A high-precision detection method for performance parameters of metal plates based on multi-angle stretching

Abstract

This invention relates to the field of material mechanical property testing technology, and more particularly to a high-precision method for detecting the performance parameters of metal sheets based on multi-angle tensile testing. By adding samples in three directions—15°, 30°, and 60°—this invention obtains denser anisotropic data points. Combined with full-field strain measurement using DIC (Distributed Induction Coefficient), it can more realistically and continuously reflect the anisotropic behavior of the sheet material in the plane, significantly improving the calculation accuracy of the Δr value. DIC technology avoids errors that may be introduced by contact measurement methods and can directly extract strain from the most stable deformation region, resulting in more reliable results. The continuous r(θ) curve obtained through function fitting provides direct and complete data support for parameter identification in advanced material constitutive models, improving the accuracy of finite element simulation analysis. This method is not only applicable to conventional steel sheets but also effective for sheets with more complex anisotropic behavior, such as aluminum alloys, magnesium alloys, and titanium alloys.

Description

Technical Field

[0001] This invention relates to the field of material mechanical property testing technology, and in particular to a high-precision method for detecting the performance parameters of metal sheets based on multi-angle stretching. Background Technology

[0002] During the rolling process, sheet metal develops a crystallographic texture, resulting in anisotropic mechanical properties, meaning it exhibits different deformation behaviors in different directions. This anisotropy has a crucial impact on the stamping performance of sheet metal, with two key parameters being: 1. Plastic strain ratio (r-value): This reflects the sheet metal's ability to resist thickness reduction during stretching. A higher r-value indicates stronger wrinkle resistance. 2. Planar anisotropy coefficient (Δr-value): This reflects the difference in r-values ​​in different directions, determining the severity of "earing" in stamped parts.

[0003] Traditional testing methods, typically based on national standards, use only three directions (0°, 45°, and 90°) for tensile testing. Strain is measured using extensometers or contact strain gauges, and the average r-value (rm) and Δr-value are then calculated. This method has the following significant drawbacks: 1. Insufficient information: Data from only three directions is insufficient to accurately describe the continuous anisotropic behavior of the sheet metal in a complex plane, leading to inaccurate predictions of the Δr-value. 2. Measurement limitations: Contact strain measurement methods struggle to accurately capture the true strain during the uniform plastic deformation stage before necking and cannot obtain the full-field strain distribution. 3. Calculation approximation: Based on data from a limited number of points, it is impossible to construct a continuous anisotropic function, limiting the support for material models.

[0004] To address the shortcomings of existing technologies, this invention proposes a high-precision method for detecting anisotropic parameters of metal sheets based on multi-angle tensile specimens and digital image correlation (DIC) non-contact full-field strain measurement technology. This method increases the number of sampling directions and utilizes DIC technology to acquire high-precision, full-field strain data, thereby achieving a more comprehensive and accurate description of the anisotropic behavior of the sheet material. Summary of the Invention

[0005] To address the technical problems existing in the background art, this invention proposes a high-precision detection method for the performance parameters of metal sheets based on multi-angle stretching. By increasing the sampling directions and utilizing DIC technology to obtain high-precision, full-field strain data, a more comprehensive and accurate description of the anisotropic behavior of the sheet material can be achieved. The technical solution adopted by this invention is as follows: A high-precision detection method for performance parameters of metal sheets based on multi-angle tension includes the following steps: Step 1: Prepare standard tensile specimens. Using the rolling direction of the metal sheet to be tested as a reference, cut rectangular flat tensile specimens at multiple angles to the rolling direction. Step 2: Sample preparation and DIC system setup. Prepare speckle patterns on the surface of each parallel segment of the sample as feature points for DIC analysis. Mount the sample on the universal tensile testing machine and adjust the position of the binocular DIC camera system so that it faces the sample surface, ensuring that the entire parallel segment is within the field of view and the image is clear. Calibrate the DIC system to establish an accurate correspondence between the image pixel coordinates and the world physical coordinates. Step 3: Synchronous Tensioning and Data Acquisition. Start the tensile testing machine and apply uniaxial tension to the specimen at a constant displacement rate. Simultaneously, the DIC camera system synchronously acquires a sequence of images of the specimen surface at a preset frame rate until the specimen breaks. The tensile testing machine synchronously records load-time data. Step 4: DIC data processing and key strain extraction. Import the sequence images acquired by the DIC camera system in Step 4 into the analysis software to calculate the full-field strain distribution data of the specimen during the entire tensile process, including the true longitudinal strain ε_l and the true transverse strain ε_w. For the specimen at each angle, select a stable deformation state in the uniform plastic deformation stage from its experimental data. Under this state, select a representative analysis region ROI in the center region of the parallel segment of the specimen. Step 5: Calculate anisotropic parameters. Calculate the stress-strain curves, the plastic strain ratio in each direction, construct the anisotropic coefficient function, and calculate the Δr value. Step Six: Result Output and Material Card Generation. Output the measured r_θ values ​​in each direction, the expression of the fitting function, the calculated r_m values, and the Δr values. The above high-precision data can be directly used to establish the constitutive model of the material, providing accurate input parameters for computer simulation of the stamping process.

[0006] As a further optimization of the above-mentioned high-precision detection method for the performance parameters of metal sheets based on multi-angle stretching, in step one, rectangular flat plate tensile specimens with six different included angles to the rolling direction are cut, with included angles of 0°, 15°, 30°, 45°, 60° and 90° respectively.

[0007] As a further optimization of the above-mentioned high-precision detection method for the performance parameters of metal sheets based on multi-angle stretching, the frame rate preset for the DIC camera system in step three is 5-10 frames per second.

[0008] As a further optimization of the above-mentioned high-precision detection method for the performance parameters of metal sheets based on multi-angle tension, in step five, the calculation of the stress-strain curve involves first extracting the average values ​​of the longitudinal true strain ε_l and the transverse true strain ε_w within the analysis region ROI; based on the volume invariance assumption formula ε_l + ε_w + ε_t = 0, the true strain in the thickness direction ε_t = -(ε_l + ε_w) is calculated; within the gauge length of the specimen, a calculation region l*w is taken, and the average strain εl_1 and εw_1 within the region are obtained through DIC calculation. Therefore, the average strain in the thickness direction is εt_1 = -(εl_1 + εw_1), and the cross-sectional area of ​​the specimen after deformation is: The stress in each direction can be calculated using the formula σ=F / A, based on the loads obtained from the single tensile test in each included angle direction.

[0009] As a further optimization of the above-mentioned high-precision detection method for the performance parameters of metal sheets based on multi-angle tension, in step five, the plastic strain ratio in each direction is calculated. According to the definition, the plastic strain ratio of each angle θ is: r_θ = ε_w / ε_t. Substituting ε_w and ε_t obtained in step four, the plastic strain ratio of each angle θ is calculated respectively.

[0010] As a further optimization of the above-mentioned high-precision detection method for the performance parameters of metal sheets based on multi-angle stretching, in step five, an anisotropic coefficient function is constructed and Δr is calculated. Using data from six directions, function fitting is used to more accurately describe the continuous change of the r value with angle θ.

[0011] Beneficial effects Compared with the prior art, the present invention has significant advantages and beneficial effects, achieving considerable technological progress and practicality, and possessing broad application value. It has at least the following advantages: 1. High precision and comprehensiveness: By adding samples in three directions of 15°, 30° and 60°, a denser anisotropic data point was obtained. Combined with DIC's full-field strain measurement, it can more realistically and continuously reflect the anisotropic behavior of the plate in the plane, and significantly improve the calculation accuracy of Δr value.

[0012] 2. Non-contact and anti-interference: DIC technology avoids the errors that may be introduced by contact measurement methods and can directly extract strain from the most stable deformation area, resulting in more reliable results.

[0013] 3. Data-driven modeling: The continuous r(θ) curve obtained by function fitting provides direct and complete data support for parameter identification of advanced material constitutive models (such as Hill'48, Barlat'89, etc.), improving the accuracy of finite element simulation analysis.

[0014] 4. High applicability: The method of this invention is not only applicable to conventional steel plates, but also effective for plates with more complex anisotropic behavior such as aluminum alloys, magnesium alloys, and titanium alloys.

[0015] 5. The method of the present invention increases the sampling direction and uses DIC technology to obtain high-precision, full-field strain data, thereby achieving a more comprehensive and accurate description of the anisotropic behavior of the plate material. Attached Figure Description

[0016] Figure 1 This is a schematic diagram showing the distribution of the specimen in six directions; Figure 2 This is a schematic diagram of the speckle pattern applied to the test specimen. Figure 3 These are schematic diagrams of the specimen before and after tension. Figure 4 This is a schematic diagram of the detection device; Figure 5 These are the stress-strain curves of the specimen in six directions; In the diagram: 1. Specimen; 2. Tensile testing machine; 3. Dynamic testing system; 4. Light source; 5. Control box; 6. Computer. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments and accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of protection. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art, without creative effort, including formal modifications to the technical solutions described in the following embodiments or equivalent substitutions of some technical features, based on the inspiration of the present invention, are within the scope of protection of the present invention.

[0018] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Specific Implementation Example 1: 1. Dumbbell-shaped specimens in six directions were cut from rolled Q420 sheet. The specimens were cleaned with acetone solution, sprayed with speckle coating, and then tested on a single-phase tensile testing machine at a tensile speed of 3 mm / min. The load-time relationship data for the specimens in the six directions were obtained, such as... Figure 1 , 2 As shown in Figures 3 and 4.

[0020] 2. Digital Image Correlation (DIC) technology was used to detect the strain of the test specimen in six directions. The DIC system employed two high-precision cameras, model XDTIC-CONST-5M, with a resolution of 2448×2048 and a lens focal length of 25mm. Taking the 180° test as an example, speckle coating was sprayed onto the specimen, which was then clamped onto the tensile testing machine. While the tensile testing machine was operating, the detection system took single-camera photos of the specimen directly in front of it at a frame / second shooting frequency.

[0021] 3. In the testing software, select the area within the gauge length of the specimen that needs to be calculated, set the grid size to 25 pixels × 25 pixels, and the step size to 25 pixels × 25 pixels. Calculate the X-direction strain and Y-direction strain at the center point of each grid. Take the average X-direction strain values ​​as follows: εw_0° = -3.64875, εw_15° = -3.42862, εw_30° = -3.35458, εw_45° = -3.87193, εw_60° = -3.9167, εw_90° = -3.85226.

[0022] The average strain values ​​are εl_0° = 8.468485, εl_15° = 8.326724, εl_30° = 8.38809, and εl_45° = 8.851228. , .

[0023] 4. Draw the stress-strain curves in six directions, such as... Figure 5 As shown, Figure 5 These are the stress-strain curves of the specimen in six directions. From the engineering stress-strain curves, it can be seen that before reaching the maximum tensile strength, the deformation at each key point in the six loading directions (0°, 15°, 30°, 45°, 60°, and 90°) is quite consistent, and the yield strength and tensile strength are almost the same. After the hardening stage, necking occurs, and the engineering stress changes significantly with increasing engineering strain.

[0024] 5. Calculate the plastic strain ratio in the six directions. , εt_15°=-(εl_15°+εw_15°)=-4.898104, εt_30°=-(εl_30°+εw_30°)=-5.03351, εt_45°=-(εl_45°+ε w_45°)=-4.979298, εt_60°=-(εl_60°+εw_60°)=-4.609912, εt_90°=-(εl_90°+εw_90°)=-4.809806, From r_θ = ε_w / ε_t, we get r0°=0.757044, r 15 °=0.699989, r 30 °=0.666449, r 45 °=0.777606, r 60 °=0.849626, r 90 °=0.800918.

[0025] 6. Construct the anisotropy coefficient function and calculate Δr The traditional three-angle method calculates Δr = (r0° + r 90 ° - 2r 45 The accuracy is limited (°) / 2. This invention utilizes data from six directions and employs function fitting to more accurately describe the continuous change of the r value with respect to angle θ.

[0026] a. Assume that the value of r changes with angle θ according to a functional relationship, for example, using the second harmonic function: r(θ) = A + B * cos(2θ) + C * sin(2θ), Where A, B, and C are undetermined coefficients.

[0027] b. Using the θ (converted to radians) in the six directions and the calculated r_θ as known data points, curve fitting is performed using the least squares method to solve for the optimal values ​​of coefficients A, B, and C.

[0028] c. Based on the fitted continuous function r(θ), the plane anisotropy coefficient Δr can be calculated more accurately. A preferred definition of Δr is the amplitude of the function r(θ), calculated as: Δr = (r_max - r_min) / 2. Where r_max and r_min are the maximum and minimum values ​​of the function r(θ) in the interval from 0° to 90°, respectively. Alternatively, they can be calculated based on the fitting coefficients: Δr = √(B 2 + C 2 ).

[0029] Furthermore, the average plastic strain ratio r_m can be obtained from the average value of the fitting function, i.e., r_m = A.

[0030] The following trigonometric function was obtained by fitting using the least squares method: r(θ)=a0+a1cos(πθ / 180)+a2cos(2πθ / 180)+a3sin(πθ / 180)+a4sin (2πθ / 180), The coefficients are: a0 = 0.774, a1 = -0.124, a2 = 0.012, a3 = -0.033, a4 = 0.045. The coefficient of determination R of the fitting function 2 ≈0.95, which can reflect the data trend well.

[0031] The average plastic strain ratio r_m = 0.766, with a maximum value of 0.862 at θ≈69.3° and a minimum value of 0.661 at θ≈26.0°. At this point, we get Δr = 0.1005 from Δr = (r_max - r_min) / 2. The above high-precision data can be directly used to establish the constitutive model of the material, providing accurate input parameters for the computer simulation of the stamping process.

[0032] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-precision detection method for performance parameters of metal sheets based on multi-angle tensile testing, characterized in that: Includes the following steps, Step 1: Prepare standard tensile specimens. Using the rolling direction of the metal sheet to be tested as a reference, cut rectangular flat tensile specimens at multiple angles to the rolling direction. Step 2: Sample preparation and DIC system setup. Prepare speckle patterns on the surface of each parallel segment of the sample as feature points for DIC analysis. Mount the sample on the universal tensile testing machine and adjust the position of the binocular DIC camera system so that it faces the sample surface, ensuring that the entire parallel segment is within the field of view and the image is clear. Calibrate the DIC system to establish an accurate correspondence between the image pixel coordinates and the world physical coordinates. Step 3: Synchronous Tensioning and Data Acquisition. Start the tensile testing machine and apply uniaxial tension to the specimen at a constant displacement rate. Simultaneously, the DIC camera system synchronously acquires a sequence of images of the specimen surface at a preset frame rate until the specimen breaks. The tensile testing machine synchronously records load-time data. Step 4: DIC Data Processing and Key Strain Extraction. The sequence of images acquired by the DIC camera system in Step 4 is imported into the analysis software to calculate the full-field strain distribution data of the specimen throughout the tensile process, including the true longitudinal strain. and lateral true strain For each angle of the specimen, a stable deformation state in the uniform plastic deformation stage is selected from its experimental data. Under this state, a representative analysis region (ROI) is selected in the central region of the parallel segment of the specimen. Step 5: Calculate anisotropic parameters. Calculate the stress-strain curves, the plastic strain ratio in each direction, construct the anisotropic coefficient function, and calculate the Δr value. Step Six: Result Output and Material Card Generation. Output the measured r_θ values ​​in each direction, the expression of the fitting function, the calculated r_m values, and the Δr values. The above high-precision data can be directly used to establish the constitutive model of the material, providing accurate input parameters for computer simulation of the stamping process.

2. The high-precision detection method for performance parameters of metal sheets based on multi-angle tension as described in claim 1, characterized in that: In step one, rectangular flat plate tensile specimens are cut at six different angles to the rolling direction, with the angles being 0°, 15°, 30°, 45°, 60° and 90° respectively.

3. The high-precision detection method for performance parameters of metal sheets based on multi-angle tension as described in claim 1, characterized in that: In step three, the preset frame rate of the DIC camera system is 5-10 frames per second.

4. The high-precision detection method for performance parameters of metal sheets based on multi-angle tension as described in claim 1, characterized in that: In step five, the stress-strain curve is calculated by first extracting the true longitudinal strain within the analysis region (ROI). and lateral true strain The average value; based on the assumption of constant volume, the formula Calculate the true strain in the thickness direction Within the gauge length of the specimen, a calculation region l*w is selected, and the average strain within the region is obtained through DIC calculation. , Then the average strain in the thickness direction is Then the cross-sectional area of ​​the specimen after deformation is: The stress in each direction can be calculated using the formula σ=F / A, based on the loads obtained from the single tensile test in each included angle direction.

5. The high-precision detection method for performance parameters of metal sheets based on multi-angle tension as described in claim 1, characterized in that: In step five, the plastic strain ratio in each direction is calculated. By definition, the plastic strain ratio at each angle θ is: The result obtained in step four and Substitute the values ​​and calculate the plastic strain ratio for each angle θ.

6. The high-precision detection method for performance parameters of metal sheets based on multi-angle tension as described in claim 1, characterized in that: In step five, an anisotropic coefficient function is constructed and Δr is calculated. Using data from six directions, function fitting is used to more accurately describe the continuous change of the r value with angle θ.

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