| As a lightweight and efficient form of orthogonal connection,composite T-joints are widely used in engineering structures in aerospace,rail transit and other fields.The safety and reliability of the composite structures during the service of aircrafts is the key issue for engineering design.Hypersonic vehicles are facing severe aerodynamic heat during flying.The thermal physical properties of materials and the generation of thermal stresses in hightemperature environments make T-joints a weak link in the structure,which greatly endanger the safety of the structure.The research on the mechanical characteristics of T-joints at room temperature can no longer meet the service requirements of hypersonic aircraft.The failure mechanism and mechanical characteristics of the joints after being attacked by laser weapons have not been effectively elucidated through experiments and simulations.This thesis studies systematically the irradiation mechanism,the tensile failure mechanism and mechanical properties of CCF800H/AC631 carbon fiber reinforced bismaleimide resin composite T-joint in the high-temperature environment after irradiation through laser irradiation test,postirradiation tensile test and simulation research under different laser parameters.The research results can provide theoretical and method support for the laser ablation effect of such materials and structures and the high-temperature mechanical failure mechanism after irradiation.The main research contents and conclusions of this thesis are as follows:(1)A simulation method for simulating the laser irradiation behavior of the composite T-joint skin was established,and the accuracy and reliability of the model were verified by ablation tests under different parameters such as laser power,irradiation time,and spot diameter.The model uses a dynamic heat source combined with USDFLD and DFLUX subroutines to obtain the temperature distribution,the morphology of ablation pits and heat-affected zone and other parameters under laser ablation,which offers an accurate prediction of the ablation behavior of the T-joint skin under laser irradiation.The irradiation effect of the T-joint skin under different parameters is different,and the ablation morphology is affected by the ply angle.The study reveals the laser ablation mechanism of carbon fiber/bismaleimide resin composites under different ply angles.(2)Based on the above irradiation model,a simulation method for simulating the tensile failure behavior of T-joints at room temperature(20 °C)is established,and the validity of the model is verified by static tensile tests at room temperature.The model uses cohesive zone elements to simulate the interlayer interface of the T-joint and the Tsai-Wu criterion is adopted to determine the failure of the material under different modes for the heat-affected zone.The accurate prediction of the damage and failure behavior of the composite T-joint under tensile load considering the damage of the skin after laser irradiation is realized.By comparing the tensile failure of T-joint without laser irradiation,the failure of the composite in the heataffected zone of the skin is an important reason for the decrease of the initial stage of the loaddisplacement curve.This model effectively evaluates the tensile residual strength of the T-joint under the influence of different factors.(3)A simulation method considering the static tensile failure behavior of composite T-joints under high temperature(200 ?)after laser irradiation was established,in which the accuracy and reliability of the model is verified through a high temperature tensile test at 200 ?.The model comprehensively considers the degradation of the material's thermal physical properties at high temperature and the effects of thermal stress.Based on the cohesion zone model,the delamination failure of the interface around the triangular area of the composite Tjoint is simulated.The Tsai-Wu criterion is adopted to determine the failure of the composite under different modes in the heat-affected zone,which realizes the accurate prediction of the damage and failure behavior of the composite T-joint under the high-temperature tensile load considering the failure of the skin after laser irradiation.The decrease in the thermal physical properties of the material and the existence of thermal stress are important reasons for the reduction of the failure load of the T-joint at high-temperature.By comparing the tensile failure of the T-joint at room temperature and high temperature without laser,the larger failure area of the heat-affected zone of the skin after laser irradiation caused a further reduction in the initial stage of the load-displacement curve.The study reveals the mechanism of tensile failure of Tjoints under high temperature after laser irradiation,which provides the experimental foundation and numerical method support for the engineering design of composite T-joints and its secondary service after laser irradiation. |
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