Strength Analysis Of Large Composite Component Containing Void And Delamination Defects Based On Multi-scale Method

Advanced composite materials have a lot of advantages such as high specific strength,high specific modulus,high heat resistance,corrosion resistance,fatigue resistance and its performance can be designed due to different demand.Since 1960s,composite materials have become important structural materials in aviation and aerospace equipment.As the application of composite materials in advanced aviation and aerospace equipment increases,the size of composite structures also increases.Its typical manufacturing defects such as voids,delamination occur more frequently.In addition,the scale gaps between defects size and structural size have become larger.At the same time,composite materials are applied in complex environments.Thermal loads will induce mesoscopic stress due to thermal mismatch between fiber and matrix and may lead to structure failure.Since then,the failure problem of composite structures under thermal-mechanical coupling loads has been receive more and more attention.The traditional analysis method assumes the macroscopic composite materials as homogeneous material and it is difficult for such method to cope with these challenges brought by multi-scale features of composite structures.Multi-scale methods are ideal methods for such problems.The main part of multi-scale analysis of composite structures lies in the establishment of different scale models and the relationship between different scales.Generally,for large composite structure with typical defects,it includes three dimensions:matrix-fiber scale,pore size,and macro-composite structure scale.To develop the multi-scale model of composites contains defects,the equivalent model on these three scales and the relationship between them need to be established.This is the critical issue for the implementation of the multi-scale model of composites contains defects.This paper is devoted to the development of fiber-matrix model,void equivalent model and delamination equivalent model on the different scales.The connection between different scales based on stress amplification factors are established as well.Thereby,multi-scale analysis of large composite structures containing manufacturing defects is presented.The main contents of the paper are as follows:1.Modelling on fiber and matrix scale:In view of the insufficient accuracy of the existing mesoscopic failure criteria,new trans-scale failure criteria based on the failure mechanism are proposed through introduce parameters such as the failure angle.The prediction accuracy is improved.Moreover,the micro mechanical model of fiber and matrix scale is established based on micromechanical method.The accuracy of the method is verified by comparing with WWFE(World-Wide Failure Exercise)test results.The "compression-lag" effect is discussed in this paper and a method to predict this effect is proposed.Prediction errors caused by not considering this effect is also discussed and for LaRC 03 criteria,this effect leads to overestimate of the failure load.2.Modelling of the equivalent model for manufacturing defects:There are a large number of manufacturing defects in composite materials,such as void defects and delamination defects.The existing description model for voids is unreasonable.The general constraints of the pore geometry are given in the paper and the reasons for the unreasonable description of the existing void defect description model are analyzed.Based on this,a method for describing void morphology based on complete parameters is proposed.The void description model including four proportional parameters and one size parameter related to the manufacturing process is proposed.The representative volume element on the void scale is established based on micro mechanical method.For the de lamination defect,the cohesive model and VCCT(Virtual Crack Closure Technique)method are selected as the layered equivalent analysis model for delamination defect based on the existing research.Failure analysis based these models was performed on a typical single stiffened plate.In order to consider the complex topography of the delamination,a new fractal cohesion model is proposed in this paper.3.The influence of manufacturing defects:Based on these models and methods,this paper discusses the effects of void and delamination defects on the properties of composite materials and structures.For the void defects,the relationship between void morphology,void fraction and the elastic modulus and strength of composites containing voids is discussed.At the same time,this paper analyzes the deficiencies in the existing studies using experiments to determine the effects of void defects.The possible errors caused by such experiments are shown.It indicates that the morphology of the voids has an important influence on the mechanical properties of composite materials,and should be considered as an independent parameter in the model the same as the void volume fraction.For the delamination defects,the mechanical properties of typical single reinforced structures are discussed considering the influence of delamination defects.The collapse loads,structural local buckling loads and delamination initial loads are predicted.The effects of the size and location of the delamination on the structure performance are also discussed.The different failure loads are due to the stress states at the delamination front when the delamination is of different location.When the delamination is located on the anti-node line,the structure failure happens at a relatively low load.4.Multi-scale analysis of large composite structures:Multi-scale analysis of large composite structures requires both computational efficiency and prediction accuracy.The scale connection relationship based on the stress amplification factors and the stiffness reduction principle of the progressive damage analysis are established.A multi-scale analysis method for large composite structures considering the effects of manufacturing defects at different scales is proposed.Taking a composite wing-box structure and a cryogenic temperature composite tank as examples,the failure process of the two typical large composite structures are analyzed with the consideration of the typical failure modes such as axial failure and leakage failure.The validity of the multi-scale analysis framework proposed in this paper is presented.