阅读说明：本技术 一种复合材料气瓶及其制备方法 (Composite material gas cylinder and preparation method thereof ) 是由 王金娜 刘魏魏 杨克伦 宗磊 江亚彬 赵龑浩 于 2021-11-16 设计创作，主要内容包括：本发明提供一种复合材料气瓶及其制备方法,复合材料气瓶包括由内至外依次设置的：内胆；粘接过渡层,粘接过渡层的制备原料包括玻璃纤维布和粘接剂,粘接剂的制备原料包括：环氧树脂、固化剂、促进剂；玄武岩纤维复合材料层,玄武岩纤维复合材料层的制备原料包括双马来酰亚胺树脂基体和玄武岩纤维增强材料。该复合材料气瓶以双马来酰亚胺为基体材料,玄武岩纤维为主要增强材料,其耐高温性能好、成本低。(The invention provides a composite material gas cylinder and a preparation method thereof, wherein the composite material gas cylinder comprises the following components in sequence from inside to outside: an inner container; the preparation raw materials of bonding transition layer include glass fiber cloth and adhesive, and the preparation raw materials of adhesive include: epoxy resin, curing agent and accelerator; the basalt fiber composite material layer is prepared from raw materials including a bismaleimide resin matrix and a basalt fiber reinforced material. The composite material gas cylinder takes bismaleimide as a base material and basalt fiber as a main reinforcing material, and has good high-temperature resistance and low cost.)

1. The composite material gas cylinder is characterized by comprising the following components in sequence from inside to outside:

an inner container;

the preparation raw materials of the bonding transition layer comprise glass fiber cloth and an adhesive, and the preparation raw materials of the adhesive comprise: epoxy resin, curing agent and accelerator;

the basalt fiber composite material layer is prepared from raw materials including a bismaleimide resin matrix and a basalt fiber reinforced material.

2. The composite gas cylinder according to claim 1, further comprising:

the carbon fiber composite material layer is arranged between the bonding transition layer and the basalt fiber composite material layer, and the preparation raw materials of the carbon fiber composite material layer comprise a bismaleimide resin matrix and a carbon fiber reinforced material;

and the temperature-resistant protective layer is arranged outside the basalt fiber composite material layer and is polyurethane type three-proofing paint.

3. The composite gas cylinder according to claim 2, characterized in that:

the thickness of the bonding transition layer is 0.3-0.8 mm;

the thickness of the carbon fiber composite material layer is 2.0-4.6 mm;

the thickness of the basalt fiber composite material layer is 11.5-16.8 mm.

4. The composite gas cylinder according to claim 2, characterized in that:

the glass transition temperature of the bismaleimide resin matrix is more than 250 ℃;

the carbon fiber reinforced material is T700 carbon fiber, the tensile strength of the carbon fiber reinforced material is more than or equal to 4900MPa, the elastic modulus of the carbon fiber reinforced material is more than or equal to 230GPa, the elongation of the carbon fiber reinforced material is more than or equal to 2.1 percent, the linear density of the carbon fiber reinforced material is more than or equal to 800tex, and the density of the carbon fiber reinforced material is more than or equal to 1.9g/cm³；

The basalt fiber reinforced material is a silane coupling agent modified basalt fiber, the tensile strength of the basalt fiber reinforced material is more than or equal to 3600MPa, the elastic modulus is more than or equal to 95GPa, the elongation is more than or equal to 2.4%, the linear density is more than or equal to 1200tex, and the density is more than or equal to 2.65g/cm³。

5. The composite gas cylinder according to claim 1,

in the preparation raw materials of the adhesive, the mass ratio of epoxy resin, curing agent and accelerator is 100: (90-130): (0.5-3);

in the preparation raw materials of the adhesive, the epoxy resin is bisphenol A epoxy resin; the curing agent is a phthalic anhydride curing agent; the promoter is N, N-dimethylbenzylamine.

6. A method of making a composite cylinder according to any one of claims 1 to 5, comprising the steps of:

s1, coating the outer surface of the inner container with the glass fiber cloth, coating the adhesive on the glass fiber cloth, curing, and polishing to obtain the bonding transition layer;

s2, winding the carbon fiber reinforced material impregnated with the bismaleimide resin matrix to the outer surface of the bonding transition layer by adopting a wet winding process to form the carbon fiber composite material layer; or the carbon fiber composite material layer is not arranged;

s3, winding the basalt fiber reinforced material impregnated with bismaleimide resin matrix to the outer surface of the carbon fiber composite material layer or the bonding transition layer by adopting a wet winding process to form the basalt fiber composite material layer;

s4, solidifying the multilayer structure obtained in the step S3 to obtain the composite material gas cylinder.

7. A method of making a composite gas cylinder according to claim 6, characterized in that:

in step S2, a multi-directional spiral winding and hoop winding alternating winding method is adopted when the carbon fiber reinforced material impregnated with the bismaleimide resin matrix is wound, and the spiral winding angle is 8-18 degrees; the winding tension is 30-40N, and the winding tension of each layer of fibers decreases from inside to outside.

8. A method of making a composite gas cylinder according to claim 6, characterized in that:

in step S3, a winding method of alternate multi-directional spiral winding and hoop winding is adopted when winding the impregnating bismaleimide resin matrix basalt fiber reinforced material, and the spiral winding angle is 10-70 degrees; the winding tension is 15-20N, and the winding tension of each layer of fibers decreases progressively from inside to outside.

9. The method for manufacturing a composite material gas cylinder according to claim 6, wherein in step S4, the curing system is as follows:

heating to 140-160 ℃ at a heating rate of 0.5-2 ℃/min, and keeping the temperature for 2-4 h; then heating to 175-195 ℃ at the heating rate of 0.5-2 ℃/min, and preserving the heat for 4-6 h; then the temperature is raised to 210-230 ℃ at the temperature raising speed of 0.5-2 ℃/min, and the temperature is kept for 6-8 h; finally cooling to 20-30 ℃ at the speed of not more than 3 ℃/min to finish the solidification.

10. The method for manufacturing a composite material gas cylinder according to claim 6, characterized by further comprising, after step S4:

s5, performing self-tightening treatment on the cured product according to self-tightening pressure;

and S6, spraying a temperature-resistant protective material outside the basalt fiber composite material layer to obtain the composite material gas cylinder.

Technical Field

The invention belongs to the technical field of composite material pressure-resistant containers, and particularly relates to a composite material gas cylinder and a preparation method thereof.

Background

The composite material high-pressure gas cylinder starts in the last 50 th century, has the characteristics of light weight, high strength, high modulus, strong designability and the like, and is widely applied to important fields of aviation, aerospace and the like. With the continuous development of aerospace technology, the requirement of manned lunar landing technology and the requirement of high-performance gas cylinders in the fields of carrier rockets, missiles and aviation, a high-performance gas cylinder shaping product with strong high and low temperature environment resistance is required.

At present, most of composite material gas cylinder base materials used in aerospace are epoxy resin base bodies, for example, Chinese patent document CN112963722A discloses a metal lining carbon fiber winding composite material gas cylinder and a manufacturing method thereof, epoxy resin is used as the base material, the use temperature range of the gas cylinder is only-40 ℃ to 60 ℃, advanced composite materials in the high-temperature aerospace field need to resist high temperature of 180 ℃ to 250 ℃, and the working temperature range of the epoxy resin base composite material gas cylinder is limited. For the reinforcing fiber of the composite material gas cylinder, glass fiber and carbon fiber are selected. Glass fibers have high strength and high modulus of elasticity, but are poor in weatherability, high in water absorption, and poor in chemical stability to acids, alkalis, and other corrosive media. The strength, modulus, weather resistance and corrosion resistance of the carbon fiber are all more remarkable than those of the glass fiber, but the cost is too high, import is relied on, and purchasing is easily limited by international situations.

Therefore, a composite gas cylinder with excellent temperature resistance and low cost suitable for the aerospace field is needed.

Disclosure of Invention

The invention aims to solve the technical problem of providing a composite material gas cylinder and a preparation method thereof.

In order to solve the above problems, an aspect of the present invention provides a composite gas cylinder, including, sequentially from inside to outside:

an inner container;

the preparation raw materials of the bonding transition layer comprise glass fiber cloth and an adhesive, and the preparation raw materials of the adhesive comprise: epoxy resin, curing agent and accelerator;

the basalt fiber composite material layer is prepared from raw materials including a bismaleimide resin matrix and a basalt fiber reinforced material.

The composite material gas cylinder adopts bismaleimide resin with the glass transition temperature of more than 250 ℃ and good technological property and high temperature resistance as a base material, has the technological property of epoxy resin and the high temperature resistance of polyimide, and can resist the high temperature of 180-250 ℃. Basalt fiber is adopted as a reinforcing material, and the basalt fiber is a novel inorganic environment-friendly green high-performance fiberDimensional material of SiO₂The metal oxide is used as a main component, and a large amount of metal oxide is also contained, so that silicate mineral structures such as pyroxene, plagioclase feldspar, olivine and the like are formed, and the metal oxide-containing silicon dioxide has good mechanical properties, higher chemical stability, more excellent weather resistance and acid and alkali resistance and wider application temperature range; and secondly, more importantly, the basalt fiber has wide sources and simple production process, and the basalt fiber is adopted to replace the conventional carbon fiber, so that the production cost can be greatly reduced, and the production cost of the composite material gas cylinder is lower.

Preferably, the method further comprises the following steps:

the carbon fiber composite material layer is arranged between the bonding transition layer and the basalt fiber composite material layer, and the preparation raw materials of the carbon fiber composite material layer comprise a bismaleimide resin matrix and a carbon fiber reinforced material;

and the temperature-resistant protective layer is arranged outside the basalt fiber composite material layer and is polyurethane type three-proofing paint.

Preferably, the basalt fiber reinforced material is a basalt fiber reinforced material treated by a silane coupling agent.

Specifically, the silane coupling agent treatment refers to the steps of preparing a silane coupling agent into an ethanol solution with the concentration of 2wt%, soaking the basalt fiber reinforced material in the ethanol solution of the silane coupling agent, and then drying and cooling to obtain the basalt fiber reinforced material treated by the silane coupling agent.

Specifically, the inner container is a metal inner container; furthermore, the metal inner container is a stainless steel inner container.

Specifically, the shape of the inner container is cylindrical or spherical.

Preferably, the thickness of the bonding transition layer is 0.3-0.8 mm;

the thickness of the carbon fiber composite material layer is 2.0-4.6 mm;

the thickness of the basalt fiber composite material layer is 11.5-16.8 mm.

The composite material gas cylinder takes the basalt fiber composite material layer as the main material and the carbon fiber composite material layer as the auxiliary material, so that the gas cylinder has good mechanical property, the working pressure of the gas cylinder can reach 35MPa, the bursting strength safety coefficient is not lower than 2.0, and the gas cylinder has the characteristics of low cost and high temperature resistance.

More preferably, the thickness ratio of the carbon fiber composite material layer to the basalt fiber composite material layer is (0-0.3): 1.

preferably, in the carbon fiber composite material layer, the weight percentage of the bismaleimide resin matrix is 30-40%.

Preferably, in the basalt fiber composite material layer, the weight percentage of the bismaleimide resin matrix is 30-40%.

Preferably, the glass transition temperature of the bismaleimide resin matrix is more than 250 ℃;

the carbon fiber reinforced material is T700 carbon fiber, the tensile strength of the carbon fiber reinforced material is more than or equal to 4900MPa, the elastic modulus of the carbon fiber reinforced material is more than or equal to 230GPa, the elongation of the carbon fiber reinforced material is more than or equal to 2.1 percent, the linear density of the carbon fiber reinforced material is more than or equal to 800tex, and the density of the carbon fiber reinforced material is more than or equal to 1.9g/cm³；

The basalt fiber reinforced material is a silane coupling agent modified basalt fiber, the tensile strength of the basalt fiber reinforced material is more than or equal to 3600MPa, the elastic modulus is more than or equal to 95GPa, the elongation is more than or equal to 2.4%, the linear density is more than or equal to 1200tex, and the density is more than or equal to 2.65g/cm³。

Preferably, in the raw materials for preparing the adhesive, the mass ratio of the epoxy resin, the curing agent and the accelerator is 100: (90-130): (0.5-3);

in the preparation raw materials of the adhesive, the epoxy resin is bisphenol A epoxy resin; the curing agent is a phthalic anhydride curing agent; the promoter is N, N-dimethylbenzylamine.

Another aspect of the present invention provides a method for preparing the composite material gas cylinder, comprising the following steps:

s1, coating the outer surface of the inner container with the glass fiber cloth, coating the adhesive on the glass fiber cloth, curing, and polishing to obtain the bonding transition layer;

s2, winding the carbon fiber reinforced material impregnated with the bismaleimide resin matrix to the outer surface of the bonding transition layer by adopting a wet winding process to form the carbon fiber composite material layer; or the carbon fiber composite material layer is not arranged;

s3, winding the basalt fiber reinforced material impregnated with bismaleimide resin matrix to the outer surface of the carbon fiber composite material layer or the bonding transition layer by adopting a wet winding process to form the basalt fiber composite material layer;

s4, solidifying the multilayer structure obtained in the step S3 to obtain the composite material gas cylinder.

Preferably, in step S1, the curing procedure is: heating to 85-115 ℃, and keeping the temperature for 2 h; then the temperature is raised to 145-165 ℃ and the temperature is kept for 4 h.

Preferably, in step S2, a multi-directional spiral winding method and a hoop winding method are used for winding the carbon fiber reinforced material impregnated with the bismaleimide resin matrix, wherein the spiral winding angle is 8 ° to 18 °; the winding tension is 30-40N, and the winding tension of each layer of fibers decreases from inside to outside.

Preferably, in step S3, a winding method of alternating multi-directional spiral winding and hoop winding is adopted when winding the bismaleimide resin matrix basalt fiber-impregnated reinforcing material, wherein the spiral winding angle is 10 ° to 70 °; the winding tension is 15-20N, and the winding tension of each layer of fibers decreases progressively from inside to outside.

Preferably, in step S4, the curing system is:

heating to 140-160 ℃ at a heating rate of 0.5-2 ℃/min, and keeping the temperature for 2-4 h; then heating to 175-195 ℃ at the heating rate of 0.5-2 ℃/min, and preserving the heat for 4-6 h; then the temperature is raised to 210-230 ℃ at the temperature raising speed of 0.5-2 ℃/min, and the temperature is kept for 6-8 h; finally cooling to 20-30 ℃ at the speed of not more than 3 ℃/min to finish the solidification.

Preferably, after step S4, the method further includes:

s5, performing self-tightening treatment on the cured product according to self-tightening pressure;

and S6, spraying a temperature-resistant protective material outside the basalt fiber composite material layer to obtain the composite material gas cylinder.

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

1. the composite material gas cylinder adopts bismaleimide resin with the glass transition temperature of more than 250 ℃ and good technological property and high temperature resistance as a base material, has the technological property of epoxy resin and the high temperature resistance of polyimide, and can resist the high temperature of 180-250 ℃;

2. the composite material gas cylinder adopts basalt fiber to replace conventional carbon fiber as a reinforcing material, takes the basalt fiber composite material layer as a main body, and the basalt fiber is a novel inorganic environment-friendly green high-performance fiber material which takes SiO₂The gas cylinder is a main component and also contains a large amount of metal oxides, and silicate mineral structures such as pyroxene, plagioclase feldspar, olivine and the like are formed, so that the gas cylinder has good mechanical properties, higher chemical stability, more excellent weather resistance, acid and alkali resistance and wider application temperature range, more importantly, the basalt fiber has wide sources and simple production process, and the production cost can be greatly reduced by adopting the basalt fiber to replace conventional carbon fiber, so that the production cost of the composite material gas cylinder is lower;

3. the preparation method of the composite material gas cylinder of the invention enables the obtained composite material gas cylinder to have better mechanical property by further regulating and controlling the winding angle of the fiber and the gradient heating and curing system in the process.

Drawings

Fig. 1 is a schematic structural view of a composite gas cylinder according to embodiment 1 of the present invention.

Wherein: 1-a stainless steel inner container; 2-bonding a transition layer; 3-a carbon fiber composite layer; 4-a basalt fiber composite layer; 5-temperature resistant protective layer.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the following embodiments of the present invention, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1

As shown in FIG. 1, the composite gas cylinder of the present embodiment has a working pressure of 35MPa and a volume of 50L, and the inner container is cylindrical. Comprises the following components which are arranged from inside to outside in sequence:

a stainless steel inner container 1;

the thickness of the bonding transition layer 2 is 0.5mm, the preparation raw materials of the bonding transition layer comprise glass fiber cloth and an adhesive, and the preparation raw materials of the adhesive comprise, by mass, 100: 90: 0.8 of epoxy resin, methyl tetrahydrophthalic anhydride, N, N-dimethylbenzylamine;

the carbon fiber composite material layer 3 is 2.5mm thick, and the preparation raw materials of the carbon fiber composite material layer comprise a bismaleimide resin matrix (the actually measured glass transition temperature is more than 250 ℃) and Dongli T700SC-1200K carbon fiber reinforced material (the linear density is 800tex, and the retested tensile strength is 5120 MPa), wherein the mass percentage of the bismaleimide resin matrix is 35%;

the basalt fiber composite material layer 4 is 12.76mm thick, and the preparation raw materials of the basalt fiber composite material layer comprise a bismaleimide resin matrix (actually measured glass transition temperature is higher than 250 ℃) and a basalt fiber reinforced material (linear density 1200tex, repeatedly measured tensile strength 3800 MPa) treated by a silane coupling agent KH-550, wherein the mass percentage of the bismaleimide resin matrix is 35%;

and the temperature-resistant protective layer 5 is polyurethane type three-proofing paint.

The preparation method of the composite material gas cylinder comprises the following steps:

s1, preparing an adhesive, fixing the stainless steel liner on a winding machine by using a winding tool, rotating at a rotating speed of 360 degrees of 2-8 r/min, coating a glass fiber surface felt on the outer surface of the stainless steel liner, coating the adhesive on the surface felt by using a brush to ensure that no wrinkles are adhered on the surface, then putting the stainless steel liner into a curing furnace, wherein the curing system is to heat up to 90 ℃, keep the temperature for 2 hours, further heat up to 150 ℃, keep the temperature for 4 hours, in the curing process, rotating the gas cylinder around an axis at a rotating speed of 2-8 r/min, polishing off surface nodules by using abrasive paper after curing is finished, flattening the surface, and measuring the interlayer adhesion transition layer to be 0.5mm by using a micrometer;

s2, after the inner container is installed at the winding part of the winding machine through the winding tool, the heating tool is opened to the winding part, preheating is carried out for half an hour, and the glue groove is heated in advance for 2-5 hours, so that the temperature in the glue groove is 120-130 ℃; cutting 1kg of bismaleimide resin into small blocks, placing the small blocks in a beaker for 2-5 hours at room temperature, heating the small blocks to 140 ℃ by an electric furnace, changing the small blocks of bismaleimide resin into a brownish red transparent liquid under continuous stirring, standing, discharging bubbles, and pouring the liquid into a glue tank; impregnating bismaleimide resin in a glue groove by a carbon fiber reinforced material; adopting a wet winding process, sequentially carrying out 89-degree hoop winding, 10-degree spiral winding, 89-degree hoop winding and 13-degree spiral winding on the carbon fiber reinforced material impregnated with the bismaleimide resin matrix to the outer surface of the bonding transition layer, wherein each carbon fiber is sequentially and correspondingly wound with the tension of 40N/40N/38N/38N to form the carbon fiber composite material layer, and the thickness of the carbon fiber composite material layer is 2.5 mm;

s3, impregnating the basalt fiber reinforced material treated by the silane coupling agent in a rubber bath with bismaleimide resin; adopting a wet winding process, winding the basalt fiber reinforced material impregnated with bismaleimide resin matrix to the outer surface of the carbon fiber composite material layer in a hoop winding manner and a spiral winding manner sequentially according to 89 °/17 °/89 °/60 °/89 °/13 °/89 °/17 °/89 °/60 °/10 °/89 °/13 °/89 °/10 °/89 °, wherein the basalt fiber reinforced material is wound to the outer surface of the carbon fiber composite material layer in a spiral winding manner, and the winding tension of each basalt fiber reinforced material is sequentially

20N/20N/20N/20N/19N/19N/19N/18N/18N/18N/17N/17N/17N/16N/16N/16N/16N/15N/15N/15N to form the basalt fiber composite material layer, wherein the thickness of the basalt fiber composite material layer is 12.76 mm;

s4, placing the multilayer structure obtained in the step S3 into a curing furnace for curing, wherein in the curing process, the gas cylinder rotates around an axis at a rotating speed of 2-8 r/min, and the curing system is as follows: heating to 140 ℃ at a heating rate of 0.5-2 ℃/min, and keeping the temperature for 2 h; then heating to 180 ℃ at the heating rate of 0.5-2 ℃/min, and preserving heat for 4 h; then heating to 220 ℃ at the heating rate of 0.5-2 ℃/min, and preserving heat for 6 h; finally cooling to 25 ℃ at a speed of not more than 3 ℃/min to finish solidification;

s5, performing 'self-tightening' treatment on the cured product according to the self-tightening pressure determined by modeling simulation;

and S6, spraying polyurethane type three-proofing paint outside the basalt fiber composite material layer to obtain the composite material gas cylinder.

The composite material gas cylinder passes 35MPa of gas tightness test and 52.5MPa of water pressure test. After the low-temperature test at minus 60 ℃ and the high-temperature test at 180 ℃, the air tightness test is qualified, and the air tightness test passes the low-temperature test and the high-temperature test.

Example 2

The other preparation steps of the composite material gas cylinder of the embodiment are the same as those of the embodiment 1, except that the preparation raw materials of the adhesive in the bonding transition layer 2 are 100: 120: 1, methyl tetrahydrophthalic anhydride, N, N-dimethylbenzylamine.

The composite material gas cylinder passes 35MPa of gas tightness test and 52.5MPa of water pressure test. After the low-temperature test at minus 60 ℃ and the high-temperature test at 180 ℃, the air tightness test is qualified, and the air tightness test passes the low-temperature test and the high-temperature test.

Example 3

The composite material cylinder of this example was fabricated in the same manner as in example 1 except that in step S3, the bismaleimide resin matrix-impregnated basalt fiber-reinforced material was alternately wound around the outer surface of the carbon fiber composite material layer in a hoop winding and a spiral winding in the order of 89 °/20 °/89 °/65 °/10 °/89 °/15 °/89/20 °/89/60 °/10 °/89/15 °/89/10 °/89 °.

The composite material gas cylinder passes 35MPa of gas tightness test and 52.5MPa of water pressure test. After the low-temperature test at minus 60 ℃ and the high-temperature test at 180 ℃, the air tightness test is qualified, and the air tightness test passes the low-temperature test and the high-temperature test.

Example 4

The composite material gas cylinder of the embodiment has the same structure and preparation method as those of the embodiment 1, and is different from the embodiment 1 in that the carbon fiber composite material layer is not arranged, and the thickness of the basalt fiber composite material layer is 16.8 mm.

The composite cylinder prepared in this example passed the airtightness test at 35MPa and the hydraulic pressure test at 52.5 MPa. After the low-temperature test at minus 60 ℃ and the high-temperature test at 180 ℃, the air tightness test is qualified, and the air tightness test passes the low-temperature test and the high-temperature test.

Example 5

The composite material gas cylinder of the present example has the same structure as that of example 1, except that in step S4, the curing system is: directly heating to 210 ℃ at the heating rate of 0.5-2 ℃/min, and keeping the temperature for 12 h; finally cooling to 20-30 ℃ at the speed of not more than 3 ℃/min to finish the solidification.

The composite cylinder prepared in this example passed 35MPa of gas tightness test and 52.5MPa of water pressure test, but failed-60 ℃ low temperature test and 180 ℃ high temperature test.

Example 6

The composite material gas cylinder of the embodiment has the same structure as that of the embodiment 1, and the difference is that in the preparation method, the winding angles are different, specifically:

in S2, the carbon fiber reinforced material impregnated with the bismaleimide resin matrix is wound to the outer surface of the bonding transition layer alternately in a hoop winding mode and a spiral winding mode sequentially in the 89 °/60 °/89 °/60 °;

in S3, the bismaleimide resin matrix impregnated basalt fiber reinforcement material is alternately wound on the outer surface of the carbon fiber composite material layer in a hoop winding manner and a spiral winding manner in the order of 89 °/45 °/89 °/60 °/89 °/45 °/89 °/60 °/10 °/89 °.

The composite cylinders prepared in this example failed the 52.5MPa hydraulic test. The strength does not meet the design requirement, and subsequent inspection is not carried out.

It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.