Decoupling method of tensile-bending stiffness in fiber preform compression molding simulation

Abstract

The application discloses a tensile-bending stiffness decoupling method in fiber preform die forming simulation, and relates to the technical field of simulation learning, and comprises the following steps: a macro finite element model composed of triangular shell elements is established; a biorthogonal fiber coordinate system aligned with the yarn direction is established in each element. Tensile, shear and bending stiffness changing with deformation state are obtained and a constitutive equation is constructed. In the element, an in-plane model controlled by tensile and shear stiffness and an out-of-plane model independently controlled by bending stiffness are respectively established to realize stiffness decoupling. In each increment step, the biorthogonal coordinate system is updated according to the deformation gradient, and after stress is calculated in the biorthogonal coordinate system based on the constitutive equation, the stress is converted to the global coordinate system. Finally, boundary conditions are applied for numerical solution to obtain deformation field, stress and strain distribution and fiber orientation evolution results. The application decouples tensile stiffness and bending stiffness, and eliminates false stiffness in a traditional model.

Description

Technical Field

[0001] This invention relates to the field of simulation learning technology, specifically to a method for decoupling tensile-bending stiffness in fiber preform molding simulation. Background Technology

[0002] In composite material molding processes, fiber preforms (such as woven fabrics and plywoods) typically undergo large deformations under mold constraints, and their molding quality directly affects the mechanical properties and service reliability of the final product. To accurately predict deformation behavior, stress distribution, and defects such as wrinkles during the molding process, finite element simulation is widely used for analyzing the preform molding process. However, fiber preforms exhibit typical mechanical anisotropy; their tensile stiffness along the fiber direction is much greater than their in-plane shear stiffness and out-of-plane bending stiffness, and these stiffness parameters evolve with fiber rotation and structural densification during molding. Traditional continuous medium shell element models, based on the assumption of coupled tensile and bending stiffness, cannot accurately reflect the "tensile stiffness, bending flexibility" mechanical characteristics of fiber preforms, leading to significant discrepancies between simulation results and actual conditions. To address this issue, existing technologies have proposed various improvement methods, such as using stacked shell elements, superimposed shell and membrane elements, and superimposed membrane and rod elements. These strategies treat the preform as an equivalent layer of elements with different mechanical behaviors along the thickness direction, sharing degrees of freedom at nodes to simulate tensile and bending responses separately. However, these methods essentially approximate decoupling by artificially piecing together units, failing to achieve independent modeling of tensile and bending stiffness within a unified mechanical constitutive framework. Therefore, simulation accuracy is highly dependent on empirical parameter adjustments, and the added nodal degree-of-freedom constraints easily introduce spurious stiffness, reducing computational efficiency and physical consistency. Furthermore, existing methods are insufficient in considering dynamic fiber orientation tracking and stiffness evolution with deformation, making it difficult to accurately predict nonlinear behavior and defect formation during the molding process. Therefore, there is an urgent need to develop a simulation analysis method capable of independently and decoupledly characterizing the tensile and bending behavior of fiber preforms at a macroscopic scale to improve the accuracy and reliability of numerical simulations of the molding process. Summary of the Invention

[0003] To independently decouple the tensile and bending behaviors of fiber preforms at a macroscopic scale, thereby improving the accuracy and reliability of numerical simulation of the molding process, this invention proposes a method for decoupling tensile-bending stiffness in fiber preform molding simulation, comprising the following steps: S1: Based on the initial geometric configuration of the fiber preform, a macroscopic finite element model composed of triangular shell elements is established; S2: Establish a biorthogonal fiber coordinate system within each triangular shell unit, with the first coordinate axis aligned with the warp direction and the second coordinate axis aligned with the weft direction; S3: Obtain the mechanical property parameters of the fiber preform, including tensile stiffness, shear stiffness and bending stiffness, as the deformation state changes, and construct the mechanical constitutive equation of the material based on the mechanical property parameters. S4: Establish an in-plane tensile-shear mechanical model controlled by tensile stiffness and shear stiffness, and an out-of-plane bending mechanical model independently controlled by bending stiffness in the triangular shell element. S5: In each computational increment step, update the biorthogonal fiber coordinate system according to the deformation gradient of the current configuration, calculate the stress components based on the mechanical constitutive equation in the updated biorthogonal fiber coordinate system, and convert the calculation results to the global coordinate system to complete the stress update; S6: Based on the stress update results, the boundary conditions and contact constraints corresponding to the molding process are applied to numerically solve the finite element model to obtain the deformation field, stress and strain distribution and fiber orientation evolution results of the fiber preform during the molding process.

[0004] This method establishes an in-plane tension-shear model and an out-of-plane bending model within a triangular shell element, each controlled by independent stiffness parameters. This achieves numerical decoupling of tensile stiffness and bending stiffness, eliminates spurious stiffness in traditional models, and enables high-precision simulation of large deformation behavior during fiber preform molding and accurate prediction of wrinkle defects, providing a quantitative basis for molding process optimization.

[0005] Furthermore, in step S1, when establishing the macroscopic finite element model composed of triangular shell elements, for the multi-layer fiber preform, an independent triangular shell element mesh is established for each layer of fabric, and contact relationships are defined between the layers.

[0006] Furthermore, in step S2, when establishing the biorthogonal fiber coordinate system, the first coordinate axis and the second coordinate axis coincide with the warp direction and the weft direction respectively in the initial geometric configuration, and the yarn direction is updated by tensor decomposition based on the deformation gradient of the current configuration in each calculation increment step.

[0007] Furthermore, in step S3, the mechanical performance parameters are obtained in the following manner: The tensile stiffness in the warp and weft directions was determined by uniaxial tensile test. In-plane shear stiffness was determined by a frame shearing test. Out-of-plane bending stiffness was determined by cantilever beam bending test; Furthermore, functional relationships were established between tensile stiffness, shear stiffness, and bending stiffness as a function of the deformation state of the fiber preform.

[0008] Furthermore, in step S3, the constructed mechanical constitutive equation is in strain-dependent stiffness form, and in each incremental calculation step, the values ​​of tensile stiffness, shear stiffness, and bending stiffness are calculated and updated according to the current strain state.

[0009] Furthermore, in step S4, an in-plane tension-shear mechanical model controlled by tensile stiffness and shear stiffness, and an out-of-plane bending mechanical model independently controlled by bending stiffness are established, specifically as follows: In a triangular shell element, the in-plane stress result is determined only by tensile strain and shear strain, and the out-of-plane bending moment result is determined only by curvature change. Tensile stiffness parameters are not included in the calculation of out-of-plane bending mechanical model, and bending stiffness parameters are not included in the calculation of in-plane tension-shear mechanical model.

[0010] Furthermore, in step S5, after the stress update is completed, the updated stress components are returned to the global coordinate system to drive the balance calculation in the next incremental calculation step.

[0011] Furthermore, in step S6, after obtaining the deformation field, stress-strain distribution, and fiber orientation evolution results of the fiber preform during the molding process, the method further includes: Based on the strain and curvature information of the neutral layer, the deformation state of the neutral layer is mapped to different positions in the thickness direction through the kinematic mapping method, so as to obtain the strain distribution, stress distribution and wrinkle defect prediction results of each layer.

[0012] Furthermore, in step S6, an explicit integration algorithm is used to numerically solve the finite element model.

[0013] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The present invention proposes a tensile-bending stiffness decoupling method in the simulation of fiber preform forming. By establishing an in-plane mechanical model controlled by tensile stiffness and an out-of-plane mechanical model independently controlled by bending stiffness inside the triangular shell unit, this decoupling modeling method makes the tensile response driven only by tensile strain and the bending response driven only by curvature change, avoiding the false additional stiffness caused by stiffness coupling, thereby improving the simulation model's ability to predict the complex deformation behavior of fiber preforms in hyperboloid forming process; (2) By establishing a fiber direction dynamic tracking mechanism based on a biorthogonal coordinate system and combining it with the secondary mechanical constitutive equation to realize the strain-dependent update of stiffness parameters, the model can truly reflect the evolution of mechanical properties caused by rotation, rearrangement and structural densification of fibers during the forming process. The model is updated in real time with the element deformation gradient, ensuring that stress calculation is always carried out in the accurate local fiber direction, thus avoiding the constitutive relationship misalignment problem caused by direction drift in traditional methods. (3) Based on the quasi-extensible properties of fibers, the strain and curvature information of the neutral layer is transferred to the thickness direction through the kinematic mapping method. This enables the reconstruction of the three-dimensional stress and strain field from the two-dimensional shell element results without increasing the computational scale. This post-processing analysis method can effectively identify the location and evolution trend of molding defects such as wrinkles and local instability, and provide quantitative basis for mold design optimization and process parameter adjustment. Attached Figure Description

[0014] Figure 1 A step-by-step diagram of the tensile-bending stiffness decoupling method in the simulation of fiber preform molding; Figure 2 A schematic diagram of the fiber braiding structure and molding die; Figure 3 This is a schematic diagram showing the mapping between the fiber braided structure and the shell unit model; Figure 4 Comparison of molding experiment and simulation (comparison of fabric deformation); Figure 5 Comparison of molding experiment and simulation (comparison of wrinkle defects); Explanation of reference numerals in the attached drawings: 1-punch, 2-pressure ring, 3-fiber fabric, 4-die. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. 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.

[0016] This invention proposes a method for decoupling tensile and bending stiffness in the simulation of fiber preform molding, aiming to solve the problems of low simulation accuracy and poor physical consistency caused by the coupling of tensile and bending stiffness in existing technologies. The core of this method lies in achieving independent and decoupled control of in-plane tensile behavior and out-of-plane bending behavior in a macroscopic finite element model through specific element property definitions, coordinate system construction, and constitutive relation separation. Figure 1 As shown, the method mainly includes the following steps: S1: Based on the initial geometric configuration of the fiber preform, a macroscopic finite element model composed of triangular shell elements is established; S2: Establish a biorthogonal fiber coordinate system within each triangular shell unit, with the first coordinate axis aligned with the warp direction and the second coordinate axis aligned with the weft direction; S3: Obtain the mechanical property parameters of the fiber preform, including tensile stiffness, shear stiffness and bending stiffness, as the deformation state changes, and construct the mechanical constitutive equation of the material based on the mechanical property parameters. S4: Establish an in-plane tensile-shear mechanical model controlled by tensile stiffness and shear stiffness, and an out-of-plane bending mechanical model independently controlled by bending stiffness in the triangular shell element. S5: In each computational increment step, update the biorthogonal fiber coordinate system according to the deformation gradient of the current configuration, calculate the stress components based on the mechanical constitutive equation in the updated biorthogonal fiber coordinate system, and convert the calculation results to the global coordinate system to complete the stress update; S6: Based on the stress update results, the boundary conditions and contact constraints corresponding to the molding process are applied to numerically solve the finite element model to obtain the deformation field, stress and strain distribution and fiber orientation evolution results of the fiber preform during the molding process.

[0017] To better illustrate the technical solution of the present invention, the following is a specific embodiment using the compression molding simulation of a carbon fiber plain weave preform to provide a detailed explanation of each step of the method of the present invention.

[0018] First, the mechanical behavior of the fiber preform was characterized and parameters were obtained. In this embodiment, the selected material was a typical carbon fiber plain weave fabric. To accurately describe its mechanical response during the molding process, it is necessary to obtain its stiffness parameters as it changes with the deformation state. The specific operations are as follows: Through a uniaxial tensile test, the fabric sample was loaded along both the warp and weft directions, and the load-displacement curve was recorded to determine its tensile stiffness. Through a frame shear test, the fabric sample was fixed within the four hinged frames, and pure shear deformation was caused by diagonal stretching. The relationship between the load and the shear angle was recorded to determine its in-plane shear stiffness. Through a cantilever beam bending test, one end of a long strip of fabric sample was fixed, causing it to bend under its own weight. By measuring its deflection curve, its out-of-plane bending stiffness was determined. It should be noted that the experiment does not only measure the stiffness value in the initial state. In order to capture the stiffness evolution of the material during the molding process, a series of correlation experiments are also required. For example, bending experiments were conducted on specimens under different pre-stretching or pre-shearing states to summarize the evolution of material mechanical behavior caused by changes in the braided structure (such as fiber rotation and structural densification). Based on the experimental data, functional relationships were established between tensile stiffness, shear stiffness, and bending stiffness as a function of the deformation state of the fiber preform (such as shear angle and tensile strain), and the material's mechanical constitutive equation was constructed accordingly. This constitutive equation adopts a strain-dependent stiffness form, which means that in subsequent simulation calculations, the material's stiffness matrix K can be dynamically updated according to the strain state at the current integration point, thus truly reflecting the stiffness evolution behavior.

[0019] Next, the simulation model is constructed. First, based on the initial planar dimensions of the carbon fiber plain weave preform, its two-dimensional geometric model is established in the finite element preprocessing software. Then, the geometric model is meshed. The key to this invention is the selection of triangular shell elements as the basic units, and in meshing, the two sides of each triangular unit are made approximately parallel to the initial warp and weft directions of the preform, respectively. This arrangement has multiple advantages: it ensures a high degree of consistency between the material's principal direction (fiber direction) and the unit's local coordinate direction, avoiding errors introduced by directional deviations; simultaneously, this edge-based direction definition method effectively avoids numerical shear locking problems caused by excessive tensile stiffness, improving numerical stability under large deformation conditions. For multilayer fabric forming problems, this invention establishes independent triangular shell element meshes for each layer of fabric and defines contact relationships between layers to simulate interlayer slippage and friction. The forming mold (such as punch 1, blank holder 2, and die 4) undergoes minimal deformation during the forming process, which can be ignored. Therefore, discrete rigid body elements are used for modeling. Contact pairs are defined between the rigid body and the fiber fabric 3, as well as between the fabric layers, and appropriate friction coefficients (such as the Coulomb friction model) are set to establish a complete forming simulation model. Figure 2 As shown.

[0020] Within each triangular shell unit, this invention further establishes a unique biorthogonal fiber coordinate system for precise fiber orientation tracking. Specifically, within each unit, a first coordinate axis is defined to align with the warp direction, and a second coordinate axis is defined to align with the weft direction. Figure 3 This demonstrates the mapping relationship between the fiber braided structure and the shell element model (where, The coordinate system is Green-Naghdi. For warp direction, (Warp direction). As clearly seen in the figure, the two sides of the triangular shell element correspond to the warp and weft directions in the weave structure, respectively, and the biorthogonal coordinate system established within the element is consistent with the fiber direction. In the initial configuration, these two coordinate axes coincide with the warp and weft directions in the global coordinate system. During the simulation, the model's geometric configuration continuously changes. To achieve dynamic tracking of the fiber direction, in each incremental step, the program first obtains the rigid body rotation information of the element based on its deformation gradient using polar decomposition or a similar tensor decomposition method. Then, using this rotation information, the initial biorthogonal fiber coordinate system is rotated and updated to ensure it always aligns with the warp and weft directions under the current configuration. In this way, the strain components under the current configuration can be accurately calculated in the updated local fiber coordinate system. Subsequently, the aforementioned subelastic constitutive equations constructed based on experimental data are called, and the corresponding stress components are calculated under the drive of these strain components. Finally, through tensor transformation, the calculated stress components are converted from the fiber local coordinate system back to the Green-Naghdi coordinate system or the global coordinate system, thus completing the stress update for this computational increment step. This process ensures that stress-strain calculations are always performed in the correct, physically meaningful fiber coordinate system, avoiding calculation errors caused by directional drift.

[0021] Next, we decouple the tensile and bending stiffness. In traditional shell elements, both in-plane and out-of-plane stiffness are determined by the same set of material parameters (such as elastic modulus) and geometric parameters (such as thickness). However, the physical nature of fiber preforms is "extremely high tensile stiffness and extremely low bending stiffness," which cannot be accurately described by traditional methods. In this invention, two independent mechanical models are established within the same triangular shell element. The first is an in-plane tensile-shear mechanical model, which is controlled only by the aforementioned measured tensile and shear stiffness. Its constitutive relation describes the relationship between membrane force (stress result) and in-plane strain (tensile strain and shear strain). The second is an out-of-plane bending mechanical model, which is independently controlled only by the aforementioned measured bending stiffness. Its constitutive relation describes the relationship between bending moment (out-of-plane bending moment result) and curvature change. At the numerical implementation level, the calculations of these two models are performed separately. This means that when calculating in-plane stress, the program only calls the tensile and shear stiffness parameters, while the bending stiffness parameters are not involved in any calculation; conversely, when calculating out-of-plane bending moment, the program only calls the bending stiffness parameters, while the tensile and shear stiffness parameters are completely ignored. By completely separating the mechanical model and stiffness parameters at the element level, this invention achieves numerical decoupling of tensile and bending stiffness. This allows the simulation model to realistically reflect the unique mechanical properties of the fiber preform at the constitutive level, fundamentally avoiding spurious stiffness introduced by coupling.

[0022] After completing the above modeling and constitutive definition, the simulation and solution stage of the forming process begins. Boundary conditions corresponding to the actual compression molding process are applied to the established finite element model. This includes: applying specified displacement boundary conditions to the punch, causing it to move downwards according to a preset stroke; applying constant pressure boundary conditions to the blank holder to control material flow and suppress wrinkling; and defining the contact properties between all contact pairs, including hard contact in the normal direction and a penalty function friction model in the tangential direction. Subsequently, an explicit integration algorithm (such as the central difference method) is used to numerically solve the model. Explicit algorithms do not require iterative convergence and are particularly suitable for solving forming problems involving complex contacts and high nonlinearity. During the solution process, the aforementioned dynamic fiber orientation tracking and stiffness decoupling mechanism are automatically executed in each increment step, thereby obtaining the mechanical response of the preform throughout the entire stamping process. The solution results include the transient and final shape changes of the preform, the stress / strain state of each element, and real-time evolution information of the fiber orientation.

[0023] Finally, the simulation results are subjected to in-depth post-processing and analysis to extract information that is instructive for engineering practice. Since the tensile stiffness of fibers is much greater than other stiffnesses, it can be approximated that fibers are immeasurable along their length. Based on this characteristic, this invention proposes a kinematic thickness direction mapping method. Specifically, the neutral layer of the shell element is used as the strain reference layer, and its strain, curvature, and normal information after molding are extracted. Then, assuming the fiber length remains constant, these deformation information of the neutral layer are linearly or nonlinearly projected onto the upper and lower surface layers along the thickness direction using purely geometric relationships. In this way, even if the model uses two-dimensional shell elements, the strain and stress distribution along the entire thickness direction can be reconstructed. By analyzing the reconstructed surface stress / strain concentration areas and curvature abrupt change areas, potential molding defects such as wrinkles and local buckling can be effectively identified and predicted. Figure 4 and Figure 5 The figure shows a comparison between the molding experiment and the simulation. It can be seen from the figure that the fiber deflection angle predicted by the simulation is... The location of wrinkles closely matches the actual molding experiment results. Specifically, by comparing with the actual molding experiment results, the error between the fiber deflection angle predicted by the model and the measured value is less than 5%, and the location of wrinkles matches the experimental observations well, which fully verifies the effectiveness of the model. Furthermore, compared with traditional laminated shell models, the computation time of the model in this invention is approximately one-quarter, demonstrating a significant computational efficiency advantage. This rich post-processing information, such as shear angle distribution cloud maps, fiber orientation evolution maps, curvature distribution maps, bending moment distribution maps, and thickness direction stress distribution maps, provides precise quantitative basis for mold structure optimization, blank holder force adjustment, and layup scheme improvement.

[0024] In summary, this invention, by constructing a complete numerical framework for decoupling tensile and bending stiffness, achieves a physical and realistic representation of the core mechanical characteristic of fiber preforms—"tensile stiffness and bending flexibility"—at the macroscopic finite element simulation level.

[0025] First, by establishing an in-plane mechanical model controlled by tensile stiffness and an out-of-plane mechanical model independently controlled by bending stiffness within the triangular shell element, the numerical distortion problem caused by the natural coupling of tensile and bending stiffness in traditional shell elements is fundamentally solved. This decoupled modeling approach ensures that the tensile response is driven solely by tensile strain and the bending response is driven solely by curvature change, avoiding spurious additional stiffness caused by stiffness coupling. This improves the simulation model's ability to predict the complex deformation behavior of fiber preforms during hyperboloid forming.

[0026] Secondly, by establishing a fiber orientation dynamic tracking mechanism based on a biorthogonal coordinate system and combining it with the subelastic constitutive equation to achieve strain-dependent updates of stiffness parameters, the model can realistically reflect the evolution of mechanical properties of fibers during the forming process due to rotation, rearrangement, and structural densification. The biorthogonal coordinate system is updated in real time with the element deformation gradient, ensuring that stress calculations are always performed in the accurate local fiber direction, avoiding the constitutive relation misalignment problem caused by orientation drift in traditional methods.

[0027] Furthermore, based on the quasi-extensible properties of fibers, the strain and curvature information of the neutral layer is transferred to the thickness direction through kinematic mapping, enabling the reconstruction of a three-dimensional stress-strain field from two-dimensional shell element results without increasing the computational scale. This post-processing analysis method can effectively identify the location and evolution trend of molding defects such as wrinkles and local instability, providing a quantitative basis for mold design optimization and process parameter adjustment.

[0028] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0029] Furthermore, in this invention, descriptions involving terms such as "first," "second," and "a" are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0030] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection or an electrical connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0031] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

Claims

1. A method for decoupling tensile-bending stiffness in simulation of fiber preform molding, characterized in that, Including the following steps: S1: Based on the initial geometric configuration of the fiber preform, a macroscopic finite element model composed of triangular shell elements is established; S2: Establish a biorthogonal fiber coordinate system within each triangular shell unit, with the first coordinate axis aligned with the warp direction and the second coordinate axis aligned with the weft direction; S3: Obtain the mechanical property parameters of the fiber preform, including tensile stiffness, shear stiffness and bending stiffness, as the deformation state changes, and construct the mechanical constitutive equation of the material based on the mechanical property parameters. S4: Establish an in-plane tensile-shear mechanical model controlled by tensile stiffness and shear stiffness, and an out-of-plane bending mechanical model independently controlled by bending stiffness in the triangular shell element. S5: In each computational increment step, update the biorthogonal fiber coordinate system according to the deformation gradient of the current configuration, calculate the stress components based on the mechanical constitutive equation in the updated biorthogonal fiber coordinate system, and convert the calculation results to the global coordinate system to complete the stress update; S6: Based on the stress update results, the boundary conditions and contact constraints corresponding to the molding process are applied to numerically solve the finite element model to obtain the deformation field, stress and strain distribution and fiber orientation evolution results of the fiber preform during the molding process.

2. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S1, when establishing a macroscopic finite element model composed of triangular shell elements, for a multi-layer fiber prefabricated body, an independent triangular shell element mesh is established for each layer of fabric, and contact relationships are defined between the layers.

3. The tensile-bending stiffness decoupling method in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S2, when establishing the biorthogonal fiber coordinate system, the first and second coordinate axes coincide with the warp and weft directions respectively in the initial geometric configuration, and the yarn direction is updated by tensor decomposition based on the deformation gradient of the current configuration in each calculation increment step.

4. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S3, the mechanical performance parameters are obtained in the following manner: The tensile stiffness in the warp and weft directions was determined by uniaxial tensile test. In-plane shear stiffness was determined by a frame shearing test. Out-of-plane bending stiffness was determined by cantilever beam bending test; Furthermore, functional relationships were established between tensile stiffness, shear stiffness, and bending stiffness as a function of the deformation state of the fiber preform.

5. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 4, characterized in that, In step S3, the constructed mechanical constitutive equation is in strain-dependent stiffness form. In each incremental calculation step, the values ​​of tensile stiffness, shear stiffness, and bending stiffness are calculated and updated according to the current strain state.

6. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S4, an in-plane tension-shear mechanical model controlled by tensile stiffness and shear stiffness, and an out-of-plane bending mechanical model independently controlled by bending stiffness are established, specifically as follows: In a triangular shell element, the in-plane stress result is determined only by tensile strain and shear strain, and the out-of-plane bending moment result is determined only by curvature change. Tensile stiffness parameters are not included in the calculation of out-of-plane bending mechanical model, and bending stiffness parameters are not included in the calculation of in-plane tension-shear mechanical model.

7. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S5, after the stress update is completed, the updated stress components are returned to the global coordinate system to drive the balance calculation in the next incremental calculation step.

8. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S6, after obtaining the deformation field, stress-strain distribution, and fiber orientation evolution results of the fiber preform during the molding process, the method further includes: Based on the strain and curvature information of the neutral layer, the deformation state of the neutral layer is mapped to different positions in the thickness direction through the kinematic mapping method, so as to obtain the strain distribution, stress distribution and wrinkle defect prediction results of each layer.

9. The method for decoupling tensile-bending stiffness in the simulation of fiber preform molding as described in claim 1, characterized in that, In step S6, an explicit integration algorithm is used to numerically solve the finite element model.

Images