Research On Multi-Scale Unit Cells And Simulation Of Low Velocity Impact Damage Of Needled Carbon/Carbon Composites

A lot of fiber was introduced into the Z-plane of needled carbon/carbon composites by barbed needles, which can overcome the fatal shortcomings of low inter-laminar strength and low delamination resistance of conventional laminated composites. Meanwhile, needled carbon/carbon composites can be applied in aeronautics and astronautics industry widely, such as rocket nozzle expansion, throat and aircraft brakes with the excellent properties such as high specific strength, outstanding designable characteristic, light weight, good thermal performance etc. However, the microstructure of needled carbon/carbon composites is extremely complicated, which results in considerable difficulties in the researches of macro-meso-mechanical properties and damage mechanisms. This thesis is focused on the establishment of unit cells, prediction of elastic properties and low velocity impact damage analysis of needled carbon/carbon composites. Research work has theoretical and practical significance. The main contents are as follows:(1) According to the observation of microstructure of needled carbon/carbon composites,the unit cell with needled regions was founded. Elastic parameters of component materials are calculated, then the finite element model was implemented by introducing periodic boundary conditions and loads on the platform of ABAQUS for predicting the mechanical properties of composites.(2) Based on the theory of Cohesive Zone, a new unit cell with interface model was established by using cohesive layers to replace needled regions. The equivalent of constitutive relationship was implemented between unit cell with needled regions and the one with interface, by adjusting parameters of mechanical components of the model until the stress results was closest to obtained by the unit cell with needled regions(3) On the basis of the unit cell with interface and lamination theory, a finite element model for simulating the low velocity impact damage in needled carbon/carbon composites was proposed. In addition, discussions were carried out to uncover the influences of punch geometry, initial impact velocity and the stack order on the damage response.