Research On Meso-structure Characterization And Mechanical Behavior Simulation Of Needled Carbon/Carbon Composites

Carbon fiber reinforced carbon matrix(C/C)composites have such advantages as low density,superior mechanical properties,higher thermal conductivity and lower thermal expansion coefficient together with good frictional performance,all of which have enabled their extensive application in many engineering structures.The needling process can improve the interlaminar properties of C/C composites by introducing fibers perpendicular to the laminar direction.Needled C/C composites replace 2D laminated composites by their high interlaminar strength and overcome the shortcomings of complicated crafts and high cost of 3D C/C composites.Hence,needled C/C composite has been widely used in aeronautic and aerospace fields,such as airplane brakes,solid rocket motor,nozzle throats and exit cones etc.In recent years,macroscopic experimental research has enhanced the understanding of the mechanical properties of needled C/C composites.However,the simulation analysis of the meso-mechanical behavior is still at the initial stage,and the research on mechanical performance prediction and damage mechanism analysis needs to be improved.In order to study the needle-punching effect on the mechanical properties of C/C composites,based on the study of meso-structure characterization,this paper carries out the simulation research of the meso-mechanical behavior.The simulation calculation is in good agreement with the actual structure and the craft,which can provide reasonable analysis and guidance for the optimization of the composite process.The main works are shown as follows:Firstly,the microstructure of high-density needled C/C composites is characterized by micro-CT and three-dimensional reconstruction.It is found that the porosity of the felt layer is much higher than that of the carbon-cloth layer and the needled zone.The internal gap of the layers made by needling is the source of large pores.High-pressure liquid phase immersion method can reduce the porosity of carbon-cloth layer,and then improve the density of C/C composites.The calculations of porosity sphericity,the elevation angle and azimuth angle show that the pore distribution of different layers are related to the arrangement of fibers in the layer,and the pores of the carbon-cloth layer are obviously distributed along the fiber distribution.Secondly,the unit cell model of needled C/C composites is established.The unit cell is divided into three sub-regions,such as carbon-cloth layer,felt layer and needled zones.Among them,the needled zones have random distribution characteristics,which is consistent with the actual structure of the composites.Based on the quadratic homogenization algorithm of Mori-Tanaka model,the material parameters of each region are calculated.Then the effective elastic performance can be predicted under the periodic boundary condition of the cube cell model.Then the mesomechanical analysis is carried out based on the random unit cell,including deformation and stress analysis,effective modulus prediction and the comparative verification of laminar model.It is found that the distribution of needled zones can cause local stress concentration in different situations.However,under the same needling density,the distribution of needled zones has little effect on the effective elastic modulus of composites.The slight decrease of the elastic modulus is mainly due to the decrease of the volume fraction of the needled zones by the partially region coincidence.Reducing the porosity of the carbon-cloth layer helps to improve the elastic properties of the composites in all directions,which will mainly depend on the improvement of the densification process.In a certain range,the increase of the needling density and depth will reduce the axial tensile modulus,but helps to enhance the shear properties and normal tensile properties.This result is very similar to the increase of the needled zone diameter.Laminate structure model can be used for the macro-mechanics analysis of the variable cross-section shell of needled C/C composites.The comparison between the laminar model and the random cell model shows that it will cause a large computational error in the analysis of internal shear stress,if the cohesive constitutive model is used to characterize the interlayer enhancement effect of needled fibers.At last,the combination of shear-lag theory and finite difference algorithm is proposed to provide a highly efficient and accurate shear-lag numerical algorithm.The needling effect on the meso-stress concentration of C/C laminates is analyzed.The increase of the hole size is shown to bring bigger stress concentrations.The broken width is more significant than the separated length for the improvement of the stress concentration degree.The fiber diameter/spacing ratio is also shown to have an important influence on the stress concentration.The high fiber diameter/spacing ratio,which means high fiber volume fraction,leads to small overload and ineffective areas.Few interactions of each needled area are found in the ordered distribution and random needling distributions are compared to the ordered one.These random distributions have a smaller overload area in size,while they get a larger ineffective area.Random needling distributions are more realistic representations of C/C laminates than the ordered distribution.The analysis of the meso-stress concentrations reveals the mechanism of the needling effect on the longitudinal tensile strength of C/C composites.The increase of the fiber volume fraction explains the reason why the tensile strength increases as needling density is increased from 20 to 40 punch/cm2.Since then the tensile strength decreases with the increase of needling density.This is not highly correlated to the reduction of the distance between each needled hole.The enlarging of each needled hole,especially the accumulation of broken fibers,during the cyclic process of overlaying laminates and punching is the prime reason for strength reduction.