Mechanical Property And Failure Mechanism Of C_f/SiC Corrugated Lattice Core Composites

Hypersonic vehicles have to maintain a high speed in the air for a long time,which requires them to withstand severe thermal environment and aerodynamic loads.Therefore,its thermal protection system materials must have light weight,high strength and toughness,extreme high temperature resistance,near zero ablation and other properties.In recent years,the Integrated Thermal Protection System(ITPS)is proposed to meet this requirement.Among many materials,C_f/SiC(C_f stands for long fiber)lattice structure composite material has become one of the most promising ITPS candidates due to its light weight,high strength,high temperature resistance,corrosion resistance and a series of excellent properties.In this dissertation,a new concept of light weight,high strength and high temperature resistance C_f/SiC corrugated lattice structure composite material is proposed according to the application requirements of the thermal protection system of hypersonic aircraft,such as light weight,bearing capacity,heat insulation and other multi-functions.The composite sandwich plate of C_f/SiC corrugated lattice structure was prepared by PIP and HP technology.The mechanical properties and failure mechanism of C_f/SiC corrugated lattice composites were systematically researched by experimental study,theoretical analysis and numerical simulation.The main contents and conclusions are as follows:A C_f/SiC corrugated lattice composite sandwich plate was designed and prepared.The mechanical properties of flat compression,lateral pressure and three-point bending were tested by experiments,and the failure process and failure mode of sandwich plate were analyzed.The mechanical properties of sandwich plates are predicted by using the equivalent theoretical mechanical model.Based on the Abaqus software platform,the numerical simulation method of mechanical properties was established,which accurately captured the maximum stress failure area of sandwich plate under the action of flat compression,lateral pressure and three-point bending load.The results of the software are in good agreement with the experimental results.The effect of vacuum heat treatment on the mechanical properties of C_f/SiC corrugated lattice composites was studied.The results show that the compression strength of sandwich plate increases first and then decreases with the increase of temperature after heat treatment,and the compression modulus decreases gradually.The mechanical properties of sandwich plates after heat treatment at1400?were greatly improved,and the compression strength and modulus were 2.84 MPa and 40.13 MPa respectively.As the heat treatment temperature increases,the fracture mode of sandwich plate changes.The sandwich plate showed brittle fracture behavior after heat treatment at 1800?.The mechanical properties of sandwich plates at 1600?decrease with the increase of heat treatment time.The compressive strength of sandwich plate decreases with the increase of ambient temperature.Compression modulus changes little,between34.61 and 40.13 MPa.Compared with untreated specimens,the compressive strength and modulus of sandwich plates after heat treatment at 1400?decreased by 39.08%and 13.76%respectively at 1800?.The effect of oxidation on the mechanical properties of C_f/SiC corrugated lattice composites was studied.The composition and microstructure evolution of C_f/SiC composites after oxidation were characterized,and the oxidation mechanism and damage mechanism of C_f/SiC composites were investigated.The results showed that the mechanical properties of C_f/SiC corrugated lattice composites decreased significantly after oxidation.When the oxidation time is 30min,the strength retention rate of C_f/SiC corrugated lattice composite sandwich plate decreases first and then increases with the increase of oxidation temperature.The compression strength retention rate of sandwich plate after oxidation for 30min at 1200?and 1600?was 75.45%and 88.49%,respectively.The strength retention rate of C_f/SiC corrugated lattice composite sandwich plate decreases with the extension of oxidation time when the oxidation temperature is 1600?.After oxidizing at 1600?for 90 min,the compression strength of sandwich plate decreased by 40.17%,and the compression modulus did not change much.The oxidation of long time changed the fracture behavior mode of sandwich plate,and the fracture mode of sandwich plate changed to brittle fracture after the oxidation for 60 min at 1600?The thermal-force coupling response of C_f/SiC corrugated lattice composite sandwich plate was studied.The differences of temperature field and stress field obtained by sequential coupling and full coupling are analyzed by comparison,and the applicable scope of different coupling modes is determined.The effects of different constraints and geometric dimensions on the thermal stress distribution of sandwich plates are investigated.It is found that the maximum thermal stress of sandwich plate is at the bonding surface of core bar and panel,except for type II constraint condition.The thermal stress in the structure can be reduced by increasing the core thickness,the core layer height or the core inclination angle without changing other geometric dimensions of the sandwich plate.In this paper,the damage and failure mechanism of C_f/SiC corrugated lattice composite under high temperature were investigated.The variation of high temperature mechanical properties of C_f/SiC corrugated lattice composite with temperature and time was revealed.The thermal-force coupling model of C_f/SiC corrugated lattice composite sandwich plate was established.Meanwhile,the effects of different boundary conditions and geometric dimensions on temperature and stress distribution of sandwich plate were obtained.