The effect of quenching and defects size on the HCF behaviour of Boron steel
Abstract This work investigates the effect of natural and artificial surface defects and quenching on the fatigue strength of a Boron steel (22MnB5). A vast experimental campaign has been undertaken to study the high cycle fatigue behaviour and more specifically the fatigue damage mechanisms observed in quenched and untreated materials, under different loading conditions and with differents artificial defects sizes (from 25 μm to 370 μm radius). In order to test the sheet metal in shear an original test apparatus is used. The critical defect size is determined to be 100 ± 50 μm. This critical size does not appear to depend on the loading type or the microstructure of the material (i.e. ferritic–perlitic or martensitic). However, for large defects, the quenched material is more sensitive to the defect size than the untreated material. For a defect size range of 100–300 μm the slope of the Kitagawa–Takahashi diagram is approximately −1/3 and −1/6 for the quenched and untreated materials respectively. A probabilistic approach that leads naturally to a probabilistic Kitagawa type diagram is developed. This methodology can be used to explain the relationship between the influence of the heat treatment and the defect size on the fatigue behaviour of this steel.
Very high cycle fatigue properties of bearing steel under axial loading condition
Abstract It has been observed to appear a step-wise or duplex S–N curve under the test of rotary bending fatigue using high strength steel. This behavior was caused by the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced fracture. The aim of this study is to clarify the S–N characteristic under an axial loading fatigue in the very high cycle fatigue regime. In order to investigate the mean-stress effects, fatigue tests were carried out in air at room temperature under three applied stress ratios of −1, 0 and 0.5 using a hour-glass shaped specimen of high carbon–chromium bearing steel, JIS SUJ2. From the results, three types of fracture mode were observed on the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without granular bright facet (GBF) area and that with GBF area around an inclusion. Fatigue lifetime for transition in the fracture mode depended on the applied stress ratio. Shape of the S–N curve was a smooth and continuous under three testing conditions in spite of the occurrence on the three types of fracture. Detail discussion for fatigue fracture behavior was made through the observation of fracture surface and from point in view of the fracture mechanics. In addition, an effect of residual stress in the specimen surface layer on the transition of fracture mode was discussed and compared with the experimental results.
Macro and microscopic approach to fracture
Chapter 1 Fatigue strength-1: Fatigue properties of auste nitic stainless steels Introduction Experimental procedures Rotary bending fatigue properties Effect of Cr/Ni equivalent on fatigue crack initiation Effect of aging on fatigue strength Effects of overload and heat treatment on axial fatigue strength Conclusions. Chapter 2 Fatigue strength-2: Fatigue properties of maraging steel Introduction Strengthening mechanism of Maraging steel Effect of reversion austenite on fatigue strength Effect of surface modification on fatigue strength Improvement of fatigue strength by a combination of microstructural control and surface modification Concluding remarks. Chapter 3 Stress analysis of crack: Highly accurate analysis of stress intensity factors by the crack tip stress method in FEM Introduction Weak points of the extrapolation methods Crack tip stress method Applications of the crack tip stress method Conclusions. Chapter 4 Fatigue crack propagation: On the history and application of the fatigue crack growth relation Introduction Early research Recent work Applications Concluding remarks. Chapter 5 Surface treatment-1: Characteristics of fatigue behavior of surface modified materials Importance of surface modification Fatigue behavior of ceramics coating steels in air Corrosion fatigue behavior of ceramics coated steel Fatigue behavior of anodized coating aluminium alloys Fatigue crack initiation and growth of plasma-assisted duplex surface treated tool steel Gigacycle fatigue behavior of shot-peened high-hardness steel. Chapter 6 Surface treatment-2: Improvement in fatigue limit of a notched specimen by local plastic deformation" Improvement of fatigue limit of the round bar specimen witt a round V groove Improvement of fatigue limit of plate specimen with a hole. Chapter 7 Environmental fracture: Macro- and micro-scopic approach to fracture of high strength steel notched bars Introduction Experimental program Microscopic approach Macroscopic approach Conclusions. Chapter 8 Failure analysis: A role of macrofractography in fracture analysis Introduction Development of microfractography Microfractography of hot forging dies Microfractography of steam turbine blade Microfractography of cargo oil tank structure of very large crude oil carrier Concluding remarks. Chapter 9 Engineering applications: Fatigue life evaluations for components of power generating equipments and strength evaluation of semi-conductor devices. (part contents)