] Example 2: 1. Based on the experimental requirements, determine the tensile ratio of DH590 steel for the biaxial tensile test under strain control as 3:1. Prepare at least one biaxial tensile specimen according to the tensile ratio. The outline shape of the overlapping area of ​​the two tensile axes of the current biaxial tensile specimen is rectangular. Draw an inscribed circle from the rectangular outline toward the center of the specimen, forming four intersection points with the two axes of the four tensile directions. Then, using the intersection points of the two axes of the four tensile directions as the center, draw two 45-degree diagonals 1 and 45-degree diagonals 2 clockwise on the two axes respectively, intersecting with the inscribed circle to form four intersection points. Finally, determine 8 intersection points on the circular outline. 2. Marking points are affixed to the circular outline of the specimen as determined in step 1. Marking points 1-4 are affixed at the intersections of the circular outline with the four tensile axes. Marking points 5-8 are affixed at the intersections of the circular outline with diagonals 1 and 2. Finally, center marking point 9 is affixed at the center point of the circular outline, ensuring that center marking point 9 is at the intersection of the two tensile axes of the specimen. Figure 3 As shown; 3. Based on the tensile ratio determined in step 1, start the biaxial tensile test in strain control mode, and output the strain-time curves of the marker points in the four tensile directions and the displacement-time curves of the marker points in the four diagonal directions respectively. 4. The average displacement increment of the marker points in the diagonal direction 9-5 obtained in step 3 before the total time of the biaxial tensile test reaches 50% is 0.00068 mm. Then, the moment when the displacement of the marker points exceeds the average displacement value of 356.2 seconds after the total time of the biaxial tensile test reaches 50% is determined as the instability moment. 5. If the displacement of the marked points after 50% of the total time of the biaxial tensile test does not exceed the average displacement, then the moment when the displacement of the marked points before 50% of the total time of the biaxial tensile test exceeds the average displacement is determined as the instability moment; and so on, to obtain the instability moments of the material in the four diagonal directions 9-5, 9-6, 9-7, and 9-8, as shown in Table 2. Table 2. Instability time acquisition (DH590)

[0027] 6. Compare the material obtained in step 4 at the instability time in the four diagonal directions. Taking the diagonal direction 9-5 where the earliest instability occurred as a reference, determine the strains 0.148 and 0.041 corresponding to the two tensile axes in the four tensile directions at the current moment as the instability strains of DH590 at the current tensile ratio of 3:1. 7. Continue to conduct biaxial tensile tests at other ratios using the strain control method following the above steps to obtain the instability strain of the material at different tensile ratios. Input the obtained instability strain into the finite element software. When the biaxial strain in the finite element analysis is lower than the instability strain, the sheet metal forming is safe; otherwise, the sheet metal will be at risk of failure.

[0028] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining biaxial tensile strain instability of sheet metal using strain control, characterized in that, Includes the following steps: Based on the experimental requirements, determine the tensile ratio of the material in the biaxial tensile test under strain control, prepare the biaxial tensile specimen, determine the outline shape of the overlapping area of ​​the two tensile axes of the specimen to be a circle or the shape after regularization by inscribed circle, and mark the intersection of the four tensile axis axes, the intersection of the two diagonal directions and the center point on the outline. Marking points are pasted at the intersection of the axes, the intersection of the diagonals, and the center point of the outline; The marker points include: Marker points 1-4: located at the intersections of the circular profile and the four extrusion direction axes; Marker points 5-8: located at the intersection of the circular outline and the two 45-degree diagonals; Marker point 9: Located at the center of the circular profile and at the intersection of the two stretching axes; Based on the stretching ratio, a biaxial tensile test is started in strain control mode, and the strain-time curves of the marked points in the stretching direction and the displacement-time curves of the marked points in the diagonal direction are output. Calculate the average displacement increment of the marked points along the diagonal direction before 50% of the total test time, and determine the moment when the displacement increment exceeds the average displacement increment as the moment of instability. By comparing the instability moments in each diagonal direction, the strain in the tensile direction corresponding to the earliest instability moment is taken as the instability strain under the current tensile ratio. The instability strain is input into the finite element software to determine the safety of sheet metal forming.

2. The method for determining biaxial tensile strain instability of sheet metal using strain control according to claim 1, characterized in that, When the outline of the biaxial tensile specimen is circular, the intersection of the four tensile direction axes and the two 45-degree diagonals with the circular outline is directly used as the mark point position; if the outline is irregular, the mark point position is determined by regularization processing through inscribed circles.

3. The method for determining biaxial tensile strain instability of sheet metal using strain control according to claim 1, characterized in that, The determination of the instability moment specifically includes: If the displacement increment after 50% of the total test time exceeds the average displacement increment before 50%, then the moment of the first exceedance shall be taken as the moment of instability. If the displacement increment after 50% of the total test time does not exceed the average displacement increment before 50%, then the moment when it first exceeds the average displacement increment before 50% of the total test time is determined as the instability moment.

4. The method for determining biaxial tensile strain instability of sheet metal using strain control according to claim 1, characterized in that, The method for determining the instability strain is as follows: taking the time corresponding to the diagonal direction of the earliest instability, extract the strain values ​​of two tensile axes in the four tensile directions at that time.

5. The method for determining biaxial tensile strain instability of sheet metal using strain control according to claim 1, characterized in that, The method also includes repeating biaxial tensile tests with different stretching ratios to obtain the instability strain values ​​of the material under multiple stretching ratios.

6. The method for determining biaxial tensile strain instability of sheet metal using strain 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 strain in the simulation analysis is lower than the instability strain corresponding to the stretching ratio, it is considered safe; otherwise, it is considered to have a risk of failure.

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