Composite curved beam surface radial stress regulation method

Abstract

The application discloses a kind of composite curved beam surface radial stress regulation methods, belong to composite stress regulation technical field, comprising the following steps: S1, determine the radial high stress area of composite curved beam;S2, prepare low-modulus high-toughness film;S3, low-modulus high-toughness film is inserted into the radial high stress area of composite curved beam;S4, using prepreg-hot press tank process is completed with low-modulus high-toughness film curved beam co-curing forming completes the regulation of radial stress of composite curved beam surface.The application uses the above-mentioned one kind of composite curved beam surface radial stress regulation method, by being implanted in curved beam stress concentration zone toughness film and forming " local flexible hinge ", effectively flatten stress peak, on the premise of not increasing weight and reducing rigid, make interlaminar failure load increase ≥20%, delamination displacement delay 30% or more;And the method is compatible with existing hot press tank process, can realize synchronous curing, need not increase process, with the engineering advantage of plug and play.

Description

Technical Field

[0001] This invention relates to the field of stress control technology for composite materials, and in particular to a method for controlling radial stress on the surface of a curved beam made of composite materials. Background Technology

[0002] Advanced composite materials, with their high specific strength, high specific stiffness, and designability, have become irreplaceable enabling materials for structural lightweighting. They have achieved large-scale penetration in high-end equipment fields such as aerospace, rail transportation, and civil engineering, and "composite materials" have been elevated to a fundamental technological route for national-level iterative upgrades. For the next generation of civil aircraft, the threshold values ​​for weight reduction, life extension, and stability enhancement in airframe structures are continuously converging, and the application of composite materials is rapidly expanding from non- / secondary load-bearing areas to core load-bearing areas such as wing box sections, center wings, and fuselage sections. Curved beams, as typical characteristic components of composite materials in civil aircraft, are densely distributed in wing spars, wing ribs, and stringer ends, serving as the hub for the transmission of bending-shear-torsion composite loads. Under bending-dominant conditions, the ultimate bearing capacity of its arc-shaped transition zone (R zone) directly locks the allowable design value of structures containing curved beams, becoming a "bottleneck" restricting further lightweighting and ultra-high reliability design of main load-bearing composite material structures.

[0003] Under bending loads, the radial tensile stress generated in the thickness direction of the curved region is the "switching factor" for the failure of composite curved beams. The circumferential and axial strengths can reach gigapascal levels due to continuous fiber support; however, the radial load can only be borne independently by the resin matrix, whose tensile strength is much lower than that of the fibers, making it the weakest link in the laminate. Figure 1 The radial stress cloud diagram and distribution curve along thickness path a shown indicate that the radial tensile stress along the thickness direction is not uniformly distributed, but exhibits a significant "sharp peak". Once this peak reaches the matrix strength, it triggers delamination of the sealing layer, causing most of the layup to prematurely cease operation before reaching its design value, resulting in low material utilization and severely restricting the lightweight design of curved beams.

[0004] To improve the flexural load-bearing capacity of composite curved beams, existing methods mainly focus on enhancing interlaminar mechanical properties. For example, this involves introducing chopped strand mat at the interface to enhance the bridging effect, thereby increasing the interfacial fracture energy; or using high-strength adhesive films and nanoparticles to improve interfacial strength and fracture energy. However, these methods typically introduce high-modulus filler phases, often leading to increased interfacial stress levels. If the processing conditions are not mature enough, this may even induce interfacial defects. Furthermore, such methods usually increase the complexity of composite molding processes, limiting their practical engineering application value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling radial stress on the surface of a composite curved beam. By implanting a tough adhesive film in the stress concentration area of ​​the curved beam to form a "local flexible hinge", the stress peaks are effectively flattened. Without increasing weight or reducing stiffness, the interlaminar failure load is increased by ≥20% and the delamination displacement is delayed by more than 30%. Moreover, this method is compatible with existing autoclave processes, can achieve synchronous curing, does not require additional processes, and has the engineering advantage of plug-and-play.

[0006] To achieve the above objectives, the present invention provides a method for controlling radial stress on the surface of a composite curved beam, comprising the following steps: S1. Determine the radial high-stress region of the composite curved beam; S2. Preparation of low-modulus, high-strength and high-toughness thin films; S3. Insert the low-modulus, high-strength, and tough thin film obtained in S2 into the radial high-stress region of the composite curved beam. S4. The prepreg-autoclave process is used to complete the co-curing molding of curved beams containing low-modulus, high-strength and tough films, thereby controlling the radial stress on the surface of the composite curved beam.

[0007] Preferably, in S1, the determination of the high-stress region depends on the inner diameter, outer diameter, thickness, radius of curvature, layup sequence, and material constitutive properties.

[0008] Preferably, the specific operation of S2 is as follows: S21. A low-modulus continuous phase is formed by toughening a high-crosslink density epoxy resin with a thermoplastic polyurethane elastomer, and then bisphenol A type epoxy resin is introduced to adjust the viscosity and modulus. S22. A silane coupling agent is added, and then a low-modulus, high-strength and tough thin film is prepared by coating-thermal melting method.

[0009] Preferably, in S22, the concentration of the silane coupling agent is 1-2 wt%, the silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane, and the thickness of the low modulus high strength and toughness film prepared is 100-150 μm with a thickness tolerance of ±5 μm.

[0010] Preferably, in S22, the modulus of the low-modulus, high-strength, and tough film prepared is 1 / 10 to 1 / 3 of that of the high-crosslinking-density epoxy resin.

[0011] Preferably, in S3, the low-modulus, high-strength and toughness film is inserted layer by layer or every other layer, and is only inserted locally between -45° / 90° or high-shear layers.

[0012] Preferably, the width of the low-modulus, high-strength and high-toughness film is 25% of the width of the composite material.

[0013] Preferably, in S3, the low modulus rigid-toughness film is laid according to the number of layers n involved in the radial high stress region determined in S1, as well as the high stress starting layer i and the high stress ending layer i+n. The low modulus rigid-toughness film is inserted into both the high stress starting layer and the high stress ending layer, and is inserted layer by layer from layer i+1 to layer i+n-1 or every other layer.

[0014] Therefore, the present invention employs the above-mentioned method for controlling radial stress on the curved surface of composite material beams, which has the following beneficial effects: (1) Stress “peak reduction” rather than “hard reinforcement”: Instead of the traditional “hard reinforcement” path of high modulus toughening phase, a low modulus and high elongation toughness film is implanted in the maximum radial tensile stress area of ​​the curved beam. The peak stress is induced to be unloaded to the high stiffness layers on both sides by “local softening”, so that the radial stress peak becomes a gentle slope and the layer triggering threshold is increased by ≥20%; (2) Zero redundancy process: The adhesive film and prepreg are laid and cured in the autoclave at the same time, completely following the supplier's established temperature-pressure curve, without adding any additional heating, pressurization or pressure holding steps, achieving "plug and play"; (3) Maximize material utilization: intercalation is only performed locally in the most dangerous 2-3mm stress zone, the amount of adhesive film used is very low, there is zero sacrifice in in-plane stiffness and weight, fatigue life is significantly improved, and the material's "load-bearing efficiency per unit weight" is significantly improved.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] Figure 1 This invention relates to a method for controlling radial stress on the surface of a composite curved beam. The background technology includes radial stress cloud diagrams and distribution curves along the thickness path a. Figure 2 This invention presents an optimized radial stress distribution characteristic along the thickness of a composite curved beam surface radial stress control method, and a comparison thereof. Figure 3 This invention relates to a method for controlling radial stress on the surface of a composite curved beam, specifically addressing the high radial stress region of a composite curved beam. Figure 4 This is a design diagram of the number and insertion position of low-modulus, high-strength and tough thin film layers for a method of controlling radial stress on a curved beam surface of composite materials according to the present invention; Figure 5 This is the finite element model configuration containing cohesive elements for a method of controlling radial stress on a curved beam surface of a composite material according to the present invention. Figure 6 This is Example 1 of the method for controlling radial stress on the surface of a composite curved beam according to the present invention, showing the stress distribution of a non-optimized curved beam under a load of 1.5 mm. Figure 7This is Example 1 of the present invention, which describes the preparation process of a composite curved beam using a method for controlling radial stress on the surface of a composite curved beam. Figure 8 This is Example 1 of a method for controlling radial stress on the surface of a composite curved beam according to the present invention, which optimizes the stress distribution of the curved beam under a displacement load of 1.5 mm. Figure 9 This is Example 1 of the method for controlling radial stress on the surface of a composite curved beam according to the present invention, showing the load-displacement curves of a non-optimized curved beam and an optimized curved beam. Detailed Implementation

[0017] This invention provides a method for controlling radial stress on the surface of a composite curved beam, comprising the following steps: S1. Determine the radial high-stress region of the composite curved beam, such as... Figure 3 As shown; S2. Preparation of low-modulus, high-strength and high-toughness thin films; S3. Insert the low-modulus, high-strength, and toughness film obtained in S2 into the radial high-stress region of the composite curved beam. The number of insertion layers and the insertion positions of the low-modulus, high-strength, and toughness film are as follows: Figure 4 As shown; S4. The prepreg-autoclave process is used to complete the co-curing molding of curved beams containing low-modulus, high-strength and tough films, thereby controlling the radial stress on the surface of the composite curved beam.

[0018] In this invention, the determination of the high-stress region in S1 depends on the inner diameter, outer diameter, thickness, radius of curvature, layup sequence, and material constitutive properties.

[0019] In this invention, the specific operation of S2 is as follows: S21. A low-modulus continuous phase is formed by toughening a high-crosslink density epoxy resin with a thermoplastic polyurethane elastomer, and then bisphenol A type epoxy resin is introduced to adjust the viscosity and modulus. S22. A silane coupling agent is added, and then a low-modulus, high-strength and tough thin film is prepared by coating-thermal melting method.

[0020] In this invention, in S22, the concentration of the silane coupling agent is 1-2 wt%, the silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane, and the thickness of the low modulus high strength and toughness film prepared is 100-150 μm with a thickness tolerance of ±5 μm.

[0021] In this invention, in step S22, the modulus of the low-modulus, high-strength and tough film prepared is 1 / 10 to 1 / 3 of that of the high-crosslinking density epoxy resin.

[0022] In this invention, in S3, the low-modulus, high-strength and toughness thin film is inserted layer by layer or every other layer, and is only inserted locally between -45° / 90° or high-shear layers.

[0023] In this invention, the width of the low-modulus, high-strength and toughness film is 25% of the width of the composite material.

[0024] In this invention, in S3, a low-modulus rigid-toughness film is laid according to the number of layers n involved in the radial high-stress region determined in S1, as well as the high-stress starting layer i and the high-stress ending layer i+n. The low-modulus rigid-toughness film is inserted into both the high-stress starting layer and the high-stress ending layer, either layer by layer from layer i+1 to layer i+n-1 or every other layer.

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0026] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0027] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0028] In this invention, unless otherwise specified, all other test materials and instruments are conventional test materials in the field and can be purchased through commercial channels.

[0029] Example 1 This invention provides a method for controlling radial stress on the surface of a composite curved beam, the technical principle of which is as follows: Figure 2 As shown, one or more low-modulus, high-elongation tough films are precisely inserted in the neighborhood of the stress peak. This flexible layer locally reduces stiffness, forcing the original concentrated load to redistribute to the boundary layers on both sides. The radial stress "peak" is flattened, and the delamination trigger threshold is thus increased. The entire process adds only one layup step, does not introduce a high-modulus phase, and balances process simplicity and reliability.

[0030] Specifically, the following steps are included: S1. Determine the radial high-stress region of the composite curved beam; simulate the composite curved beam model using ABAQUS finite element simulation. The dimensions, constraints, and loading conditions of the finite element model are consistent with the experimental settings, such as... Figure 5 As shown. The loading rod is simulated as a rigid cylinder. The upper cylinder is constrained by a reference point. x U z Directional displacement and all rotational degrees of freedom, by applying U y Displacement boundary conditions in the directional direction transfer the load to the L-shaped laminate. The lower cylinder is fully constrained in all degrees of freedom. Frictionless surface-to-surface contact is used to simulate the interaction between the rigid cylinder and the specimen. The finite element simulation of the specimen is based on the plane strain assumption, and all plies are discretized using four-node bilinear plane strain reduced integral quadrilateral elements (CPE4R). To balance computational efficiency and accuracy, a coarser mesh is used in regions far from the R region. In the numerical model, zero-thickness cohesive elements (CEs) are introduced in the critical regions of the L-shaped laminate. Coh2D4 type cohesive elements are used to simulate delamination behavior. Figure 5 The refined finite element mesh and zero-thickness cohesive element arrangement of the critical bending region are shown in detail. The elements in this region are radially arranged, with the innermost element being 42 μm wide and the outermost element being 50 μm wide, and the element height uniformly set at 60 μm. Delamination defects are simulated by removing cohesive elements. To account for potential interactions on the defect surfaces or re-contact of internal layers after interface failure, contact pairs with appropriate friction coefficients are defined between all internal layers, using an average friction coefficient of 0.5. The stress distribution of the non-optimized curved beam under a 1.5 mm load is shown below. Figure 6 As shown, the high-stress region is concentrated in the central area of ​​the curved beam, with stress decreasing significantly from this core area towards the inner and outer edges. This indicates that under this bending load, the middle layer of the curved beam is the main stress-bearing and transmission path, and also a potential critical area for failure initiation.

[0031] S2. Preparation of low modulus high strength and toughness film; The prepared low modulus high strength and toughness film must meet the following requirements: (1) Interface compatibility: It has excellent interfacial wetting and adhesion properties with the resin matrix to ensure no defect transmission between layers. (2) Modulus matching: Experiments show that when the modulus of the low modulus high strength and toughness film is controlled in the range of 1 / 10-1 / 3 of the modulus of the resin matrix, the stress redistribution effect is significant. (3) Both toughness and strength: The elongation at break is significantly higher than that of the matrix resin, and the tensile strength is not less than 80% of the strength of the matrix, which maintains load-bearing redundancy while inhibiting the initiation of interfacial layer cracks. (4) Synchronous process: The curing process of the low modulus high strength and toughness film is similar to that of composite materials.

[0032] Its preparation method includes the following steps: S21. A low-modulus continuous phase is formed by toughening a high-crosslink density epoxy resin with thermoplastic polyurethane elastomer. Then, bisphenol A type epoxy resin is introduced to adjust the viscosity and modulus. The final modulus can be reduced to 1 / 10-1 / 3 of the matrix resin, while the tensile strength can still be maintained at ≥80% of the matrix strength, and the elongation at break is increased by 3-5 times. S22. A silane coupling agent is added, and then a low-modulus, high-strength and tough thin film is prepared by coating-thermal melting method.

[0033] In S22, the concentration of the silane coupling agent is 1-2wt%, and the silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane. The thickness of the low-modulus, high-strength and tough film prepared is 100-150μm with a thickness tolerance of ±5μm. The addition of the silane coupling agent can form a covalent "bridge" between the film and the epoxy matrix, increasing the interlayer shear strength by more than 30% and ensuring no defect transmission.

[0034] The curing agent used in the coating-hot melt method is a dicyandiamide / organic urea system. The stepped curing regime of 120°C / 1h + 150°C / 2h completely overlaps with the typical co-curing window of epoxy composites. The difference in the coefficient of linear expansion between the curing agent and the low-modulus, high-strength, and tough film is <1×10⁻⁶. -5 K -1 To avoid thermal mismatch defects.

[0035] The typical properties of the film prepared according to the above route are as follows: Tensile modulus 0.8-2.0 GPa (matrix 10-30%), tensile strength ≥70 MPa (≥80% matrix), elongation at break ≥15% (3-5 times that of matrix), type II interlaminar fracture toughness G ⅡC Improvement >100%, Type I interlaminar fracture toughness G IC Improvement >50%.

[0036] By using an integrated solution of TPU-toughened epoxy, interface coupling, precision thickness, and co-curing, low-modulus, high-strength, and synchronously processed interlayer stress-reducing films can be achieved in engineering applications at low cost.

[0037] S3. Insert the low-modulus, high-strength, and high-toughness thin film obtained in S2 into the radial high-stress region of the composite curved beam.

[0038] In S3, the low-modulus high-strength and toughness film is inserted layer by layer or every other layer, and is only inserted locally between -45° / 90° or high shear layers. The width of the low-modulus high-strength and toughness film is 25% of the width of the composite material.

[0039] Based on the number of layers n involved in the radial high-stress region determined by S1, as well as the high-stress starting layer i and the high-stress ending layer i+n, the low-modulus rigid-tough film is laid. The low-modulus rigid-tough film is inserted into both the high-stress starting layer and the high-stress ending layer, either layer by layer from layer i+1 to layer i+n-1 or every other layer.

[0040] S4. The prepreg-autoclave process is used to complete the co-curing molding of curved beams containing low-modulus, high-strength and tough films, thereby controlling the radial stress on the surface of the composite curved beam.

[0041] The preparation process of composite curved beams is as follows: Figure 7 As shown. First, prepreg and PTFE film are laid up on a convex Invar mold, with vacuum pre-compaction performed every six layers; then, the layup is transferred to a concave Invar mold for fixation; subsequently, it is sealed with a vacuum bag and vacuumed to remove air bubbles; then, the packaged part is placed in an autoclave for curing using the curing curve recommended by the prepreg supplier; after curing, the formed composite material plate is removed, and the plate is cut and trimmed into L-shaped finished components.

[0042] The ASTM D6415 / D6415M standard, "Strength of Fiber-Reinforced Polymer Matrix Composite Beams," was used to conduct a four-point bending comparison test on optimized (containing a low-modulus, high-strength, and toughness adhesive film) and non-optimized curved beams. The stress contour plot of the optimized (containing a low-modulus, high-strength, and toughness adhesive film) curved beam under a 1.5 mm displacement load is shown below. Figure 8 Its load-displacement curve is as follows Figure 9 ,Depend on Figure 8 It can be seen that the stress in the central region of the curved beam is effectively dispersed to both sides of the adhesive film, and the stress concentration area is transferred from the center of the curved beam to the edge, thus shifting the high-stress area outward and significantly improving structural safety. Figure 9 It can be seen that the maximum load of the non-optimized curved beam is 71 N / mm, while the maximum load of the optimized curved beam is 87.4 N / mm, representing a 23% increase in the maximum load. This is because the flexible hinge formed at the adhesive film causes a redistribution of bending moment, resulting in the "unloading" of bending moment in the central region and the transfer of stress to the edge of the adhesive film. This achieves a "peak-shaving" effect in the interlayer of high-stress areas, thereby significantly reducing the risk of overall structural failure caused by displacement loads.

[0043] Therefore, the present invention adopts the above-mentioned method for radial stress control of curved beam surface of composite material. By implanting a tough adhesive film in the stress concentration area of ​​the curved beam to form a "local flexible hinge", the stress peaks are effectively flattened. Without increasing weight or reducing stiffness, the interlaminar failure load is increased by ≥20% and the delamination displacement is delayed by more than 30%. Moreover, the method is compatible with the existing autoclave process, can achieve synchronous curing, does not require additional processes, and has the engineering advantage of plug and play.

[0044] 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for controlling radial stress on the surface of a composite curved beam, characterized in that: Includes the following steps: S1. Determine the radial high-stress region of the composite curved beam; S2. Preparation of low-modulus, high-strength and high-toughness thin films; S3. Insert the low-modulus, high-strength, and tough thin film obtained in S2 into the radial high-stress region of the composite curved beam. S4. The prepreg-autoclave process is used to complete the co-curing molding of curved beams containing low-modulus, high-strength and tough films, thereby controlling the radial stress on the surface of the composite curved beam.

2. The method for controlling radial stress on a composite curved beam surface according to claim 1, characterized in that: In S1, the determination of high-stress regions depends on the inner diameter, outer diameter, thickness, radius of curvature, layup sequence, and material constitutive properties.

3. The method for controlling radial stress on a composite curved beam surface according to claim 1, characterized in that: The specific operation of S2 is as follows: S21. A low-modulus continuous phase is formed by toughening a high-crosslink density epoxy resin with a thermoplastic polyurethane elastomer, and then bisphenol A type epoxy resin is introduced to adjust the viscosity and modulus. S22. A silane coupling agent is added, and then a low-modulus, high-strength and tough thin film is prepared by coating-thermal melting method.

4. The method for controlling radial stress on a composite curved beam surface according to claim 3, characterized in that: In S22, the concentration of the silane coupling agent is 1-2 wt%, and the silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane. The thickness of the low-modulus, high-strength and tough film prepared is 100-150 μm, with a thickness tolerance of ±5 μm.

5. The method for controlling radial stress on the surface of a composite curved beam according to claim 3, characterized in that: In S22, the modulus of the low-modulus, high-strength and tough film prepared is 1 / 10 to 1 / 3 of that of the high-crosslinking-density epoxy resin.

6. The method for controlling radial stress on the surface of a composite curved beam according to claim 1, characterized in that: In S3, the low-modulus, high-strength and toughness thin films are inserted layer by layer or every other layer, and are only inserted locally between -45° / 90° or high-shear layers.

7. The method for controlling radial stress on the surface of a composite curved beam according to claim 1, characterized in that: The width of the low-modulus, high-strength and high-toughness film is 25% of the width of the composite material.

8. The method for controlling radial stress on the surface of a composite curved beam according to claim 1, characterized in that: In S3, based on the number of layers n involved in the radial high stress region determined in S1, as well as the high stress starting layer i and the high stress ending layer i+n, a low modulus rigid-tough membrane is laid. The low modulus rigid-tough membrane is inserted into both the high stress starting layer and the high stress ending layer, either layer by layer from layer i+1 to layer i+n-1 or every other layer.

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