A Study Of Nonlinear Models Of Composites And Its Applications To Composite Multi-bolt Joints

Along with the application of composites on aircrafts,huge amounts of bolts were used to assemble the composite-composite or the composite-metal components.The vicinity of the assembling location is the weakest and most vulnerable area in the aircraft structures.However,when designing and analyzing composite bolted joints in industry,sometimes the supposed effect of weight reduction brought by the application of composites was not obvious.This was due to the over simplification and inaccurate representation of composite materials,which was caused by the engineering method and progressive damage model on the hypothesis of linearizing composite materials.So the damage mechanism of composites is the priority issue to be solved in the designing of composite bolted joints.This dissertation could be divided into two parts: firstly,propose the nonlinear models basing on the physical phenomenon of composite material;secondly,employ the nonlinear models respectively to study the mechanical response,damage evolution and failure modes of composite multi-bolt joints From Chapter 2 to Chapter 4,three mechanical constitutions in combination with corresponding failure criteria are respectively presented to study the load-displacement response,damage evolution and failure modes of composite bolted joints,and they are respectively the intra-laminar continuum damage mechanics(CDM)model,the intra-laminar combined elastoplastic damage model and the phenomenological intra-laminar elastoplastic model.Based on the previous research,the phenomenological intra-laminar elastoplastic model presented in Chapter 4 was embedded into the fatigue model of composites.Furthermore,including the Lemaitre metallic fatigue damage model,the composite-metal combined fatigue damage was used to study fatigue damage evolution of the composite/metal hybrid bolted joints.In Chapter 2,the classic CDM model proposed by Ladevèze is simplified and modified,and then this model is applied for simulation of composite double-lap bolted joints.The engineering method,the progressive damage model commonly seen in the research of bolts joints and the CDM model were respectively introduced.The intra-laminar CDM model presented in Chapter 2 mainly refered to the continuum damage of matrix under tension and the fibre/matrix shear on the hypothesis that the nonlinear behavior in transverse and shear direction of fibre-reinforced polymer composites is induced by the micro-cracks accumulation of the matrix.As for the damage in fibre direction,Hashin's criterion was initially called to calculate the fibre failure initiation in each element,and then the fibre damage variable was given as a function of current strain,initial failure strain and ultimate failure strain.On the basis of introducing the concepts of damage variable,effective stress,strain energy after micro damage initiation and the damage matrix of material,the modified intra-laminar CDM model is finally presented in detail.This CDM model was validated by comparisons between predictions and experimental results.At last,the load-displacement response,stiffness reduction,damage evolution and failure models of composite double-lap,multi-bolt joints were studied using this CDM model.The simulation results were in pretty good agreement with experimental results.In Chapter 3,an elastoplastic CDM damage model is proposed by assuming that the nonlinear behavior of composites was caused by the coupling effects of micro-cracks and plasticity of the matrix.This model combined micro damage and macro damage post-failure: the intra-laminar damage model predicted the continuum micro damage before material's failure,while the Puck's criterion,Hashin's criterion and mesh independent Crack Band approach were used to simulate the macro damage after material's failure.Firstly this model was employed to predict the intra-laminar stress-strain responses,the predictions were in good agreements with experiments.The mesh independent effect was also validated by comparing finite element models with different mesh sizes.The validations indicated that this model was capable of not only representing the mechanical response of composites but also decreasing the mesh dependent effects.Furthermore,this model was employed to study the secondary effects,stiffness reduction of structure,damage evolution and failure modes of composite single-lap multi-bolt joints.This model showed better capability than the previous CDM model in Chapter 2 on the aspect of predicting matrix damage near the hole.At last,according the comparisons,choosing the elastoplastic model as the constitutive model of the bolt achieved better results than the elastic model.In Chapter 4,further hypothesis is made that the nonlinear phenomenon was caused by plasticity mechanism of matrix only,and based on this assumption,a new elastoplastic model is developed to represent the nonlinear mechanism of composites.In this model,the yield function presented in Chapter 3 was modified and a new plastic hardening function was developed.Then this elastoplastic model was applied to simulate the macro nonlinear behavior of the matrix,shear and angle-ply laminates,predictions accurately represented the nonlinear feature of composites and matched the experiments very well.It indicated that this plasticity model was capable of modeling the nonlinear behavior of fibre reinforced polymer composites.In combination with the Puck's criterion and the mesh-independence approach,we simulated the damage evolution of the notched plate and studied the mechanical behavior of composite/metal countersunk multi-bolt joints under the quasi-static loading.Good agreements were achieved between the simulations and experiments.Chapter 5 mainly focuses on the fatigue analysis of composite laminates and composite-metal hybrid bolted joints.In this chapter,a progressive fatigue damage model was developed involving the elastoplastic model in Chapter 4,the normalized fatigue life model,the strength reduction and stiffness reduction fatigue model.Under T300/QY8911 material system,this fatigue damage model was used to predict the fatigue life of laminates,and to study the stiffness reduction,hole's deformation,damage evolution and failures of composite multi-bolt joints.Subsequently,in combination with Lemaitre high cycle and low cycle fatigue model,the fatigue damage evolution of composite-metal hybrid bolted joints was studied.Compared with the experiments,the simulation results accurately predicted the experimental failure modes and rationally explained the special experimental phenomena of hybrid joints.