| The hydraulic system as the core power source of an aircraft,which hydraulic pipelines throughout the whole body of the aircraft as if the blood vessels in the human body,is one of the important components of the aircraft hydraulic system.The aircraft hydraulic pipelines are the main channels to transport the hydraulic oil working medium,and the important links to build the hydraulic control loop.As the modern aviation hydraulic system is developing in the direction of high pressure,large load and light weight,the strength margin of hydraulic pipelines is becoming less and less,and the fatigue problem is becoming more and more prominent.How to avoid or delay the structural damage of the hydraulic pipelines caused by fatigue has become a common problem faced by both military and civil aviation industries.Therefore,many scholars at home and abroad have carried out in-depth research on the performance of the aircraft hydraulic pipelines,but mainly researches focus on analyzing the dynamic response under the action of a single excitation.Most of the researches on the fatigue strength characteristics of the aircraft hydraulic piping structures are based on tests with high economic and time costs.There is still no effective systematic evaluation method for the fatigue strength of the aircraft hydraulic pipeline structures under the effect of the composite excitations.In view of this,this thesis takes the aircraft hydraulic pipeline structures as the research objects,focuses on their fatigue strength characteristics under the composite excitations of fluid pulse and random vibration.Based on the research and comparative verification of the fatigue strength analysis methods of the structures under a single excitation and the dynamics simulation analysis of the hydraulic pipelines,a comprehensive framework for evaluating the fatigue strength of the aircraft hydraulic pipelines under the composite excitations has been formed,which will attempt to provide some reference for the establishment of evaluation criteria of fatigue strength for the aircraft hydraulic pipelines.The main work and research results of the thesis are as follows:(1)Research and comparative verification of structural fatigue strength analysis methods under the effect of a single excitation.The correction capability of various averaged stress correction models is compared and verified by means of the available fatigue test data of domestic aviation metal materials,and suitable models for different metal materials are selected.The characteristics of two random vibration fatigue analysis methods in the frequency domain and time domain are examined and compared with each other by means of a 7075-T6 aluminium alloy semi-circular notched specimen.The above researches provide a basis for the selection of fatigue strength evaluation methods for the aircraft hydraulic pipeline structures.(2)Strength evaluation research based on joint multi-threshold criterion.Three threshold criteria based on the average stress correction model are derived for various types of aerospace metal materials,and then a joint multi-threshold criterion for the strength of the aircraft hydraulic pipeline structures is proposed,and the evaluation process is elaborated with the results of dynamic stress simulation analysis and risk node selection of a typical aircraft hydraulic pipeline structure model.(3)Study on a comprehensive fatigue strength evaluation method for aircraft hydraulic pipelines under the effect of the composite excitations.A coupled equivalent force is defined to superimpose the random vibration response and fluid pulse response of the risk nodes of the pipeline.The fatigue lives of the risk nodes are estimated by combining the statistical results of rainflow counting,Miner’s criterion and S-N curves,and a method is proposed to quantitatively predict the fatigue life of the pipeline structures under the effect of the composite excitations.On the basis of the above research,a comprehensive fatigue strength evaluation framework based on the joint multi-threshold strength evaluation and the fatigue life time domain sample estimation method is established for the aircraft hydraulic pipelines under the composite excitations.(4)Design and implementation of fatigue strength evaluation software for the aircraft hydraulic pipelines.The comprehensive fatigue strength evaluation method for the aircraft hydraulic pipelines with complex processes is modularly encapsulated,and a prototype system of dedicated evaluation software for the fatigue strength of the aircraft hydraulic pipelines is studied and developed.Firstly,the feasibility and requirements of the software development are analysed;secondly,the design details of the software are highlighted,including the architecture design,the core class diagram and the database design;then,the implementation details of the software are introduced,including the software development tools,the key technologies involved in the development and the effect of the software implementation;finally,the testing of the software prototype system is completed through application cases to verify its effectiveness and practicality. |
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