Fatigue Life Prediction And Reliability Analysis Of Notched Components Under Size Effect

With the society development,for higher functional needs and the requirement of assembling,major equipment with magnify size and complex configurations have been utilized nowadays.It is generally known that fatigue limit of structure normally decreases with its size/volume.But,considering time-consuming and cost of major equipment testing with full-scale,smooth and small specimens are designed to use in realistic test.It is crucial to study size effect for an available passage from small and smooth specimens to full-scale components.A key factor affecting size effect is the presence of defects due to manufacturing,processing or mechanical damage occurring during service.Fractography has revealed that structural defects are normally regarded as the primary reason for fatigue crack initiation,which influence not only the fatigue life of cast alloys by triggering premature failure,but also considerable scatter of fatigue life by increasing the variability of properties.Furthermore,another dominant factor cannot be ignored is the geometric discontinuities(like notch).Affected by inhomogeneous stress,even under uniaxial loading,a complex multiaxial stress field appears geometrical discontinuities or near the notch root region.However,an efficient approach considering both the effects notch and size in project design of major equipment is still lacking and highly desirable.As aforementioned,coupling with the existing size effect research,this thesis is summarized as follows:(1)Induction and summary of size effect.Four main size effects are reviewed to present a guide for assessing the strength of engineering components containing defects,inclusions and inhomogeneities.Firstly,the history of inherent defect and its failure mechanism is briefly outlined and the fatigue failure analysis based on fracture mechanics and statistical theory are emphasized.Subsequently,attention is focused on statistical size effect and geometrical size effect,and briefly overviews production size effect and surface size effect.(2)Probabilistic model of fatigue life and statistical size effect under defects.A probabilistic model is presented to account for the influence of manufacturing defects and statistical size effect on fatigue life,based on the damage mechanism of surface initial crack and Weibull distribution.Experimental data of three cast irons and aluminum alloys are utilized for model validation,and the proposed model provides more accurate predictions than three size effect models.(3)Probabilistic model of size effect based on high stressed volume method.A methodology considering the effect of notch and size to analyze the fatigue life distribution of specimens with various geometries using the highly stressed volume approach is presented.Experimental data of three aluminum and titanium alloys are utilized for model validation and comparison.Fatigue lives of three materials with different geometries are evaluated respectively,and predicted results indicate that proposed model predictions agree well with the probabilistic scatter band of experimental results.(4)Fatigue life prediction and reliability analysis of notched specimens under uncertainty.For assessing the influence of random loads and the uncertainty of defect size on the fatigue life of notched structures,fatigue life prediction and reliability analysis of notched structures under size effect by inputting different distributions of loads and crack length.