| Reducing the structural weight and improving the efficiency of the structure are the eternal themes of the spacecraft development.As a medium for storing cryogenic propellants,the cryogenic tank accounts for more than 60%of the dry weight,which directly determines the overall lightweight level of the vehicle.Compared with the metal tanks,carbon fiber reinforced resin matrix composite tanks could achieve a structural weight reduction of more than 20%,which is the core to improve the efficiency of transportation and lead the upgrade of the aerospace industry.However,restricted by the air circulation heating mode and the complex structural characteristics of the components,it’s easy to cause the non-uniform distribution of the thermal and pressure field in the manufacturing process of large composite tanks,which induces the generation of curing defects and non-synergistic evolution of the deformation and performance,leading to the weakening of the overall structure and the existence of cryogenic propellants permeation through the tanks,which becomes the bottleneck restricting the application of composite tanks in spacecraft engineering.Therefore,in order to meet the urgent requirement for high comprehensive performance composite tanks to improve the spacecraft carrying capacity,this paper innovatively integrated the composites microwave curing process with the mechanical vibration energy field,and a vibration-microwave coupled out of autoclave curing process was proposed.On the one hand,the characteristic of microwave selective heating was further used to achieve rapid and uniform heating of composites,which greatly reduced energy consumption and solved the difficulty of uneven temperature field and asynchronous curing of complex components caused by the traditional electric heating mode.On the other hand,based on the team’s previous research to introduce the vibration energy field to greatly reduce the dependence of the components molding quality on the curing pressure,the vibration pretreatment was combined with the microwave curing process to overcome the contradiction that microwave generators was difficult to operate continuously at high pressure,while vacuum and low pressure conditions could not effectively reduce or eliminate internal defects in components.The main research work and achievements were as follows:（1）In combination with the T800/#602 composite material system for aerospace cryogenic propellants tank,the chemical cross-linking reaction equations of bisphenol A epoxy resins containing silane coupling agents were studied and developed by introducing epoxy group ring-opening chain-extending reaction.The differences in microwave absorbing ability of different constituents of composite materials were identified and the heating mechanism of microwave-cured composites was systematically clarified:Under the combined action of resonance effect,skin effect and tunneling effect,the carbon fibers in the microwave field would generate strong induced currents both on the surface and inside,tunneling currents would also be formed between the fibers,and the microwave energy would be consumed and absorbed through their own impedances.Therefore,the microwave absorption and response speed of carbon fibers were much higher than that of bisphenol A epoxy resins.Under the action of the same microwave power（2k W）,the heating rate of fibers was as high as25.6℃/min,which was about 3 times that of resins,and fibers could transfer the energy through conduction to the surrounding resins to promote their curing reaction.Further research found that compared with the traditional electric heating-air circulation transferring mode,the composite laminate cured by microwave had a higher degree of conversion of functional groups at the fiber-resin interface,and the content of hydroxyl functional groups generated after curing was 10.41%,which was 5 times that of thermal cured samples.The microwave cured laminate also had a better interfacial bonding performance,and the interlaminar shear strength increased by 10.74%.The work provided research foundations and analysis bases for the research of the vibration-microwave componund molding process.（2）The influence of geometric structure and feeding mode of microwave curing equipment on the temperature field uniformity of composite components was studied,and the calculation model for the correlation between the electric field location and the temperature field distribution of composites during the microwave heating was established.The regulation of different microwave feeding modes and mechanical structure optimization on the temperature field distribution of microwave heating composite materials was detected.The results showed that the heating rate of the composite component was only proportional to the square of the electric field intensity in the cavity at a constant microwave frequency.Therefore,the electric field inside could adequately map the temperature field distribution of the composite component.Further research found that the increase in the number of microwave generators and the symmetrical setting along the cavity could significantly improve the uniformity of the electric field,and the introduction of the mode stirrers promoted the regular changes of the electric field in the cavity,which both effectively solved the problem of asynchronous heating of the components caused by the uneven distribution of the electric field.On this basis,a design scheme of microwave curing system for uniform heating of composite components was proposed:Under the optimized heating mode of symmetrical distribution of seven generators and 6r/min stirring speed,the temperature distribution inside the component was relatively uniform,and the maximum temperature difference was reduced to 3.6℃ compared with 17.4℃ before optimization,with a decrease of 79.31%.The research work provided temperature field regulation scheme and curing system design scheme support for aerospace composite components.（3）Aiming at the problem of the failure of the whole tank caused by the overall performance weakening of the structure and the permeation of cryogenic propellants due to the difficulty in eliminating the internal defects of the composites under microwave vacuum and low pressure forming conditions,the evolution of the curing defects under the introduction of vibration pretreatment and different microwave heating rates was studied and identified:The introduction of vibration pretreatment could effectively eliminate the delaminateion within the microwave-cured component and inhibit the formation as well as growth of voids,with a reduction in porosity of over 60%at different heating rates.A method for testing and characterizing the permeability of aerospace cryogenic tanks considering the real service environment was proposed and a corresponding testing system was developed.The macro/mirco interfacial properties was studied by combining the short beam three-point bending method and the fiber push-in test based on Nanoindentation and the correlation relationship between the curing defects and the properties（including permeation resistance）under the regulation of composite energy fields was defined:Both the interlaminar bonding property and the fiber-resin interfacial bonding property of the composite component showed obvious non-linear relationship with curing defects.When the porosity declined from 1.25%to 0.41%,the interlaminar shear strength increased by 18.71%and the interfacial shear strength improved by 30.85%.The cryogenic permeation rate of the component under the traditional vacuum differential pressure testing method was significantly different from the results under 0.5MPa high differential pressure service condition,which verified the necessity of the new cryogenic permeability testing method proposed in this article.Accordingly,the correlation between the permeation rate and the porosity was identified:An exponential decrease in the cryogenic permeation rate of the composite component with the drop of the porosity,and when the porosity decreased from 1.25%to 0.41%,the cryogenic permeation rate of component declined from4.36×10-6Pa·m3/sto 1.72×10-7Pa·m3/s.The research work provided new ideas and technical support for high quality out-of-autoclave manufacturing of aerospace composite components.（4）The engineering trial production of vibration pretreatment-microwave curing complex surface components of aerospace composites was carried out,and the profile accuracy,curing defects,interfacial bonding properties as well as normal/cyogenic permeability of the cured components were tested,characterized and analyzed.The results showed that the maximum deformation of complex curved component formed by the compound curing process was about 0.40mm,which was less than the surface accuracy requirement of±1.5mm.The average porosity of the component was only 0.39%,the average interlaminar shear strength reached 71.51MPa and the normal along with cryogenic permeation rate under 0.5MPa differential pressure conditions was3.41×10-6Pa·m3/s and7.21×10-8Pa·m3/srespectively,which all arrived at the same level as the results of 0.6MPa autoclave molding component,verifying that the vibration pretreatment-microwave compound curing process proposed in this paper was suitable for the deformation-performance cooperative manufacturing of complex surface composite components.In this paper,by combining theoretical analysis with simulation modeling,platform construction and experimental verification,the scientific and technical problems in the deformation-performance cooperative manufacturing environmental regulation of aerospace composite components under the action of vibration-microwave energy field were systematically studied,and the traditional forming and manufacturing modes were changed,providing the high efficiency,low consumption and high quality out-of-autoclave curing manufacturing theories and technical methods of large scale composite components. |
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