Damage Mechanism of the Acceleration of Intergranular Cracking of Stainless Steel SUS316LN Under Creep Loading at Elevated Temperatures

2020 ◽  
Author(s):  
Chongzhe Gu ◽  
Yukako Takahashi ◽  
Hideo Miura
Keyword(s):  
Plastic Deformation ◽  
Grain Boundaries ◽  
Creep Test ◽  
Elevated Temperatures ◽  
Creep Damage ◽  
Degradation Process ◽  
Damage Mechanism ◽  
Dominant Factor ◽  
Total Density ◽  
Intergranular Cracking

Abstract In this study, the simple creep test and intermittent creep test of SUS316LN, which has become a preferred materials for the structural components used in Boiling Water Reactors and Sodium Cooled-fast Reactors, were carried out to investigate the damage evolution. The effect of doping nitrogen into conventionally used SUS316L on the creep and fatigue strength has been proved in the comparison experiment between SUS316L and SUS316LN. At elevated temperatures, however, intergranular cracking still appeared under the application of low tensile stress. The mechanism of intergranular cracking at elevated temperatures, however, has not been clarified quantitatively yet. Therefore, in this research, EBSD method was applied to investigate the degradation process of the crystallinity around grain boundaries in this alloy from the viewpoint of the change of micro texture and atom arrangement. IQ (Image Quality) values, which indicates the average sharpness of the obtained diffraction pattern, were used for the evaluation of the local total density of defects. KAM (Kernel Average Misorientation) value was used for the evaluation of local plastic deformation in this study. In the creep test, the crystallinity decreased monotonically with the increase of creep damage. Combined with ΔKAM value, it was concluded that the accumulation of dislocations along specific grain boundaries and the difference of the magnitude of plastic deformation between two nearby grains were the dominant factors of intergranular cracking. Large difference of the magnitude of plastic deformation between two grains accelerated the accumulation of dislocations around the grain boundary. Therefore, the large difference of Schmid Factor between nearby grain is the dominant factor which determines the place where intergranular cracking starts to occur.

Molecular Dynamics Analysis of the Acceleration of Intergranular Cracking of Ni-Base Superalloy Caused by Accumulation of Vacancies and Dislocations Around Grain Boundaries

2020 ◽  
Author(s):  
Ryo Kikuchi ◽  
Shujiro Suzuki ◽  
Ken Suzuki
Keyword(s):  
Molecular Dynamics ◽  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fatigue Loading ◽  
Transgranular Fracture ◽  
Dynamics Analysis ◽  
Intergranular Cracking ◽  
Creep Fatigue ◽  
Schmid Factor

Abstract Ni-based superalloys with excellent high temperature strength have been used in advanced thermal power plants. It was found that grain boundary cracking is caused in the alloy under creep-fatigue loading due to the degradation of the crystallinity of grain boundaries and the grain boundary cracking degrades the lifetime of the alloy drastically. In order to clarify the mechanism of intergranular cracking, in this research, static and dynamic strains were applied to a bicrystal structure of the alloy perpendicularly to the grain boundary using molecular dynamics analysis. In addition, the effect of the accumulation of vacancies in the area with high-density of dislocations on the strength of the bicrystal structure was analysed. It was found that the fracture mode of the bicrystal structure changed from ductile transgranular fracture to brittle intergranular one as strong functions of the combination of Schmid factor of the two grains and the density of defects around the grain boundary. The local heavy plastic deformation occurred around the grain boundary with large difference in Schmid factor between nearby grains and the diffusion of the newly grown dislocations and vacancies was suppressed by the large strain field due to the large mismatch of the crystallographic orientation between the grains. The accumulation of vacancies accelerated the local plastic deformation around the grain boundary. Therefore, the mechanism of the acceleration of intergranular cracking under creep-fatigue loading was successfully clarified by MD analysis.

scholarly journals Creep Damage Evolution Process of Asphalt Binder Based On Viscoelastic Characteristics

2021 ◽  
Author(s):  
Tian Shuang ◽  
QI Xiaofei ◽  
shan liyan ◽  
Liu Shuang ◽  
Wang Yajie
Keyword(s):  
Creep Test ◽  
Damage Evolution ◽  
Asphalt Binder ◽  
Creep Property ◽  
Creep Damage ◽  
Damage Mechanism ◽  
Model Parameters ◽  
Frequency Sweep ◽  
Viscoelastic Parameters ◽  
Two Stages

Abstract The creep damage evolution of asphalt binder plays a significant role in investigating the formation mechanism of rutting, a common type of distress at high temperature for asphalt pavements. However, the reliability of existing creep damage parameters is under questioned, and these parameters cannot accurately illustrate change law of intrinsic microstructure about asphalt binder. In this paper, a new testing protocol is given access to study the evolution of viscoelastic parameters during creep damage. It is completed by inserting the frequency sweep during creep test. The frequency sweep curve clusters are fitted using the generalized Kelvin-Voigt model for obtaining the change law of model parameters. Based on the change law and sensitivity analysis of model parameters, (E2 + E3)/2 is proposed as the creep damage variable. According to the curve of (E2+E3)/2 versus loading time, two stages during the creep test can be identified: an approximate constant value in phase Ⅰ and a linear decrease in phase Ⅱ. Intrinsic differences about creep property of binders can be determined by this new proposed parameter. Above results not only ensure better understanding of the creep damage mechanism of binders, but also lay the theoretical foundation on predicting the anti-rutting performance of binders.

Inner Pressure/Bending Creep Test on Circumferentially Welded Large Diameter Pipe of Grade 91 Steel: Creep Damage Mechanism and Applicability of NDE

2016 ◽  
Author(s):  
Satoshi Nishinoiri ◽  
Yukio Takahashi ◽  
Hiroyuki Fukutomi ◽  
Masatsugu Yaguchi
Keyword(s):  
Creep Test ◽  
Welded Joints ◽  
Welded Joint ◽  
Large Diameter ◽  
Ultrasonic Inspection ◽  
Creep Damage ◽  
Damage Mechanism ◽  
Joint Surface ◽  
Internal Defects ◽  
Inner Pressure

In this study, an inner pressure/bending creep test on a circumferentially welded large diameter 9Cr steel pipe was conducted at a temperature of 650°C to investigate the creep damage mechanism and applicability of nondestructive evaluation techniques to the detection of creep damage. The test was interrupted before steam leakage based on a change in axial strain at the surface of the welded joint. Surface cracks were observed at the lower part of the specimen along the circumferentially welded joint where axial tensile stress was applied. As a result of phased array ultrasonic inspection, indications of internal defects were first detected along welded joints where the failure life ratio was 54% with the range of indication increasing with test time. The indications of internal defects were in good agreement with observations of sectional views in welded joints.

Creep-Damage-Induced Deterioration of the Strength of Ni-Base Superalloy due to the Change of its Microstructure

2017 ◽  
Author(s):  
Hayato Sakamoto ◽  
Ken Suzuki ◽  
Hideo Miura
Keyword(s):  
Power Plants ◽  
Elevated Temperatures ◽  
Creep Damage ◽  
Test Sample ◽  
Test System ◽  
Electron Back Scatter Diffraction ◽  
Dominant Factor ◽  
Jet Engines ◽  
Creep Tests ◽  
Kikuchi Pattern

Ni-base superalloys are widely used for various power plants and jet engines. Since the operating temperature of thermal plants and equipment has been increasing to improve their thermal efficiency for decreasing the emission of carbon-dioxide, the initially designed microstructure was found to change gradually during their operation. Since this change of microstructure should deteriorate the strength of the materials, sudden unexpected fracture should occur during the operation of the plants and equipment. Therefore, it is very important to clarify the dominant factor of the change of the microstructure and the relationship between the microstructure and its strength for assuring the stable and reliable operation of the plants and equipment. In this study, the change of the strength of a grain and a grain boundary of Ni-base superalloys caused by the change of their microstructure was measured by using a micro tensile test system in a scanning ion microscope. A creep test was applied to bulk alloys at elevated temperatures and a small test sample was cut from the bulk alloy with different microstructure caused by creep damage by using focused ion beams. The test sample was fixed to a silicon beam and a micro probe, respectively, by tungsten deposition. Finally, the test sample was thinned to 1μm and the sample was stretched to fracture at room temperature. The change of the order of atom arrangement of the sample was evaluated by applying electron back-scatter diffraction (EBSD) analysis quantitatively. In this study, the quality of grains in Ni-base superalloys was analyzed by using image quality (IQ) value calculated by using Hough transform of the observed Kikuchi pattern. It was found that the order of atom arrangement was deteriorated monotonically during the creep tests and this deterioration corresponded to the change of the microstructure clearly. Both the yield strength and the ultimate tensile strength of a grain in the alloys decreased drastically with the change of the microstructure, in other words, the IQ value of the grains. There was a clear relationship between the IQ value of a grain and its strength. Therefore, this IQ value is effective for evaluating the crystallinity of the alloys and the remained strength of the damaged alloys. The change of the microstructure was dominated by the strain-induced anisotropic accelerated diffusion of component elements of the alloys and the activation energy of the diffusion was determined quantitatively as a function of temperature and the applied stress.

EVALUATION OF DAMAGE DEVELOPMENT IN STEELS SUBJECTED TO PRIOR DEFORMATION: DESTRUCTIVE AND NONDESTRUCTIVE TECHNIQUES

2009 ◽  
Vol 01 (03n04) ◽  
pp. 479-499 ◽  
Author(s):  
ZBIGNIEW L. KOWALEWSKI ◽  
SŁAWOMIR MACKIEWICZ ◽  
JACEK SZELĄŻEK ◽  
KRYSTYNA PIETRZAK ◽  
BOLESŁAW AUGUSTYNIAK
Keyword(s):  
Plastic Deformation ◽  
Elevated Temperatures ◽  
Damage Identification ◽  
Creep Damage ◽  
Damage Evaluation ◽  
Material Degradation ◽  
Damage Development ◽  
Nondestructive Methods ◽  
Tension Tests ◽  
Magnetic Techniques

Creep damage due to constant loading at elevated temperatures and damage due to plastic deformation at room temperature are assessed using destructive and nondestructive methods in steels (40HNMA and P91). In the destructive methods the standard tension tests were carried out after prestraining and variations of the selected tension parameters were taken into account for damage identification. In order to assess damage development during the creep and plastic deformation the tests for both steels were interrupted for a range of selected strain magnitudes. Ultrasonic and magnetic techniques were used as the nondestructive methods for damage evaluation. The last step of the experimental programme contained microscopic observations. The results show that ultrasonic and magnetic parameters can be good indicators of material degradation and can help to locate the regions where material properties are changed due to prestraining. A good correlation of mechanical and selected nondestructive parameters identifying damage was achieved for both tested steels.

Degradation of the Strength of a Grain Boundary of Ni-Base Superalloys Under Creep-Fatigue Loading at Elevated Temperature

2020 ◽  
Author(s):  
Yukako Takahashi ◽  
Yifan Luo ◽  
Kenta Ishihara ◽  
Shujiro Suzuki ◽  
Hideo Miura
Keyword(s):  
Grain Boundary ◽  
Tensile Test ◽  
Grain Boundaries ◽  
Elevated Temperatures ◽  
Base Alloy ◽  
Tensile Load ◽  
Test System ◽  
Uniaxial Tensile ◽  
Total Density ◽  
Creep Fatigue

Abstract The degradation of the strength of a grain boundary was measured by using a micro tensile test in a scanning electron microscope. The change of the crystallinity of grain boundaries during creep-fatigue tests of Ni-base alloy such as Alloy 617 and 625 at elevated temperatures was monitored by electron back-scatter diffraction analysis. The image quality (IQ) value obtained from the analysis, which indicates the total density of defects, was applied to the quantitative evaluation of the crystallinity. It was clearly observed that the accumulation of defects occurred at grain boundaries which were perpendicular to the loading direction and consisted of grains with large difference of Schmid factor. Bicrystal specimens with different crystallinity were cut from the tested samples and the strength of the bicrystal specimens were measured by using the micro tensile test system. It was confirmed that the strength of a grain boundary decreased monotonically by about 50% with the decrease of IQ value, in other words, the increase in the total density of various defects such as vacancies and dislocations. On the other hand, the effective yielding stress of grains increased monotonically with the decrease of the IQ value. This is because the increase in the total density of these defects suppresses the movement of dislocations, in other words, plastic deformation. Therefore, there were three independent strengths, the strength of two grains and that of a grain boundary which consisted of the bicrystal specimen. Since the strength of grains increased, at the same time, that of a grain boundary decreased monotonically with the decrease of the IQ value, it was confirmed that there was critical IQ value at which the fracture mode of a bicrystal specimen changed from conventional transgranular cracking to intergranular cracking under the application of uniaxial tensile load.

Dynamic Embrittlement - Diffusion-Induced Intergranular Cracking

2006 ◽  
Vol 258-260 ◽  
pp. 192-198
Author(s):  
Ulrich Krupp
Keyword(s):  
Grain Boundary ◽  
Elevated Temperatures ◽  
Low Cycle Fatigue ◽  
Hold Time ◽  
Fatigue Loading ◽  
Damage Mechanism ◽  
Boundary Structure ◽  
Grain Boundary Engineering ◽  
Intergranular Cracking ◽  
Dynamic Embrittlement

The present paper is about dynamic embrittlement as a generic damage mechanism. It involves grain-boundary diffusion of an embrittling species at elevated temperatures under the influence of mechanical stress. The embrittling species, either coming from the material itself or from the environment, reduces the grain-boundary cohesion and, hence, causes time-dependent intergranular fracture. Evidence of the technical significance of dynamic embrittlement is given by two examples, stress-relief cracking in steels and hold-time cracking during low-cycle-fatigue loading of nickel-base superalloys. There is an obvious relationship between the grain-boundary structure and the local susceptibility to dynamic embrittlement. This was proven by mechanical experiments on bicrystals and grain-boundary-engineering-type-processed specimens.

Local plastic deformation in the vicinity of grain boundaries in Fe–3 mass% Si alloy bicrystals and tricrystal

2014 ◽  
Vol 49 (14) ◽  
pp. 4698-4704 ◽  
Author(s):  
Sadahiro Tsurekawa ◽  
Yuta Chihara ◽  
Kyohei Tashima ◽  
Seiichiro Ii ◽  
Pavel Lejček
Keyword(s):  
Plastic Deformation ◽  
Grain Boundaries ◽  
Local Plastic ◽  
Local Plastic Deformation

scholarly journals Simulation of thermal cycle aging process on fiber-reinforced polymers by extended finite element method

2017 ◽  
Vol 52 (14) ◽  
pp. 1947-1958 ◽  
Author(s):  
Sergio González ◽  
Gianluca Laera ◽  
Sotiris Koussios ◽  
Jaime Domínguez ◽  
Fernando A Lasagni
Keyword(s):  
Finite Element ◽  
Degradation Process ◽  
Damage Mechanism ◽  
Fiber Reinforced Polymers ◽  
Model Parameters ◽  
Suitable Model ◽  
Fiber Reinforced ◽  
Aging Effects ◽  
Extended Finite Element ◽  
Element Analysis

The simulation of long life behavior and environmental aging effects on composite materials are subjects of investigation for future aerospace applications (i.e. supersonic commercial aircrafts). Temperature variation in addition to matrix oxidation involves material degradation and loss of mechanical properties. Crack initiation and growth is the main damage mechanism. In this paper, an extended finite element analysis is proposed to simulate damage on carbon fiber reinforced polymer as a consequence of thermal fatigue between −50℃ and 150℃ under atmospheres with different oxygen content. The interphase effect on the degradation process is analyzed at a microscale level. Finally, results are correlated with the experimental data in terms of material stiffness and, hence, the most suitable model parameters are selected.

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