Fundamental Research On Microwave Curing Of CFRP Components

Carbon fiber reinforced polymer(CFRP)composite,due to its high strength-to-weight and stiffness-to-weight ratios,has become the preferred material for high-end aerospace equipment.Curing is a vital process for manufacturing CFRP components,by which materials and structures are formed synchronously,but the mismatched thermal expansion between fiber and resin,layer and layer,mold and part,as well as the curing shrinkage of resin will inevitably produce large and uneven stress inside the component.Temperature is an important parameter that needs to be precisely controlled during the curing process and is closely related to the cure induced stress of CFRP components.However,autoclaves,in which the material is heated by circulating airflow,is theoretically unable to regionally manipulate the temperature distribution of the CFRP component so that it cannot actively control the cure induced stress field.As a result,part deformation will be easily caused,and mechanical properties are difficult to be guaranteed.Focusing on the above problems,this thesis investigates several key technologies for microwave curing of CFRP components.The main achievements are described as follows:1.The microwave response and its mechanism of CFRP laminates are investigated.Since it is difficult to study the response of different CFRP laminates to microwaves at arbitrary incident angles using traditional methods,the range of refraction angle of anisotropic CFRP laminates to microwaves at arbitrary incident angles is analyzed,inspired by the refraction characteristics of electromagnetic waves in isotropic conductors.It is found that no matter what angle the microwave is incident on the surface of the CFRP laminate from the air,the refracted wave will propagate approximately along the surface normal direction.Accordingly,an analytical model is established for calculating the electromagnetic properties of CFRP laminates.The microwave reflection,transmission,and loss mechanism of the CFRP laminate are investigated.The impedance mismatch between the multi-directional CFRP laminate and the air is proved to be the primary reason for the total reflection of microwaves and heating inefficiency.2.A meta-surface based microwave heating method for CFRP components is proposed.To solve the problem that multi-directional CFRP laminates,from which the aerospace CFRP components are made,cannot be heated by microwave because of impedance mismatch,this thesis presents a new idea of introducing a low-loss artificial electromagnetic meta-surface on the surface of the CFRP component,so that the combination of the meta-surface and the CFRP component can obtain a matched impedance with air through electromagnetic resonance.As the resonance frequency of the combination of the meta-surface and the CFRP component can be customized,a zone-based heating method for CFRP components is further proposed by using multiple microwaves with different frequencies.On this basis,a finite element model is established to simulate the electromagnetic properties of the combination of the meta-surface and the CFRP component.The structural parameters of the meta-surface are optimized based on the parameter scanning method.High-efficiency microwave heating of CFRP components is achieved,as well as the precise zone-based heating.3.A temperature field control method with heating pattern compensation during the microwave curing process is proposed.To mitigate the cold and hot spots on the surface of the CFRP component during the microwave curing process,which leads to uneven temperature distribution,it is found that for the same heating system equipped with multiple microwave sources,when the temperature and cure degree of the CFRP are unchanged there is a fixed relationship between the power ratio of each source and the heating rate ratio(heating pattern)of each point on the CFRP surface.On this basis,a new idea of cotrolling temperature field by heating pattern compensation is proposed,which breaks through the limitations of existing methods with the random compensation principle.An online-learning and decision-making process control model is built using the neural network and PID algorithm,which greatly improves the temperature uniformity of the CFRP component during the microwave curing process.Based on the above research,a microwave heating prototype system with multiple frequencies is developed,in which CFRP laminates and C-shaped components are cured by zone-based microwave heating.Results of testing cases from laboratory and companies indicate that the cure induced stress and deformation are reduced by an average of 29.6% and 55.8%,while the mechanical properties are improved by varying degrees,among which the bending strength and interlaminar shear strength of the microwave cured CFRP laminates are increased by an average of 11.1% and 11.3%,compared with the autoclave cured counterparts.