Abstract:Synthesis of aluminum-containing SiC（N） ceramic precursors is the key process of fabrication of high-strength SiC fibers. With a brief classification of polysilane， this paper summarizes the synthesis and characterization methods of aluminum-containing SiC（N） ceramic precursors， including using solid polysilane， liquid polysilane and polysilazane. Meanwhile， the influences of aluminum on ceramics are discussed from three aspects. Furthermore， the future perspectives of aluminum-containing SiC（N） ceramic precursors are proposed.

Abstract:Preceramic polymers can be processed or shaped using conventional polymer-forming techniques， which show unique advantages in the preparation of high-performance ceramics and composites. The application of 3D printing technology in ceramic precursor molding provides a new possibility for the preparation of ceramic parts with complex structure. In this paper， the research progress on preparation of SiCO， SiCN， SiC， as well as B， Zr and other elements containing silicon-based ceramic precursors by 3D printing in recent years are systematically summarized from the aspects of precursor resin types， 3D printing technologies and their applications. The challenges and research direction faced by 3D printing ceramic precursors are pointed out in combination with the future development needs.

Abstract:Rubber sealing materials are important components of the spacecraft. However， they are still limited by their aging tendency and failure risk in the complex service environment.Therefore，it is of great significance to explore the storage life of rubber sealing materials. Combined with the applications of rubber sealing materials in space products， the research status and main technical methods of the storage life are introduced， and the typical prediction methods of accelerated storage life of sealing materials are analyzed.Moreover，the future research directions are summarized according to the urgent practical needs of space explorations.

Abstract:Nonmetallic composite is a kind of high-performance material with low density， high stiffness and high strength. At present， it has become an indispensable key structural material in aerospace satellites. But at the same time， this kind of material is also an anisotropic heterogeneous material which is very difficult to process. Using the traditional contact method， it is prone to produce problems such as edge collapse， delamination， fuzzing， tearing and so on. As an advanced manufacturing technology gradually moving towards practicality， laser manufacturing technology has a series of advantages， such as strong material removal ability， high machining accuracy and controllable damage. It is an ideal method to realize the high-performance machining of nonmetallic composites and meet the existing and future needs. Focusing on carbon fiber composites， aramid fiber composites and ceramic matrix composites， which are widely used in aerospace field， this paper systematically summarizes the research and application progress of laser processing non-metallic composites. The technology branch involves cutting， hole drilling， milling， etching， cleaning and other material removal manufacturing technologies. Finally， the future research focus and engineering application prospect of laser processing methods for nonmetallic composites are summarized and prospected.

Abstract:In order to improve the load-bearing capacity of the composite double-bolt hybrid bolted/bonded joints， model was built on ABAQUS and the feasibility was verified based on experiments and finite element methods. Then， the accuracy of proxy model was studied， and a multi-parameter optimization design was carried out on the plate width， end distance and hole distance of hybrid joints based on genetic algorithm. The results show that the quadratic polynomial surrogate model has the highest accuracy. Compared with the load performance of the conventional double-bolt hybrid bolted/bonded joints obtained in the test， the load-bearing capacity of the structure after parameter optimization has been increased by 44.35%.

Abstract:In order to increase structural stiffness of propeller blade，stacking orientation in different regions of the composite propeller blade was optimized respectively. The blade was divided into 4 zones according to its thickness and load condition. Stacking optimization was based on draping simulation results of different zones in the propeller blade，fiber orientation in draping simulation was 0° ，45° ，90° and -45°. Compared with the initial layup scheme of［0/45/0/-45］_sn，the optimized blade layup scheme made the main direction of the laminate close to the line connecting the centre points of each section of the blade. The 1st and 3rd natural frequencies of propeller blade were raised by more than 25% ，the 2nd natural frequency was raised by more than 5%. Displacement of propeller blade under uniformly distributed pressure loads was reduced by 50%，and displacement under thermal loads was reduced by 25%.The expected objective of increasing structural stiffness of blade is achieved by layup optimization. The method of optimization by region not only utilizes the designability of composite materials but also improves the optimization efficiency. It is suitable for layup optimization of complex composite structures.

Abstract:To study loss mechanisms of magnetic loss radar absorbing coatings （RACs）， two kinds of carbonyl iron powders and a kind of ferrite were used. The permeability and permittivity were tested by coaxial method， the attenuation constant， phase constant， wavelength， and attenuation loss of microwave with frequencies at 1~18 GHz in RACs were investigated. And the input impedance of RACs on microwave were calculated. The results show that the attenuation loss of microwave at 18 GHz by type S absorbing coating with thickness of 2 mm is -24.9 dB， which shows the electromagnetic energy is reduced to only 0.3% of the original energy. These frequencies of maximum of real input impedance Re（Z_in） of RACs stay the same with frequencies of maximum of reflection loss of radar absorbing materials separately. The impedance matching of real input impedance Re（Z_in） of RACs with intrinsic impedance of free space is not the only factor of reflection loss of radar absorbing coatings.

Abstract:In order to deeply understand the curing reaction process of 3，3-bis（azidomethyl）oxetane/tetrahydrofuran copolymer （PBT） binder and polyisocyanate curing agent （N100）， the modulus changes during the entire isothermal curing process and the viscosity changes before gelation for PBT/N100 binder system was studied by using a rheometer. The results show that the entire isothermal curing process of PBT/N100 binder system follows the autocatalytic kinetic model. According to the change of modulus with curing time， the curing process of PBT/N100 binder system is divided into reaction-control stage， gelation stage and diffusion-control stage. Dual-Arrhenius model can be used to describe the changes of viscosity in the reaction- control stage. The activation energy for viscosity and the activation energy for reaction are 36.271 kJ/mol and 54.882 kJ/mol， respectively. The Eyring model is used to calculate the thermodynamic parameters ΔH and ΔS during the curing process， which are 52.07 kJ/mol and -125.55 J/（mol·K）， respectively. It is basically confirmed that the ideal isothermal curing temperature for PBT/N100 binder system is 60 ℃， and the curing time is about 160 h.

Abstract:The C/SiC-SiO₂ composites with different SiO₂/SiC ratios were prepared by sol-gel-PIP process. The effect of SiO₂ content on the microstructure and mechanical properties of the composites was investigated. The results show that the tensile and compressive strength of C/SiC-SiO₂ composites with the mass fraction of SiO₂ no more than 25% are close to those of C/SiC composites. When the mass fraction of SiO₂ added exceeds 25%， the strength and modulus of composites decrease with increasing of SiO₂ content. It is worthy to mention that the fabrication cost of C/SiC-SiO₂ composite with 25% SiO₂ content is 24% lower than that of C/SiC composite， which provides a new technical approach for the rapid and low-cost preparation of C/SiC composites.

Abstract:Removal of tooling by electrolysis was one of the key procedure in hot isostatic pressing （HIP） of complex titanium alloy components. Based on the current studies， there were few works on the properties of titanium alloy prepared by HIP in sulfuric acid environment.The effects of different electrolysis time on the performance of HIP TA15 titanium alloy such as surface passive film， hydrogen content and mechanical properties under 40% sulfuric acid environment were studied. The results show that the thickness of passive film on the surface of TA15 titanium alloy increases gradually at 15， 30， 45， 60 d respectively， and reaches to 175.3 μm at 60 d.The increase of hydrogen content is not significant at 15 d with an average of less than 2×10^-5.After 30 d， the average hydrogen content increases remarkably and reaches to 5.4×10^-4 at 60 d. The yield and tensile strength of HIP TA15 titanium alloy decreases gradually with the prolonging of electrolysis time， while the plasticity is basically unchanged. The surface qualities and mechanical properties of HIP titanium alloy are effected by the time of electrolysis in sulfuric acid environment， subsequent process control need to be strengthened.

Abstract:The weld-bonding test was carried out based on Box-Behnken Design response surface method， the acoustic emission signals in the stretching process of weld-bonding， spot-welding and adhesive joints were collected， and the signal was decomposed and reconstructed by wavelet packet. The weld nugget failure fracture was analyzed based on the hits history diagram， and the damage models of three kinds of joints were established through the cumulative hits. The results show that the three types of joints have abundant acoustic emission signals in the yield stage and fracture stage. Adhesive layer failure and weld nugget failure can be distinguished by acoustic emission signal characteristics. And the failure frequency of the adhesive layer is mainly concentrated in 0 to 62.5 kHz. When the weld nugget fails， the frequency is mainly concentrated in 31.25 to 281.25 kHz.The acoustic emission hits history diagram can reflect the failure process of the nugget on the whole. The established damage model can better characterize the damage state of the specimen.

Abstract:Based on the I-II mix-mode high cycle fatigue crack propagation tests of AA7075-T6 aluminum alloy under different loading angles， the equivalent stress intensity factor （SIF） amplitude of the mix-mode crack tips was calculated by using M-integral in FRANC3D. By processing the data with seven-point increasing polynomial， the growth rate curve of mix-mode crack tip was calculated. The mix-mode fatigue properties were analyzed and the path and fracture properties of mix-mode crack propagation were discussed. The result of the experiment shows the parameters of Paris formula of pure type I fatigue crack propagation rate curve in the stage of stable growth and proves that， the lifespan of the type I fatigue crack propagation is the longest， and the life of mix-mode fatigue crack propagation have suffered various degrees of reduction. The numerical analysis and experimental results of the cracking direction of the mix-mode fatigue crack are compatible with the theory， and the fracture of the mix-mode fatigue is characterized by brittle fracture.

Abstract:The single factor milling test of SiC_f/SiC composites prepared by PIP was carried out with PCD milling cutter. The effects of milling process parameters on the cutting force and surface roughness after machining were analyzed. The formation mechanism of machined surface morphology of SiC_f / SiC composites was discussed by SEM analysis of machined surface and fiber fracture. The results show that the surface roughness and cutting force have the same change trend. High spindle speed and small cutting width are conducive to obtain the machined surface with small surface roughness. The material removal method near the hole area is different from that far away from the hole area. The in-plane deflection and interlaminar buckling of fibers in materials lead to a variety of fracture modes of fibers.

Abstract:The wing beam of medium fixed-wing unmanned aerial vehicle was a carbon fiber composite laminated structure. Although the conventional female mold-prepreg molding method could greatly improve the dimensional stability of the shape， it is difficult to ensure the positive pressure at the rounded corners and the phenomenon of bridging and bubble wrapping due to its forming even when the silicon rubber core mold was introduced. In this paper， the combined adaptive equalizing plate auxiliary tooling and the male mold-prepreg forming method were used to ensure the outer surface quality of the wing beam parts， and not only improve the inner surface quality of the wing beam parts significantly， but also simplify the process obviously， which greatly avoids the phenomenon of bridging and bubble wrapping caused by the forming. Through the furnace production and performance test， the bending strength， bending modulus and interlaminar shear strength of unidirectional prepreg are 1 549 MPa， 110 GPa and 87 MPa， respectively.

Abstract:In order to control the chip formation and improve the chip removal quality of CFRP suction type internal chip removal system， a two-dimensional right angle cutting force model was established. The chip formation process was studied， and the chip shape was predicted and classified.KEYENCE VHX-1000 was used to observe the chips. The results show that the main causes of chip formation are chip bending fracture and shear failure. The chip size is generally between 1.02~1.80 mm. The chip shape is mainly divided into strip chip， micro round chip and meter chip. When the temperature reaches the glass transition temperature of resin， the chip will deform， C-shaped chip and fiber will appear the pull-out phenomenon， the proportion of chips with different shapes is strip chip， micro round chip， C-shaped chip and fiber pull-out chip， except for the rice chip which can not be estimated. The experimental results are in good agreement with the theoretical analysis.