Mechanical Properties And Damage Mechanism Of SiC Ceramic Containing Pre-existing Defects Under Uniaxial Compression

Ceramics are widely used in aerospace,precision machinery and microelectronics for their excellent mechanical,thermal and chemical properties.However,it inevitably introduces various random defects during the powder compaction and sintering preparation.And microcracks and microholes,which are varied in size,shape and distribution,are the most typical among these random defects.During the processing and service of materials,the compressive stress on the material can cause internal damage and failure to the material under the influence of defects.The defects is the source of internal crack propagation in materials.Its randomly distribution can lead uncontrollable effects on the material strength,service performance and reliability.Therefore,the study on the mechanism of meso-damage and fracture evolution of ceramic materials under external loads(especially under compression)is of importance for the performance testing,evaluation and service performance of ceramic materials.In this thesis,based on the discrete element method,it studies the influence of internal defects under uniaxial compression on mechanical properties and cracking modes of SiC ceramic.It is significant for further revealing the damage and failure mechanism and mechanical response of ceramic materials containing internal defects under compression.This thesis is divided into six chapters.The main research contents of each chapter are as follows:In chapter one,the background and significance of the study was introduced.And it briefly reviewed the crack propagation,mechanical properties and numerical simulation of brittle materials containing defects.At the same time,the discrete element method and its application in ceramic materials was described.Finally,it summarized the source of the topic and the research content.Chapter two established a model of a SiC ceramic containing a single pre-existing flaw on basis of the discrete element method.It focused on the effects of the flaw inclination angle on the mechanical properties and cracking process of the specimen under uniaxial compression.It also analysed the evolution of the force-chain field,displacement field and stress field around the pre-existing flaw in the process from the load to failure.And the influence of pre-existing flaw size in the model was discussed.Based on the hyperelliptic equations,chapter three established discrete element models of SiC ceramics containing three different shapes of single-hole defect.It was concerned about the effects of the aspect ratio and shape of the hole on the crack propagation and mechanical properties under the different hole inclination angles.And it also analysed the stress field and strain field around the pre-existing hole in the process from the load to failure.The effects of the hole size on the mechanical response were discussed.Considering the interaction relationship between microcrack defects at different positions and orientations,chapter four studied the propagation and coalescence of two parallel and non-parallel microcracks.Based on the application of the Mori-Tanaka method in the effective modulus calculation of microcracked bodies,the microcrack density function was introduced.Finally,the failure modes and mechanical properties of specimens with different crack densities were studied.Chapter five analysed the coalescence of two parallel and non-parallel elliptical hole defects at different positions and inclination angles.In addition,the hole density function was introduced based on the Mori-Tanaka method.The influence of hole density on the failure mode and strength of the specimen was studied.And the change of the effective modulus of the specimen under different hole density was compared.In chapter six,the contents of this thesis were systematically summarized.Then,the potential future work was also discussed.