Parametric Modeling Of Needling Process And Constitutive Relationship Of Needled Composite

The needling(or needle-punching)technology can be used to produce needled carbon fiber preforms.For the needled preforms,the in-plane fibers are transferred to out-of-plane direction to enhance the delamination resistance capacity.And thus,needled preforms show better capacity than 2D reinforcements.The needling process has the advantages of simple process,low cost and displays outstanding designability characteristic.By needling process,large-scaled prforms with complex shapes can be produced.For now,needling technology has been adopted to manufacture fabric reinforcement for advanced composites and the needled composites have been widely used in airplane brake disc,solid rocket nozzle throats,exit cones high temperature furnace and crucible etc.However,the fiber architecture of the needled pre form is complicated and the microstructures of needling regions are always uncertain.And thus,the macroscopic mechanical properties of needled composite are scattered.This dissertation researches the microstructures and macroscopic mechanical properties of needled composite by experiments.The correlation between needling parameters,fiber structures and composite properties are studied.A macroscopic constitutive model is established to describe the nonlinear mechanical behavior of the composite.The uncertain macroscopic properties of needled composite are also analyzed in this dissertation.The needling process of the non-woven cloth/carbon fiber net laminated preform is introduced by this dissertation.The microstructures of needled preform s and C/CSi C composites have been observed by SEM and optical microscopes.Sufficient experiments are performed to investigate the macroscopic behavior and microscopic damage mechanisms of the composite.The fiber architectures of needled composites are determined by needling parameters such as needling density,depth and needling distribution.According to the needling process,a parametric modeling method is proposed to explicitly describe the distribution of needling regions in the composite and obtain the period ic unit cell of needled composite.Based on the microstructures of the needled composite and considering the uncertain fiber architectures of needling regions,this dissertation classifies the microstructures of needled composite into four typical microstru ctures and established four representative volume elements to predict the effective stiffness properties of local regions.Then the stiffness properties of the composite can be predicted by using average volume method based on the effective properties of local regions.And the effect of needling density,depth and distribution on the material performances are studied further.Macroscopic plastic deformations and secant modulus degradation of the needled composite were observed in the experiments.The evolution of plastic strains are determined by associated flow rule and a power type plastic hardening law.An exponential damage state function and three damage variables are formulated to characterize stiffness degradation for the composite in each material di rection.The effect of temperature on the tensile behavior of the composite is considered in this model by introducing a thermal damage variable.A combined elastoplastic damage model is then established to analyze the nonlinear mechanical behavior of need led C/C-Si C composites.Nonlinear stress-strain curves for the composite under off-axis tensile and shear loadings can be well described by this model.Furthermore,this model is embedded into Abaqus user defined material subroutine.And the efficiency of the constitutive model is verified by analyzing the mechanical behavior of a composite plate containing a center-hole subjected to tensile load.The uncertainty mechanical properties of needled composite is related to the needling parameters.Finite element models in specimen sizes have been established to predict the uncertain properties of the needled composite,in which the random distribution of the needling regions are considered.The stiffness and strength of the needling regions are supposed to obey uniform random distribution.Coefficient of variation of the modulus,strength and failure strain for the composite can be predicted.The influence of specimen size and needling parameters such as needling density,depth and distribution on the coefficient of variation for composite properties have been investigated.This dissertation establishes the correlation between needling parameters,fiber structures and macroscopic properties of the needled composite.The stiffness,strength as well as nonlinear mechanical properties of the needled composite can be predicted.In addition,the influence of specimen size and needling parameters on the coefficient of dispersion for the macroscopic properties can be clarified.This dissertation would be helpful to guide the manufacturing and design of needled composites in the engineering.