Research On The Damage Coupling Mechanism Of Plain-woven Ceramic Matrix Composites

As innovative high temperature structural materials,plain woven ceramic matrix composites are very suitable to be used in the hot sections of aircrafts and spacecrafts,which have increasing requirements on the structural weight and extreme temperature.Under different stress states,understand and acquisition of the materials' damage mechanism and mechanical behaviors is very essential to optimize the preparation process and promote them for engineering applications.In service process,materials located in composite components always suffer complex stress states.The study on the damage mechanisms and mechanical behaviors of materials subjected to single axial stress has not been able to meet the needs of engineering design.Meanwhile,under the complex stress state,there are strong coupling effects between various types of damage process respectively caused by different stress components.The damage coupling effects can accelerate the damage evolution,transform the failure mode and decrease the mechanical properties of materials significantly.This could ultimately lead to a sharp decrease in the structural strength and service life of composite components.Therefore,the damage coupling effects deserve a targeted research.The researches were mainly performed on the 2D-C/SiC and 2D-SiC/SiC composites fabricated by chemical vapor infiltration method(CVI)in this thesis.The basic room temperature damage mechanisms and mechanical behaviors of materials were investigated systematically.The axial tensile hysteresis model was constructed and used to calculate the constituent properties of materials.Under in-plane shear loading conditions,a fiber bending-bearing mechanism was proposed,and the microscopic shear damage mechanisms and evolution model were built.Under off-axial loading conditions,the damage coupling effects of material were characterized and studied by the strain deviations.Besides,the decoupling analysis on the damage coupling effects of material was carried out by alternate loading experiments.The main contents and conclusions of this paper are listed as follows:(1)With various loading procedures,the axial tensile and compressive damage mechanisms and mechanical behaviors of 2D-C/SiC composite and 2D-SiC/SiC composite were obtained and analyzed comparatively.The axial tensile hysteresis model was constructed by shear-leg theory,combined with test results,values of four material constituent parameters(matrix cracking stress,residual stress,interfacial debond energy and sliding stress)were calculated out.Meanwhile,the influences of fiber properties and interface layer thickness on these parameters were studied and discussed.The calculating results show that 2D-SiC/SiC composite has bigger matrix cracking stress,smaller residual stress,smaller interfacial debond energy and smaller sliding stress.With a thicker interface layer,2D-C/SiC composite has smaller residual stress,interfacial debond energy and sliding stress,but the values of matrix cracking stress remains unchanged.(2)The in-plane shear damage evolution of 2D-C/SiC composite and 2D-SiC/SiC composite subjected to various loading procedures were analyzed comparatively.Based on the stress-strain behaviors and SEM(Scanning Electron Microscope)results of fracture surfaces,a fiber bending-bearing mechanism was proposed.And combined with the crack closure effect,the shape of shear hysteresis loops was elucidated well.Meanwhile,the microscopic shear damage mechanisms,damage evolution processes and shear mechanical properties under various loading procedures were also compared and analyzed.By considering the distribution of matrix cracks,two damage variables were defined to build a shear damage evolution model.The analysis results show that the differences between these two materials' shear damage behaviors and properties are mainly caused by the variation in the thermal residual stress within materials.Under in-plane shear loading conditions,the bridging fibers between crack surfaces withstand complex shear-bending load to transfer shear load.Compared to the monotonic loading conditions,complex loading procedure makes 2D-SiC/SiC composite has a faster damage evolution process and lower shear strength.During shear damage process,the initiation stress for matrix cracks in the 45° direction of 2D-C/SiC composite is much lower than 2D-SiC/SiC composite,but the initiation stress for matrix cracks in the 0°/90° direction of these two materials has the same value.(3)The influences of off-axial angle on the stress-strain behaviors,damage evolution,failure mode and mechanical properties of 2D-C/SiC composite were studied using off-axial tensile and compressive tests.By the strain deviations between the stress-strain behaviors of material subjected to single axial loads and off-axial loads,the tension-shear and compression-shear damage coupling effects in the fiber bundle directions of material were characterized and analyzed.SEM techniques were used to observe the fractured surfaces of specimens and identify the action mechanisms of the damage coupling effects.Research results are listed as follows: the damage coupling effect between tension and shear is mutual accelerative;the shear damage accelerates the compressive damage;but the effect of compressive stress components on the shear damage is suppressive.The strength of the coupling effects is controlled by the values of stress components.With increasing off-axial angle(0°~45°),the damage coupling effects increase the nonlinearity of material's stress-strain behaviors,decrease the tensile and compressive strengths of material,accelerate the damage evolution and make the failure mode of material change from axial tensile or compressive failure to the in-plane shear failure mode.Furthermore,based on the stress-strain data obtained from 0° and 45° tensile tests,a model was built to predict the off-axis tensile stress-strain behaviors of material.The prediction has good agreement with test results.(4)The axial alternate loading experiments were designed and preformed on the 2D-C/SiC composite.The quantitative decoupling analysis on the tension-shear and compression-shear damage coupling effects of material was carried out by alternate tension-shear and compression-shear loading tests.The effects of initial tensile and shear damage on the subsequent shear and tensile damage evolution were also studied.Experimental results show that the introduced tensile damage makes the shear modulus of material decrease by 48% before fracture,and the introduced shear damage makes the tensile modulus of material decrease by 68%.Likewise,the introduced shear damage makes the compressive modulus of material decrease by 63%,but the introduced compressive damage nearly has no effect on the shear modulus of material.The influences of the initial introduced tensile damage on the material's subsequent shear damage evolution exist in the whole process,and remain unchanged with increasing shear damage.However,the influences of the initial introduced shear damage on the material's subsequent tensile damage evolution only exist before crack saturation point,which are “covered” and become smaller with increasing tensile damage.