Stress Analysis And Progressive Failure Process Of Adhesively Bonded Composite Structures

Composite laminates have many advantages,such as high specific strength,high specific stiffness,excellent fatigue resistance,good high temperature performance,good thermal stability and good designability.They are widely used in aircraft structures.Bonding is the main way of aircraft composite structure connection.It is very important to study the strength and failure mechanism of composite structure for aircraft structure safety.In this paper,the stress distribution and damage propagation of single lap and double lap bonded composite structures are studied from the perspective of aerospace applications.The main research contents of this paper include:(1)Considering the influence of overlap length,axial tension in unit width of laminate,equivalent tensile modulus of bonding zone and equivalent inertia moment on load eccentricity coefficient in bending moment,a theoretical stress analysis model of composite single-lap bonded structure is established by using elastic mechanics analysis method considering load eccentricity;the composite laminate and adhesive material are equivalent to linear elastic materials.Based on the classical elastic theory,a theoretical model for stress analysis of composite double lap bonded structures is established.(2)The strains of the upper and lower surfaces of the laminates are measured by experiments,and the stress calculation methods based on classical elastic theory and classical laminate theory are proposed according to the measured strains.The shear stress distribution of the rubber layer calculated by the theoretical model is compared with that calculated by the stress calculation method.The results show that the shear stress distribution curve calculated by the theoretical model is in good agreement with the shear stress distribution curve calculated by the stress calculation method,which verifies the validity of the theoretical model.The peak shear stress of the adhesive layer appears at the end of the joint;under the same overlap length,the shear stress of the adhesive layer increases with the increase of the load level;under the same tensile load,when the overlap length reaches a certain value,the peak shear stress of the adhesive layer will not increase with the increase of the overlap length.(3)The effects of lap length and adhesive layer thickness on the tensile properties of composite bonded structures were studied by experiments,and the corresponding load-displacement curves and failure modes were obtained.The test results show that the bearing capacity of composite bonded structures increases with the increase of lap length.When the lap length reaches a certain value,the ultimate load will not be increased by increasing the lap length.The ultimate load of the structure decreases with the increase of the thickness of the adhesive layer.The failure modes of the adhesive layer are mainly bonding failure,cohesion failure of the thin layer and a mixture of two failure modes.(4)A finite element model for progressive damage analysis of composite bonded structures is established.An improved failure criterion and its corresponding material degradation model are selected to establish the progressive damage model of composite laminates,and the adhesive layer failure is simulated by using the cohesion model based on bilinear constitutive law.The validity of the finite element model is verified by comparing with the experimental results.The stress distribution of single lap bonded composite structures under different tensile loads is obtained by this model,which is in good agreement with the results of theoretical model analysis,and further verifies the validity of the theoretical model of stress analysis.At the same time,the model can also simulate the process of damage initiation,damage extension and ultimate failure of composite bonded structures.