A method for calculating the pre-stretching amount of a skin stretch incremental composite forming process

Abstract

The present application relates to the technical field of skin stretching, and particularly relates to a pre-stretching amount calculation method of skin stretching gradual composite forming process, comprising the following steps: obtaining skin material to be used by a target machining part, and obtaining forming target parameters of the target machining part; constructing a calculation formula of optimal pre-stretching amount; obtaining a strain hardening index through a unidirectional stretching experiment of the skin material, obtaining a forming limit of the skin material in a plane strain state through a forming limit experiment of the skin material, obtaining a comprehensive working condition coefficient according to the forming target parameters, obtaining a safety correction factor according to a preset skin processing path and a safety margin, and measuring an initial length of the skin, and then calculating the optimal pre-stretching amount according to the calculation formula of the optimal pre-stretching amount. The present application improves the calculation efficiency and calculation accuracy of the pre-stretching amount.

Description

Technical Field

[0001] This invention relates to the field of skin stretching technology, and in particular to a method for calculating the pre-stretching amount in a progressive composite forming process for skin stretching. Background Technology

[0002] Skin stretching and incremental forming are two important methods for manufacturing complex curved sheet metal parts. Skin stretching applies uniform tensile strain to the sheet metal, causing work hardening, which effectively suppresses springback after subsequent forming and unloading, improving shape freeze-thaw stability. Incremental forming, on the other hand, achieves flexible manufacturing of complex three-dimensional shapes through localized, point-by-point deformation without the need for specialized molds. The "skin stretching-incremental composite forming" process, which combines the two, first obtains a preform with a uniformly hardened state and initial curvature through stretching, and then completes the shaping of local fine features through incremental forming, combining the advantages of high precision and high flexibility.

[0003] However, determining the initial pre-stretch amount is a key technical challenge in this composite forming process. If the stretch amount is insufficient, the work hardening effect will be weak, the elastic zone inside the sheet will account for a large proportion, and the springback cannot be effectively controlled, affecting dimensional accuracy. If the stretch amount is too large, it will excessively consume the plasticity reserve of the material, increasing the risk of breakage in subsequent progressive forming. At present, the determination of the pre-stretch amount mainly relies on the operator's experience, finite element simulation iteration, or a large number of "trial and error-correction" physical experiments, lacking a direct quantitative calculation method that can comprehensively consider the material constitutive, geometric characteristics, and failure limit. Existing methods have the following inherent defects: (1) Strong subjectivity and poor consistency: the experience method varies from person to person, and the process stability cannot be guaranteed. (2) Long development cycle and high cost: whether it is numerical simulation or physical trial and error, repeated iterations are required, especially for high-value aerospace materials, the cost is unbearable. (3) Lack of theoretical guidance and blind optimization: existing methods have failed to establish an explicit theoretical relationship between the stretch amount and the intrinsic properties of the material (such as forming limit and hardening ability) and the geometric characteristics of the part, and the optimization lacks a clear direction. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in related technologies. To this end, this invention provides a method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching, solving the technical problem of low pre-stretch amount calculation efficiency leading to high part processing costs in the prior art, and improving the calculation efficiency and accuracy of the pre-stretch amount.

[0005] This invention provides a method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching, comprising the following steps: Obtain the skin material to be used for the target part to be processed, and obtain the forming target parameters of the target part to be processed; The formula for calculating the optimal pre-stretch amount is as follows: ,in, This represents the optimal pre-stretch amount for the skin. This is the initial length of the skin. As a safety correction factor, This represents the forming limit of the skin material under plane strain. The maximum forming angle between the local surface of the target part and the horizontal direction, used for skin fabrication. For strain enhancement index, This is the comprehensive operating condition coefficient; The strain hardening index was obtained through uniaxial tensile tests on the skin material. The forming limit of the skin material under plane strain was obtained through forming limit experiments. Based on the forming target parameters, obtain The comprehensive operating condition coefficient is obtained based on the parameters of the skin processing equipment. The safety correction factor is obtained based on the preset skin processing path and safety margin. The initial length of the skin was measured. Then, the optimal pre-tension amount is calculated according to the formula for calculating the optimal pre-tension amount.

[0006] A further improvement of the pre-stretch calculation method for the progressive composite forming process of skin stretching of the present invention lies in that the calculation formula for the optimal pre-stretch amount includes the following steps: Hollomon power-law hardening model Describe the plastic constitutive relation of the skin material, where, True stress, For true plastic strain, For strain enhancement index, The strength coefficient; The skin forming limit constraint in the progressive composite forming process of skin stretching is: ,in, For pre-stretch true strain, The true strain consumed during the incremental forming stage. As a safety correction factor, This represents the forming limit of the skin material under plane strain. The skin stability constraint in the progressive composite forming process of skin stretching is: , This is the comprehensive operating condition coefficient; Skin thickness variation ,in, The thickness after skin forming. The initial thickness of the skin. The angle between a local surface of the formed skin and the horizontal direction. The value is (0, 90°); The maximum forming angle between the local surface of the target part and the horizontal direction in skin fabrication θ max Decide The calculation formula is: ,because When the angle is between (0, 90°), cos θ The value is less than 1. If it is negative, take As the true strain consumed in the incremental forming stage, we obtain ; Theoretical optimal pre-stretch strain for: min{} means taking the smaller of the two calculated values ​​within the parentheses; Optimal stretch of skin The calculation formula is ,in, L 0 represents the initial length of the skin.

[0007] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching in this invention lies in the following: The value range is 0.8 to 0.9.

[0008] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching of the present invention is that when ≤45° The value range is 0.85 to 0.9.

[0009] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching of the present invention is that when >45° The value range is 0.8 to 0.85.

[0010] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching in this invention lies in the following: The value range is 1.1 to 1.5.

[0011] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching in this invention lies in that, when the processing accuracy of the skin processing equipment is ≤±1% and the positioning accuracy is ≤±0.05mm, The value range is 1.1 to 1.3.

[0012] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching in this invention lies in that, when the processing accuracy of the skin processing equipment is > ±1% and the positioning accuracy is > ±0.05mm, The value range is 1.3 to 1.5.

[0013] A further improvement of the pre-stretching calculation method for the progressive composite forming process of skin stretching of the present invention is that the skin material is a metal material.

[0014] This invention's pre-stretch calculation method comprehensively considers the intrinsic properties of the skin material, the target forming parameters, actual production conditions (comprehensive working condition coefficient), and safety margin (safety correction factor). By constraining both the material forming limit and the working condition through a min function, it avoids calculation deviations dominated by a single factor, ensuring a high degree of match between the pre-stretch calculation results and the actual forming process, significantly improving the scientific rigor and accuracy of the pre-stretch setting. By matching the optimal pre-stretch amount with the material forming limit and the target forming angle, the strain distribution during the skin stretching process can be precisely controlled. This avoids forming defects such as local wrinkling and excessive springback caused by insufficient pre-stretch, while preventing the risk of local necking and cracking caused by excessive pre-stretch. It fully taps the forming potential of the material, expands the formable range of complex skin parts, and significantly improves the surface accuracy and forming integrity of the parts. By quantifying process factors such as equipment parameters, processing paths, and safety margins into comprehensive operating condition coefficients and safety correction factors, the pre-stretching calculation is deeply integrated with actual production conditions, reducing reliance on manual experience. This maintains the stability and repeatability of the pre-stretching process across different batches or processing scenarios with different equipment, effectively reducing forming defect rates and scrap rates. Pre-stretching control avoids excessive material thinning and part scrapping caused by over-stretching, while also reducing subsequent trimming and shaping processes due to insufficient pre-stretching, significantly improving skin material utilization and reducing raw material loss and rework costs. The introduced parameters, such as the strain strengthening index and comprehensive operating condition coefficient ξ, are adaptable to skin materials of different materials and states (such as aerospace aluminum alloys and titanium alloys), and are compatible with forming targets and processing equipment of varying complexity, providing a universal pre-stretching calculation scheme for the stretching forming of diverse skin parts. By adjusting parameters such as the safety correction factor and comprehensive operating condition coefficient, the process can be refined and optimized, providing a solid theoretical foundation and quantitative tools for the digital control and intelligent parameter matching of the skin stretching progressive composite forming process.

[0015] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of a method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching provided by the present invention.

[0018] Figure 2 It is a true stress-true plastic strain fitting diagram.

[0019] Figure 3 It is the forming limit of the skin material under plane strain. A schematic diagram.

[0020] Figure 4 It is the maximum forming angle between the local surface of the target part and the horizontal direction in the skin fabrication process. A schematic diagram. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. The following embodiments are used to illustrate this invention but should not be used to limit the scope of this invention.

[0022] The following is combined with Figure 1 The present invention describes a method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching, comprising the following steps: Obtain the skin material to be used for the target part to be processed, and obtain the forming target parameters of the target part to be processed; The formula for calculating the optimal pre-stretch amount is as follows: ,in, This represents the optimal pre-stretch amount for the skin. This is the initial length of the skin. As a safety correction factor, This represents the forming limit of the skin material under plane strain. The maximum forming angle between the local surface of the target part and the horizontal direction, used for skin fabrication. For strain enhancement index, This is the comprehensive operating condition coefficient; The strain hardening index was obtained through uniaxial tensile tests on the skin material. The forming limit of the skin material under plane strain was obtained through forming limit experiments. Based on the forming target parameters, obtain The comprehensive operating condition coefficient is obtained based on the parameters of the skin processing equipment. The safety correction factor is obtained based on the preset skin processing path and safety margin. The initial length of the skin was measured. Then, the optimal pre-tension amount is calculated according to the formula for calculating the optimal pre-tension amount.

[0023] This invention's pre-stretch calculation method comprehensively considers the intrinsic properties of the skin material, the target forming parameters, actual production conditions (comprehensive working condition coefficient), and safety margin (safety correction factor). By constraining both the material forming limit and the working condition through a min function, it avoids calculation deviations dominated by a single factor, ensuring a high degree of match between the pre-stretch calculation results and the actual forming process, significantly improving the scientific rigor and accuracy of the pre-stretch setting. By matching the optimal pre-stretch amount with the material forming limit and the target forming angle, the strain distribution during the skin stretching process can be precisely controlled. This avoids forming defects such as local wrinkling and excessive springback caused by insufficient pre-stretch, while preventing the risk of local necking and cracking caused by excessive pre-stretch. It fully taps the forming potential of the material, expands the formable range of complex skin parts, and significantly improves the surface accuracy and forming integrity of the parts. By quantifying process factors such as equipment parameters, processing paths, and safety margins into comprehensive operating condition coefficients and safety correction factors, the pre-stretching calculation is deeply integrated with actual production conditions, reducing reliance on manual experience. This maintains the stability and repeatability of the pre-stretching process across different batches or processing scenarios with different equipment, effectively reducing forming defect rates and scrap rates. Pre-stretching control avoids excessive material thinning and part scrapping caused by over-stretching, while also reducing subsequent trimming and shaping processes due to insufficient pre-stretching, significantly improving skin material utilization and reducing raw material loss and rework costs. The introduced parameters, such as the strain strengthening index and comprehensive operating condition coefficient ξ, are adaptable to skin materials of different materials and states (such as aerospace aluminum alloys and titanium alloys), and are compatible with forming targets and processing equipment of varying complexity, providing a universal pre-stretching calculation scheme for the stretching forming of diverse skin parts. By adjusting parameters such as the safety correction factor and comprehensive operating condition coefficient, the process can be refined and optimized, providing a solid theoretical foundation and quantitative tools for the digital control and intelligent parameter matching of the skin stretching progressive composite forming process.

[0024] Furthermore, the calculation formula for the optimal pre-stretch amount includes the following steps: Based on elastoplastic mechanics, the Hollomon power-law hardening model is adopted. Describe the plastic constitutive relation of the skin material, where, True stress, For true plastic strain, For strain enhancement index, The strength coefficient; Based on the Keeler-Goodwin forming limit theory, the forming limit of skin materials under plane strain is... As the overall deformation safety boundary, the skin forming limit constraint in the skin stretching progressive composite forming process is: ,in, For pre-stretch true strain, The true strain consumed during the incremental forming stage. As a safety correction factor, This represents the forming limit of the skin material under plane strain. Based on the Considère plastic instability criterion, to ensure that no necking occurs during the pre-stretching stage, the skin stability constraint in the progressive composite forming process of skin stretching is as follows: , This is the comprehensive operating condition coefficient; Based on the progressive forming cosine law, the geometric complexity of the target machined part is quantified as strain consumption; based on the progressive forming cosine law, the skin thickness change is... ,in, The thickness after skin forming. The initial thickness of the skin. The angle between a local surface of the formed skin and the horizontal direction. The value is (0, 90°); The maximum forming angle between the local surface of the target part and the horizontal direction in skin fabrication θ max Decide The calculation formula is: ,because When the angle is between (0, 90°), cos θ The value is less than 1. If it is negative, take As the true strain consumed in the incremental forming stage, we obtain This formula shows that the steeper the feature (the larger θmax), the more plastic reserves are consumed in the incremental forming stage, and the less "safety margin" is left for pre-stretching; Combining the two constraints mentioned above and Theoretical optimal pre-stretch strain for: min{} means taking the smaller of the two calculated values ​​within the parentheses; Based on the theory of large deformation, the theoretically optimal pre-tension strain is the natural logarithm of the ratio of the instantaneous length to the initial length of the skin: ,in, L final This refers to the final length of the skin after pre-stretching. L 0 represents the initial length of the skin. Taking the exponents of both sides of the above formula: ,because ,Will Fully integrate the optimal stretch of the skin. The calculation formula is ,in, L 0 represents the initial length of the skin.

[0025] Furthermore, To account for nonlinearity in composite forming paths, The value range is 0.8 to 0.9. When ≤45°, which is suitable for composite forming processes with high path linearity and simple strain history. The value range is 0.85 to 0.9. When >45°, which is relevant for composite forming processes with complex paths and significant nonlinearity. The value range is 0.8 to 0.85.

[0026] Furthermore, the comprehensive operating condition coefficient A safety factor greater than 1 is used to compensate for the effects of friction, equipment precision, and process fluctuations; for operating conditions where equipment is in good condition and mold lubrication is stable, Take a value of 1.1 to 1.3; for applications with complex operating conditions or requiring high safety margins, ξ Take 1.3~1.5. When the processing accuracy of the skinning equipment is ≤±1% and the positioning accuracy is ≤±0.05mm, The value range is 1.1~1.3. When the processing accuracy of the skin processing equipment is >±1% and the positioning accuracy is >±0.05mm, The value range is 1.3 to 1.5.

[0027] Preferably, the skin material is a metallic material. The metallic material may be an aluminum alloy or a titanium alloy.

[0028] The optimal pre-stretching amount optimization formula proposed in this invention contains multiple parameters, the accuracy of which directly affects the reliability of the calculation results. The calibration methods for each key parameter are as follows: (1) Strain hardening index n The stress-strain curves were obtained through uniaxial tensile tests and converted into true stress-true plastic strain curves. The Hollomon power-law hardening model was then used. The result was obtained through fitting.

[0029] Specimens and equipment: Tensile specimens of 5052-H32 aluminum alloy sheet were prepared according to GB / T 228.1, with a gauge length of 50 mm, a parallel section width of 12.5 mm, and a sheet thickness of 1.5 mm; a 100 kN electronic universal testing machine was used; the extensometer gauge length was 50 mm; and the beam speed was 2 mm / min.

[0030] Operating steps: First, clamp the specimen and install the extensometer; second, collect engineering stress and strain data until uniform deformation is complete; finally, convert the engineering stress-strain data into true stress-true plastic strain data and fit the curve to obtain n. Formulas for converting engineering data into true stress and true strain: , ,in For engineering stress, For engineering contingency, The true stress of the t-th test, Let be the true plastic strain of the t-th test. The uniaxial tensile test data are shown in Table 1.

[0031] Table 1 Uniaxial tensile test data

[0032] According to Hollomon's power-law hardening model The fitting yielded n=0.196, and the true stress-true plastic strain fitting graph is shown below. Figure 2 As shown.

[0033] (2) Forming limit of skin material under plane strain state The FLD curve was obtained through the standard forming limit test (GB / T24171, hemispherical punching test), and the plane strain state (secondary strain) was read. The principal strain corresponding to 0) value.

[0034] Operating procedures: After the crack initiation is achieved through stamping, the principal and secondary strains in the vicinity of the crack are read through mesh analysis, and the strain limit points are connected to form the FLD curve. The experimental data of the forming limit are shown in Table 2.

[0035] Table 2 Forming Limit Test Data

[0036] FLD curve as follows Figure 3 As shown, by Figure 3 It can be seen that the current strain When =0, the corresponding principal strain limit value is =0.32.

[0037] (3) The maximum forming angle between the local surface of the target part made of skin and the horizontal direction. The determination is made by measuring the 3D CAD model of the target part. In the progressive forming region, the maximum value of the angle between the tangent of the formed surface and the original skin plate plane at all points is taken, such as... Figure 4 As shown.

[0038] (4) Safety correction factor Used to compensate for deviations between the theoretical model and actual conditions. Recommended value range: 0.8~0.9. For parts with simple deformation paths, a value of 0.85~0.9 is suitable; for parts with complex deformation paths, a value of 0.8~0.85 is suitable.

[0039] (5) Comprehensive working condition coefficient For operating conditions where the equipment is in good condition and the mold lubrication is stable, Take a value of 1.1 to 1.3; for applications with complex operating conditions or requiring high safety margins, Take 1.3~1.5.

[0040] (6) Initial length of skin The original dimensions of the skin along the pre-stretch direction are obtained by direct measurement.

[0041] The following application scenario demonstrates how to use the pre-stretch calculation method proposed in this invention to calculate the optimal stretch amount in the skin stretching-progressive composite forming process.

[0042] Example 1: Forming of aerospace aluminum alloy skin parts Step 1: Determine basic information about parts and processes Part features: Fuselage skin with smooth transitions, double curvature, and local shallow bosses.

[0043] Material: Aerospace-grade 5052-H32 aluminum alloy with good formability.

[0044] Initial skin dimensions: length =1200mm, width 600mm, thickness 1.5mm.

[0045] Step 2: Determine material performance parameters Strain hardening index n: Standard specimens were prepared and uniaxial tensile tests were conducted according to the national standard GB / T 228.1 Metallic materials, tensile testing—Part 1: Test methods at room temperature. The obtained true stress-true plastic strain curve data were analyzed using the Hollomon power-law hardening model. Nonlinear regression fitting yielded n=0.196 for this batch of materials.

[0046] Forming limit of skin material under plane strain state Based on the national standard GB / T24171 "Determination of Forming Limit Curves for Thin Sheets and Strips of Metallic Materials", the forming limit curve (FLD) of this material was determined and plotted using hemispherical punching tests combined with digital image and mesh analysis techniques. The principal strain limit values ​​corresponding to the plane strain state were determined from the graph. =0.32.

[0047] Step 3: Extract the geometric parameters of the part Analyze the 3D CAD model of the part, focusing on the local shallow boss feature completed by progressive forming. Measurement. =40°.

[0048] Step 4: Determine the safety correction factor and comprehensive operating condition coefficient. Considering the stable condition of equipment and good lubrication at the production site, but to retain a certain safety margin to cope with minor fluctuations in material properties, a comprehensive operating condition coefficient is selected. =1.2. Due to the simplicity of the deformation path, the safety correction factor is... =0.9.

[0049] Step 5: Calculate the incremental forming geometric strain consumption According to the formula The calculation yields: .

[0050] Step 6: Calculate the available plastic allowance based on the total forming limit. This means that after meeting the incremental forming geometry requirements, the material still has approximately 2.15% of the actual strain margin available for pre-stretching.

[0051] Step 7: Consider pre-stretching uniformity stability constraints Based on the Considère plastic instability criterion, to ensure that no necking occurs during the pre-tensioning stage, the pre-tensioning strain must meet the following requirements: If the strain hardening index of the material is n=0.196, then... .

[0052] Step 8: Calculate the theoretical optimal pre-tension amount ΔL Substitute the parameter values ​​into the pre-stretching calculation formula provided by this invention: The final calculation result is: Therefore, according to the method of the present invention, the optimal stretching amount ΔL of the aluminum alloy skin part in the skin stretching-progressive composite forming process is approximately 26.08 mm.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching, characterized in that, Includes the following steps: Obtain the skin material to be used for the target part to be processed, and obtain the forming target parameters of the target part to be processed; The formula for calculating the optimal pre-stretch amount is as follows: ,in, This represents the optimal pre-stretch amount for the skin. This is the initial length of the skin. As a safety correction factor, This represents the forming limit of the skin material under plane strain. The maximum forming angle between the local surface of the target part and the horizontal direction, used for skin fabrication. For strain enhancement index, This is the comprehensive operating condition coefficient; The strain hardening index was obtained through uniaxial tensile tests on the skin material. The forming limit of the skin material under plane strain was obtained through forming limit experiments. Based on the forming target parameters, obtain The comprehensive operating condition coefficient is obtained based on the parameters of the skin processing equipment. The safety correction factor is obtained based on the preset skin processing path and safety margin. The initial length of the skin was measured. Then, the optimal pre-tension amount is calculated according to the formula for calculating the optimal pre-tension amount.

2. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 1, characterized in that, The calculation formula for the optimal pretension amount includes the following steps: Hollomon power-law hardening model Describe the plastic constitutive relation of the skin material, where, True stress, For true plastic strain, For strain enhancement index, The strength coefficient; The skin forming limit constraint in the progressive composite forming process of skin stretching is: ,in, For pre-stretch true strain, The true strain consumed during the incremental forming stage. As a safety correction factor, This represents the forming limit of the skin material under plane strain. The skin stability constraint in the progressive composite forming process of skin stretching is: , This is the comprehensive operating condition coefficient; Skin thickness variation ,in, The thickness after skin forming. The initial thickness of the skin. The angle between a local surface of the formed skin and the horizontal direction. The value is (0, 90°); The maximum forming angle between the local surface of the target part and the horizontal direction in skin fabrication θ max Decide The calculation formula is: ,because When the angle is between (0, 90°), cos θ The value is less than 1. If it is negative, take As the true strain consumed in the incremental forming stage, we obtain ; Theoretical optimal pre-stretch strain for: min{} means taking the smaller of the two calculated values ​​within the parentheses; Optimal stretch of skin The calculation formula is ,in, L 0 represents the initial length of the skin.

3. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 1, characterized in that, The value range is 0.8 to 0.

9.

4. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 3, characterized in that, when ≤45° The value range is 0.85 to 0.

9.

5. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 3, characterized in that, when >45° The value range is 0.8 to 0.

85.

6. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 1, characterized in that, The value range is 1.1 to 1.

5.

7. The method for calculating the pre-stretching amount in a progressive composite forming process for skin stretching according to claim 6, characterized in that, When the processing accuracy of the skinning equipment is ≤±1% and the positioning accuracy is ≤±0.05mm, The value range is 1.1 to 1.

3.

8. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 6, characterized in that, When the processing accuracy of the skinning equipment is > ±1% and the positioning accuracy is > ±0.05mm, The value range is 1.3 to 1.

5.

9. The method for calculating the pre-stretch amount in a progressive composite forming process for skin stretching according to claim 1, characterized in that, The skin material is a metallic material.

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