| Carbon fiber composites provide a good choice to solve the lightweight problem of rail vehicles for their excellent performances such as high specific modulus,high specific strength and anti-fatigue property.The discontinuity of the fiber in the composite joints leads to the prominent stress concentration problem.The interrelated damage modes exist in the joint,the weak zone of the structure where the failure is easy to occur.The study on the analysis of the failure modes and failure law as well as the bearing capacity and life of the joint is of great importance to the evaluation of the whole structure.The commonly used lap joint for carbon fiber composite structures has bending effect due to eccentric loading at the end of the joint,which has an adverse effect on the load carrying capacity of the structure.Besides,T700 grade carbon fiber composites are widely used in the bearing structures of the rail vehicle,and the BMI resin has excellent heat resistance as well as high strength and modulus.However,the joint application research for carbon fiber/BMI resin composite is not sufficient.Aiming at the above problems,the carbon fiber/BMI resin composite(T700/5429)flat-joggle-flat joints were taken as the research objects,and the stress distribution,load carrying capacity and failure modes of the joint were studied from three aspects: test,theoretical analysis and numerical simulation.The longitudinal,transverse and in-plane shear static tensile tests provided basic mechanical parameters of the unidirectional plate for stress analysis and load carrying capacity prediction of the flat-joggle-flat joints.At the same time,the longitudinal,transverse and in-plane shear fatigue performance data of the carbon fiber/BMI resin composite unidirectional plate were supplemented.Compared with the single lap hybrid joint,flat-joggle-flat bonded joint and flat-joggle-flat mechanical joint,the superiority of the load carrying capacity for the flat-joggle-flat hybrid joint was verified by tests.Meanwhile,the static tensile load carrying capacities,fatigue lives and failure modes of the flat-joggle-flat hybrid joints were explored under different edge to diameter ratios,width to diameter ratios,bolt torques and adhesive thicknesses.Stress analysis is an important session for structural load carrying capacity calculation and failure analysis.Based on the linear elastic one-dimensional beam model and the elastic deformation theory of laminated curved beam,the formula of the adhesive peel stress and its shear counterpart at the end of the flat-joggle-flat bonded joint overlap zone were derived.The established semi-analytical method provided a tool for the quick and convenient calculation of the adhesive stresses of the flat-joggle-flat bonded joint.Based on the finite element software ABAQUS,the adhesive stresses of the overlap zone and the hole edge stresses of laminated plate of flat-joggle-flat hybrid joints were calculated under different parameters,and the causes of fracture load change of the hybrd joints under different parameters were analyzed.On the basis of stress analysis,considering the progressive damage characteristics of the laminate and the elastoplastic constitutive law of the adhesive layer,a finite element analysis model was established based on ABAQUS to predict the tensile static load carrying capacity and failure modes of three kinds of FJF joints.The correctness of the model was verified by the test results.Adopting the orthogonal design,the tensile static load carrying capacities and failure modes of the joints under the combination of the edge to diameter ratio,width to diameter ratio,adhesive thickness and stacking were compared using the prediction model.The systematic tests,theoretical and numerical simulation study provide a reliable basis for the engineering design and application of carbon fiber composite flat-joggle-flat joints. |
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