| With the introduction of the advanced design concept of―Integrated thermalprotection-insulation and structure‖, the impact resistance of thermal protection system in thecomplex service environment, including high temperature, micrometeoroid and so on, is beingpaid great attention in the developed countries. As a widely applied material in the field ofthermal protection,2D plain-woven C/SiC composite material (2D-C/SiC) overcomes theweaknesses of high brittleness and poor reliability of traditional ceramic material, and hasmany advantages such as high specific strength, high specific modulus, high temperatureresistance, antioxidation and low density etc. Therefore, it is of great significance to carry outthe experiment and simulation studies on the impact performance of2D-C/SiC in order towell describe the mechanical behavior of this material in the severe environment. However,the typical powder behavior of2D-C/SiC under the high-speed impact load is a big challengeto numerical simulation. As a typical meshfree method, Smoothed Particle Hydrodynamics(SPH) meshless method represents the entire physical model through a series of discreteparticles, which do not need to connect each other via the grid, and thus has obviousadvantages in the simulations with free surface, deformable boundary, impact, explosion andother dynamic large deformations. Unfortunately, SPH still has some inherent defects, andthen it is essential to improve and develope the fast high-accuracy simulation methodology inorder to obtain accurate description of the mechanical behavior and the damage process.In this thesis, high-speed impact resistance of2D-C/SiC is revealed through experimentaland numerical studies, and then an improved FPM meshless numerical methodology isproposed. The main contributions of this paper are summarized as follows:1. By using the electrical gun and the air gun with a self-designed SimultaneousHeating-Cooling System (SHCS), the impact experiments of2D-C/SiC plate by the steel ballwith the velocity range of79m/s-219m/s at both high temperature and room temperature andthe Mylar flyer with the velocity range of3400m/s-9300m/s at room temperature are carriedout. During the experiments, the expansions of debris clouds are recorded with the high-speedcamera; the fragments are collected with prepared PMI foam plates; the history of free surfacevelocity is measured with a laser Doppler velocimeter; and finally the damaged specimens areinspected with an ultrasonic scanning system.2. Based on the experimental results, the damage evolution process and mechanisms underimpact load are investigated. It is indicated that,(1) there are four energy conversion forms,i.e. fragments spalling, delamination, fiber fracture and matrix powdering, in which the last two are the major damage forms of2D-C/SiC under impact load.(2) The critical penetratingenergy of3mm2D-C/SiC plate is assessed to be2.66J, and the variation of the damage areawith the impact energy shows a―three stages‖phenomenon, i.e. the sharp increasing stage,the linear increasing stage and the stable stage.(3) The damage degree changes between roomtemperature and high temperature at the critical impact energy of approximately13J. At hightemperature, the critical penetrated energy of3mm2D-C/SiC plate is higher and the damagedegree with impact energy lower than13J is lighter than those at room temperature, whichrepresents the high impact resistance of2D-C/SiC at high temperature under low-energyimpact. However, when the impact energy exceeds13J, serious damage is found at hightemperature.3. Based on the orthotropic constitutive material model in the commercial softwareAutodyn, the material parameters to describe the impact response of2D-C/SiC are derived. Inthe low-speed and hypervelocity numerical simulations, the rationality and accuracy of thecalculation results are compared and verified with experimental results based on the structureand the axial velocity of the debris cloud, the damage modes and the free surface velocity ofspecimens. For the spherical projectile, according to the features of2D-C/SiC debris clouds,the residual velocity of projectiles, the axial average velocity and the dispersion angle of thedebris cloud are chosen as the main characterization parameters. Taken the sphericalprojectile-plate impact model as the object, the impact resistance of2D-C/SiC is analyzedunder a variety of working conditions, and the limit penetration depth formula of2D-C/SiCunder the impact by steel ball is proposed. For the Mylar flyer, the relationships of freesurface velocity to the diameter of the flyer and the thickness of the plate are discussedrespectively. For the Whipple structure, the debris clouds of2D-C/SiC and Al under theimpact by Al ball are compared, and the typical high-energy area is found in the fore-end ofthe2D-C/SiC debris cloud, which is the major threat to the aircraft body.4. The Finite Particle Method (FPM), which is a generalized SPH method, is analyzedcomprehensively. Its numerical advantages, including the high-accuracy near the boundary,free choice of basis functions and so on, are investigated through numerical examples.Meanwhile, the numerical defects of FPM, including computing instability and longcomputing time, are also indicated.5. In order to overcome the defects of FPM, an improved FPM methodology is proposedbased on the Taylor series, and matrix decomposition theory. Under both one-dimensional andtwo-dimensional cases, the Specified Finite Particle Method (SFPM) and Generalized FiniteParticle Method (GFPM) are derived, and the computing stability of these methods is proved. The numerical results show that,(1) SFPM and GFPM inherited the advantages of highnumerical accuracy of traditional FPM in the whole domain, and can achieve C1and C2orderconsistency;(2) Meanwhile these two methods could greatly reducing the computing time,which is of great significance for large-scale numerical analysis;(3) The numerical accuracyof SFPM is controllable, which could be improved by increasing the number of particles inthe computational domain;(4) The convenient high-dimensional and higher-orderexpansibility of SFPM and GFPM are investigated through theoretical analysis at the end ofthis thesis. |
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