CN108561660B - Connection joint and method for fiber reinforced thermoplastic composite continuous pipe - Google Patents

Abstract

A connection joint and method for fiber reinforced thermoplastic composite continuous pipe are realized by adopting the connection of convex joint and concave joint, the convex joint, the concave joint and the composite continuous pipe body are connected into a whole. The manufacturing process of the convex surface joint and the concave surface joint is synchronously and continuously carried out with the production process of the composite continuous pipe body, the concave surface joint at one end of one composite continuous pipe is connected with the convex surface joint at one end of the other composite continuous pipe through bolts, two sealing rings are arranged between the convex surface joint and the concave surface joint, after the two composite continuous pipes are connected, the parts contacted with internal conveying media are all inner liners, the inner liners are all anti-corrosion thermoplastic plastics, therefore, the connecting joint cannot reduce the anti-corrosion performance of the pipeline, the pipeline pipe diameters are consistent, the problem that the pipe diameters of mechanical buckling type metal joint parts are reduced is solved, the conveying efficiency is not influenced, the inside of the pipeline connected by adopting the method is all anti-corrosion thermoplastic plastics, the on-site connection is easy to realize, and the installation speed is high.

Description

A fiber-reinforced thermoplastic composite continuous pipe connection joint and method

Technical Field

The invention belongs to the field of non-metallic composite pipes, and in particular relates to a connection joint and a method for a fiber-reinforced thermoplastic composite continuous pipe.

Background technique

Non-metallic pipes have excellent corrosion resistance and have become an important development direction for corrosion protection of oil and gas field gathering and transportation pipelines. Non-metallic pipes for oil and gas fields have developed from the initial glass fiber reinforced plastic pipes to various types of non-metallic composite pipes, including coilable fiber reinforced thermoplastic composite continuous pipes, steel skeleton composite pipes, flexible composite pipes, plastic alloy composite pipes, etc. The non-metallic composite continuous pipe generally adopts a three-layer structure of inner lining layer, reinforcement layer and outer protective layer. The existing non-metallic composite continuous pipe is mainly connected by mechanical crimping metal joints, that is, the metal joint is first inserted into the inner lining layer of the composite pipe body, and then the metal joint is composited with the non-metallic pipe body by external crimping or internal expansion. For example, the utility model patent with Chinese patent publication number CN201925632 U discloses a crimping composite pressure pipe joint and a pipeline connection structure, which belongs to the crimping metal joint connection form. There are local metal channels in the entire non-metallic transmission pipeline connected by crimping metal joints. The appearance of local metal channels will bring four unfavorable factors. First, the metal material is in direct contact with the transported medium. If the metal material is not properly selected, it will be corroded by the transported medium, which will affect the service life of the entire pipeline and increase the operation risk. Second, since the metal joint is inserted into the non-metallic composite continuous pipe, the diameter of the channel formed by the metal joint is smaller than the diameter of the entire pipeline, affecting the transportation efficiency of the pipeline. Third, the bonding force between the crimped metal joint and the non-metallic pipe body is only provided by the friction between the two. The bonding strength is limited and much smaller than the tensile strength of the non-metallic pipe body. There is a risk of joint slippage during construction. Fourth, for non-metallic composite pipelines used to transport high-pressure natural gas, the gas medium will slowly penetrate into the structural layer of the composite pipe from the interface between the metal joint and the inner lining of the composite pipe, causing failure of the structural layer of the composite pipe.

To this end, the prior art has also proposed some new connection methods, such as the invention patent with Chinese patent publication number CN 103267192A, which discloses a flexible composite pipe joint and its connection method. The patent proposes to use thermoplastic plastic materials to make the joint, the joint and the composite pipe body are connected by ultrasonic welding, and the joints are connected by flanges. Through experiments, it was found that the joint and its connection method proposed in the invention are only suitable for lower working pressures, because there is no structure similar to the composite pipe body reinforcement layer at the joint, and it is only made of uniform plastic material, and the pressure bearing performance of the joint is much lower than that of the composite pipe body.

Summary of the invention

The present invention aims to solve the disadvantages of the prior art by providing a fiber reinforced thermoplastic composite continuous pipe connection joint and method, so as to solve the disadvantages of mechanically crimping metal joints used to connect non-metallic composite continuous pipes.

To achieve the above object, the present invention adopts the following technical solutions:

A connection joint for a fiber-reinforced thermoplastic composite continuous pipe comprises a convex joint arranged at the end of a first composite continuous pipe body and a concave joint arranged at the end of a second composite continuous pipe body, the convex joint cooperates with the concave joint, wherein the convex joint comprises a convex metal inlay; the first composite continuous pipe body and the second composite continuous pipe body have the same structure, the first composite continuous pipe body comprises an inner lining layer, the outer side of the inner lining layer is wrapped with a reinforcing layer, and the outer side of the reinforcing layer is wrapped with an outer protective layer; the convex metal inlay is sleeved on the end of the inner lining layer of the first composite continuous pipe body, and a composite material of fiber and thermosetting resin is arranged on the outside of the convex metal inlay and the outer protective layer of the first composite continuous pipe body; the concave joint comprises a concave metal inlay cooperated with the convex metal inlay, the concave metal inlay is arranged at the end of the inner lining layer of the second composite continuous pipe body, and a composite material of fiber and thermosetting resin is arranged on the outside of the concave metal inlay and the outer protective layer of the second composite continuous pipe body.

A further improvement of the present invention is that the convex metal inlay includes a convex body, a convex shoulder is provided at the end of the convex body, and a plurality of annular protrusions are provided on the outer wall of the convex body; the concave metal inlay includes a concave body, a concave shoulder is provided at the end of the concave body, and the concave shoulder matches the convex shoulder.

A further improvement of the present invention is that the number of the annular protrusions on the convex body and the annular protrusions on the concave body are both 3 to 5; the annular protrusions on the convex body and the concave body are arranged at equal intervals, and the distance between two adjacent annular protrusions is 40 to 60 mm.

A further improvement of the present invention is that a plurality of grooves for changing the winding direction and angle of the composite material are provided on the annular protrusion on the convex body that is farthest from the convex shoulder; and a plurality of grooves for changing the winding direction and angle of the composite material are provided on the edge of the annular protrusion on the concave body that is farthest from the concave shoulder.

A further improvement of the present invention is that a plurality of screw holes are evenly opened around the convex shoulder, and a plurality of screw holes are evenly opened around the concave shoulder, and the convex joint is connected to the concave joint by bolts passing through the screw holes on the concave shoulder and the convex body.

A further improvement of the present invention is that a sealing ring is provided between the convex shoulder and the concave shoulder.

A further improvement of the present invention is that the winding angle of the composite material of the fiber and the thermosetting resin is 65° to 85°, and the winding thickness is 8 to 50 mm.

A method for connecting a joint, wherein the manufacturing process of the convex joint is synchronously and continuously carried out with the production process of the composite continuous pipe body, after the convex metal inlay is installed on the end of the inner lining layer of the first composite continuous pipe, the end of the inner lining layer connected to the convex metal inlay is thickened and flanging processed, and then the fiber continuous with the reinforcement layer and the thermosetting resin are composited and wound on the composite continuous pipe body and the convex metal inlay, so that the convex metal inlay is completely placed in the composite material of the fiber and the thermosetting resin, and the convex joint is manufactured after curing, and the reinforcement layer and the outer protective layer are wrapped on the inner lining layer in sequence;

The manufacturing process of the concave joint is carried out synchronously and continuously with the production process of the composite continuous pipe body. After the concave metal inlay is installed on the end of the inner lining layer of the second composite continuous pipe, the end of the inner lining layer connected to the concave metal inlay is thickened and flanging processed, and then it is compounded with the continuous fiber and thermosetting resin of the reinforcement layer and then wound on the composite continuous pipe body and the concave metal inlay, so that the concave metal inlay is completely placed in the composite material of the fiber and the thermosetting resin. After curing, the concave joint is completed, and the reinforcement layer and the outer protective layer are wrapped on the inner lining layer in sequence; the convex joint is connected to the concave joint with bolts.

A further improvement of the present invention is that the angle of winding of the composite material of the fiber and the thermosetting resin is 65° to 85°, and the thickness of the winding is 8 to 50 mm; and the material of the convex metal inlay is steel.

A further improvement of the present invention is that the composite material of fiber and thermosetting resin is prepared by the following process: the fiber is impregnated in a thermosetting resin; wherein the fiber is glass fiber, carbon fiber, basalt fiber, polyester fiber or aramid fiber; and the thermosetting resin is epoxy resin, unsaturated polyester resin or vinyl ester resin.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention realizes the connection of fiber reinforced thermoplastic composite continuous pipes by providing a convex joint and a concave joint matched with the convex joint, and a convex metal inlay is sleeved on the end of the inner lining layer of the first composite continuous pipe body, and the concave metal inlay is provided at the end of the inner lining layer of the second composite continuous pipe body, and the concave metal inlay cooperates with the convex metal inlay. The present invention has a strong working pressure, and the joint will not slip during the construction project, and has good pressure bearing performance.

Furthermore, the male connector is connected to the female connector by bolts passing through the female shoulder and the male body, which is easy to install.

The present invention proposes a method for connecting a coilable fiber reinforced thermoplastic composite continuous pipe, which is realized by connecting a convex joint and a concave joint. The convex joint, the concave joint and the composite continuous pipe body are connected as a whole, so the tensile strength of the joint after connection is not lower than the tensile strength of the composite pipe body. The metal inlay in the concave joint has an annular protrusion that increases the interface area with the fiber and the thermosetting resin, which can greatly increase the bonding strength between the metal inlay and the composite continuous pipe, avoiding the problem of easy disengagement of similar mechanical buckle-type joints. The concave joint at one end of a coilable fiber reinforced thermoplastic composite continuous pipe and the convex joint at one end of another coilable fiber reinforced thermoplastic composite continuous pipe are connected. The face joints are connected by bolts, and two sealing rings are installed between the convex joint and the concave joint. After the two composite continuous pipes are connected, the parts in contact with the internal conveying medium are all lining layers, and the lining layers are all corrosion-resistant thermoplastic plastics. Therefore, the connecting joints will not reduce the corrosion resistance of the pipeline, and the pipeline diameters are consistent. There is no problem of the diameter of some mechanical crimping metal joints becoming smaller, and the transportation efficiency is not affected. The interior of the pipeline connected by this method is all corrosion-resistant thermoplastic plastics. Not only is the interior of the pipeline resistant to corrosion by the conveying medium, but the partially exposed martensitic stainless steel on the outside of the pipeline can also prevent external corrosion of the pipeline joints by the external environment. The service life is long, and this method is easy to connect on site and has a fast installation speed.

Furthermore, in the present invention, the end of the lining layer connected to the convex metal inlay is thickened and flanging is performed, which can increase the sealing performance between the lining layer and the convex joint end face. The end of the lining layer connected to the concave metal inlay is thickened and flanging is performed, which can increase the sealing performance between the lining layer and the concave joint end face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic longitudinal cross-sectional view of a composite coiled tubing with a convex joint;

FIG2 is a schematic diagram of a longitudinal section of a composite coiled tubing with a concave joint;

FIG3 is a schematic diagram of a composite coiled tube connection.

Among them: 1. lining layer; 2. reinforcement layer; 3. protective layer; 4. convex metal inlay; 5. annular protrusion; 6. screw hole; 7. concave metal inlay; 8. first sealing ring; 9. bolt; 10. second sealing ring; 11. convex body; 12. convex shoulder; 13. concave body; 14. concave shoulder.

Detailed ways

The technical solution of the present invention is further described in detail below through embodiments.

Referring to Fig. 1, Fig. 2 and Fig. 3, a connection joint of a coilable fiber reinforced thermoplastic composite continuous pipe provided by the present invention comprises a convex joint arranged at the end of a first composite continuous pipe body and a concave joint arranged at the end of a second composite continuous pipe body, the convex joint cooperates with the concave joint, wherein the convex joint comprises a convex metal inlay 4; the first composite continuous pipe body and the second composite continuous pipe body have the same structure, the first composite continuous pipe body comprises an inner lining layer 1, the outer side of the inner lining layer 1 is wrapped with a reinforcement layer 2, and the outer side of the reinforcement layer 2 is wrapped with an outer protective layer 3; the convex metal inlay 4 is sleeved on the end of the inner lining layer 1 of the first composite continuous pipe body, and the outer side of the convex metal inlay 4 and the outer protective layer 3 of the first composite continuous pipe body are provided with a composite material of fiber and thermosetting resin; the convex metal inlay 4 comprises a convex body 11, the end of the convex body 11 is provided with a convex shoulder 12, and the outer wall of the convex body 11 is provided with 3 to 5 annular protrusions 5;

The concave joint includes a concave metal inlay 7 matched with the convex metal inlay 4, the concave metal inlay 7 is arranged at the end of the inner lining layer 1 of the second composite continuous pipe body, and the outer side of the concave metal inlay 7 and the outer protective layer 3 of the second composite continuous pipe body is provided with a composite material of fiber and thermosetting resin. The concave metal inlay 7 includes a concave body 13, and a concave shoulder 14 is arranged at the end of the concave body 13, and the concave shoulder 14 matches the convex shoulder 12.

The annular protrusions 5 on the convex body 11 and the annular protrusions 5 on the concave body 13 are both 3 to 5; the annular protrusions 5 on the convex body 11 and the concave body 13 are arranged at equal intervals, and the distance between two adjacent annular protrusions 5 is 40 to 60 mm. The annular protrusions 5 on the outer wall of the convex body 11 are used to enhance the bonding strength between the convex metal inlay 4 and the composite continuous pipe body. The annular protrusions 5 on the outer wall of the concave body are used to enhance the bonding strength between the concave metal inlay 7 and the composite continuous pipe body.

The annular protrusion on the convex body 11 farthest from the convex shoulder 12 is provided with four grooves for changing the winding direction and angle of the composite material; the annular protrusion edge on the concave body 13 farthest from the concave shoulder 14 is provided with four grooves for changing the winding direction and angle of the composite material.

The convex shoulder 12 is evenly provided with a plurality of screw holes 6 in the circumferential direction, and the concave shoulder 14 is evenly provided with eight screw holes 6 in the circumferential direction. The convex joint is connected to the concave joint by bolts passing through the screw holes 6 on the concave shoulder 14 and the convex body 11 .

A sealing ring is arranged between the convex shoulder 12 and the concave shoulder 14 .

The winding angle of the composite material of the fiber and the thermosetting resin is 65° to 85°, and the winding thickness is 8 to 50 mm.

The connection method of the above-mentioned connection joint is as follows:

The manufacturing process of the convex joint is synchronously and continuously carried out with the production process of the composite continuous pipe body. After the convex metal inlay 4 is installed on the end of the lining layer 1 of the first composite continuous pipe body, the end of the lining layer 1 connected to the convex metal inlay 4 is thickened and flanging processed, and then the fiber continuous with the reinforcement layer 2 and the thermosetting resin are composited and wound on the first composite continuous pipe and the convex metal inlay 4 at a certain angle and a certain thickness, so that the convex metal inlay 4 is completely placed in the composite material of the fiber and the thermosetting resin. After curing, the convex joint is completed, and the reinforcement layer 2 and the outer protective layer 3 are wrapped on the lining layer 1 in sequence;

The manufacturing process of the concave joint is the same as that of the convex joint, except that the convex metal inlay 4 is replaced by the concave metal inlay 7. Specifically, the manufacturing process of the concave joint is carried out synchronously and continuously with the production process of the composite continuous pipe body. After the concave metal inlay 7 is installed on the end of the inner lining layer 1 of the second composite continuous pipe, the end of the inner lining layer 1 connected to the concave metal inlay 7 is thickened and flanging processed, and then the fiber continuous with the reinforcement layer 2 and the thermosetting resin are composited (the fiber and the thermosetting resin are composited to impregnate the fiber with the thermosetting resin, and the fiber and the thermosetting resin composite material is obtained after the impregnation). After that, the concave metal inlay 7 is wrapped at a certain angle and a certain thickness on the second composite continuous pipe body and the concave metal inlay 7, so that the concave metal inlay 7 is completely placed in the composite material of the fiber and the thermosetting resin. After curing, the concave joint is completed, and the reinforcement layer 2 and the outer protective layer 3 are wrapped on the inner lining layer 1 in sequence; the convex joint is connected to the concave joint by bolts.

The composite material of fiber and thermosetting resin is wound on the convex metal inlay 4 and the outer protective layer 3, the winding angle of the composite material of fiber and thermosetting resin is 65° to 85°, and the winding thickness is 8 to 50 mm.

The material of the convex metal inlay 4 is steel, preferably martensitic stainless steel.

The fiber material of the composite material of fiber and thermosetting resin is preferably the same continuous fiber as the fiber material of the reinforcing layer of the composite continuous pipe body.

The composite material of fiber and thermosetting resin is prepared by the following process: the fiber is impregnated in the thermosetting resin, wherein the fiber is inorganic or organic fiber, preferably glass fiber, carbon fiber, basalt fiber, polyester fiber or aramid fiber; the thermosetting resin is preferably epoxy resin, unsaturated polyester resin or vinyl ester resin.

In the present invention, the end of the lining layer 1 connected to the convex metal inlay 4 is thickened and flanging-processed, so as to increase the sealing performance between the lining layer and the end face of the convex joint.

The end of the lining layer 1 connected to the concave metal inlay 7 is thickened and flanging-processed, so as to increase the sealing performance between the lining layer and the end face of the concave joint.

The convex joint is connected to the concave joint by bolts passing through the screw holes 6 on the concave shoulder 14 and the convex body 11. When connected, a first sealing ring 8 is arranged between the convex surface of the convex shoulder 12 and the concave surface of the concave shoulder 14, and a second sealing ring 10 is arranged between the convex shoulder 12 and the concave shoulder 14.

The material of the concave metal inlay 7 is the same as that of the convex metal inlay 4. After the convex joint of one coilable fiber reinforced thermoplastic composite continuous pipe is connected with the concave joint of another coilable fiber reinforced thermoplastic composite continuous pipe, the coilable fiber reinforced thermoplastic composite continuous pipes can be connected. The size of the convex joint can match the size of the concave joint to achieve a satisfactory sealing effect.

Example 1: Connection method of DN80mm PN26MPa coilable glass fiber reinforced high-density polyethylene composite continuous pipe

The composite continuous pipe inner lining layer 1 is made of high-density polyethylene, the reinforcement layer 2 is glass fiber wound on the inner lining layer 1, and the outer protective layer 3 is polyethylene coated on the reinforcement layer 2. The convex joint is composed of three parts: the inner lining layer 1 of the composite continuous pipe, the convex metal inlay 4, and the composite material of glass fiber and thermosetting resin. The inner lining layer 1 is high-density polyethylene, and the convex metal inlay 4 is made of 1Cr13 martensitic stainless steel. The 1Cr13 martensitic stainless steel convex metal inlay is processed with three evenly distributed annular protrusions. The production process of the convex joint is synchronously and continuously carried out with the composite continuous pipe body production (winding) process. After the 1Cr13 martensitic stainless steel convex metal inlay is installed at the end of the high-density polyethylene inner lining layer 1 of the composite continuous pipe body, the high-density polyethylene inner lining layer connected to the convex metal inlay is 1 The end is thickened and flanging treated, and a composite material of glass fiber and epoxy resin is wound on the installed 1Cr13 martensitic stainless steel convex metal inlay and the pipe body reinforcement layer 2, wherein the epoxy resin, calculated by mass, includes 100 parts of 6207 epoxy, 138 parts of 647 anhydride, 1.5 parts of dimethylaniline, 8 parts of liquid nitrile and 8 parts of acetone, and the winding angle is 75°. After winding, it is cured at 85°C for 12 hours to complete the production; the production process of the concave joint is the same as that of the convex joint.

The convex joint of one composite continuous pipe is fastened with the concave joint of another composite continuous pipe by bolts 9, and a sealing ring is added during the connection to achieve the connection of the entire pipeline. After the connection, a verification test was carried out, including a long-term hydrostatic pressure test and a gas sealing test. The pressure value of the long-term hydrostatic pressure is 39MPa, and the pressure is maintained for 24 hours. There is no leakage at the pipe body and joints. The gas pressure value of the gas sealing test is 33MPa, and the pressure is maintained for 8 hours. There is no leakage at the pipe body and joints.

Claims (6)

1. The connecting joint of the fiber reinforced thermoplastic composite continuous pipe is characterized by comprising a convex joint arranged at the end part of a first composite continuous pipe body and a concave joint arranged at the end part of a second composite continuous pipe body, wherein the convex joint is matched with the concave joint, and the convex joint comprises a convex metal inlay (4); the first composite continuous pipe body and the second composite continuous pipe body have the same structure, the first composite continuous pipe body comprises an inner liner (1), the outer side of the inner liner (1) is wrapped with a reinforcing layer (2), and the outer side of the reinforcing layer (2) is wrapped with an outer protective layer (3); the convex metal inlay (4) is sleeved at the end part of the inner liner (1) of the first composite continuous pipe body, and a composite material of fiber and thermosetting resin is arranged outside the convex metal inlay (4) and the outer protective layer (3) of the first composite continuous pipe body; the concave surface joint comprises a concave surface metal inlay (7) matched with the convex surface metal inlay (4), the concave surface metal inlay (7) is arranged at the end part of the inner liner (1) of the second composite continuous pipe body, and the outer sides of the concave surface metal inlay (7) and the outer protective layer (3) of the second composite continuous pipe body are provided with composite materials of fiber and thermosetting resin;

the convex metal inlay (4) comprises a convex body (11), a convex shoulder (12) is arranged at the end part of the convex body (11), and a plurality of annular bulges (5) are arranged on the outer wall of the convex body (11); the concave metal inlay (7) comprises a concave body (13), a concave shoulder (14) is arranged at the end part of the concave body (13), and the concave shoulder (14) is matched with the convex shoulder (12);

The number of the annular protrusions (5) on the convex body (11) and the number of the annular protrusions (5) on the concave body (13) are 3-5; annular protrusions (5) on the convex body (11) and the concave body (13) are arranged at equal intervals, and the distance between two adjacent annular protrusions (5) is 40-60 mm;

The annular bulge furthest from the convex shoulder (12) on the convex body (11) is provided with a plurality of grooves for changing the winding direction and angle of the composite material; the annular convex edge of the concave body (13) farthest from the concave shoulder (14) is provided with a plurality of grooves for changing the winding direction and angle of the composite material;

A plurality of screw holes (6) are uniformly formed in the circumferential direction of the convex shoulder (12), a plurality of screw holes (6) are uniformly formed in the circumferential direction of the concave shoulder (14), and the convex connector is connected with the concave connector through bolts penetrating through the screw holes (6) in the concave shoulder (14) and the convex body (11).

2.A connector for a fibre reinforced thermoplastic composite continuous tube according to claim 1, characterised in that a sealing ring is arranged between the convex shoulder (12) and the concave shoulder (14).

3. The connector of a fiber reinforced thermoplastic composite continuous pipe according to claim 1, wherein the composite material of the fiber and the thermosetting resin is wound at an angle of 65-85 degrees and a thickness of 8-50 mm.

4. A method for connecting a joint according to claim 1, characterized in that the manufacturing process of the convex joint is carried out synchronously and continuously with the production process of the composite continuous pipe body, after the convex metal inlay (4) is installed on the end part of the inner liner (1) of the first composite continuous pipe, thickening and flanging the end part of the inner liner (1) connected with the convex metal inlay (4), then adopting fiber and thermosetting resin continuous with the reinforcing layer (2) to be compounded and then winding on the composite continuous pipe body and the convex metal inlay (4), so that the convex metal inlay (4) is completely placed in the composite material of the fiber and the thermosetting resin, and the manufacturing of the convex joint is completed after curing, and simultaneously, the reinforcing layer (2) and the outer protective layer (3) are sequentially wrapped on the inner liner (1);

The manufacturing process of the concave surface joint is synchronously and continuously carried out with the production process of the composite continuous pipe body, after the concave surface metal inner inlay (7) is installed on the end part of the inner liner (1) of the second composite continuous pipe, the end part of the inner liner (1) connected with the concave surface metal inner inlay (7) is thickened and flanging, then the fiber continuous with the reinforcing layer (2) and the thermosetting resin are compounded and then wound on the composite continuous pipe body and the concave surface metal inner inlay (7), so that the concave surface metal inner inlay (7) is completely placed in the composite material of the fiber and the thermosetting resin, the manufacturing of the concave surface joint is completed after curing, and meanwhile, the reinforcing layer (2) and the outer protection layer (3) are sequentially wrapped on the inner liner (1); the male adapter is connected to the female adapter by bolts.

5. The connection method according to claim 4, wherein the angle of winding the composite material of the fiber and the thermosetting resin is 65-85 degrees, and the thickness of winding is 8-50 mm; the convex metal inlay (4) is made of steel.

6. The method of joining according to claim 4, wherein the composite of fibers and thermosetting resin is prepared by: impregnating fibers in a thermosetting resin; wherein the fiber is glass fiber, carbon fiber, basalt fiber, polyester fiber or aramid fiber; the thermosetting resin is epoxy resin, unsaturated polyester resin or vinyl ester resin.