Method for manufacturing a honeycomb sandwich structure and the structure thereof.

Abstract

Provided is a novel method for manufacturing a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers. The method comprises: (i) charging a mold for primary molding with a honeycomb core and at least two first prepregs; (ii) obtaining a primary molded article of a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers by pressurizing and heating the charged mold for primary molding with a press machine; (iii) charging a mold for secondary molding with the primary molded article and at least two second prepregs; and (iv) obtaining a secondary molded article of the honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and second CFRP layers are stacked on the two respective first CFRP layers by pressurizing and heating the charged mold for secondary molding with the press machine.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a honeycomb sandwich structure and the structure itself. This application claims priority based on Japanese Patent Application No. 2022-133293, filed in Japan on 24 August 2022, and Japanese Patent Application No. 2023-070926, filed in Japan on 24 April 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a surge in the development of unmanned aerial vehicles (UAVs), which are required to be able to fly at higher altitudes, that is, to fly in spaces with lower air density. One way to achieve this is to rotate the wings at a higher speed to generate greater lift, and for this purpose, it is necessary to increase the rigidity of the wings. On the other hand, high-speed rotation of wings increases noise, so to avoid this and achieve the above objective, one might consider increasing the thickness of the wings to increase the lift per rotational speed. However, if the weight increases with the thickness of the wings, the effect of increasing lift will be offset. Therefore, it is necessary to thicken the wings without significantly increasing the weight. To meet the above requirements, a wing with a honeycomb sandwich structure has been proposed, in which a honeycomb core is sandwiched between layers of CFRP (Carbon Fiber Reinforced Plastics) (Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-112849 [Overview of the project] [Problems that the invention aims to solve]

[0004] Patent Document 1 describes a method for manufacturing a laminate, which includes a first step of manufacturing a punched honeycomb structure by pressing a punching die from a direction intersecting the opening surface of the cells of the honeycomb material, and a second step of laminating a first surface layer and a second surface layer before hardening or solidification onto the honeycomb structure obtained by punching out a honeycomb core, and then hardening or solidifying the first surface layer and the second surface layer before hardening or solidification. In this method, the first and second layers of the epidermis are laminated onto a pre-prepared honeycomb structure. Depending on the shape of the honeycomb structure, wrinkles may form in the first and second layers during lamination, or the pressure on the surface of the laminate may not be uniform. Furthermore, Patent Document 1 describes a method for manufacturing a laminate in which a hardened or solidified third and fourth epidermal layer and a punched honeycomb core are prepared in advance, and the honeycomb core, prepreg, third and fourth epidermal layers are charged into a mold, compressed, and heated to obtain a honeycomb sandwich structure. This method yields materials with fewer surface voids in the third and fourth epidermal layers, but voids generated from the prepreg tend to accumulate between the third or fourth epidermal layer and the prepreg because they cannot pass through the third or fourth epidermal layer. Furthermore, if the mold used in the manufacturing process of the honeycomb sandwich structure does not match the shape of the third and fourth epidermal layers due to shrinkage during hardening or solidification of the third and fourth epidermal layers, there is a risk of uneven thickness occurring in the first and second epidermal layers. Furthermore, there are four additional processes: the manufacturing of the third epidermal layer, the manufacturing of the fourth epidermal layer, the punching of the honeycomb core, and the manufacturing of the honeycomb sandwich structure. These processes are time-consuming and labor-intensive, resulting in low productivity and high production costs.

[0005] The objective is to provide a novel manufacturing method for a honeycomb sandwich structure in which a honeycomb core is sandwiched between layers of CFRP (carbon fiber reinforced polymer). Another objective is to provide a honeycomb sandwich structure with excellent rigidity. Furthermore, one of the objectives is to provide a honeycomb sandwich structure with an attractive appearance. [Means for solving the problem]

[0006] One aspect of the present invention is a method for manufacturing a honeycomb sandwich structure, comprising: (i) charging a primary molding die with a honeycomb core and at least two first prepregs; (ii) pressurizing and heating the charged primary molding die with a press to obtain a primary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers; (iii) charging a secondary molding die with the primary molded product and at least two second prepregs; and (iv) pressurizing and heating the charged secondary molding die with a press to obtain a secondary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and a second CFRP layer is laminated on each of the two first CFRP layers.

[0007] Another aspect of the present invention is a structure in which a honeycomb core is sandwiched between CFRP layers, The honeycomb sandwich structure is one in which the value A, determined by the following measurement method, is between 0.01 and 0.10. (Method for measuring value A) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. Using the image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are acquired pixel by pixel within a 20 mm range perpendicular to the thickness direction of the honeycomb core in the cross-sectional image. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the innermost surface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. A and the average value D of the aforementioned distance B Calculate the value and find the value A from the following formula (1). A=D A / D B ...Equation (1) Provided that, as described above, the measurement of value A is performed on the thickest part of the honeycomb sandwich structure. Even if value A is outside the above range in any portion other than the thickest portion, the structure shall be deemed to fall within this embodiment as long as value A satisfies the above range in the thickest portion.

[0008] Another aspect of the present invention is the honeycomb sandwich structure described above, wherein the UD layer is disposed adjacent to the outer surface side of the honeycomb sandwich structure relative to the fabric layer, and a value B determined by the following measurement method is 0.01 or more and 0.10 or less. (Measurement Method for Value B) A cross-section obtained by cutting the thickest part of the honeycomb sandwich structure parallel to the thickness direction of the honeycomb core is imaged. Using the image processing software ImageJ, within a 20 mm range in the direction perpendicular to the thickness direction of the honeycomb core in the cross-sectional image, the outer surface coordinates of the honeycomb sandwich structure and the interface coordinates between the fabric layer and the UD layer are acquired for each pixel. For each point on the outer surface coordinates, the distance between the outer surface coordinates and the interface coordinates located in the normal direction to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distances C and the average value D of the distances D are calculated, and value B is obtained from the following formula (2). B=D C / D D ···Formula (2) Provided that, as described above, the measurement of value B is performed on the thickest part of the honeycomb sandwich structure. Even if value B is outside the above range in any portion other than the thickest portion, the structure shall be deemed to fall within this embodiment as long as value B satisfies the above range in the thickest portion.

[0009] Another aspect of the present invention is a honeycomb sandwich structure, which is a structure in which a honeycomb core is sandwiched between CFRP layers, the CFRP layers include a fabric layer having a woven fiber reinforcing material, and the standard deviation of distances for fabric yarns of each divided layer, determined by the following measurement method for the fabric layer, is 30 μm or less. (Method for measuring standard deviation of distance) When one of the orthogonal fabric yarns of the fabric is defined as yarn A and the other as yarn B, a cross-section of the CFRP layer cut in the direction orthogonal to the yarn B is imaged, the cross-section is binarized using the image processing software ImageJ, and the position of the center of gravity of the cross-section of each yarn B is calculated. In each of the divided layer on the surface layer side relative to the yarn A and the divided layer on the honeycomb core side in the fabric layer, for the five adjacent yarn Bs, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is obtained respectively, and the standard deviation thereof is calculated.

[0010] Another aspect of the present invention is a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein at least one of the CFRP layers sandwiching the honeycomb core has a fabric layer containing a fabric fiber reinforcing material disposed in the CFRP layer closest to the honeycomb core side, and the standard deviation of the distance for the fabric yarns of each divided layer obtained by the following measurement method for the fabric layer is 30 µm or less. (Method for measuring standard deviation of distance) When one of the orthogonal fabric yarns of the fabric closest to the honeycomb core side is defined as yarn A and the other as yarn B, a cross-section of the CFRP layer cut in the direction orthogonal to the yarn B is imaged, the cross-section is binarized using the image processing software ImageJ, and the position of the center of gravity of the cross-section of each yarn B is calculated. In each of the divided layer on the surface layer side relative to the yarn A and the divided layer on the honeycomb core side in the fabric layer closest to the honeycomb core side, for the five adjacent yarn Bs, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is obtained respectively, and the standard deviation thereof is calculated.

[0011] Another aspect of the present invention is a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a fabric layer containing a fabric fiber reinforcing material, and the standard deviation of the angle obtained by the following measurement method for the fabric yarns of the fabric layer is 1.5 degrees or less. (Method for measuring standard deviation of angle) When one of the orthogonal threads in the fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each thread B cross-section is calculated. For each of the 10 adjacent threads B in the fabric layer, the angle between the major axis direction of the elliptical approximation of the cross-section of thread B and a reference line that is parallel to the tangent to the outer surface at the nearest point of contact with thread B on the outer surface curve of the honeycomb sandwich structure and passes through the centroid of thread B is determined, and the standard deviation is calculated. [Effects of the Invention]

[0012] A novel manufacturing method for a honeycomb sandwich structure, in which a honeycomb core is sandwiched between layers of CFRP (carbon fiber reinforced polymer), is provided. Furthermore, a honeycomb sandwich structure with excellent rigidity is provided. In addition, a honeycomb sandwich structure with a good appearance is provided. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a cross-sectional view of a wing for an unmanned aerial vehicle that can be manufactured using the manufacturing method according to the embodiment. [Figure 2] Figure 2 shows a honeycomb core that can be used in the manufacturing method according to the embodiment. Figure 2(A) is a perspective view, and Figure 2(B) is a plan view. [Figure 3] Figure 3 is a cross-sectional view showing an example of charging a mold for primary molding with a honeycomb core and a first prepreg. [Figure 4] Figure 4 is a cross-sectional view showing an example of charging a mold for primary molding with a honeycomb core and a first prepreg. [Figure 5] Figure 5 is a cross-sectional view showing the process of charging the mold for secondary molding with the primary molded product and the second prepreg. [Figure 6] Figure 6 is a schematic cross-sectional view showing the inside of the mold during the primary molding process. [Figure 7] Figure 7 is a schematic cross-sectional view showing the inside of the mold during secondary molding. [Figure 8]Figure 8 is a cross-sectional view showing an example of a portable mold that can be used in the manufacturing method according to the embodiment. [Figure 9] Figure 9 is a cross-sectional view showing an example of a press machine that can be used in the manufacturing method according to this embodiment. [Figure 10] Figure 10 is a plan view showing the lower plate section of the press machine. [Figure 11] Figure 11 is a plan view (viewed from below) of the upper plate section of the press machine. [Figure 12] Figure 12 is a cross-sectional view showing a charged portable mold being pressed by a press machine. [Figure 13] Figure 13 is a cross-sectional view showing an example of a press machine that can be used in the manufacturing method according to this embodiment. [Figure 14] Figure 14 is a cross-sectional view showing an example of a press machine that can be used in the manufacturing method according to this embodiment. [Figure 15] Figure 15 is an example of a binarized cross-sectional view of the woven fabric layer cross-section of the honeycomb sandwich structure related to Experimental Example 1. [Figure 16] Figure 16 is an example of a binarized cross-sectional view of the woven fabric layer of the honeycomb sandwich structure related to Experimental Example 2. [Figure 17] Figure 17 is an explanatory diagram showing the angle between the long axis of the fiber bundle cross-section (cross-section of yarn B) and the reference line. [Figure 18] Figure 18 is an explanatory diagram of the CFRP layer including the woven fabric layer. [Modes for carrying out the invention]

[0014] 1. Honeycomb sandwich structure The manufacturing method according to the embodiment relates to a method for manufacturing a honeycomb sandwich structure, and can be preferably used, for example, in the manufacture of a wing for an unmanned aerial vehicle having a honeycomb sandwich structure. Figure 1 is a cross-sectional view of a wing for an unmanned aerial vehicle that can be manufactured using the manufacturing method according to the embodiment. Wing 1 is a honeycomb sandwich structure, and each wing has a structure in which a honeycomb core 13 is sandwiched between two CFRP layers 10, each consisting of a first CFRP layer 11 and a second CFRP layer 12.

[0015] The honeycomb core 13 is processed into a predetermined shape by cutting or by pressing as disclosed in Japanese Patent Publication No. 2021-112849. The material of the metal foil constituting the honeycomb core 13 is an aluminum alloy (typically A5052, an Al-Mg aluminum alloy, or A3003, an Al-Mn aluminum alloy), but other metals, such as stainless steel, may also be used.

[0016] The cell size and thickness of the honeycomb core are defined as shown in Figure 2. The cell size d of the honeycomb core 13 is preferably 2 to 20 mm. The thickness of the metal foil constituting the honeycomb core 13 is preferably 20 to 120 μm. The maximum thickness C of the honeycomb core 13 is, for example, 1 to 50 mm, preferably 2 to 10 mm, and more preferably 2 to 7 mm.

[0017] The material of the first CFRP layer 11 and the second CFRP layer 12 is CFRP. CFRP is a composite material in which fiber reinforcement containing carbon fibers is impregnated with a cured thermosetting resin composition. The honeycomb core 13 may be embedded in the first CFRP layer 11, in which case carbon fiber cutting may occur at the embedded portion. The honeycomb core 13 does not penetrate the second CFRP layer 12, and no cutting of carbon fibers by the honeycomb core 13 occurs in the second CFRP layer 12. The first CFRP layer 11 may contain more voids than the second CFRP layer 12. The second CFRP layer 12 may contain no voids, or only a small number of them.

[0018] The honeycomb sandwich structure that can be manufactured using the manufacturing method according to the embodiment is not limited to wings for unmanned aerial vehicles. The manufacturing method according to the embodiment can also be preferably used to manufacture honeycomb sandwich structures used as parts for space equipment, household appliances, automobiles, ships, etc., or as building materials. In a honeycomb sandwich structure manufactured using the manufacturing method according to the embodiment, the specifications of the honeycomb core, such as the material, cell size, foil thickness, and maximum thickness, are appropriately determined according to the application. In addition to metal honeycomb cores made of metals such as aluminum alloys and stainless steel, FRP (Fiber Reinforced Plastics) honeycomb cores can also be used, such as paper honeycomb cores and aramid honeycomb cores made of aramid fiber paper impregnated with phenolic resin. It is preferable to charge the honeycomb core into the mold with the first prepreg while it is still in a flat state, and then form it into the desired shape together with the first prepreg by pressing. Alternatively, the honeycomb core may be processed in advance to the desired shape or a shape close to the desired shape, and then charged into the mold with the first prepreg.

[0019] As described later, in this embodiment, the honeycomb sandwich structure is formed by curing the first prepreg in primary molding, and then curing the second prepreg in secondary molding. During primary molding, the first prepreg is pressed against the mold by a honeycomb core, resulting in parts of the first prepreg being pressed against the mold and parts not being pressed. On the other hand, during secondary molding, the second prepreg is pressed against the mold across the entire surface of the hardened first molded product, thus reducing the disruption of the orientation of the fiber reinforcement material.

[0020] A honeycomb sandwich structure manufactured using the manufacturing method according to one embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the distances between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the aforementioned fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each cross-section of thread B is calculated. For each of the divided layers in the fabric layer, both on the surface side and on the honeycomb core side, the shortest distance from the centroid position to the outer surface of the honeycomb sandwich structure is determined for five adjacent threads B, and the standard deviation is calculated. Normally, it is not possible to determine which of the orthogonal threads in the woven fabric within the CFRP layer is the warp thread and which is the weft thread. However, either the warp thread or the weft thread can be designated as thread A and thread B, and in either case, the standard deviation of the aforementioned distance is 30 μm or less.

[0021] When a CFRP layer contains multiple woven fabric layers, the standard deviation of the distance measured for each divided layer of the woven fabric yarn is 30 μm or less in all cases. If the standard deviation of the aforementioned distance is 30 μm or less, the woven yarns are arranged along the outer surface of the honeycomb sandwich structure, and the strength of the honeycomb sandwich structure tends to be uniform. From these viewpoints, the standard deviation of the distance is preferably 25 μm or less, and more preferably 20 μm or less.

[0022] The woven fabric layer only needs to be present on at least one of the CFRP layers sandwiching the honeycomb core. Furthermore, it is preferable that the woven fabric layer be located in the CFRP layer closest to the honeycomb core. Since the woven fabric layer located in the CFRP layer closest to the honeycomb core is pressed against the mold in the thickness direction of the honeycomb core, fiber disorder tends to be reduced. In addition to being closest to the honeycomb core, the woven fabric layer is also preferably located in the outermost layer of the CFRP layer. Placing it in the outermost layer makes it easier to improve the appearance. In CFRP layers that include a woven fabric layer, unidirectional (UD) fiber reinforcement and nonwoven fabric fiber reinforcement can be combined with the woven fabric. The choice of material should depend on the desired physical properties.

[0023] Figure 18 is an explanatory diagram showing an example of a CFRP layer including a woven fabric layer, specifically a CFRP layer containing four woven fabric layers. A divided layer in a woven fabric refers to the respective layers obtained when yarn B on the surface side of yarn A is designated as layer A, and yarn B on the honeycomb core side of yarn A is designated as layer B. In Figure 18, each woven fabric layer is designated as layer 1, layer 2, layer 3, and layer 4 from the surface side, with "1A" referring to layer A in layer 1, and "1B" referring to layer B in layer 1, and so on for 2A, 2B, 3A, 3B, 4A, and 4B.

[0024] (Detailed explanation of how to measure the standard deviation of distance) A small piece is cut from the honeycomb sandwich structure, allowing observation of a structural cross-section including the fabric layer, perpendicular to the fabric thread B. This piece is embedded in epoxy resin, and the cross-section is then polished using a rotary polishing machine (Stoluas Co., Ltd., Degramin-25, grit #4000) to prepare a sample. The polished cross-section is imaged at 80x magnification using a laser microscope (VHX-8000, manufactured by Keyence Corporation). Using the image processing software ImageJ, the distance per pixel is obtained from the scale bar of the acquired cross-sectional image. After converting the coordinates from pixel numbers to a length scale, the outer surface of the honeycomb sandwich structure is approximated by lines or spline curves to obtain the outer surface curve. Next, the contours of the fiber bundles (thread B) oriented perpendicular to the image plane are approximated by spline curves, and the cross-sections of the fiber bundles (thread B) are extracted by binarization. From the cross-sections of all extracted threads B, the standard deviation of the shortest distance from the centroid coordinate (centroid position) to the outer surface is obtained for five adjacent threads B in each divided layer. The method of converting coordinates from pixel numbers to length scales using the image processing software ImageJ, and the method of obtaining the outer surface curve of the honeycomb sandwich structure, will also be employed in the measurement method described later.

[0025] A honeycomb sandwich structure manufactured using the manufacturing method according to one embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, wherein at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcing material in the CFRP layer closest to the honeycomb core, and the standard deviation of the distance between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the fabric closest to the honeycomb core is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each thread B cross-section is calculated. For each of the divided layers on the surface side and the divided layer on the honeycomb core side of the fabric layer closest to the honeycomb core, the shortest distance from the centroid position to the outer surface of the honeycomb sandwich structure is determined for five adjacent threads B, and the standard deviation is calculated. Note that either the warp thread or the weft thread may be designated as thread A or thread B, and in either case, the standard deviation of the aforementioned distance is 30 μm or less.

[0026] If the standard deviation of the distance obtained for the weaving yarn of the weaving layer closest to the honeycomb core is 30 μm or less, then the weaving located in the weaving layer closest to the honeycomb core is uniformly arranged along the outer surface of the honeycomb sandwich structure, and the strength of the honeycomb sandwich structure tends to be uniform. From these perspectives, the standard deviation of the distance in the fabric layer closest to the honeycomb core is preferably 25 μm or less, and more preferably 20 μm or less.

[0027] A honeycomb sandwich structure manufactured using the manufacturing method according to one embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the angles of the woven yarns of the woven fabric layer, determined by the following measurement method, is 1.5 degrees or less. When the CFRP layer includes a plurality of fabric layers, the standard deviation of said angle measured for the fabric yarn of each fabric layer is 1.5 degrees or less in all cases. When the standard deviation of said angle is 1.5 degrees or less, the surface of the fabric is arranged along the outer surface of the honeycomb sandwich structure, and the strength of the honeycomb sandwich structure tends to be uniform. From these viewpoints, the standard deviation of said angle is preferably 1.2 degrees or less, and more preferably 1.0 degrees or less.

[0028] (Method for measuring standard deviation of angle) When one of the orthogonal fabric yarns of said fabric is defined as yarn A and the other is defined as yarn B, a cross-section cut in the direction orthogonal to said yarn B in the CFRP layer is imaged, said cross-section is binarized using the image processing software ImageJ, and the position of the center of gravity of the cross-section of each said yarn B is calculated. For each of 10 adjacent said yarn B in said fabric layer, as shown in Figure 17, an angle A formed by the major axis direction B obtained by elliptical approximation of the cross-section (fiber bundle cross-section 18) of said yarn B, and a reference line C that is parallel to the tangent to the outer surface at the closest point between said yarn B and the outer surface curve of the honeycomb sandwich structure and passes through the center of gravity of said yarn B, is obtained, and the standard deviation thereof is calculated. It should be noted that either the warp yarn or the weft yarn may be used as yarn A or yarn B, and in either case, the standard deviation of said angle is 1.5 degrees or less.

[0029] A honeycomb sandwich structure manufactured using the manufacturing method according to an example embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, wherein the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the innermost surface of the CFRP layer, obtained by the following measurement method A (μm), the average value D of said distance B divided by (μm) gives a value A of 0.01 or more and 0.10 or less. (Method for measuring standard deviation and average value of the distance between said outer surface and said innermost surface) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. The cross-section is then binarized using the image processing software ImageJ. Within a 20 mm range in the direction perpendicular to the thickness direction of the honeycomb core in the image of the cross-section, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are acquired pixel by pixel. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the innermost surface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. A and the average value D of the aforementioned distance B Calculate.

[0030] By employing a two-stage molding process consisting of primary and secondary molding, as described later, the thickness of the first-stage prepreg can be reduced. This makes it easier to maintain the straightness of the prepreg even when it is compressed by the honeycomb core, thus reducing value A. Because there is less variation in the thickness of the CFRP layer, the rigidity of the honeycomb sandwich structure is increased. The fact that the aforementioned value A is between 0.01 and 0.10 is a characteristic of two-stage molding.

[0031] A honeycomb sandwich structure manufactured using the manufacturing method according to one embodiment preferably includes a CFRP layer (woven layer) having woven fiber reinforcement and a CFRP layer (UD layer) having unidirectional (UD) fiber reinforcement. The inclusion of a woven fabric layer prevents the honeycomb sandwich structure from cracking, and the inclusion of a UD layer increases rigidity in specific directions.

[0032] In a honeycomb sandwich structure manufactured using the manufacturing method according to one embodiment, the UD layer is positioned adjacent to the outer surface of the honeycomb sandwich structure, on the outer surface side of the honeycomb sandwich structure, and the standard deviation D of the distance from the outer surface of the honeycomb sandwich structure to the interface between the woven layer and the UD layer is determined by the following measurement method. C (μm) is the average value D of the distance. D The value B obtained by dividing by (μm) is between 0.01 and 0.10. (Method for measuring the average value of the distance, which is the standard deviation of the distance from the outer surface to the interface) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. The cross-section is then binarized using the image processing software ImageJ. Within a 20 mm range in the direction perpendicular to the thickness direction of the honeycomb core in the image of the cross-section, the outer surface coordinates of the honeycomb sandwich structure and the interface coordinates between the woven fabric layer and the UD layer are acquired pixel by pixel. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the interface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. C and the average value D of the aforementioned distance D Calculate.

[0033] By using a two-stage molding process, the straightness of each layer improves, thus increasing the rigidity of the honeycomb sandwich structure. The fact that the aforementioned value B is between 0.01 and 0.10 is a characteristic of two-stage molding.

[0034] 2. Method for manufacturing a honeycomb sandwich structure The manufacturing method according to the embodiment will be explained using the case of manufacturing wing 1 as an example. (1) Primary molding As materials, a honeycomb core is prepared, either in a flat plate form or processed into a predetermined shape by cutting or pressing (as disclosed in Japanese Patent Publication No. 2021-112849), and at least two first prepregs are prepared. The first prepreg is a sheet consisting of a reinforcing material made of carbon fibers and a thermosetting resin matrix. The resin matrix is ​​preferably an epoxy resin composition. Examples of first prepregs include, but are not limited to, UD prepregs, woven prepregs, nonwoven prepregs, and sheet molding compounds (SMCs). Preferably, at least two of the first prepregs include at least one of a UD prepreg and a woven prepreg.

[0035] As shown in Figures 3 and 4, the primary molding die 20 is charged with a honeycomb core 13 and at least two first prepreg sheets 14. In other words, the honeycomb core 13 and at least two first prepreg sheets 14 are placed in the molding cavity formed when the primary molding die 20, which consists of a lower mold 21 and an upper mold 22, is closed. The honeycomb core 13 is placed in the molding cavity with its thickness in the vertical direction, and at least one first prepreg 14 is placed on each of the honeycomb surfaces, i.e., on the top and bottom of the honeycomb core 13. Figure 3 shows an embodiment in which a honeycomb core 13 that has been processed in advance to a desired shape or close to a desired shape is used, and Figure 4 shows an embodiment in which a flat honeycomb core 13 is used.

[0036] It is preferable to apply a release agent to the portion of the mold 20 used for primary molding that forms the molding cavity. Applying a release agent makes it easier to remove the primary molded product from the mold 20 used for primary molding.

[0037] The number of first prepreg sheets positioned above and below the honeycomb core is preferably between one and five, and more preferably three or fewer. Fewer first prepreg sheets result in a less undulating prepreg structure in contact with the honeycomb core, thus increasing rigidity. From this perspective, it is preferable that the number of first prepreg sheets be three or fewer. On the other hand, when obtaining large structures, thickness is required compared to obtaining small structures, so a larger number of first prepreg sheets makes it easier to obtain rigidity and strength. For this reason, from this viewpoint, it is preferable that the number of first prepreg sheets be between 5 and 30.

[0038] The ends of the first prepreg 14, which are positioned below the honeycomb core 13, may wrap around to the top of the honeycomb core 13. The ends of the first prepreg 14, which are positioned above the honeycomb core 13, may wrap around to the bottom of the honeycomb core 13. The thickness direction of the honeycomb core 13 refers to the height direction of the multiple hexagonal prism cells that make up the honeycomb core 13.

[0039] The type and number of first prepregs 14 placed below the honeycomb core 13 and the first prepregs 14 placed on top of the honeycomb core 13 may be the same or different. When two or more UD prepregs are stacked as the first prepreg 14 below the honeycomb core 13, they may be laminated so that the fiber directions coincide, or they may be laminated in a cross-ply or angle-ply manner. The same applies when two or more UD prepregs are stacked as the first prepreg 14 on top of the honeycomb core 13.

[0040] An adhesive film can be placed between the honeycomb core 13 and the first prepreg 14.

[0041] This may include areas where the number of first prepreg layers positioned above and below the honeycomb core is partially excessive. By partially increasing the number of first prepreg 14 layers, it becomes easier to increase the rigidity in a specific direction while keeping the weight increase of the honeycomb sandwich structure to a minimum. For partially placed prepregs, UD prepregs are preferred, and from the viewpoint of bending rigidity, the fiber direction of the UD prepregs is oriented in the direction of the longer length of the structure.

[0042] The first prepreg is hardened by applying pressure and heating to the charged primary molding die 20 using a press machine to obtain a primary molded product. As shown in the example in Figure 4, when a flat honeycomb core is used, the charged primary molding die 20 is pressurized and heated using a press machine to shape the honeycomb core into the desired form while curing the first prepreg to obtain a primary molded product. In this case, since the honeycomb core is in a flat state, the first prepreg is easy to position and less prone to wrinkling. Furthermore, the honeycomb core can be used to uniformly pressurize the first prepreg.

[0043] The primary molded product is a honeycomb sandwich structure in which a honeycomb core is sandwiched between two first CFRP layers. The first CFRP layers are cured products of the first prepreg. To shorten the curing time of the first prepreg, it is preferable to heat the press machine to the molding temperature by the time the primary molding die is fully charged. The molding temperature may be between 110°C and 120°C, 120°C and 135°C, 135°C and 145°C, 145°C and 150°C, or 150°C and 180°C.

[0044] (2) Secondary molding For secondary molding, at least two more second prepregs are prepared. The second prepreg, like the first prepreg, is a sheet consisting of a carbon fiber reinforcing material and a thermosetting resin matrix. The resin matrix is ​​preferably an epoxy resin composition. Examples of second prepregs include, but are not limited to, UD prepregs, woven prepregs, nonwoven prepregs, and sheet molding compounds (SMCs). Preferably, at least two second prepregs include at least one of a UD prepreg and a woven prepreg. For the outermost layer of prepreg, woven prepreg is preferred because the fibers at the edges are less likely to become disordered.

[0045] As shown in Figure 5, the mold 30 for secondary molding is charged with the primary molded product 2 and at least two second prepreg sheets 15. In other words, the primary molded product 2 and at least two second prepreg sheets 15 are placed in the molding cavity formed when the mold 30 for secondary molding, which consists of a lower mold 31 and an upper mold 32, is closed. The mold 30 for secondary molding differs from the mold 20 for primary molding in that it is larger due to the thickness added by the second prepreg layer.

[0046] If a mold release agent is applied to the mold during the primary molding process, the release agent will adhere to the primary molded product, resulting in poor adhesion to the secondary prepreg. Therefore, it is necessary to remove the release agent adhering to the primary molded product. As a method for removing the mold release agent, polishing or degreasing treatments such as sandblasting may be performed.

[0047] The primary molded product 2 is placed in the molding cavity such that the thickness direction of the honeycomb core 13 contained in the primary molded product 2 is in the vertical direction, and at least one second prepreg 15 is placed above and below the primary molded product 2. The end of the second prepreg 15 placed below the primary molded product 2 may wrap around to the top of the primary molded product 2. The end of the second prepreg 15 placed above the primary molded product 2 may wrap around to the bottom of the primary molded product 2. In this way, the fiber reinforcement material of the second prepreg 15 connects the top and bottom of the primary molded product 2, thereby increasing the strength of the molded product and obtaining a good appearance without a visible boundary between the top and bottom.

[0048] The type and number of second prepregs 15 placed below the primary molded product 2 and the second prepregs 15 placed on top of the primary molded product 2 may be the same or different. When two or more UD prepregs are stacked as a second prepreg 15 beneath the primary molded product 2, they may be laminated so that the fiber directions coincide, or they may be laminated in a cross-ply or angle-ply configuration. The same applies when two or more UD prepregs are stacked as a second prepreg 15 on top of the primary molded product 2.

[0049] The second prepreg is hardened by applying pressure and heating the charged secondary molding die 30 using a press machine to obtain a secondary molded product, i.e., the target molded product. The secondary molded product is a honeycomb sandwich structure in which a honeycomb core 13 is sandwiched between two first CFRP layers 11 and a second CFRP layer 12 laminated on each of them. The second CFRP layers are cured products of the second prepreg. To shorten the curing time of the second prepreg, it is preferable to heat the press machine to the molding temperature by the time the secondary molding die is fully charged. The molding temperature may be between 110°C and 130°C, 130°C and 135°C, 135°C and 145°C, 145°C and 150°C, or 150°C and 180°C.

[0050] (3) Effects In the primary molding process, as shown in Figure 6 (left), the first prepreg 14 is placed between the honeycomb core 13 and the mold 20. When the mold 20 is pressed with a press machine, the first prepreg 14 is pressed against the surface of the mold 20 by the honeycomb core 13. However, as shown in Figure 5 (right), only the edges of the metal foil constituting the honeycomb core 13 are in contact with the first prepreg 14, so the entire first prepreg 14 is not pressed uniformly against the surface of the mold 20. Therefore, the shape of the mold 20 (the shapes of the surfaces 21a and 22a of the lower mold 21 and upper mold 22, respectively) is not sufficiently reflected in the surface shape of the first CFRP layer 11. In addition, because the first prepreg 14 is not sufficiently pressed, voids are likely to form inside the first CFRP layer 11. Furthermore, as shown in Figure 6 (right), the honeycomb core 13 may become embedded in the first CFRP layer 11, which may result in the cutting of carbon fibers.

[0051] In the secondary molding process, as shown in Figure 7 (left), the second prepreg 15 is placed between the primary molded product 2 and the mold 30. As shown in Figure 7 (right), when the mold 30 is pressed with a press machine, the second prepreg 15 is pressed against the surface of the mold 30 by the first CFRP layer 11 of the primary molded product 2. Since the entire surface of the first CFRP layer 11 is in contact with the second prepreg 15, the second prepreg 15 is pressed uniformly throughout. As a result, the shape of the mold 30 (the shapes of the surfaces 31a and 32a of the lower mold 31 and upper mold 32, respectively) is sufficiently reflected in the surface shape of the second CFRP layer 12. Furthermore, voids are less likely to form inside the second CFRP layer 12. In addition, carbon fiber cutting due to the honeycomb core biting into the second CFRP layer 12 does not occur.

[0052] According to the manufacturing method described in one embodiment, a honeycomb sandwich structure can be obtained in two steps, enabling efficient production. Furthermore, because the second prepreg can be uniformly pressed between the hardened first CFRP layer and the mold, variations in thickness and distortion of the second CFRP layer can be reduced, resulting in higher rigidity.

[0053] 3. Preferred Embodiment (1) Use of portable molds In the manufacturing method described in item 2 above, it is preferable to use a portable mold at least in the secondary forming stage, preferably in both the primary and secondary forming stages. The portable mold can be charged outside the press machine, for example, in a room at a temperature of 17-28°C. The temperature of the portable mold during charging is preferably the same as the ambient temperature.

[0054] For example, a portable mold that has been heated for the purpose of drying after cleaning may be charged before its temperature drops to the ambient temperature. However, even in this case, it is preferable to wait until the temperature of the portable mold drops to 40°C or below before charging, in order to prevent wrinkles from forming in the prepreg during handling. In particular, it is important to position UD prepregs and woven prepregs within the portable mold in a way that prevents wrinkles from forming. This is because wrinkles disrupt the orientation of the fiber reinforcement, which can lead to a decrease in the mechanical properties after curing. If the temperature of the portable mold is 40°C or lower, or even 28°C or lower, even if the charging process takes a long time, the temperature of the prepreg will not rise during that time, preventing the surface tackiness from increasing, thus making it less likely for wrinkles to form in the prepreg.

[0055] When using portable molds, it is preferable to preheat the charged portable mold before applying pressure and heating with the press machine. This reduces the time required to heat the portable mold in the press machine, thereby increasing the number of molding operations that can be performed per day with a single press machine. Preheating is performed within a range that does not significantly reduce the viscosity of the resin matrix of the prepreg. Specifically, the temperature of the portable mold after preheating is preferably between 40°C and 80°C, and more preferably between 60°C and 80°C. If the temperature of the portable mold is 80°C or lower, it can be touched with gloved hands, so there is no problem with transportation, etc.

[0056] (2) Portable mold The material of the portable mold may be carbon steel or alloy steel (steel with added chromium, molybdenum, tungsten, vanadium, etc.) which have been conventionally used as steel materials for plastic molding. However, since the portable mold does not have an internal heat source and is heated by a heat source provided in the press machine, aluminum alloy or beryllium copper are more preferable materials. These alloys have a high thermal conductivity of 100 W / (m·K) or more at 20°C.

[0057] Aluminum alloys are particularly preferred because they have high thermal conductivity and are lightweight. At 20°C, the specific gravity of steel is around 7.9, and that of beryllium copper exceeds 8, while that of aluminum alloys is typically 2.7 to 2.8. Examples of aluminum alloys include, but are not limited to, the 7000 series aluminum alloy (Al-Zn-Mg alloy), which is considered to have the highest strength among aluminum alloys, and the 2000 series aluminum alloy (Al-Cu alloy), which has strength comparable to steel.

[0058] The thermal conductivity of aluminum alloys varies depending on the heat treatment, but at 20°C, the thermal conductivity is said to be 130-190 W / (m·K) for A2017 (duralumin), 150 W / (m·K) for A7003, and 130 W / (m·K) for A7075 (super duralumin). In contrast, even chromium-molybdenum steel, which has relatively high thermal conductivity, has a thermal conductivity of approximately 60 W / (m·K) at 20°C, and carbon steel has an even lower thermal conductivity.

[0059] The weight of the portable mold is preferably between 2 kg and 30 kg. A weight within this range is convenient for transporting the portable mold. However, the weight of the portable mold is not limited to this range.

[0060] The portable mold may consist of a bottom plate 40, an upper plate 50, and an intermediate plate 60, as shown in Figure 8 as an example. The number of intermediate plates is not limited to three; it may be two or fewer, or four or more. The planar shape of each plate is, for example, rectangular. Handles can be attached to the sides of any plate as needed. All plates have the same outer shape and dimensions in plan view, allowing them to be perfectly stacked on top of each other.

[0061] When the bottom plate 40, the intermediate plate 60, and the top plate 50 are stacked so that they fit snugly on top of each other, molding cavities 70 are formed between the bottom plate 40 and the intermediate plate 60 directly above it, between adjacent intermediate plates 60, and between the top plate 50 and the intermediate plate 60 directly below it. In one example, one adjacent plate may be provided with an alignment protrusion and an alignment recess, respectively, so that they interlock only when the two plates are stacked so that they fit together perfectly.

[0062] The bottom surface of the lowest plate and the top surface of the top plate are both completely flat. When all the plates are stacked, the bottom surface of the lowest plate and the top surface of the top plate are parallel to each other. In one example, multiple molding cavities may be formed between adjacent plates. In one example, at least a portion of the plates constituting the portable mold may have an insert structure and be composed of a master mold and an insert mold.

[0063] (3) Depressurization In the manufacturing method described in item 2 above, it is preferable to use a portable mold at least in the secondary molding stage, and to reduce the pressure in the space around the portable mold when heating and pressurizing the portable mold with a press machine. Figure 9 shows an example of a press machine that allows for reduced pressure in the space around a portable mold when heating and pressurizing the mold.

[0064] The press machine 100 includes a lower plate section 110, an upper plate section 120 positioned above the lower plate section 110 and facing the lower plate section 110, and a pressurizing mechanism (only the cylinder 132 of the pressurizing mechanism is shown in Figure 9) that moves the lower plate section 110 and the upper plate section 120 toward each other. The lower plate section 110 consists of a horizontally positioned lower substrate 111 and a lower heating plate 112 attached to the upper surface of the lower substrate 111. The upper plate section 120 consists of a horizontally positioned upper substrate 121 and an upper heating plate 122 attached to the lower surface of the upper substrate 121. In Figure 9, the frame for fixing the upper plate section 120 and the like is not shown.

[0065] The pressurizing mechanism is preferably hydraulic because it allows for easy pressure control. The cylinder 132 of the pressurizing mechanism may be positioned to act on the upper plate 120 rather than the lower plate 110. The lower heating plate 112 and the upper heating plate 122 each have a built-in heater 131. Examples of heaters 131 include, but are not limited to, steam heaters, oil heaters, electric heaters, and electromagnetic induction heaters.

[0066] An annular lower wall member 113 is fixed to the upper surface of the lower heating plate 112. As shown in the plan view in Figure 10, the lower wall member 113 has a structure in which four vertical walls, each perpendicular to the upper surface of the lower heating plate 112, are connected to form a rectangular ring, and is positioned slightly inward from the circumferential surface of the lower heating plate 112. The lower heating plate 112 and the lower wall member 113 may be integrated without any joints. The lower wall member 113 is provided with a groove that encircles its outer surface, and a vacuum sealing ring 114 is fitted into this groove.

[0067] An annular upper wall member 123 is fixed to the lower surface of the upper heating plate 122. As shown in the plan view (viewed from below) in Figure 11, the upper wall member 123 has a structure in which four vertical walls, each perpendicular to the lower surface of the upper heating plate 122, are connected to form a rectangular ring, and is arranged along the circumferential surface of the upper heating plate 122. The upper heating plate 122 and the upper wall member 123 may be integrated without any seams. The upper wall member 123 is provided with an exhaust hole 124 that penetrates one of the vertical walls. The exhaust hole 124 is connected to a vacuum pump 133.

[0068] As shown in Figure 12, when the charged portable mold is placed on the lower heating plate and the pressurizing mechanism is activated to bring the lower plate portion 110 closer to the upper plate portion 120, the lower wall member 113 and the upper wall member 123 interlock via the vacuum sealing ring 114, forming a closed space S surrounded by the lower heating plate 112, the lower wall member 113, the upper heating plate 122, and the upper wall member 123. This closed space S can be depressurized by activating the vacuum pump 133.

[0069] The closed space S is depressurized by operating the vacuum pump 133 so that the pressure after depressurization is preferably 0.1 Pa or less, more preferably 0.01 Pa or less. When applied to the aforementioned secondary molding process, reducing the pressure in the closed space S prevents air bubbles between the first CFRP layer and the second prepreg from becoming voids after molding. Furthermore, since the resin matrix of the second prepreg is degassed, void formation within the second CFRP layer is also prevented. When multiple second prepregs are laminated, air bubbles between adjacent second prepregs can also be prevented from becoming voids after molding, making it easier to obtain a good appearance.

[0070] In a modified version of the press machine, as shown in Figure 13, the lower wall member 113 may be omitted, and the lower heating platen 112 and the upper wall member 123 may be configured to fit together via a vacuum sealing ring 114.

[0071] In other variations of the press machine, as shown in Figure 14, the lower wall member 113 and the upper wall member 123, which interlock with each other via a vacuum sealing ring, may be fixed to the upper surface of the lower substrate 111 and the lower surface of the upper substrate 121, respectively. The configuration of a press machine in which a depressurized closed space is formed between the lower heating plate and the upper heating plate is not limited to those exemplified above.

[0072] Figures 12-14 illustrate a portable mold that does not have an intermediate plate and consists only of a lower mold (bottom plate 40) and an upper mold (top plate 50). However, this can be replaced with a portable mold that has an intermediate plate.

[0073] 4. Summary of Embodiments [Embodiment 1] (i) Charge the mold for primary molding with a honeycomb core and at least two first prepregs, (ii) By pressurizing and heating the charged primary molding die with a press machine, a primary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers is obtained. (iii) Charge the mold for secondary molding with the primary molded product and at least two second prepregs, (iv) By pressurizing and heating the charged secondary molding die with a press machine, a secondary molded product having a honeycomb sandwich structure is obtained in which the honeycomb core is sandwiched between the two first CFRP layers and a second CFRP layer is laminated on each of the two first CFRP layers, A method for manufacturing a honeycomb sandwich structure containing [a specific component]. [Embodiment 2] A method for manufacturing a honeycomb sandwich structure according to Embodiment 1, wherein at least one of the two first prepregs is arranged on each of the two sides of the honeycomb surface of the honeycomb core. [Embodiment 3] The manufacturing method according to Embodiment 2, wherein the first prepreg arranged on each of the two sides of the honeycomb surface of the honeycomb core is one to five sheets. [Embodiment 4] The manufacturing method according to Embodiment 2, wherein the number of first prepregs arranged on each of the two sides of the honeycomb surface of the honeycomb core is 5 to 30. [Embodiment 5] A manufacturing method according to any one of Embodiments 1 to 4, which includes a portion of the first prepreg where the number of sheets is partially increased. [Embodiment 6] A manufacturing method according to any one of Embodiments 1 to 5, wherein the honeycomb core is obtained in a desired shape by applying pressure with a press machine. [Embodiment 7] A manufacturing method according to any one of Embodiments 1 to 6, wherein the mold for secondary molding is a portable mold. [Embodiment 8] The manufacturing method according to Embodiment 7, wherein the material of the portable mold is an aluminum alloy. [Embodiment 9] A manufacturing method according to Embodiment 7 or 8, wherein the temperature of the secondary molding die is 40°C or lower when the secondary molding die is charged. [Embodiment 10] A manufacturing method according to Embodiment 9, wherein the charging of the secondary molding die is performed in a room at a temperature of 17 to 28°C. [Embodiment 11] A manufacturing method according to any one of Embodiments 7 to 10, wherein the mold for primary molding is a portable mold. [Embodiment 12] The manufacturing method according to Embodiment 11, wherein the material of the portable mold is an aluminum alloy. [Embodiment 13] A manufacturing method according to Embodiment 11 or 12, wherein the temperature of the primary molding die is 40°C or lower when the primary molding die is charged. [Embodiment 14] A manufacturing method according to Embodiment 13, wherein the charging of the primary molding die is performed in a room at a temperature of 17 to 28°C. [Embodiment 15] A manufacturing method according to any one of Embodiments 7 to 14, wherein the space around the charged secondary molding die is depressurized when the charged secondary molding die is pressed and heated in a press machine. [Embodiment 16] A manufacturing method according to Embodiment 15, wherein the space around the secondary molding die is depressurized when the charged primary molding die is pressurized and heated in a press machine. [Embodiment 17] A structure in which a honeycomb core is sandwiched between CFRP layers, A honeycomb sandwich structure in which the value A, determined by the following measurement method, is between 0.01 and 0.10. (Method for measuring value A) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. Using the image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are acquired pixel by pixel within a 20 mm range perpendicular to the thickness direction of the honeycomb core in the cross-sectional image. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the innermost surface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. A and the average value D of the aforementioned distance B Calculate the value and find the value A from the following formula (1). A=D A / D B ...Equation (1) [Embodiment 18] The honeycomb sandwich structure according to Embodiment 17, wherein the CFRP layer includes a woven layer which is a CFRP layer having woven fiber reinforcement and a UD layer which is a CFRP layer having unidirectional (UD) fiber reinforcement. [Embodiment 19] A honeycomb sandwich structure according to Embodiment 18, wherein the UD layer is arranged adjacent to the outer surface side of the honeycomb sandwich structure than the woven fabric layer, and the value B, determined by the following measurement method, is 0.01 or more and 0.10 or less. (Method for measuring value B) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. Using the image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the interface coordinates between the woven fabric layer and the UD layer are acquired pixel by pixel in a 20 mm range perpendicular to the thickness direction of the honeycomb core in the cross-sectional image. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the interface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. C and the average value D of the aforementioned distance D Calculate and find the value B from the following formula (2). B=DC / D D ...Equation (2) [Embodiment 20] A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the distances between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the aforementioned fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each cross-section of thread B is calculated. For each of the divided layers in the fabric layer, both on the surface side and on the honeycomb core side, the shortest distance from the centroid position to the outer surface of the honeycomb sandwich structure is determined for five adjacent threads B, and the standard deviation is calculated. [Embodiment 21] A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the distances between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the fabric closest to the honeycomb core is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each thread B cross-section is calculated. For each of the divided layers on the surface side and the divided layer on the honeycomb core side of the fabric layer closest to the honeycomb core, the shortest distance from the centroid position to the outer surface of the honeycomb sandwich structure is determined for five adjacent threads B, and the standard deviation is calculated. [Embodiment 22] A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the angles of the woven yarns of the woven fabric layer, determined by the following measurement method, is 1.5 degrees or less. (Method for measuring the standard deviation of angles) When one of the orthogonal threads in the fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each thread B cross-section is calculated. For each of the 10 adjacent threads B in the fabric layer, the angle between the major axis direction of the elliptical approximation of the cross-section of thread B and a reference line that is parallel to the tangent to the outer surface at the nearest point of contact with thread B on the outer surface curve of the honeycomb sandwich structure and passes through the centroid of thread B is determined, and the standard deviation is calculated.

[0074] 5. Experimental Results The results of experiments conducted by the inventors are described below.

[0075] (Experimental Example 1) We prepared an aluminum alloy honeycomb core and eight sheets of woven fabric prepreg. The mold for primary molding was charged with a honeycomb core and a first prepreg, which is a woven fabric prepreg with one layer placed above and one below the honeycomb core. The first prepreg was cured by applying pressure and heating using a press machine to obtain a primary molded product. Next, the mold for secondary molding was charged with the primary molded product and the second prepreg, which is a woven fabric prepreg with three pieces placed above and below the primary molded product. The second prepreg was hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0076] (Experimental Example 2) We prepared an aluminum alloy honeycomb core and eight sheets of woven fabric prepreg. The mold for secondary molding was charged with a honeycomb core and four layers of woven prepreg placed above and below the honeycomb core. By applying pressure and heat using a press, the woven fabric prepreg was hardened to obtain a honeycomb sandwich structure.

[0077] Tables 1 and 2 show the results of determining the standard deviation of the shortest distance from the centroid of yarn B to the outer surface of the honeycomb sandwich structure, and the standard deviation of the angle between the major axis of the elliptical cross-section of yarn B and the reference line, using the method described above, for each woven yarn of the honeycomb sandwich structure obtained in Experimental Examples 1 and 2. In Table 1, 1A to 4B represent the divided layer on the surface side (layer A) and the divided layer on the honeycomb core side (layer B) of each of the four woven yarn layers, respectively. In Table 2, 1 to 4 represent each of the four woven yarn layers. Furthermore, in Experimental Examples 1 and 2, Figures 15 and 16 show the binarized cross-sectional views of the fabric layer when the standard deviation of the shortest distance from the centroid of yarn B to the outer surface of the honeycomb sandwich structure and the standard deviation of the angle between the major axis of the elliptical approximation of the cross-section of yarn B and the reference line were determined.

[0078] [Table 1]

[0079] [Table 2]

[0080] (Experimental Example 3) We prepared an aluminum alloy honeycomb core, four sheets of woven prepreg, and two sheets of UD prepreg. The mold for primary molding was charged with a honeycomb core and a first prepreg, which is a UD prepreg with one sheet placed above and one below the honeycomb core. The first prepreg was cured by applying pressure and heating using a press machine to obtain a primary molded product. Next, the mold for secondary molding was charged with the primary molded product and the second prepreg, which is a woven fabric prepreg with two pieces placed above and below the primary molded product. The second prepreg was hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0081] (Experimental Example 4) We prepared 46 sheets of aluminum alloy honeycomb core, UD prepreg, and 6 sheets of woven fabric prepreg. The mold for primary molding was charged with a honeycomb core and a first prepreg consisting of 21 sheets of UD prepreg and 2 sheets of woven prepreg arranged above and below the honeycomb core. The first prepreg was cured by applying pressure and heating using a press machine to obtain a primary molded product. Next, the mold for secondary molding was charged with the primary molded product and a second prepreg, in which two UD prepregs and one woven fabric prepreg were arranged above and below the primary molded product. The second prepreg was hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0082] (Experimental Example 5) We prepared an aluminum alloy honeycomb core, four sheets of woven prepreg, and two sheets of UD prepreg. The molding die consisted of a honeycomb core, with one sheet of UD prepreg placed above and below the honeycomb core, and two sheets of woven prepreg placed on top of each. The prepreg was hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0083] For the honeycomb sandwich structures obtained in Experimental Examples 3-5, the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the innermost layer of the CFRP layer was calculated using the method described above. A The average value D of the aforementioned distance BTable 3 shows the results of calculating the value A by dividing by . Furthermore, for the honeycomb sandwich structures obtained in Experimental Examples 3-5, the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the interface between the woven fabric layer and the UD layer was calculated using the method described above. C The average value D of the aforementioned distance D The value B obtained by dividing by is also shown in Table 3.

[0084] [Table 3]

[0085] Although the present invention has been described above with reference to specific embodiments, each embodiment is presented as an example and does not limit the scope of the present invention. Each embodiment described herein can be modified in various ways within the scope in which the effects of the invention are achieved, and can be combined with features described in other embodiments to the extent that is feasible. [Explanation of Symbols]

[0086] 1. Wing (honeycomb sandwich structure) 2 Primary molded product 10 CFRP layers 11 First CFRP layer 12 Second CFRP layer 13 Honeycomb core 14 First prepreg 15. Second prepreg 16 Honeycomb core side 17 Outer surface of honeycomb sandwich structure 18. Fiber bundle cross-section (cross-section of yarn B) 20. Die for primary molding 21 Lower mold 22 Upper mold 30. Die for secondary molding 31 Lower mold 32 Upper mold 40 Bottom type (bottom plate) 50 Top type (top plate) 60 Intermediate Plate 70 molded cavities 100 press machines 110 Lower plate part 111 Lower board 112 Lower heating plate 113 Lower wall member 114 Vacuum sealing ring 120 Upper plate section 121 Upper circuit board 122 Top heating plate 123 Upper wall member 131 Heater 132 Cylinder of pressurizing mechanism 133 Vacuum pump A angle B Long axis of the fiber bundle C reference line

Claims

1. A structure in which a honeycomb core is sandwiched between CFRP layers. A honeycomb sandwich structure in which the value A, determined by the following measurement method, is between 0.01 and 0.

10. (Method for measuring value A) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. Using the image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are acquired pixel by pixel in a 20 mm range perpendicular to the thickness direction of the honeycomb core in the cross-sectional image. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the innermost surface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. Ａ and the average value D of the aforementioned distance Ｂ Calculate the value and find the value A from the following formula (1). A = D Ａ / D Ｂ ・・・Form (1)

2. The honeycomb sandwich structure according to claim 1, wherein the CFRP layer comprises a woven layer which is a CFRP layer having a woven fiber reinforcing material and a UD layer which is a CFRP layer having a unidirectional (UD) fiber reinforcing material.

3. The honeycomb sandwich structure according to claim 2, wherein the UD layer is positioned adjacent to the outer surface of the honeycomb sandwich structure more than the woven fabric layer, and the value B, determined by the following measurement method, is 0.01 or more and 0.10 or less. (Method for measuring value B) The thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and a cross-section is imaged. Using the image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the interface coordinates of the woven fabric layer and the UD layer are acquired pixel by pixel in a 20 mm range perpendicular to the thickness direction of the honeycomb core in the cross-sectional image. For each point of the outer surface coordinates, the distance between the outer surface coordinate and the interface coordinate located in the direction normal to the outer surface at that outer surface coordinate is measured, and the standard deviation D of the distance is calculated. Ｃ and the average value D of the aforementioned distance Ｄ Calculate the value and find the value B from the following formula (2). B=D Ｃ / D Ｄ ・・・Form (2)

4. A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the distances between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the aforementioned fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each cross-section of thread B is calculated. For each of the divided layer on the surface side of thread A and the divided layer on the honeycomb core side of the woven fabric layer, the shortest distance from the centroid position to the outer surface of the honeycomb sandwich structure is determined for five adjacent threads B, and the standard deviation is calculated.

5. A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the distance between the woven yarns of each divided layer, determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of distance) When one of the orthogonal weaving threads in the fabric closest to the honeycomb core is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each cross-section of thread B is calculated. For each of the five adjacent threads B in the fabric layer closest to the honeycomb core, the shortest distance from the centroid to the outer surface of the honeycomb sandwich structure is determined, and the standard deviation is calculated.

6. A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, wherein the CFRP layers include a woven fabric layer having a woven fiber reinforcing material, and the standard deviation of the angles of the woven yarns of the woven fabric layer, determined by the following measurement method, is 1.5 degrees or less. (Method for measuring the standard deviation of an angle) When one of the orthogonal threads of the fabric is designated as thread A and the other as thread B, a cross-section of the CFRP layer cut in the direction orthogonal to thread B is imaged, the cross-section is binarized using the image processing software ImageJ, and the centroid position of each thread B cross-section is calculated. For each of the 10 adjacent threads B in the fabric layer, the angle between the major axis direction of the elliptical approximation of the cross-section of thread B and a reference line that is parallel to the tangent to the outer surface at the nearest point of contact with thread B on the outer surface curve of the honeycomb sandwich structure and passes through the centroid of thread B is determined, and the standard deviation is calculated.

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