Multidirectional Damage Mechanics Of Composite Materials Under Quasi-static And Fatigue Loading

At present,composite materials are widely employed in the field of aircraft,automobiles and ships and so on due to the high specific strength and stiffness.However,the increasing use of composites raises higher demand on the research on composite damage mechanisms.This dissertation is dedicated to analytical and experimental research on composites damage mechanisms under quasi-static loading and fatigue loading.On the basis of damage mechanisms,the multidirectional damage mechanical model for composite materials is established.The influence of different damages on composite stiffness properties is analyzed in the model,damage evolution and stiffness degradation are predicted,and experimental results are adopted to verify the proposed model.Main damage mechanisms of composite material are introduced in detail,including the initiation and evolution feature of each damage mechanism,and the influence on the composite material mechanical properties.Composite damage mechanics models are reviewed systematically,several representative damage models at present are introduced in this dissertation,including damage characterization models and damage evolution models.The damage characterization models are divided into three categories,which are micro damage mechanical models,macro damage mechanical models and meso damage mechanical models based on their analysis scaler.Also,the damage characterization models are divided into two categories,which are damage evolution models under quasi-static loading and under fatigue loading respectively based on their analysis object.A multidirectional damage model based on continuum damage mechanics for fiber-reinforced composite laminates under quasi-static loading is proposed.The evolution and influence of three main damage mechanisms,including transverse matrix cracking,local delamination,and fiber breakage,on the multidirectional stiffness properties of composite laminates is analyzed.The damaged mechanical behavior of elementary ply in laminates is modeled based on the framework of macro damage mechanics.Relations between micro-level damage variables and macro-level damage variables are established based on their definitions.Damage evolution laws of transverse matrix cracking,local delamination,and fiber breakage are derived from micro and macro damage mechanics respectively.Several key factors,including initial defect,damage interactions and residual thermal strain are considered in the model.Then the prediction results are compared with experimental results to verify the proposed model.Then a multidirectional damage model for composite laminates under fatigue loading is proposed.The micromechanics method is utilized based on the framework of multidirectional damage mechanics to characterize the damaged behavior of composite laminates under fatigue loading in the model.The initial matrix crack initiation life is defined as the material fatigue property and deduced based on micromechanics method.Then the relation between initial matrix crack initiation life and critical energy release rate is established to obtain the distribution of initial matrix crack initiation life in cracked plies,and then evolution of matrix crack density and stiffness degradation are predicted.Several key factors,including initial defect in composites,initiation length and damage interactions are considered in the model.The prediction results are compared with experimental results to verify the model.A damage model for woven composites under fatigue loading is proposed in the dissertation.Compared with composite laminate,it is more difficult to analysis damaged mechanical behavior of woven composites at micro level due to the more complicated structure.Hence a homogenization method is utilized based on the framework of multidirectional damage mechanics,and a micro-mechanical shear leg model is adopted to characterize the influence of damage on woven composites.An energy release rate deduced from micromechanics is defined as the damage evolution variable in the damage evolution model,and a Paris-law like equation is utilized to predict the damage evolution in woven composites.The prediction results are compared with experimental results to verify the proposed model.