Fatigue Analyses And Life Prediction Methods Of Notched Members:From Materials To Structures

Modern engineering components are developing towards extreme dimensions,loads and parameters;consequently,structural strength’s safety redundancy gets lower,leading to higher failure probability and increasing fault cases of parts.Therefore,the importance of conducting structural fatigue studies has become increasingly prominent.Unfortunately,multiple factors affect fatigue life/strength,and in particular,most of them cannot be described effectively and quantitatively based on the physics of failure;as a result,it is hard to raise reliable theories and methods for accurate fatigue life/strength assessment.In practice,to ensure the service safety of components,conservative lifing models are generally utilized during design procedures;nevertheless,conservative life/strength estimation will compress design space,lead to excessive redundant design and reduce equipments’ service performance.Many laboratory tests and engineering cases show that fatigue failure originates from geometrical discontinuous features(collectively referred to as notches in fatigue analysis),such as holes,grooves,transition fillets,etc.Notches will lead to local stress concentration;therefore,material elements within the highly stressed volume are more likely to show dislocation glide under cyclic loading and form micro-cracks,which gradually initiate and propagate until final structural failure.When dealing with problems with multiple influencing factors,a common engineering practice is to develop highprecision models to cover the main influencing factors and correct them with appropriate safety factors to describe the impact of remaining influencing factors and cognitive uncertainties.Given that fatigue failure of engineering structures generally occurs on notched structures,this thesis focuses on the notch effect — the main challenge in accurate fatigue assessment and life determination;and also considers critical factors,including size effect,mean stress effect and probabilistic aspects of fatigue.By combining experimental research,theoretical modelling and engineering applications,this thesis intends to provide valid theoretical and methodological support for fatigue strength analysis and life prediction of complex notched structures in engineering practice.The main work and conclusions of the thesis are as follows:(1)Clarify the influencing mechanism of size effect on fatigue damage evolution of notched structuresBased on fatigue tests of smooth specimens and notched specimens of different scales,made by Al 7075-T6511 alloy for aircraft fuselage manufacturing,the influence of size effect on the fatigue failure process of notched structures was clarified.It was found that the failure mode of this material gradually changes from fatigue damage to ductile fracture.The life fractions of initiation and propagation periods of smooth and notched specimens were recorded.By analyzing their cross sections with scanning electron microscopy,the average fatigue striation width,on the whole,was observed to increase with the increasing specimen scale and loading level,and the microscopic evidence of size effect on fatigue strength of notched structures,which works by altering the stress distribution,was identified.(2)Investigate the mean stress effect on fatigue strength of notched structuresTo uncover the mean stress effect on the fatigue failure behaviour of notched structures,fatigue tests of notched specimens of GH4169 alloy for disks were designed and carried out under different stress ratios.Microstructure analyses were conducted on their cross sections to clarify the failure mechanism of GH4169 notched specimens under various load levels.The talent of the Walker and the modified Walker stress–life models to correlate notch fatigue test data under different stress ratios were checked.(3)Develop a generalized total strain energy density–lifetime equation and raise the probabilistic energy field strength method.A generalized total strain energy density–life equation and a standard process for determining relevant model parameters were proposed,and the fatigue test data of Al7075-T6511 alloy and a wrought iron were used for verification.Based on it,aiming at the notch fatigue scenarios with relatively fixed failure sites,the energy field intensity approach which introduces the volume ratio of the effective damage zone to correct the size effect was proposed.The dispersion characteristics of the Al 7075-T6511 fatigue test data were extracted and then used to extend the energy field intensity approach to a probabilistic model.Finally,model prediction results were compared with the notch fatigue data under different ratios/load levels,which shows promising correlations.(4)Establish a global damage theory for probabilistic fatigue life prediction of notched structures under size effect.Focusing on the design requirements of complex engineering structures under multisite damage circumstances,a global damage theory for probabilistic fatigue life prediction of notched structures was established,which comprehensively considers notch effect,size effect,multiaxial fatigue and data dispersion.Based on the fatigue test data of smooth specimens of different dimensions,the effectiveness of the proposed model in fatigue dispersion description and fatigue performance transformation under size effect was verified.Then,the fatigue test data of TA19 notched specimens was used to check its validity to characterize the variation tendency of fatigue life and its dispersity with varying load and/or scale.(5)Propose energy parameters-based critical distance theories and apply them to fatigue life prediction of a high-pressure turbine disk.Catering to the demand for fatigue life prediction of notched structures under asymmetrical loadings,the damage parameters that can stably describe the damage distribution under various stress ratios were screened.Based on them,energy parametersbased critical distance theory for notch fatigue life prediction was put forward,which was verified with the aforementioned fatigue test data of GH4169 notched specimens.Finite element analyses of the high-pressure turbine disk of an aeroengine show that,under three typical working conditions,the fatigue critical point always lies at the interior root of the mortise.Finally,by extracting the cyclic responses near the critical point and inputting them into the proposed model,fatigue lives were predicted.