Composite Material Winding Process

 

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.

 

Each of the three methods has its 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 a 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 certain shape of product.

 

Schematic diagram of fiber winding process 1-1.

 

 

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 silk and circumferential silk to long wooden poles and then impregnating 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 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, wet and semi-dry:

 

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.

 

Compared with the Wet Method, 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

Storing and transporting chemical corrosive liquids, such as alkalis, salts, acids, etc., steel tanks are prone to rot and leakage, and the service life is very short. 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

Fiber winding road 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

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 shells, liquid rocket engine shells, FRP pressure vessels, deep-water external pressure shells, etc.

 

FRP wound pressure pipes can be filled with liquids and gases, and will not leak or break under certain pressure, such as seawater reverse osmosis pipes and rocket launch tubes.

 

The excellent properties of advanced composite materials have successfully applied various specifications of rocket engine shells 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 shells as small as a few centimeters in diameter and large transport rocket engine shells as large as 3 meters in diameter.

 

Repair Method of FRP Wound Pipes

 

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

 

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%.

 

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.

 

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.

 

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:

 

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.

 

The viscosity of the resin is too large. 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-based active diluents. The diluent for furan resin and phenolic resin is ethanol.

 

Improper selection of reinforcement material, the type of reinforcement material used should be reconsidered.

 

Improper operation process, appropriate dipping, brushing, rolling angle and other process methods should be selected according to the different types of resin and reinforcement material.

 

The Delamination Phenomenon of the Product is Caused by the Following Reasons:

1. a) The fiber fabric is not 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

 

Delamination of FRP Pipes: Reasons:

① The tape is too old;

② The amount of tape is too little or uneven;

③ The temperature of the hot roller is too low, the resin is not melted well, and the tape cannot stick to the tube core well;

④ The tension of the tape is small;

⑤ Too much oily demoulding is used, which stains the core cloth.

 

Solution:

① The glue content of the tape and the soluble resin content must meet the quality requirements;

② Adjust the temperature of the hot roller a little higher so that the tape is soft and sticky when it passes through the hot roller, and can firmly stick to the tube core;

③ Adjust the tension of the tape;

④ Do not use oily demoulding agent or reduce its dosage.

 

The Inner Wall of the Glass Fiber Reinforced Plastic Tube is Bubbling

 

The Reason is: the lead cloth is not close to the tube core.

 

Solution:

When operating, pay attention to make sure to keep the lead cloth close and flat on the tube core. The main reason for the bubbling or wrinkling of the glass fiber reinforced plastic tube 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 tube. When the tube is heated and cured, its residual volatilization expands due to heat, causing bubbling in the tube.

 

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 tube 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.

 

Reasons for the failure of the glass fiber reinforced plastic tube to withstand voltage:

① The tension of the tape is insufficient during rolling, the rolling temperature is low or the rolling speed is fast, so that the adhesion between the cloth and the cloth is not good, and there is a lot of volatile residue in the tube;

② The tube is not completely cured.

 

Solutions:

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

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

 

Issues to be Noted:

 

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 on installed FRP pipes and repairing pipe cracks, it is required to be completely dry in the factory and the resin and fiber cloth used during construction need to be cured for 7-8 hours. It is generally difficult to meet this requirement for on-site construction and repair.

 

The 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 are delayed by making a 0.5mm thick resin-rich layer and UV absorption (processed in the factory) on their surface. As the operation time goes by, the resin-rich layer and ultraviolet absorber will be damaged, thus affecting its service life.

 

The depth of soil cover is required to be high. Under the general roadway, the shallowest soil cover of SN5000 grade FRP pipe is not less than 0.8m; the deepest soil cover is not more than 3.0m; the shallowest soil cover of SN2500 grade FRP pipe is not less than 0.8m; the deepest soil cover is not more than 1.2m (the minimum and deepest soil cover of 12mm thick steel plate coil pipe 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.

 

At present, there has been no report on the large-scale use of FRP pipes by large water companies across the country. Since FRP pipes are new pipes, it is still unknown how long they will last.

 

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 (leakage, bursting), which seriously pollutes the environment and affects the water injection rate. After on-site investigation and analysis, the leakage is mainly caused by the following reasons.

 

Influence of FRP performance Since FRP is a composite material, the 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 the resin, resin diluent and curing agent, FRP rubber formula, etc.

 

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

 

Environmental influence, mainly the 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 impact and detection methods, the performance of FRP will decline, and there will be very few local non-standard pipe walls, 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. It is a product quality problem.

 

External damage FRP pipes have strict regulations during long-distance transportation and loading and unloading construction. If soft slings are not used, wooden boards are not used for long-distance transportation, and the pipeline of the transport vehicle exceeds the carriage by more than 1.5M, during the backfilling of the construction, it is 0.20mm away from the pipe. Using 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 occurred.

 

Design problems High pressure water injection pressure and large vibration, FRP pipe: The pipes are staggered, and the sudden changes in the axial and lateral directions generate thrust, causing the thread to disjoint and burst. In addition, in the connection parts of steel conversion joints, metering stations, wellheads, flow meters and FRP pipes, due to different vibration materials, FRP pipes leak.

 

Construction Quality Issues. 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, 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 the reasons for the leakage of FRP pipes.

 

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 basic water conservancy construction every year, causing pipeline damage and leakage.

 

Operational Errors. The injection pressure is high and the impact is large. FRP pipes cannot withstand the impact of loads. After being 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” and J/QH0789-2000 “Construction and acceptance specifications for buckled FRP pipelines”. Harbin Star FRP Co., Ltd.’s “Installation Instructions for Threaded FRP Pipeline Systems” and reference to GB1350235-97 “Industrial Metal Pipeline Engineering Construction and Acceptance Specifications” prevent common quality problems, ensure the construction of each process, and ensure construction quality. Preventive measures are proposed for the above 6 causes of leakage (see Table 1).

 

Solution

After the FRP pipe leaks, measures should be taken immediately to prevent environmental pollution. The most effective construction method is to cut and taper and connect them with steel conversion joints. The main process is to stop production → find the leak point → excavate → recycle sewage → on-site thread installation → install steel turn → weld → test pressure → backfill the trench → put into production. Construction pipe connection method

 

Construction Precautions:

 

Before cutting and tapering, according to the construction requirements of the HSE system, a warning tape should be drawn in the center area, and warning signs must be placed when entering the construction section. After the leak 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 pipeline is buried 1m deep), and the excavation is at least 20m on both sides from the leak point.

 

On-site thread installation

On-site thread installation process: cutting → taper cutting → bonding on-site threads → heating and curing. It is best to cut more than 0.3m away from the leakage point, select a suitable ratchet grinding machine (the manufacturer is equipped with special tools), the cone surface must be clean, free of grease, dust, and moisture, and the adhesive must be evenly mixed.

 

The on-site thread and the pipe end are bonded to drive out the bubbles on the bonding surface, and then tighten 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 turns. 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 0.3m above the natural ground.

 

Conclusions and Suggestions

 

High-pressure FRP pipelines are applied to the production of Jianghan oil injection wells and some water injection trunk lines, which solves the corrosion and perforation of pipelines, reduces pollution, extends the service life of pipelines, and saves investment.

 

Through implementation, the construction process of high-pressure FRP pipeline leak repair is standardized, the injection time rate is improved, safe production is guaranteed, and civilized construction is achieved. Since 2005, the average leak repair 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 progress 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 1. The reinforcing material is not completely impregnated with the matrix resin, and a part of the air remains in the fiber material, which is sealed by the solidified resin around it. 2. The problem of the glue itself. One is that the glue is mixed with air during the preparation process, and the air is not completely removed in time; in addition, the glue produces small molecules due to chemical reactions during gelation and curing, and these low molecular weight substances cannot escape in time. Measures to reduce voids

 

Optimize Materials

Select raw materials that match each other according to the characteristics of 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 using the resin, initiators, promoters, cross-linking agents, powdered fillers, flame retardants, antistatic agents and pigments will be added. When adding and mixing, a lot of air will be brought in, and measures must be taken to eliminate it.

 

Adjust the Glue Tank

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

 

Rolled Products

When white silk yarn is wrapped around the core mold, this phenomenon can only be eliminated by rotating the core mold. It must be eliminated by rolling the factory roller. Rolling is not only conducive to impregnation, but also makes the product dense, allowing the excess glue to flow to the lacking parts or flow 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. This phenomenon exists at the end and the barrel.

 

If the equipment is roughly manufactured, the precision 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, which is prone to fiber 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 suspension of the winding fiber.

 

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

 

Inspection of Fiber-Wound Composite Products

 

For fiber-wound composite products, the following aspects should be taken into consideration.

 

Appearance Inspection

 

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

 

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

 

Concavities (or wrinkles): The surface of the corrosion-resistant layer should be smooth and flat, and the thickness of the convex and concave parts of the reinforcement layer should not exceed 20% of the thickness.

 

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

 

Dimension inspection In accordance with the requirements of the drawings, use measuring tools with appropriate accuracy and range to inspect the dimensions of the product.

 

Inspection of Curing Degree and Lining Micropores

 

On-site Inspection

 

The surface of the composite product should not feel sticky when touched.

 

Dip clean cotton yarn in acetone and place it on the surface of the product to observe whether the cotton yarn changes color.

 

Use your hand or a coin to knock on the product to see if the sound is fuzzy or crisp. If the hand feels sticky, the cotton yarn changes color, and the sound is fuzzy, the surface curing of the product is considered unqualified.

 

Simple test of furan composite solidity 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 smooth and intact surface of the sample and the lack of color change of acetone are used as signs of curing.

 

Inspection and testing of product curing degree The curing degree of the composite material is indirectly assessed by testing the Barcol hardness. A Barcol hardness tester, whose model can be HBa-1 or GYZJ934-1, is used to convert the measured Barcol hardness into an approximate curing degree. The Barcol hardness of a winding composite product with relatively ideal curing is generally 40-55. The curing degree of the product can also be accurately tested according to the relevant provisions of GB2576-89.

 

Lining micropore detection When necessary, the composite material lining is sampled and inspected by spark detector or micropore detector.

 

Product Performance Test

 

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

 

5. Damage 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 surface of composite material products is sticky. The main reasons are as follows:

 

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

 

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 wax, other methods (such as adding cellophane or polyester film) can also be used to isolate the surface of the product from the air.

 

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.

 

For unsaturated polyester resin, 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:

 

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 groove type.

 

The viscosity of the resin is too high. When stirring or brushing, the air bubbles brought into the resin cannot be driven out. It is necessary to add an appropriate amount of diluent. The diluent for unsaturated polyester resin is styrene; the diluent for epoxy resin can be selected from inactive or glycerol ether-type active diluents such as ethanol, acetone, toluene, and xylene. The diluent for furan resin and phenolic resin is ethanol.

 

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

 

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

 

The reasons for the delamination of products are as follows:

 

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

 

The tension of the fabric is insufficient during the winding process, or there are too many bubbles.

 

The amount of resin is insufficient or the viscosity is too high, and the fiber is not soaked.

 

The formula is unreasonable, resulting in poor bonding performance, or the curing speed is too fast or too slow.

 

During post-curing, the process conditions are not suitable (generally premature thermal curing or too high temperature). Regardless of the cause of the delamination, the delamination part must be completely removed, and the resin layer outside the defective area must be polished with an angle grinder or polishing machine, with a width of not less than 5cm, and then the layer is re-laid 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 Specimen Preparation and Performance Testing

 

Composite materials 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 manuals or material specifications provided by manufacturers based on the material (or brand) when selecting materials. Composite materials are more accurate than materials, and 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 has been laid for the selection of raw materials. At present, the results predicted by micromechanical methods are still limited and can only be qualitatively estimated. The performance data required for the design of composite components needs to be obtained by basic performance tests, which is crucial to the 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 compressive properties in the directions of 0, 90 and 45 degrees, and the interface properties between the fiber and the resin are characterized by bending and interlayer 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, and GB3355-82, the unidirectional fiber composite flat plate is made, and then the fiber composite flat plate is processed into the sample size and quantity required by various test methods.

 

Production of Unidirectional Fiber Composite Flat Plate

The winding method is to make the fiber drawn from the yarn frame pass through the tensioner, glue groove, yarn guide roller, and winding nozzle in turn, and then wind it onto 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.

 

Do not use a demoulding agent to prevent the demoulding agent from migrating to the surface of the composite material and causing difficulties in the bonding process of the aluminum sheet. When a small amount of silicone ester is used to cover 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 be kept 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 surface dust, impurities and other foreign matter. 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 reinforcing sheets with aluminum plates, 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 placed in an acid treatment solution and heated to 65 degrees, 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 liquid is sulfuric acid: distilled water: potassium dichromate = 10:30:1. Before bonding the reinforcing sheet, polish the surface of the specimen with 100-mesh 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.

 

Because the tensile strength of unidirectional composite materials in the fiber direction is very high, the reinforcing sheet is easy to slip during the tensile test. It is recommended to use epoxy 618: 200# polyamide: diglycidol: imidazole = 100:80:15:2. The specimen and the reinforcement sheet are coated with adhesive and contact pressure is applied 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 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 sanded 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.

 

2. Preparation of NOL ring specimens 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 all borne 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 to wind a single ring on a special combination mold, and the other is to first wind the ring around the cylinder and then cut it into a ring. The outer surface of the ring can be processed or not.

 

The processed ring has high strength and small discrete coefficient. The unprocessed ring has an uneven surface, high surface resin content, and 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.