KR20220096026A - High-pressure tank and making method thereof - Google Patents

Abstract

Provided is a pressure vessel, comprising: a liner accommodating a gas; a composite material layer formed outside the liner; and a shear stress absorbing layer having viscoelasticity between the liner and the composite material layer. An objective of the present invention is to provide the pressure vessel capable of imparting additional durability to the liner.

Description

Pressure vessel and its manufacturing method {HIGH-PRESSURE TANK AND MAKING METHOD THEREOF}

The present invention relates to a pressure vessel. More specifically, the present invention relates to a pressure vessel capable of preventing breakage of a liner due to a shear fatigue load generated at an interface between a liner and a composite layer, and a method for manufacturing the same.

A pressure vessel is stored and transported in a pressurized state in which various gases such as oxygen, natural gas, and hydrogen are inside, and an airtight structure is required to prevent leakage of the stored gas.

For pressure vessels, Type I made of metal, Type II, which is lighter by adding a composite material to the metal liner, and Type III, in which the composite material is reinforced with a full wrap on the metal liner are mainly used. Type IV pressure vessels with reinforcement are mainly being developed.

Composite-reinforced pressure vessels usually include a liner and a composite layer formed by a filament winding method on the outside of the liner. When high-pressure gas is repeatedly charged and discharged, a shear load is generated between the liner and the composite layer.

When the shear load occurs, cracks may occur due to friction between the liner and the composite material layer, and the pressure vessel is ruptured, thereby reducing the lifespan.

A method of dispersing stress by treating a release agent or applying a lubricant to suppress adhesion between the liner and the composite layer reinforcing the liner is disclosed, but occurs at the entire outer peripheral surface of the liner and the interface between the liner and a metal boss selected from a different material A pressure vessel in consideration of the stress required has not yet been disclosed.

Therefore, there is an urgent need to develop a pressure vessel that effectively increases the lifespan of the pressure vessel by more effectively dispersing fatigue stress during charging and discharging in a pressure vessel having a different material boss.

Background art of the present invention is described in Korean Patent Application Laid-Open No. 10-2020-0090455.

The liner of the pressure vessel of the present invention increases the durability of the liner by giving resistance to buckling deformation due to expansion and contraction during charging and discharging of high-pressure gas, and additional durability to the liner by adjusting the thickness of the shear stress absorbing layer This is to provide a pressure vessel that can give

Another object of the present invention is to provide a method for manufacturing a pressure vessel capable of effectively dispersing stress by forming a direct shear stress absorbing layer through surface treatment and spray application after forming a liner.

The above and other objects of the present invention can all be achieved by the present invention described below.

1. One aspect of the present invention relates to a pressure vessel.

The pressure vessel may include a liner for accommodating a gas; and a composite material layer formed outside the liner, and a shear stress absorbing layer having viscoelasticity between the liner and the composite material layer.

2. In the first embodiment, the shear stress absorbing layer may satisfy the following equation:

[Equation 1]

M ₂ < M ₁ , M ₃

In Equation 1, M ₂ is the modulus of elasticity of the shear stress absorbing layer, M ₁ is the modulus of elasticity of the liner, and M ₃ is the modulus of elasticity of the composite layer.

[Equation 2]

E ₂ > E ₁ , E ₃

In Equation 2, E ₂ is the elongation of the shear stress absorbing layer, E ₁ is the elongation of the liner, and E ₃ is the elongation of the composite layer.

3. In embodiments 1 or 2, the liner may have a reactive functional group formed on its surface, or irregularities may be formed on its surface.

4. In any one of 1 to 3, the shear stress absorbing layer is silicone, polyurethane, polyurea, styrene-butadiene-rubber (SBR), nitrile-butadiene-rubber (NBR), and ethylene-propylene- It may be one or more types of rubber (EPDM).

5. In any one of embodiments 1 to 4, the shear stress absorbing layer may have a thickness of 1 mm to 5 mm.

6. In any one of the embodiments 1 to 5, the shear stress absorbing layer may have a tensile strength of 10 Mpa or less according to ASTM D638, and an elongation of 150% or more.

7. Another aspect of the present invention relates to a method for manufacturing a pressure vessel.

The pressure vessel manufacturing method comprises the steps of (a) surface-treating a liner; (b) forming a shear stress absorbing layer on the liner; and (c) forming a composite layer on the shear stress absorbing layer.

8. In the seventh embodiment, the surface treatment of the liner may be performed by one of plasma, corona, flame, sanding, grinding, and polishing.

9. In the 7th or 8th embodiment, the shear stress absorbing layer may be formed to have a uniform thickness over the upper portion of the connection interface between the boss and the liner, the boss, and the entire outer circumferential surface of the liner.

10. In any one of the embodiments 7 to 9, step (b) may be formed by spraying the shear stress absorbing layer to a thickness of 1 mm to 5 mm.

The present invention can greatly increase the durability of the liner by alleviating the buckling phenomenon of the liner that may occur during charging and discharging of the pressure vessel.

In particular, it is possible to increase the durability of the pressure vessel by preventing the buckling phenomenon and damage of the liner due to the difference in thermal expansion coefficient at the interface between the composite layer and the liner with different physical properties. By adjusting the thickness, it is possible to increase the manufacturing efficiency of the pressure vessel.

In addition, it provides a pressure vessel manufacturing method with increased efficiency of the pressure vessel manufacturing process through a simple process of forming a shear stress absorbing layer directly by a spray coating method after manufacturing a liner and forming a composite material layer through filament winding.

1 is a perspective view showing a partially cut-away surface of a pressure vessel according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of the pressure vessel according to FIG. 1 .
3 is a view showing the magnitude and direction of the shear force acting on the liner and the composite material of FIG. 1, respectively.
4 is a process flow diagram of a method for manufacturing a pressure vessel according to another embodiment of the present invention.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. However, the following drawings are provided only to help the understanding of the present invention, and the present invention is not limited by the drawings. In addition, since the shape, size, ratio, angle, number, etc. disclosed in the drawings are exemplary, the present invention is not limited to the illustrated matters.

Like reference numerals refer to like elements throughout. In addition, in describing the present invention, if it is determined that a detailed description of a related known technology may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted.

When 'including', 'having', 'consisting', etc. mentioned in this specification are used, other parts may be added unless 'only' is used. When a component is expressed in the singular, cases including the plural are included unless otherwise explicitly stated.

In interpreting the components, it is construed as including an error range even if there is no separate explicit description.

When the positional relationship between two parts is described with ‘on’, ‘on’, ‘on’, ‘beside’, etc., unless ‘directly’ or ‘directly’ is used, between the two parts One or more other portions may be located.

Positional relationships such as 'upper', 'top', 'lower', and 'bottom' are only described based on the drawings, and do not represent absolute positional relationships. That is, the positions of 'upper' and 'lower' or 'upper surface' and 'lower surface' may be changed according to the observed position.

In the present specification, "a to b" representing a numerical range is defined as "≥a and ≤b".

Hereinafter, a pressure vessel according to an embodiment of the present invention will be described with reference to the drawings.

1 is a perspective view showing a partially cut-away surface of a pressure vessel according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view of the pressure vessel according to FIG. 1 .

1 and 2 , the pressure vessel according to the present invention includes a liner 100 , a shear stress absorbing layer 300 , and a composite material layer 400 .

The liner 100 accommodates gases such as hydrogen, oxygen, natural gas, and nitrogen.

The liner 100 is selected to be non-reactive with the gas.

The liner 100 may be a thermoplastic resin including at least one of nylon resin, ethylene vinyl alcohol resin, polyamide resin, polyester resin, polyolefin resin, polycarbonate resin, polyphenylene ether resin, and polystyrene resin. When the liner 100 is made of the above type of material, it has corrosion resistance, can have resistance to deformation and breakage of the pressure vessel due to pressure generated during gas filling, can implement excellent mechanical strength, and processability Because it is good, it can be manufactured by molding the liner in various ways. In a specific embodiment, it may be a nylon resin (PA11) which is advantageous for weight reduction and has high gas barrier properties.

The liner 100 is not limited in shape as long as gas can be introduced and stored therein.

The liner 100 may be in the form of a cylinder having a dome shape at both ends, and it is also possible to select a container in the form of a ball or a donut.

Although the shape of the liner 100 is limited in consideration of the stress, when the shear stress absorbing layer 300 is provided, the stress can be effectively dispersed throughout the liner 100, so the shape of the liner 100 can be determined more freely. can

In an embodiment the thickness of the liner is between 1 mm and 30 mm.

When the liner 100 is provided with a thickness within the above range, it is possible to lighten the pressure vessel and maintain a constant gas barrier property. It is difficult to prevent the phenomenon, and when it exceeds the above range, there is a problem in that the effect of forming the shear stress absorbing layer 300 is reduced.

The liner 100 may have reactive functional groups or irregularities formed on its surface.

The liner 100 is chemically or physically surface-treated after molding.

In an embodiment, when the liner 100 is made of the above type of polymer resin, reactive functional groups such as -OH and -COOH may be formed on the surface of the liner by plasma treatment.

In an embodiment, the surface-treated liner may have irregularities and a surface roughness (Ra, roughness) of 1.5 μm to 2.5 μm.

When the unevenness is formed within the above range, the shear stress absorbing layer 300 of the liner 100 may be effectively adhered.

In an embodiment, when the liner 100 is non-polar, the surface treatment may be performed to effectively increase the adhesive force with the shear stress absorbing layer 300 . When the liner 100 is surface-treated, the adhesive force with the shear stress absorbing layer 300 increases, so that the shear stress absorbing layer 300 is not separated from the surface of the liner 100 .

When filled with high-pressure gas, a general liner expands rapidly and generates stress in the tangential direction of the force applied to the pressure. , when the buckling phenomenon occurs, friction occurs with the outermost composite layer, and the liner and the composite layer are separated.

Since the liner 100 is surface-treated and the shear stress absorbing layer 300 having viscoelasticity is adhered to the liner 100, the liner 100 can be fixedly supported to prevent buckling of the liner 100 .

In an embodiment, the liner 100 includes a boss 200 on one side.

The boss part 200 is formed of a metal, which is a material different from that of the liner 100 . Specifically, it is provided with aluminum or stainless steel capable of supporting a pressure of 700 bar to 900 bar.

The boss part 200 may be formed by fastening after manufacturing the liner 100 by blow molding, or by bonding after forming the liner by rotational molding, and injection after inserting the boss part 200 into the mold. It is also possible that the compression type liner 100 is formed at one time.

Like the liner 100 , the outer peripheral surface of the boss 200 is chemically or physically surface-treated after molding.

Of the outer peripheral surface of the boss part 200, only the interface portion 210 in contact with the shear stress absorbing layer 300 may be surface-treated.

When the boss part 200 is stainless steel or aluminum, it is preferable to surface-treat the surface area 210 by treating the surface by a physical method such as grinding or sanding rather than plasma treatment.

The shear stress absorbing layer 300 is uniformly formed on the entire outer surface of the boss part 200 , the upper interface formed by bonding the liner 100 and the boss part 200 , and the entire outer peripheral surface of the liner 100 .

The shear stress absorbing layer 300 is adhesively disposed on the interface between the liner 100 and the boss 200 even when the boss 200 is made of a different material.

The shear stress absorbing layer 300 is formed on the entire outer peripheral surface of the boss part 200 and the liner 100 and has viscoelasticity.

The shear stress absorbing layer 300 is provided to have viscoelasticity between the liner 100 and the composite material layer 300 to absorb the shear fatigue load at the interface between the liner 100 and the composite material layer 300 and dissipate energy. to prevent buckling.

The shear stress absorbing layer 300 satisfies Equations 1 and 2 below.

[Equation 1]

M ₂ < M ₁ , M ₃

In Equation 1, M ₂ is the modulus of elasticity of the shear stress absorbing layer, M ₁ is the modulus of elasticity of the liner, and M ₃ is the modulus of elasticity of the composite layer.

[Equation 2]

E ₂ > E ₁ , E ₃

In Equation 2, E ₂ is the elongation of the shear stress absorbing layer, E ₁ is the elongation of the liner, and E ₃ is the elongation of the composite layer.

The shear stress absorbing layer 300 satisfies Equations 1 and 2, and has a lower elastic modulus and higher elongation than the liner 100 and the composite material layer 400. Adhesion between the liner 100 and the composite material layer 400 It can absorb and dissipate the shear fatigue load.

In an embodiment, the shear stress absorbing layer 300 is one of silicone, polyurethane, polyurea, styrene-butadiene-rubber (SBR), nitrile-butadiene-rubber (NBR), and ethylene-propylene-rubber (EPDM). may be more than

The shear stress absorbing layer 300 can be adhered not only to the outer peripheral surface of the liner 100 but also to the metal boss 200, and can be adhered by painting or spray application.

The shear stress absorbing layer 300 has a thickness of 1 mm to 5 mm.

When the thickness of the shear stress absorbing layer 300 is less than 1 mm, it is difficult to dissipate energy by sufficiently absorbing the stress generated in the liner 100 and the shear fatigue load transmitted between the liner and the composite layer 400, and the adhesive force As this is reduced, a problem of separation from the liner 100 may occur. When it exceeds 5 mm, it is difficult to form a uniform thickness, and it is disadvantageous to weight reduction of the pressure vessel, and there is a problem in that the manufacturing cost is increased.

The shear stress absorbing layer 300 has a tensile strength of 10 Mpa or less and an elongation of 150% or more at a specimen thickness of 1 mm to 5 mm according to ASTM D638. In a specific embodiment, the tensile strength is 1 MPa to 10 Mpa, more preferably 7 to 8 MPa.

When the tensile strength of the shear stress absorbing layer 300 exceeds 10 MPa, the stress generated in the liner 100 and the boss 200 cannot be effectively absorbed, and it is difficult to distribute the shear fatigue load.

In an embodiment, the shear stress absorbing layer 300 has an elongation of 150% or more according to ASTM D638. in an embodiment from 150% to 500%. When the elongation is less than 150%, the shear stress absorbing layer 300 does not change in response to the expansion of the liner 100, so that the fatigue load dispersing ability is deteriorated.

3 is a view showing the magnitude and direction of the shear force acting on the liner and the composite material of FIG. 1, respectively.

Referring to FIG. 3 , when the pressure vessel 1000 is charged, a stress S is generated due to the rapid expansion of the liner 100 , but the composite layer 400 receives a relatively small stress s, and even during discharge. Although the composite material layer 400 is subjected to a smaller stress (s) than the stress (S) formed in the liner 100, a shear fatigue load is generated, but when the shear stress absorbing layer 300 is formed within the above range, the liner ( 100) and the composite material layer 400 can absorb and dissipate the shear fatigue load in a state of being adhered to it.

In an embodiment, when the liner 100 is in the form of a cylinder having a dome shape at both ends, the stress generated in the cylinder is much greater than that of the dome. Since stress must be absorbed from the entire outer circumferential surface of the cylinder portion of the liner 100, when the reinforcing material layer and the release layer are disposed in the portion where the stress is concentrated, the manufacturing efficiency of the pressure vessel is greatly reduced, but the shear stress absorbing layer 300 having viscoelasticity ) is applied directly to the outside of the liner to form uniformly, it is possible to effectively prevent the buckling phenomenon caused by the stress generation of the liner 100 while reducing the work man-hours, thereby preventing the occurrence of shear fatigue load.

The composite layer 400 is disposed on the shear stress absorbing layer 300 .

In a specific embodiment, the composite layer 400 is formed by impregnating carbon fiber, glass fiber, or synthetic polyamide fiber with an epoxy resin, etc., and then winding it on the shear stress absorbing layer 300 .

Since the composite layer 400 has a different elongation and a coefficient of thermal expansion from the liner 100 , shear stress occurs due to the expansion of the liner when the pressure vessel is charged and discharged with a high-pressure gas. When the shear stress absorbing layer 300 is disposed between the composite material layer 400 and the liner 100, the shear fatigue load can be dissipated at the interface due to the difference in physical properties between the liner 100 and the composite material layer 400. .

Another aspect of the present invention relates to a method for manufacturing a pressure vessel.

4 is a process flowchart of a method for manufacturing a pressure vessel according to another embodiment of the present invention.

The pressure vessel manufacturing method comprises the steps of (a) surface-treating a liner; (b) forming a shear stress absorbing layer on the liner; and (c) forming a composite layer on the shear stress absorbing layer.

Referring to FIG. 4 , first, a liner having a boss is manufactured, and the liner is surface-treated (S100).

In a specific embodiment, a liner material is put into a mold, and a liner is manufactured using a rotor molding that rotates by heating, and the boss part made of a stainless material is joined.

The surface treatment of the liner is performed by one of plasma, corona, flame, sanding, grinding, and polishing. With this type of surface treatment, reactive functional groups may be formed on the outer circumferential surface of the liner, or unevenness may be formed to effectively adhere the shear stress absorbing layer.

In one embodiment, it is desirable to plasma treat the surface of the liner to form reactive functional groups on the surface of the liner.

A portion of the surface of the outer surface of the boss may be subjected to grinding or sanding to increase surface roughness, thereby forming irregularities.

A shear stress absorbing layer is formed on the upper portion of the liner (S200).

In an embodiment, in S200, the shear stress absorbing layer may be formed by spray coating. When the shear stress absorbing layer is directly formed by spray coating, the working efficiency can be increased and the shear stress absorbing layer can be formed with a uniform thickness.

The thickness of the shear stress absorbing layer may be 1 mm to 5 mm. In the case of forming the shear stress absorbing layer within the above range, it adheres to the liner to support the composite layer, and while increasing the gas barrier property, effectively dissipates the shear fatigue load occurring between the composite layers to prevent buckling of the liner, and pressure Increases the durability of the container.

The shear stress absorbing layer is formed on the upper portion of the connection interface between the boss portion and the liner, the boss portion, and the outer peripheral surface of the liner portion to a uniform thickness.

A composite layer is formed on the shear stress absorbing layer (S300).

The composite layer is formed by a filament winding method in which carbon fibers are impregnated with an epoxy resin and then wound on a liner.

In another embodiment, when the composite layer is wound on the liner in the form of a towpreg, it may be subjected to surface treatment such as plasma treatment to increase adhesion with the shear stress absorbing layer.

Hereinafter, preferred examples are presented to help the understanding of the present invention, but the following examples are only illustrative of the present invention and the scope of the present invention is not limited to the following examples.

Example 1. Preparation of pressure vessel

Nylon resin (PA11, Arkema) was put into the mold, the liner was manufactured to a thickness of 2 mm by rotor molding, and the surface was treated by plasma treatment (200 W, 3 minutes) in a plasma generator to adjust the surface roughness to 2.5 μm.

A boss part made of SUS316L was prepared, and the flange and neck parts were sanded and surface-treated so that the average roughness (Ra) of the center line was 50 µm.

The boss part was bonded to the liner for 700 bar, polyurethane resin was sprayed on, dried at 90° C. for 20 minutes, and this was repeated to form a shear stress absorbing layer with a thickness of 5 mm.

The carbon fiber was impregnated with epoxy and wound on the liner by the filament winding method, and the carbon fiber filament was wound in a certain direction (hoop 0°, helical ±45°) to a thickness of about 100 mm to form a composite material layer. The vessel was put into a curing furnace and cured at 110° C. for 2 hours, and the pressure vessel was recovered.

Example 2.

A pressure vessel was prepared in the same manner as in Example 1, except that the shear stress absorbing layer was formed to be 0.5 mm.

Example 3.

A pressure vessel was prepared in the same manner as in Example 1, except that the shear stress absorbing layer was formed to be 10 mm.

Comparative Example 1.

A pressure vessel was prepared in the same manner as in Example 1, except that the shear stress absorbing layer was not formed.

Comparative Example 2.

A pressure vessel was prepared in the same manner as in Example 1 using a fluorine-based release agent instead of the shear stress absorbing layer.

Comparative Example 3

A pressure vessel was prepared in the same manner as in Example 1 by impregnating the composite layer with a viscoelastic material.

Comparative Example 4.

A pressure vessel was prepared in the same manner as in Example 1, except that the shear stress absorbing layer was formed to have a thickness of 5 mm, and the elongation did not reach 150% using the epoxy resin composition.

Experimental Example 1. Burst test and pressure repetition test

In order to check whether the liner is damaged due to repeated charging and discharging of high-pressure gas, a rupture test and repeated pressure test were performed in accordance with the Ministry of Land, Infrastructure and Transport Notice No. It was confirmed that the actual burst pressure was higher than the minimum burst pressure (2.25 times the maximum filling pressure) by filling water at room temperature and gradually applying pressure. The pressure repetition experiment was performed by repeatedly applying pressure from a pressure of 2 MPa or less to a pressure of 1.25 times or more of the working pressure.

type Bursting pressure (standard: 1,575 bar) Cycle Test
Standard: 45,000 times Burst pressure (bar) result Example 1 1,750 satisfied satisfied Example 2 1,710 satisfied leakage Example 3 1,880 satisfied satisfied Comparative Example 1 1,510 below standard leakage Comparative Example 2 1,600 satisfied leakage Comparative Example 3 1,700 satisfied leakage Comparative Example 4 1,510 below standard leakage

Referring to Table 1, in the pressure vessel according to Example 1, by design, the durability of the liner was maintained even after pressurization to a minimum bursting pressure or more, so that cracks did not occur. It was confirmed that the durability of the pressure vessel was increased by effectively distributing the shear stress fatigue load, which increased the burst pressure. However, when it is formed thickly with a shear stress absorption of 10 mm, the weight of the pressure vessel at 700 bar is increased by 5% or more, which is unfavorable for reducing the weight of the pressure vessel.

In Comparative Example, when a shear stress absorbing layer is not formed, when a fluorine-based release agent is used, when winding by coating a viscoelastic material on the composite layer, and when the elongation does not reach 150% due to different viscoelasticity, the pressure vessel after repeated pressurization A leak was confirmed and a deformation of the liner was predicted. In particular, when the experiment of discharging all the filling fluid into the pressure vessel was repeated, the contraction of the pressure vessel was observed before reaching the standard number of repetitions (45,000 repeated pressurization at 10-minute intervals).

Therefore, when the shear stress absorbing layer having high viscoelasticity is uniformly applied to the boss portion and the entire liner, it is possible to prevent breakage due to interfacial separation at the interface between the boss portion and the liner, and to effectively disperse the stress concentrated in the cylinder portion of the liner. , it prevents buckling, and also effectively distributes the shear fatigue load with the composite layer, greatly increasing the durability of the pressure vessel.

Up to now, the present invention has been mainly examined in the examples. Those of ordinary skill in the art to which the present invention pertains will understand that the present invention can be implemented in modified forms without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments are to be considered in an illustrative rather than a restrictive sense. The scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

1000: pressure vessel 100: liner
200: boss part 210: interview part
300: shear stress absorbing layer 310: interface
400: composite layer

Claims (10)

a liner containing gas; And a pressure vessel having a composite layer formed on the outside of the liner,
A pressure vessel having a shear stress absorbing layer having viscoelasticity between the liner and the composite layer.
The pressure vessel according to claim 1, wherein the shear stress absorbing layer satisfies the following equation:
[Equation 1]
M ₂ < M ₁ , M ₃
(In Equation 1, M ₂ is the modulus of elasticity of the shear stress absorbing layer, M ₁ is the modulus of elasticity of the liner, and M ₃ is the modulus of elasticity of the composite layer)
[Equation 2]
E ₂ > E ₁ , E ₃
(In Equation 2, E ₂ is the elongation of the shear stress absorbing layer, E ₁ is the elongation of the liner, and E ₃ is the elongation of the composite layer)
The pressure vessel according to claim 1, wherein the liner has reactive functional groups or irregularities formed on its surface.
The method of claim 1, wherein the shear stress absorbing layer is at least one of silicone, polyurethane, polyurea, styrene-butadiene-rubber (SBR), nitrile-butadiene-rubber (NBR), and ethylene-propylene-rubber (EPDM). A pressure vessel, characterized in that.
The pressure vessel according to claim 1, wherein the shear stress absorbing layer has a thickness of 1 mm to 5 mm.
The pressure vessel according to claim 1, wherein the shear stress absorbing layer has a tensile strength of 10 Mpa or less and an elongation of 150% or more according to ASTM D638.
(a) surface treatment of the liner;
(b) forming a shear stress absorbing layer on the liner; and
(c) forming a composite layer on the shear stress absorbing layer; a pressure vessel manufacturing method comprising a.
The method according to claim 7, wherein the surface treatment of the liner is performed by one of plasma, corona, flame, sanding, grinding, and polishing.
The method according to claim 7, wherein the shear stress absorbing layer is formed on the upper portion of the connection interface between the boss portion and the liner, the boss portion, and the outer peripheral surface of the liner portion with a uniform thickness.
The method of claim 7, wherein in step (b), the shear stress absorbing layer is applied to have a thickness of 1 mm to 5 mm.

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