Research Progress On Quality Control Of Composite Wind Blades Formed by RTM Process

 

 

The formation mechanism of bubble defects in the typical RTM molding process of composite wind turbine blades is analyzed from the perspective of process principles and process influencing parameters. At the same time, the current research status of bubble defect control in RTM process at home and abroad is reviewed. Finally, the quality control research of RTM molding process of composite wind turbine blades is prospected.

 

As an emerging green and environmentally friendly energy technology, wind power generation has been widely used in the development of renewable energy. Wind turbine blades (referred to as wind turbine blades) are the most critical and basic components for capturing wind energy. Due to the harsh and long-term service environment, they are required to have sufficient fatigue strength and mechanical properties. At the same time, they also require low manufacturing costs and easy installation and maintenance.

 

At present. Composite materials composed of glass fiber, carbon fiber and thermosetting/plastic resin have been widely used in wind turbine blades due to their high specific strength, specific stiffness, good corrosion resistance and designability. There are many molding processes for composite fan blades, mainly including hand lay-up, compression molding, prepreg molding, pultrusion, fiber winding, resin transfer molding (RTM), vacuum infusion molding, vacuum assisted resin infusion (VARI), etc.

 

Among the many molding processes for fan blades, the RTM process uses overall closed mold molding, which can produce composite blades with high overall surface finish, high size and shape accuracy, and can make the entire blade molded in one time without secondary bonding, which can save tooling and improve molding efficiency and manufacturing costs.

 

In addition, when using the RTM process to mold composite fan blades, the closed mold can ensure that the harmful volatiles in the resin are leaked, which has little damage to the environment and the health of workers. Therefore, the RTM process is currently the only composite molding process that meets international environmental protection requirements, and is often used as the preferred molding process for composite fan blades.

 

For the quality control research of composite fan blades molded by RTM process, people often pay more attention to the shape molding accuracy and rationality of the structural design of the fan blades, but not enough attention to the internal molding quality of the fan blades. Due to the harsh and long-term service environment of composite fan blades, the internal molding quality of the blades must be guaranteed to ensure that they can exert excellent mechanical properties in complex service environments.

 

In the process of RTM molding of composite fan blades, the factors affecting the internal quality of the blades are mainly bubbles, glue deficiency, delamination (or cracks), dry spots, preform deformation, etc. Among them, bubbles are the most common. The presence of a large number of bubbles seriously reduces the shear and bending properties of the blades.

 

Since the manufacturing of composite fan blades in the RTM process is a manufacturing process in which both material forming and property occur simultaneously, the molding of the product must achieve form-property synergy, and internal quality control must also be given enough attention.

 

Aiming at the typical RTM molding process of composite fan blades, this paper analyzes the main defect that affects its quality control, the formation mechanism of bubbles, from the perspective of its process principle and process influencing parameters.

 

At the same time, the current research status of bubble defect control in the RTM process at home and abroad is reviewed. Finally, the quality control research of the RTM molding process of composite fan blades is prospected.

 

RTM Process Principle

 

Resin transfer molding (RTM) is a low-cost molding method widely used by foreign manufacturers. Its basic principle is: first, a preform of a reinforcement material designed according to performance and structural requirements (generally glass fiber, carbon fiber or glass fiber/carbon fiber mixture) is laid in the mold cavity, and a special low-viscosity resin system is injected into the closed cavity by injection molding equipment. The exhaust system keeps the resin flowing smoothly, thereby exhausting all the gas in the cavity and thoroughly soaking the fiber. The mold heating system heats and solidifies the resin to form a component. The process flow is shown in Figure 1.

 

 

Formation Mechanism of Bubbles Inside Composite Fan Blades

 

In the manufacturing process of RTM composite wind turbine blades, the resin flows in the fiber-reinforced preform under the action of a certain filling pressure and temperature. There are two main ways of flow:

(1) the resin flows in the gap between the fiber bundles;

(2) the resin flows in the gap between the fiber filaments inside the fiber bundles.

 

The gap between the fiber bundles is relatively large, and the bubbles formed are generally relatively large, while the gap between the fiber filaments is much smaller, so the bubbles formed are generally called small bubbles or microbubbles. During the filling process, the flow of resin is mainly affected by two driving forces: dynamic pressure and capillary force. The inconsistency of the two driving forces causes the flow rate of the resin between the fiber bundles and the fiber filaments to differ. The advanced or lagging resin flow eventually leads to the generation of large and small bubble defects.

 

Under certain external conditions such as materials, mold tooling, and blade structure, the filling process of the resin is mainly affected by temperature and filling pressure. When the temperature is constant, if the filling pressure (i.e. dynamic pressure) is low, when it is lower than the capillary force between the fiber filaments, the capillary force will play a dominant role in the flow of the resin. At this time, the resin flowing in the fiber bundle will flow to the gap between the fiber bundles, and the smaller filling pressure will cause the flow front of the resin to lag behind, and the air that has not been discharged will be encapsulated between the resin flow front and the infiltrating resin, forming a large bubble defect.

 

When the filling pressure is constant, if the temperature is high, the viscosity of the resin will decrease with the increase of temperature, and its fluidity will also be improved. Under the combined effect of dynamic pressure and capillary force, the resin flow rate between fiber filaments will be higher than the resin flow rate between fiber bundles, and eventually large bubbles will be formed between fiber bundles.

 

The formation mechanism of small bubble defects is exactly the opposite of that of large bubbles. It is caused by dynamic pressure being higher than capillary force. Small bubble defects mainly exist in the gaps between fiber filaments. The specific formation mechanism of large and small bubble defects is shown in Figure 2.

 

 

Current Status Of Research On Bubble Defects At Home Snd Abroad

 

In the manufacturing process of RTM composite fan blades, the main factors affecting the bubble defects inside the blades include resin viscosity, reinforcement material structure and performance, porosity, mold surface quality, mold temperature and injection pressure. Domestic and foreign scholars have achieved rich research results in terms of experiments and theories on the relationship between various influencing factors and the formation or elimination of bubble defects.

 

• Theoretical Study of Bubble Defects

 

Deng Yanping in China analyzed the flow model of existing resins, the formation and discharge mechanism of bubbles in the process, and studied the gap between the existing models and the heterogeneous porous fiber medium system. Feng Wu used multi-layer plaid cloth as the object to simulate the formation mechanism of bubbles by numerical methods.

 

Shao Xueming used the finite element control volume method to numerically simulate the diffusion of resin and the bubble formation process in the multi-layer woven fabric of textile composite preforms. The results of theoretical analysis and numerical simulation were basically consistent. Pamas abroad established a one-dimensional model of bubble inclusion during the flow process for the resin flow perpendicular to the fiber bundle direction, described the morphology of the resin flow front in the large pores between fiber bundles and the small pores in the fiber bundle, and proposed the influence of the fiber bundle structure on bubble formation.

 

Chen used the equal refractive index technology, enhanced microscope and high-power camera to track the flow front of the resin glue, and found two forms of bubbles in the RTM process: cylindrical microbubbles in the fiber bundle and spherical large bubbles between the fiber bundles, and proposed the mechanism of bubble formation through image analysis technology. Kang et al. conducted a theoretical analysis of the flow parallel and perpendicular to the fiber bundle, and established models to describe and predict the formation of bubbles.

 

Patel explored the fiber wetting and bubble formation problems during RTM filling. They studied the formation mechanism of bubbles in the filling flow through flow visualization experiments, proposed that bubble formation is related to capillary force and the contact angle of liquid-fiber-air, and established a resin matrix flow model in porous media to explain the bubble formation mechanism. Can et al. considered the effect of surface tension in their cylindrical bubble formation model in the fiber bundle, making the model closer to reality.

 

3.2 Experimental Study of Bubble Defects

 

Feng Wu in China took multi-layered gingham as the research object. On the basis of theoretical analysis, he studied the bubble formation of multi-layered gingham cross-section through visualization experiments. Qin Wei used the cavitation effect of ultrasound to reduce the viscosity and surface tension of the resin, improve the wettability of the fiber and the resin, thereby controlling the bubbles inside the part and improving the performance.

 

Peterson abroad used glass tubes with different diameters and bends to study the flow of resin and proposed the concept of capillary action number. Molna et al. took microscopic photos of the resin flowing through unidirectional fiber fabrics at high and low flow rates. The results showed that when the flow rate was low, the fluid flow rate between the fiber filaments was faster than that between the fiber bundles. This was because of the influence of capillary force. When the flow rate was high, the influence of capillary force was small, and the fluid flow rate between the fiber filaments was significantly slower than that between the fiber bundles.

 

Lundstrom et al. studied the effect of vacuum assistance on bubble formation in the RTM molding process-the effect of vacuum degree on bubble content and the size of the bubble-containing area, and determined the bubble volume content by optical microscopy and image analysis technology. Hull described the areas and types of bubbles that are prone to form in FRP, and found that the bubbles formed between and within the fiber bundles may be round, or elongated into elliptical cavities parallel to the fiber bundles. The size of these bubbles is related to the pores between and within the fiber bundles.

 

Judd concluded through research that for every 1% of bubbles in the product, the interlaminar shear strength of the composite material will decrease by 7%. It can be seen that the presence of bubble defects is extremely detrimental to the mechanical properties of the composite material, not only reducing the bending strength, durability and fatigue resistance of the composite product, but also increasing the sensitivity to climate and the dispersion of hygroscopicity and strength properties.

 

3.3 Elimination of Bubble Defects

 

In the study of eliminating bubble defects, researchers have conducted a lot of research mainly from the aspects of material selection, molding manufacturing environment and molding process. Li Caiqiu in China believes that lower resin viscosity is conducive to the elimination of bubbles, and points out that the viscosity at a temperature of 25°C is preferably 0. 5-1.5 Pa·s. In addition, in the selection of reinforcing materials, different types of reinforcing materials are preferably selected in the horizontal and vertical directions.

 

Therefore, when using the RTM process to mold composite fan blades, the use of glass fiber and carbon fiber to reasonably design the ply can not only produce products with higher strength than traditional glass fiber reinforced fan blades, but also have more ideal internal molding quality. Qin Wei, Wang Wei and others invented the ultrasonic exhaust method and the pressure shock wave exhaust method respectively, which realize the exhaust of bubbles under the premise of increasing the infiltration effect of fiber and resin.

 

Baig abroad found that the use of mechanical vibration can effectively reduce the effective viscosity of the resin and shorten the filling time. Lundstrom pointed out that the use of vacuum-assisted molding can effectively extract the air in the mold cavity during resin filling, thereby increasing the wettability of the fiber and obtaining a product with lower porosity.

 

Outlook

 

With the rapid development of the wind power industry, wind turbine blades are developing towards lighter weight, larger size and lower cost.

 

RTM process and the VARTM process, RTM-Light process, SCRIIM process derived from RTM process have been more widely used in the manufacturing field of composite wind turbine blades.

 

However, no matter which process is used, the internal molding quality control of the blade is the top priority, and the control of bubble defects is often the key factor in measuring the process quality.

 

At present, the research on molding quality of RTM process is still under exploration. Therefore, in future research, the following aspects should be focused on the exploration of composite wind turbine blades:

 

(1) Material System:

Develop a resin system with low viscosity, low volatility and high curing efficiency. At the same time, in order to meet the current requirements for blade size and strength, a mixed reinforcement method of glass fiber and carbon fiber is adopted, which can not only ensure the service performance and service life of the blade in harsh environments, but also reduce manufacturing and maintenance costs.

(2) Mold Tooling:

Optimize the design of mold tooling, and use simulation software (such as Mold flow finite element software) to further optimize the process parameters such as the number, position and flow rate of mold filling channels. At the same time, according to the structural characteristics of different fan blades, design local heat source supplement to ensure uniform temperature inside the tooling, so as to facilitate good infiltration of fiber and resin.

(3) Control Process:

Strictly control key process factors such as temperature and mold filling pressure, and adjust process parameters in time at different stages of mold filling to ensure that air and volatiles are discharged from the mold cavity to the maximum extent.

(4) Online Monitoring:

Develop a comprehensive online monitoring system to monitor process parameters such as temperature, pressure, viscosity, and resin curing degree in real time. At the same time, establish a specific quantitative relationship model between bubble defects and process parameters, and use the online monitoring system to adjust process parameters in time to reduce the porosity of the product.