Micromechanical Analysis On Relative Problems Of Tensile And Impact Failure In Composite Materials

Composite materials have been widely used in various engineering fields,such as automotive,aerospace industry etc.,in the past few decades.The failure of composite materials under loads is usually influenced by several damage mechanisms,and a variety of damage mechanisms mainly depend on microstructure and properties of composite.Investigation on damage mechanism of composite materials plays a key role in the improvement of their structure and overall performance.Therefore,analysis and prediction models of failure problem in composite material have very important significance.Several failure problems of composite materials under tensile or impact loads are considered in this thesis,and the corresponding theoretical models are proposed.In addition,the mechanical properties of composite materials are studied by numerical method.The research background and significance of the topic are first introduced in this thesis,and the research status of problems related to the tensile and impact failure in composite materials is reviewed.The main contents of this thesis are also described.A theoretical prediction of the interfacial debonding length and fiber pull-out length in fiber reinforced polymer-matrix composites is carried out based on the single fiber pull-out model,and the effects of different material parameters are studied.The stress field and displacement field of fiber and matrix are obtained using the dual phase region model,and the relation between the pullout length and debonding length of fiber is derived.The interface debonding criterion of fiber reinforced composites is given considering an interface debonding process,and the relation between the interfacial debonding length and applied loads is obtained.Based on single fiber pull-out model,the shear-lag model for the hybrid-fiber reinforced brittle-matrix composites is proposed.The solution of stresses and displacements is given based on the dual displacement domain model.Relation between the pull-out length and the debonding length of fibers is presented,and the interface debonding criterion of hybrid-fiber reinforced composites is obtained using the energy release rate relation in an interface debonding process.A series of parameters studies are preformed to analyze the effects of fiber hybridization on the properties of the hybrid-fiber reinforced composite.The damage behavior of hybrid glass/carbon composites under high-velocity impact is predicted by finite element method,and the effect of fiber hybridization on impact damage behavior of laminates is also investigated.The Hashin damage model is adopted to model the damage initiation of composite laminates,and the bilinear form of damage evolution law based on the effective displacement is employed to describe the damage evolution of laminates.The residual velocity of impact projectile is approximately shown a linearly decreasing trend with the increasing of the thickness of glass fabric ply.As the proportion of glass fabric ply in the hybrid laminates increases,the impact resistance of laminates increased gradually.A micromechanical model is developed to investigate the impact damage of composite laminates based on micro structure and various failure models of composite laminates.Impact damage mechanisms of laminate are analyzed by using the micromechanical model proposed and the effects of impact energy and laminated type on impact damage behavior of laminates are investigated.The kinetic energy of the impact projectile shows a nonlinearly decreasing trend in the impact damage process.In the early stage of impact loading,the kinetic energy has a more rapidly decrease trend,and as the thickness of laminate resisted impact loading near the impact point decreased,the descending speed of the kinetic energy is gradually reduced.Due to the damage of the surrounding matrix near the impact point caused by the fiber deformation,the surface damage area of laminate is larger than the area of impact projectile.The shape of the damage area is roughly rectangle or elliptical with the major axis extending parallel to the fiber direction in the surface layer of laminate.A micromechanical model is established to study interfacial failure properties of particle reinforced composites based on the cohesive zone model,and the particle is arranged on the body-centered cubic distribution.The interfacial strength and failure mechanisms of particle reinforced metal-matrix composites are predicted using the micromechanical model developed.The different deformation between particle and matrix will occur under loading due to the difference of material properties.The interfacial debonding between particle and matrix will appear when the stress concentration level reaches up to the interfacial strength.With the increasing of the tensile deformation,the size of the void formed gradually increases at both sides of the particle.The interfacial strength and the fracture energy play a key role in the interfacial debonding of particle reinforced composites.