Competing Failure Methods And Application For Aircraft Mechanisms

There are development tendencies of precision,large-sized and comprehensive both for civil aircraft and military aircraft.Those characters and tendencies are necessary to meet the demands of economic level and war situation in modern terms.The accomplishments of aircraft motion mechanism functions are important conditions for reducing flight failure rate,flight safety and flight missions.Besides,the comprehensive and diversification of mechanisms lead to multiple failure modes which may competing mutually.Therefore,the aircraft mechanism reliability analysis with competing failures considered is drawing the attention of researchers.In general,this thesis focus on the competing failure problems of practice aircraft and the reliabilities of motion mechanism are also studied on the basis of competing failure principle.To start with,the multiple failure modes of motion mechanism are defined based on which an assessment method for aircraft competing failure is established.Then instant-shift hard failure threshold and intermission periods are taken into consideration to make up for the deficiencies of current models.As a consequence,two engineering mechanisms used in flight vehicle are investigated by proposed methods and models.The details of this thesis are composed by the following parts:(1)Reliability assessment methods for multi-performance mechanisms are established based on competing failures.First of all,relative theories and current research situations are expounded based on the development history and modeling methods of competing failures,which illustrates the research level and current problems of multi-functions aircraft competing failures.What’s more,some conceptions related to mechanism performance failures are defined,including the conception of mechanism performance failure,the reliability evaluation indexes,the characters of performance failures,failure causes and etc.Then basic theories of competing failures are summed up,such as basic theories of catastrophic failures and degradation failures.On the basis of basic theories,the application range and merit or demerit are studied for existing models.Afterwards,the reliability assessment based on competing failures methods are formed,consisted by three parts,mechanism construction and workin g principle analysis,competing failures modeling and reliability analysis.Finally,a case(four-bar mechanism)is studied using proposed methods.(2)A novel competing failures model considering instant-shifting hard failure threshold is established.Instant-shifting hard failure threshold illustrates the decreasing ability of mechanism part to resist the shocks from environment,which is confirmed by two engineering cases(a pair of gears and a crank connecting link).The instant-shifting thresholds are common among engineering practices.The shifting parts of hard thresholds include two parts,the natural part and the part relating to shocks.Then the mathematical derivations,including catastrophic failures,degradation failures and mechanism system reliability evaluations,are conducted,based on which modeling process and engineering significance are also discussed.At last,a micro-electromechanical system(MEMS)is studied as a case,demonstrating the practicability of the proposed model.(3)A novel competing failure model considering intermission periods as well as the preventive maintenance policy based on it are studied.The mechanism systems have a feature that working periods and intermission periods interact with each other.Exiting models cannot describe this fact properly,which motivates the appearing of this novel model.Different changing rates and shock rates are introduced to model the various characters of working periods and intermission periods.After that the model characters are described and mathematical expressions are also conducted.Besides,the preventive maintenance policies are studied in view of the model considering intermission periods.The presenting maintenance policies are preventive replacement by which the advantages of proposed model can be demonstrated.Similarly,the typical MEMS is chosen as a case to verify the model,demonstrating usefulness of proposed reliability and maintenance methods.(4)The competing failure process and reliability of an aircraft mechanis m lock system and an aircraft cabin door link mechanism are thoroughly studied.Respectively,related chapters introduce the compositions and structures,working principle and failure modes for both systems.Competing failure establishments come next.For the aircraft mechanism lock system,it is influenced catastrophic failure,open-lock-performance failure and close-lock-performance failure jointly.Meanwhile the aircraft cabin door link mechanism has two failure modes,the insufficient kinematic precision and the blocking of actuator cylinder.The failure modes of both systems are competing with others respectively.Proper competing failure modes are selected to research these two systems.As a consequence,the reliability as well as its degradation regulation is obtained.Besides the sensitiveness of some critical parameters are also studied.This thesis comprehensively studies the failure features for multi-performance mechanisms as well as the theories of competing failures.As a consequence,a research method for all kinds of multi-failure-mode mechanisms is formed.Making up the drawbacks in current models,instant-hard failure threshold and intermission are considered to establish novel models.The proposed models have much more advantages during practice using.Analysis results of the two engineering mechanisms can provide strong support for the reliability prediction and maintenance.The results also demonstrate the practicability of proposed reliability assessment methods and competing failures models.