Effect of Ply Orientation on the Mechanical Performance of Carbon Fibre Honeycomb Cores

Citation: Kondratiev, A.; Píštˇek, V.; Gajdachuk, V.; Kharchenko, M.; Nabokina, T.; Kuˇcera, P.; Kuˇcera, O. Effect of Ply Orientation on the Mechanical Performance of Carbon Fibre Honeycomb Cores. Polymers 2023,15, 2503. https://doi.org/ 10.3390/polym15112503 Academic Editor: Yang Li Received: 6 March 2023 Revised: 4 May 2023 Accepted: 26 May 2023 Published: 29 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Effect of Ply Orientation on the Mechanical Performance of Carbon Fibre Honeycomb Cores Andrii Kondratiev 1,* , Václav Píštˇek 2, Vitaliy Gajdachuk 3, Maksym Kharchenko 4, Tetyana Nabokina 3, Pavel Kuˇcera 2and Ondˇrej Kuˇcera 2,* 1 Department of Materials Science and Engineering of Composite Structures, O.M. Beketov National University of Urban Economy in Kharkiv, Marshal Bazhanov Str. 17, 61002 Kharkiv, Ukraine 2Institute of Automotive Engineering, Brno University of Technology, Technická2896/2, 616 69 Brno, Czech Republic 3Department of of Rocket Design and Engineering, National Aerospace University “Kharkiv Aviation Institute”, Chkalova Str. 17, 61070 Kharkiv, Ukraine 43D METAL TECH LLC, Simi Khokhlovyh Str. 11/2, 04119 Kyiv, Ukraine *Correspondence: [email protected] (A.K.); [email protected] (O.K.); Tel.: +420-541-142-267 (O.K.) Abstract: Carbon fibres used as a honeycomb core material (subject to a proper in-depth analysis of their reinforcement patterns) allows solving the thermo-dimensional stability problem of the units for space systems. Based on the results of numerical simulations with the support of finite element analysis, the paper provides an evaluation of the accuracy of analytical dependencies for the determination of the moduli of elasticity of a carbon fibre honeycomb core in tension/compression and shear. It is shown that a carbon fibre honeycomb reinforcement pattern has a significant impact on the mechanical performance of the carbon fibre honeycomb core. For example, for honeycombs measuring 10 mm in height, the maximum shear modulus values corresponding to the reinforcement pattern of ± 45 ◦ exceed the minimum values for a reinforcement pattern of 0 ◦ and 90 ◦ by more than 5 times in the XOZ plane and 4 times for the shear modulus in the YOZ plane. The maximum modulus of the elasticity of the honeycomb core in the transverse tension, corresponding to a reinforcement pattern of ± 75 ◦ , exceeds the minimum modulus for the reinforcement pattern of ± 15 ◦ more than 3 times. We observe a decrease in the values of the mechanical performance of the carbon fibre honeycomb core depending on its height. With a honeycomb reinforcement pattern of ± 45 ◦ , the decrease in the shear modulus is 10% in the XOZ plane and 15% in the YOZ plane. The reduction in the modulus of elasticity in the transverse tension for the reinforcement pattern does not exceed 5%. It is shown that in order to ensure high-level moduli of elasticity with respect to tension/compression and shear at the same time, it is necessary to focus on a reinforcement pattern of ± 64 ◦ . The paper covers the development of the experimental prototype technology that produces carbon fibre honeycomb cores and structures for aerospace applications. It is shown by experiments that the use of a larger number of thin layers of unidirectional carbon fibres provides more than a 2-time reduction in honeycomb density while maintaining high values of strength and stiffness. Our findings can permit a significant expansion of the area of application relative to this class of honeycomb cores in aerospace engineering. Keywords: thermo-dimensional stability; modulus of elasticity; finite element analysis; experimental prototype technology 1. Introduction Modern structural materials and their design and technology concepts, as well as wide the possibilities of materials science and computer technologies, reveal unique reserves for improvements in the efficiency of structures in almost all areas of technology [ 1 , 2 ]. In particular, these reserves can be used in designing aerospace structures [ 3 , 4 ]. A specific Polymers 2023,15, 2503. https://doi.org/10.3390/polym15112503 https://www.mdpi.com/journal/polymers Polymers 2023,15, 2503 2 of 17 feature of structures operating in open space, which are meant for the precision coordination of spacecraft interactions with ground objects, is the necessity to meet very stringent requirements, including the provision of their thermo-dimensional stability [ 5 , 6 ]. For example, the thermo-dimensional stability of the structural elements of spacecraft can be measured in fractions of a millimetre [7,8]. Sandwich structures with various types of cores (including honeycomb structures that are commonly used in flying vehicles) are known for their high thermo-dimensional stability [ 9 ]. The unique set of strengths, processes, and operational characteristics observed in honeycomb cores determined their extensive use and priority over other materials [10]. Honeycombs based on carbon fibre have been increasingly used in recent years, which allows significant improvements in the strength and stiffness of honeycomb structures and can extend their service life [ 11 , 12 ]. With the maximum specific strength and stiffness, a carbon fibre honeycomb core features the minimum coefficient of linear thermal expansion [ 13 , 14 ]. Combined with face sheets made of carbon fibre composites, the carbon fibre honeycomb core allows producing chemically homogeneous and dimensionally stable structures that are widely used in the aerospace industry [ 15 , 16 ]. A carbon fibre honeycomb core is a new structural material for aerospace engineering. In [ 17 ], a brief analysis of an application of 50 types of carbon-fibre-reinforced honeycomb composites was carried out. It is noted that one of the potential applications of this aggregate is its application in lightweight solar panels. A triaxial filler of the UCF-98-3/8-4.5 type was used on the X-33 apparatus. A carbon fibre honeycomb core was used as a baffle and lid in an LH2 tank. It is expected that this filler will also be used in antenna reflectors and other spacecraft components, which should provide high strength and thermal stability in operations. Currently, there are many unsolved problematic tasks regarding improvements in the properties of carbon fibre honeycomb cores and structures [ 18 , 19 ]. For example, the use of carbon fibre composites as materials for honeycomb cores creates opportunities for regulating their consolidated physico-mechanical characteristics within a wide range by changing the reinforcing fibers’ laying angle to obtain an optimal combination of properties of the sandwich structure as a whole [20,21]. The optimal design of honeycomb composite structures requires the use of the consolidated physico-mechanical characteristics of cores [ 22 , 23 ]. Modern tools of engineering analyses allow for the direct finding of the stress–strain behaviour of such structures without a replacement of the core used by some solid orthotropic material, i.e., without its “smearing” [ 24 , 25 ]. However, the labour intensity of this approach, most likely, is justified only in cases where verification analysis is required [ 26 , 27 ]. In practical terms, designers have always used and continue to use analytical models of honeycomb structures, allowing them to theoretically calculate the consolidated physico-mechanical characteristics of cores, expressed via their geometric parameters and the properties of the material that they are made of [ 28 ]. The use of these models to obtain a deliberately approximate result mainly allows a further directive adjustment of the result [ 29 ]. From this, it follows that the use of analytical models and expressions implemented to determine physico-mechanical characteristics is a reliable and effective method for the optimization of the parameters of sandwich composite units for various applications, taking specific features of the structures into account [30,31]. Whereas these characteristics, expressed via the geometric and physico-mechanical properties of isotropic materials for the honeycomb cores and several other cores, were obtained long ago [ 28 , 29 ], similar consolidated characteristics of composite cores are not found in the literature. The bearing capacity of the panel and shell structures based on these materials is established in most cases by experiments [ 32 ]. For example, several companies produce honeycombs based on UCF-145-3/8-0.8and UCF-121-1/4-3.0-grade carbon filler, and specific physico-mechanical characteristics are given below (Table 1) [33]. Polymers 2023,15, 2503 3 of 17 Table 1. Specific physico-mechanical characteristics of carbon fibre honeycomb cores with a reinforcement pattern of ±30◦produced by a number of companies. Characteristics UCF-145-3/8-0.8 UCF-121-1/4-3.0 Density, kg/m312.8 48.05 Specific compressive strength, km 2.07 5.73 Specific shear strength parallel to adhesive strips, km 1.35 4.41 Specific shear strength perpendicular to adhesive strips, km 0.77 2.71 Specific modulus of elasticity in shear parallel to adhesive strips, km 1042. 397 Specific modulus of elasticity in shear perpendicular to adhesive strips, km 659 279 It is known that the values of the physico-mechanical characteristics of a honeycomb core significantly depend on the level of the operating load and the peculiar features of this load’s accommodation by individual elements of the honeycomb’s cell [ 34 ]. Attempts to achieve a satisfactory convergence of theoretical and experimental data related to the physico-mechanical characteristics of honeycomb cores are reduced to updating the known mathematical models of honeycombs [ 35 ] because of the impact of various technical deficiencies of a core [36]. The need for the continuous improvement of these structures contributes to active theoretical and experimental research aimed at the development of methods for designing this product class and the creation of experimental structures for operations in outer space [37]. The properties of the composite material and geometric parameters of the carbon fibre core were optimized in [ 38 ]. The paper shows that the compressive and shear strength of the core is well predicted by micromechanical fracture models. Technological methods for improving the shear performance of honeycomb cores reinforced with carbon fibre are described in [ 39 ]. Paper [ 40 ] proposes analytical models for the prediction of thermal conductivity and the strength of the composite core under compression, showing an acceptable agreement with the experiment. Paper [ 41 ] deals with the methods for reinforcing carbon fibre honeycombs, which provide higher strength due to the use of the curved wall topology technique and explores the effect of the geometric parameters of the core and characteristics of the composite materials used on the compressive strength. Analytical expressions for predicting the stiffness and strength of hexagonal carbon fibre honeycomb cores in transverse compression and shear are proposed in [42]. In most cases, depending on the criticality of honeycomb structures, a conclusion on the values of the given physico-mechanical characteristics of the core is given only based on the results of full-scale tests of honeycombs, which are rather expensive when a carbon filler is used for the honeycombs [ 43 ]. Paper [ 44 ] reviews the experimental results of the determination of the physico-mechanical characteristics of honeycombs made of various materials. It is shown that carbon fibre honeycombs feature a relatively stable linear stress–strain curve, and mechanical characteristics can be found based on the elastic model of the honeycomb’s behaviour. The experimental study of the effect of the compressive load on the behaviour of carbon fibre honeycomb cores with different cell shapes is conducted in [ 45 ]. Specimens of the cores were made by the corrugation of carbon fibre composite pieces with their subsequent gluing. Twenty-seven groups of specimens with different honeycomb cell shapes and thicknesses and varying heights with respect to the core were tested under the action of the transverse force. The results showed that the thickness of the cell was one of the main parameters affecting the mechanical performance of the cores, while the honeycomb’s height also has a minor impact on the mechanical performance of the cores. According to the results of the experiment, the density of the obtained carbon fibre honeycomb cores lies within the range of 157–282 kg/m3. Polymers 2023,15, 2503 4 of 17 In connection with the above, analyzing the accuracy of analytical dependencies for the determination of the mechanical performance of carbon fibre honeycomb cores and their applicability at various stages of sandwich structure designs is an urgent task. 2. Materials and Methods Analytical dependencies for the determination of the given physico-mechanical characteristics of a composite honeycomb core were obtained according to the pattern of a uniform distribution with respect to the volume of a typical honeycomb block element. These dependencies allow the theoretical calculation of the consolidated physico-mechanical characteristics of the cores, expressed via their geometric parameters and the properties of the composite material that they are made of. The characteristics of the composite were determined based on the mathematical models of the reinforcement theory. It is assumed that the reinforcing fibres of the composite material are laid symmetrically with respect to the middle surface of the package. The accuracy of analytical dependencies for the determination of the moduli of elasticity of the carbon fibre honeycomb core in tension/compression and shear, described by the adopted analytical dependencies, was assessed according to the results of a series of numerical simulations in the finite element analysis software ANSYS Mechanical 2021 R2. To determine the moduli of shear elasticity of the carbon fibre core with a regular hexagonal cell, shear tests conducted on twinned specimens using the stretching method were simulated using finite element analysis software. Modelling was carried out for 162 full cells of honeycombs. The finite element model was fixed along lateral unloaded plates for all linear displacements. Steel was taken as the material for the external plates of the shear test’s fixture models, with a central plate and edge plates measuring 10 mm and 5 mm thick, respectively. For the determination of the modulus of elasticity of a carbon fibre honeycomb core in transverse tension/compression, modelling was carried out on a single specimen using the finite element model described above. The external plate was loaded with a pressure of 0.1 MPa, with a fixation on the other plate for all linear displacements. A four-node multilayer quadrangular shell element exhibiting bending and membrane properties for spatial analyses (ShellL type and six degrees of freedom, including three translational and three rotational ones) was used in the process of generating a finite element grid. A study of the convergence of the numerical solution showed that normal and shear stresses varied slightly (by 5% at most) with this number of finite elements in the models. Analysis of the quality of the constructed finite-element models did not reveal any critical errors. The physico-mechanical characteristics of the carbon filler for honeycombs adopted for numerical simulations in the finite element analysis software corresponded to the KMU-4E material (manufacturer: Federal State Unitary Enterprise All-Russian Scientific Research Institute of Aviation Material, Moscow, Russia). Displacement patterns were used as boundary conditions in the process of finding the values of the given mechanical characteristics of the carbon fibre honeycomb core. Considering that the presence of edge effects causes the non-uniformity of these patterns, displacement values averaged over all nodes were used as the estimated values. The values of the moduli of elasticity were determined by standard methods for the considered test configurations. During these studies, the technology and equipment for the manufacture of a carbon fibre honeycomb core with a cell face of 5 mm were developed. The technology is based on the block method. The used carbon fillers are listed below: unidirectional fibre of ELUR-PA grade (manufacturer: Argon LLC, Balakovo-1, Moscow, Russia) impregnated with an ENFB epoxy binder (manufacturer: Federal State Unitary Enterprise All-Russian Scientific Research Institute of Aviation Material, Moscow, Russia) with a 0.13 mm thick monolayer; IMS-65 high-modulus carbon filler (manufacturer: Toho Tenax Co. Ltd., Tokyo, Japan) impregnated with an ENFB epoxy binder (manufacturer: Federal State Unitary Enterprise All-Russian Scientific Research Institute of Aviation Material, Moscow, Russia) with a 0.02 mm thick monolayer; sparse fibre of TC-36S-12K grade (manufacturer: TAIRYFIL, Congleton, UK) impregnated with an ENFB epoxy binder (manufacturer: Federal State Unitary Enterprise All-Russian Scientific Research Institute of Aviation Material, Moscow, Polymers 2023,15, 2503 5 of 17 Russia) with a 0.095 mm thick monolayer. The results of the experimental studies were obtained in the form of averaged values of the specific physical and mechanical parameters of six series of carbon-fibre-core pilot samples, and they were obtained in laboratory conditions using standard equipment, devices, and tools. The validity of the conclusions is confirmed by using newly developed reliable mathematical models, and these models are compared with the results of numerical and experimental studies. 3. Theoretical Background It is known [ 28 , 38 ] that the critical physico-mechanical characteristics of honeycomb cores are the moduli of elasticity in tension/compression in the Ez direction transverse to the panel’s surface and shear moduli Gxz and Gyz in the transverse direction (Figure 1). Figure 1. Adopted coordinate system and geometric parameters of the honeycomb core. The expressions for their theoretical definitions obtained in previous studies are widely used in the practice of designing honeycomb structures [ 28 , 29 ]. For a honeycomb core with a regular hexagonal cell, these dependencies take the following form: Ez=1.54δc acEm, Gxz =0.866δc acGm, Gyz =0.577 δc acGm, (1) where δc and ac are the thickness and size of the honeycomb’s face, respectively; Em and Gm are the modulus of elasticity and shear modulus of the honeycomb core’s filler material. The use of the carbon filler as a material for honeycombs creates opportunities to vary their physico-mechanical characteristics within a wide range by changing the laying angle of reinforcing fibres into a symmetrical structure. The variation of the above angle in the interval of 0 ◦≤ϕ≤ 90 ◦ allows obtaining an optimal combination of the stiffness properties of the honeycomb core. These dependencies should be completely valid for carbon fibre honeycombs as well if we substitute Em=Ecm zϕ , Gm=Gcm xzϕ , and Gm=Gcm yzϕ from the formulas in (1) into the adopted coordinate system (Figure 1). The dependencies of the physico-mechanical characteristics of symmetrically reinforced composite materials representing a special case of dependencies that follow from the general mechanics of composites [46,47] are written as follows: Ecm xϕ=1 δΣ B11 −B2 12 B22 !;Ecm zϕ=1 δΣ B22 +B2 12 B11 !;Gcm xzϕ=B33 δΣ ;Gcm yzϕ=B33 δΣ , (2) where the effective stiffness coefficients of the composite package should be as follows [ 48 ]. Polymers 2023,15, 2503 6 of 17 B11 =δΣhE1cos4ϕ+2E1µ21 sin2ϕcos2ϕ+E2sin4ϕ+G12 sin22ϕi; B12 =δΣhE1+E2sin2ϕcos2ϕ−G12 sin22ϕ+E1µ21sin4ϕ+cos4ϕi; B22 =δΣhE1sin4ϕ+2E1µ21 sin2ϕcos2ϕ+E2cos4ϕ+G12 sin22ϕi; B33 =δΣE1+E2−2E1µ21sin2ϕcos2ϕ+G12 cos2ϕ; E1=E1 1−µ12µ21 ;E2=E2 1−µ12µ21 (3) Here, E1 , E2 , G12 , and µ12 denote the elastic characteristics of the carbon-fiber-reinforced plastic (moduli of elasticity along and across fibres, shear modulus, and Poisson’s ratio, respectively); δΣ=δc denotes the total thickness of the composite package. Poisson’s ratio µ21 can be determined from the experiment simultaneously with E2 or from the condition of the existence of elastic potential E1µ21 =E2µ12. Let us consider the possibility of using dependencies (1) as related to the determination of the consolidated mechanical characteristics of the carbon fibre honeycomb core in order to reduce the costs and time for experimental or virtual research. Figure 2shows the representative element of the honeycomb core used for the determination of its consolidated mechanical characteristics according to the formulas in (1) . Figure 2. Representative element of the honeycomb core for the determination of its consolidated moduli of elasticity: (a)—Ez; (b)—Gxz; (c)—Gyz. As shown in Figure 2, under the action of transversal forces Pz on the representative element, all ends of the faces (AB,BC,CD,DE,EF,FK and KB) receive the same relative strain, εz , depending on reinforcing angle ϕ , which leads to the strict equality, Ez , of the honeycomb core relative to module Ecm zϕ from Formula (2) within the plane of its face with a constant coefficient of 1.54 δc/ac. Analysis of the element shown in Figure 2b allows establishing shear strain γxz for the representative element of the honeycomb’s core, and it is strictly equal to these strains in the plane of the double AB face and faces CD and FK. The relative shear in four faces (BC,DE,EF and KB) should be equal to angle γxz due to the observance of the law of parity for the tangential stresses in these faces and faces AB,CD and FK. Based on these considerations, the strict equality of Gxz in the XOZ plane to shear modulus Gcm xzϕ in the face planes with a constant coefficient of 0.866 δc/acshould be valid. Analysis of the honeycomb element in Figure 2c allows us to assume that shear strain γyz for the representative element of the honeycomb core is strictly equal to those in fictitious faces CK and DF only. In this case, an assumption about the equality of honeycomb shear modulus Gyz in the YOZ plane relative to shear modulus Gcm yzϕ in Formula (2) with a coefficient of 0.577 δc/acrequires experimental verification. 4. Numerical Implementation Such verification was performed based on a series of numerical simulations with the use of finite element modelling. For the determination of the moduli of shear elasticity of the carbon fibre honeycomb core with a regular hexagonal cell, shear tests of twinned specimens by using the stretching method were simulated using finite element analysis software [49]. Modelling was carried out for 162 full cells of honeycombs. Polymers 2023,15, 2503 7 of 17 In the process of determining shear modulus Gxz in the XOZ plane, double faces of the specimen were oriented in the direction of the loading performed along the middle plate with a force of 200 N (Figure 3a). Figure 3. Generated finite element models of carbon fibre honeycomb specimens that are 10 mm high and corresponding deformed states: ( a )—for determining the shear modulus Gxz ; ( b )—for determining the shear modulus Gyz ; ( c )—for determining the modulus of elasticity in transverse tension Ez. To determine shear modulus Gyz in the XOY plane, modelling was carried out on the same finite element model, but it was carried out using loading in a different direction along the middle plate with a force of 200 N (Figure 3b). The finite element model was fixed along the lateral unloaded plates for all linear displacements. Steel was used as the material for the external plates used in shear test fixture models, with a central plate and edge plates measuring 10 mm and 5 mm thick, respectively. For the determination of the modulus of elasticity, Ez , of the carbon fibre honeycomb core in transverse tension/compression, modelling was carried out on a single specimen of the finite element model described above. The external plate was loaded with a pressure of 0.1 MPa, with a fixation on the other plate for all linear displacements (Figure 3c). The physico-mechanical characteristics of the carbon filler for honeycombs adopted for further numerical simulations in the finite element analysis software are presented in Table 2[33,50]. Table 2. Physico-mechanical characteristics of the carbon filler for honeycombs adopted for further numerical simulations. CFRP Grade Prepreg Thickness (Monolayer), δ, mm Modulus of Elasticity along the Fibres, E1, GPa Modulus of Elasticity across the Fibres, E2, GPa Shear Modulus G12, GPa Poisson’s Ratio, µ12 Density, ρ, kg/m3 CMU-4E 0.1 115 28.3 5.5 0.25 1550 Figure 3shows examples of the generated finite element models of specimens used to determine the moduli of elasticity of the carbon fibre honeycomb core, corresponding boundary conditions and deformed states of the virtual specimens obtained from the numerical simulations. Polymers 2023,15, 2503 8 of 17 Graphs in Figure 4show the change in the moduli of elasticity of the carbon fibre honeycomb core, as calculated according to the formulas in (1), where the points obtained with the use of finite element analysis are plotted. Figure 4. Graph of the dependency of the carbon fibre honeycomb core’s modulus of elasticity Ez and shear moduli Gxz and Gyz on its reinforcing angle: denotes analytical determination; denotes data obtained from numerical simulations using finite element models. Analysis of the results allows us to establish the following. The values of both the modulus of elasticity and shear moduli ECE z , GCE xz and GCE yz obtained on the basis of the technologies of finite element analysis exceed the corresponding values determined by analytical Formula (1) EA z , GA xz and GA yz . Such excess over the corresponding analytical values for shear moduli, close to the constant value for different reinforcing angles, is equal to GCE xz /GA xz ≈ 1.07 and GCE yz /GA yz ≈ 1.14. For the modulus of elasticity, Ez , the excess varies from 1.25 at ϕ=± 30 ◦ to 1.8 at ϕ=± 45 ◦ , and then it decreases again to 1.32 at ϕ=± 60 ◦ and 1.03 at ϕ=± 80 ◦ , which indicates some difference in the law of variation of Ez from those found by analytical Formula (1). 5. Experimental Research The technology and equipment for the manufacture of a carbon fibre honeycomb core with a cell face of 5 mm were developed in the process of research. The technology is based on the block method [33,50]. Primary blanks with a reinforcement pattern of ± 45 ◦ , providing the maximum mechanical performance of a honeycomb core in shear, were formed using various carbon fillers. The used carbon fillers are listed below: • Fibre of ELUR-P-A grade impregnated with an ENFB epoxy binder with a 0.13 mm thick monolayer; • IMS-65 high-modulus carbon filler impregnated with an ENFB epoxy binder with a 0.02 mm thick monolayer; • TC-36S-12K-grade sparse fibre impregnated with an ENFB epoxy binder with a 0.095 mm thick monolayer. After a proper layup, the blanks were corrugated using a punch (Figure 5a). Corrugated blanks (Figure 5b) were assembled into a honeycomb block on the assembly fixture (Figure 5c). After heat treatment, formed rods were removed from the assembled honeycomb block, cut around the perimeter (Figure 6a) and ground to the required height (Figure 6b). Four series of specimens with a height of h= 10 mm were made from the obtained blocks comprising carbon fibre honeycomb cores to determine their physicomechanical characteristics. Series 1 comprises a carbon fibre honeycomb core with a hexagonal honeycomb cell, where its wall contains two layers of unidirectional carbon fibres comprising 0.13 mm thick ELUR-P-A carbon tape impregnated with an ENFB epoxy binder. First, a specified even number of prepreg sheets was formed by laying and rolling on two layers of carbon tape Polymers 2023,15, 2503 9 of 17 with fibres oriented at an angle of ± 45 ◦ relative to the prepreg axis. The outer surfaces of both layers of the ELUR-P-A tape were duplicated with a fluoroplastic film that is 40 µ m thick. Prepreg pressing was not performed. Prepreg was corrugated (Figure 5a) on a matrix preheated to 150 ◦ C under a pressure of 0.3 MPa, and holding was carried out for 5–6 min. For this purpose, key punches with special metal tips were used. The corrugated prepregs were formed into a honeycomb block and cured using metal rods. At this time, BK-25 glue was applied onto the tangent surfaces of prepregs, and they were laid to provide the matching orientation of unidirectional carbon fibres on them. After that, the honeycomb block was cured in the oven with an increase in temperature at a rate of 0.8 to 1.2 ◦ C/min: first to 130 ◦ C, kept at that temperature for 30 min and then the temperature was raised to 175 ◦C at a rate of 1.8 to 2.2 ◦C/min; it was then held for 3 h at that temperature. Figure 5. Equipment for the manufacture of a carbon fibre honeycomb core: ( a )—forming punches; (b)—fragment of the corrugated blank; (c)—device for honeycomb gluing. Polymers 2023,15, 2503 16 of 17 14. Sugiyama, K.; Matsuzaki, R.; Ueda, M.; Todoroki, A.; Hirano, Y. 3D printing of composite sandwich structures using continuous carbon fiber and fiber tension. Compos. Part A Appl. Sci. Manuf. 2018,113, 114–121. [CrossRef] 15. Vargas-Rojas, E.; Nocetti-Cotelo, C.A. 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