Modeling And Analysis On Secondary-Damage Of Composite Bolted Joints

Composite laminates consisting of continuous carbon-fiber reinforced plies are now widely employed in the main bearing components of aircraft by the virtues of light weight,high strength and outstanding designability.However,considering the limitations of forming process and the demands of design,process,transportation,maintenance and other aspects,a large number of bolted joints are still necessary.Delamination induced by many uncertain factors during the forming and processing of composite components,is prone to propagate and come into secondary-damage when suffering assembly-induced-stress in assembling process.The assembly-induced-stress and secondary-damage can cause unpredictable damage for aircrafts during service,which seriously affects the performance reliability and even results in disaster.In terms of the above problems,with the support by related research content of the National 973 Project,numerical analysis for the effects of connection hole machining quality and component forming quality on assembly-induced-stress distribution of composite bolted joints is carried out.The influences of perpendicularity error,assembly gap and embedded delamination parameters on loading capacity and delamination propagation of composite bolted joints are investigated with experimental test and numerical simulation,so as to provide theoretical reference for performance evaluation and damage prediction of large aeronautic carbon fiber composite structures.The main research and achievements in this paper are as follows:(1)Combining progressive damage analysis which is based on Olmedo improved Chang-Lessard failure criterion and continuum damage degradation criterion and cohesive zone model,finite element model of carbon fiber composite laminate containing embedded delamination under compression is established and experimentally verified.The simulation analysis and experimental results are in favorable consistency,which verifies the effectiveness of the proposed model.(2)Numerical simulation for the effects of connection hole machining quality and component forming quality on assembly-induced-stress distribution of composite bolted joints is conducted.It is found that perpendicularity error is the dominant factor among connection hole machining quality which causes structural stress concentration.When perpendicularity error is 1°,the maximum stress of connection area can increase by 41.3% compared to composite structure without perpendicularity error.As for component forming quality,assembly gap will result in sharp increase of connection area stress,for example,the maximum stress of connection area can reach 600 MPa for a 4 mm thick composite component with 1.0 mm assembly gap during normal assembly condition(clamping load of 8000 N).(3)By conducting tensile properties test and numerical simulation of double-bolt single-lap composite joints,the bearing capacity and failure morphology are obtained.The numerical simulation and experiment results are comparative analyzed,which verifies the validity of the model and reveals the influence mechanism of perpendicularity error and embedded delamination parameters on bearing capacity and delamination propagation of composite bolted joints.(4)Numerical simulation of composite bolted joints containing embedded delamination and assembly gap is carried out.The numerical analysis successfully reproduces delamination propagation which is in good agreement with the experimental results,and the relative error of the limit bearing capacities between prediction and test is 5.1%,verifying the validity of the model.The influence of embedded delamination parameters on the load-bearing performance and delamination propagation of composite bolted joints with 2.0 mm assembly gap during the two kinds of assembly sequences is explored.