Research On The Influence Of Temperature And Humidity Upon The Strength Of Composite Bolted Joints

Composite bolted connections are widely used in aircraft construction due to their good reliability,easy assembly and disassembly,and recycle.However,composite bolted joints often work under complicated conditions because of the complicated flight environments.This thesis made a theoretical and experimental investigation to the influence of temperature and humidity upon the strength of composite bolted joints.The progressive damage analysis method is used in the numerical studies.The main research topics include:(1)Experiments of moisture absorption of laminates were performed and the absorption vs.square root time curves were plotted to study the behavior of moisture absorption of laminates.(2)Tensile and compressive experiments of unnotched laminates under room temperature dry conditions,hygrothermal conditions and cold temperature dry conditions were performed to analyze the influence of temperature and humidity on the mechanical property of laminates.The results show that the compressive strength declines when it is under hygrothermal conditions,and the tensile strength declines when it is under cold temperature dry conditions.(3)Strength texts of composite bolted connections under room temperature dry conditions,hygrothermal conditions and cold temperature dry conditions were performed,and the load-displacement curves were got to analyze the influence of temperature and humidity upon the strength of composite bolted joints.Results show that the failure mode of composite bolted joints is extrusion failure when it is under room temperature dry conditions and cold temperature dry conditions,while the failure mode of composite bolted joints is extrusion-layered failure when it is under hygrothermal conditions.(4)UMAT subroutines of three failure criteria,namely,the three-dimensional Hashin criterion,the maximum stress criterion and the mixed criterion,and in three environmental conditions were written.Three-dimensional finite element models of single-bolt,double shear composite bolted joints were developed to predict their failure loads by ABAQUS with using the self-written UMAT subroutines.The initiation and propagation of damages in composite bolted joints in three environmental conditions were discussed.The distributions of damage after the material failed were obtained.The simulated results were compared with the experimental data.It is shown that the progressive damage analysis method is reliable.