Effect of orientation of small defects on fatigue limit of steels (original) (raw)
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Effect of small defect orientation on fatigue limit of carbon steels
Fatigue & Fracture of Engineering Materials & Structures, 2015
In order to clarify the effect of defect orientation on the fatigue limit of two types of steels, JIS‐S15C and JIS‐S45C, a small semi‐circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 °, 30 ° or 60 ° with respect to the plane normal to the loading axis, but all of them had the same defect size, = 188 µm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. In all the combinations of the materials and tilt angles, a non‐propagating crack was found at or just below the fatigue limit, that is, the fatigue limit was determined by the non‐propagation condition of a crack initiated from the defect. In both steels, the fatigue limit was found to be nearly independent of the tilt angle for the same value of projected size , which was in good agreement with the prediction by the parameter model. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the ...
THE INFLUENCE OF SMALL DEFECTS ON THE FATIGUE LIMIT OF 1045 STEEL
This work investigates the influence of small defects on the fatigue limit of SAE 1045 steel. The tests were performed under uniaxial force-controlled loading at room temperature. The study defect has a cylindrical shape with √í µí±í µí±í µí±í µí± = 400 í µí¼í µí±. It is shown that the Murakami-Endo formula for fatigue limit prediction of materials containing defects works well for the investigated material.
2016
The influence of defect on fatigue resistance of components is carried out by using a propagation threshold approach to estimate the high cycle fatigue resistance of components from data of intrinsic properties of the material. The Chapetti´s threshold curve for fatigue crack propagation is used, which includes the short crack regime. The approach accounts for the effective driving force applied to the crack as the difference between the total driving force applied to the component, K, and the threshold for fatigue crack propagation of the material, K th. Application to analyze the influence of plate thickness on fatigue resistance of welded joints is presented.
Metall Mater Trans a, 1997
The continuous variations in crack shape or aspect ratio, a/c (a is the crack depth and c is the halfsurface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue crack growth of small cracks in a titanium (Ti-8Al) alloy. The significance of the aspect ratio variations in explaining the ''anomalous'' small-crack behavior was evaluated. The aspect ratio data were determined from the measurements of crack compliance, made using a laser interferometric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the order of a few grain diameters. The experimental a/c data were compared with the patterns of crack aspect ratio variation, obtained from theoretical simulations. The simulated data were generated by assuming alternate crack propagation at the surface and at the depth, the details of which are presented in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally observed variations. After incorporating the a/c variations in ⌬K calculations, the scatter in the growth data of small cracks was significantly reduced and was found to be of the same order as in large cracks. Additionally, it is shown in this study that the conventional methods of analysis of small-crack data, performed with an assumption of a/c ϭ 1, can result in significant errors in ⌬K calculation and an increased level of scatter in small-crack growth data. Small cracks also were found to exhibit low closure levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks, often referred to as anomalous, are due to the assumption of a/c ϭ 1 in situations of large variations in aspect ratio, the use of conventional methods of data analysis, and the lower levels of crack closure found naturally in small cracks.
Observation of Fatigue Crack Initiation and Growth in
Quantifying the low-cycle fatigue damage accumulated in nuclear power plant components is one of the important issues for aged plants. In this study, detailed observations of crack initiation and growth were made using scanning electron microscopy in order to correlate the crack size and the magnitude of the fatigue damage. Type 316 stainless steel specimens were subjected to the strain-controlled axial fatigue test (strain range: 1.2%) in air at room temperature. The test was interrupted several times in order to observe the specimen surface. The spatial distribution of inhomogeneously accumulated damage by cyclic loading was identified by crystal orientation measurements using the electron backscatter diffraction technique. Cracks were initiated at grain boundaries and slip steps, where relatively large damage accumulated. The changes in the number of cracks and their length were quantified. The crack growth rates were well correlated with the strain intensity factor. The change in crack size during the fatigue test was predicted using the obtained growth rate and assumed initial crack size. The fatigue lives estimated by the crack growth prediction agreed well with those obtained experimentally. It was concluded that the fatigue damage could be estimated from the crack size measured in plant components.
Experimental investigations on the growth of small fatigue cracks in naval steel
Fatigue & Fracture of Engineering Materials and Structures, 2007
The concept of damage tolerance is now largely employed to evaluate the fatigue life of structures. However, part of this fatigue relies on the initiation and growth of small cracks. The fatigue behaviour of a naval structural steel (S355NL) was investigated. In order to characterize the behaviour of short and long cracks, tests were performed under constant amplitude loading for several load ratios between-1.0 and 0.5. A major part of fatigue life is constituted by short crack initiation and propagation.
Fatigue Crack Growth under Mode I, II and III for Plane-strain and Plane-stress Conditions
Procedia Engineering, 2014
Fatigue crack growth (FCG) could be encountered in many mechanical components, which can be made from either thin or thick steel plates (or shells) and, therefore, be subjected to a plane-stress or a plane-strain condition, respectively. The loads applied in a solid body containing a narrow notch or a sharp crack will induce a yield zone near its tip with a dimension that will depend on the mechanical properties of the material, as well as on the thickness of the body, the crack length and the magnitude of the loads applied. Crack propagation can then occur under mode I, II, III or mixed-mode for general loading. The paper presents J I , J II and J III integral functions, which were correlated with the elastic stress intensity factors K I , K II and K III , for thin and thick CT specimens. The evaluation of J-Integral values was carried out for different crack lengths, along the crack front, and using the Finite Element Method (FEM), with collapsed nodes and midside nodes dislocated to ¼ of the edge's length, in order to simulate the crack tip singularity. Interaction between in-plane, in-plane sliding and out-of-plane modes are also discussed in the paper. In addition, FCG rates under mode I, mode III and a mixed-mode (mode I+III) were experimentally determined, at room temperature, for a high-strength Cr-Mn austenitic stainless steel.
High resolution synchrotron X-ray tomography has been used to obtain 3D images of arrested cracks initiated at small artificial defects located on the surface of cylindrical steel specimens subjected to mode I fatigue loading. These defects consist in small semi-circular slits tilted at 0°, 30° or 60° with respect to the plane normal to the loading axis; all of them had the same defect size, area = 188 μm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. Arrested cracks initiated from the notch were observed for all tilt angles at the surface of samples cycled at the fatigue limit (stress amplitude at which the specimen does not fail after 1×10 7 cycles). High resolution synchrotron X-ray tomography has been used to obtain 3D images of those small defects and non-propagating cracks (NPC). Despite the fact that steel is a highly attenuating material for X rays, high resolution 3D images of the cracks and of the initiating defects were obtained (0.65 m voxel size). The values of surface crack length measured by tomography are the same as those obtained by optical microscope measurements. The area values present the same tendency as the surface length of NPC, i.e. larger non-propagating cracks areas were observed in the softer steel. In the extreme case of 60º tilted defect, the crack fronts appear much more discontinuous with several cracks propagating in mode I until arrest.