| As one of the key components of the aircraft,the small-size fasteners have a direct impact on the reliability and safety of the aircraft during its fatigue performance.Most of the small-sized components on the aircraft are components with a diameter of less than10mm,and their failure mode is mostly fatigue failure under tensile-pull cyclic loading and the surface quality of components has always been the main cause of fatigue failure.Fatigue resistance can be improved by surface modification with enhancing the surface quality of components,forming a gradient structure on the surface of components and compressive residual stress.In previous research,the author’s research group found that the tensile and compressive fatigue limit of Inconel 718 alloy after USRP showed a trend of decreasing with the decrease of diameter,which we named the abnormal fatigue limit size effect.Therefore,in this paper,a high-cycle tension-tension fatigue experiment of different diameters of specimens is conducted on aged Inconel 718 treated with USRP at room temperature to study the effect of USRP on the tension-tension fatigue behaviour of the superalloy.The mechanism of the abnormal size-effect in the fatigue limit of the Inconel718 superalloy after the USRP is explored.This study provides a theoretical basis and guidance for the application and improvement of the USRP process for manufacturing butt bolts of the Inconel 718 superalloy.The following main conclusions are drawn:(1)The fatigue limit of specimen AR-3.4 at room temperature is 757 MPa,while the fatigue limit of USRP-3.4 is only 514 MPa,which is considerably lower than that of AR-3.4.At the same time,after USRP treatment,the fatigue limit decreases with the decrease of sample diameter(USRP-3.4:514 MPa,USRP-4.5:604 MPa,USRP-7.0:856 MPa),and there is obvious abnormal fatigue limit size effect.(2)Under the combined action of compressive residual stress and work hardening,fatigue cracks are all initiated inside the sample.The smaller the size of the sample,the closer the crack initiation position is to the center,the greater the probability that the diameter of the crack is larger than the A value.Therefore,the initiation of internal cracks is not only related to the uneven distribution of grain size and large angle grain boundaries but are also closely related to the high elastic incompatible stress between annealing twins and the matrix in large grains.The high elastic incompatible stress near the twin interface induces strain localization to form a resident slip band,resulting in crack initiation,and extends along the{1 1 0}facet family,forming the cleavage large facet feature.(3)The coverage area of quasi-cleavage features in the crack growth stage increases with the increase of the diameter ratio A.The fatigue life under the load near the fatigue limit increases first and then decreases with the increase of the sprout-diameter ratio A,and there is an obvious peak value.(4)FIB technology was used to intercept and analyze the dislocation configuration of fatigue small crack wake at different crack initiation positions.Microscopic analysis shows that the plastic deformation degree of the small crack tip with different sprouting diameter ratio is different due to the different stress triaxiality,and the small crack tip with a larger sprouting diameter ratio A has a higher stress triaxiality.Under high stress triaxiality,the plastic deformation of the small crack tip is more constrained,so there is a lower dislocation density and a smaller plastic zone.As a result,the critical fracture stressσ_fat the tip of the small crack is reduced,and the small fatigue crack is induced to expand rapidly under lower cyclic loading. |
PDF Full Text Request