Study Of The Fatigue/Damage Load Spectrum Enhancement Of The Aircraft's Metal Structure

Full scale aircraft structure fatigue test is the most complicated,expensive and time-consuming work in aircraft structure fatigue/damage tolerance airworthiness compliance test.Aggravated load spectrum in fatigue accelerating tests of the full scale aircrafts is researched in this paper by the systematic study on the aggravated load spectrum in fatigue,aggravated load spectrum in crack propagation and aggravated load spectrum in widespread fatigue damage.A software platform for selecting coefficients of the aggravated load spectrum and predicting the original spectrum's fatigue/damage life is established based on the corresponding aggravated load tests.Though above research,the predicting of the original spectrum's fatigue/damage life and selecting coefficients of the aggravated load spectrum in fatigue accelerating tests of the full scale aircrafts can be carried out effectively.Specific contents are as below:(1)The aggravation experiment of the fatigue/damage load spectrum of the aircraft's metal structure are studied by 26 pieces of 30 Cr Mn Si Ni2 A test specimen belonging to fatigue aggravated load spectrum tests,67 pieces of LY12_CZ test specimen belonging to crack propagation aggravated load spectrum tests and 18 pieces of LY12_CZ test specimen with one rows of 7 holes belonging to widespread fatigue damage aggravated load spectrum tests.(2)The aggravation of the random load spectrum is studied.Two methods to predict the life of the random load spectrum are established,including equivalent nominal stress method and local strain-strain method which both based on structural detail fatigue eigenvalues.Fatigue aggravated load spectrum test is picked to verify the life prediction models.The result shows that the equivalent nominal stress method have better consistency with the experiments than the local stress-strain method.The curves of the aggravated coefficients-fatigue life and aggravated coefficients-aggravated spectrum life/original spectrum life are drawn.According to the ratio of the shortening test time,proper aggravated coefficients can be selected.The curves of aggravated coefficients-original spectrum life/ aggravated spectrum life are drawn.If the aggravated coefficients and aggravated spectrum life are given,the original spectrum life can be estimated.(3)The crack propagation aggravated load spectrum is studied.The Paris model and Willenborg-chang model are picked to predict the crack propagation life under the constant load spectrum and random load spectrum.The two models are verified by the crack propagation aggravated load spectrum test and the result shows a good consistency.Based on this model,when the aggravated coefficients increase,the crack propagates quickly.Besides,the shorter the critical crack is,the shorter the crack propagation life is.With the unitization processing of the small board and the big board under the constant load spectrum and random load spectrum,the curves of aggravated coefficients-aggravated spectrum life /original spectrum life have a good consistency when load coefficients increase.With this conclusion,when analyzing large specimen under aggravated random load spectrum,the small specimen with the same material under aggravated constant load spectrum can be used to determine aggravated coefficients selecting model and crack propagation life prediction model of original spectrum.Then based on several big scale specimen aggravated crack propagation life tests,the original spectrum life can be determined,which saves the tests' cost and time.(4)The widespread fatigue damage aggravated load spectrum is studied.The Paris model and Willenborg-chang model are picked to predict the crack propagation life under the constant load spectrum and random load spectrum.The two models are verified by the widespread fatigue damage aggravated load spectrum test and the results show that calculating results and experimental results have good consistency.Under the aggravated load,the analysis on the crack mutual impact factors and crack propagation shows that when aggravated coefficients increase and mutual impact factors stay unchanged,the crack propagates faster and crack propagation life decreases.Using the curves of aggravated coefficients-aggravated spectrum life /original spectrum life and the curves of aggravated coefficients-original spectrum life/ aggravated spectrum life can select aggravated load coefficients and predict original spectrum life.(5)On the base of the metal structure fatigue/damage load spectrum theory and experiment,for the first time,the software platform which is used to select coefficients of the aggravated load spectrum and predict the original spectrum's life is established.The platform adopts C++ language,and is divided into five modules.They are database management,the aggravated load spectrum,crack propagation aggravated load spectrum,widespread fatigue damage aggravated load spectrum and result monitor.The database management module includes file name,item remark,load spectrum location,model parameters and etc.The aggravated load spectrum,crack propagation aggravated load spectrum and widespread fatigue damage aggravated load spectrum modules mainly include fatigue crack propagation model,material parameters,specimen size data,the original load spectrum and load spectrum amplification coefficient and etc.The result monitor outputs fatigue(crack propagation)life,the curves of aggravated coefficients-aggravated spectrum life /original spectrum life and the curves of aggravated coefficients-original spectrum life /aggravated spectrum life.The module also displays figures,selects the aggravated coefficients and predicts the original spectrum life.