IMPACT OF NOTCH GEOMETRY ON THE FATIGUE LIFE OF BS 970-4 GRADE 349S52 STAINLESS STEEL

Field: Engineering (Physical Sciences)

Components in engineering applications often have discontinuities and abrupt change in cross sections which may be present owing to their functional requirements like oil holes, grooves, keyways etc. This would result in localization of high stresses when these components are subjected to loading. Situation becomes more hazardous to the material if the loading is not static and varying in magnitude with time. This fatigue phenomenon reduces the resistance of the material under fluctuating stresses. Fatigue can be defined as a failure taking place by the formation and growth of cracks due to repeated stresses. Fatigue design is considered to be complex as the failure sometimes occurs abruptly without any indication about the initiation of the failure. It is evident from experience that, around 80% of structural failures is due to insufficient fatigue design. Prodigious work is done in the field of fatigue design but there is still lot of scope in this area. C.S.Yen et.al. Reviewed lot on the literature and concluded that, the fatigue notch-sensitivity of a metal member depends upon three different factors namely, the basic material characteristics, the degree of material homogeneity, and the geometry of the member (C.S. Yen, 1952). M.Makkonnen showed that when the notch gets sharper, the magnitude of the plastic portion of the strain starts to play an important role in the fatigue crack initiation, and the fatigue limit is lower than that prediction is by statistical and geometric size effects. In those cases, fatigue limits should be arrived, by assuming the notch to be an initial crack with the notch depth being considered as the depth of the crack and the fatigue limit is computed to this crack by banking upon linear elastic fracture mechanics and the stress intensity factor range threshold (M. Makkonen, 2003). Yoshiaki Akiniwa et. al. demonstrated that, for specimens with circumferential notch, fatigue fracture starts from the surface or very near the surface. The slip deformation is often responsible for the crack initiation in high cycle and very high cycle regimes (Yoshiaki Akiniwa, 2006). A.J.McEvily et.al. proved that, for holes of radii less than 1 mm in the steel investigated, the notch fatigue factor, KF, is dependent upon crack closure and for holes of radii in the range of 1–5 mm in the steel investigated; the analysis indicates that KF is constant and dependent upon the ratio σmax/σy (A.J. McEvily, 2008). M. Zehsaz et.al. Showed that the volumetric approach gives good results in predicting the fatigue life of the notched specimens. The effect of notch radius for different notched specimens was investigated to observe the stress concentration factor, notch strength reduction factor, and fatigue life of the specimens (M. Zehsaz, 2010). G.H.Majzoobi et.al. demonstrated that notch geometry has profound effect on fatigue life of materials. For high strength steel this reduction is roughly about 50%. For low strength -steel alloy, however, the reduction depends on fatigue life and varies from 20% for low cycle fatigue tests up to 75% for high cycles fatigue tests. The maximum and minimum fatigue life reduction occurs for the V-shape and U-shape notches, respectively (G.H. Majzoobi, 2010). Baohua Nie et. al. concluded that fatigue life improves with the increase in the crack initiation depth. The scatter of the fatigue property should be carefully considered in fatigue design (Baohua Nie, 2018). M.L. Aggarwal et. al. developed a numerical model using stress approach to predict the fatigue life of a shot-peened mechanical component (M.L. Aggarwal, 2006). The current research work aims at finding the pressure that the selected material can sustain in tension at a fixed stress level and fatigue life. Specimens are fabricated with different geometries of notches on them. The width, depth and central angle of the notch are varied and the fatigue life is found out using finite element method (R. Marimuthu, 2014) (J. Jagadesh Kumar, 2017). The design of experiments using Taguchi L9 orthogonal array is selected to know the impact of each parameter on the pressure bearing capacity and thereby on the fatigue life. Numerous finite element method runs are conducted in an iterative approach using ANSYS 18.1 to get the target values of stress and fatigue life.