Bending test method

Abstract

The invention discloses a bending test method, which adopts a comprehensive technical solution of'pressure heads with different bending radiuses, fixed stroke loading, crack existence judgment and grid division with different scale distances in a sample bending ridge area ', so as to accurately measure limit cold bending angles and resilience values under different bending radiuses and surface bending fracture strains under different scale distances. And quantitative evaluation of the bending performance of the material is realized. Based on conventional test equipment and a clamping device, the test can be simply and efficiently completed on the basis that the test cost is not increased, and important engineering significance and wide engineering application prospects are achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of mechanical property testing of metal materials, and particularly relates to a test method for determining the bending angle, springback amount and surface bending fracture strain of a metal material. BACKGROUND

[0002] With the continuous improvement of the whole vehicle collision safety standards, in order to cope with the technical challenges brought by safety regulations, the proportion of application of advanced high-strength steel in the lightweight design of automobile body is increasing, especially the large use of hot-formed parts. However, in the automobile collision, such as the bending deformation of A-pillar and B-pillar, it is a typical plane strain state failure under bending deformation, and the higher the material strength, the greater the risk of material failure. Therefore, the bendability of the material has become one of the key indicators for measuring the mechanical properties of advanced high-strength steel.

[0003] How to evaluate the bending resistance of advanced high-strength steel, especially ultra-high-strength steel? At present, the internationally widely used technical standard is VDA238-100 technical standard (hereinafter referred to as "VDA standard"), which measures the limit sharp cold bending angle to evaluate the bending resistance of the material, especially it is an effective technical means for evaluating hot-formed materials.

[0004] However, there are at least two problems in the VDA standard: first, the test termination condition, stop loading until the pressure value reaches the closing threshold value, which means that the pressure head drops by 30N or 60N after reaching the maximum load; second, the measurement result, the limit sharp cold bending angle is a physical quantity, not an intrinsic quantity of the material. If the purpose of the test is only to qualitatively evaluate the bending resistance of the material, the VDA standard can meet the technical requirements. If the purpose of the test is to quantitatively evaluate the bending resistance of the material, the VDA standard is difficult to meet.

[0005] For example, for the measurement result, people hope to get the surface bending fracture strain under the limit sharp bending, therefore, on the basis of VDA standard, the numerical image correlation method technology, referred to as DIC technology, is introduced to measure the strain distribution of the bending surface of the sample, which is a common technical solution, as detailed in the reference (Evaluation of the VDA238-100 Tight Radius Bending Test using Digital Image Correlation Strain Measurement, doi:10.1088 / 1742-6596 / 896 / 1 / 012075), but there are still technical problems, first, in order to cooperate with the DIC technology to measure the bending surface of the sample, a new test system needs to be developed, which obviously greatly increases the cost of test equipment; second, using DIC technology, a layer of thick speckle needs to be sprayed on the surface of the sample, since the sample and the speckle are two materials with different properties, generally speaking, the matrix material has failed, but the speckle is still complete, or in other words, when the crack is observed from the speckle, the matrix material has already failed, which will lead to the bending surface fracture strain measured by DIC technology being larger than the actual value.

[0006] For example, in the patent document (application number 2020105503958, a bending failure strain measurement method), a laser marking method is used to make mark lines with a spacing of 0.5 mm and a line width of 0.1 mm from the middle of the sample to both sides. The inventor believes that laser marking will artificially introduce surface defects in the bending area, and when bending deformation occurs, the crack will easily expand along the mark line, which will lead to the measured bending surface fracture strain being smaller than the actual value.

[0007] At the same time, the VDA standard does not involve the evaluation of material resilience, which uses theoretical calculation to obtain the bending angle before unloading as the test result through four parameters of support roller radius, sample thickness, punch displacement and support roller distance, and does not pay attention to the bending angle or resilience after resilience.

[0008] Therefore, the inventor of the present application is aware of the above technical defects in the VDA standard and its existing technical solutions, and deeply understands the test requirements, and proposes the present application, which aims to realize the quantitative evaluation of the bending resistance of the material based on the existing technical conditions.

[0009] Improvement of VDA238-100 test method

[0010] In order to overcome the technical defects of the current VDA standard, the inventor comprehensively uses "a set of different bending radius punches + fixed stroke loading + crack determination", to realize the accurate determination of the limit sharp bending angle, which is specified as follows:

[0011] First, a set of different bending radius indenter. In the actual engineering problem, it is necessary to know the minimum bending radius of the material that can achieve 180° bending, and for the bending radius that cannot achieve 180° bending, it is necessary to know how much the bending angle can be achieved. In the existing VDA standard, two bending radii of 0.2mm and 0.4mm are specified for different thicknesses, which cannot meet the needs of engineering practice. Therefore, the improved VDA method proposes to conduct comprehensive testing on a set of different bending radius indenter.

[0012] Second, fixed stroke loading. According to the closing threshold of VDA standard (referring to the indenter descending 30N or 60N after reaching the maximum load), does it exactly correspond to the critical state of material failure? Is it suitable for all advanced high-strength steels? Can it distinguish different components of the same brand? There is significant uncertainty. At the same time, the limit cold bending angle calculated based on the loading displacement corresponding to the maximum load does not have a corresponding post-bending sample, and the bending sample after the test has a significant bending crack on the surface, which has exceeded the surface critical bending crack. Therefore, the basic idea of the improved VDA method is to determine the "condition" from the "result", that is, to select the crack "result" of the sample bending edge position after the test is completed, to meet the critical penetrating crack, and then directly measure the bending angle after unloading, and if it does not meet the requirement, continue the test until it meets the requirement. Therefore, the test method matched with the improved VDA method needs to be changed, that is, the fixed stroke loading automatic termination test replaces the program load determination termination test. The fixed stroke loading refers to the displacement of the indenter motion reaching the set stroke value to automatically terminate the test, and the two are completely equivalent test provisions in the test method.

[0013] Third, crack determination. VDA standard does not determine the crack state of the sample, but critical crack determination is a prerequisite for accurate determination of the limit sharp cold bending angle and bending fracture strain. Therefore, the idea of the improved VDA method is to determine only the "yes" and "no" of the crack, and the "yes" and "no" of the crack state is a qualitative judgment, which can be accurately determined and has strong operability. According to the fixed stroke loading, the bending state of the sample with and without cracks can be easily obtained. However, according to the judgment idea of the crack "yes" and "no", the maximum fixed loading stroke S1 of the sample without cracks will be naturally sought during the test, which is reasonable. At the same time, the fixed loading stroke S2 of the sample with cracks will be increased, which is unreasonable. Therefore, in the improved VDA method, in order to avoid artificially increasing the fixed loading stroke S2, only the relationship between the fixed loading strokes S1 and S2 of the crack "yes" and "no" needs to be limited, such as S2≤S1+0.5mm. Then, the average value of the two is set to carry out the fixed stroke loading bending test, and the obtained sample is the effective sample for measuring the bending angle, and its crack state is not determined.

[0014] Meanwhile, the ultimate cold bending angle measured according to the VDA standard is a physical parameter, not an intrinsic parameter of the material. It can only qualitatively evaluate the bending resistance of the material and cannot provide valuable input parameters for the simulation model. Furthermore, the ultimate cold bending angle is highly correlated with the sample thickness.

[0015] To overcome the technical shortcomings of existing technologies that measure bending fracture strain based on the VDA standard, the inventors have comprehensively utilized a clamping device, multiple sets of scribing blocks, and a scribing pen to draw a grid that can characterize different gauge lengths in the bending edge region of the sample. This enables accurate measurement of surface bending fracture strain, regardless of the sample thickness. Summary of the Invention

[0016] 1. The technical problem solved by the present invention

[0017] The technical problem to be solved by this invention is to adopt a comprehensive technical solution of "indenters with different bending radii + fixed stroke loading + crack presence and absence determination + grids with different gauge lengths for dividing the bending edge area of ​​the sample" to accurately measure the ultimate cold bending angle, springback amount and surface bending fracture strain under different bending radii, so as to achieve quantitative evaluation of the bending performance of materials.

[0018] 2. Technical solution of the present invention

[0019] To achieve the technical problem to be solved by this invention, this invention provides a bending test method, wherein:

[0020] A test method for determining the bending forming limit, characterized by comprising the following steps:

[0021] Step 1: Select a set of pressure heads, wherein the set of pressure heads has different bending radii r;

[0022] Step 2: Prepare a batch of pre-bending samples of the material to be tested;

[0023] Step 3: Select indenters one by one from a set of indenters and perform bending tests on the pre-bent specimens to obtain effective specimens under different bending radii. The bending test adopts "fixed stroke loading and crack presence / absence determination". The fixed stroke loading refers to the loading method in which the indenter stops moving when the set fixed loading stroke is reached. The fixed loading stroke S(r) of the effective specimen is equal to the average fixed loading stroke of two adjacent specimens with "crack" and "no" cracks.

[0024] Step 4: Determine the bending angle α1 of the effective sample before unloading. r or / and the bending angle α2 after unloading r ;

[0025] Step 5: Plot the bending radius r as the abscissa and the bending angle α1 as the ordinate. ror α2 r Using the vertical axis as the ordinate, plot the bending forming limit curve of the tested material.

[0026] A test method for determining bending springback, characterized by comprising the following steps:

[0027] Step 1: Select a set of pressure heads, wherein the set of pressure heads has different bending radii r;

[0028] Step 2: Prepare a batch of pre-bending samples of the material to be tested;

[0029] Step 3: Select indenters one by one from a set of indenters and perform bending tests on the pre-bent specimens to obtain effective specimens under different bending radii. The bending test adopts "fixed stroke loading and crack presence / absence determination". The fixed stroke loading refers to the loading method in which the indenter stops moving when the set fixed loading stroke is reached. The fixed loading stroke S(r) of the effective specimen is equal to the average fixed loading stroke of two adjacent specimens with "crack" and "no" cracks.

[0030] Step 4: Determine the bending angle α1 of the effective sample before unloading. r and the bending angle α2 after unloading r ;

[0031] Step 5: Calculate the springback amount Δα under different bending radii. r The rebound amount refers to the change in bending angle of the effective sample before and after unloading, and its calculation formula is as follows:

[0032] Δα r =α1 r -α2 r (1)

[0033] Where, α1 r It is the bending angle of the effective sample before unloading, α2 r It is the bending angle of the effective sample after unloading;

[0034] Step 6: Plot the bending radius r as the abscissa and the springback amount Δα as the ordinate. r Use the vertical axis to plot the bending springback curve of the tested material.

[0035] Preferably, the test method for determining the bending forming limit or bending springback is characterized in that:

[0036] In step one, the selected set of bending radii r of the pressure heads are: 0.4mm, 0.8mm, 1.2mm, 1.6mm, 2.0mm, 2.5mm, and 3.0mm;

[0037] In step two, a pre-bent specimen is prepared according to the technical requirements of VDA238-100 standard;

[0038] In step three, the indenter applies a load to the pre-bent sample parallel or perpendicular to the rolling direction, and the difference between the fixed loading travel of the two samples with and without cracks is less than or equal to 0.5 mm;

[0039] In step four, the bending angle of the effective sample is measured according to the definition in the VDA238-100 standard.

[0040] A test method for determining gauge effect, characterized by comprising the following steps:

[0041] Step one, selecting a set of indenters, the set of indenters having different bending radii r;

[0042] Step two, preparing a batch of pre-bent samples of the material to be tested;

[0043] Step three, selecting an indenter from the set of indenters one by one, and performing a bending test on the pre-bent sample to obtain an effective sample under different bending radii, the bending test using "fixed travel loading and crack presence / absence determination", the fixed travel loading referring to a loading method in which the indenter stops moving when a set fixed loading travel is reached, and the fixed loading travel S(r) of the effective sample being equal to the average of the fixed loading travels of the two adjacent samples with and without cracks;

[0044] Step four, for each bending radius, preparing a set of pre-bent samples with gauges, the set of gauges referring to a grid drawn on the surface of the pre-bent sample to represent integer gauges of 1 mm, 2 mm,..., n (mm), the grid having a bending ridge as a symmetry axis, the bending ridge being required to be drawn at the center of the pre-bent sample surface, and the length of the edge of the grid perpendicular to the bending ridge being defined as a gauge;

[0045] Step five, for each bending radius, using the fixed loading travel S(r) of the effective sample to perform a bending test on the selected set of pre-bent samples with gauges, and when placing the sample, the bending ridge is required to face away from the indenter and be strictly centered with the indenter;

[0046] Step six, for each bending radius r, measuring the deformation arc length of the gauges in the direction perpendicular to the bending ridge after unloading, and calculating the surface bending strain ε l r n , and the calculation formula is as follows:

[0047]

[0048] wherein, l nL is the gauge length before bending deformation, and is an integer length 1, 2, …, n, L is the gauge length before bending deformation, and is an integer length 1, 2, …, n, n L is the gauge length before bending deformation, and is an integer length 1, 2, …, n,

[0049] Step seven, for the same gauge length L n The maximum bending strain of the deformed bending sample under different bending radii r The surface bending fracture strain under this gauge length

[0050] Step eight, taking the gauge length L n The surface bending fracture strain as the abscissa The surface bending fracture strain as the abscissa, and the bending deformation gauge length effect curve of the measured material as the ordinate.

[0051] Preferably, the test method for determining the gauge length effect is characterized in that:

[0052] In step one, the bending radii r of the selected group of indenters are: 0.4mm, 0.8mm, 1.2mm, 1.6mm, 2.0mm, 2.5mm, 3.0mm;

[0053] In step two, the pre-bending sample is prepared according to the technical requirements of VDA238-100 standard;

[0054] In step three, the indenter maintains parallel or perpendicular to the rolling direction to apply load to all pre-bending samples, and the difference between the fixed load travel of adjacent "crack" and "no crack" samples is less than or equal to 0.5mm.

[0055] Preferably, the test method for determining the gauge length effect is characterized in that: in step four, n is equal to 3, and the grid division method is as follows:

[0056] First, prepare a fixture device that can clamp the cut edges on both sides of the sample, and prepare three groups of marking blocks with widths of: first group: 30mm, 29.5mm, 30.5mm, 29mm, 31mm; second group: 30mm, 29mm, 31mm, 28mm, 32mm; third group: 30mm, 28.5mm, 31.5mm, 27mm, 33mm;

[0057] Second, use the three groups of marking blocks to obtain a group of pre-bending samples with gauge lengths of 1mm, 2mm and 3mm, and the process of dividing the grid on the surface of the sample is as follows:

[0058] If the second group of marking blocks is selected first,

[0059] (a) Select a sample, draw an arrow on the surface of the sample to indicate the rolling direction, place the sample on the cushion, and use the clamping device to clamp the sample, and require that the placement direction of the sample is consistent with the direction of the bending edge line drawn in the next step and the direction of the pressure head acting on the pre-bending sample;

[0060] (b) Select a 30mm wide reticle block and place it close to the fixed end of the clamping device, use a hooking pen to draw two non-penetrating middle line segments on the two ends of the sample or draw a penetrating line segment on the sample as a bending edge line;

[0061] (c) Select a 29mm and 31mm wide reticle block respectively and place it close to the fixed end of the clamping device, and use a hooking pen to draw a penetrating line segment on the surface of the sample;

[0062] (d) Take out the sample from the clamping device, rotate the sample by 90° and clamp it again;

[0063] (e) Select a 28mm, 30mm and 32mm wide reticle block respectively and place it close to the fixed end of the clamping device, and use a hooking pen to draw a penetrating line segment on the surface of the sample to form three square grids on the middle position of the sample surface, which can represent the pre-bending sample with a 2mm gauge length;

[0064] Continue to select another two groups of reticle blocks:

[0065] Repeat the processes of (a) to (e) respectively to obtain pre-bending samples with a 1mm and 3mm gauge length;

[0066] Third step, for each bending radius, repeat the second step, and prepare a group of pre-bending samples with a gauge length of 1mm, 2mm and 3mm.

[0067] Preferably, the test method for determining the gauge length effect is characterized in that, in step six, the arc length measurement method after the gauge length deformation is: after the sample is unloaded, a strip-shaped note paper is used to tightly adhere to the three arcs of the bending deformation sample, and a hooking pen is used to mark the two end points of the arcs on the note paper; then the note paper is flattened, and a vernier caliper is used to measure the distance between the two end points of each arc; and the average value of the three arc lengths is taken as the length of the gauge length after the bending deformation.

[0068] 3、The beneficial effects of the present application

[0069] The present application provides a bending test method, which has the following three main beneficial effects:

[0070] First, the precise measurement of the limit sharp cold bending angle under different bending radii is realized. The fixed stroke loading method is adopted, and the crack judgment method is combined, so that the critical state of the bending crack on the surface of the sample can be accurately obtained, and the bending forming limit curve and the bending springback curve drawn have quantitative guiding significance for part design.

[0071] Second, the precise determination of the surface bending fracture strain under different gauges is realized. The comprehensive technical solution of "indenter with different bending radii + fixed stroke loading + crack judgment + sample bending edge line area different gauge grid drawing" is adopted, so that the surface bending fracture strain under different gauges is accurately determined, and the gauge effect curve drawn can be directly used for setting the material failure parameters of the simulation model and predicting bending failure.

[0072] Third, the deficiencies of the prior art solutions are avoided. Based on the conventional test equipment and clamping device, the test test can be simply and efficiently completed without increasing the test cost, and the operability of the test method and the repeatability of the test result are high.

[0073] In summary, through the implementation of the present application, the technical deficiencies in the traditional test method are solved, and the application space of the bending test is improved, so the present application has important engineering practical significance and broad engineering application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0074] The present application will be further described in detail below with reference to the accompanying drawings.

[0075] Figure 1 is the schematic view of the cross-sectional shape of the indenter with different bending radii of embodiment 1 of the present application;

[0076] Figure 2 is the schematic view of three crack states of embodiment 1 of the present application;

[0077] Figure 3 is the schematic view of the bending forming limit curve of the measured material before and after unloading of embodiment 1 of the present application;

[0078] Figure 4 is the schematic view of the bending springback curve of the measured material of embodiment 2 of the present application;

[0079] Figure 5 is the schematic view of the gauge effect curve of the measured material of embodiment 3 of the present application;

[0080] Figure 6 is the schematic view of the top view and side view cross section of the sample after clamping of embodiment 4 of the present application;

[0081] Figure 7-a is a schematic top view of the clamped state of the reticle block with a width of 30 mm according to embodiment 4 of the present application;

[0082] Figure 7 -b is a schematic top view of the sample after drawing curved edges according to embodiment 4 of the present application;

[0083] Figure 8 -a is a schematic top view of the placed state of the reticle block with a width of 29 mm according to embodiment 4 of the present application;

[0084] Figure 8 -b is a schematic top view of the placed state of the reticle block with a width of 31 mm according to embodiment 4 of the present application;

[0085] Figure 8 -c is a schematic top view of the sample with two through-line segments according to embodiment 4 of the present application;

[0086] Figure 9 is a schematic top view of the sample after rotating 90° according to embodiment 4 of the present application;

[0087] Figure 10 is a schematic top view of the sample with three through-line segments according to embodiment 4 of the present application;

[0088] Figure 11 is a schematic top view of a set of pre-bent samples with a gauge of 1 mm, 2 mm and 3 mm according to embodiment 4 of the present application.

[0089] Explanation of reference signs:

[0090] 1 - clamping fixed end of the clamping device;

[0091] 2 - clamping mobile end of the clamping device;

[0092] 3 - sample;

[0093] 4 - spacer block;

[0094] 5 - base of the clamping device;

[0095] 6 - marking of the rolling direction on the sample;

[0096] 7 - reticle block with a width of 30 mm;

[0097] 8 - curved edge;

[0098] 9 - reticle block with a width of 29 mm;

[0099] 10 - reticle block with a width of 31 mm. DETAILED DESCRIPTION

[0100] The technical solutions in the present application will be described clearly and completely below in combination with the drawings in the present application.

[0101] Example 1

[0102] This example takes a GI plate with a tensile strength of 980 level and a thickness of 1.4 mm as an example, and uses the test equipment under the VDA238-100 technical standard, including the following steps:

[0103] Step one, select a group of indenter bending radii r in turn: 0.2mm, 0.4mm, 0.8mm, 1.5mm, 2.0mm, 2.5mm, 3.0mm indenter, the cross-sectional shape of the indenter with different bending radii is as shown in Figure 1 .

[0104] Step two, according to the technical requirements of VDA238-100 technical standard, prepare a batch of bending samples of the tested material for extreme sharp cold bending test, use laser cutting or wire cutting, the sample size is 60mm×60mm.

[0105] Step three, according to the bending radius of a group of indenter, select the indenter from small to large in turn, and carry out bending test on the pre-bending sample to obtain effective samples under different bending radii. The specific method is: the bending test adopts "fixed stroke loading and crack presence or absence judgment", the fixed stroke loading refers to the loading method that the indenter stops moving when the set fixed loading stroke is reached, the fixed loading stroke S(r) of the effective sample is equal to the average value of the fixed loading strokes of the two samples with and without cracks in the adjacent bending edge line area, and the absolute value of the difference between the two loading strokes is equal to 0.2mm, mark the middle crack state of the effective sample as "OK"(in general, the crack state can not be judged, and "with" can be taken as "OK" in actual operation), the crack state is recommended to be observed with the aid of a stereomicroscope, taking the bending radius of 0.2mm as an example, three kinds of surface bending crack states are shown in Figure 2 ; During the bending test, the indenter is parallel to the rolling direction of the pre-bending sample, and the detailed test data are shown in Table 1. In Table 1, since the bending radius is 2.0mm, the tested material has realized 180° bending without bending crack, and the bending radii of 2.5mm and 3.0mm are cancelled.

[0106] Step four, calculate the bending angle of the effective sample before unloading by using the theoretical formula in VDA238-100 technical standard (or China Automobile Engineering Society group standard: C\SAE 154-2020 extreme sharp cold bending performance test method for ultra-high strength automobile steel plate) , or the bending angle of the effective sample after unloading can also be directly measured This example uses theoretical calculation, at the same time, measures the bending angle of the effective sample after unloading The calculation and measurement results are shown in Table 1.

[0107] Table 1 Test data record under different bending radii

[0108]

[0109] Step five, draw the bending forming limit curve with the bending radius r of the effective sample as the abscissa and the bending angle before or after unloading of the effective sample as the ordinate. The bending forming limit curve of the measured material before or after unloading is as shown in Figure 3 Since the bending angle is related to the thickness of the sample, the bending forming limit curve needs to clearly mark the thickness of the tested material.

[0110] From the bending forming limit curve after unloading in this embodiment, the following conclusions can be drawn: (1) the area below the curve represents safety and the area above the curve represents cracking; (2) before 180° bending can be achieved, the critical bending angle of this material is considered as a constant value in engineering, which is independent of the bending radius; (3) to achieve 180° bending, the bending radius of this material is 2.0 mm, and the average critical bending angle is 86.8°, which has the following physical meaning in engineering: for part design, if the bending radius is greater than or equal to 2.0 mm, it can be bent and deformed at will, and if the bending radius is less than or equal to 1.5 mm, the bending angle cannot be greater than 80° (considering a certain bending forming safety factor).

[0111] Example 2

[0112] This embodiment is a continuation of Example 1. The two examples are a whole in terms of test tasks, and are consistent from Step one to Step four. Therefore, this embodiment is described in detail from Step five based on Example 1, including the following steps:

[0113] Step five, calculate the rebound amount Δα under different bending radii r , wherein the rebound amount refers to the change amount of the bending angle before and after unloading of the effective sample, and the calculation formula is as follows:

[0114] Δα r = α1 r - α2 r (1)

[0115] Wherein, α1 r is the bending angle before unloading of the effective sample, and α2 r is the bending angle after unloading of the effective sample. The calculation results of the rebound amount under different bending radii are shown in Table 1.

[0116] Step six, draw the bending rebound curve of the measured material with the bending radius r as the abscissa and the rebound amount Δα r as the ordinate, as shown in Figure 4The bending resilience curve of the measured material is shown. The bending resilience curve shows the change of resilience of the material under different bending radii, and its characteristics are as follows: the higher the curve, the greater the resilience, which indirectly indicates that the material strength or yield ratio is higher, and in the meantime, in the Figure 4 0.2mm bending radius is not considered, the resilience increases with the increase of the bending radius.

[0117] In this embodiment, since the critical bending angle before and after resilience can be determined, the calculated resilience is actually the resilience in the critical state, which has practical guiding significance for part resilience control.

[0118] Example 3

[0119] This embodiment is also a continuation of Example 1, and the two examples are consistent in test tasks from Step 1 to Step 3. Therefore, this embodiment is described in detail from Step 4 based on Example 1, including the following steps:

[0120] Step 4: For the bending radii of 0.2mm, 0.4mm, 0.8mm and 1.5mm (since 2.0mm can achieve 180° bending without cracks, it is not included in this test range) of a group of indenter, three pre-bending samples are selected as a group of gauges, and a grid with gauges of 1mm, 2mm and 3mm is drawn in the parallel bending edge region, and the drawn grid is required to be symmetrical to the bending edge.

[0121] Step 5: For each bending radius, determine the corresponding effective sample fixed loading stroke S(r). As shown in Table 1, S(0.2mm) is equal to 10.6mm, S(0.4mm) is equal to 10mm, S(0.8mm) is equal to 10.4mm, and S(1.5mm) is equal to 10.6mm. The same loading stroke is used for the pre-bending samples selected for a group of gauges (1mm, 2mm and 3mm) to perform fixed stroke loading bending test. When the sample is placed, the bending edge is required to face away from the indenter and be strictly centered with the indenter. The bending sample after gauge deformation is obtained, and its crack state is equivalent to the "OK" state of the effective sample in Step 3 of Example 1.

[0122] Step 6: For the bending radii of 0.2mm, 0.4mm, 0.8mm and 1.5mm, the deformation arc length of 1mm, 2mm and 3mm gauges in the direction perpendicular to the bending edge is measured after unloading, and the surface bending strain The calculation formula is as follows:

[0123]

[0124] Wherein, l nThe gauge length before bending deformation is 1 mm, 2 mm and 3 mm, The gauge length before bending deformation is 1 mm, 2 mm and 3 mm, n The bending deformation arc length after unloading under the action of bending radius r, and the data of each item under different bending radii and gauge lengths are recorded in Table 2.

[0125] Table 2 Surface bending strain test data under different bending radii and gauge lengths

[0126]

[0127] In Table 2, under the 1 mm gauge length, only the bending radius of 0.2 mm is subjected to bending test, the reason is that under the 1 mm gauge length, the difference between the measured and calculated results is very small, which can be considered equal in engineering, so in this embodiment, only one is selected for measurement, which is not true for 2 mm and 3 mm gauge lengths. The surface bending strain of different bending radii is not the same, and each bending radius needs to be tested. Because under different bending radii, the uniform deformation degree of 2 mm and 3 mm gauge lengths reaching the failure state is different, the maximum value indicates that the uniform deformation degree is the highest and is closest to the true bending fracture strain. In theory, as long as the indenter of different bending radii is sufficient, the bending radius of the indenter corresponding to the entire gauge length is just the bending fracture when the deformation is uniform.

[0128] Step seven, for the same gauge length l n The maximum bending strain under different bending radii r of the deformed bending sample is taken as the surface bending fracture strain under this gauge length From Table 1, the maximum bending strain corresponding to the 1 mm gauge length is 58%, the maximum bending strain corresponding to the 2 mm gauge length is 44%, and the maximum bending strain corresponding to the 3 mm gauge length is 35%.

[0129] Step eight, taking the gauge length l n as the horizontal coordinate, and taking the surface bending fracture strain as the vertical coordinate, the bending deformation gauge length effect curve of the measured material is drawn, as shown in Figure 5 Due to the relativity of the definition of surface bending strain, the determination of strain will have a gauge length effect, that is, the strain value will decrease with the increase of the gauge length, which is an objective physical phenomenon.

[0130] The bending deformation gauge length effect curve based on this embodiment is independent of the bending radius of the indenter and the thickness of the pre-measured material, which is an intrinsic parameter of the material, and is objective and quantitative test data. By combining it with the equivalent plastic strain forming limit diagram, local cracking can be predicted in part forming simulation.

[0131] Example 4

[0132] In the embodiment 3, the test result is reliable and effective to a great extent depending on the accuracy and convenience of drawing the grid on the sample surface, therefore, the embodiment is described in detail on how to effectively draw the grid on the sample surface in the embodiment 3, including the following steps:

[0133] Firstly, a fixture device is prepared to clamp the cutting edges on both sides of the sample, and three sets of marking blocks are prepared, each with a length of 80 mm and a thickness of 1 mm, and the widths are as follows:

[0134] The first set is used for grid division with a 1 mm gauge: 29.5 mm, 30.5 mm, 29 mm, 30 mm, 31 mm;

[0135] The second set is used for grid division with a 2 mm gauge: 29 mm, 31 mm, 28 mm, 30 mm, 32 mm;

[0136] The third set is used for grid division with a 3 mm gauge: 28.5 mm, 31.5 mm, 27 mm, 30 mm, 33 mm.

[0137] In the embodiment, the reason for using only three sets of marking blocks to obtain a 1 mm, 2 mm and 3 mm gauge is based on the VDA bending test, which is difficult to achieve uniform deformation in a 4 mm or more gauge.

[0138] Secondly, a pre-bending sample with a 1 mm, 2 mm and 3 mm gauge is obtained by using the three sets of marking blocks, and the process of dividing the grid on the sample surface is as follows:

[0139] If the second set of marking blocks is selected first:

[0140] (a) Select a sample (3) and draw an arrow on the sample surface to indicate the rolling direction. Place the sample on the cushion block (4) and clamp it with the fixture device. The arrow marked on the sample is parallel to the fixed end (1) of the clamping device. The top view and side view of the clamped sample are shown in Figure 6 , and the moving end (2) of the clamping device can move left and right.

[0141] (b) Select a marking block (7) with a width of 30 mm and place it tightly against the clamping fixed end (1). The top view of the clamped state is shown in Figure 7 -a. Use a 0.05 mm marking pen to draw two line segments (8) on both ends of the sample tightly against the marking block (7). The line segments can not be connected, which is convenient for aligning the indenter with the sample during the bending test to ensure that the bending edge (8) is the middle position of the bending deformation area of the sample. The top view of the sample after drawing the bending edge is shown in Figure 7 -b.

[0142] (c) Select a marking block (9) with a width of 29 mm and place it tightly against the clamping fixed end (1). The top view of the placement state is shown inFigure 8 -a as shown, using 0.05mm hooking pen close to the reticle block (9) in the sample surface draw a through line segment; then, select the width of 31mm reticle block (10) close to the clamping fixed end (1), the top view of the placement state as shown in Figure 8 -b as shown, using 0.05mm hooking pen close to the reticle block (10) in the sample surface draw a through line segment; draw two line segments of the sample top view as shown in Figure 8 -c as shown.

[0143] (d) the sample is taken out from the clamping device, the sample is rotated 90° and re-clamped, the top view of the clamping state as shown in Figure 9 .

[0144] (e) in turn select the width of 28mm reticle block (11), the width of 30mm reticle block (7), the width of 32mm reticle block (12) close to the clamping fixed end (1) using 0.05mm hooking pen close to the reticle block (9) in the sample surface draw a through line segment, the top view of the three line segments as shown in Figure 10 , and three 2mm square grids are formed in the middle of the sample.

[0145] Continue to select the first group and the second group of reticle blocks:

[0146] Repeat the processes of (a) to (e) respectively to obtain a group of pre-bending samples as shown in Figure 11 , which have drawn grids on the sample surface that can represent 1mm, 2mm and 3mm gauge lengths.

[0147] Step three, for each indenter bending radius, repeat step two, and prepare a group of pre-bending samples with gauge length of 1mm, 2mm and 3mm.

[0148] In step six of example 3, the arc length measurement method after the gauge length deformation is: after the sample is unloaded, a strip of note paper is used to closely adhere to the three arc lines of the bending deformation sample in turn; 0.05mm hooking pen is used to mark the two end points of the arc line on the note paper; the note paper is flattened, and the distance between the two end points of each arc line is measured by using a vernier caliper, and the average of the three arc lengths is taken as the length after the deformation of the gauge length.

[0149] The above embodiments are only exemplary embodiments of the present application and are not used to limit the present application, the protection scope of the present application is defined by the claims. Various modifications or equivalent replacements made by those skilled in the art to the present application within the essential protection scope of the present application also fall within the protection scope of the present application.

Claims

1. A test method for determining the bending forming limit, characterized in that, Includes the following steps: Step 1: Select a set of pressure heads, wherein the set of pressure heads has different bending radii r; Step 2: Prepare a batch of pre-bending samples of the material to be tested; Step 3: Select indenters one by one from a set of indenters and perform bending tests on the pre-bent specimens to obtain effective specimens under different bending radii. The bending test adopts "fixed stroke loading and crack presence / absence determination". The fixed stroke loading refers to the loading method in which the indenter stops moving when the set fixed loading stroke is reached. The fixed loading stroke S(r) of the effective specimen is equal to the average fixed loading stroke of two adjacent specimens with "crack" and "no" cracks. Step 4: Determine the bending angle of the effective sample before unloading. or / and the bending angle after unloading Step 5: Using the bending radius r as the x-axis and the bending angle as the y-axis... or Using the vertical axis as the ordinate, plot the bending forming limit curve of the tested material.

2. A test method for determining bending springback, characterized in that, Includes the following steps: Step 1: Select a set of pressure heads, wherein the set of pressure heads has different bending radii r; Step 2: Prepare a batch of pre-bending samples of the material to be tested; Step 3: Select indenters one by one from a set of indenters and perform bending tests on the pre-bent specimens to obtain effective specimens under different bending radii. The bending test adopts "fixed stroke loading and crack presence / absence determination". The fixed stroke loading refers to the loading method in which the indenter stops moving when the set fixed loading stroke is reached. The fixed loading stroke S(r) of the effective specimen is equal to the average fixed loading stroke of two adjacent specimens with "crack" and "no" cracks. Step 4: Determine the bending angle of the effective sample before unloading. and the bending angle after unloading Step 5: Calculate the springback amount Δα under different bending radii. r The rebound amount refers to the change in bending angle of the effective sample before and after unloading, and its calculation formula is as follows: in, It is the bending angle of the effective sample before unloading. It is the bending angle of the effective sample after unloading; Step 6: Plot the bending radius r as the abscissa and the springback amount Δα as the ordinate. r Use the vertical axis to plot the bending springback curve of the tested material.

3. The test method for determining bending forming limit or bending springback according to claim 1 or 2, characterized in that: In step one, the selected set of bending radii r of the pressure heads are: 0.4mm, 0.8mm, 1.2mm, 1.6mm, 2.0mm, 2.5mm, and 3.0mm; In step two, a pre-bent specimen is prepared according to the technical requirements of VDA238-100 standard; In step three, the pressure head applies a load to the pre-bent specimen in a manner that is parallel or perpendicular to the rolling direction, and the difference in fixed loading stroke between two adjacent specimens with and without cracks is less than or equal to 0.5 mm. In step four, the bending angle of the valid specimen is determined according to the definition in VDA238-100 standard.

4. A test method for determining the gauge length effect, characterized in that, Includes the following steps: Step 1: Select a set of pressure heads, wherein the set of pressure heads has different bending radii r; Step 2: Prepare a batch of pre-bending samples of the material to be tested; Step 3: Select indenters one by one from a set of indenters and perform bending tests on the pre-bent specimens to obtain effective specimens under different bending radii. The bending test adopts "fixed stroke loading and crack presence / absence determination". The fixed stroke loading refers to the loading method in which the indenter stops moving when the set fixed loading stroke is reached. The fixed loading stroke S(r) of the effective specimen is equal to the average fixed loading stroke of two adjacent specimens with "crack" and "no" cracks. Step 4: For each bending radius, prepare a set of pre-bending specimens with a set of gauge lengths. The set of gauge lengths refers to a grid that can represent integer gauge lengths of 1 mm, 2 mm, ..., n (mm) drawn on the surface of the pre-bending specimen. The grid is symmetrical about the bending edge. The bending edge is required to be drawn in advance at the exact center of the surface of the pre-bending specimen. The length of the side of the grid perpendicular to the bending edge is defined as the gauge length. Step 5: For each bending radius, use the fixed loading stroke S(r) of the effective specimen to perform a bending test on a set of pre-bending specimens of the selected gauge length with a fixed stroke loading. When placing the specimen, the bending edge should face away from the indenter and be strictly aligned with the indenter. Step 6: For each bending radius r, after unloading, measure the deformation arc length of the gauge length in the direction perpendicular to the bending edge, and calculate the surface bending strain. The calculation formula is as follows: Among them, l n The gauge length before bending deformation is 1, 2, ..., n. For l n The arc length of bending deformation after unloading under the action of bending radius r; Step 7: For the same gauge length l n The maximum bending strain of the deformed bending specimen under different bending radii r was measured. Surface bending fracture strain at this gauge length Step 8: Using gauge length l n As the horizontal axis, the surface bending fracture strain As the vertical axis, plot the gauge length effect curve of the bending deformation of the tested material.

5. The test method for determining the gauge length effect according to claim 4, characterized in that: In step one, the selected set of bending radii r of the pressure heads are: 0.4mm, 0.8mm, 1.2mm, 1.6mm, 2.0mm, 2.5mm, and 3.0mm; In step two, a pre-bent specimen is prepared according to the technical requirements of VDA238-100 standard; In step three, the pressure head applies load to all pre-bent specimens in a manner parallel or perpendicular to the rolling direction, and the difference in fixed loading stroke between two adjacent specimens with and without cracks is less than or equal to 0.5 mm.

6. The test method for determining the gauge length effect according to claim 5, characterized in that: In step four, n equals 3, and the grid division method is as follows: The first step is to prepare a clamping device to hold the cut edges on both sides of the sample. Simultaneously, prepare three sets of marking blocks with the following widths: Set 1: 30mm, 29.5mm, 30.5mm, 29mm, 31mm; Set 2: 30mm, 29mm, 31mm, 28mm, 32mm; Set 3: 30mm, 28.5mm, 31.5mm, 27mm, 33mm. The second step involves using three sets of marking blocks to obtain a set of pre-bent specimens with gauge lengths of 1mm, 2mm, and 3mm. The process of drawing a grid on the specimen surface is as follows: If the second group of marking blocks is selected first... (a) Select a sample, draw an arrow on the sample surface to indicate the rolling direction, place the sample on the pad, and use a clamping device to clamp the sample. The placement direction of the sample should be consistent with the direction of the bending ridge line drawn in the next step and the direction of the pressure head acting on the pre-bent sample. (b) Select a 30mm wide marking block and place it close to the fixed end of the clamping device. Use a fine-line pen to draw two line segments that do not pass through the middle at both ends of the specimen or draw a line segment that passes through the specimen as the bending edge. (c) Select marking blocks with widths of 29 mm and 31 mm respectively and place them close to the fixed end of the clamping device. Use a fine-line pen to draw a line segment through the surface of the sample on each marking block. (d) Remove the specimen from the clamping device, rotate the specimen 90° and then clamp it again; (e) Select marking blocks with widths of 28mm, 30mm and 32mm in sequence and place them close to the fixed end of the clamping device. Use a fine-line pen to draw a line segment through the surface of the sample on each marking block. Three square grids are formed in the middle of the sample surface, which can characterize the pre-bent sample with a gauge length of 2mm. Continue by selecting two more sets of marking blocks: Repeat steps (a) to (e) to obtain pre-bent specimens that can characterize 1 mm and 3 mm gauge lengths; The third step is to repeat the second step for each bending radius, and prepare a set of pre-bending specimens with gauge lengths of 1mm, 2mm and 3mm.

7. The test method for determining the gauge length effect according to claim 6, characterized in that: In step six, the method for measuring the arc length after gauge length deformation is as follows: After the sample is unloaded, a strip of sticky note is used to tightly attach to the three arcs of the bent and deformed sample in sequence, and the two ends of the arcs are marked on the sticky note with a fine-tipped pen; then the sticky note is flattened, and the distance between the two ends of each arc is measured with a vernier caliper; the average value of the three arc lengths is taken as the length of the gauge length after bending and deformation.

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