Research On Mechanical Properties And Failure Mechanism Of Composite Material Structure Of Light Aircraft

Fiber-reinforced composite materials with excellent mechanical properties such as high specific strength,high specific modulus and low specific gravity can effectively reduce the structural quality and improve the overall strength of the structure,and have become an important material for the manufacture of main load-bearing components on aircraft.The performance of composite bonded connection,composite bolted connection and low-velocity impact resistance of composite materials are important issues faced in the manufacture and service of all-composite aircraft.In this paper,the finite element method（FEM）is used to study the tensile strength and damage distribution of composite bonded single-lap joint structures（SLJs）,summarize the tensile strength and crack propagation laws of composite bolted joint structures,and reveal the failure mechanism of composite laminates subjected to low-velocity impact.The major studies are shown as follows:（1）The tensile strength and adhesive layer degradation law of SLJs were explored through the composite material sudden drop damage model.The finite element model of SLJs with different stacking sequences and adhesive layer thicknesses is established,and the validity of the model is verified by the experimental results in the literature.The results show that the damage modes and failure laws of the adhesive layer are related to the angle of the composite material layup.When the angle of the composite material adhesion on both sides of the adhesive layer is 0°,the adhesive layer will degenerate and form an elliptical ring;When the angle is 45°or-45°,the adhesive layer will degenerate and form two droplets symmetrical in the upper and lower centers;When the angle is 90°,the adhesive layer will degenerate and form an hourglass shape.When the 0°layup is adjacent to the adhesive layer,the structure has the largest ultimate failure load and no damage appears in the composite materials;When the 90°layup is adjacent to the adhesive layer,the structure has the lowest ultimate failure load,and the composite material will have matrix damage,fiber damage and delamination.With the thickness of the adhesive layer between 0.05mm and 0.5mm,the adhesively bonded single-lap joints have better connection performance.If the thickness of the adhesive layer is greater than 0.5mm,the connection performance of the SLJs will be greatly reduced.（2）The extend finite element method（XFEM）is applied to study the tensile strength and crack propagation laws of composite bolted structures.First,the equivalent modulus of composite laminates was calculated by the equivalent mechanics theory of composite materials and compared with the calculation results in the literature.Secondly,the finite element model of the composite bolted connection structure is established and compared with the experimental results in the literature to verify the accuracy of the finite element model.The results show that the crack starts at the front end of the centerline of the bolt hole and propagates to the edge of the composite laminate in a V shape.Finally,the effects of pre-tightening force,bolt hole diameter,number of bolts and lap joint on the tensile strength of composite bolted structures are studied.（3）A three-dimensional composite elastic progressive damage model is constructed.Firstly,the original 2D energy evolution model is extended to 3D by introducing the strain component along the thickness direction into the matrix damage evolution model.Then,the low-velocity impact finite element model of composite laminates is established,and the mechanical response and damage distribution of composite laminates under different impact energy are studied.The effectiveness of the proposed three-dimensional composite elastic progressive damage model is verified by comparing with the experimental results in the literature.The results show that the elastic progressive damage model of 3D composites considering σ₃₃ and ε₃₃ is beneficial to improve the prediction accuracy of delamination area and inter-laminar damage distribution;Increasing the number of±45°plies in the design of the stacking sequence of composite laminates can reduce the maximum impact load and damage area,and improve the overall strength and impact resistance of the composite laminate.（4）The influence of plastic deformation on the tensile properties of composite bonded structures and the low-velocity impact mechanical response of composite laminates was investigated.Based on the elastic-plastic constitutive model of composites and the three-dimensional energy evolution model proposed above,a three-dimensional elastic-plastic progressive damage model of composites is constructed,and the tensile properties of SLJs with different overlap lengths and stacking sequences and the mechanical response of composite laminates subjected to low-velocity impact are studied.The consistency between the finite element results and the experimental data suggests that the 3D composite elastic-plastic progressive damage model proposed in this paper can describe the progressive damage process of composite materials more accurately,and can overcome the inherent defects of the 3D Hashin criterion and improve the accuracy of numerical simulation.Furthermore,the application scope of the existing elastic constitutive model of composite materials is expanded.