The Experts below are selected from a list of 5049 Experts worldwide ranked by ideXlab platform
F P E Dunne - One of the best experts on this subject based on the ideXlab platform.
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is stored energy density the primary meso scale mechanistic driver for fatigue crack nucleation
International Journal of Plasticity, 2018Co-Authors: Bo Chen, Jun Jiang, F P E DunneAbstract:Abstract Fatigue crack nucleation in a powder metallurgy produced nickel alloy containing a non-Metallic Inclusion has been investigated through integrated small-scale bend testing, quantitative characterisation (HR-DIC and HR-EBSD) and computational crystal plasticity which replicated the polycrystal morphology, texture and loading. Multiple crack nucleations occurred at the nickel matrix-Inclusion interface and both nucleation and growth were found to be crystallographic with highest slip system activation driving crack direction. Local slip accumulation was found to be a necessary condition for crack nucleation, and that in addition, local stress and density of geometrically necessary dislocations are involved. Fatemi-Socie and dissipated energy were also assessed against the experimental data, showing generally good, but not complete agreement. However, the local stored energy density (of a Griffith-Stroh kind) identified all the crack nucleation sites as those giving the highest magnitudes of stored energy.
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on the mechanistic basis of fatigue crack nucleation in ni superalloy containing Inclusions using high resolution electron backscatter diffraction
Acta Materialia, 2015Co-Authors: Jun Jiang, F P E Dunne, Tiantian Zhang, Jie Yang, Ben T BrittonAbstract:Abstract A series of interrupted three-point bend low-cycle fatigue tests were carried out on a powder metallurgy FHG96 nickel superalloy sample containing non-Metallic Inclusions. High resolution electron backscatter diffraction (HR-EBSD) was used to characterise the distribution and evolution of geometrically necessary dislocation (GND) density, residual stress and total dislocation density near a non-Metallic Inclusion. A systematic study of room temperature cyclic deformation is presented in which slip localisation, cyclic hardening, ratcheting and stabilisation occur, through to crack formation and microstructurally-sensitive propagation. Particular focus is brought to bear at the Inclusion–matrix interface. Complex inhomogeneous deformation structures were directly observed from the first few loading cycles, and these structures were found not to vary significantly with increasing number of cycles. A clear link was observed between crack nucleation site and microstructurally-sensitive growth path and the spatially-resolved sites of extreme values of residual stress and GND density.
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slip localization and fatigue crack nucleation near a non Metallic Inclusion in polycrystalline nickel based superalloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2015Co-Authors: Tiantian Zhang, Jun Jiang, Ben T Britton, Barbara A Shollock, F P E DunneAbstract:Fatigue crack nucleation at a non-Metallic agglomerate Inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the Metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.
Keiro Tokaji - One of the best experts on this subject based on the ideXlab platform.
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effect of loading condition on very high cycle fatigue behavior in a high strength steel
International Journal of Fatigue, 2010Co-Authors: Masaki Nakajima, Hisatake Itoga, Keiro Tokaji, Toshihiro ShimizuAbstract:Abstract Very high cycle fatigue tests were conducted under rotating bending and axial loading using a high carbon chromium steel, JIS SUJ2, in order to evaluate the effect of loading condition on subsurface fracture with a fish-eye. In very high cycle region where subsurface crack was generated at a non-Metallic Inclusion, the fatigue lives under axial loading were shorter than those under rotating bending. This was attributed to the maximum size of non-Metallic Inclusions included in the control volume, because the sizes of non-Metallic Inclusions that acted as fracture origin were larger under axial loading than under rotating bending. Under both loading conditions, the Kmax values for crack initiation from non-Metallic Inclusion decreased with increasing fatigue life and were almost similar irrespective of loading condition, while the threshold K values where the transition from the granular area to the flat area in fish-eye took place depended on the size of the granular area. The transition condition was similar under both loading conditions.
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experimental estimation of crack initiation lives and fatigue limit in subsurface fracture of a high carbon chromium steel
International Journal of Fatigue, 2006Co-Authors: Masaki Nakajima, Hisatake Itoga, Nami Kamiya, Keiro TokajiAbstract:Abstract In order to clarify subsurface crack initiation life and fatigue limit, two-step variable amplitude loading tests were conducted using fine-particle bombarded specimens of a high carbon chromium steel, JIS SUJ2. Particular attention was paid to the granular area formed around a non-Metallic Inclusion in fish-eye. Based on the experimental finding that the granular area was not seen in short life regime at high stress levels, but observed in long life regime at low stress levels, the subsurface crack initiation was estimated by examining whether the granular area was present or not after testing. It was found that the subsurface crack nucleation occurred at the early stage of fatigue life, i.e., 5% or so, as a result of low–high sequence tests with different cycle ratios at low stress level. Furthermore, the results, which were obtained under similar low–high sequences performed until 10 9 cycles at six low stress levels followed by high stress level applied to failure, revealed that the fatigue limit for N = 10 9 would be around 650 MPa in the present material.
Kazuaki Shiozawa - One of the best experts on this subject based on the ideXlab platform.
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influence of Inclusion size on s n curve characteristics of high strength steels in the giga cycle fatigue regime
Fatigue & Fracture of Engineering Materials & Structures, 2009Co-Authors: Junke Zhang, Kazuaki ShiozawaAbstract:Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-Metallic Inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal Inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface Inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 106 cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal Inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of Inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of Inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface Inclusion-induced fracture depends on the Inclusion size.
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effect of non Metallic Inclusion size and residual stresses on gigacycle fatigue properties in high strength steel
Advanced Materials Research, 2008Co-Authors: Kazuaki ShiozawaAbstract:Fatigue fracture of some high-strength steels occurs at small defect in the subsurface zone of a material at low stress amplitude level and in a high-cycle region of more than 106 cycles (gigacycle fatigue life), whereas surface fatigue crack initiation occurs at high-stress amplitude and low cycles. There is a definite stress range where the crack initiation site changes from a surface to a subsurface defect, giving a step-wise S-N curve or a duplex S-N curve. From the experimental results, fatigue fracture mode was classified into three types, such as, surface Inclusion induced fracture mode, subsurface Inclusion induced fracture mode without granular bright facet (GBF) area and that with the GBF, depending on stress amplitude level and stress ratio. The GBF area was observed in the vicinity of a non-Metallic Inclusion at the fracture origin inside the fish-eye in gigacycle fatigue regime. It was made clear from the discussion with fracture mechanics that the transition of fracture mode was affected by compressive residual stresses on the specimen surface. Fracture-mode transition diagram was proposed through the experimental and theoretical investigation. Also, from the evaluation of the fatigue life based on the estimated subsurface crack growth rate from the S-N data, effect of Inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface Inclusion-induced fracture depended on the Inclusion size was provided.
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subsurface crack initiation and propagation mechanism in high strength steel in a very high cycle fatigue regime
International Journal of Fatigue, 2006Co-Authors: Kazuaki Shiozawa, Yuuichi Morii, Seiichi Nishino, Liantao LuAbstract:Abstract The mechanism of subsurface crack initiation and propagation in high strength steel in a very high-cycle fatigue region was studied by a computational simulation using a fracture surface topographic analysis (FRASTA) method for specimens of a high carbon chromium bearing steel with data obtained from rotary bending fatigue testing in air. Distinctive features of the fracture surface formed in the vicinity of a non-Metallic Inclusion at the fracture origin inside the fish-eye zone, the GBF area, was observed in detail using a scanning probe microscope and a three-dimensional SEM for comparing the microstructures of the materials. The GBF area, in which a rich carbon distribution was detected by electron probe microanalysis, revealed a rough granular morphology compared with the area inside the fish eye. It was clearly simulated by the FRASTA method in which during the fatigue process multiple microcracks are initiated dispersively by decohesion of spherical carbide from the matrix around a non-Metallic Inclusion, and coalesce with each other into the GBF area. In the study, the mechanism for the formation of the GBF area was proposed as the ‘dispersive decohesion of spherical carbide’ model.
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s n curve characteristics and subsurface crack initiation behaviour in ultra long life fatigue of a high carbon chromium bearing steel
Fatigue & Fracture of Engineering Materials & Structures, 2001Co-Authors: Kazuaki Shiozawa, L Lu, S. IshiharaAbstract:The S–N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-Metallic Inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The Inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S–N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S–N curve composed of two different S–N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S–N curve are discussed.
Masaki Nakajima - One of the best experts on this subject based on the ideXlab platform.
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effect of loading condition on very high cycle fatigue behavior in a high strength steel
International Journal of Fatigue, 2010Co-Authors: Masaki Nakajima, Hisatake Itoga, Keiro Tokaji, Toshihiro ShimizuAbstract:Abstract Very high cycle fatigue tests were conducted under rotating bending and axial loading using a high carbon chromium steel, JIS SUJ2, in order to evaluate the effect of loading condition on subsurface fracture with a fish-eye. In very high cycle region where subsurface crack was generated at a non-Metallic Inclusion, the fatigue lives under axial loading were shorter than those under rotating bending. This was attributed to the maximum size of non-Metallic Inclusions included in the control volume, because the sizes of non-Metallic Inclusions that acted as fracture origin were larger under axial loading than under rotating bending. Under both loading conditions, the Kmax values for crack initiation from non-Metallic Inclusion decreased with increasing fatigue life and were almost similar irrespective of loading condition, while the threshold K values where the transition from the granular area to the flat area in fish-eye took place depended on the size of the granular area. The transition condition was similar under both loading conditions.
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experimental estimation of crack initiation lives and fatigue limit in subsurface fracture of a high carbon chromium steel
International Journal of Fatigue, 2006Co-Authors: Masaki Nakajima, Hisatake Itoga, Nami Kamiya, Keiro TokajiAbstract:Abstract In order to clarify subsurface crack initiation life and fatigue limit, two-step variable amplitude loading tests were conducted using fine-particle bombarded specimens of a high carbon chromium steel, JIS SUJ2. Particular attention was paid to the granular area formed around a non-Metallic Inclusion in fish-eye. Based on the experimental finding that the granular area was not seen in short life regime at high stress levels, but observed in long life regime at low stress levels, the subsurface crack initiation was estimated by examining whether the granular area was present or not after testing. It was found that the subsurface crack nucleation occurred at the early stage of fatigue life, i.e., 5% or so, as a result of low–high sequence tests with different cycle ratios at low stress level. Furthermore, the results, which were obtained under similar low–high sequences performed until 10 9 cycles at six low stress levels followed by high stress level applied to failure, revealed that the fatigue limit for N = 10 9 would be around 650 MPa in the present material.
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an observation of fish eye fracture process in high strength steel suj2
Transactions of the Japan Society of Mechanical Engineers. A, 1999Co-Authors: Masaki Nakajima, Tatsuo Sakai, Toshihiro ShimizuAbstract:Long-term fatigue tests have been conducted on a high strength steel, SUJ2, in order to clarify the fatigue behaviour in long life region, more than 107 cycles. The fatigue failure at the stress level under the ordinary fatigue limit occurred in long life region. Fracture mechanism in this region is fish-eye type, i. e. inner defect induced failure, while surface slip type fracture is found in short fatigue life region. At the center of fish-eye non-Metallic Inclusion is recognized, which is surrounded by an indefinite and grainy region. However, the fish-eye without grainy region was also observed exceptionally. Therefore, a fish-eye is divided usually into three parts, i. e. Inclusion, grainy region and inner crack. Most of the Inclusions generating the fish-eye were located within 0.1mm from the specimen surface. The sizes of Inclusions calculated by the √(area) method are almost constant to the stress level and the number of cycles. The initial stress intensity factor range, ΔKini, at which crack was generated from the Inclusion, is almost the same with increasing fatigue life, i, e. 2.3∼3.5 MPa √(m). On the other hand, the sizes of fish-eyes calculated by the √(area) method are scattered to the stress level and the number of cycles. Based on the results, a model of the fatigue failure at the stress level under the fatigue limit is proposed.
Liantao Lu - One of the best experts on this subject based on the ideXlab platform.
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subsurface crack initiation and propagation mechanism in high strength steel in a very high cycle fatigue regime
International Journal of Fatigue, 2006Co-Authors: Kazuaki Shiozawa, Yuuichi Morii, Seiichi Nishino, Liantao LuAbstract:Abstract The mechanism of subsurface crack initiation and propagation in high strength steel in a very high-cycle fatigue region was studied by a computational simulation using a fracture surface topographic analysis (FRASTA) method for specimens of a high carbon chromium bearing steel with data obtained from rotary bending fatigue testing in air. Distinctive features of the fracture surface formed in the vicinity of a non-Metallic Inclusion at the fracture origin inside the fish-eye zone, the GBF area, was observed in detail using a scanning probe microscope and a three-dimensional SEM for comparing the microstructures of the materials. The GBF area, in which a rich carbon distribution was detected by electron probe microanalysis, revealed a rough granular morphology compared with the area inside the fish eye. It was clearly simulated by the FRASTA method in which during the fatigue process multiple microcracks are initiated dispersively by decohesion of spherical carbide from the matrix around a non-Metallic Inclusion, and coalesce with each other into the GBF area. In the study, the mechanism for the formation of the GBF area was proposed as the ‘dispersive decohesion of spherical carbide’ model.
