Study On Meso-mechanics Model And Oxidation Behavior Of Needled-punched Carbon/carbon Composites

Needle-punched carbon/carbon composite(NP C/C composite)is a heat-resisted material with quasi 3D structure.Due to its balance between fabricating cost and inter-laminar mechanical properties,it has found extensive applications in thermal protecting structures.The needle-punched process inevitably introduces local defects in this composite,e.g.needle hole,fiber breaking and fiber deflecting in both thickness and planar directions.The overall mechanical properties of NP C/C composite largely depend on the densities and distribution patterns of these defects.So,a refined mechanical model should characterize these local features.NP C/C composite is usually used in structures experiencing high temperature.Oxidation of carbon material in high temperature might degrade its mechanical properties significantly.So,oxidation behavior and residual moduli and strength are essential research issues for this composite.The main works and research achievements in this doctoral dissertation are summarized as follows.1)Cross sectional imaging method is employed to obtain series meso-scope images of NP C/C composite.In order to get high quality images(so reconstructed model with good precision),a fixture is designed to improve the flatness and parallelism of cross section of the specimen during layer-by-layer sanding.Maximum entropy method is used to get proper threshold gray level in image for identifying phases of fiber bundle,matrix and pore.After noise reduction and pixel binarization the fiber bundle/matrix boundary is enhanced.This method is capable of dealing with the problem of low gray level contrast between fiber bundle and matrix,which is a difficulty encountered in image recognition of C/C composites.A method of reconstruction meso-scale FE model of composites based upon binarization images is proposed.For improving the precision of simulation,a node moving technology is suggested to smooth the zigzag boundary between two different phases of composite in FE model.The reconstructed FE model developed here can be used directly as input data of general FE package ABAQUS.2)Proper periodic boundary conditions are applied to the reconstructed FE model of NP C/C composite and progressive damages are simulated under tension and compression by ABAQUS.A criterion of Hashin is employed for tensile and compressive failure estimations of fiber bundle.The failure of matrix is judged by maximum stress law.The failure models mentioned above and the moduli degradations for the damaged elements are implemented in the platform of ABAQUS through user's subroutine of UMAT.The meso-scope progressive damages for NP C/C composite under various loads are simulated.The reconstructed FE model provides an effective method of mechanical property assessment of this composite.3)Based on the meso-scope observation of NP C/C specimens,the composite is generalized as five typical patterns.They are regions of without defect,needling entirely in matrix,needing through fiber bundle,needing through fiber bundle and breaking some fiber,needling through edge of fiber bundle and breaking some fiber,respectively.Five sub-cells are proposed to represent these five defects and the average moduli and damage evolutions on these sub-cells are predicted.The needle hole is simplified as elliptical inclusion and the stress concentration around it is estimated using meso-mechanical theory.The progressive damage of this composite is represented by the step-by-step failures of these five sub-cells in a representative volume unit(RVU).The RVU assembled with sub-cells and represents the least periodic needing pattern.Compared with reconstructed meso-scope FE model,this sub-cell model can also characterize the main damage features.Meanwhile it significantly reduces calculation time at cost of some detailed damage description.Using this sub-cell model,the effect of needling hole distribution on mechanical properties of this composite is investigated.For different needling distributions,the discrepancies for moduli and strength predictions are found.4)A curved beam model is established to characterize the tension(compression)/bending coupling of fiber bundles in NP C/C composite which are curved or deflected locally during punching process.The matrix bonding fiber bundle together is simplified as three-freedom spring.Stiffness degradations of fiber bundle and matrix caused by defects of broken fiber and pore in matrix are considered in the model.The gradually failures of the curved beam elements and spring elements can also represent the meso-scope damage evolution process of NP C/C composite in another way.5)The oxidation experiment of NP C/C composite is conducted in the environment of the open air with temperature of 600?.The curve of mass lose rate versus high temperature exposure time is measured.A fitting formula relating mass lose rate and original density of this composite is presented.Surface morphology is observed using microscope and open pore rate is measured by water absorption method before and after oxidation experiment,respectively.Tensile,compressive and bending moduli and strengths for NP C/C composite experiencing different oxidation time are tested.The residual mechanical properties versus oxidative mass loss are fitted and they are found can be described with hyperbolic functions.