Analysis of FRP Winding Process and Production Precautions

 

The fiber winding process is one of the manufacturing processes of resin-based composite materials. The main forms of winding are hoop winding, plane winding and spiral winding. The three methods have their own characteristics. The wet winding method is the most widely used because of its relatively simple equipment requirements and low manufacturing cost.

 

The fiber winding process is one of the main manufacturing processes of resin-based composite materials. It is a composite material molding method that uses special winding equipment to impregnate continuous fibers or cloth tapes with resin glue and then continuously, evenly and regularly wind them on the core mold or lining under the conditions of controlled tension and predetermined linear shape, and then solidify them under a certain temperature environment to form a product of a certain shape. Schematic diagram of fiber winding molding process:

 

 

There are three main forms of winding (Figure 1-2): hoop winding, plane winding and spiral winding. The hoop winding reinforcement material is continuously wound on the mandrel at a direction close to 90 degrees (usually 85-89 degrees) with the axis of the mandrel. The plane winding reinforcement material is continuously wound on the mandrel in a direction tangent to the two ends of the mandrel and in the plane. The spiral winding reinforcement material is also tangent to the two ends of the mandrel, but is continuously wound on the mandrel in a spiral state.

 

 

The development of fiber winding technology is closely related to the development of reinforcing materials, resin systems and process inventions. Although there was a process of adding longitudinal bamboo filaments and circumferential silk to long wooden poles and then impregnating them with lacquer to make long weapon poles such as halberds and halberds in the Han Dynasty, it was not until the 1950s that fiber winding technology truly became a composite material manufacturing technology.

 

In 1945, the fiber winding technology was first used to successfully manufacture a springless wheel suspension device, and the first fiber winding machine was invented in 1947. With the development of high-performance fibers such as carbon fiber and aramid fiber and the emergence of microcomputer-controlled winding machines, the fiber winding process has developed rapidly as a composite material manufacturing technology with a high degree of mechanized production. Since the 1960s, it has been applied in almost all possible fields.

 

According to the different chemical and physical states of the resin matrix during winding, the winding process can be divided into three types: dry method, wet method and semi-dry method:

 

Dry Method

Dry winding uses pre-impregnated yarn tapes that are in the B stage after being pre-impregnated. Pre-impregnated yarn tapes are manufactured and provided in special factories or workshops. In dry winding, the pre-impregnated yarn tape must be heated and softened on the winding machine before being wound onto the core mold. Since the glue content, tape size and quality of the pre-impregnated yarn tape can be tested and screened before winding, the quality of the product can be controlled more accurately. The production efficiency of dry winding is high, the winding speed can reach 100-200m/min, and the working environment is also relatively clean. However, the dry winding equipment is relatively complex and expensive, and the interlayer shear strength of the wound product is also low.

 

Wet Winding

Wet winding is to bundle and dip the fibers, and then directly wind them on the core mold under tension control, and then solidify them into shape. The equipment for wet winding is relatively simple, but since the yarn tape is wound immediately after being dipped in glue, it is difficult to control and inspect the glue content of the product during the winding process. At the same time, the solvent in the glue solution is easy to form defects such as bubbles and pores in the product when it solidifies, and the tension is also difficult to control during winding. At the same time, workers operate in an atmosphere of solvent evaporation and an environment with flying fiber hairs, and the working conditions are poor.

 

Semi-Dry Method

Compared with the wet process, the semi-dry method adds a set of drying equipment from the fiber dipping to the winding to the core mold to basically drive out the solvent in the yarn tape glue. Compared with the dry method, the semi-dry method does not rely on a complete set of complex pre-impregnation process equipment. Although the glue content of the product is not easy to accurately control during the process like the wet method and there is one more set of intermediate drying equipment than the wet method, the labor intensity of the workers is greater, but the defects such as bubbles and pores in the product are greatly reduced. The three methods have their own characteristics. The wet winding method is the most widely used because of its relatively simple equipment requirements and low manufacturing cost. The advantages and disadvantages of the three winding process methods are compared in Table 1-1.

 

 

Main Applications of Winding Molding Process

 

FRP Storage Tanks

For storing and transporting chemical corrosive liquids, such as alkalis, salts, acids, etc., steel tanks are prone to rot and leakage, and have a short service life. The cost of switching to stainless steel is high, and the effect is not as good as composite materials. Using fiber winding underground oil FRP storage tanks can prevent oil leaks and protect water sources. Double-wall composite FRP storage tanks and FRP pipes made by fiber winding process have been widely used in gas stations

 

FRP Pipes

FRP pipe products are widely used in refinery pipelines, petrochemical anti-corrosion pipelines, water pipelines, natural gas pipelines and solid particle (such as fly ash and mineral) pipelines due to their high strength, good integrity, excellent comprehensive performance, easy to achieve efficient industrial production, and low comprehensive operating costs.

 

FRP Pressure Products

 

The fiber winding process can be used to manufacture FRP pressure vessels (including spherical containers) and FRP pressure pipe products that withstand pressure (internal pressure, external pressure or both).

FRP pressure vessels are mostly used in military industry, such as solid rocket engine casings, liquid rocket engine casings, FRP pressure vessels, deep-water external pressure casings, etc. FRP wound pressure pipes can be filled with liquids and gases, and will not leak or break under certain pressure, such as seawater desalination reverse osmosis pipes and rocket launch tubes.

 

The excellent properties of advanced composite materials have enabled the successful application of various specifications of rocket engine casings and fuel tanks prepared by fiber winding technology, becoming the main direction of engine development now and in the future. They include attitude adjustment engine casings as small as a few centimeters in diameter and large transport rocket casings as large as 3 meters in diameter.

 

Repair Methods for FRP Wound Pipes

 

The Surface of Composite Products is Sticky, and the Main Reasons are as Follows:

 

1. a) High Humidity in the Air. Since water vapor has a delaying and inhibitory effect on unsaturated polyester resins and epoxy resins, it can even cause permanent stickiness on the surface and incomplete long-term curing of products. Therefore, it is necessary to ensure that the production of composite products is carried out under a relative humidity of less than 80%.

 

1. b) Too little paraffin is added to the unsaturated polyester resin or the paraffin does not meet the requirements, resulting in the inhibition of oxygen in the air. In addition to adding an appropriate amount of paraffin, other methods (such as adding cellophane or polyester film) can also be used to isolate the surface of the product from the air.

 

1. c) The amount of curing agent and accelerator does not meet the requirements, so when preparing the glue, the amount should be strictly controlled according to the formula specified in the technical documents.

 

1. d) For unsaturated polyester resin, too much styrene evaporates, resulting in insufficient styrene monomer in the resin. On the one hand, this requires that the resin cannot be heated before gelling, and on the other hand, the ambient temperature should not be too high (usually 30 degrees Celsius is appropriate), and the ventilation volume should not be too large.

 

There Are Too Many Bubbles In The Product. The Reasons Are as Follows:

 

1. a) The bubbles are not completely driven out. Each layer of laying and winding must be repeatedly rolled with a roller. The roller should be made into a circumferential serrated or longitudinal grooved shape.

 

1. b) The viscosity of the resin is too high. When stirring or brushing, the air bubbles brought into the resin cannot be driven out. An appropriate amount of diluent needs to be added. The diluent for unsaturated polyester resin is styrene; the diluent for epoxy resin can be ethanol, acetone, toluene, xylene and other inactive or glycerol ether active diluents. The diluent for furan resin and phenolic resin is ethanol.

 

1. c) Improper selection of reinforcing materials, the type of reinforcing materials used should be reconsidered.

 

1. d) Improper operation process, appropriate dipping, brushing, rolling angle and other process methods should be selected according to the different types of resins and reinforcing materials.

 

The Reasons For The Delamination Of Products Are As Follows:

 

1. a) The fiber fabric has not been pre-treated or the treatment is insufficient.
2. b) The fabric has insufficient tension or too many bubbles during the winding process.
3. c) The amount of resin is insufficient or the viscosity is too high, and the fiber is not soaked.
4. d) The formula is unreasonable, resulting in poor bonding performance, or the curing speed is too fast or too slow.
5. e) During post-curing, the process conditions are not suitable (generally premature thermal curing or too high temperature).

 

Regardless of the reasons for the delamination, the delamination part must be completely removed, and the resin layer outside the defective area must be polished off with an angle grinder or polisher, with a width of not less than 5cm, and then the layer must be re-laid according to the process requirements.

 

For the above defects, no matter what the reason, appropriate measures should be taken to completely eliminate them to meet the quality requirements.

 

Causes and solutions for delamination of FRP pipes

 

Causes of delamination of FRP pipes caused by delamination of FRP pipes:

 

Reasons:

 

The tape is too old;

The amount of tape is too little or uneven;

The temperature of the hot roller is too low, the resin is not well melted, and the tape cannot stick to the tube core well;

The tension of the tape is small;

The amount of oily release agent is too much, which stains the core cloth.

 

Solution:

The glue content of the tape and the soluble resin content must meet the quality requirements;

Adjust the temperature of the hot roller a little higher, so that when the tape passes through the hot roller, the tape is soft and sticky, and can firmly stick to the tube core;

Adjust the tension of the tape;

Do not use oily release agent or reduce its amount. The inner wall of the FRP pipe is bubbling

 

The Reason is:

The main reason is that the guide cloth is not close to the tube core.

Solution: Pay attention during operation and make sure to keep the guide cloth close and flat on the tube core.

 

The main reason for the bubbling or wrinkling of the FRP pipe after curing is that the volatile content of the tape is too high, and the rolling temperature is low, the rolling speed is fast, and the volatiles of the tape have no time to evaporate, so that they remain in the pipe. When the pipe is heated and cured, its residual volatiles expand due to heat, causing bubbling in the pipe.

 

Solution: Control the volatile content of the tape, appropriately increase the rolling temperature, and slow down the rolling speed. The reason for the wrinkling of the pipe after curing is that the tape has a high glue content.

 

Solution: Appropriately reduce the glue content of the tape and reduce the rolling temperature. FRP pipe voltage resistance is unqualified.

 

Causes:

 

① Insufficient tension of the tape during rolling, low rolling temperature or fast rolling speed, resulting in poor adhesion between the tapes and a large amount of volatile residue in the pipe;

② Incomplete curing of the pipe.

 

Solutions:

① Increase the tension of the tape, increase the rolling temperature or slow down the rolling speed;

② Adjust the curing process to ensure that the pipe is completely cured.

 

Issues to Note:

 

Due to its low density and light material, FRP pipes are very easy to float when installed in areas with high groundwater levels. It is necessary to consider anti-floating measures such as setting up piers or rainwater runoff diversion.

 

In the construction of opening tees and repairing cracks in installed FRP pipes, it is required to have completely dry conditions similar to those in the factory and the resin and fiber cloth used during construction need to be cured for

7-8 hours. However, it is generally difficult to meet this requirement for on-site construction and repair.

 

Existing underground pipeline detection equipment is mainly used to detect metal pipelines, while non-metallic pipeline detection instruments are expensive. Therefore, FRP pipes cannot be detected after being buried, and other subsequent construction units are very likely to dig and damage the pipelines during construction.

 

FRP pipes have poor UV protection. At present, exposed FRP pipes delay aging time by making a 0.5mm thick resin-rich layer and UV absorber (processed in the factory) on their surface. As the operating time goes by, the resin-rich layer and UV absorber will be damaged, thus affecting their service life.

 

The depth of soil cover is required to be high. Generally, the shallowest soil cover of SN5000 grade FRP pipes under the roadway is not less than 0.8m; the deepest soil cover is not more than 3.0m; the shallowest soil cover of SN2500 grade FRP pipes is not less than 0.8m; the deepest soil cover is not more than 1.2m (the minimum and deepest soil covers of 12mm thick steel plate coils are 0.7m and 4.0m respectively).

 

The backfill soil must not contain hard objects such as bricks and stones larger than 50mm to avoid damaging the outer wall of the pipe.

 

There is no report on the large-scale use of FRP pipes by large water companies across the country. Since FRP pipes are new pipes, their service life is still unknown.

 

Causes, treatment methods and prevention measures for leakage of high-pressure FRP pipes

 

Analysis of Leakage Causes

 

FRP pipes are continuous glass fiber reinforced thermosetting resin pipes. They are fragile and cannot withstand external impact. They are affected by internal and external factors during use and sometimes leak (seepage, bursting), which seriously pollutes the environment and affects the water injection rate. After on-site investigation and analysis, the main reasons for leakage are as follows.

 

Influence of FRP Performance

 

Since FRP is a composite material, its materials and processes are seriously affected by external conditions. The main influencing factors are as follows:

 

The influence of the type and curing degree of synthetic resin, mainly the quality of resin, resin diluent and curing agent, FRP rubber formula, etc.

 

The influence of the structure of FRP components and glass fiber materials. The complexity of FRP components directly affects the quality of processing technology. Different materials and different medium requirements will also cause the processing technology to be complicated.

 

Environmental influence, mainly production medium, atmospheric temperature, humidity environment, etc.

 

The Influence of Processing Plan.

Whether the processing plan is reasonable directly affects the construction quality. Due to factors such as materials, personnel operation, environmental influence and detection methods, the performance of FRP will decline.

 

There will be a very small number of pipe walls that do not meet the standards, and there will be hidden cracks on the internal and external screws. These phenomena are difficult to find during inspection and will only be revealed during use. They are product quality issues.

 

External Damage There are Strict Regulations for FRP Pipes During Long-distance Transportation and Loading and Unloading Construction.

 

If soft slings are not used, wooden boards are not used for long-distance transportation, the pipeline of the transport vehicle exceeds the carriage by more than 1.5M, and during backfilling, the distance from the pipe is 0.20mm. Stones, bricks, and direct backfilling will cause external damage to the FRP pipe. During construction, it was not discovered in time, and the pressure overload caused leakage.

 

Design Problems:

 

The high pressure of high-pressure water injection is high and the vibration is large.

 

FRP pipes: The pipes are staggered, and the sudden changes in the axial and lateral directions generate thrust, causing the threads to disjoint and burst. In addition, the steel conversion joints, metering stations, wellheads, flow meters and FRP pipe connections are different due to vibration materials, causing FRP pipes to leak.

 

Construction Quality Problems:

 

The construction of FRP pipes directly affects the service life. The construction quality is mainly reflected in the fact that the burial depth does not meet the design, the protective casing is not worn when crossing roads and drainage channels, and the casing is not added with a centralizer, thrust seat, fixed support according to the specifications, and the work and materials are reduced. These are all causes of FRP pipe leakage.

 

External Factors:

 

The FRP water injection pipeline passes through a wide area, most of which are near farmland or drainage ditches. The marker piles have been stolen after a long service life. Rural towns use mechanization for water conservancy infrastructure construction every year, causing pipeline damage and leakage.

 

Operational Errors.

 

The water injection pressure is high and the impact is large. The FRP pipe cannot withstand the impact of the load. After it is put into use, the operator misinterprets the process and holds the pressure, and the operation is unbalanced, which will cause the FRP pipeline to leak.

 

Prevention and Control Measures

 

According to SY/T6267-1996 “High-pressure glass fiber pipeline”, J/QH0789-2000 buckle FRP pipeline construction and acceptance specifications”. Harbin Star FRP Co., Ltd. “Threaded connection FRP pipeline system installation instructions”, and refer to GB1350235-97 “Industrial metal pipeline engineering construction and acceptance specifications”, prevent quality common problems, grasp the construction of each process, and ensure the quality of construction. For the above 6 reasons for leakage, prevention and control measures are proposed (see Table 1).

 

 

Solution

 

After the glass fiber reinforced plastic pipeline leaks, measures must be taken immediately to prevent environmental pollution.

 

The most effective construction method is to cut and taper and connect with steel conversion joints. The main process is to stop production → find the leakage point → excavation → recycle sewage → on-site thread installation → install steel turn → welding → pressure test → backfill the trench → put into production. Construction pipe connection method (see Figure 1)

 

 

Construction Precautions:

 

Before cutting and tapering, according to the construction requirements of the HSE system, warning tapes should be pulled in the center area, and warning signs must be placed when entering the construction section. After leakage occurs, cut off the water injection source to reduce the pressure to zero. After excavation, recycle sewage in time to prevent trench collapse and injure people.

 

After the FRP pipe is sawed off, the lifting height shall not exceed 1m, and the angle shall not be greater than 10°. When cutting and tapering, it is safe and convenient to construct on the ground. The maximum difference is more than 2m (the pipe is buried 1m deep), and the excavation is at least 20m on both sides from the leakage point.

 

On-Site Thread Installation

On-site thread installation process: cutting → taper cutting → bonding on-site thread → heating and curing. The cutting distance from the leakage point should be at least 0.3m. Select a suitable ratchet grinding machine (the manufacturer provides special tools). The taper surface must be clean, free of grease, dust, and moisture. The adhesive must be stirred evenly. Bond the on-site thread to the pipe end to drive out the bubbles on the bonding surface and tighten it by hand. The curing time of the adhesive is determined according to the ambient temperature. The ambient temperature and curing time are shown in Table 2.

 

 

In winter, the construction temperature is low and the water injection shutdown time cannot exceed 24 hours. The electric belt heating curing method can shorten the construction time. According to construction experience and the characteristics of the adhesive, the best effect is achieved in 3-4 hours of curing, which also controls the total construction shutdown time within 8 hours. The electric belt heating is controlled at 30-32℃ for 3 hours and cooling for 0.5 hours. The power requirement of the electric belt is

 

 

When Installing a Steel Conversion Joint, the external thread and the internal thread of the steel conversion joint must be clean and evenly coated with sealing grease. If there is no torque wrench, tighten it by hand and then tighten it two more times. If there is a torque wrench, tighten it according to the approximate torque table (see Table 4).

 

 

Welding Workers Must be Certified Before Taking Up Their Posts.

During the welding process, the steel conversion joint must be cooled down to a temperature not exceeding 40°C, otherwise the screws on site will be burned and leakage will occur.

 

Backfill The Trench.

Within 0.2m around the pipeline, backfill with sand or soft soil to a height of 0.3m above the natural ground.

 

Conclusions and Suggestions

 

High-pressure FRP pipelines are used in the production of water injection wells and some water injection trunk lines in Jianghan Oilfield, which solves the corrosion and perforation of pipelines, reduces pollution, prolongs the service life of pipelines, and saves investment.

 

Through implementation, the construction process of high-pressure FRP pipeline leak repair is standardized, the water injection time rate is improved, safe production is guaranteed, and civilized construction is achieved. Since 2005, the average number of leak repairs has reached 47 times, and the annual increase in crude oil production has reached more than 80 tons.

 

At present, for medium and high-pressure FRP pipelines (0.25 MPa~2.50MPa), the use of cone making and steel conversion joints to repair leaks takes a long time and is not corrosion-resistant. With the advancement of science and technology, high-strength resins, initiators, curing agents, accelerators and reinforcing materials with excellent performance are constantly produced. The use of adhesive interfaces for medium and high-pressure FRP pipelines needs to be studied in the next step.

 

Solutions to Winding Product Series Problems

 

After the production of FRP winding products is completed, various problems will occur in the quality of the products. These problems can be effectively eliminated and avoided after targeted analysis of factors such as raw materials, additives, and process. The following is a common problem in winding products-voids.

 

Basic Types of Voids

 

The bubbles are inside the fiber bundle, wrapped by the fiber bundle, and formed along the direction of the fiber bundle.

 

The voids mainly appear in the pits between the layers and the resin accumulation area. Analysis of the causes of voids

 

The reinforcing material is not completely impregnated with the matrix resin, and a part of the air remains in the fiber material, which is enclosed by the solidified resin around it.

 

Problems with the glue itself. First, the glue liquid was mixed with air during the preparation process, and the air was not completely removed in time; in addition, the glue liquid produced small molecules due to chemical reactions during gelation and curing, and these low molecular weight substances failed to escape in time. Measures to reduce voids

 

Optimize Materials

Select raw materials that match each other according to the characteristics of the raw materials.

 

Strengthen Impregnation

Impregnation is an important link in the composite material molding process and is the key to the process of how many bubbles or voids appear. Therefore, impregnation must be strengthened to reduce bubbles and improve product quality.

 

Control the Mixing

Before use, initiators, promoters, cross-linking agents, powdered fillers, flame retardants, antistatic agents and pigments will be added to the resin. When adding and mixing, a lot of air will be brought in, and measures must be taken to remove it.

 

Adjust the Glue Tank

Impregnation is an important process in the manufacture of FRP/composite materials. If the impregnation of the glass fiber roving is not good or the glue is not sufficient, white silk will occur after passing through the glue tank.

 

Roller-Pressed Products

When white silk yarn is entangled on the core mold, it is impossible to eliminate this phenomenon by rotating the core mold alone. It must be eliminated by rolling the factory roller. Rolling is not only conducive to impregnation, but also can make the product dense, so that the excess glue flows to the lacking parts or flows away, reducing gaps or bubbles, making the product more fitting, denser and more excellent in performance.

 

Reduce Bridging

The so-called bridging refers to the phenomenon that the rubber yarn of the product is suspended, which exists at the end and the barrel.

 

If the equipment is roughly manufactured, the accuracy is poor, and the operation is unstable, the yarn arrangement will be tight and sparse, overlap and separate, and the original regular wiring cannot be achieved, and the fiber is prone to suspension. At this time, timely maintenance and equipment improvement should be carried out.

 

The actual yarn width must be adjusted to be equal to or close to the yarn width calculated by design.

 

Control the glue content.

 

The fiber number, twist, resin viscosity and fiber surface treatment all have a certain influence on the aerial of the wound fiber.

 

The ambient temperature also has a certain influence on the aerial of the fiber.

 

Inspection and repair of fiber wound products

 

Inspection of fiber wound composite products

 

For fiber wound composite products, the following aspects should be inspected.

 

Appearance Inspection

 

Bubbles: The maximum bubble diameter allowed on the surface of the corrosion-resistant layer is 5MM. If there are less than 3 bubbles with a diameter of no more than 5mm per square meter, no repair is required. Otherwise, the bubbles should be scratched and repaired.

 

Cracks: There shall be no cracks with a depth of more than 0.5mm on the surface of the corrosion-resistant layer. There shall be no cracks with a depth of more than 2mm on the surface of the reinforcement layer.

 

Concavity (or Wrinkles): The surface of the corrosion-resistant layer should be smooth and flat, and the thickness of the convex and concave part of the reinforcement layer should not be greater than 20% of the thickness.

 

Whitening: There shall be no whitening on the corrosion-resistant layer, and the maximum diameter of the whitening area of ​​the reinforcement layer shall not exceed 50mm.

 

Dimension Inspection

 

According to the requirements of the drawings, use measuring tools with appropriate accuracy and range to inspect the dimensions of the product. 3. Inspection of curing degree and lining micropores

 

On-site Inspection

 

1. a) The surface of the composite product does not feel sticky when touched by hand.
2. b) Dip clean cotton yarn in acetone and place it on the surface of the product to observe whether the cotton yarn changes color.
3. c) Is the sound produced by knocking the product with your hand or a coin unclear or crisp?

If the hand feels sticky, the cotton yarn changes color, and the sound is unclear, the surface curing of the product is considered unqualified.

 

Simple Inspection of Curing Degree of Furan Composite Materials

Take a sample and immerse it in a beaker filled with a small amount of acetone, seal it, and soak it for 24 hours. The curing is marked by a smooth and intact surface of the sample and no color change in acetone.

 

Inspection and Testing of Product Curing Degree

The curing degree of composite materials can be indirectly assessed by testing the Barcol hardness. The Barcol hardness tester, which can be HBa-1 or GYZJ934-1, is used to convert the measured Barcol hardness into an approximate curing degree. The Barcol hardness of wound composite materials with relatively ideal curing is generally 40-55. The curing degree of products can also be accurately tested in accordance with the relevant provisions of GB2576-89.

 

(4) Lining Micropore Detection

When necessary, the composite lining is sampled and inspected by an electric spark detector or a micropore detector.

 

Product Performance Testing

 

The thermal, physical and mechanical properties of the product are tested according to the test content required by the work instruction document and the specified test standards to provide a basis for the acceptance of the product.

 

Destruction Detection

 

When necessary, the product needs to be subjected to non-destructive testing such as ultrasonic scanning, X-ray, CT, thermal imaging, etc. to accurately analyze and determine the internal defects of the product.

 

Product Defect Analysis, Control Measures and Repair

 

The Main Reasons for the Stickiness Of Composite Products are as Follows:

 

1. a) High humidity in the air. Since water vapor has a delaying and inhibitory effect on unsaturated polyester resins and epoxy resins, it can even cause permanent stickiness on the surface and incomplete long-term curing of products. Therefore, it is necessary to ensure that the production of composite products is carried out under a relative humidity of less than 80%.

 

1. b) Too little paraffin is added to the unsaturated polyester resin or the paraffin does not meet the requirements, resulting in the inhibition of oxygen in the air. In addition to adding an appropriate amount of paraffin, other methods (such as adding cellophane or polyester film) can also be used to isolate the surface of the product from the air.

 

1. c) The amount of curing agent and accelerator does not meet the requirements. Therefore, when preparing the glue, the amount should be strictly controlled according to the formula specified in the technical documents.

 

1. d) For unsaturated polyester resins, too much styrene volatilization causes insufficient styrene monomer in the resin. On the one hand, this requires that the resin cannot be heated before gelling, and on the other hand, the ambient temperature should not be too high (usually 30 degrees Celsius is appropriate), and the ventilation volume should not be too large.

 

There are too many bubbles in the product. The reasons are as follows:

 

1. a) The bubbles are not completely driven out. Each layer of laying and winding must be rolled repeatedly with a roller. The roller should be made into a circumferential serrated or longitudinal grooved shape.

 

1. b) The viscosity of the resin is too high. When stirring or brushing, the air bubbles brought into the resin cannot be driven out. An appropriate amount of diluent needs to be added. The diluent for unsaturated polyester resin is styrene; the diluent for epoxy resin can be ethanol, acetone, toluene, xylene and other inactive or glycerol ether active diluents. The diluent for furan resin and phenolic resin is ethanol.

 

1. c) Improper selection of reinforcing materials, the type of reinforcing materials used should be reconsidered.

 

1. d) Improper operation process, the appropriate process methods such as dipping, brushing, rolling angle, etc. should be selected according to the different types of resins and reinforcing materials.

 

The Delamination Phenomenon Of The Product Is Caused By The Following Reasons:

 

1. a) The fiber fabric has not been pre-treated or the treatment is insufficient.

 

1. b) The fabric has insufficient tension during the winding process, or there are too many bubbles.

 

1. c) Insufficient amount of resin or too high viscosity, the fiber is not soaked.

 

1. d) Improper formulation leads to poor bonding performance, or too fast or too slow curing speed.

 

1. e) In post-curing, the process conditions are not suitable (generally premature thermal curing or too high temperature).

 

Regardless of the cause of the delamination phenomenon, the delamination part must be completely removed, and the resin layer outside the defect area must be polished with an angle grinder or polisher, with a width of not less than 5cm, and then re-layered according to the process requirements.

 

For the above defects, regardless of the cause, appropriate measures should be taken to completely eliminate them to meet the quality requirements.

 

Typical Winding Composite Material Specimen Preparation and Performance Testing

Composites are often anisotropic materials, and their design and analysis methods are different from those of metal materials. The anisotropic properties of composite materials lead to differences in performance test methods from metal materials. For traditional materials, designers can obtain performance data from the manual or material instructions provided by the manufacturer based on the material (or brand) when selecting the material. It is more accurate to say that composite materials are structures rather than materials. Their performance is related to many factors such as resin matrix, reinforcement materials, process conditions, storage time and environment.

 

It is very necessary to test the performance of raw materials before designing composite materials, but it cannot be said that the performance data required for design has been mastered. It can only be considered that the foundation for the selection of raw materials has been laid.

 

At present, the results predicted by micromechanics methods are still limited and can only be estimated qualitatively. The performance data required for the design of composite components needs to be obtained by basic performance tests, which is crucial for design work. Composite material performance testing is the basis for material selection, evaluation of reinforcement materials, resin matrix, interface performance, molding process conditions and manufacturing technology level, as well as product design.

 

Unidirectional Fiber Composite Flat Plate

The elastic properties of unidirectional composite materials are characterized by tensile and compression properties in the directions of 0 degrees, 90 degrees and 45 degrees, and the interface performance between fiber and resin is characterized by bending and interlaminar shear tests.

 

In order to evaluate the material performance, according to the specific requirements of national standards GB3354-82, GB3856-83, GB3356-82, GB3357-82, GB3355-82, the unidirectional fiber composite material flat plate is made, and then the fiber composite material flat plate is processed into the sample size and quantity required by various test methods.

 

Production of Unidirectional Fiber Composite Material Flat Plate

 

The winding method is to make the fiber drawn from the yarn frame pass through the tensioner, glue groove, yarn guide roller, winding nozzle in turn, and wind it to the surface of the core mold, and finally solidify it into shape. The national standard stipulates that the template size is 270mm X 270mm. The template can be wound to make two flat plates (front and back sides) at a time, and can process the number of specimens required for tensile, compression, bending, interlayer shear and other performance tests.

 

It is best to use polytetrafluoroethylene cloth as a demoulding sheet to cover the mold surface when making a flat plate, without using a demoulding agent to prevent the demoulding agent from migrating to the surface of the composite material and bringing difficulties to the bonding process of the aluminum sheet. When a small amount of silicone ester is used to spread the demoulding sheet on the mold surface, it can be well flattened to avoid the surface of the specimen being uneven after curing.

 

During the winding process, the tension of each yarn is adjusted by the tension control device, generally 5%-8% of the fiber strength is appropriate, and the tension should remain stable. The winding speed does not exceed 30r/min. During the process, the glue tank can be heated to reduce the viscosity of the resin and improve the resin’s wetting performance on the fiber. The fiber should be evenly distributed on the mold surface until the required thickness, and the number of winding layers is calculated according to the following formula.

 

After winding, place the upper and lower pressure plates and thickness pads, pre-tighten the bolts, and leave for about 2 hours to allow excess resin to flow out. Tighten the bolts evenly to make the upper and lower pressure plates of the mold contact the pads, and use a sharp blade to cut the fibers at both ends of the core mold to release the winding tension and avoid the board from warping and deformation after curing. However, cutting the fibers too early will cause the fibers to be arranged non-straight.

 

After the mold is placed in the oven, it is cured according to the prescribed system, with a temperature control accuracy of ±3°C, and naturally cooled to room temperature. The mold is disassembled, and the prepared flat plate is taken out to form specimens of various specifications.

 

Processing Of The Specimen

After the unidirectional fiber composite flat plate is prepared, remove the dust, impurities and other foreign matter on the surface. According to the specimen size and quantity required by the national standard, considering the cutting edge width, use a special pencil to mark the fiber direction and 0 degree, 90 degree, 45 degree tensile, compression, and bending specimen processing areas on the flat plate, and use a diamond grinding wheel to cut the specimen on a special milling machine. The shape and size of the specimen refer to the corresponding national standard.

 

Bonding Of Reinforcing Sheet

The reinforcing sheet is made of a glass fiber composite material plate with a thickness of 2-3mm orthogonal ply or an aluminum plate with a thickness of 1-3mm. When making the reinforcing sheet with an aluminum plate, the surface of the reinforcing sheet should be flat, and the surface oxide layer should be removed by grinding with sandpaper. The reinforcing sheet should be heated to 65 degrees in the acid treatment solution and placed for 15 minutes, then rinsed with tap water, and then baked at 105 degrees for 2 hours to remove moisture. Do not touch the surface of the reinforcing sheet after treatment with your hands. The weight ratio of the treatment solution is sulfuric acid: distilled water: potassium dichromate = 10:30:1.

 

Before bonding the reinforcing sheet, grind the surface of the specimen with 100-grit sandpaper, wipe off the surface dust, remove the release agent and surface grease with ethyl acetate, and then wash the specimen with acetone and cool it to dry. Since the tensile strength of unidirectional composite materials in the fiber direction is very high, the reinforcement sheet is easy to slip during the tensile test. It is recommended to use epoxy 618: 200# polyamide: diglycidyl ether: imidazole = 100: 80: 15: 2. Apply adhesive to the specimen and the reinforcement sheet and apply contact pressure to cure. The curing system is: the room temperature rises to 60 degrees, keeps warm for 2 hours, then rises to 120 degrees, and cools to room temperature after keeping warm for 8 hours. The strength of the tensile specimens perpendicular to the fiber direction and the 45-degree eccentric tensile specimens is low, and room temperature adhesives can be used.

 

After the reinforcement sheet is cured, the surface of the test part of the specimen is polished with sandpaper to expose the fiber layer, and the surface oil is removed with acetone and ethyl acetate. The measurement direction line is drawn, and the strain gauge is pasted with 502 adhesive. The pasting direction of the strain gauge is consistent with the performance direction of the material being tested. After the adhesive is cured, the specimen production work is completed.

 

Production Of NOL Ring Samples

 

Fiber-wound high-pressure vessels and solid engine casings and other internal pressure vessels are usually designed using grid theory analysis. The grid theory ignores the matrix stiffness and the load is borne entirely by the fiber. The NOL ring test method is developed on this basis.

 

NOL ring samples are usually wound on a special small winding machine. There are two methods for manufacturing NOL rings:

One is single-ring winding in a special combination mold, and the other is first winding the ring toward the cylinder and then cutting it into a ring.

 

The outer surface of the ring can be processed or not. The processed ring has high strength and a small discrete coefficient. The unprocessed ring has an uneven surface, a high surface resin content, and a large dimensional deviation. The strength of the cylinder-cut ring is lower than that of a single wound ring. This is because when the ring is cut from the cylinder, some continuous fibers are cut off, reducing the bearing capacity of the ring. In actual tests, a single wound ring is usually used. The performance of the ring is related to the winding tension, curing system, fiber moisture content, fiber surface state, resin system, etc., and attention should be paid during manufacturing. For detailed sample preparation and testing, see GB2578-89; GB1458 and GB1461.