Research On Aircraft Structural Fatigue Management Program System And Key Technologies

Structural fatigue damage design has experienced several stages,including safe-life design,fail-safe design,damage-tolerance design and reliability design.The purpose of these methods is to improve the performance,the safety,the reliability and the economy of the aircraft.Considering the complexity of the aircraft,a certain area may face multiple kinds of risk at the same time,and the load situation and the corresponding stress distribution are also very complicated.In addition,there are many factors affecting the fatigue fracture,and the fatigue critical part presents a certain irregularity.So the dispersion of fatigue life is quite large.However,in modern aircraft design higher requirements of the safety and economy are proposed,which makes the damage tolerance design a key technology.The inspection based on damage-tolerance and the appropriate fatigue management program(FMP)and the development of the continuous airworthiness program has also become the highlight of the airworthiness assessment.To this end,this paper proposes an aircraft structural fatigue management program system(ASFMPS)for damage management and maintenance during aircraft service,and studies the key technologies supporting the ASFMPS.This research not only has academic value,but also has the engineering application value.This dissertation consists of two parts:The first part studies the theoretical system of the aircraft structure fatigue management program.Aiming at the lack of operational process and engineering judgment in the aspect of fatigue management of aircraft structure in China,the paper focus on the ASFMPS,and proposes the framework of ASFMPS.Under the framework of the system,the damage management and maintenance measures that should be taken in different service phases of the aircraft are given.The inspection program,structure replacement/maintenance program and the widespread fatigue damage inspection/maintenance program are the main content of the damage assessment and management.The evaluation/maintenance scheme for the environmental deterioration,accidental damage,local fatigue damage and the widespread fatigue damage has been developed,and its effectiveness and feasibility have been illustrated by an engineering example.In the second part,the paper studies several key technologies in the aircraft structural fatigue management program system.The concept of the non-crack damage is proposed.Aiming at the difficulties and existing problems in the research of non-crack damage,the paper puts forward the characteristic element,and the formulation ideas of the non-crack damage prediction and diagnosis system are given.The assessment system of the environmental deterioration,accidental damage and fatigue damage is presented according to the structural inspection program of ASFMPS.The fuzzy synthetic evaluation method is applied to determine the inspection interval,and its feasibility is verified,which has good engineering application value.The validity of the initial structure inspection program is judged by the damage tolerance rating(DTR)and the average probability of the crack detection is used which simplifies the calculation process and improves the calculation efficiency.For the widespread fatigue damage(WFD),a probabilistic and deterministic method is presented,and a numerical example is used to verify the method.Based on the evaluation of WFD,the maintenance strategies for preventing the WFD are given,and the formulation of the maintenance activites is also proposed.The residual strength analysis and the crack growth analysis methods related to fatigue assessment are studied.Based on the notch theory,a relatively conservative crack growth rate model is proposed,and the validity of the model is verified by relevant experimental data.The calculated data error is analyzed and the relevant conclusions are obtained.The crack propagation rate data and crack growth life of the new model are given by numerical simulation,and compared with the experimental values.It is proved that the new model is relatively conservative and has certain engineering application value without giving priority to economy.The physical meanings of the three crack propagation rate parameters in the new model are discussed.It is proved that these parameters can effectively describe the process experienced by the fatigue element in the crack edge and the plastic influence of crack propagation.