Research On Fracture Behaviors Of Particle Reinforced Composite Materials

Due to the simple processing technology, particle reinforced composites arewidely used in many fields, such as aerospace, nuclear energy, constructionindustry, automobile, etc. In the application of particle reinforced composites,fracture is one of the most common failure modes, which significantly influencesthe service lives of the composite materials. Therefore, it is especially important tounderstand the fracture behavior of these types of composites. Due to the limitationof the mathematical tool, analytical method can not deal with the fracture problemsof complex materials. The experimental method requires the high cost and can notbe widely used either. As a result, the numerical method becomes a more powerfultool. There are many restrictions to solve the problems with both strongdiscontinuities (crack) and weak discontinuities (particle) using the traditionalnumerical methods, the accuracy and the efficiency is difficult to be obtainedsimultaneously. In this thesis, a novel numerical method is developed based on thetraditional finite element method (FEM) and the extended finite element method(XFEM) for the calculation of the crack tip characteristics and the modeling of thefracture behaviors of particle reinforced composites. The influences of materialproperty parameters and geometric parameters on crack growth trajectories areinvestigated. The main contents are given as follows:Firstly, the basic analytical methods and research status of fracture mechanicsof particle reinforced composites are reviewed. The characteristics of the crack tipfields are emphasized when the crack interacts with the material interface. Secondly,we focus on the related numerical methods and their applications. The advantagesand disadvantages of each numerical method are discussed. Finally, the XFEM isbriefly introduced and the possible improvements of this method are also pointedout.In order to verify the high efficiency of the XFEM, three types of commonlyused fracture criterions are discussed, and then the growth of an edge crack in afunctionally graded beam is simulated. However, it can be found that there aresome difficulties to obtain the accurate stress intensity factors (SIFs) when thecrack tip tends to the interface. If we treat the nodes around the crack tip with aparticular method, the solution process will not be unified and the difficulty of thecalculation will be increased. In this thesis, an improved extended finite elementmethod without tip enriched functions is developed, which overcomes the difficulties mentioned above and at the same time, the core advantages of theXFEM are maintained.The interaction between a propagating crack and a particle is simulated by theproposed method and the corresponding normalized energy release rates are alsopresented. The effects of “repelling” or “attracting” from the particle on crackgrowth trajectories are discussed simultaneously. The influence of the interfaceflaw on crack growth trajectories and fracture parameters is investigated. There isno doubt that the complex particle distribution problems are more significant inpractice compared with the case of a single particle. Based on the previous work,the interaction between a propagating crack and particle clusters is simulated usingthe proposed numerical method. Then we investigate the influence of particlenumber, distance and distribution on fracture parameters at the main crack tips.Finally, SIFs and initial angles at the main crack tips are obtained for randomlydistributed elliptical particles with various volume fractions.A new domain expression of the interaction integral is derived for thecomputation of dynamic mixed-mode SIFs. The derived expressions have two mainadvantages:1) the interaction integral does not contain any terms related to thederivatives of material properties;2) the expression is still valid even when theintegral domain contains material interfaces. As a result, the interaction integralderived in this thesis can be used to evaluate dynamic fracture parametersaccurately when the crack tip tends to material interfaces. The interaction integralcombined with the proposed numerical method is used to resolve several typicaldynamic fracture problems. The aim is to verify the effectiveness and domainindependence of the interaction integral. Then, the influence of particle number,location and properties on mixed-mode dynamic SIFs at the main crack tip isstudied.The fatigue crack growth in particle reinforced composites is investigated andthe influences from particles on the fatigue lives of these composites are studied.The crack growth trajectories under the case of two kinds of particle distributionforms are given. The influence of particle distributions on the fatigue crack growthrates is also investigated. The numerical results imply that the average crackgrowth rate seems to be increased with the increased clustering.