Research On Microstructure And Mechanical Behavior Of Three-dimensional Six-directional Braided SiC_f/SiC Composite

Reducing the weight of aero-engine turbine structural parts and increasing the temperature tolerance,and then increasing its thrust-to-weight ratio are the core ways to improve the performance of new military aircraft.At present,the service environment of new aircraft is more demanding,and the service temperature and force-thermal performance of traditional nickel-based and cobalt-based superalloy materials are close to their limits and cannot meet the requirements of the turbine outer ring.The three-dimensional braided SiC_f/SiC composite material not only has the characteristics of high temperature resistance,light weight and oxidation resistance,but also fundamentally overcomes the defect of easy delamination of laminated composite materials,and has"near net shape"and performance"tailor-made body"feature,has become an ideal candidate material for the new turbine outer ring.However,limited by the high manufacturing cost of SiC_f/SiC ceramic matrix composites and the complexity of the three-dimensional braided structure,the characterization of the microstructure and mechanical behavior of the three-dimensional braided SiC_f/SiC ceramic matrix composites is still in the exploratory stage.Based on the four-step 1×1 braiding technology and the precursor impregnation pyrolysis(PIP)composite process for the outer ring components of aviation turbines,this paper prepares three-dimensional six-directional braided SiC_f/SiC composites,focus on the fine-scale reconstruction of the meso-structure,experimental characterization,and high-fidelity numerical simulation,in order to provide data support for the design of the hot end structure of the turbine outer ring.The main research contents and conclusions of the thesis are as follows:(1)Use Micro-CT three-dimensional tomography technology to scan the complete sample of three-dimensional six-way braided SiC_f/SiC composite material,and use VG Studio Max to extract yarn morphology and porosity characteristics to obtain the authenticity of four-way yarn,five-way yarn and six-way yarn Filling factor,carried out statistical analysis of pores.On this basis,considering the cross-sectional size of the yarn bundle,the extrusion correlation and the pore characteristics,a three-dimensional six-directional braided SiC_f/SiC composite material with pore defects was constructed through the Rand stochastic algorithm.Research shows that the established model is in good agreement with the real structure,and provides enough support for the subsequent numerical prediction of the mechanical properties of the three-dimensional six-way braided SiC_f/SiC composite.(2)The universal testing machine was used to carry out the longitudinal tensile,longitudinal three-point bending,transverse tensile and transverse three-point bending tests of the three-dimensional six-directional braided SiC_f/SiC composite material,and obtained the stress-strain curve and key mechanical characteristic values,and passed HX-5000 ultra-depth of field three-dimensional microscope and SU1510 scanning electron microscope analyzed the fracture damage morphology,clarified the failure mode and damage mechanism,and constructed the tension/bending relationship.Studies have shown that the three-dimensional six-directional braided SiC_f/SiC composite material exhibits anisotropic characteristics.The longitudinal tensile strength,tensile modulus,bending strength and flexural modulus are 10.37 times,4.62times,5.06 times and 1.45 times that of the transverse direction,respectively.Under tensile load,the longitudinal sample cracks expand along the six-direction yarn in a"Z"shape,and the transverse specimen cracks propagated along the braid axis,causing the material to fracture at a lower strength,showing ductile fracture characteristics.Under the three-point bending load,the longitudinal and transverse sample cracks both expand from the tensile side to the compression side along the thickness direction,and form different fracture morphologies on both sides,which eventually leads to ductile fracture of the longitudinal and transverse samples,however,the strength and toughness of the longitudinal specimen is obviously better than that of the transverse specimen.(3)Based on the constructed internal unit cell model of the three-dimensional six-directional braided SiC_f/SiC composite with pores,the component material damage coefficients are introduced and their elastic constants are modified.Combined with the three-dimensional Hashin criterion,the maximum stress criterion and the continuous damage constitutive model,a three-dimensional six-directional constitutive model is developed.Prediction of stiffness and strength of braided SiC_f/SiC composite under uniaxial tensile load.The results show that when the fiber is damaged to 4/5 of the original parameter(ξ_f=0.8),the theoretical values of the longitudinal and transverse elastic modulus are in good agreement with the experimental results.At the same time,the longitudinal tensile stress-strain curve and damage morphology are in good agreement with the test results.Among them,the four-way yarn is the first to be damaged,and the damage area is concentrated in the contact part of the four-way yarn with other yarns and the matrix;as the tensile strain increases,the five-way yarn and the matrix begin to have initial damage;when the maximum load is approaching,obvious failure units appeared in the five-direction yarn and the matrix;subsequently,the damage pattern showed a zigzag failure,which verified the accuracy of the numerical model.