Fml panel-gradient pmi foam sandwich structure and design method thereof

Abstract

The application discloses an FML panel-gradient PMI foam sandwich structure and a design method thereof, and relates to the technical field of composite material structures.The FML panel-gradient PMI foam sandwich structure comprises an upper FML panel, a lower FML panel and a gradient PMI foam core layer arranged between the upper FML panel and the lower FML panel; and the design method comprises the following steps: S1, FML panel prefabrication; S2, gradient PMI foam core layer preparation; S3, core layer interlayer hot pressing and compounding; S4, interface adhesive coating; S5, vacuum bag assisted curing and compounding; and S6, demolding and post-processing.The FML panel-gradient PMI foam sandwich structure realizes the optimized distribution of structural performance along the thickness direction, relieves the interface stress concentration, and improves the bending load-carrying capacity, the impact resistance and the damage tolerance of the sandwich structure without significantly increasing the weight.

Description

Technical Field

[0001] This invention relates to the field of composite material structure technology, and in particular to an FML panel-gradient PMI foam sandwich structure and its design method. Background Technology

[0002] Sandwich structures typically consist of upper and lower panels and a core material in between. The panels primarily bear bending moments, while the core layer handles shear forces and provides lightweighting and energy absorption. In high-performance sandwich structures, fiber-reinforced metal laminates (FMLs) combine the high toughness of metal materials with the high specific strength of fiber composites, exhibiting significant advantages in impact resistance, fatigue resistance, and crack propagation resistance. They have been gradually applied to critical load-bearing components such as wing skins and fuselage structures, becoming one of the important forms of sandwich structure panels.

[0003] Commonly used foam core materials include polyurethane (PU), polyvinyl chloride (PVC), polystyrene (PS), polyetherimide (PEI), and polymethacrylimide (PMI). Among them, PMI foam not only has excellent compressive and shear strength, but also effectively absorbs impact energy and maintains stable mechanical properties at high temperatures, making it one of the ideal choices for constructing the core layer of high-performance sandwich structures.

[0004] However, traditional PMI foam core layers mostly adopt a homogeneous design, meaning that the core layer has a uniform density and single performance along the thickness direction, which cannot optimize the performance for different stress areas of the structure: the compressive strength of the pressure area is not fully utilized, the shear area cannot effectively reduce weight, and the stiffness mismatch between the panel and the core layer can easily lead to stress concentration at the interface, which can cause failure modes such as debonding, thus restricting the further improvement of the overall load-bearing efficiency of the sandwich structure.

[0005] To overcome the limitations of traditional homogeneous core layer designs, a gradient design approach is proposed, incorporating continuously or hierarchically varying physical parameters (such as density, pore size, composition, and elastic modulus) within the core material to achieve an optimized spatial distribution of material properties. This approach not only effectively improves the material's strength, stiffness, and energy absorption capacity but also minimizes weight and enhances the overall structural performance. Density gradient design, in particular, can address the stiffness mismatch between the panel and core layers in traditional sandwich structures, reducing damage caused by interfacial shear stress and improving structural design flexibility to meet optimized load-bearing capacity requirements under various working conditions. However, existing research largely focuses on single density gradient forms. A systematic approach is still lacking for various implementation forms of density gradients (symmetric, asymmetric, positive, negative gradients, etc.) and their synergistic design with high-performance panel materials (such as FML). Corresponding quantitative design methods and supporting manufacturing processes also require further improvement. Summary of the Invention

[0006] To address the technical problems of existing sandwich structures, such as the difficulty of adapting to complex stress environments due to the homogeneous design of the foam core layer, the susceptibility of stress concentration and debonding at the interface between the panel and the core layer, the limited variety of density gradient design forms, and the lack of a systematic solution for collaboration with FML panels, this invention provides an FML panel-gradient PMI foam sandwich structure and its design method. This structure includes an upper FML panel, a lower FML panel, and a gradient PMI foam core layer disposed between them. The core layer exhibits a density gradient distribution along its thickness direction, with gradient forms including symmetrical gradients, asymmetrical gradients, positive gradients, and negative gradients. This invention performs gradient design of the foam core layer density along the thickness direction, combined with interfacial adhesive bonding between the panel and the core layer and vacuum bag-assisted curing composite processes, to achieve optimized distribution of structural performance along the thickness direction, alleviate interfacial stress concentration, and improve the bending load-bearing capacity, impact resistance, and damage tolerance of the sandwich structure without significantly increasing weight. This invention also provides a quantitative design method for density gradient parameters and supporting manufacturing processes, applicable to lightweight high-performance engineering fields such as aerospace, rail transportation, and construction.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: an FML panel-gradient PMI foam sandwich structure, comprising an upper FML panel, a lower FML panel, and a gradient PMI foam core layer disposed between the upper FML panel and the lower FML panel, wherein the gradient PMI foam core layer is distributed in a density gradient form along the thickness direction;

[0008] The upper FML panel or the lower FML panel is composed of two metal layers and a single fiber reinforcement layer disposed between the two metal layers;

[0009] The gradient PMI foam core layer is composed of an upper high-density area, a medium-density area, and a lower high-density area arranged sequentially from top to bottom; the upper high-density area and the lower high-density area are respectively adjacent to the interface of the upper FML panel and the interface of the lower FML panel.

[0010] The upper FML panel, lower FML panel and the upper and lower interfaces of the gradient PMI foam core layer are connected by an adhesive layer.

[0011] Furthermore, a foam interlayer transition layer is provided between adjacent density regions of the upper high-density region, the medium-density region, and the lower high-density region.

[0012] Furthermore, the density gradient form includes one or more combinations of symmetric gradient, asymmetric gradient, positive gradient, negative gradient, and hierarchical gradient or continuous gradient;

[0013] The symmetrical gradient: The density distribution of the gradient PMI foam core layer along the thickness direction is symmetrical with respect to the middle surface of the core layer; for example, the density is high on both sides and low in the middle, or the density is low on both sides and high in the middle.

[0014] Asymmetric gradient: The density distribution of the gradient PMI foam core layer along the thickness direction is asymmetric relative to the middle surface of the core layer, with different density change amplitudes or rates on both sides, which can be flexibly designed according to the structural stress requirements.

[0015] Positive gradient: The gradient PMI foam core layer monotonically increases from bottom to top along the thickness direction;

[0016] Negative gradient: The gradient of the PMI foam core layer decreases monotonically from bottom to top along the thickness direction, and is a mirror image of the positive gradient.

[0017] Furthermore, the gradient PMI foam core layer has a high density in the areas that are respectively attached to the upper FML panel interface and the lower FML panel interface, and a low density in the medium density area.

[0018] Furthermore, the gradient PMI foam core layer is distributed in the form of a density gradient along the thickness direction. Through continuous gradient design or layered gradient design along the thickness direction, the density parameters of each region are quantitatively controlled according to design requirements. The density distribution function is expressed as follows: ,in These are the coordinates in the thickness direction. For the initial density, These are the gradient coefficients.

[0019] To better achieve the objectives of this invention, the present invention also provides a layered foaming mold for fabricating the FML panel-gradient PMI foam sandwich structure of this invention. The layered foaming mold includes:

[0020] The layered foaming mold body is composed of a mold base plate and a mold side wall forming a foaming cavity.

[0021] To better achieve the objectives of this invention, this invention further provides a design method for an FML panel-gradient PMI foam sandwich structure, comprising the following steps:

[0022] Step S1, FML panel prefabrication: Select aluminum alloy sheet with a thickness of 0.3mm-0.6mm as the metal layer, and carbon fiber / epoxy resin prepreg as the fiber reinforcement layer. Lay them in the order of "metal layer / fiber reinforcement layer / metal layer" or "metal layer / fiber reinforcement layer / ... / fiber reinforcement layer / metal layer" to prepare the upper FML panel and the lower FML panel respectively. After the layup is completed, place the laminated structure in a vacuum bag for vacuum treatment, and control the vacuum degree at -0.08 to -0.095MPa. Use hot pressing or autoclaving process to heat and cure under a molding pressure of 0.3 to 0.6MPa, control the curing temperature at 120 to 180℃, and keep warm for 1 to 2 hours to allow the epoxy resin to impregnate the fiber reinforcement layer and complete the curing reaction. After curing, cool to room temperature, demold, and polish the panel surface for later use.

[0023] Step S2, Gradient PMI Foam Core Layer Preparation: According to the design requirements, the density distribution of the gradient PMI foam core layer in the thickness direction is determined to adopt a layered gradient structure. The foam density of each area is controlled by adjusting the raw material formula, foaming agent content and foaming process parameters. The core layer density range is set to 50-200 kg / m³.

[0024] Step S3, hot-pressing composite of core layer: The gradient PMI foam core layer prepared in step S2 is subjected to hot-pressing composite treatment. The hot-pressing temperature is controlled at 150-180℃, the hot-pressing pressure is controlled at 0.2-0.5MPa, and the core layer is held under pressure for 120 minutes before cooling and molding.

[0025] Step S4, Interface Adhesive Coating: Place the prepared gradient PMI foam core layer on the lower FML panel, uniformly coat the adhesive layer at the interface between the lower FML panel and the gradient PMI foam core layer, and then lay the upper FML panel to form a sandwich structure prefabricated body.

[0026] Step S5, Vacuum Bag-Assisted Curing and Composite: On the surface of the sandwich structure preform obtained in step S4, a release film, a breathable felt, and a vacuum bag film are laid in sequence, and the surface is sealed with sealing tape at the edge of the tooling platform. Vacuum is drawn by connecting a vacuum pipeline through a vacuum connector, and the vacuum degree is controlled at -0.08 to -0.095 MPa. Hot pressing or autoclaving process is adopted, and the curing temperature is maintained at 0.2 to 0.5 MPa molding pressure and 80 to 140℃ for 1 to 2 hours to allow the adhesive layer to flow fully and fill the interface micropores, complete the curing reaction, and realize the overall composite between the upper FML panel, the gradient PMI foam core layer and the lower FML panel.

[0027] Step S6, Demolding and Post-processing: After curing, allow it to cool naturally to room temperature, demold, and perform edge trimming, dimensional trimming, and surface cleaning to obtain the finished FML panel-gradient PMI foam sandwich structure.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) Achieve structural performance gradient optimization and lightweight design: By designing the density gradient of the PMI foam core layer, the sandwich structure has local mechanical properties that match the stress area (compression area, shear area), which improves the utilization rate of materials while meeting the load-bearing strength requirements, achieves structural lightweighting, and enhances the overall mechanical performance.

[0030] (2) Enhanced impact and fatigue resistance: The FML panel itself has excellent impact resistance, fatigue resistance and crack propagation resistance; the high-density area near the panel in the gradient PMI foam core layer can resist local indentation in the initial stage of impact, while the low-density area in the middle will preferentially undergo progressive crushing to continuously absorb impact energy. The synergistic effect of the two further reduces the peak impact force and enhances the overall impact resistance of the sandwich structure, making it suitable for high-performance fields such as aviation, rail transportation, automobiles, and construction.

[0031] (3) Interface stress relief: The gradient transition of core layer density along the thickness direction effectively alleviates the interface shear stress concentration caused by the sudden change in stiffness between the panel and the core layer, reduces the risk of debonding failure, and improves structural reliability.

[0032] (4) Design flexibility and controllability: The density gradient forms (symmetric, asymmetric, positive and negative gradients) and two implementation methods (layered / continuous) provided by this invention can be flexibly combined according to the stress requirements of different working conditions, which facilitates the precise and functional design of the structure; the supporting vacuum pressure molding, layered foaming and vacuum bag assisted curing composite process parameters are clear, with good engineering repeatability, which is conducive to large-scale manufacturing. Attached Figure Description

[0033] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0034] Figure 1 This is a three-dimensional structural diagram of the sandwich structure of the present invention;

[0035] Figure 2 This is a schematic diagram of the cross-sectional structure of the sandwich structure of the present invention along the thickness direction;

[0036] Figure 3 This is a graph showing the density distribution of the gradient PMI foam core layer along the thickness direction (taking a symmetrical gradient as an example).

[0037] Figure 4This is a comparative schematic diagram of four density gradient forms in the foam core layer of the present invention: symmetrical gradient, asymmetrical gradient, positive gradient, and negative gradient.

[0038] Figure 5 The diagram shows two typical layup structures of the FML panel of the present invention; wherein, (a) is metal / fiber / metal; and (b) is metal / fiber / ... / metal.

[0039] Figure 6 This is a schematic diagram of the layered foaming mold structure for the gradient PMI foam core layer of the present invention;

[0040] Figure 7 This is a schematic diagram of the vacuum bag-assisted adhesive bonding and curing composite process of the present invention;

[0041] Figure 8 This is an enlarged schematic diagram of the interface structure of the sandwich structure of the present invention; wherein, (a) is the interface between the FML panel and the foam core layer; and (b) is the interface between different molded foam core layers.

[0042] Figure 9 This is a process flow diagram of the manufacturing method of the gradient PMI foam sandwich structure of the present invention.

[0043] Figure 10 This is a schematic diagram of the impact response analysis of the sandwich structure in this invention.

[0044] Figure 11 This is an impact force-displacement curve diagram of different foam core layer sandwich structures of the present invention.

[0045] The attached figures are labeled as follows: 1. Upper FML panel; 11. Metal layer; 12. Fiber reinforcement layer; 2. Gradient PMI foam core layer; 21. Upper high-density zone; 22. Medium-density zone; 23. Lower high-density zone; 24. Interlayer transition layer; 3. Lower FML panel; 4. Adhesive layer; 5. Mold body; 51. Mold base plate; 52. Mold sidewall; 6. Vacuum bag film; 7. Sealing tape; 8. Breathable felt; 10. Separating membrane; 13. Vacuum connector; 100. Sandwich structure prefabricated body; 40. Impact material. Detailed Implementation

[0046] 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 embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0047] Example 1: See Figure 5In this embodiment, the upper FML panel 1 and the lower FML panel 3 are produced using a vacuum pressure curing process. Each FML panel is formed by resin impregnation, lamination, and curing of a metal layer 11 and a fiber reinforcement layer 12. The layup structure can be flexibly configured according to design requirements such as strength, stiffness, and lightweighting. The lamination structure of the FML panel includes, but is not limited to, the following two forms:

[0048] (1) Single-layer fiber-reinforced structure: formed by alternating two metal layers 11 and a single-layer fiber-reinforced layer 12, with the layup sequence being "metal layer / fiber-reinforced layer / metal layer", such as Figure 5 As shown in (a), this structure has few layers and a simple manufacturing process, making it suitable for applications with high requirements for lightweighting or that can withstand general impact loads.

[0049] (2) Multilayer fiber-reinforced structure: formed by alternating layers of metal layers 11 and multilayer fiber-reinforced layers 12, with the layup sequence being "metal layer / fiber-reinforced layer / ... / fiber-reinforced layer / metal layer", such as Figure 5 As shown in (b), the layup angle of each fiber reinforcement layer 12 can be designed according to the direction of force, for example, by using combinations such as 0° / 90° orthogonal layup and ±45° symmetrical layup, to improve the load-bearing capacity and impact resistance of the FML panel in different directions. By increasing the number of fiber reinforcement layers and optimizing the layup angle, this structure can significantly improve the overall strength, stiffness, and damage tolerance of the panel, making it suitable for structural areas subjected to complex loads, high stress concentrations, or high impact resistance requirements.

[0050] After the layup is completed, the laminated structure is placed in a vacuum bag for vacuum pressure curing, allowing the resin to fully impregnate each layer and cure completely, forming an FML panel with high interlayer bond strength and stable mechanical properties. The thickness, metal material, and fiber reinforcement layer type of the FML panel can be adjusted according to different application requirements: for areas subjected to large impact loads, the thickness of the FML panel can be appropriately increased or higher strength fiber reinforcement materials can be selected; for areas with high lightweight requirements, a thinner FML panel can be used.

[0051] See Figure 2 , Figure 3 , Figure 4 The gradient PMI foam core layer features a density gradient design, with the gradient PMI foam core layer 2 exhibiting a density gradient distribution along its thickness direction. Figure 2 , Figure 3 It can be achieved through layered gradient design or continuous gradient design, and the density variation trend along the thickness direction can be designed in combination with the impact direction of the FML panel.

[0052] The layered gradient design divides the core layer into multiple density regions along its thickness (upper high-density region 21, medium-density region 22, and lower high-density region 23). The density within the same region remains relatively consistent, while density variations between different regions are more pronounced. A transition layer 24 between adjacent density regions can be provided to alleviate interfacial stress concentration. The number of regions in this layered structure is not limited to three; more density regions can be defined based on actual load-bearing and lightweight requirements. The continuous gradient design allows for a smooth and continuous change in density along the thickness direction, avoiding abrupt density changes between layers. Linear gradients, exponential gradients, or other continuous change forms can be used. Linear gradients ensure uniform and continuous density changes along the thickness direction, simplifying process control. Exponential gradients result in faster density changes near the core layer surface and slower changes within the core, ensuring both surface impact resistance and overall lightweighting. These density variations can be achieved by continuously adjusting the amount of foaming agent, mold temperature gradient, or segmented pressurization processes, which facilitates precise adjustment of interfacial stress distribution and reduces the risk of delamination failure.

[0053] like Figure 4 As shown, the specific forms of the density gradient of the gradient PMI foam core layer include: (a) Symmetric gradient: The density distribution is symmetrical with respect to the mid-plane of the core layer thickness, and the density near the interface between the upper FML panel 1 and the lower FML panel 3 is... The density in the middle of the core layer is ,and (b) Asymmetric gradient: The density distribution is asymmetric with respect to the thickness of the core layer, with different magnitudes or rates of density change on both sides; (c) Positive gradient: The density along the thickness direction ( Figure 4 (d) Positive gradient along the z-axis: monotonically increasing; negative gradient: density along the thickness direction ( Figure 4 The gradient (in the positive z-axis direction) decreases monotonically and is a mirror image of the positive gradient. During design, an appropriate gradient form or a combination of multiple gradient forms can be selected according to the actual stress conditions of the structure (such as symmetrical bending loads and asymmetrical impact loads) to balance the structural load-bearing capacity, impact resistance, and lightweight requirements.

[0054] See Figure 8 In section (a), the interface design between the upper FML panel 1, the lower FML panel 3, and the gradient PMI foam core layer is shown. The upper FML panel 1, the lower FML panel 3, and the gradient PMI foam core layer 2 are connected by an adhesive layer 4. Figure 8 As shown in (a), the adhesive layer 4 has a thickness of d, and is used in conjunction with the vacuum bag film 6 to assist in the curing process, ensuring full adhesion of the interface and improving the bonding strength between the panel and the core layer. The thickness d of the adhesive layer 4 can be adjusted according to the thickness of the FML panel, the foam density, and the interface load requirements to reduce the problem of insufficient adhesion between the panel and the foam core layer in conventional sandwich structures.

[0055] For interface treatment of foam regions with different densities, please refer to [link / reference]. Figure 8 In section (b), for the connection between different density regions within the gradient PMI foam core layer 2, such as between the upper high-density region 21 and the medium-density region 22, interlayer bonding can be achieved by adhesive bonding, hot-pressing composite, integral foaming molding, or a combination thereof. If necessary, a foam interlayer transition layer 24 can be provided. Figure 8 In section (b), the stiffness abrupt change is mitigated, and interface defects such as bubbles and voids that are prone to occur between layers of conventional foam core materials are avoided, thereby improving the overall strength of the structure.

[0056] Quantitative design of materials and structures: During the design phase, numerical simulation and mechanical analysis methods, combined with stress analysis, are used to determine the specific thickness, density gradient form, and distribution parameters of the FML panel and the gradient PMI foam core layer 2. Higher density foam (e.g., upper high-density zone 21, lower high-density zone 23) is used in the compression region to increase local stiffness; lower density foam (e.g., medium-density zone 22) is used in the shear region to reduce weight and improve energy absorption capacity. The density distribution function can be expressed as:

[0057]

[0058] in, These are the coordinates in the thickness direction. For the initial density, These are the gradient coefficients.

[0059] Since the elastic modulus and density of PMI foam material satisfy a power function relationship, it can be expressed as:

[0060]

[0061] in, The elastic modulus of the foam. The density of the foam. This refers to a material-related index.

[0062] Therefore, the elastic modulus of the gradient PMI foam core layer 2 changes continuously along the thickness direction, and its shear modulus... It also exhibits a gradient distribution, according to the shear constitutive relation:

[0063]

[0064] in, For shear stress, This represents the shear modulus in the thickness direction. This represents shear strain.

[0065] Gradient shear modulus distribution is beneficial for achieving smooth load transfer in the core and interface regions, mitigating abrupt changes in stiffness between regions of different densities, reducing interface shear stress concentration, and thus improving the overall stability and impact resistance of sandwich structures.

[0066] Under impact loading conditions, the impactor has an initial velocity of ,quality Upon impact with the surface of the upper FML panel 1, its initial kinetic energy can be expressed as:

[0067]

[0068] During the impact, part of the initial kinetic energy of the impactor 40 is converted into the elastic recovery energy of the structure. Part of it is converted into structural energy absorption. (Including the crushing deformation of the gradient PMI foam core layer 2, local damage to the FML panel, and interface energy dissipation, etc.), its energy relationship can be approximately expressed as:

[0069]

[0070] Under low-speed impact conditions, the contact force F between the impactor 40 and the upper FML panel 1 can be approximately described using Hertzian contact theory:

[0071]

[0072] in, For contact stiffness coefficient, For the impact indentation depth (i.e., the local displacement at the contact point between the impactor 40 and the panel, see...), Figure 10 (a)

[0073] Combining the equation of motion of the impactor 40, the contact force can be determined. Indentation depth Simplified estimation of changes over time:

[0074]

[0075] Furthermore, the crushing strength of the gradient PMI foam core layer 2 With density The relationship between them satisfies an exponential function, which can be expressed as:

[0076]

[0077] in, The core layer crushing strength, For the corresponding location density, For closed-cell PMI foam materials, the crush strength index is... .

[0078] Since the upper high-density region 21 and the lower high-density region 23 are adjacent to the upper FML panel 1 and the lower FML panel 3 respectively, the high-density region near the panel has higher crushing strength, which can effectively resist local indentation in the initial stage of impact and protect the panel from being penetrated; while the medium-density region 22 has relatively lower crushing strength, and progressive crushing preferentially occurs when the impact energy is transferred to the middle of the core layer. Through the plastic buckling, collapse and compaction of the core layer material, the impact energy is continuously absorbed, thereby reducing the peak impact force Fmax, prolonging the impact time, and achieving a smoother force-displacement response. The impact energy absorbed by the core layer can be approximately expressed as the integral of the crushing stress along the crushing depth:

[0079]

[0080] Where A is the effective area of ​​the impact contact zone.

[0081] The corresponding impact physics model and typical impact force-displacement response curves are as follows: Figure 10 As shown: During the loading stage, the curve rises sharply first and then flattens out (corresponding to progressive core crushing), and during the unloading stage, it rebounds along a lower path to the residual indentation. The area enclosed by the loading-unloading curves represents the impact energy absorbed by the structure. See Figure 10 (b)

[0082] See Figure 9 The process flow shown in this invention, a design method for an FML panel-gradient PMI foam sandwich structure, includes the following steps:

[0083] S1, FML panel prefabrication: A thin aluminum alloy sheet with a thickness of 0.3mm to 0.6mm is selected as the metal layer 11, and carbon fiber / epoxy resin prepreg is selected as the fiber reinforcement layer 12. The layers are stacked in the order of "metal layer / fiber reinforcement layer / metal layer" or "metal layer / fiber reinforcement layer / ... / fiber reinforcement layer / metal layer" to prepare the upper FML panel 1 and the lower FML panel 3 respectively.

[0084] After the layup is completed, the laminated structure is placed in a vacuum bag for vacuum treatment, with the vacuum degree controlled between -0.08 MPa and -0.095 MPa. Hot pressing or autoclaving is then used to cure the structure under a molding pressure of 0.3 MPa to 0.6 MPa, with the curing temperature controlled between 120℃ and 180℃ and the holding time between 1 h and 2 h. This allows the resin to fully impregnate the fiber reinforcement layer 12 and complete the curing reaction. After curing, the structure is cooled to room temperature, demolded, and the surface of the FML panel is polished for later use.

[0085] S2. Gradient PMI foam core layer preparation (layered foaming): The density distribution pattern in the thickness direction of the core layer is determined according to the design requirements. In this embodiment, a layered gradient structure is adopted. The foam density in each area is controlled by adjusting the raw material formula, foaming agent content and foaming process parameters. The core layer density range is set to 50 kg / m³~200 kg / m³.

[0086] Preferably, the upper high-density region 21 and lower high-density region 23 near the interface between the upper FML panel 1 and the lower FML panel 3 have a density of 120 kg / m³ to 200 kg / m³, and the medium-density region 22 has a density of 50 kg / m³ to 100 kg / m³. The foaming temperature is controlled at 180℃ to 220℃, and the foaming time is controlled at 20 min to 40 min. Figure 6 The mold body 5 of the layered foaming mold shown is defined by the mold base plate 51 and the mold side wall 52, which together define the foaming cavity. Figure 6 The mold body 5, mold base plate 51 and mold side wall 52 of the layered foaming mold shown define the cavity, and each density zone is poured and foamed in layers in the order of ①②③.

[0087] S3. Core Layer Interlayer Hot Press Composite: The gradient PMI foam core layer 2 prepared in step S2 is subjected to interlayer hot press composite treatment. The hot pressing process further improves the bonding strength between different density regions. The hot pressing temperature is controlled at 150℃~180℃, and the hot pressing pressure is controlled at 0.2 MPa~0.5 MPa. After holding the pressure for a certain time, it is cooled and formed. If necessary, a transition layer 24 can be set between adjacent density regions to obtain a gradient PMI foam core layer 2 with good integrity and stable interlayer bonding.

[0088] S4. Interface adhesive coating: Place the prepared gradient PMI foam core layer 2 on the lower FML panel 3, and uniformly coat the adhesive layer 4 at the interface between the FML panel and the foam core layer. Epoxy structural adhesive, modified epoxy structural adhesive, or other structural adhesives suitable for connecting the FML panel and the gradient PMI foam core layer 2 are preferred. The amount of adhesive coating is controlled to be 150 g / m² to 300 g / m². Then, lay the upper FML panel 1 to form a sandwich structure prefabricated body 100.

[0089] S5, Vacuum Bag Film Assisted Curing and Composite: Refer to... Figure 7The process shown involves sequentially laying an isolation membrane 10 and a breathable felt 8 on the surface of the sandwich structure prefabricated body 100, covering it with a vacuum bag film 6, and sealing it with sealing tape 7 to the edge of the mold body 5 of the layered foaming mold. Vacuum is then drawn by connecting a vacuum pipeline through a vacuum connector 13, with the vacuum degree controlled at -0.08 MPa to -0.095 MPa. Hot pressing or autoclaving is used to cure the material. The material is kept at a molding pressure of 0.2 MPa to 0.5 MPa and a curing temperature of 80℃ to 140℃ for 1 to 2 hours to allow the adhesive layer 4 to flow fully and fill the interfacial micropores, completing the curing reaction and achieving a high-strength integral composite between the upper FML panel 1, the gradient PMI foam core layer 2, and the lower FML panel 3.

[0090] S6. Demolding and post-processing: After curing, allow it to cool naturally to room temperature, demold, and perform necessary edge trimming, dimensional trimming, and surface cleaning to obtain the finished FML panel-gradient PMI foam sandwich structure.

[0091] In the sandwich structure obtained by the above preparation method, the gradient PMI foam core layer 2 forms a preset density gradient distribution along the thickness direction. The upper high-density region 21 and the lower high-density region 23, which are close to the upper FML panel 1 and the lower FML panel 3, provide higher local support and interface bearing capacity for the FML panel. The medium-density region 22 reduces the structural mass and improves the energy absorption capacity while ensuring the integrity of the structure. The gradient transition between different density regions achieves continuous change in stiffness, which is conducive to the smooth transfer of load in the core layer and interface region, reduces interface stress concentration, and improves the overall stiffness, impact resistance and structural stability of the sandwich structure.

[0092] The effectiveness of this embodiment is verified as follows: Finite element simulation analysis (low-speed impact simulation based on ABAQUS software) and experimental tests show that, without significantly increasing the mass, the sandwich structure prepared by the above method reduces the peak impact contact force Fmax by about 15% to 30%, increases the impact energy Ea absorbed by the structure by about 20% to 40%, significantly reduces the impact damage area and the back bulge deformation, and no obvious debonding phenomenon occurs at the interface. This verifies the feasibility and effectiveness of the FML panel-gradient PMI foam sandwich structure and design method of the present invention in terms of impact resistance.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An FML panel-gradient PMI foam sandwich structure, characterized in that, It includes an upper FML panel (1), a lower FML panel (3), and a gradient PMI foam core layer (2) disposed between the upper FML panel (1) and the lower FML panel (3), wherein the gradient PMI foam core layer (2) is distributed in the form of a density gradient along the thickness direction; The upper FML panel (1) or the lower FML panel (3) is composed of two metal layers (11) and a single fiber reinforcement layer (12) disposed between the two metal layers (11); The gradient PMI foam core layer (2) is composed of an upper high-density area (21), a medium-density area (22), and a lower high-density area (23) arranged sequentially from top to bottom; the upper high-density area (21) and the lower high-density area (23) are respectively adjacent to the interface of the upper FML panel (1) and the interface of the lower FML panel (3); The upper FML panel (1), the lower FML panel (3), and the upper and lower interfaces of the gradient PMI foam core layer (2) are connected by an adhesive layer (4).

2. The FML panel-gradient PMI foam sandwich structure according to claim 1, characterized in that, A foam interlayer transition layer (24) is provided between adjacent density regions of the upper high-density region (21), the medium-density region (22), and the lower high-density region (23).

3. The FML panel-gradient PMI foam sandwich structure according to claim 1, characterized in that, The density gradient forms include one or more combinations of symmetric gradients, asymmetric gradients, positive gradients, negative gradients, and hierarchical gradients or continuous gradients. The symmetrical gradient: The density distribution of the gradient PMI foam core layer (2) along the thickness direction is symmetrical with respect to the middle surface of the core layer; Asymmetric gradient: The density distribution of the gradient PMI foam core layer (2) along the thickness direction is asymmetric relative to the middle surface of the core layer, and the density change amplitude or rate on both sides is different; Positive gradient: The gradient PMI foam core layer (2) increases monotonically from bottom to top along the thickness direction; Negative gradient: The gradient PMI foam core layer (2) decreases monotonically from bottom to top along the thickness direction, and is a mirror image of the positive gradient.

4. The FML panel-gradient PMI foam sandwich structure according to claim 2, characterized in that, The gradient PMI foam core layer (2) has a high density in the area that is attached to the interface of the upper FML panel (1) and the interface of the lower FML panel (3), and a low density in the medium density area (22).

5. The FML panel-gradient PMI foam sandwich structure according to claim 3, characterized in that, The gradient PMI foam core layer (2) is distributed in the form of a density gradient along the thickness direction. Through continuous gradient design or layered gradient design along the thickness direction, the density parameters of each region are quantitatively controlled according to the design requirements. The density distribution function is expressed as follows: ,in These are the coordinates in the thickness direction. For the initial density, These are the gradient coefficients.

6. A layered foaming mold, characterized in that, Fabricating the FML panel-gradient PMI foam sandwich structure according to any one of claims 1 to 6, comprising: The layered foaming mold (5) body is composed of a mold base plate (51) and a mold side wall (52) forming a foaming cavity.

7. A design method for an FML panel-gradient PMI foam sandwich structure based on any one of claims 1-6, characterized in that, Includes the following steps: Step S1, FML panel prefabrication: Select aluminum alloy sheet with a thickness of 0.3mm-0.6mm as metal layer (11), and carbon fiber / epoxy resin prepreg as fiber reinforcement layer (12). Lay the layers in the order of "metal layer / fiber reinforcement layer / metal layer" or "metal layer / fiber reinforcement layer / ... / fiber reinforcement layer / metal layer" to prepare upper FML panel (1) and lower FML panel (3) respectively. After the layup is completed, place the laminated structure in a vacuum bag (6) for vacuum treatment. The vacuum degree is controlled at -0.08MPa to -0.095MPa. Use hot pressing or autoclaving process to heat and cure under a molding pressure of 0.3 MPa to 0.6MPa. The curing temperature is controlled at 120℃ to 180℃ and the heat preservation time is 1h to 2h to allow the epoxy resin to impregnate the fiber reinforcement layer and complete the curing reaction. After curing, cool to room temperature, demold and polish the panel surface for later use. Step S2, Gradient PMI foam core layer preparation: According to the design requirements, the density distribution form of the gradient PMI foam core layer (2) in the thickness direction is determined. A layered gradient structure is adopted. The foam density of each area is controlled by adjusting the raw material formula, foaming agent content and foaming process parameters. The core layer density range is set to 50 kg / m³~200 kg / m³. Step S3, hot pressing composite of core layer: The gradient PMI foam core layer (2) prepared in step S2 is subjected to hot pressing composite treatment. The hot pressing temperature is controlled at 150℃~180℃, the hot pressing pressure is controlled at 0.2 MPa~0.5 MPa, and the core layer is held under pressure for 120 minutes and then cooled and formed. Step S4, Interface adhesive coating: Place the prepared gradient PMI foam core layer (2) on the lower FML panel (3), and uniformly coat the adhesive layer (4) at the interface between the lower FML panel (3) and the gradient PMI foam core layer (2). Then lay the upper FML panel (1) to form a sandwich structure prefabricated body (100). Step S5, Vacuum bag-assisted curing and composite: On the surface of the sandwich structure preform (100) obtained in step S4, a separation film (10), a breathable felt (8), and a covering vacuum bag film (6) are laid in sequence, and sealed to the edge of the mold body (5) of the layered foaming mold with sealing tape (7). Vacuum pipes are connected through vacuum connector (13) to draw vacuum, and the vacuum degree is controlled to be -0.08 MPa to -0.095 MPa. Hot pressing or autoclaving process is adopted, and the adhesive layer (4) is kept at a molding pressure of 0.2 MPa to 0.5 MPa and a curing temperature of 80℃ to 140℃ for 1h to 2h, so that the adhesive layer (4) can flow fully and fill the interface micropores, complete the curing reaction, and realize the overall composite between the upper FML panel (1), the gradient PMI foam core layer (2) and the lower FML panel (3). Step S6, Demolding and Post-processing: After curing, allow it to cool naturally to room temperature, demold, and perform edge trimming, dimensional trimming, and surface cleaning to obtain the finished FML panel-gradient PMI foam sandwich structure.

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