A test method for determining the plastic strain ratio

Abstract

This invention discloses a test method for determining the plastic strain ratio. By introducing the concept of a finite body and specifying the consistency of the measurement point positions before and after deformation, and using conventional measuring tools and different test conditions, the plastic strain ratio is determined, ensuring the stability of the data measurement results, improving test efficiency and reliability, and eliminating the problem of large fluctuations in the r-value measurement results caused by many uncontrollable factors in existing standards. It achieves accurate determination of the r-value in a low-cost, simple, and efficient manner. Therefore, this invention has important engineering practical significance and broad engineering application prospects.

Description

Technical Field

[0001] This invention belongs to the field of mechanical property testing of metallic materials, and in particular relates to a test method for determining the plastic strain ratio of metallic materials. Background Technology

[0002] The plastic strain ratio, or r-value, of sheet metal is an important mechanical property indicator for metallic materials. It characterizes the ability of a metal material to thin in the thickness direction during plastic deformation. When r = 1, the flow intensity is equal in the width and thickness directions; when r < 1, the material easily deforms and thins in the thickness direction; when r > 1, the material easily deforms in the length and width directions during stamping, but has strong resistance to deformation in the thickness direction. It has wide applications in many sectors of the national economy, including metallurgy, automobiles, home appliances, aviation, and aerospace.

[0003] Existing standards (ISO 10113-2020, GB / T 5027-2016, ASTM-E517-19) define the plastic strain ratio as: under uniaxial tensile stress, i.e., according to the uniaxial tensile test method specified in GB / T 228.1—2010, the ratio of the true plastic strain in the width direction to the true plastic strain in the thickness direction of the specimen, as shown in the following formula:

[0004]

[0005] In the formula, ε a It represents the true plastic strain in the thickness direction; ε b This is the true plastic strain in the width direction. However, existing standards unanimously agree that the deformation in the length direction is easier and more accurate to measure than the deformation in the thickness direction. Equation (1-2) obtained from the condition of constant volume is used to calculate the plastic strain ratio r:

[0006]

[0007] In the formula, L0 is the original gauge length of the specimen, L is the gauge length of the specimen after the specified strain, b0 is the original width within the gauge length of the specimen, and b is the width of the specimen after the specified strain. Formula (1-1) is called the direct method, and formula (1-2) is called the indirect method.

[0008] The existing standards specify three methods for determining the r-value, all of which are indirect methods:

[0009] 1. Manual Measurement Method: Before the tensile test, manually mark the original gauge length with an accuracy of 0.2%. Then, measure the width of the original sample at at least three locations within the gauge length, with an accuracy controlled within ±0.005 mm. Finally, average the three width values. Next, apply a uniaxial tensile load to the sample to the set strain. After unloading, measure the gauge length and width using the same method as above, and calculate the r-value. This method is only applicable to materials with uniform plastic deformation.

[0010] 2. Semi-automatic measurement method: The difference from manual measurement is that semi-automatic measurement uses an axial extensometer to measure the strain along the sample length, while the same steps as manual measurement are used to measure the sample width before and after deformation (still requiring measurements at three locations and taking the average). It is important to note that the axial plastic strain used to calculate the r-value requires subtracting the elastic strain from the total strain measured by the extensometer under loading. This method is only applicable to materials with uniform plastic deformation.

[0011] 3. Fully Automated Measurement Method: This method automatically measures the deformation of the sample in two directions (axial and width directions) during uniaxial tensile loading using both axial and transverse extensometers. Therefore, the sample does not need to be loaded to a set deformation and then unloaded; it can be directly loaded to the elongation corresponding to the maximum force or to break at the moment of fracture. This method allows users to measure the r-value at multiple deformations or to take the average r-value within a certain deformation range.

[0012] In practical engineering applications of existing standards, the fluctuation of r-value measurement data is very large, which is an industry consensus. Specimen size, specimen preparation, clamping alignment, testing equipment, dimensional measurement, extensometer, etc., all have a significant impact on the results. In particular, the use of the extensometer introduces systematic errors. Its gauge length, clamping position and form, relative sliding between the blade and the specimen surface, and clamping effect of the width measuring device all significantly affect the test results. The fundamental reason is that the calculation formulas (1-1) and (1-2) for the plastic strain ratio are extremely sensitive to the accuracy of the test data, which is also an industry consensus.

[0013] Because many factors are uncontrollable in the process of plastic strain ratio measurement, the measurement results fluctuate greatly and cannot be accurately evaluated or calculated. Therefore, how to establish an accurate method for measuring plastic strain ratio has become an urgent problem to be solved.

[0014] The inventors discovered that fluctuations exceeding 0.01 mm in length and width measurements, and exceeding 0.005 mm in thickness measurements, significantly impact the calculated r-value. Therefore, it is essential to control the measurement accuracy within 0.005 mm and 0.002 mm respectively (which is very close to the achievable accuracy range of the measuring instrument). Technically, such high accuracy cannot be achieved using the three measurement methods specified in existing standards. Furthermore, through careful study of the existing standard documents, the inventors revealed the problems with the existing standards from both mechanical understanding and testing technology perspectives, primarily in the following seven aspects:

[0015] First, the issue of understanding the plastic strain ratio.

[0016] In plasticity mechanics, only Poisson's ratio exists, defined as the ratio of the true plastic strain in the width direction to the true plastic strain in the tensile direction. The concept of the r-value, however, appears in plastic forming and is defined as the ratio of the true plastic strain in the width direction to the true plastic strain in the thickness direction. According to the condition of constant volume, Poisson's ratio and r-value are equivalent. In plasticity mechanics, for metallic materials, Poisson's ratio is an intrinsic parameter of the material, a constant value, meaning it is independent of the gauge length and strain level. Therefore, the r-value is also independent. This provides a theoretical basis for the application of the direct method, because with current technology, thickness measurement can only be performed after unloading, and cannot be performed in real time during deformation.

[0017] The existing standard r-value measurement results do not support the conclusion that "r-value is independent of the gauge length and strain level of the specimen". However, the inventors have found that the r-value measurement results based on the direct method can well support the conclusion that "r-value is independent of the gauge length and strain level of the specimen". Conversely, for general metallic materials, it can be used to check and evaluate the reliability of the r-value measurement results, that is, the test method has the function of logical self-consistency.

[0018] Second, there is the problem of the lack of a concept of finite bodies.

[0019] In existing standards, there is no concept of a finite body; the object of measurement is directly considered to be the specimen. However, due to the use of an extensometer in the indirect method (which requires a large space to arrange), a finite body is actually defined, the size of which is "gauge length * specimen width * thickness". That is, this finite body is equivalent to the gauge length region of the specimen. Therefore, in essence, existing standards equate the r value of the specimen to the r value of the material being measured.

[0020] The initial thickness of metal sheets is not absolutely uniform; fluctuations at the micrometer level are normal, such as ±10 μm. These micrometer-level fluctuations significantly affect the calculated r-value. Therefore, if the object being measured is a sample, there is reason to believe that the measured thickness is inaccurate. Which location's thickness truly represents the entire sample's thickness? This is a problem. Furthermore, in existing standards, the indirect method based on the condition of constant volume is used, measuring the r-value of the sample, not the material's r-value. Are the two equal? ​​This is an even greater problem. Given the high sensitivity of r-value calculations to measurement data, there is no reason to assume they are equal.

[0021] The specimen is merely a carrier of a finite body; the specimen is not the object of measurement. The object of measurement is the finite body. The r-value is an intrinsic parameter of the material, not the r-value of the specimen. Rather, it is the r-value of a finite body represented by a local area on the specimen, which is closer to the true r-value of the material. Therefore, the concept of a finite body is the premise and foundation of the entire testing scheme, and all subsequent technical solutions must revolve around the finite body.

[0022] If the concept of a finite body is explicitly introduced, and one or more local regions on the specimen are selected and defined as finite bodies, then as long as the finite body is appropriately small—for example, its length, width, and height are all equal to the thickness of the specimen—then measuring the thickness can characterize the true thickness of the material. Therefore, it is possible to determine the r-value using the direct method. Furthermore, a finite body defined by an appropriately small local region is closer to the object of measurement for characterizing the r-value of the material, and it can also characterize the measurement of the r-value under local necking strain levels. Therefore, how to define a finite body on the specimen is one of the most crucial elements of this invention.

[0023] Third, the issue of applying the condition of constant volume.

[0024] According to the definition of the r-value, the direct method is more appropriate for calculation. However, perhaps due to the limitations of early measurement tools, the indirect method had to be used. Although thickness is a directly relevant factor, it is not present in the data measurement and calculation. The indirect method relies on the premise of constant volume, which is essentially an assumption. In the indirect method, it is treated as an objective condition, inevitably leading to calculation errors, or rather, requiring extremely high measurement accuracy. In fact, this error can be quantified, but it is selectively avoided in existing standards.

[0025] The inventors, through research on the constant volume condition, discovered that, given the high sensitivity of the r-value to the accuracy of measurement data, the constant volume condition is not suitable for direct inclusion in the calculation of the r-value. However, it is more appropriate to use it in the direct method to judge the reasonableness of the r-value measurement results. In the specific practice of this invention, the greater the deviation of the r-value from the true value, the greater the deviation of the constant volume condition. Therefore, it can be used to detect and correct anomalies in the data measurement results to ensure the reliability of the test results. Furthermore, if the measurement data fully satisfies the constant volume condition, the r-values ​​measured by the direct method and the indirect method are very close. However, in many cases, this is not strictly satisfied; the former has better r-value stability, while the latter's r-value stability is less satisfactory.

[0026] Fourth, the issue of marking the location of measurement points.

[0027] Existing standards, when using manual measurement methods, stipulate that gauge length and width should be based on measured values, not nominal values. However, there are no clear technical regulations regarding the consistency of measurement point positions before and after deformation. From a measurement perspective, manual measurement methods require maintaining consistency in the positions of measurement points before and after deformation. However, in practice, this consistency is merely subjective; that is, while maintaining consistency as much as possible before and after testing, deviations are inevitable.

[0028] The inventors have discovered that without clearly defined technical measures to address this issue, the fluctuation in r-value measurements using the indirect method is unacceptable, while the direct method exhibits significant fluctuations. The fundamental reason for this is that the lines marked on the sample have widths, and the points have sizes. Under a video microscope, deviations in the selection of measurement points on the lines and points significantly affect the stability of the r-value measurement results. Therefore, "how to ensure the consistency of the measurement point positions before and after deformation" requires clear technical regulations (existing standard manual measurement methods involve the first marking, while the new technology requires a second marking on top of the first marking), which is another crucial element of this invention. Furthermore, it has been found that while the r-value measured using the indirect method is not as stable as the direct method when secondary marking is present, it is still acceptable in engineering practice. Therefore, this invention does not exclude the indirect method, but considers that the existing standard indirect method has technical defects (the size of the finite body A is too large, making it difficult to meet the condition of constant volume), and needs improvement.

[0029] Fifth, the reliability issue of the same state.

[0030] According to existing testing and product standards, there are generally no fewer than three test samples, and the average value is taken as the r-value of the tested material. This is a simple repetition of the same state. Since the r-value is highly sensitive to the measurement accuracy of the test data, the key issue is not the number of samples, but the uniformity of the sample state. This can lead to problems with the reliability of the test results. For example, the three measured r-values ​​may all be greater than the true value, or they may all be less than the true value. Taking the average of the three r-values ​​will then deviate from the true value.

[0031] To address the issue that "the r-value is highly sensitive to the measurement accuracy of test data," the inventors discovered that using samples in different states to determine the r-value has high reliability. This is because fluctuations in the r-value under different states are more likely to be biased towards the fluctuations of the true r-value. Taking the average of the r-values ​​under different states is more likely to be biased towards the true value. For example, using samples of different widths to define a single finite body, or defining multiple finite bodies of different sizes on the same sample, to obtain different measurement states is also a technical highlight of this invention.

[0032] Sixth, the complexity of the measurement system.

[0033] The plastic strain ratio was first proposed in 1950 by Lankford et al. of Carnegie-Illinois Steel Company in the United States. Due to the difficulty in accurately measuring the thickness change of thin plates under the technical conditions at that time, and the excessive sensitivity of the direct method to thickness measurement error, the indirect method was recommended to determine the r value, and this recommendation has continued to this day. For example, ISO 10113-2020 still recommends the use of the indirect method—"Since it is easier and more accurate to measure the change in length than to measure the change in thickness, the plastic strain ratio is calculated using the relationship derived from the condition of constant volume." This makes extensometers indispensable, increasing the complexity of the testing system.

[0034] However, during testing, the inventors discovered that under current technological conditions, the direct method using a micrometer to measure thickness can fully meet the accuracy requirements for thickness measurement. Using a video microscope to measure width and length offers high precision and convenience, providing low-cost and convenient testing methods for determining the r-value using the direct method. Of course, measurements using a micrometer and video microscope can only measure the length, width, and thickness of a finite body after unloading, but they also have a significant technical advantage: the measurement results directly match the concept of true plastic strain, while the indirect method requires subtracting elastic deformation, increasing the complexity of data processing.

[0035] Seventh, the problem of missing r-values ​​for uniaxial compression.

[0036] The existing standard only specifies the determination of the r-value in the uniaxial tensile direction, but not in the uniaxial compression direction. The fundamental reason is that the indirect method requires the use of an extensometer, which requires a certain amount of installation space. The uniaxial compression test of thin plates generally requires the installation of anti-buckling devices, which would conflict with the space requirement. The direct method, on the other hand, does not require the use of an extensometer and can be used for both uniaxial tension and uniaxial compression.

[0037] Through preliminary testing, the inventors discovered that the r-values ​​of the same material under uniaxial tension and uniaxial compression are different. The fundamental reason is that the uniaxial tensile hardening curve and the uniaxial compressive hardening curve of the material are not the same. The greater the deformation, the greater the difference between the two. Since the symmetry of the hardening curve under tension and compression is only an ideal assumption, the equality of the r-values ​​in the two directions is also just an assumption. There is now sufficient reason to believe that this assumption is not valid. Therefore, the r-value also needs to be measured in the compression direction.

[0038] In summary, the inventors completed this invention by thoroughly studying the various problems existing in the existing standards and proposing targeted technical measures. Summary of the Invention

[0039] 1. The technical problem solved by the present invention

[0040] To address the technical deficiencies in existing standards, namely the large fluctuations in measurement results caused by numerous uncontrollable factors in the process of determining the plastic strain ratio according to existing standards, the purpose of this invention is to provide a test method for determining the plastic strain ratio based on conventional measuring tools that can eliminate the influence of uncontrollable factors, thereby achieving a significant improvement in detection accuracy, precision, and reliability in a low-cost, simple, and efficient manner.

[0041] 2. Technical solution of the present invention

[0042] To achieve the technical problem to be solved by this invention, this invention provides a test method for determining the plastic strain ratio, characterized by comprising the following steps:

[0043] Step 1: Prepare several samples with a gauge length of L0, an original width of W0, and an original thickness of t0. The samples have the same sampling direction.

[0044] Step 2: Mark one or more local regions on the sample. Define the marked local region as a finite body A. The original length of the finite body A is l0, the original width is w0, and the original thickness is t0. Its length direction is parallel to the tensile direction of the sample, and l0 is less than L0 and w0 is less than or equal to W0.

[0045] Step 3: Mark the measurement points in three directions of the finite body A, and measure and record the measured values ​​of its original thickness t0, original width w0 and / or original length l0;

[0046] Step 4: Set a loading stroke and perform a uniaxial tensile test or uniaxial compression test with fixed stroke loading. After reaching the loading stroke, unload and remove the specimen. The fixed stroke loading refers to a loading method in which the testing machine automatically stops moving when it reaches the set loading stroke, and the set loading stroke does not exceed the critical value at which the finite body A undergoes non-uniform deformation.

[0047] Step 5: Measure and record the measured values ​​of the thickness t1, width w1 and / or length l1 of the finite body A after deformation, wherein the location of the measurement point is consistent with the measurement point marked in Step 3;

[0048] Step 6: Calculate the plastic strain ratio r of finite body A using the measured values ​​according to the following formula:

[0049]

[0050] or / and:

[0051]

[0052] Where, ε l Let ε be the true plastic strain of finite body A along its length. w Let ε be the true plastic strain of finite body A in the width direction. t Let A be the true plastic strain of the finite body A in the thickness direction.

[0053] Preferably, the test method for determining the plastic strain ratio is characterized in that: in step two, the method for calibrating the local area is as follows: First, within the gauge length of a surface of the specimen, draw a line of symmetry in the width direction and a line of symmetry in the tensile direction, the two lines of symmetry intersecting at point O; Second, arrange a square grid with a side length of l0 within the gauge length, the geometric center of the square grid coinciding with point O, and the side length taking the range of [t0, W0]; If multiple local areas of different sizes are to be calibrated simultaneously on the same specimen, the second step is repeated.

[0054] The preferred method for determining the plastic strain ratio is characterized in that:

[0055] In step three, the original thickness t0 of finite body A is measured and recorded using a micrometer, with all measurement points being point O; the original length l0 and original width w0 of finite body A are measured and recorded using a video microscope. During the measurement, measurement points are selected at the intersection of the outer or inner edge of the grid with the symmetry lines a and b, and their positions are marked.

[0056] In step five, the thickness t1 of the finite body A after deformation is measured and recorded using a micrometer; the length l1 and width w1 of the finite body A after deformation are measured and recorded using a video microscope, and the positions of the measurement points are selected by referring to or comparing with the measurement point positions marked in step three.

[0057] In both of the above steps, the measurement accuracy is 0.001 mm. If the original width w0 of the finite body A is equal to the width W0 of the sample, a micrometer can also be used to measure its width before and after deformation. The position of the measurement point is determined with reference to the position of the symmetry line b in the tensile direction.

[0058] Preferably, the test method for determining the plastic strain ratio is characterized in that: in step two, the method for arranging a square grid on the specimen is as follows:

[0059] The first step is to make a marking block. The length and width of the marking block are equal to the length and width of the sample. A square hole with a side length of l0 is cut on it. The square hole is positioned to coincide with the geometric center of the marking block surface and is arranged along the length of the marking block.

[0060] The second step is to fully attach the marking block to the sample in the length and width directions, and use a fine-line pen to draw lines around the square hole to obtain a square grid with a side length of l0.

[0061] If you want to arrange square grids of different sizes on the same sample at the same time, repeat the above two steps.

[0062] Preferably, the test method for determining the plastic strain ratio is characterized in that: in step one, the gauge length L0 of the specimen is equal to 50 mm, and the width W0 is within the range of [t0, 12 mm].

[0063] Preferably, the test method for determining the plastic strain ratio is characterized in that: in step two:

[0064] If only one square grid is arranged for a single specimen, then at least three specimens of different widths should be used. Repeat steps one to six to determine the plastic strain ratio of each finite body A, and then take the average value as the plastic strain ratio of the material being tested.

[0065] If at least three square grids of different sizes are arranged on a specimen, then a single specimen can be used to determine the plastic strain ratio of each finite body A, and then the average value can be taken as the plastic strain ratio of the material being tested.

[0066] Preferably, the test method for determining the plastic strain ratio is characterized by: using three specimens with different widths of 4mm, 8mm and 12mm, or four specimens with different widths of 3mm, 6mm, 9mm and 12mm, and arranging only one square grid on each specimen, the side length l0 of which is equal to the width W0 of the specimen, measuring the plastic strain ratio of the finite body A on each specimen, and then taking the average value as the plastic strain ratio of the tested material.

[0067] Preferably, the test method for determining the plastic strain ratio is characterized in that: a 12mm wide specimen is used, with three square grids having side lengths of 4mm, 8mm and 12mm respectively, or four square grids having side lengths of 3mm, 6mm, 9mm and 12mm respectively, the plastic strain ratio of each finite body A is measured, and then the average value is taken as the plastic strain ratio of the tested material.

[0068] Furthermore, the test method for determining the plastic strain ratio is characterized in that: at least three different fixed loading strokes are used, and steps one to six are repeated to determine the plastic strain ratio respectively, and then the average value of the plastic strain ratio under each fixed loading stroke is taken as the plastic strain ratio of the material being tested.

[0069] Furthermore, the test method for determining the plastic strain ratio is characterized in that: after step five, an incremental loading with a fixed stroke of 0.5 mm is applied to the unloaded sample. After the test, step five is performed again, and the above incremental loading is repeated at least twice. The plastic strain ratio of each unloaded sample is measured in step six. Then, the average value of the plastic strain ratio of the same sample under different unloading states is taken as the plastic strain ratio of the tested material.

[0070] Furthermore, the test method for determining the plastic strain ratio is characterized by: sampling from three directions—parallel to the rolling direction, perpendicular to the rolling direction, and at a 45° angle to the rolling direction—and repeating steps one through six to measure the plastic strain ratios r0 and r2 in each sampling direction. 45 and r 90 The r0 mentioned above is the plastic strain ratio parallel to the rolling direction, r 45 For a plastic strain ratio at a 45° angle to the rolling direction, r 90 This represents the plastic strain ratio perpendicular to the rolling direction.

[0071] Furthermore, the test method for determining the plastic strain ratio is characterized by: calculating the weighted average value of the plastic strain ratio. The anisotropy of the plastic strain ratio Δr is calculated using the following formulas:

[0072]

[0073] Where r0 is the plastic strain ratio parallel to the rolling direction; r 45 The plastic strain ratio is at a 45° angle to the rolling direction; r 90 This represents the plastic strain ratio perpendicular to the rolling direction.

[0074] 3. Beneficial effects of the present invention

[0075] This invention provides a test method for determining the plastic strain ratio, which has the following beneficial effects:

[0076] First, by introducing the concept of a finite body, the object of measurement was clearly defined, laying the foundation for the formulation of new technical solutions. Furthermore, by selecting a finite body of appropriate size, deviations of the measurement data from the condition of constant volume were effectively controlled, reducing calculation errors.

[0077] Secondly, by marking the measurement point positions twice, the stability of the data measurement results is ensured, eliminating the problem of large fluctuations in the r-value determination results caused by the instability of extensometer measurement data in the existing standard.

[0078] Third, by using conventional measuring tools instead of extensometers, the efficiency and reliability of the test are improved, the range of r-value determination is expanded, and the test results are logically consistent, avoiding the technical risk that the r-value measured by existing standards may deviate from the true value.

[0079] In summary, by implementing this invention, the influence of uncontrollable factors is eliminated, and a significant improvement in the accuracy, precision, and reliability of r-value measurement is achieved in a low-cost, simple, and efficient manner. This invention also solves the technical defects existing in the current standard. Therefore, this invention has important engineering practical significance and broad engineering application prospects. Attached Figure Description

[0080] The present invention will now be described in further detail with reference to the accompanying drawings.

[0081] Figure 1 This is a schematic diagram of four samples with different widths according to Embodiment 1 of the present invention;

[0082] Figure 2 This is a schematic diagram of the symmetry line of the 12mm wide sample in Embodiment 1 of the present invention;

[0083] Figure 3 This is a schematic diagram of a 12mm square hole marking block according to Embodiment 1 of the present invention;

[0084] Figure 4 This is a schematic diagram of a 12mm wide sample of a calibrated finite body A according to Embodiment 1 of the present invention;

[0085] Figure 5This is a schematic diagram of the measurement point positions marked in the tensile direction according to Embodiment 1 of the present invention;

[0086] Figure 6 This is a schematic diagram of the gauge length versus r-value curves for two calculation methods in Embodiment 1 of the present invention;

[0087] Figure 7 This is a schematic diagram of four finite bodies A calibrated on the same sample in Embodiment 2 of the present invention;

[0088] Figure 8 This is a schematic diagram of the selection of measurement points before and after deformation of finite body A in Embodiment 2 of the present invention;

[0089] Figure 9 This is a schematic diagram of the gauge length versus r-value curves for the two calculation methods in Embodiment 2 of the present invention;

[0090] Figure 10 This is a schematic diagram of the gauge length versus r-value curves for the two calculation methods in Embodiment 3 of the present invention;

[0091] Figure 11 This is a schematic diagram of the sampling direction and r-value curves for the two calculation methods in Embodiment 3 of the present invention.

[0092] Explanation of reference numerals in the attached figures:

[0093] 1—Specimen clamping section; 2—Specimen transition section; 3—Specimen gauge length section; 4—Width direction symmetry line a; 5—Tension direction symmetry line b; 6—Square hole with a side length of 12mm on the gauge block; 7—Limited body A with a gauge length of 12mm; 8—Limited body A with a gauge length of 9mm; 9—Limited body A with a gauge length of 6mm; 10—Limited body A with a gauge length of 3mm. Detailed Implementation

[0094] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings.

[0095] Example 1

[0096] This embodiment takes a hot-rolled pickled steel plate with a tensile strength of 780 MPa and a thickness of 3 mm as an example. Multiple specimens of different widths are used to test its plastic strain ratio, including the following steps:

[0097] Step 1: Take samples from the steel plate parallel to the rolling direction (0°) and process four different widths of specimens using wire cutting (edges are not ground). At least three specimens of each width are processed. The specimen width is 24mm, the length is 116mm, the gauge length L0 is 50mm, the original thickness t0 is 3mm, and the original widths W0 are 3mm, 6mm, 9mm, and 12mm respectively. These are all nominal values. The four different width specimens are shown below. Figure 1 As shown.

[0098] Step 2: On the surface of the specimen, draw a line of symmetry a in the width direction and a line of symmetry b in the tensile direction. The two lines of symmetry intersect at point O. Taking a gauge length of 12mm as an example, the lines of symmetry for a 12mm wide specimen are as follows: Figure 2 As shown; simultaneously, within the gauge length of the specimen, a local region is marked and defined as a finite body A. The specific method is as follows:

[0099] The first step is to fabricate marking blocks. The length and width of each marking block are equal to the length of the sample (116 mm) and the width (24 mm). A square hole is cut along the tensile direction of the sample, coinciding with the geometric center of the marking block surface. In this embodiment, four marking blocks were fabricated, with side lengths of the square holes being 3 mm, 6 mm, 9 mm, and 12 mm respectively. These are all nominal values. Taking a marking block with a 12 mm square hole as an example, the marking block with a 12 mm square hole is as follows... Figure 3 As shown.

[0100] The second step is to select one of the marking blocks and fit it completely against the sample with the same square hole side length in both length and width directions. Use a fine-tipped pen to draw lines around the square hole to obtain a square grid. Define this square grid area as a finite body A. The characteristic of this finite body A is that the two sides in the tensile direction coincide with the upper and lower sides in the width direction of the sample. In actual operation, the upper and lower sides do not need to be drawn.

[0101] Repeat the above process of arranging the grid, preparing three specimens for each width, for a total of 12 specimens. The nominal dimensions of the original length l0, original width w0, and original thickness t0 of the four different finite bodies A are 3mm*3mm*3mm, 6mm*6mm*3mm, 9mm*9mm*3mm, and 12mm*12mm*3mm, respectively. Taking the 12mm gauge length as an example, the 12mm wide specimen of finite body A is calibrated as follows: Figure 4 As shown. The 12 samples were coded as follows: the 3 mm wide samples were coded as A1, A5 and A9; the 6 mm wide samples were coded as A2, A6 and A10; the 9 mm wide samples were coded as A3, A7 and A11; and the 12 mm wide samples were coded as A4, A8 and A12.

[0102] Step 3: Mark the measurement points of finite body A, and measure and record the measured values ​​of the original width w0, original length l0, and original thickness t0 of finite body A. The intersection point O of the two lines of symmetry is the measurement point in the thickness direction. The measurement points on both sides of the sample width direction are determined with reference to the location of the line of symmetry b. Use a micrometer (a flat-head micrometer with a 2mm diameter is recommended) to measure the measured values ​​of the thickness t0 and original width W0 of finite body A; use a video microscope to measure the measured value of the original length l0 of finite body A. Select measurement points at the intersection of the outer edge of the grid and the line of symmetry a, and mark the measurement point positions. For example, the two measurement points P1 and P2 marked in the tensile direction are as follows: Figure 5 As shown in the figure, the measurement accuracy is required to reach 0.001 mm. The measured values ​​of the length, width and height (thickness) of the finite body A of the 12 calibration samples are shown in Table 1.

[0103] Table 112 Measured values ​​of length, width and height of finite body A before deformation

[0104]

[0105] Step 4: Set a loading stroke and conduct a fixed-stroke loading uniaxial tensile test. Fixed-stroke loading refers to a loading method where the tensile end of the tensile testing machine stops moving when it reaches the set loading stroke. The set loading stroke does not exceed the critical value at which non-uniform deformation occurs in the finite body A. Uniform deformation means that no local necking occurs in the width and thickness directions of the finite body A. In this embodiment, the loading stroke is set as follows: Select an uncalibrated specimen and conduct a uniaxial tensile test. Break the specimen directly and obtain the load-displacement curve. Before the curve drops downwards or in the stable segment of the tensile load, determine the loading stroke for the tensile test. Select three loading strokes at 0.5mm intervals: 4.0mm, 4.5mm, and 5.0mm. For each loading stroke, conduct a fixed-stroke loading uniaxial tensile test on four calibrated specimens of different sizes, obtaining 12 unloaded specimens.

[0106] Step 5: Measure and record the measured values ​​of the length l1, width w1 and thickness t1 of each finite body A after deformation, as shown in Table 2. The positions of the measurement points are consistent with the measurement points marked in Step 3.

[0107] Step 6: Calculate the plastic strain ratio r for each finite body A using the following formula:

[0108]

[0109] and:

[0110]

[0111] Where, ε wε represents the true plastic strain of finite body A in the width direction; l Let ε be the true plastic strain of finite body A along its length. t For the true plastic strain of finite body A in the thickness direction, equation (1) is called the direct method, and equation (2) is called the indirect method. The measurement results according to the two calculation methods are shown in Table 3.

[0112] Table 212 Measured values ​​of length, width and height of finite body A after deformation of finite body A for finite body A.

[0113]

[0114] Table 312 shows the calculation results of the r values ​​for finite bodies A.

[0115]

[0116] The direct method was used, and the average r-value of 12 finite body A samples in the tensile tests, 0.582, was taken as the test result for the material's r-value. The indirect method was used, and the average r-value of 12 finite body A samples in the tensile tests, 0.506, was taken. Based on the data in Table 3, the gauge length versus r-value curves for the two calculation methods were plotted as follows: Figure 6 As shown in the image.

[0117] In this embodiment, if multiple samples of different widths are used and the r-value is determined using the direct method, only the thickness and width need to be measured using a micrometer. Therefore, using a video microscope to measure the deformation in the length direction and marking the measurement points is a non-essential technical feature. However, if secondary marking is performed, it can be used to verify the condition of constant volume. At the same time, for the determination of r-value under conditions above room temperature, secondary marking may have operational feasibility issues. Therefore, the method in this embodiment is more suitable.

[0118] Example 2

[0119] This embodiment uses the same materials as Embodiment 1, but employs a method of arranging multiple square grids on the same upper sample. A necessary technical feature is the secondary marking of measurement points using a video microscope to measure deformation along the length direction. The determination of its plastic strain ratio includes the following steps:

[0120] Step 1: Take samples from the steel plate at a 45° angle to the rolling direction. Process at least three samples using wire cutting. The sample width is 24 mm, the length is 116 mm, the gauge length L0 is 50 mm, the original thickness t0 is 3 mm, and the original gauge length width W0 is 12 mm. These are all nominal values. Samples should be... Figure 1 As shown in (d).

[0121] Step 2: On the surface of the specimen, draw a line of symmetry a in the width direction and a line of symmetry b in the tensile direction. The two lines of symmetry intersect at point O. The lines of symmetry for a 12mm wide specimen are shown below. Figure 2 As shown; simultaneously, within the gauge length of the specimen, four local regions are marked and defined as finite bodies A, respectively. The specific method is as follows:

[0122] The first step is to fabricate marking blocks. The length and width of each marking block are equal to the length of the sample (116 mm) and the width (24 mm). A square hole is cut along the tensile direction of the sample, coinciding with the geometric center of the marking block surface. In this embodiment, four marking blocks are fabricated, with side lengths of the square holes being 3 mm, 6 mm, 9 mm, and 12 mm respectively. These are nominal values. Taking a marking block with a 12 mm square hole as an example, the marking block with a 12 mm square hole is as follows... Figure 3 As shown;

[0123] The second step involves selecting one of the gauge blocks and aligning it perfectly with the sample in both length and width directions. Using a fine-tipped pen, lines are drawn around the square hole to create a square grid. This grid arrangement process is repeated on the same sample. Each square grid region is defined as a finite body A, resulting in four finite bodies A of different specifications. The nominal dimensions of their original length l0, original width w0, and original thickness t0 are 3mm*3mm*3mm, 6mm*6mm*3mm, 9mm*9mm*3mm, and 12mm*12mm*3mm, respectively. Taking a gauge length of 12mm as an example, the four finite bodies A calibrated on the same sample are as follows: Figure 7 As shown.

[0124] Three samples were calibrated and coded repeatedly. The three 12mm wide samples were coded as B1, B2 and B3. The finite body A on sample B1 was coded as B1-3mm, B1-6mm, B1-9mm and B1-12mm respectively; the finite body A on sample B2 was coded as B2-3mm, B2-6mm, B2-9mm and B2-12mm respectively; the finite body A on sample B3 was coded as B3-3mm, B3-6mm, B3-9mm and B3-12mm respectively.

[0125] Step 3: Mark the positions of the measurement points of each finite body A, and measure and record the measured values of the original width w0, original length l0, and original thickness t0 of each finite body A. The intersection point O of the two symmetry lines is the measurement point in the thickness direction, and the measurement points on both sides in the width direction of the specimen are determined by referring to the position of symmetry line b; use a micrometer (a flat-headed micrometer with a recommended diameter of 2 mm) to measure the measured value of the thickness t0 of the finite body A; use a video microscope to measure the measured values of the original length l0 and original width w0 of each finite body A. At the intersection position of the outer edge of the grid and symmetry line a, select measurement points and mark the positions of the measurement points. The specific method is as follows: Using the measurement and annotation function of the software supporting the video microscope, taking the stretching direction as an example, on the picture (0) of the finite body A before deformation, select the point marking position as the marking point, as Figure 8 shown in -a. When measuring the picture of the finite body A again after deformation, select points by referring to the marked measurement point positions on the picture (0), as Figure 8 shown in -b; The measurement accuracy requirement reaches 0.001 mm. The measured values of the length, width, and height (thickness) of the 12 finite bodies A calibrated on the three specimens are shown in Table 4.

[0126] Table 4 Measured values of the length, width, and height of the 12 finite bodies A calibrated on the three specimens before deformation

[0127]

[0128] Table 5 Measured values of the length, width, and height of the 12 finite bodies A calibrated on the three specimens after deformation

[0129]

[0130] Step 4: Set a loading stroke and conduct a uniaxial tensile test with fixed-stroke loading. The fixed-stroke loading refers to a loading method in which the tensile end of the testing machine stops moving when it reaches the set loading stroke, and the set loading stroke does not exceed the critical value of non-uniform deformation of the finite body A. The uniform deformation means that there is no local necking in the width direction and thickness direction of the finite body A.

[0131] The method for setting the loading stroke in this embodiment is as follows: Select an uncalibrated specimen and perform a uniaxial tensile test. Break the specimen directly and obtain the load-displacement curve. Before the curve drops downwards or during the stable phase of the tensile load, determine the loading stroke for the tensile test. Select three loading strokes at 0.5mm intervals: 4.0mm, 4.5mm, and 5.0mm. For each loading stroke, perform a fixed-stroke loading uniaxial tensile test on four calibrated specimens of different specifications to obtain four unloaded specimens. As an alternative, only one B1 specimen is used, with a set of finite bodies A coded as B1-3mm, B1-6mm, B1-9mm, and B1-12mm respectively. First, a fixed stroke load of 4.0mm is applied, and after unloading, the length, width, and thickness of each finite body A are measured. Then, a fixed stroke load of 0.5mm is applied, and after unloading, the length, width, and thickness of each finite body A are measured (equivalent to directly applying a 4.5mm fixed stroke load to B2). Continue applying a fixed stroke load of 0.5mm, and after unloading, the length, width, and thickness of each finite body A are measured (equivalent to directly applying a 4.5mm fixed stroke load to B3).

[0132] Step 5: Measure and record the measured values ​​of the length l1, width w1 and thickness t1 of each finite body A after deformation, as shown in Table 5. The positions of the measurement points are consistent with the measurement points marked in Step 3.

[0133] Table 6 shows the calculation results of the r values ​​of the 12 finite bodies A calibrated on the three samples.

[0134]

[0135] Step 6: Calculate the plastic strain ratio r for each finite body A using the following formula:

[0136]

[0137] and:

[0138]

[0139] Where, ε w ε represents the true plastic strain of finite body A in the width direction; l Let ε be the true plastic strain of finite body A along its length. t Let A be the true plastic strain in the thickness direction of the finite body A. The result calculated according to Equation (1) is the direct method, and the result calculated according to Equation (2) is the indirect method. The calculation results are shown in Table 6.

[0140] Using the direct method, the average r-value of 12 finite bodies A was taken as 1.243, which was used as the test result for the material being tested. Using the indirect method, the average r-value of 12 finite bodies A was taken as 1.036. Based on the data in Table 6, the gauge length versus r-value curves for the two calculation methods were plotted as follows: Figure 9 As shown in the image.

[0141] Example 3

[0142] This embodiment uses the same materials as Example 1, and the weighted average value of the plastic strain ratio is measured. The anisotropy of the plastic strain ratio Δr is determined by the following steps:

[0143] Step 1: Samples are taken from the steel plate in three directions: parallel to the rolling direction (0°), at 45° to the rolling direction, and perpendicular to the rolling direction (90°). Specimens are machined using wire cutting, with at least one sample machined in each direction. The specimen width is 24 mm, the length is 116 mm, the gauge length L0 is 50 mm, the original thickness t0 is 3 mm, and the original gauge length width W0 is 12 mm. These are all nominal values. The specimens are as follows... Figure 1 As shown in (d).

[0144] Step 2: On the surface of the specimen, draw a line of symmetry a in the width direction and a line of symmetry b in the tensile direction. The two lines of symmetry intersect at point O. The lines of symmetry for a 12mm wide specimen are shown below. Figure 2 As shown; simultaneously, within the gauge length of the sample, four local regions are marked and defined as four finite bodies A, respectively. The specific method is as follows:

[0145] The first step is to fabricate marking blocks. The length and width of each marking block are equal to the length of the sample (116 mm) and the width (24 mm). A square hole is cut along the tensile direction of the sample, coinciding with the geometric center of the marking block surface. In this embodiment, four marking blocks are fabricated, with side lengths of the square holes being 3 mm, 6 mm, 9 mm, and 12 mm respectively. These are nominal values. Taking a marking block with a 12 mm square hole as an example, the marking block with a 12 mm square hole is as follows... Figure 3 As shown;

[0146] Step 2: Select one of the marking blocks in sequence, fully fit it with the specimen in the length and width directions, and use a marking pen to draw lines around the square hole to obtain a square grid. Repeat the process of arranging the grid on the same specimen, and define each square grid area as a finite body A. Four different specifications of finite body A are obtained, and the nominal dimensions of their original length l0, original width w0, and original thickness t0 are: 3mm * 3mm * 3mm, 6mm * 6mm * 3mm, 9mm * 9mm * 3mm, and 12mm * 12mm * 3mm. Taking the 12mm width of the gauge section as an example, the four different specifications of finite body A calibrated on the same specimen are as Figure 7 shown.

[0147] Calibrate the specimens in three directions respectively and carry out coding. The specimens in the parallel rolling direction (0°), at 45° to the rolling direction, and perpendicular to the rolling direction (90°) are coded as A1, B1, and C1 in sequence. The finite body A on specimen A1 is coded as A1-3mm, A1-6mm, A1-9mm, and A1-12mm respectively; the finite body A on specimen B1 is coded as B1-3mm, B1-6mm, B1-9mm, and B1-12mm respectively; the finite body A on specimen C1 is coded as C1-3mm, C1-6mm, C1-9mm, and C1-12mm respectively.

[0148] Step 3: Mark the positions of the measurement points of each finite body A, measure and record the measured values of the original width w0, original length l0, and original thickness t0 of each finite body A. The intersection point O of the two symmetric lines is the measurement point in the thickness direction, and the measurement points on both sides in the width direction of the specimen are determined by referring to the position of the symmetric line b; use a micrometer (a flat-headed micrometer with a recommended diameter of 2mm) to measure the measured value of the thickness t0 of the finite body A; use a video microscope to measure the measured values of the original length l0 and original width W0 of each finite body A. At the intersection position of the outer edge of the grid and the symmetric line a, select the measurement points and mark the positions of the measurement points.

[0149] The specific method is as follows: Taking the tensile direction as an example, use the measurement and marking function of the software supporting the video microscope. On the picture (0) of the finite body A before deformation, take the marked point selection position as the marking point, as Figure 8 shown in -a. When measuring the picture of the finite body A after deformation again, select points according to the marked measurement point positions on the picture (0), as Figure 8 shown in -b; the measurement accuracy requirement reaches 0.001mm. The measured values of the length, width, and height (thickness) of the 12 finite body A calibrated on the specimens in three directions are shown in Table 7.

[0150] Table 7 Measured values of the length, width, and height of the 12 finite body A calibrated on the specimens in three directions before deformation

[0151]

[0152] Table 8 shows the measured length, width, and height of the 12 finite bodies A marked on the specimens in three directions after deformation.

[0153]

[0154] Step 4: Set a loading stroke and perform a fixed-stroke loading uniaxial tensile test. The fixed-stroke loading refers to a loading method in which the tensile end of the tensile testing machine stops moving when it reaches the set loading stroke. The set loading stroke does not exceed the critical value at which the finite body A undergoes non-uniform deformation. The uniform deformation means that the finite body A does not experience local necking in either the width or thickness direction.

[0155] The method for setting the loading stroke is as follows: Select an uncalibrated specimen and perform a uniaxial tensile test. Break the specimen directly and obtain the load-displacement curve. Before the curve drops downward or in the stable section of the tensile load, determine the loading stroke of the tensile test to be 4.0 mm. Perform a uniaxial tensile test with fixed stroke loading on the calibrated specimens in three directions respectively to obtain the specimen after unloading.

[0156] Step 5: Measure and record the measured values ​​of the length l1, width w1 and thickness t1 of each finite body A after deformation, as shown in Table 8. The positions of the measurement points are consistent with the measurement points marked in Step 3.

[0157] Step 6: Calculate the plastic strain ratio r for each finite body A using the following formula:

[0158]

[0159] and:

[0160]

[0161] Where, ε w ε represents the true plastic strain of finite body A in the width direction; l Let ε be the true plastic strain of finite body A along its length. t Let A be the true plastic strain in the thickness direction of the finite body A. The result calculated according to Equation (1) is the direct method, and the result calculated according to Equation (2) is the indirect method. The calculation results are shown in Table 9.

[0162] Table 9 shows the calculation results of the r values ​​of the 12 finite bodies A calibrated on the three samples.

[0163]

[0164] Based on the data in Table 9, the curves of gauge length versus r-value for the two calculation methods are plotted as follows: Figure 10As shown. Using the direct method, parallel to the rolling direction (0°), the average r value of four finite bodies A is taken, r0 equals 0.535; at 45° to the rolling direction, the average r value of four finite bodies A is taken, r... 45 Equal to 1.287; perpendicular to the rolling direction (90°), take the average of the r values ​​of four finite bodies A, r 90 Equals 0.782. Using the direct method, parallel to the rolling direction (0°), the average r value of four finite bodies A is taken, r0 equals 0.550; at 45° to the rolling direction, the average r value of four finite bodies A is taken, r... 45 Equal to 1.331; perpendicular to the rolling direction (90°), take the average of the r values ​​of four finite bodies A, r 90 It equals 0.982. Based on the average results of the three directions from the direct and indirect methods, the sampling direction versus r-value curves for the two calculation methods are plotted as follows: Figure 11 As shown.

[0165] Step 7: Calculate the weighted average value of the plastic strain ratio according to the following formula. Plastic strain ratio anisotropy Δr:

[0166]

[0167] Where r0 is the plastic strain ratio of the material being tested parallel to the rolling direction; r 45 r is the plastic strain ratio of the material being tested at a 45° angle to the rolling direction; 90 This is the plastic strain ratio of the material being tested perpendicular to the rolling direction.

[0168] The weighted average of plastic strain ratios was calculated using the direct method. The plastic strain ratio is equal to 0.847, and the anisotropy of the plastic strain ratio Δr is equal to 0.129; the weighted average of the plastic strain ratio is calculated using the indirect method. The value is equal to 0.939, and the anisotropy of the plastic strain ratio Δr is equal to -0.086.

[0169] Based on the above three embodiments, the following points can be summarized:

[0170] First, the r-values ​​measured by the direct method of this invention under different conditions have good stability, and the direct method is recommended as the preferred method. The stability of the indirect method is acceptable in engineering, and it can also be consistent with the direct method on the basis of strictly marking the measurement points twice.

[0171] Second, for determining the r-value of a specific material in a certain direction, it is recommended to use three 12mm wide specimens, with three grids arranged on each specimen (such as 6mm, 9mm and 12mm or 4mm, 8mm and 12mm), and apply three different fixed loading strokes respectively. The average value of the nine finite bodies A is taken as the r-value of the material being tested.

[0172] Third, the r-values ​​(for automotive steel) measured by the direct method of this invention under different conditions show that the r-value is independent of both "gauge length and loading stroke (strain level)," which is consistent with mechanical theory, but contradicts Note 3 in GB / T 5027-2016 Test Method for Plastic Strain Ratio (R-value) of Thin Metal Sheets and Strips. If this invention is used, and finite body A is confirmed to meet the uniform deformation requirement and the r-value is indeed related to the strain level, then the representation method of GB / T 5027-2016 can be adopted, and the number of fixed loading strokes can be increased.

[0173] Three additional points regarding the embodiments:

[0174] First, when using a micrometer to measure thickness and width, the measurement results will fluctuate to some extent. In order to make the fluctuation range of the measured r value smaller, it is appropriate to correct the thickness measurement result by 0.002 mm and the width measurement result by 0.005 mm.

[0175] Second, regarding the influence of coating thickness on coated materials, the inventors found that if the measurement results strictly meet the condition of constant volume, the coating thickness has no effect on the r value. However, if the deviation of the constant volume condition is large, the coating thickness has a certain influence on the r value. This indicates that the coating thickness is not an objective reason affecting the r value, that is, the r value is unrelated to the coating thickness.

[0176] Third, in GB / T 5027-2016 Test Method for Plastic Strain Ratio (R-value) of Thin Metal Sheets and Strips, only the determination of r-value under tensile conditions is specified. The indirect method specified therein cannot be used for the determination of r-value under uniaxial compression conditions because it uses an extensometer. Since the uniaxial compression test of thin sheets generally requires an additional buckling-resistance device, the direct method of this invention is more suitable for the determination of r-value under uniaxial compression conditions. Since the determination of r-value under uniaxial compression and uniaxial tension is completely similar when implementing this invention (the clamping section (2) and transition section (3) of the specimen may differ, but the gauge length section (1) is the same), therefore, this specification only provides examples based on uniaxial tensile tests.

[0177] The above embodiments are merely exemplary embodiments of the present invention and are not intended to limit the present invention. The scope of protection of the present invention is defined by the claims. Various modifications or equivalent substitutions made by those skilled in the art within the substantial scope of protection of the present invention also fall within the scope of protection of the present invention.

Claims

1. A test method for determining the plastic strain ratio, characterized by, Includes the following steps: Step 1: Prepare several samples with a gauge length of L0, an original width of W0, and an original thickness of t0. The samples have the same sampling direction. Step 2: Mark one or more local regions on the sample. Define the marked local region as a finite body A. The original length of the finite body A is l0, the original width is w0, and the original thickness is t0. Its length direction is parallel to the tensile direction of the sample, and l0 is less than L0 and w0 is less than or equal to W0. Step 3: Mark the measurement points in three directions of the finite body A, and measure and record the measured values ​​of its original thickness t0, original width w0 and / or original length l0; Step 4: Set a loading stroke and perform a uniaxial tensile test or uniaxial compression test with fixed stroke loading. After reaching the loading stroke, unload and remove the specimen. The fixed stroke loading refers to a loading method in which the testing machine automatically stops moving when it reaches the set loading stroke, and the set loading stroke does not exceed the critical value at which the finite body A undergoes non-uniform deformation. Step 5: Measure and record the measured values ​​of the thickness t1, width w1 and / or length l1 of the finite body A after deformation, wherein the location of the measurement point is consistent with the measurement point marked in Step 3; Step 6: Calculate the plastic strain ratio r of finite body A using the measured values ​​according to the following formula: or / and: wherein ε l is the true plastic strain of the finite body A in the length direction, ε w is the true plastic strain of the finite body A in the width direction, and ε t is the true plastic strain of the finite body A in the thickness direction.

2. The test method for determining the plastic strain ratio in accordance with claim 1, wherein: In step two, the method for calibrating local areas is as follows: First, within the gauge length of a surface of the sample, draw a line of symmetry in the width direction and a line of symmetry in the tensile direction, with the two lines of symmetry intersecting at point O; Second, arrange a square grid with a side length of l0 within the gauge length, the geometric center of which coincides with point O, and the side length ranges from [t0, W0]; If multiple local areas of different sizes are to be calibrated simultaneously on the same sample, repeat step two.

3. The test method for determining the plastic strain ratio according to claim 2, characterized in that: In step three, the original thickness t0 of finite body A is measured and recorded using a micrometer, with all measurement points being point O; the original length l0 and original width w0 of finite body A are measured and recorded using a video microscope. During the measurement, measurement points are selected at the intersection of the outer or inner edge of the grid with the symmetry lines a and b, and their positions are marked. In step five, the thickness t1 of the finite body A after deformation is measured and recorded using a micrometer; the length l1 and width w1 of the finite body A after deformation are measured and recorded using a video microscope, and the positions of the measurement points are selected by referring to or comparing with the measurement point positions marked in step three. In both of the above steps, the measurement accuracy is 0.001 mm. If the original width w0 of the finite body A is equal to the width W0 of the sample, a micrometer can also be used to measure its width before and after deformation. The position of the measurement point is determined with reference to the position of the symmetry line b in the tensile direction.

4. The test method for determining the plastic strain ratio in accordance with claim 3, wherein: In step two, the method for arranging a square grid on the sample is as follows: The first step is to make a marking block. The length and width of the marking block are equal to the length and width of the sample. A square hole with a side length of l0 is cut on it. The square hole is positioned to coincide with the geometric center of the marking block surface and is arranged along the length of the marking block. The second step is to fully attach the marking block to the sample in the length and width directions, and use a fine-line pen to draw lines around the square hole to obtain a square grid with a side length of l0. If you want to arrange square grids of different sizes on the same sample at the same time, repeat the above two steps.

5. The test method for determining the plastic strain ratio in accordance with claim 4, wherein: In step one, the gauge length L0 of the sample is equal to 50 mm, and the width W0 ranges from [t0, 12 mm].

6. The test method for determining the plastic strain ratio in accordance with claim 5, wherein: In step two: If only one square grid is arranged for a single specimen, then at least three specimens of different widths should be used. Repeat steps one to six to determine the plastic strain ratio of each finite body A, and then take the average value as the plastic strain ratio of the material being tested. If at least three square grids of different sizes are arranged on a specimen, then a single specimen can be used to determine the plastic strain ratio of each finite body A, and then the average value can be taken as the plastic strain ratio of the material being tested.

7. The test method for determining the plastic strain ratio in accordance with claim 6, wherein: Three different widths of specimens (4 mm, 8 mm, and 12 mm) or four different widths (3 mm, 6 mm, 9 mm, and 12 mm) are used, with only one square grid arranged on each specimen. The side length l0 is equal to the width W0 of the specimen. The plastic strain ratio of the finite body A on each specimen is measured, and the average value is taken as the plastic strain ratio of the material being tested.

8. The test method for determining the plastic strain ratio in accordance with claim 6, wherein: A 12mm wide specimen is used, with three square grids having side lengths of 4mm, 8mm, and 12mm, or four square grids having side lengths of 3mm, 6mm, 9mm, and 12mm. The plastic strain ratio of each finite body A is measured, and the average value is taken as the plastic strain ratio of the tested material.

9. Test method for determining the plastic strain ratio according to claim 7 or 8, characterized in that: Using at least three different fixed loading strokes, repeat steps one through six to measure the plastic strain ratio. Then, take the average value of the plastic strain ratio under each fixed loading stroke as the plastic strain ratio of the material being tested.

10. A test method for determining the plastic strain ratio in accordance with claim 7 or 8, characterised in that: After step five, apply an incremental load of 0.5 mm to the unloaded specimen. After the test, repeat step five and repeat the incremental load at least twice. Then, measure the plastic strain ratio of each specimen after unloading according to step six. Finally, take the average value of the plastic strain ratio of the same specimen under each unloading state as the plastic strain ratio of the tested material.

11. A test method for determining the plastic strain ratio in accordance with claim 7 or 8, characterised in that: Respectively from parallel rolling direction, vertical rolling direction and 45° with rolling direction three directions sampling, repeating step one to step six, measuring the plastic strain ratio r0, r 45 and r 90 of each sampling direction, the r0 is parallel rolling direction plastic strain ratio, r 45 is 45° with rolling direction plastic strain ratio, r 90 is vertical rolling direction plastic strain ratio.

12. The test method for determining the plastic strain ratio in accordance with claim 11, wherein: Calculate the weighted average value of plastic strain ratio r and the anisotropy of plastic strain ratio Δr using the following formulas: wherein r0 is the plastic strain ratio in the parallel rolling direction; r 45 r45 is the plastic strain ratio at 45° to the rolling direction; and 90 r90 is the plastic strain ratio perpendicular to the rolling direction.

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