Study On Damage Evolution Of Carbon Fiber Composite Component During Jointing

Carbon fiber composite has high specific strength,high specific stiffness,fatigue resistance,and corrosion resistance,therefore,the amount of composite materials on the aircraft is gradually increasing.The application of composite materials on large aircraft has developed small and secondary bearing components to large load-bearing components.The joint between composite bearing components mainly relies on bolt joints.However,due to the limitation of the molding process and processing technology,crack defects,and delamination damage often occurs during the forming and processing of composite component,which will inevitably affect the joint performance,occurs damage evolution phenomenon,which will seriously affect the performance and working life of composite bearing components.In view of the above problems,with the support by related research content of the National 973 Project,this paper carried out the numerical analysis and related experiments for the effects of crack defects and delamination defects on joint performance of composite bolted joints.In this paper,the variation and damage evolution of the bolt joint performance of carbon fiber composite components with crack or delamination defects were investigated from the aspects of experimental and numerical simulation,it is hoped to provide some references for the design of the joint structure of carbon fiber composite components and the prediction of the bearing capacity when there is damage.The main work of this paper is as follows:(1)The specific structure and size of the unit cell structure are observed by an optical microscope.The SolidWorks 3D modeling software is used to establish the fiber bundle and matrix model consistent with the measurement results,and the 3D unit cell model is obtained.The established three-dimensional unit cell model is imported into the finite element simulation software ABAQUS.Under the tensile load,the unit cell model is distorted and deformed.The stress concentrated on the fiber curvature and the cross-section of the matrix,which accurately revealed deformation and stress distribution of the two-dimensional weaving structure under tensile load.Combined with the progressive damage analysis and cohesion theory model,using Hashin's three-dimensional failure criterion and stiffness degradation criterion,the external subroutine UMAT is programmed to assign the material properties and stiffness degradation,and simulate the state and damage of the composite when subjected to load.(2)The propagation form and fracture mode of crack defects in the two-dimensional braided structure are studied.Scanning electron microscopy and direct observation revealed that cracks begin to expand along the intersection of warp and weft fibers,brittle fracture of warp fibers,and fall off of resin from fibers.Based on the force-displacement curve and critical stress intensity factor under tensile load,it is found that with the increase of crack length,the bearing capacity of single-layer plate decreases,and the stress intensity at the crack tip is serious.The typical single-bolt joint mode in the composite connection is used as a representative to analyze the influence of the crack defects in the layer on the connection performance.It is found that the joint performance is reduced when there is a crack in the joint structure,and the joint performance was further reduced in the presence of double cracks.The bearing capacity decreases with the existence of cracks,but the fracture failure form of the joint structure,the fracture mode of the fiber,and the crack propagation direction remain basically unchanged.(3)The compression finite element simulation example of composites with delamination defects is compared with the compression experiment to verify the correctness of the combination of the progressive damage analysis method and cohesion model.By comparing the experimental results and the simulation results,it can be found that both of them can be effectively matched the specific process of delamination propagation and bearing performance.It is verified that the damage judgment criterion and stiffness degradation criterion adopted by the paper is completely correct and reliable.The influence of the existence of layered defects on tensile bearing performance and compression performance,the evolution of delamination damage under load,the buckling mode and the form of fracture under load are investigated by the establishment of composite material single plate tensile and compression tests and numerical simulation.Comparing the experimental and finite element simulation results,both the bearing performance and the damage evolution process have good consistency,and the errors are within 5%.It is found that the delamination defects have little effect on the tensile bearing performance of the single plate,and the delamination damage does not show obvious propagation.However,under the compressive load,the performance of the laminates is significantly reduced,and the bearing performance is different by 31.3 %.The delamination defects are further propagation and the fracture modes are completely different.The typical single-bolt single-lap joint mode in the composite joint is as a representative,the influence of the delamination defect on the joint performance is analyzed by experiment and numerical simulation.The analysis found that the influence of delamination defects on the single-bolt single-lap joint performance is consistent with the single-plate test.That is,when the joint structure is under tensile load,the bearing performance remains basically unchanged,and the prefabricated delamination defects are not further propagation.Under the compressive load,the bearing performance is obviously reduced,the bearing performance is different by 13.2%,and the delamination defect also has the damage evolution behavior.The results of numerical simulation and experiment can obtain good consistency,and the error is within 5%.The correctness of the influence of layered defects on the single-bolt single-lap joint structure is further verified.