An underwater acoustic metamaterial and a preparation method thereof

Abstract

The application discloses a kind of water pressure resistant low reflection high absorption underwater acoustic metamaterial and preparation method, belong to underwater acoustic metamaterial technical field.Periodic arrangement of superstructure material includes cell unit, cell unit presents sandwich structure form, upper and lower surface layer is continuous fiber reinforced resin panel, intermediate layer is composed of three parts with specific wall thickness three-dimensional corrugated curved frame, viscoelastic sound-absorbing polymer filled in frame and specific outer shape size cavity, three-dimensional corrugated curved frame and polymer present mutual continuous structure that is penetrated.Cells are distributed into square or hexagon, cell cycle size, thickness can be adjusted according to incident sound wave length.The corrugated curved surface geometry parameter of the application can be designed, and the acoustic performance and mechanical properties of underwater sound-absorbing material can be flexibly adjusted, to meet the requirements of high hydrostatic pressure resistance, wide frequency high absorption, low reflection, impact resistance and other structure function integration, to provide support for the design of low acoustic target intensity shell of underwater platform.

Description

Technical Field

[0001] This invention belongs to the field of underwater acoustic metamaterials, specifically relating to a water pressure resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. Background Technology

[0002] The acoustic coating layer on the surface of an underwater platform can absorb sound waves from active sonar and reduce its own acoustic target reflection intensity, playing a crucial role in the platform's underwater concealment. The performance of underwater sound-absorbing materials is determined by both the intrinsic properties of the material and its sound-absorbing structure. Typical underwater acoustic absorbing materials are mainly low-modulus viscoelastic polymers, including modified rubber and modified polyurethane. By adjusting the elastic modulus and damping loss of the material, its broadband sound absorption can be improved. Simultaneously, the presence of cavities of specific sizes, resonant mass blocks, or localized resonant structures within the material can also enhance the low-frequency broadband sound absorption effect from an acoustic structural perspective. However, viscoelastic polymers and their composite structures have weak resistance to water pressure. Under high hydrostatic pressure, on the one hand, the increased modulus of viscoelastic polymers leads to increased reflectivity and decreased sound absorption; on the other hand, the internal cavities undergo large deformations, resulting in a shift in absorption frequency and degradation of acoustic performance. Therefore, it is urgent to effectively improve the underwater pressure-bearing capacity of underwater acoustic absorbing materials while maintaining high absorption and low reflection.

[0003] Foam structures and lattice structures are the most commonly used mechanical reinforcement structures. Foam structures, such as aluminum foam, have high strength and can form a load-bearing frame in three-dimensional space, but controlling the porosity and pore size of the foam remains challenging. Lattice structures, by designing and arraying different lattice unit cells, form a regular structure that repeats continuously in all directions in three-dimensional space. Most lattice structure unit cells are rods composed of straight metal rods, such as face-centered cubic, body-centered cubic, and diamond structures. However, these unit cells composed of straight rods exhibit sharp turns at the joints, leading to severe stress concentration at these joints under water pressure. This makes them prone to premature failure at the nodes when subjected to cyclic loading, which is detrimental to their long-term service life. Furthermore, lattice structures are typically fully continuous between incident and emitted waves, easily forming acoustic channels, and they also exhibit strong reflections in specific directions, which is unfavorable for sound absorption.

[0004] Three-dimensional corrugated surface structures possess advantages such as high specific stiffness, streamlined contours, and zero average curvature. In terms of mechanical load-bearing, they provide uniform pressure resistance. The streamlined surface profile eliminates significant stress concentration within the structure, allowing for the uniform bearing of larger loads and maintaining long-term stability under high hydrostatic pressure. This effectively overcomes the limitation of traditional sound-absorbing polymers being susceptible to water pressure. The three-dimensional corrugated surface sandwich structure is an effective sound-absorbing load-bearing structure, providing both a mechanical load-bearing framework and a sound-absorbing polymer filling framework. Furthermore, the design of cavities with different shapes and sizes within the load-bearing framework allows for the utilization of the cavity's resonance and longitudinal / transverse wave conversion characteristics, effectively improving the sound absorption coefficient and expanding the sound absorption bandwidth. This achieves integrated sound absorption and load-bearing capabilities, demonstrating promising application prospects in practical engineering. Summary of the Invention

[0005] The purpose of this invention is to provide a water pressure resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method, which solves the shortcomings of current underwater acoustic absorbing polymers such as weak mechanical load-bearing capacity, easy deformation of the sound-absorbing pores inside the polymer, and decreased sound absorption performance under high hydrostatic pressure, and provides a new technical solution for the design of water pressure resistant broadband sound-absorbing materials.

[0006] To achieve the above objectives and solve the above technical problems, the present invention provides the following technical solution:

[0007] A water-pressure resistant, low-reflection, and high-absorption underwater acoustic metamaterial is characterized by comprising periodically arranged cellular units, which have a sandwich structure. The upper and lower surface layers are continuous fiber-reinforced resin panels, and the middle layer consists of a three-dimensional corrugated surface mechanical support frame with a set wall thickness and a viscoelastic sound-absorbing polymer filled within the mechanical support frame. The three-dimensional corrugated surface frame and the polymer have an interpenetrating continuous structure. The periodic distribution of the cells is in the form of squares or hexagons, and the size and thickness of the cell period can be adjusted according to the wavelength of the incident sound wave.

[0008] The profile of the three-dimensional corrugated surface is determined by an implicit function. The average curvature of the surface is zero or close to zero, which can effectively scatter the incident sound waves uniformly to all directions. The corrugated surface divides the space into two interconnected regions. By controlling the wall thickness of the corrugated surface, mechanical load-bearing frames with different volume ratios can be obtained. The volume ratio of the mechanical load-bearing frame varies between 5 vol% and 100 vol%. The wall thickness of the three-dimensional corrugated surface can also be gradient-varying, thus causing the volume ratio to also exhibit a gradient change, with the gradient value varying between 5 vol% and 100 vol%.

[0009] Furthermore, the design of the three-dimensional corrugated surface includes the following steps:

[0010] ① Determine the shape of the three-dimensional corrugated surface as square or cylindrical, discretize the coordinates of the target three-dimensional space, and generate the coordinate dataset [x,y,z] required to draw the three-dimensional corrugated surface;

[0011] ② Determine the implicit function of the corrugated surface This function controls the contour change of the corrugated surface unit, with a period of L, where L ranges from 1mm to 100mm. The period is anisotropic in different directions of the coordinate system. This implicit function ensures that the dimensions of the corrugated surface structure in different directions have at least one complete period.

[0012]

[0013]

[0014] ③ By setting the value t of the isosurface corresponding to the implicit function, the implicit function of the corrugated surface is made to satisfy... or It can control the contour of the corrugated surface to present a specific wall thickness and volume ratio, with a corresponding wall thickness of 2t.

[0015] ④ Based on the implicit function, the isosurface of the three-dimensional ripple surface is drawn in the discrete coordinate dataset [x,y,z]. Triangular elements are used to mesh the isosurface, and the face and vertex data [F,V] corresponding to the triangular elements are obtained and output for surface modeling analysis and structural processing.

[0016] ⑤ By performing gradient interpolation on the implicit function values ​​in the gridded spatial coordinate dataset [x,y,z], a three-dimensional wavy surface with gradient changes in wall thickness and volume ratio is obtained.

[0017] Furthermore, the water pressure resistance of the underwater acoustic supermaterial can be adjusted by designing the wall thickness of the three-dimensional corrugated surface; the greater the wall thickness, the stronger the water pressure bearing capacity.

[0018] Furthermore, when cavities are added inside the three-dimensional corrugated surface structure layer, the underwater acoustic supermaterial exhibits superior low-frequency broadband sound absorption performance. The cavities are located on the central axis of the cell, and their cross-sections perpendicular to the central axis are square or circular, with the side length or diameter of the cross-section being 0.2 to 0.85 times the side length of the cell. The thickness of the cavity along the central axis is 0.1 to 0.85 times the thickness of the cell. In step 1 of the design of the three-dimensional corrugated surface, when the corrugated surface contains cavities of different shapes and sizes, the coordinate dataset [x,y,z] required for the three-dimensional corrugated surface is drawn by removing the discrete points inside the cavities in the coordinate dataset [x,y,z].

[0019] Furthermore, the three-dimensional corrugated surface is printed using additive manufacturing, and the materials used for the three-dimensional corrugated surface include aluminum alloy, titanium alloy, high-modulus resin, and high-modulus ceramic.

[0020] Furthermore, the upper and lower panels of the cell are made of glass fiber reinforced resin material, which is a high-transmittance material for sound waves, with a surface thickness of 1mm to 4mm. This is used to ensure that the pressure is uniformly transmitted to the three-dimensional corrugated curved frame and to reduce the deformation of the internal viscoelastic polymer.

[0021] Furthermore, when the underwater acoustic metamaterial is applied to a steel plate, the underwater acoustic metamaterial exhibits better high absorption and low reflection performance.

[0022] Furthermore, when the underwater acoustic metamaterial is placed directly under the condition of being submerged in water at both ends, the underwater acoustic metamaterial exhibits better high sound absorption and low reflection performance.

[0023] Furthermore, the underwater acoustic metamaterial exhibits better absorption performance for large-angle oblique incident acoustic waves ranging from 0° to 60°.

[0024] This invention also provides a method for preparing a water pressure-resistant, low-reflection, high-absorption underwater acoustic metamaterial, comprising the following steps:

[0025] ① A three-dimensional corrugated surface frame with characteristic wall thickness and external dimensions is obtained by additive manufacturing;

[0026] ② Place the three-dimensional corrugated surface frame in a mold made of polytetrafluoroethylene or a machined metal mold, and cast it with a modified polymer. After the polymer is cured at room temperature or by heating, it is demolded to obtain a corrugated surface structure with embedded solid viscoelastic polymer, which is the core material of the sandwich material.

[0027] ③ When the core material contains cavities, the three-dimensional corrugated surface frame is additively manufactured in two parts and modified polymer is poured into each part. The two parts of the three-dimensional corrugated surface frame can be linearly cut or curved cut. During pouring, the cavities are formed using a pre-set mold. The two load-bearing frames are combined and assembled to obtain the three-dimensional corrugated surface core material containing cavities.

[0028] ④ By winding and vacuum thermoplasticizing a layer of fiber-reinforced resin material panel on the surface and circumference of the three-dimensional corrugated core material, the underwater sound-absorbing material of the present invention is obtained.

[0029] Furthermore, a viscoelastic polymer is filled inside the three-dimensional corrugated surface frame. The materials are rubber, polyurethane, and their modified polymers. After impregnation and curing, a sound-absorbing structure is obtained by fusing the three-dimensional corrugated surface frame with the viscoelastic polymer.

[0030] The benefits of this invention are as follows:

[0031] 1. This invention proposes a water-pressure resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. It uses a periodic three-dimensional corrugated surface as a load-bearing frame. Thanks to the smooth outline of the corrugated surface, it can achieve a uniform pressure bearing effect in terms of mechanical load bearing. The streamlined surface outline does not have obvious stress concentration inside the structure, so it can uniformly bear a larger load and maintain long-term stability under high hydrostatic pressure, thereby effectively solving the limitation of traditional sound-absorbing polymers being not resistant to water pressure.

[0032] 2. This invention proposes a water-pressure-resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. First, by designing the volume ratio of the water-pressure-resistant structure on the incident surface, the impedance of the sound-absorbing composite material can be matched with the impedance of water, allowing sound waves to enter the material's interior as much as possible. Second, the corrugated surface's zero-average-curvature profile reduces the reflection of incident sound waves while uniformly scattering them in all directions. Finally, the viscoelastic damping polymer filled within the corrugated surface structure provides efficient broadband sound absorption.

[0033] 3. The present invention proposes a water pressure resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. It has a simple structure, readily available raw materials, and mature preparation process. The corrugated surface geometric parameters are highly designable, and the acoustic and mechanical properties of the underwater sound-absorbing material can be flexibly adjusted to meet the integrated structural and functional requirements of high hydrostatic pressure resistance, wide-band high absorption, low reflection, impact resistance, lightweight and high strength. This provides support for the design and development of low-sound-target strength shells for underwater platforms. Attached Figure Description

[0034] Figure 1 It is a schematic diagram of the underwater sound-absorbing metamaterial containing a three-dimensional corrugated surface pressure-bearing frame. (a) is a schematic diagram of the sound-absorbing structural cell, where: 1-upper fiber-reinforced resin panel; 2-middle three-dimensional corrugated surface load-bearing frame; 3-viscoelastic polymer filled in the middle three-dimensional corrugated surface; 4-bottom fiber-reinforced resin panel; (b) is a diagram of the corrugated surface structure in the cell.

[0035] Figure 2 It is a three-dimensional corrugated surface load-bearing frame structure with a gradient of volume fraction, and all cell thicknesses are 50 mm. (a) represents a linear variation of volume fraction from 20 vol% to 60 vol%; (b) represents a linear variation of volume fraction from 40 vol% to 80 vol%; and (c) represents a linear variation of volume fraction from 5 vol% to 100 vol%.

[0036] Figure 3 This is the acoustic model of the periodic cells in Example 1, where both the incident and exit ends of the sound waves are in a water-backed environment.

[0037] Figure 4This refers to the maximum displacement of corrugated sandwich materials with different wall thicknesses under different hydrostatic pressures in Example 1.

[0038] Figure 5 This is the variation of the sound absorption coefficient of corrugated sandwich materials with different wall thicknesses under normal pressure with frequency in Example 1.

[0039] Figure 6 This is the variation of the reflection coefficient of corrugated sandwich materials with different wall thicknesses under normal pressure with frequency in Example 1.

[0040] Figure 7 This is a comparison of the sound absorption coefficient of a corrugated sandwich material with a wall thickness of 5.14 mm under normal pressure and 5 MPa water pressure as a function of frequency in Example 1.

[0041] Figure 8 This is a comparison of the reflection coefficient of a corrugated sandwich material with a wall thickness of 5.14 mm under normal pressure and 5 MPa water pressure as a function of frequency in Example 1.

[0042] Figure 9 This is Example 1, showing the change in sound absorption coefficient of a corrugated sandwich material with a wall thickness of 5.14 mm with frequency under different oblique incident angles.

[0043] Figure 10 This is Example 1, showing the variation of the reflection coefficient with frequency of a corrugated curved sandwich material with a wall thickness of 5.14 mm under different oblique incident angles.

[0044] Figure 11 This is a cellular model of a corrugated sandwich material with a wall thickness of 5.14 mm embedded in a cavity inside a load-bearing frame, as shown in Example 1.

[0045] Figure 12 This describes the change in the sound absorption coefficient of a corrugated sandwich material with a wall thickness of 5.14 mm before and after it is embedded in the cavity inside the load-bearing frame, as shown in Example 1.

[0046] Figure 13 In Example 2, the corrugated curved sandwich material cell model with a wall thickness of 2.55mm contains a cuboid cavity of 15mm×15mm×30mm.

[0047] Figure 14 This is Example 2, showing the change in sound absorption coefficient with frequency before and after the corrugated curved sandwich material with a wall thickness of 2.55mm is introduced into the cavity.

[0048] Figure 15 The water pressure resistant sandwich specimens actually prepared in Example 3 are shown in (a) as corrugated curved core material with a wall thickness of 6.1 mm and (b) as specimens after the surface is covered with glass fiber reinforced resin panels.

[0049] Figure 16 In Example 3, the sound absorption coefficient of the actually prepared hydrostatic pressure resistant specimen changes with frequency under different hydrostatic pressures.

[0050] Figure 17 In Example 3, the reflection coefficient of the water pressure resistant specimen actually prepared under different hydrostatic pressures varies with frequency.

[0051] Figure 18 This is Example 4, showing the change in the sound absorption coefficient of the water-pressure resistant interlayer material with frequency under different panel thicknesses.

[0052] Figure 19 This is Example 4, showing the variation of the reflection coefficient of the water-pressure resistant interlayer material with frequency under different panel thicknesses. Detailed Implementation

[0053] This invention aims to provide a water-pressure resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. It adopts a sandwich structure, with the core material using a corrugated curved surface support structure. By designing the wall thickness, volume duty cycle, and gradient changes of the corrugated curved surface, and optimizing the shape and size parameters of the internal cavities of the corrugated curved surface, it achieves compatibility between low-frequency broadband sound absorption and high water pressure resistance. It can be used in environments where both ends are submerged in water, or it can be applied to a steel plate. Furthermore, it maintains good broadband sound absorption performance even with large-angle oblique incidence.

[0054] To better understand the present invention, the present invention will be explained and described in detail below with reference to the accompanying drawings and embodiments.

[0055] This invention provides a water-pressure-resistant, low-reflection, high-absorption underwater acoustic metamaterial and its preparation method. The material comprises periodically arranged cellular units, which exhibit a sandwich structure. The upper and lower surface layers are continuous fiber-reinforced resin panels, and the middle layer consists of a three-dimensional corrugated surface mechanical support frame with a predetermined wall thickness, and a viscoelastic sound-absorbing polymer filled within the mechanical support frame. The three-dimensional corrugated surface frame and the polymer exhibit an interpenetrating continuous structure. The periodic distribution of the cells is square or hexagonal, and the size and thickness of the cell period can be adjusted according to the incident sound wave wavelength.

[0056] The profile of the three-dimensional corrugated surface is determined by an implicit function. The average curvature of the surface is zero or close to zero, effectively scattering incident sound waves uniformly in all directions. The corrugated surface divides space into two interconnected regions. By controlling different wall thicknesses of the corrugated surface, mechanical load-bearing frames with different volume percentages can be obtained, varying between 5 vol% and 100 vol%. The wall thickness of the three-dimensional corrugated surface can also be gradient-varied, resulting in a gradient-varied volume percentage, with the gradient value also varying between 5 vol% and 100 vol%. Figure 2As shown, the corrugated curved support frame and resin panel allow the water pressure load to be applied more evenly to the cells, thus avoiding large deformation of the viscoelastic polymer cells under high water pressure and maintaining their efficient sound absorption characteristics.

[0057] Furthermore, the design method for three-dimensional corrugated surfaces includes the following steps:

[0058] ① Determine the shape of the 3D corrugated surface as either square or cylindrical, and discretize the coordinates of the target 3D space to generate the coordinate dataset [x, y, z] required for drawing the 3D corrugated surface. When the corrugated surface contains cavities of different shapes and sizes, this can be achieved by removing discrete points within specific cavities from the coordinate dataset [x, y, z].

[0059] ② Determine the implicit function of the corrugated surface This function controls the contour change of the corrugated surface unit, with a period of L. In this invention, L ranges from 1mm to 100mm, and the period can be anisotropic in different directions of the coordinate system. This implicit function can ensure that the dimensions of the corrugated surface structure in different directions have at least one complete period.

[0060]

[0061]

[0062] ③ By setting the value t of the isosurface corresponding to the implicit function, the implicit function of the corrugated surface is made to satisfy... or It can control the contour of the corrugated surface to present a specific wall thickness and volume ratio, with a corresponding wall thickness of 2t.

[0063] ④ Based on the implicit function, the isosurface of the three-dimensional ripple surface is drawn in the discrete coordinate dataset [x,y,z]. Triangular elements are used to mesh the isosurface, and the face and vertex data [F,V] corresponding to the triangular elements are obtained and output for surface modeling analysis and structural processing.

[0064] ⑤ By performing gradient interpolation on the implicit function values ​​in the gridded spatial coordinate dataset [x,y,z], a three-dimensional wavy surface with gradient changes in wall thickness and volume ratio can be obtained.

[0065] Furthermore, the three-dimensional corrugated surface is processed using additive manufacturing methods, with materials including aluminum alloy, titanium alloy, high-modulus resin, and high-modulus ceramics.

[0066] Furthermore, a viscoelastic polymer is filled inside the three-dimensional corrugated surface frame. The materials are rubber, polyurethane, and their modified polymers. After impregnation and curing, a sound-absorbing structure that fuses the three-dimensional corrugated surface frame with the viscoelastic polymer can be obtained.

[0067] Furthermore, the upper and lower panels of the cell are made of glass fiber reinforced resin material, which is a high-transmittance material for sound waves, with a surface thickness of 1mm to 4mm. This is used to ensure that the pressure is uniformly transmitted to the three-dimensional corrugated curved frame and to reduce the deformation of the internal viscoelastic polymer.

[0068] Furthermore, the water pressure resistance can be adjusted by designing the three-dimensional corrugated curved wall thickness; the greater the wall thickness, the stronger the water pressure bearing capacity.

[0069] Furthermore, when cavities are added inside the three-dimensional corrugated surface structure layer, the material exhibits excellent low-frequency broadband sound absorption performance. Based on the scattering characteristics of sound waves by the cavities and the conversion characteristics of longitudinal waves to transverse waves, the low-frequency broadband sound absorption performance can be further improved when the cavity diameter exhibits multi-level variations. The cavities are located on the central axis of the cell, and their cross-sections perpendicular to the central axis are square or circular, with the side length or diameter of the cross-section being 0.2 to 0.85 times the side length of the cell. The thickness of the cavity along the central axis is 0.1 to 0.85 times the thickness of the cell. In step 1 of the design of the three-dimensional corrugated surface, when the corrugated surface contains cavities of different shapes and sizes, the coordinate dataset [x,y,z] required for the three-dimensional corrugated surface is drawn by removing discrete points within the cavities from the coordinate dataset [x,y,z].

[0070] Meanwhile, this invention provides a method for preparing a water pressure-resistant, low-reflection, high-absorption underwater acoustic metamaterial, comprising the following base material preparation steps:

[0071] ① A three-dimensional corrugated surface frame with characteristic wall thickness and external dimensions is obtained by additive manufacturing;

[0072] ② Place the three-dimensional corrugated surface frame in a mold made of polytetrafluoroethylene or a machined metal mold, and cast it with a modified polymer. After the polymer is cured at room temperature or by heating, it is demolded to obtain a corrugated surface structure with embedded solid viscoelastic polymer, which is the core material of the sandwich material.

[0073] ③ When the core material contains cavities, the three-dimensional corrugated surface frame is additively manufactured in two parts and modified polymer is poured into each part. The two parts of the three-dimensional corrugated surface frame can be linearly cut or curved cut. During pouring, the cavities are formed using a pre-set mold. The two load-bearing frames are combined and assembled to obtain the three-dimensional corrugated surface core material containing cavities.

[0074] ④ By winding and vacuum thermoplasticizing a layer of fiber-reinforced resin material panel on the surface and circumference of the three-dimensional corrugated core material, the underwater sound-absorbing material of the present invention is obtained.

[0075] Furthermore, when sound-absorbing materials are applied to steel plates, underwater acoustic materials exhibit high absorption and low reflection properties.

[0076] Furthermore, when the sound-absorbing material is placed directly under the condition of being submerged in water at both ends, the underwater acoustic material exhibits high sound absorption and low reflection performance.

[0077] Furthermore, it exhibits excellent absorption effects for large-angle obliquely incident sound waves ranging from 0° to 60°.

[0078] In existing technologies, porous sound-absorbing polymers have poor hydrostatic pressure resistance, limited low-frequency broadband sound absorption performance, and require application to a steel outer shell. This invention proposes a water-pressure-resistant, low-reflection, high-absorption underwater acoustic metamaterial. It employs a three-dimensional periodic corrugated surface with a unique shape, zero average curvature, and adjustable wall thickness and spatial distribution as the main mechanical load-bearing framework. Fiber-reinforced resin panels enhance water pressure resistance. This material can be applied to steel outer shells or in applications with water-backed ends, providing a new option for underwater sound-absorbing materials.

[0079] The water pressure-resistant, low-reflection, and high-absorption underwater acoustic metamaterial proposed in this invention has a simple structure, readily available raw materials, mature preparation process, and highly designable parameters, thus making it highly feasible for implementation. Four examples of specific implementations of this invention are given below.

[0080] Example 1

[0081] The unit cell has a side length of 50 mm and a thickness of 62 mm. The thickness of both the upper and lower fiber-reinforced resin panels in the sandwich structure is 1 mm. The fiber-reinforced resin panels are made of glass fiber reinforced resin with an elastic modulus of 18 GPa, a Poisson's ratio of 0.15, and a density of 1600 g / cm³. 3 The corrugated core material is 60mm thick and made of aluminum alloy with a density of 2700kg / m³. 3 The Young's modulus is 70 GPa, Poisson's ratio is 0.3, and the equivalent isotropic loss factor is 0.001. The viscoelastic polymer used is a modified rubber with a density of 1000 kg / m³. 3 The Young's modulus is 10 MPa, Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.5. The characteristic density of water is 1000 kg / m³. 3 The speed of sound is 1500 m / s.

[0082] like Figure 3As shown, the acoustic properties of a periodic cellular structure in a water-backed environment with both the incident and exit ends of the sound wave are modeled using the finite element method. In the model, the cell period is 50 mm, the thickness is 62 mm, and the corrugated surface wall thickness is 2.55 mm. The influence of different corrugated surface wall thicknesses on its water pressure resistance and sound absorption / reflection performance is analyzed. The design method of the corrugated surface structure includes the following steps:

[0083] ① Determine the shape of the three-dimensional corrugated surface as square, and discretize the three-dimensional coordinates x∈[0,50mm], y∈[0,50mm], z∈[0,60mm] to generate the coordinate dataset [x,y,z] required to draw the three-dimensional corrugated surface.

[0084] ② Determine the implicit function g(x,y,z) of the corrugated surface, with a period of 50mm.

[0085] ③ By setting different isosurface values ​​t for the implicit function, the implicit function can satisfy g(x,y,z)-t. 2 =0, corresponding to a wall thickness of 2t. In this embodiment, the wall thicknesses are 1.48mm, 2.55mm, 3.34mm, 4.30mm, 5.14mm, 6.08mm, and 7.00mm, respectively. Through calculation, the corresponding volume percentages of the corrugated surface are approximately 10vol%, 15vol%, 20vol%, 25vol%, 30vol%, 35vol%, and 40vol, respectively.

[0086] ④ Based on the implicit function, the isosurface of the three-dimensional ripple surface is drawn in the discrete coordinate dataset [x,y,z]. Triangular elements are used to mesh the isosurface, and the face and vertex data [F,V] corresponding to the triangular elements are obtained and output for surface modeling and analysis.

[0087] like Figure 4 The variation of maximum displacement of corrugated surface sandwich materials with different wall thicknesses under different hydrostatic pressures is presented to analyze the material's water pressure resistance. It can be seen that the underwater acoustic material can effectively withstand a hydrostatic pressure of 10 MPa, exhibiting excellent load-bearing performance. The maximum deformation of the material increases approximately linearly with increasing hydrostatic pressure, indicating that the material is within the elastic deformation range and has not failed. Increasing the corrugated surface wall thickness helps improve the load-bearing performance. When the wall thickness of the corrugated surface load-bearing frame is 1.48 mm, the maximum displacement of the material at 3 MPa and 10 MPa is 0.79 mm and 2.67 mm, respectively, compared to an overall thickness of 62 mm, corresponding to deformations of 1.27% and 4.31%, respectively. When the wall thickness of the corrugated surface load-bearing frame is 7 mm, the maximum displacement of the material at 3 MPa and 1.80 mm is 0.54 mm and 1.80 mm, respectively, with deformations of 0.87% and 2.90%, respectively. Figure 5 and Figure 6The sound absorption and reflection coefficients of corrugated sandwich materials with different wall thicknesses under normal pressure vary with frequency. It can be seen that the water-pressure resistant sandwich material exhibits good sound absorption performance in a wide frequency range of 1kHz to 10kHz, while maintaining a low reflectivity. With increasing frequency, the sound absorption coefficient gradually increases, eventually approaching or exceeding 0.8; simultaneously, with increasing wall thickness of the three-dimensional corrugated surface, the high sound absorption frequency range of the sandwich material gradually shifts to higher frequencies. Regarding the reflection coefficient, a large reflection occurs at low frequencies when the wall thickness is 3.34mm, possibly due to resonance in the sandwich material; the reflection coefficients remain below 0.5 for the other three thicknesses.

[0088] Figure 7 and Figure 8 The sound absorption and reflection coefficients of a corrugated sandwich material with a wall thickness of 5.14 mm under normal pressure and 5 MPa water pressure are compared with frequency. It can be seen that the sandwich material has excellent water pressure resistance, and the sound absorption is significantly improved in the range of 1 kHz to 5 kHz. However, the reflection coefficient in this frequency range also increases. Figure 9 and Figure 10 The sound absorption coefficient and reflection coefficient of a 5.14mm corrugated sandwich material vary with frequency under different oblique incident angles. It can be seen that the corrugated sandwich material exhibits good stability in sound absorption performance under oblique incident angles from 0° to 60°. This is mainly due to the curved surface characteristics of the corrugated frame. Since the average curvature of the corrugated surface is zero at all points, it can effectively and uniformly scatter incident sound waves in all directions, achieving efficient sound absorption. In the frequency range of 4.5kHz to 10kHz, the sound absorption coefficient decreases slightly with increasing incident angle, but remains above 0.7. Simultaneously, in the frequency range of 1.0kHz to 10kHz, the reflection coefficient decreases slightly with increasing incident angle.

[0089] Figure 11 The cell model is a corrugated sandwich material with a wall thickness of 5.14 mm embedded in a cavity inside a load-bearing frame. The cavity is a cylindrical cavity with a diameter of 15 mm and a height of 15 mm, located at the lower part of the cell's central axis. Figure 12 The changes in sound absorption coefficient before and after embedding the material into the cavity are shown. A comparison of the sound absorption coefficient and reflection coefficient with frequency under normal pressure and 5 MPa water pressure is presented. It can be seen that the sound absorption coefficient of the thick sandwich material with the cavity remains basically unchanged in the 5.5 kHz to 10 kHz frequency range, but the sound absorption in the 1 kHz to 5.5 kHz frequency range is significantly improved. This is mainly because the cavity resonates on one hand and converts the incident longitudinal sound wave into a transverse wave for efficient sound absorption on the other. It is foreseeable that optimizing the shape and size of the cavity can further improve the low-frequency sound absorption performance.

[0090] In summary, the underwater sound-absorbing material proposed in this invention has excellent low-frequency broadband sound absorption performance. When the sound-absorbing material is placed directly under the condition of being submerged in water at both ends, the underwater acoustic material has high sound absorption and low reflection performance, and has a good absorption effect on large-angle obliquely incident sound waves from 0° to 60°. At the same time, adding cylindrical cavities inside the corrugated curved surface structure layer can effectively improve the low-frequency broadband sound absorption performance.

[0091] Example 2

[0092] The cell has a side length of 50mm and a thickness of 51mm. The fiber-reinforced resin panel of the underwater acoustic pressure-resistant material has a thickness of 1mm, an elastic modulus of 18GPa, a Poisson's ratio of 0.15, and a density of 1600g / cm³. 3 The corrugated curved core material has a thickness of 50mm and a wall thickness of 2.55mm. It is made of aluminum alloy with a density of 2700kg / m³. 3 The Young's modulus is 70 GPa, Poisson's ratio is 0.3, and the equivalent isotropic loss factor is 0.001. The viscoelastic polymer used is modified polyurethane with a density of 960 kg / m³. 3 The Young's modulus is 20 MPa, Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.6. The characteristic density of water is 1000 kg / m³. 3 The sound velocity is 1500 m / s. The bottom of the corrugated curved load-bearing frame is directly bonded to a 30mm thick steel plate, such as... Figure 13 As shown, a 15mm × 15mm × 30mm cuboid cavity is introduced inside the corrugated curved support frame, located at the center of the cavity's central axis, to enhance low-frequency sound absorption performance. Figure 14 The diagram shows the changes in sound absorption with frequency before and after the introduction of the cavity. It can be seen that when the sound-absorbing material is applied to the steel plate, the underwater acoustic material exhibits high absorption and low reflection performance, with an absorption coefficient exceeding 0.8 above 3kHz. Introducing a cavity within the sandwich material significantly improves the material's broadband sound absorption performance. Optimizing the shape and size of the cavity can further enhance the broadband sound absorption performance.

[0093] Example 3

[0094] The underwater acoustic pressure-resistant material has a diameter of 118mm and a thickness of 62mm. The upper and lower fiber-reinforced resin panels are 1mm thick, with an elastic modulus of 18GPa, a Poisson's ratio of 0.15, and a density of 1600g / cm³. 3 The corrugated curved surface structure has a thickness of 60mm and a wall thickness of 6.1mm. It is made of aluminum alloy with a density of 2700kg / m³. 3 The Young's modulus is 70 GPa, Poisson's ratio is 0.3, and the equivalent isotropic loss factor is 0.001. The viscoelastic polymer used is modified polyurethane with a density of 960 kg / m³. 3The Young's modulus is 20 MPa, Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.6. The characteristic density of water is 1000 kg / m³. 3 The speed of sound is 1500 m / s.

[0095] The preparation process of the water pressure resistant, low-reflection, and high-absorption underwater acoustic material includes the following steps: First, an aluminum alloy corrugated curved surface structure with a diameter of 116 mm and a thickness of 60 mm is printed using additive manufacturing. The corrugated curved surface structure is then placed in a circular hollow mold made of polytetrafluoroethylene or metal. Second, a modified polymer is poured into the mold, cured at room temperature, and then demolded to obtain a corrugated curved surface structure with an embedded solid viscoelastic polymer, which is the core material of the sandwich material. Figure 15 As shown in a. Finally, a layer of fiber-reinforced resin material is wound onto the surface and circumference of the three-dimensional corrugated core material using winding and vacuum thermoforming methods, thus obtaining the underwater sound-absorbing material of the present invention, such as... Figure 15 As shown in b.

[0096] like Figure 16 and 17 The figure shows the changes in broadband acoustic wave absorption and reflection performance of the sandwich material with frequency under different hydrostatic pressures. It can be seen that, thanks to the mechanical bearing characteristics of the corrugated surface, the hydrostatic pressure has little effect on the acoustic performance of the material. The underwater acoustic material has good coefficients in the frequency range of 1kHz to 10kHz, and the reflection coefficient is basically kept below 0.3, that is, the reflection of sound wave energy is less than 10%, which is beneficial to reduce its own sound wave reflection intensity.

[0097] Example 4

[0098] The cell side length is 40mm. The thickness of the fiber-reinforced resin panel of the underwater acoustic pressure-resistant material is 1mm, 2mm, and 4mm respectively. The elastic modulus is 18GPa, Poisson's ratio is 0.15, and the density is 1600g / cm³. 3 The corrugated core material has a thickness of 42mm and a wall thickness of 3.34mm (50mm), and is made of steel. The viscoelastic polymer is modified polyurethane with a density of 960kg / m³. 3 The Young's modulus is 20 MPa, Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.6. The characteristic density of water is 1000 kg / m³. 3 The speed of sound is 1500 m / s. For example... Figure 18 and Figure 19The figure shows the variation of the sound absorption coefficient and reflection coefficient of the sandwich material with frequency under different panel thicknesses. It can be seen that adjusting and optimizing the thickness of the sandwich panel can effectively improve the low-frequency broadband sound absorption performance. This is mainly because the panel itself has a significant impact on the input impedance of the sandwich material, thus affecting its sound absorption and reflection characteristics. When the panel thickness is 2mm, the sound absorption coefficient of the sandwich material in the 1kHz~5.5kHz frequency band is significantly improved.

Claims

1. A type of underwater acoustic metamaterial, characterized in that, It includes periodically arranged cell units, which have a sandwich structure. The upper and lower surface layers are continuous fiber-reinforced resin panels, and the middle layer consists of a mechanical load-bearing frame with a set wall thickness and a viscoelastic sound-absorbing polymer filled within the mechanical load-bearing frame. The three-dimensional corrugated surface frame and the polymer have an interpenetrating continuous structure. The cells are periodically distributed in squares or hexagons, and the size and thickness of the cell period can be adjusted according to the wavelength of the incident sound wave. The three-dimensional corrugated surface profile is determined by an implicit function. The average curvature of the surface is zero or close to zero, which can effectively scatter the incident sound wave uniformly to all directions. The corrugated surface divides the space into two interconnected regions. By controlling the wall thickness of the corrugated surface, mechanical load-bearing frames with different volume ratios can be obtained. The volume ratio of the mechanical load-bearing frame varies between 5 vol% and 100 vol%. The wall thickness of the three-dimensional corrugated surface is gradient-varying, which in turn causes the volume ratio to also exhibit gradient-varying characteristics, with the ratio gradient value varying between 5 vol% and 100 vol%. The design of the three-dimensional corrugated surface includes the following steps: ① Determine whether the shape of the three-dimensional corrugated surface is square or cylindrical, discretize the coordinates of the target three-dimensional space, and generate the coordinate dataset [x,y,z] required to draw the three-dimensional corrugated surface; ② Determine the implicit function of the corrugated surface This function controls the contour variation of the corrugated surface element, with a period of [value missing]. L, L The value range is 1mm to 100mm, and the period is anisotropic in different directions of the coordinate system. This implicit function ensures that the dimensions of the corrugated surface structure in different directions have at least one complete period. ③ By setting the value of the isosurface corresponding to the implicit function t The implicit function of the corrugated surface satisfies or The contour of the corrugated surface is controlled to exhibit a specific wall thickness and volume ratio, with a corresponding wall thickness of 2. t ; ④ Based on the implicit function, the isosurface of the three-dimensional ripple surface is drawn in the discrete coordinate dataset [x,y,z]. Triangular elements are used to mesh the isosurface, and the face and vertex data [F,V] corresponding to the triangular elements are obtained and output for surface modeling analysis and structural processing. ⑤ By performing gradient interpolation on the implicit function values ​​in the gridded spatial coordinate dataset [x,y,z], a three-dimensional wavy surface with gradient changes in wall thickness and volume ratio is obtained.

2. The underwater acoustic metamaterial according to claim 1, characterized in that, The water pressure resistance of underwater acoustic metamaterials can be adjusted by designing the wall thickness of the three-dimensional corrugated surface; the greater the wall thickness, the stronger the water pressure bearing capacity.

3. The underwater acoustic metamaterial according to claim 2, characterized in that, When cavities are added inside the three-dimensional corrugated surface structure layer, the underwater acoustic metamaterial exhibits superior low-frequency broadband sound absorption performance. The cavities are located on the central axis of the cell, and their cross-sections perpendicular to the central axis are square or circular, with the side length or diameter of the cross-section being 0.2 to 0.85 times the side length of the cell. The thickness of the cavity along the central axis is 0.1 to 0.85 times the thickness of the cell. In step 1 of the design of the three-dimensional corrugated surface, when the corrugated surface contains cavities of different shapes and sizes, the coordinate dataset [x,y,z] required for the three-dimensional corrugated surface is drawn by removing the discrete points inside the cavities in the coordinate dataset [x,y,z].

4. The underwater acoustic metamaterial according to claim 3, characterized in that, The three-dimensional corrugated surface is printed using additive manufacturing. The materials of the three-dimensional corrugated surface include aluminum alloy, titanium alloy, high-modulus resin, or high-modulus ceramic.

5. The underwater acoustic metamaterial according to claim 1, characterized in that, The upper and lower panels of the cell are made of glass fiber reinforced resin, which is a high-transmittance material for sound waves. The surface thickness is 1mm to 4mm. This material is used to uniformly transmit pressure to the three-dimensional corrugated curved frame and reduce the deformation of the internal viscoelastic polymer.

6. The underwater acoustic metamaterial according to claim 1, characterized in that, When underwater acoustic metamaterials are applied to steel plates, they exhibit better performance.

7. The underwater acoustic metamaterial according to claim 1, characterized in that, When the underwater acoustic metamaterial is placed directly under the condition of being submerged in water at both ends, the underwater acoustic metamaterial exhibits high sound absorption and low reflection properties.

8. The underwater acoustic metamaterial according to claim 1, characterized in that, The underwater acoustic metamaterial exhibits better absorption performance for large-angle oblique incidence acoustic waves ranging from 0° to 60°.

9. A method for preparing the underwater acoustic metamaterial according to any one of claims 1-8, characterized in that, Includes the following steps: ① A three-dimensional corrugated surface frame with characteristic wall thickness and external dimensions is obtained by additive manufacturing. ② Place the three-dimensional corrugated surface frame in a mold made of polytetrafluoroethylene or a machined metal mold, and cast it with a modified polymer. After the polymer is cured at room temperature or by heating, it is demolded to obtain a corrugated surface structure with an embedded solid viscoelastic polymer, which is the core material of the sandwich material. ③ When the core material contains cavities, the three-dimensional corrugated curved surface frame is additively manufactured in two parts and modified polymer is poured into each part. The two parts of the three-dimensional corrugated curved surface frame are linearly cut or curved cut. During pouring, the cavities are formed using a pre-set mold. The two load-bearing frames are combined and assembled to obtain the three-dimensional corrugated curved surface core material containing cavities. ④ By winding and vacuum thermoplasticizing a layer of fiber-reinforced resin material panel on the surface and circumference of the three-dimensional corrugated core material, a hydroacoustic metamaterial is obtained.

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