Growth Of Creep Cavities Research Articles

Stress states and growth behavior of bridged cracks at creep regime

In this study growth behavior of bridged cracks, resulting from the growth of pre-nucleated creep cavities with diffusional and dislocation-assisted mechanisms, is investigated numerically. The elements bridging the crack are assumed to be elastic; the bridging behavior ranges from full development of the bridging zones to failure of the bridging elements during the course of crack growth. The results indicate that the bridging traction significantly relaxes even with the overall creep deformation alone. The rate of this relaxation is not influenced by the rate of crack growth. However, the rate of change in the bridging zone length or the density of the bridging elements in the bridging zone strongly affects both the maximum value and the distribution of the traction in the bridging zone. A much weaker stress singularity than the ones described by K or C * was found ahead of the bridged cracks in the creep regime. In this weak singularity region the cavities, located at increasing distance from the crack tip, grow at similar high rates to each other.

Evaluation of component creep behaviour by means of a threshold approach

Creep deformation as well as damage evolution in nut bolt assemblies have been investigated. A damage model was developed, based on a combination of Kachanov theory and critical creep strain threshold. Exceeding the creep strain threshold cause initiation and growth of creep cavities. The model together with a modified Norton‐Bailey creep law was implemented in the user subroutine Creep of the FE code Abaqus. Verification of the computation was done by means of comparison with creep tests of nut bolt assemblies.

Crystallographic features and growth of creep cavities in a Ni–18Cr–18Fe alloy

The crystallographic nature and growth of creep cavities in a Ni–18Cr–18Fe alloy were examined as part of the study of behavior of nickel base superalloyys at high temperatures. Creep experiments were carried out at 973 K and 52 MPa, and grain boundary cavitation damage was examined in polished sections, as well as by scanning electron microfractography of surfaces exposed after hydrogen charging. Well developed cavities, whose development tended to favor one side of the grain boundary, were observed and identified with vacancy condensation on {111} planes. The preference is attributed to cavity nucleation in the grain in which the carbide-matrix interface is incoherent and vacancy condensation is energetically favored. A model for cavity growth patterned after crystal growth is suggested and discussed.

Grain Boundary Self-Diffusion of Alloy 800 as Affected by Sulphur, Phosphorous and Carbon

Using a radioactive tracer method the bulk and grain boundary diffusion of 59 Fe was determined in industrial Alloy 800 and melts of Alloy 800 with additional P and S in the temperature range 800 to 1000 °C. The use of the approximation of Suzuoka was confirmed by autoradiographs. In Alloy 800H the activation energy of grain boundary diffusion of 59 Fe is (209 ± 17)kJ/mol. Dissolved elements especially P increase the activation energy of the grain boundary diffusion of Fe by their segregation to the grain boundaries. In addition the influence of the grain boundary diffusion on the growth of creep cavities was investigated in the same materials, and the chemical composition of the creep cavities and grain boundaries were analysed by Auger electron spectroscopy (AES). For Alloy 800 + 0.088 wt-%P an enrichment of about 14 at-%P was observed at the grain boundaries. The addition of P clearly enhances the creep strength of Alloy 800; this can probably be explained by precipitation of Cr 23 C 6 on the grain boundaries during the creep process.

Creep cavity growth under interaction between lattice diffusion and grain-boundary diffusion

A model and a method of numerical analysis of the cavity growth in grain boundaries in metals due to lattice diffusion are proposed. The growth behavior simulated is compared to that due to grain-boundary (GB) diffusion. The simulation method is extended to analyze the growth under the interaction between lattice diffusion and GB diffusion. The growth rate calculated under the interaction is approximately equal to the linear sum of those due to the pure lattice diffusion and the pure GB diffusion.

Study On Mechanical Factor Of Grain BoundaryCavity Nucleation Under Creep-fatigue Condition

Creep-fatigue damage at high-temperature on an austenitic stainless steel is characterized by nucleation and growth of creep cavities on grain boundaries. We had carried out continuous observation of the cavity growth process under the creep-fatigue condition by using a high-temperature fatigue machine combined with a scanning electron microscope(SEM) and grasped the cavity growth mechanisms. In this study, in order to clarify a mechanical factor on cavity nucleation under the creep-fatigue condition, creep-fatigue tests under unsymmetrical triangle waveform were conducted on Type304 stainless steel and a cross section of the failure specimens were examined by the SEM. As a result, a volumetric density of damaged grain boundary induced by cavities, which was expressed as a function of time-dependent strain rate, was proposed based on the SEM observation results. The volumetric density of damaged grain boundary under trapezoidal waveform with tensile strain hold could be predicted by the proposal equation with good accuracy.

Characteristics of interface cracks in creep regime

In this study, the growth behavior of interface cracks in bimaterials and in graded/layered materials at creep regime was studied. In the first part of the study, the evolution of the time-dependent stress states ahead of the stationary interface cracks was elucidated using a finite element model. In the second part, the observed stress states were incorporated into a creep crack growth model resulting from the growth of creep cavities ahead of the interface cracks. The growth model includes the effects of material deposition resulting from the growth of creep cavities on the crack tip stress fields. The results indicate that in graded/layered materials under identical applied loading, both the location of the interface crack and the extent of the transitional layer strongly influence the amplitude of the stress field ( C*) at steady state. Also, the time required to achieve the steady state varies considerably with these parameters. The growth rates of the interface cracks are much faster than the growth rate of a homogeneous crack in the fastest creeping sector of that interface. Due to large variation in the distribution of the stresses ahead of the interface cracks at creep regime, depending upon the location, the creep crack growth rates will be significantly different from each other under identical loading for a given graded/layered material.

Distribution Properties of Grain Boundary Cavity Nucleation under Creep-Fatigue Condition.

Creep-fatigue damage at high-temperature on SUS 304 austenitic stainless steel is characterized by nucleation and growth of creep cavities on grain boundaries. We had carried out the continuous observation of the cavity growth under creep-fatigue condition and clarified its mechanisms. In this study, in order to clarify distribution properties of cavity nucleation under the creep-fatigue condition, creep-fatigue tests with CP waveform were conducted on SUS 304 stainless steel and the failure specimens were examined by scanning electron microscope. As a result, it was found that the cavitated grain boundaries exsisted almost normal to a stress axis regardless of tensile strain-rate and the volumetric density of damaged grain boundary increased monotonously with the inelastic strain range. The volumetric density of damaged grain boundary, whchi is expressed as a function of time-dependent strain, was proposed based on test results of CP waveform condition and the volumetric density under strain-hold waveform condition could be predicted with good accuracy.

Theoretical Consideration of Cavity Growth Process under Creep-Fatigue Conditions.

The major factor dominating creep and creep-fatigue failure at high temperature for SUS304 stainless steel is the nucleation and growth of creep cavities on grain boundaries. In this study, previously proposed cavity growth models for creep conditions were expanded to give models for creep-fatigue condition. The analytical cavity growth rate was compared with experimental data obtained in our prevlcus study. As a result, it was found that the cavity growth process under stress-controlled creep-fatigue conditions continuously observed by the SEM could be explained using the cavity growth rate equation consisting of the sum of constrained diffusion-controlled and deformation-controlled growth rates. Also, a cavity growth mechanism map, which can be applied to various load patterns, was developed based on the models for cavity growth in creep conditions.

高温強度 改良９Ｃｒ‐１Ｍｏ鋼のクリープ疲労寿命に及ぼす長時間ひずみ保持の影響

Modified 9Cr-1Mo steel is the principal candidate material of a steam generator in a fast breeder reactor because of its superior high-temperature property. In this study, the influence of long-term strain hold on the failure life of Mod. 9Cr-1Mo steel was investigated and the applicability of existing creep-fatigue life evaluation methods was disccussed based on the experimental results. Creep-fatigue tests with hold time up to 10 hours per cycle at either tension or compression maximum strain were conducted under strain control of 0.5% and 1.0% at 550°C. Although failure life reduction occurred by introducing short hold period in the compression hold tests, the degree of reduction became smaller with increasing hold period. On the other hand, the failure life reduction became larger with increasing hold period in the tension hold tests. It was found that the failure life reduction in the compression hold tests was mainly due to the accumulation of tensile strain at the center of a specimen and that in the tension hold tests it was caused by intergranular damage in terms of creep cavity nucleation and growth from observation of the failure specimens. The time fraction rule adopted in a design code gave longer life compared to the experimental life, especially at low strain level. The failure life was well predicted by the ductility exhaustion method and the nonlinear damage accumulation model.

Environmentally-enhanced cavity growth in nickel and nickel-based alloys

Environmental factors have a strong effect on the elevated-temperature failure behavior of nickel-based alloys. It has been proposed that this effect is due to the reactions of oxygen with carbon in the interior of creep cavities. Such reactions can lead to quite high internal gas pressures, sufficient to result in substantial increases in the cavity growth rates. This hypothesis is investigated by carrying out detailed calculations for a simple system which take into account the coupled effects of oxygen diffusion into the cavity and concurrent cavity growth. The results show that creep cavity growth may or may not be affected by internal, gas-producing reactions, depending upon the nature of the carbon-containing particle, the ratio of the grain boundary oxygen diffusivity to the self-diffusivity of nickel, and upon other factors as well.

Continuous Observation of Cavity Growth and Coalescence by Creep-Fatigue Tests in SEM

Structural components operating at high temperatures in power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to understand microscopic damage evolution under creep-fatigue conditions. In this study, static creep and creep-fatigue tests with tensile hold-time were conducted on a SUS 304 stainless steel by using a high-temperature fatigue testing machine combined with a scanning electron microscope (SEM), and cavity growth and coalescence behaviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observations was made for comparison with theoretical growth models. As a result, it was found that cavities nucleate at random and grow preferentially on grain boundaries in a direction almost normal to the stress axis. Under creep condition, the cavities grew monotonously on grain boundaries while remaining an elliptical shape. On the other hand, under creep-fatigue conditions, the cavities grew due to the effect of the local strain distribution around the grain boundary due to cyclic loading and microcracks of one grain-boundary length were formed by coalescence of the cavities. Also, cavity nucleation and growth rates under the creep-fatigue condition were more rapid than those under the static creep condition and the constrained cavity growth model coincided well with the experimental data for creep.

Grain Size Effect on Creep Deformation of Alumina‐Silicon Carbide Composites

A study of the flexural creep response of aluminas reinforced with 10 vol% SiC whiskers was conducted at 1200° and 1300°C at stresses from 50 to 230 MPa in air to evaluate the effect of matrix grain size. The average matrix grain size was varied from 1.2 to 8.0 μm by controlling the hot‐pressing conditions. At 1200°C, the creep resistance of alumina composites increases with an increase in matrix grain size, and the creep rate (at constant applied stress) exhibits a grain size exponent of approximately 1. The stress exponent of the creep rate at 1200°C is approximately 2, consistent with a grain boundary sliding mechanism. On the other hand, the creep deformation rate of 1300°C was not sensitive to the alumina grain size. This was seen to be a result of enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increases. Observations also indicated that the prevalent site for nucleation and growth of creep cavities in coarsegrained materials is at two‐grain junctions (grain faces), whereas in fine‐grained materials cavities nucleate primarily at triple‐grain junctions (grain edges). Electron microscopy studies revealed that the content of any amorphous phase present at whisker‐alumina interfaces is independent of alumina grain size (and hot‐pressing conditions). In addition, the alumina grain boundaries are quite devoid of amorphous phase(s). This variation in amorphous phase content does not appear to be a factor in the present creep results.

Analysis of Creep Crack Growth at High-Temperature Components by Diffusive Growth Model of Grain Boundary Cavities (I)-Effect of Grain Boundary Cavitation on Stress Field and Crack Growth Rate-

The crack growth under creep condition is one of the major damage mechanisms which determines remaining life of the component operating at high temperatures. In this paper, the creep crack growth by grain boundary cavitation is studied, which is frequently observed failure mechanism for creep brittle materials. As a result of diffusive growth of creep cavities, it is shown that the crack-tip stress field is modified from the original stress distribution by the amount of singularity attenuation parameter which is function of crack growth rate and material properties. Also, the stress relaxation at crack-tip results in the extension of cavitating area by the load dump effect to meet the macroscopic force equilibrium conditdion.

Creep cavity nucleation and growth in 12Cr–Mo–V steel

Quantitative measurements of the number of cavities per unit area, mean cavity size, and cavitated area fraction are carried out on creep tested specimens of a 12Cr–Mo–V martensitic steel. Both interrupted and ruptured specimens are included. Cavities initiate at an early stage of tests. The number of cavities, mean cavity size, and cavitated area fraction exhibit a continued increase with increasing strain and time. At rupture, the number of cavities is about 30 000 mm−2, and the mean cavity size has a maximum value of about 2·5 μm.The cavitated area fraction is approximately 0·1% at 1% strain, 0·5% at 2% strain, and 10% at rupture. Cavity initiation and growth are analysed regarding temperature, stress, strain, and strain rate. A constrained cavity growth model is discussed and compared with the experimental results.MST/3097

Continuous Observation of Cavity Growth and Coalescence by Creep-Fatigue Tests in SEM.

Structural components operating at high temperatures in power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to understand microscopic damage evolution under creep-fatigue conditions. In this study, static creep and creep-fatigue tests with tensile holdtime were conducted on SUS304 stainless steel by using a high-temperature fatigue machine combined with a scanning electron microscope (SEM), and cavity growth and coalescence behaviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observation was made for comparison with theoretical growth models. As a result, it was found that grain boundary cavities nucleate at random and grow preferentially on grain boundaries in a direction almost normal to the stress axis. Under the creep condition, the cavities grow monotonously on grain boundaries while they remain the elliptical shape. On the other hand, under the creep-fatigue condition the cavities grow with an effect of local strain distribution around the grain boundary due to cyclic loading and the micro cracks of one grain-boundary length were formed by coalescence of the cavities. Also, cavity nucleation and growth rates for creep-fatigue were more rapid than those for static creep and the constrained cavity growth model coincided well with the experimental data for creep.

The link between creep rupture and creep crack growth for a power-law creeping material: A monkman-grant analogue

Engineering materials under small loads at ambient temperatures creep and ultimately rapture by the nucleation growth and inter-linkage of creep cavities along grain boundaries. Cavities are generally observed on grain boundaries normal to the stress axis, and mechanisms of cavity nucleation and growth have direct consequences on creep rupture (CR) properties. Often, by studying the rupture time (t[sub j]), and its stress and temperature (T) dependencies, information on the cavitation mechanisms can be deduced. Within the similar temperature and stress ranges, a crack usually propagates by the nucleation and growth of creep cavities ahead of the crack tip, and the creep crack growth (CCG) rates are directly related to the mechanisms of cavity nucleation and growth under crack tip stresses. Therefore, attempts were made to correlate the crack velocity (v) with the load parameters, such as the stress intensity factor (K[sub I]) or C integral, which characterize the crack tip stress and strain fields in elastic and power-law creeping materials, respectively. If CR and CCG proceed ;by the same mechanisms of cavity nucleation and growth for a given material, a link between the two phenomena is expected because CCG is equivalent to the successive CR of crack tip ligaments. Inmore » the present paper, within the framework of quasi-equilibrium cavity growth and crack growth under extensive creep conditions, a link between the two phenomena is sought for a power-law creeping material, then its meaning and potential applications are discussed.« less

Models for coupled diffusive/strain controlled growth of creep cavities

The importance of intergranular creep cavitation to high-temperature failure processes in metals and ceramics has been well-recognized for some time now. In general, creep cavity growth at elevated temperature is thought to occur by one of two processes. The first of these is diffusive growth, whereby matter is transported from the cavity surface and is deposited on the grain boundary. The second is strain-controlled growth, in which the cavity grows entirely as a consequence of creep deformation of the surrounding material under the action of an applied stress. Several models of cavity growth have been proposed in which these processes are coupled to each other and occur simultaneously. These models have attained some currency and have been used in several studies involving the growth of creep cavitation. The purpose of the present note is to investigate in some detail one particular class of these models.

Creep Damage Detection for a Steam Turbine Rotor Material after Long-Term Service Using Ultrasonic Technique.

The present paper describes an experimental study on a physical meaning of nondestructive creep damage detection for a steam turbine rotor material after long-term service using an ultrasonic technique. As a result of microstructure observation and density measurements, it was made clear that change in ultrasonic parameters such as attenuation and noise value was found to be attributed to nucleation, growth and coalescence of creep cavities with increasing creep damage. The ultrasonic technique was found to be valid for predicting remanent creep life of a fossil fuel plant steam turbine rotor.

ＳＵＳ３０４長時間クリープ疲労寿命評価

The long term creep-fatigue life of type 304 stainless steel was evaluated by the creep-fatigue life prediction method based on a linear damage fraction rule. The displacement controlled creep-fatigue tests were carried out, and the time to failure of longer than 10000 hours was obtained. The creep damage of long term creep-fatigue was evaluated by taking into account the stress relaxation behavior with elastic follow-up during the hold period. The relationship between life reduction of creep-fatigue and fracture mode was provided by the creep cavity growth. The results of this study are summarized as follows;(1) The long term creep-fatigue data can be reasonably evaluated by the present method. The predicted lives were within a factor of 3 of the observed ones.(2) The present method provides the capability to predict the long term creep-fatigue life at lower temperatures as well as that at the creep dominant temperature.(3) The value of creep damage for the long term creep-fatigue data increased by elastic follow-up. The creep-fatigue damage diagram intercepted between 0.3 and 1 can represent the observed creep-fatigue damages.(4) The cavity growth depends on the hold time. The fracture of long term creep-fatigue is caused by the intergranular cavity growth. The intergranular fracture of creep-fatigue is initiated by the cavity growth and followed by the microcrack propagation along grain boundaries starting from creep cavities.