Damage Initiation Mechanism And Optimization Method Around Interference-fit Joint Of CFRP Structures

CFRP(Carbon Fiber Reinforced Plastics)components are widely applied in aviation and aerospace industries because of their advantages in terms of weight reduction and energy saving.The interference-fit joint technology of CFRP components can improve the fatigue performance substantially and extend the service life of the aircraft.However,CFRP is a kind of heterogeneous and inhomogeneous brittle material.The installation process of interference-fit bolt will induce complicated damage around the hole,which affects the quality of joint directly.The interference-fit joint of CFRP components hasn't developed a professional technological process at present.The empirical technological parameters and arbitrary process control restrict the application and development of interference-fit joint technology in CFRP components which decreases the assembly quality of aircraft structures.In allusion to these problems,some researches focus on the installation process of interference-fit bolt in CFRP laminates.The damage initiation mechanisms are analyzied in micro-scale by local micro-macro FE model.The effects of interference percentage(IP)on the squeezed damage and delamination damage around the hole are discussed based on the stress distribution analytical model and critical thrust force(CTF)prediction model.Then,the process control methods of interference-fit damage are proposed.The main works and contributions of this research are as follows:(1)A local micro-macro FE modeling method is proposed to simulate and analyze the damage initiation mechanisms of fibers,matrix and interface in micro-scale based on the damage response models programed by user defined material subroutine(UMAT).The complicated 3D instance is simplified as two 2D problems in plane and through thickness reducing the computational work.The vicinity of 30° angle between normal pressure and fiber orientation is the weakest portion in CFRP laminate,then interfacial debonding and matrix plastic deformation propagates to the both sides.The epoxy matrix on the top surface of CFRP laminate is damaged first and easily during the interference-fit pin installation process,then the damage propagates downward with the increase of interference value until the support zone by the block is damaged.The damage mechanisms of CFRP laminate are detected by scanning electron microscope(SEM).(2)A general analytical model of stress distributions in each individual layer around the joint is established,and the effect of IP on the squeezed damage is discussed by the modified Hashin's damage criteria.Considering the anisotropy of CFRP laminate,the isotropy of metallic bolt and its elastic deformation,the contours of pin and hole after deformation are ellipse-like shapes,and the periodic nonuniform stress distributions are more accurate compared with those of rigid pin.It's observed that the stress magnitudes increase with IP and material stiffness linearly before damage,but the peak of circumferential stress is larger than that of radial stress.The trends of stress curves of each individual layer are consistent due to the same property of each lamina,but the phase delay is depended on the respective ply orientation.Then,the fiber and matrix tensile and compression damage and interface debonding damage mechanisms are simulated and analyzed.The critical interference percentages(CIPs)decrease from 1.10% to 0.85% with the increase of bolt diameter.(3)The thrust force model in the whole inserting process of interference-fit bolt is established.The effect of IP on delamination damage is discussed,after CTFs and CIPs of each interlaminar interface without delamination are predicted based on the energy balance equation.The installation process of interference-fit bolt is divided into 6 stages.The thrust force which induced delamination damage directly in the whole inserting process is modeled by the contact force on the chamfer and friction force on the bolt.It's observed that the CTF and CIP decrease with the interlaminar interface moving downward,which indicates that the closer to the bottom of CFRP laminates,the easier to be delamination.Then,the delamination and CIPs are verified by FE simulation with various IPs based on the cohesive zone model.(4)The process control methods of squeezed damage and delamination are proposed based on the research above,and the inhibiting effect of tightening torque on the damage evolution is discussed.Firstly,the strains and squeezed damage of CFRP laminate are tested by interference-fit bolt installation experiments.Strict control of IP is an effective method to control the squeezed damage,because IP is the main factor affected the stress and damage.Then,the effect of friction performance on delamination damage is analyzed by FE method.Reducing the friction coefficient between the laminate and bolt can reduce the damage level.Finally,the effects of tightening torque on the stress distribution and structural strength are investigated by FE simulation and experiments respectively.It's observed that tightening torque could improve the strength of CFRP laminates with interference-fit joint,but the advantages reduce with the increase of tightening torque.