Core-sheath multi-structure FRP bolt and method for manufacturing the same

Abstract

This invention provides a resin bolt and a method for manufacturing the same, which simplifies the manufacturing process while achieving sufficient strength by constructing the bolt body from a material other than fiber-reinforced resin. [Solution] The resin bolt comprises a bolt body having a threaded portion and a core portion positioned inserted along the axis of the bolt body. The bolt body is made of thermoplastic resin, and the core portion is made of pultruded fiber-reinforced resin. The inner surface of the bolt body and the outer surface of the core portion are fused, bonded, or compressed, or a combination thereof, so that the bolt body and the core portion are integrally formed. A typical manufacturing method includes the steps of molding the bolt body, creating the core portion by pultruded fiber-reinforced resin, inserting the core portion into a through hole in the bolt body, and rubbing the core portion and the bolt body together. The frictional heat generated by the friction melts the inner surface of the thermoplastic resin and fuses it to the outer surface of the fiber-reinforced resin.

Description

Technical Field

[0001] The present invention relates to a resin bolt using fiber reinforced plastic (FRP) and a method for manufacturing the same, and more particularly to a multi-structured resin bolt in which fiber reinforced plastic is used for a core part and a thermoplastic resin is used for a bolt body serving as a sheath, and a method for manufacturing the same.

Background Art

[0002] Generally, metal bolts are used for fastening members on which shear loads act. However, since metal bolts are corroded by rust and the like, attempts have been made to improve corrosion resistance by using bolts made of synthetic resin. However, since bolts made of synthetic resin are insufficient in terms of strength, bolts using fiber reinforced plastic have been developed. For example, a bolt in which both a core part and an outer skin are made of fiber reinforced plastic has been proposed (see Patent Document 1). However, in such a configuration, since thread cutting is involved in forming a threaded part, there has been a concern that fibers are cut and the strength is reduced.

[0003] Therefore, a concavo-convex structure is formed by spirally winding a fiber string or tape around the outer periphery of a core part in which fibers are arranged in the axial direction, impregnated with a thermosetting resin, and then compression molded by a molding die to form a threaded part, so that the concavo-convex structure by the string or the like enters the threaded part. A fiber reinforced plastic bolt has been developed (see Patent Document 2). In addition, a fiber reinforced plastic bolt has been developed in which an outer skin in which fibers perpendicular to the axial direction are arranged is formed on the outer periphery of a main body part (core part) in which fibers are arranged in the axial direction, and the fibers of the outer skin enter the main body part (core part) (Patent Document 3).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] The aforementioned prior art (Patent Documents 2 and 3) is designed to avoid cutting of fibers in the threaded portion of a bolt. In both cases, the outer shell is made of fiber-reinforced resin in which the fibers are arranged in a direction different from the fiber direction of the core, and the threaded portion (threads, etc.) is formed by molding with a synthetic resin impregnation.

[0006] However, in the fiber-reinforced resin bolt with the above configuration, the fiber direction of the core and the fiber direction of the outer sheath are different, making it essential to perform a process to form the outer sheath around the outer circumference of the core. This process involves wrapping with string or other materials, or manufacturing prepregs, which are extremely cumbersome steps.

[0007] Furthermore, since both the core and outer sheath of these fiber-reinforced resin bolts are made of fiber-reinforced resin, the threads are formed using a mold with thermosetting resin, and consequently, the threads had to be formed as the final step. This is because the threads cannot be formed until the outer sheath portion, which is to form the threads, has been integrated with the core. However, if there is any misalignment of the threads during the molding of the threads, the intended threads cannot be produced, requiring precise machining accuracy. It should be noted that fiber-reinforced resin bolts of any kind basically consist of a core and outer sheath made of fiber-reinforced resin, and multi-structures with materials other than fiber-reinforced resin have not been considered until now due to concerns about reduced strength.

[0008] The present invention has been made in view of the above points, and its objective is to provide a resin bolt and a method for manufacturing the same that can obtain sufficient strength while simplifying the manufacturing process by constructing the outer shell (bolt body having a threaded portion) from a material other than fiber-reinforced resin. [Means for solving the problem]

[0009] Therefore, as a result of diligent research, the inventors of this application have made it possible to strengthen resin bolts by using a material other than fiber-reinforced resin for the outer shell while using fiber-reinforced resin for the core, and have invented a high-strength resin bolt. In other words, the inventors of this application have invented a core-sheath multi-structure FRP bolt and a method for manufacturing the same, with the following configuration.

[0010] The first configuration of the invention relating to a core-sheath multi-structure FRP bolt comprises a bolt body having a threaded portion and a core portion positioned in a state inserted along the axis of the bolt body, wherein the bolt body is made of thermoplastic resin and the core portion is made of pultruded fiber-reinforced resin, and the bolt body and the core portion are integrally formed by fusion, bonding, or crimping or a combination thereof between the inner surface of the bolt body and the outer surface of the core portion.

[0011] In the multi-structured FRP bolt with the core and sheath configuration described above, the core is made of fiber-reinforced resin, while the bolt body is made of thermoplastic resin. Therefore, both can be manufactured separately and then ultimately integrated by fusion, adhesive, or compression, or a combination thereof. By joining the two by fusion or other means, they are integrated in a tightly fitted state, eliminating gaps between them and eliminating spaces where deformation in the bending direction is permitted when shear stress is applied. This improves the overall shear strength. In the above configuration, the bolt body made of thermoplastic resin can be molded by injection molding or 3D printing, allowing for a simple manufacturing process. In addition to fusion, the two can also be joined by compression, where the core is pressed into the bolt body. Alternatively, they can be bonded using a material with an adhesive effect when inserted into the bolt body, or a combination of compression and adhesive may be used. In any case, it is sufficient that no gaps are formed at the joint surface between the two and that they are integrated.

[0012] A second configuration of the invention relating to a core-sheath multi-structure FRP bolt is that, in the first configuration, the bolt body contains at least one type of reinforcing fiber, such as carbon fiber, recycled carbon fiber, or glass fiber, in the thermoplastic resin.

[0013] The multi-structured core-sheath FRP bolt described above can compensate for insufficient strength due to the thermoplastic resin alone by incorporating reinforcing fibers. In particular, the configuration containing recycled carbon fibers can be effective for applications of carbon fibers that have recently been considered difficult to recycle. Furthermore, since the thermoplastic resin containing these materials can be molded by injection molding or 3D printing, just like ordinary thermoplastic resins, it can be easily manufactured in this case as well.

[0014] A third configuration of the invention relating to a core-sheath multi-structure FRP bolt is that, in the first configuration, the bolt body and the core generate frictional heat between the inner surface of the bolt body and the outer surface of the core, melting the inner surface of the thermoplastic resin and fusing it to the outer surface of the fiber-reinforced resin.

[0015] In the multi-structured FRP bolt with the core and sheath configuration described above, frictional heat is generated by friction between the bolt body and the core. This heat melts the thermoplastic resin, and ultimately fuses the thermoplastic resin and the fiber-reinforced resin together. As a result, no gaps are formed at the joint surface between the two, resulting in a strong, integrated state and improved shear strength. To generate frictional heat between the bolt body and the core, one method is to fix the bolt body and rotate the core at high speed around its axis, or to apply ultrasonic vibration to either the bolt or the core.

[0016] A fourth configuration of the invention relating to a core-sheath multi-structure FRP bolt is that, in any of the first to fourth configurations, the outer diameter of the core portion is 1 / 2 or more of the outer diameter of the bolt body.

[0017] With the core-sheath multi-structure FRP bolt described above, the shear strength of the pultruded fiber-reinforced resin constituting the core can be fully utilized, allowing for high shear strength regardless of the type of material (thermoplastic resin) used for the bolt body. Furthermore, by setting the outer diameter of the core to half the outer diameter of the bolt body as the lower limit, the proportion of pultruded fiber-reinforced resin, which allows for the use of continuous fibers at a high rate, can be increased, thereby maintaining shear strength. In other words, by making the outer diameter of the core at least half the outer diameter of the bolt body, fiber-reinforced resin is used in more than 25% of the cross-sectional area, allowing the shear strength of the fiber-reinforced resin to be fully utilized and improving the overall strength. While no upper limit is specifically set, it should be within a reasonable range that does not cause the threads formed on the bolt body to collapse. That is, since the bolt body is formed in a substantially cylindrical shape, the strength of the shaft is affected by the thickness of the body. Similarly, the strength of the threaded portion (the part where the threads are formed) also depends on the thickness of the body (the base where the threads are formed) of the threaded portion. Therefore, for example, if the outer diameter of the core (inner diameter of the cylindrical shape) is set such that the wall thickness of the body of the screw thread is equal to or greater than 1 / 2 of the difference between the outer diameter (nominal diameter) and groove diameter of the screw thread (thread height), then strength exceeding the external force acting on the screw thread can be obtained. Also, since the screw thread is composed of threads and grooves, a defect occurs in the cross-sectional area. If the defect rate is 20%, then the total cross-sectional area including the core will be 80% of the cross-sectional area of ​​the shaft where the screw thread is not formed, and this can be converted to a diameter of approximately 90% (square root of 0.8 = 0.8944). If the inner diameter of the cylindrical shape (outer diameter of the core) is adjusted by the equivalent of 10% to compensate for this 10% difference, it will be able to withstand the external force acting on the thread, and the upper limit can be calculated as 4 / 5 of the total (outer diameter of the shaft). However, if carbon fibers or the like are included in the thermoplastic resin, a different calculation method may be used depending on the state of the fibers. Furthermore, when the outer diameter of the core is reduced (to half the diameter of the bolt body), the proportion of thermoplastic resin used increases. However, when using thermoplastic resin containing recycled carbon fiber, this contributes to increasing the amount of recycled carbon fiber used.

[0018] A fourth configuration of the invention relating to the core-sheath multi-structure FRP bolt is that, in any of the first to fourth configurations described above, the bolt body is a molded product produced by injection molding or 3D printing.

[0019] With the core-sheath multi-structure FRP bolt described above, the bolt body can be molded relatively easily, and the molded product can function as a sheath by being pre-formed with a through hole that penetrates in the axial direction. When the core is fitted into the through hole and the two are joined together, they become a strong, integrated unit and can have the desired shear strength.

[0020] On the other hand, the first configuration of the present invention relating to a method for manufacturing a core-sheath multi-structure FRP bolt includes a body molding step of molding the bolt body into a shape having a through hole into which the core can be fitted; a core molding step of providing the core by drawing the fiber-reinforced resin; a cutting step of cutting the length of the core to be longer than the length of the through hole in the bolt body so that a driving force transmission region or a fixing region for fixing is secured; a core insertion step of fitting the core into the through hole in the bolt body; and a friction step of generating friction between the bolt body and the core using the driving force transmission region or fixing region of the core, wherein the friction step is characterized in that frictional heat is generated by rotating the core at high speed around its axis, or by applying ultrasonic vibration to either the core or the bolt body to generate frictional heat, melting the inner surface of the thermoplastic resin and fusing it to the outer surface of the fiber-reinforced resin.

[0021] According to the manufacturing method described above, the bolt body and the core are molded separately, and after inserting the core into the bolt body, the contact surfaces of the two are rubbed together, and the resulting frictional heat can be applied to the contact surface. At that time, if the bolt body is fixed and rotational driving force is transmitted to the driving force transmission region of the core, and the core is rotated at high speed around the axis, high-temperature frictional heat can be obtained. Alternatively, the bolt body may be rotated around the axis using the fixed region of the core. Alternatively, while the bolt body is fixed, ultrasonic vibration may be applied to the core using the driving force transmission region of the core, or the core may be fixed using the fixed region of the core, and ultrasonic vibration may be applied to the bolt body to generate high frictional heat. As the contact surface becomes hot due to the frictional heat, a part of the thermoplastic resin (contact surface) can be partially melted. Due to this melting of the thermoplastic resin, the two contact surfaces fuse together, and an integrated FRP bolt can be obtained as a whole. Any burrs that form on the thermoplastic resin due to melting, or the driving force transmission region of the core, can be removed as unnecessary parts after fusion.

[0022] A second configuration of the present invention relating to a method for manufacturing a core-sheath multi-structure FRP bolt is characterized by including a body molding step of forming the bolt body into a shape having a through hole into which the core can be fitted; a core molding step of forming a core with a diameter larger than the diameter of the through hole provided in the bolt body by drawing the fiber-reinforced resin; a cutting step of cutting the core while matching its length to the length of the through hole in the bolt body; and a core press-fitting step of press-fitting the core into the through hole in the bolt body.

[0023] According to the manufacturing method of the above structure, since the bolt body and the core part are formed individually and the outer diameter of the core part is configured to be larger than the diameter of the through hole of the bolt body, the core part can be integrated by press-fitting it into the through hole of the bolt body. By press-fitting, the thermoplastic resin is elastically deformed so as to slightly expand the through hole, and the restoring force at that time can be used to utilize the shrinking force against the core part. As a result, the periphery of the core part is entirely crimped. Although the diameter of the core part is set to be larger than the diameter of the through hole, the difference can be set to be the degree of tight fit in the so-called fitting tolerance. Also, by using a resin (for example, epoxy resin, etc.) having an adhesive effect at the time of insertion, it becomes possible to strongly integrate by the synergistic effect of crimping and adhesion.

[0024] The third configuration of the present invention related to the manufacturing method of the core-sheath multi-structure FRP bolt includes a core part forming step of providing the core part by drawing the fiber reinforced resin, a cutting step of cutting the core part into a predetermined length, and a main body forming step of injecting a thermoplastic resin or a thermoplastic resin containing reinforcing fibers after installing the core part of the predetermined length cut by the cutting step at the center of the shaft part of the injection mold to form the bolt body.

[0025] According to the manufacturing method of the above structure, a core part with a predetermined length is prepared in advance. In an injection mold, the core part is arranged like a core, and molten resin is injected so as to fill the thermoplastic resin around it, whereby the two can be molded as an integrated body in a fused state. Note that the "predetermined length" of the core part cut by the cutting process is a length appropriately adjusted according to the state when installed as a core in the injection mold. For example, when installed in a state where it matches the length of the bolt body, it becomes the length of the bolt body. When installed with both ends protruding from the bolt body, it becomes longer than the length of the bolt. Also, for example, when molding so as to cover the whole with the bolt body (thermoplastic resin) (so that the tips at both ends of the core part are buried in the thermoplastic resin), it can be made shorter than the length of the bolt. Even when molding in a state where the core part is covered with the thermoplastic resin (the core part is buried in the bolt body), in the substantially used range, its cross-section is such that the thermoplastic resin is in the outer skin (sheath), and the fiber reinforced resin is arranged in the core part (core), so this can also be recognized as a multi-structure by the core-sheath.

Effect of the Invention

[0026] According to the core-sheath multi-structure FRP bolt of the present invention, the bolt body is composed of a thermoplastic resin rather than a fiber reinforced resin and can be molded separately from the core part. Therefore, it is possible to form the thread part (thread ridge, etc.) on the surface during the molding of the bolt body. That is, since the thread part is not composed of a fiber reinforced resin as in the prior art, a resin bolt can be constituted without going through a complicated manufacturing process. Also, since the core part is constituted by a fiber reinforced resin by draw molding, the fiber direction is arranged along the longitudinal direction, and a core part excellent in shear strength can be constituted. And by inserting this core part with an appropriate ratio of the outer diameter with respect to the entire shaft part of the bolt, the shear strength of the shaft part of the entire resin bolt can be improved.

[0027] On the other hand, according to the manufacturing method of the core-sheath multi-structure FRP bolt of the present invention, the bolt body and core, which are individually molded, can be integrated by joining them in an appropriate manner. Therefore, by pre-forming a threaded portion (threads, etc.) on the surface of the bolt body and a through hole in the center, the core can be integrated by inserting it into the through hole. In this case, the joining method can be fusion bonding, adhesive bonding, crimping, or a combination thereof. Furthermore, when the core is placed in an injection mold and thermoplastic resin is injected, fusion bonding can be performed simultaneously with the molding of the bolt body, enabling manufacturing with fewer steps. [Brief explanation of the drawing]

[0028] [Figure 1] (a) shows the overall structure of an embodiment of a core-sheath multi-structure FRP bolt, and (b) is an explanatory diagram showing a longitudinal section. [Figure 2] This is an explanatory diagram showing the configuration of the jig used in the testing machine for the experiment. [Figure 3] This is an explanatory diagram showing the composition of the test specimen. [Figure 4] This graph shows the measurement results from the test machine in Experiment 1. [Figure 5] This graph shows the measurement results from the test machine in Experiment 1. [Figure 6] This is an explanatory diagram showing the procedure of the first embodiment of a method for manufacturing a core-sheath multi-structure FRP bolt. [Figure 7] This is an explanatory diagram showing the procedure of a second embodiment of a method for manufacturing a core-sheath multi-structure FRP bolt. [Modes for carrying out the invention]

[0029] The embodiments of the present invention will be described below with reference to the drawings. First, an embodiment relating to a core-sheath multi-structure FRP bolt will be described, followed by an embodiment of the manufacturing method.

[0030] <Embodiment of a core-sheath multi-structure FRP bolt> Figure 1 shows an embodiment of the invention relating to a core-sheath multi-structure FRP bolt. Figure 1(a) shows a schematic of the overall structure of this embodiment, and Figure 1(b) shows a longitudinal cross-section of this embodiment. The core-sheath multi-structure FRP bolt 1 of this embodiment is composed of a bolt body 2 and a core 3. The bolt body 2 is a molded product made by molding thermoplastic resin by an appropriate method, and the core 3 is a rod-shaped member made by drawing and molding fiber-reinforced resin. The axis of the bolt body 2 has a through hole 20 for inserting the core 3, and the core-sheath multi-structure FRP bolt 1 of this embodiment is an integrated product with the core 3 inserted into this through hole 20.

[0031] The bolt body 2 is based on a versatile design having a shaft portion 21 and a head (such as a hexagonal head) 22 at its base end, and can be used, for example, as a hexagonal bolt. These bolt shapes are formed to conform to the standards for hexagonal bolts. In principle, this embodiment is configured as a so-called half-threaded bolt, with the shaft portion 21 being divided into a bolt shank (a portion without a threaded shape) 23 and a threaded portion 24 having a male thread. The reason for adopting a half-threaded design is to provide a bolt used exclusively for maintaining shear strength, that is, to allow shear stress to act on the bolt shank 23. The threaded portion 24 conforms to standards, and, like general metal bolts, has a male thread formed on the shaft portion 21 with a screw groove.

[0032] Examples of thermoplastic resins used for the bolt body 2 include vinyl chloride resin (PVC resin), vinylidene chloride resin, vinyl acetate resin, polyvinyl alcohol resin, polystyrene resin, acrylonitrile-styrene resin (AS resin), acrylonitrile-butadiene-styrene resin (ABS resin), acrylic resin, methacrylic resin, polyethylene resin, polypropylene resin, various thermoplastic polyamide resins, polyacetal resin, polycarbonate resin, polyethylene terephthalate resin, polyethylene naphthalate resin, polybutylene naphthalate resin, polybutylene terephthalate resin, polyarylate resin, polyphenylene ether resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, and polylactic acid resin (PLA resin). From the viewpoint of physical properties, nylon may also be used. Examples of nylon include PA6 and PA66.

[0033] To obtain adequate shear strength, it is preferable to select at least one of the thermoplastic resins exemplified above from polyvinyl chloride resin, polylactic acid resin, nylon, polypropylene resin, and polycarbonate resin. These thermoplastic resins may contain additives such as stabilizers, flame retardants, pigments, and fillers as needed, and may be used individually or in mixtures of two or more.

[0034] To improve shear strength, at least one type of reinforcing fiber can be selected, such as carbon fiber, recycled carbon fiber, glass fiber, or other reinforcing fibers. When recycled carbon fiber is included, in addition to improving strength, it can also contribute to the reuse of carbon fiber.

[0035] The core 3 is made of a fiber-reinforced resin formed by extrusion molding. The reinforcing fibers used in the fiber-reinforced resin are preferably selected from at least one of the following: inorganic fibers such as carbon fibers, glass fibers, alumina fibers, boron fibers, ceramic fibers, and metal fibers; or organic fibers such as plant fibers, aramid fibers, polyoxymethylene fibers, aromatic polyamide fibers, poly(p-phenylenebenzobisoxazole) fibers, and ultra-high molecular weight polyethylene fibers. Carbon fibers or glass fibers are particularly preferred, and carbon fibers are even more preferred.

[0036] The core portion 3 is integrated with the bolt body 2 by being inserted into a through hole 20 formed in the axis of the bolt body 2. In this case, as a means of maintaining the bonding force between the two, firstly, there is a method of press-fitting the core portion 3 into the through hole 20. Press-fitting can be done by making the outer diameter of the core portion 3 slightly larger than the inner diameter of the through hole 20 and forcibly inserting it. When the core portion 3 is press-fitted into the through hole 20, the bolt body 2 (mainly the shaft portion 21) elastically deforms as the through hole 20 expands, and due to its restoring force, it becomes compressed against the outer surface of the core portion 3.

[0037] Furthermore, as a second means of maintaining the bonding force between the two, there is a method of melting the inner surface of the through hole 20 to fuse them together. Since fusion requires heating only the inner surface of the through hole 20, frictional heat is generated between the contacting surfaces. With the core 3 inserted into the through hole 20 of the bolt body 2, frictional heat can be generated between the contacting surfaces by rotating only the core 3 at high speed in a circular motion. In this case, because the core 3 is rotated at high speed, the core 3 inserted into the through hole 20 is not in a press-fit state, but is in a state that allows for a moderate amount of sliding motion.

[0038] Furthermore, a third means of maintaining the bonding strength between the two is to bond them together using a resin with adhesive properties. For example, an epoxy resin or the like can be impregnated into the surface of the core 3 and then inserted into the through hole 20. If the bonding strength is insufficient, this method can be used in combination with the aforementioned press-fitting.

[0039] As described above, this embodiment is configured as a partial thread in principle and is used so that a shear load acts on the bolt shank 23. Therefore, the clamping strength (tensile strength in the axial direction) only needs to be strong enough to prevent the clamped object from coming loose. As described above, the bolt body 2 on which the threaded portion 24 is formed is made of thermoplastic resin, so the strength of the threaded portion (thread) depends on the strength of the thermoplastic resin material. Therefore, when using it, the required clamping force can be achieved by using it in a way that increases the range in which the threaded portion is screwed (the range in the longitudinal direction of the threaded portion that is used for screwing).

[0040] <Experimental Example 1> As described above, the core-sheath multi-structure FRP bolt 1 of this embodiment is solely intended to improve shear strength. Therefore, a test specimen was prepared and an experiment was conducted to measure the maximum shear load. The testing machine used for the experiment was a tensile testing machine (1000kN universal testing machine). As shown in Figure 2, a jig was used in which plate-shaped working parts (steel plates) A ​​and B, which are held by chucks on both sides and to which tensile force is transmitted, are sandwiched between two auxiliary plates (steel plates) C and D, and a shear stress is applied between them. The tensile load was converted to a shear load for measurement. The lower working part B of the jig was held by the fixed-side chuck of the universal testing machine (tensile testing machine), and the upper working part A was held by the rising-side chuck. The upper parts of the auxiliary plates C and D were firmly fixed with two metal bolts and nuts E and F, and the lower parts of the auxiliary plates C and D were fixed with the FRP bolt (test specimen) 1 of the embodiment under test. In this embodiment, the auxiliary plates C and D are lifted up as the upper working part (moving side) A rises relative to the lower working part (fixed side) B, which is fixed by the FRP bolt (test piece) 1. Therefore, when an upward tensile force acts on the two auxiliary plates C and D, shear stress acts between the lower working part (fixed side) B and the auxiliary plates C and D. The plate thickness of the working parts A and B is in the range of 9 mm to 12 mm, and the plate thickness of the auxiliary plates C and D is 12 mm, with a plate width of 80 mm.

[0041] To measure the shear strength using the above-described test fixture, a test specimen 10 was prepared as shown in Figure 3. The test specimen 10 had threaded sections 24 on both sides and a bolt shank 23 in the center. The outer shell (bolt body) 2 was molded from several types of thermoplastic resin, with a through hole in the axial center. A carbon fiber reinforced polymer (CFRP) core 3, constructed by pultrusion molding, was inserted into this through hole. The two were integrated by gently press-fitting them together to prevent any gaps from forming between the through hole in the outer shell 2 and the core 3.

[0042] The above test specimens 10 used were one made of polylactic acid (PLA) resin (test specimen 1) and one made of polyvinyl chloride (PVC) resin (test specimen 2). Both test specimens 1 and 2 had M16 threads (16 mm diameter metric threads) for the threaded portion 24. For test specimen 1, the outer shell (bolt body) was molded using a 3D printer for experimental purposes, with the length of the bolt shank 23 (L1) being 26 mm and the lengths of the threaded portions 24 on both sides (L2, L3) being 32 mm. The outer diameter (φ1) of the outer shell 2 was 16 mm, and the outer diameter (φ2) of the core portion 3 was 8 mm, which is half the length of the outer shell 2. By making the length of the bolt shank 23 (L1) 26 mm, if the working portion B (plate thickness 9-12 mm) is placed in the center, regions for applying shear stress can be provided on both sides of the working portion B.

[0043] On the other hand, for test piece 2, a commercially available resin bolt (M16 with a head) was used. A bolt with a shank length of 20 mm and a threaded portion length of 40 mm was used, and a through hole was drilled in the center of the shaft, into which carbon fiber reinforced polymer (CFRP) was inserted. The polyvinyl chloride bolt was used with the underside of the head in contact with one of the auxiliary plates C.

[0044] Five test specimens were prepared for each configuration, and the same test was performed on all of them. For comparison, five resin bolts without a core (without through-holes) were prepared and the same test was performed on them.

[0045] The experimental results for the test specimens and comparative examples described above are shown in the table below, and a graph showing the displacement meter data from the testing machine is shown in Figure 4. Note that the maximum shear load (kN) is the average value of five measured values, and the shear strength (MPa) is a calculated value based on the measured values ​​(shear strength = maximum shear load / cross-sectional area).

[0046] [Table 1]

[0047] As is clear from the results above, when comparing test specimens 1 and 2 with comparative examples 1 and 2, the specimens using polylactic acid (PLA) resin (test specimen 1 and comparative example 1) showed an increase in strength of approximately 10%, while the specimens using polyvinyl chloride (PVC) (test specimen 2 and comparative example 2) exhibited more than twice the strength. Although the improvement in strength was lower because polylactic acid (PLA) resin is stronger than polyvinyl chloride (PVC), both specimens were able to achieve shear strengths exceeding 60 MPa, indicating that they are high strength for resin bolts.

[0048] <Experimental Example 2> Experiments were also conducted with materials other than those mentioned above. The experiment used nylon (PA6) (test specimen 3). Test specimen 3 did not have a bolt shank 23, and the entire surface of the outer shell was made up of threads. For reference, a similar shape made of polylactic acid (PLA) resin (reference example 1) was also experimented with. The threads were M16 (16 mm diameter metric thread), as in Experiment 1. The outer shells of both were molded using a 3D printer. For comparison, comparative examples 3 and 4 were made using a 3D printer, and the core was not inserted (made entirely of resin). Five of each test specimen and comparative example were prepared, and the experiments were conducted using the same tensile testing machine and testing fixture as in Experiment Example 1.

[0049] The experimental results for the test specimens and comparative examples described above are shown in the table below, and a graph showing the displacement meter data from the testing machine is shown in Figure 5. In this experiment as well, the maximum shear load (kN) is the average of five measured values, and the shear strength (MPa) is a calculated value.

[0050] [Table 2]

[0051] From the above results, it was found that even when using nylon (PA6) (test specimen 3 and comparative example 3), it is possible to obtain a product that exhibits approximately twice the strength. Furthermore, the above evaluation was based on a comparison between Reference Example 1 and Comparative Example 4, which used polylactic acid (PLA), and a strength increase of approximately 10% was confirmed. Therefore, it is judged that a similar effect can be obtained even in the case of a configuration with a bolt shank, as in Experimental Example 1.

[0052] Based on a comprehensive assessment of the above experimental examples, it was found that by using thermoplastic resins, not limited to the resins used in the experiments, and by using materials widely used as fiber-reinforced resins, not limited to carbon fiber reinforced polymers (CFRP), for the core, it is possible to obtain bolts with higher strength than bolts made entirely of resin. Furthermore, since the outer diameter of the core used in the experiments was half the outer diameter of the bolt shank, it was found that by making the outer diameter of the core larger than half of that (a larger outer diameter), it is possible to obtain strength exceeding the results shown above.

[0053] <Summary> As described above, the embodiments and experimental results for the core-sheath multi-structure FRP bolt are as follows: By using a thermoplastic resin bolt body 1 as a sheath to form the outer shell, the bolt has the appearance of a resin bolt, while the fiber-reinforced resin (FRP) core is positioned at the center of the bolt body 1. This results in an FRP bolt with a multi-structure made of different materials, divided into core and sheath. While resin bolts made solely of thermoplastic resin tend to have relatively low strength (shear strength), using this as a sheath and arranging fiber-reinforced resin as the core dramatically improves the overall strength (shear strength). Furthermore, since the core 2 made of fiber-reinforced resin (FRP) provides strength, and the bolt body (sheath portion) 1 made of thermoplastic resin allows for easy molding of the bolt shank 23 and threaded portion 24, it becomes easier to manufacture compared to manufacturing the entire bolt from fiber-reinforced resin, as shown in the embodiments relating to the manufacturing method described later. As a result, a reduction in manufacturing costs can be expected, and the bolt can become more versatile.

[0054] It should be noted that the above embodiments and the FRP bolts fabricated for experimental purposes are merely examples of the present invention, and the present invention is not limited to these embodiments. It is also conceivable that the components may be modified as appropriate, or other components may be added.

[0055] For example, while only polylactic acid (PLA) resin, polyvinyl chloride (PVC) resin, and nylon 6 (PA6) were used as thermoplastic resins in the experimental FRP bolts, other resins can be used, as is clear from the experimental results above. Furthermore, as long as the resin can be easily integrated with the fiber-reinforced resin (FRP) used as the core 2, it is not limited to thermoplastic resins, and thermosetting resins and other synthetic resins can also be used.

[0056] Furthermore, while the above embodiments and experimental FRP bolts have been described primarily in which no other materials are added to the material of the bolt body 1, other materials can be added to the resin used within an appropriate range. For example, in the description of the embodiments, reinforcing fibers such as carbon fiber, recycled carbon fiber, and glass fiber were given as examples, but it is not limited to reinforcing fibers, and other resins or metal materials may be added.

[0057] <Embodiment relating to the manufacturing method> Next, an embodiment of a manufacturing method for producing the aforementioned core-sheath multi-structure FRP bolt 1 will be described. This embodiment is a manufacturing method for realizing the configuration of the embodiment relating to the core-sheath multi-structure FRP bolt described above. Two embodiments relating to the manufacturing process are given as examples, but are not limited to these: an embodiment in which the bolt body and the core are rubbed against each other after the core is inserted and the frictional heat generated is used to fuse them together (first embodiment), and an embodiment in which thermoplastic resin is injected after the core is placed in an injection mold (second embodiment).

[0058] <First embodiment relating to the manufacturing method> Figure 6 shows the procedure for manufacturing a core-sheath multi-structure FRP bolt according to this embodiment. As shown in Figure 6, the manufacturing method according to this embodiment involves separately manufacturing the bolt body and the core, and then inserting the core into the bolt body to manufacture an integrated core-sheath multi-structure FRP bolt.

[0059] First, regarding the manufacturing of the bolt body, a resin molded body is produced (body molding process). The resin molded body can be produced by injection molding or 3D printing. In this case, if the injection mold or 3D data is pre-formed with a through hole that penetrates the axis, the molded product can be used as is. On the other hand, if the bolt body is molded without a through hole, or if a commercially available resin bolt is used, the same type of bolt body can be produced by adding a process to drill a through hole (drilling process).

[0060] On the other hand, regarding the manufacture of the core, a rod-shaped core is produced from fiber-reinforced resin by pultrusion molding (core molding process). Pultrusion molding generally involves impregnating a fiber base material made of thread-like fibers with resin, heat-curing it, and then passing it through a pultrusion die to finish the process. At this time, by using a pultrusion die of a predetermined shape and size, a rod-shaped fiber-reinforced resin of a desired size (e.g., desired outer diameter) and desired shape (e.g., circular cross-section) can be obtained.

[0061] Since the pultrusion process for fiber-reinforced resin is continuous (continuous pultrusion), the pultruded fiber-reinforced resin is cut to the desired length before use (cutting process). The desired length at this time is longer than the length of the through hole formed in the bolt body. This is to ensure a driving force transmission area for rotating the core in a later process.

[0062] As described above, once the bolt body and core are formed, the core is inserted into the through-hole of the bolt body (core insertion step). The insertion of the core into the through-hole is done manually, by lightly tapping one end of the core with a wooden mallet or the like. It is preferable that the core be inserted slowly, as in a later step the core will be rotated while the bolt body is fixed. At this time, the driving force transmission area will protrude from the bolt body. The driving force transmission area may protrude from both ends of the bolt body (the head end and the threaded end), or from just one end. If it protrudes from both ends of the bolt body, one end may be used for support and the other for driving.

[0063] After inserting the core into the through-hole of the bolt body, friction is generated between the core and the bolt body to produce frictional heat (friction process). One method for generating frictional heat is to fix the bolt body and rotate the core at high speed around its axis (circumferential direction). The bolt body can be fixed, for example, by holding the bolt shank with a vise. The core can also be rotated by an electric motor or the like, which has a rotation axis on the extension of the core's axis. If an actuator with a chuck at the tip of the rotation axis of the electric motor or the like is used, the core can be easily rotated around its axis. This rotation generates frictional heat between the surface of the through-hole of the bolt body and the surface of the core, and as the temperature rises, the thermoplastic resin of the bolt body partially melts, and then, by cooling, the two are fused together. Alternatively, the driving force transmission region may function as a fixed region, and the bolt body may be rotated while the core is fixed. Furthermore, by utilizing ultrasonic vibration, frictional heat may be generated by applying ultrasonic vibration to either the core or the bolt body. In this case, in addition to transmitting ultrasonic vibration using the driving force transmission region of the core, the driving force transmission region may be used as a fixed region to transmit ultrasonic vibration to the bolt body. In either case, if the thermoplastic resin of the bolt body can be partially melted by the action of frictional heat, the core can be fused.

[0064] As described above, the bolt body and core are integrally constructed, completing the core-sheath multi-structure FRP bolt. Since the core, used as the driving force transmission area (or fixing area), protrudes from the bolt body, the final step involves cutting off the excess portion of the core (the part protruding from the bolt body). This final step also includes removing any burrs that may have formed due to the melting of the thermoplastic resin.

[0065] By going through the processes described above, it is possible to manufacture a core-sheath multi-structure FRP bolt in which the bolt body and core are made of different materials and are integrated together. With this manufacturing method, the complicated process of wrapping string-like or tape-like fiber-reinforced resin around the threaded portion (especially the threads) to fill the fibers, as is done with conventional fiber-reinforced resin bolts, is eliminated, and the fiber-reinforced resin bolt can be completed by an extremely simple method.

[0066] <Second embodiment relating to the manufacturing method> Next, a second embodiment of the manufacturing method for a core-sheath multi-structure FRP bolt will be described. The procedure for the manufacturing method according to this embodiment is shown in Figure 7. As shown in Figure 7, the manufacturing method according to this embodiment involves first manufacturing the core portion, and then using the manufactured core portion to complete an integrated core-sheath multi-structure FRP bolt by injection molding.

[0067] First, regarding the production of the core, a rod-shaped core is produced by pultrusion molding of fiber-reinforced resin, similar to the first embodiment (core molding process). Pultrusion molding is performed by passing the material through a pultrusion die to achieve a predetermined shape and size. In this embodiment as well, since pultrusion molding is performed by continuous pultrusion, the core is cut to a predetermined length (cutting process). Here, the predetermined length to be cut is basically determined to match the length of the bolt body, as there is no need to provide any extra parts (such as a driving force transmission area), and in this case, there is no need to cut the bolt body after molding.

[0068] As described above, after the core is completed, the cut core is placed in an injection mold, and then thermoplastic resin is injection molded onto it (body molding process). When placing the core in an injection mold, it can be used like a core. In this case, both ends of the core can be held by, for example, aligning the axis vertically and clamping it between the upper and lower molds. If it is difficult to uniformly fill the area around a core of a predetermined length with resin, the core may be made longer than the bolt body and placed in the injection mold with both ends supported. In this case, a final step of cutting both ends can be added after injection molding. Alternatively, the core may be made shorter than the length of the bolt body and the entire core may be covered with thermoplastic resin. In this case, the core will be molded embedded in the bolt body, but since the area where the core is not present is part of both ends of the bolt body (the head tip and the threaded tip), it will not affect the strength of the finished FRP bolt. Even in such cases, the cross-section of the main part essentially consists of a multi-structure of core and sheath, and this also falls under the category of multi-structure FRP bolts with a core and sheath.

[0069] By going through the processes described above, the thermoplastic resin melted during injection molding covers the surface of the core, making it possible to manufacture a core-sheath multi-structure FRP bolt in which the bolt body and core made of different materials are integrated. Even in this embodiment, since it does not require the complicated processes of conventional fiber-reinforced resin bolt manufacturing methods, fiber-reinforced resin bolts can be easily manufactured.

[0070] <Variation> As described above, embodiments (first and second embodiments) relating to the manufacturing method of a core-sheath multi-structure FRP bolt have been explained, but core-sheath multi-structure FRP bolts can be manufactured by other methods. For example, a method of pressing the core into the through hole, or a method of bonding. When pressing or bonding the core into the through hole, it is a variation of the first embodiment and basically follows the same procedure as the first embodiment (see Figure 6). That is, the bolt body and the core are manufactured separately and integrated when the core is inserted into the bolt body.

[0071] In this case, when the core is pressed into the through hole, the outer diameter of the core is made larger than the inner diameter of the through hole in the bolt body, and the core is pressed into the through hole during the core insertion process. Press-fitting means inserting while applying sufficient pressure, and this is done by forcibly inserting it using a hydraulic hammer or the like. Because the outer diameter of the core is made larger than the inner diameter of the through hole, the bolt body elastically deforms to expand the through hole (compressed in the thickness direction), and this restoring force acts around the core.

[0072] On the other hand, when bonding the core to a through hole, the core surface is impregnated with an adhesive resin, the core is inserted into the through hole, and the adhesive resin is cured through a predetermined heat treatment or drying treatment. A thermosetting resin can be used as the adhesive resin.

[0073] Furthermore, even when the bolt body and core are integrated by adhesive bonding, it is crucial to ensure that there is no gap between the surface of the through-hole and the surface of the core. If a gap is formed between the two, a deformable area will be created on one of them (especially the thermoplastic resin), which can cause breakage when subjected to shear stress. Therefore, both press-fitting and adhesive bonding may be used in combination. Basically, the primary method is to integrate them through the adhesive effect, while the elastic force (restoring force) of the thermoplastic resin helps to adhere the thermoplastic resin to the surface of the core. In this case, the inner diameter of the through-hole and the outer diameter of the core may be adjusted to allow for gentler press-fitting than in the case of integration by compression bonding alone.

[0074] <Summary> As described above, embodiments and modifications relating to the manufacturing method of a core-sheath multi-structure FRP bolt allow for easy integration of the bolt body and core made of different materials. The core-sheath multi-structure FRP bolt in this integrated state has the configuration shown in the above-described embodiment of the FRP bolt. Therefore, since a thermoplastic resin with low shear strength on its own can be used as the outer shell while a fiber-reinforced resin can be placed as the core, it is possible to manufacture an FRP bolt with improved overall strength (shear strength).

[0075] The above embodiments and modifications are merely examples of the present invention, and the present invention is not limited to these embodiments. It is possible to modify each component as appropriate or to add other components.

[0076] For example, in the first embodiment of the manufacturing method, the bolt body and the core are manufactured separately, and then the core is inserted into the through hole to integrate them. Therefore, the material used for the bolt body is not limited to thermoplastic resin, but other synthetic resins can be used. [Explanation of symbols]

[0077] 1. Core-sheath multi-structure FRP bolt 2 Bolt body 3 core 10 Experimental FRP bolts 20 Through holes 21. Shaft portion of the bolt body 22 Head of the bolt body 23 Bolt Shank 24 Screw part

Claims

1. It comprises a bolt body having a threaded portion and a core portion positioned to be inserted along the axis of the bolt body, The bolt body is made of thermoplastic resin, The core portion is made of fiber-reinforced resin that has been extruded. A core-sheath multi-structure FRP bolt characterized in that the inner surface of the bolt body and the outer surface of the core are integrally formed by fusion, bonding, crimping, or a combination thereof.

2. The core-sheath multi-structure FRP bolt according to claim 1, wherein the bolt body contains at least one type of reinforcing fiber, such as carbon fiber, recycled carbon fiber, or glass fiber, in the thermoplastic resin.

3. The core-sheath multi-structure FRP bolt according to claim 1, wherein frictional heat is generated between the inner surface of the bolt body and the outer surface of the core, melting the inner surface of the thermoplastic resin and fusing it to the outer surface of the fiber-reinforced resin.

4. The core-sheath multi-structure FRP bolt according to any one of claims 1 to 3, wherein the outer diameter of the core portion is 1 / 2 or more of the outer diameter of the bolt body.

5. The core-sheath multi-structure FRP bolt according to any one of claims 1 to 3, wherein the bolt body is a molded product produced by injection molding or 3D printing.

6. A method for manufacturing a core-sheath multi-structure FRP bolt according to claim 1 or 2, A body forming step in which the bolt body is formed to have a through hole in the inside into which the core portion can be fitted, A core molding step in which the core portion is formed by drawing the fiber-reinforced resin, A cutting step of cutting the core portion to a length longer than the length of the through hole in the bolt body, so as to ensure a driving force transmission area that enables the transmission of driving force or a fixing area for fixing, A core insertion step in which the core is inserted into the through hole of the bolt body, A friction step is performed to generate friction between the bolt body and the core using the driving force transmission region or the fixing region of the core. Includes, The friction process involves generating frictional heat by rapidly rotating the core around its axis, or by applying ultrasonic vibrations to either the core or the bolt body to generate frictional heat, thereby melting the inner surface of the thermoplastic resin and fusing it to the outer surface of the fiber-reinforced resin. A method for manufacturing a core-sheath multi-structure FRP bolt, characterized by the following features.

7. A method for manufacturing a core-sheath multi-structure FRP bolt according to claim 1 or 2, A body forming step in which the bolt body is formed to have a through hole in the inside into which the core portion can be fitted, A core forming step in which the fiber-reinforced resin is drawn to form a core with a diameter larger than the diameter of the through hole provided in the bolt body, A cutting step in which the length of the core portion is cut to match the length of the through hole in the bolt body, A core press-fitting step in which the core is pressed into the through hole of the bolt body. A method for manufacturing a core-sheath multi-structure FRP bolt, characterized by including [a specific component].

8. A method for manufacturing a core-sheath multi-structure FRP bolt according to claim 1 or 2, A core molding step in which the core portion is formed by drawing the fiber-reinforced resin, A cutting step of cutting the core portion to a predetermined length, The core portion of a predetermined length, cut in the aforementioned cutting step, is placed in the center of the shaft portion of the injection molding die, and then a thermoplastic resin or a thermoplastic resin containing reinforcing fibers is injected to form the bolt body in a main body molding step. A method for manufacturing a core-sheath multi-structure FRP bolt, characterized by including [a specific component].

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