Creep-fatigue Loading Research Articles

高温強度 ２．２５Ｃｒ‐１Ｍｏ鋼溶接部のクリープ疲労条件下におけるタイプＩＶき裂発生と寿命評価

Under the circumstance that aging of boiler components proceeds and some “Type IV” cracking incidents were recently reported in boiler weldment parts, development of an accurate remaining life evaluation method for the weldment parts is an important subject for the utilities. In this study, in order to clarify a cause of the “Type IV” cracking under creep-fatigue loading condition and to develop a life evaluation method for boiler weldment parts, creep-fatigue tests on the heat affected zone (HAZ) simulated materials, base metal, weld metal and weld joint of 2.25Cr-1Mo steel, and elastic-plastic and creep analysis for the weld joint was conducted. As a result, initiation of many cavities and “Type IV” cracking were observed in a fine grain region of the weld joint specimen failured under the creep-fatigue loading. It was found from the comparison between experimental evidences and the analytical results that “Type IV” cracking was caused by two major reasons. One of them is accumulation of creep strain during strain hold in the fine grain region is larger than that in other region suggesting progress of creep damage in the fine grain region prior to other regions. The other one is existence of multiaxial tensile field within the fine grain region caused reduction of failure ductility. Crack initiation portion and failure life under the creep-fatigue test could be well predicted by the non-linear damage accumulation model based on finite element analysis using conventional elastic-plastic and creep theory.

高速炉構造用ＳＵＳ３１６のクリープおよび疲労強度特性に及ぼすナトリウム環境の影響

To clarify the effect of sodium environment on mechanical properties of FBR Grade Type 316SS (316FR), creep and fatigue tests were carried out both in sodium and in air at elevated temperatures. The creep strength in sodium was slightly lower than that in air at the stresses corresponding to the rupture time of 1000 to 10000 hours. At the lower stresses the strength was equivalent in both the environments. The fatigue strength in sodium was higher than that in air at total strain ranges above 0.5%, and was equivalent to that in air at the strain ranges below 0.5%. Metallurgical examinations showed that the strength reduction in sodium was caused by the precipitation of σ-phase on grain boundaries close to the surface of a specimen, and that in the case of creep tests at the lower stresses, rupture in sodium was caused by grain boundary cracks in the bulk of a specimen. In the main loading mode of FBR's, which is creep-fatigue loading at low stress levels with long hold periods, the mechanism of failure is considered to be similar to that of creep at low stress levels rather than creep at high stress levels or fatigue. Therefore, it was considered that the sodium environment has limited effects on the creep-fatigue strength of 316FR in FBR's. This was verified by the fact that internal grain boundary cracks were also found in the specimens that were subjected to creep-fatigue tests in sodium.

Deformation Behaviour Of Polycrystalline (IN78LC) And Single Crystal (SC16) Nickel Base Superalloys Under Creep-fatigue Loading

Deformation behaviour of polycrystalline (IN738LC) and single crystal (SCI6) superalloys has been investigated under creep-fatigue loading at 1223 K. Both the alloys show qualitatively a similar deformation behaviour: In the compressive part of the cycles a stable cyclic stress response was observed under all test conditions. The stress amplitude in compression increases with decreasing hold stress. The average creep rate during the hold periods as a function of number of load cycles shows a three stage behaviour: A rapid increase in the creep rate in the first stage. This increase in the creep rate is dependent on the levels of hold stresses. The second stage is characterised by a much slower increase in the creep rate with increasing number of cycles. At higher hold stress levels the creep rate remains approximately constant. In the last stage, the creep rate increases again rapidly. Based on the results of microscopic examinations this behaviour is discussed in the light of dislocation substructure, internal stresses, stability of precipitate morphology and formation of macro cracks.

A comparison of high temperature defect assessment methods

Cracked high temperature components which are subjected to creep or creep-fatigue loading may fail by crack growth, net section rupture or a combination of both processes. In this paper, models are presented for describing these modes of failure in terms of fracture mechanics concepts, limit analysis methods and cumulative damage laws. It is shown that these models form the basis of a number of high temperature defect assessment procedures that are available for plant. These procedures are then applied to a semi-elliptical defect in a plate which is subjected to creep-fatigue loading. It is found that the predictions are sensitive to the crack initiation criteria assumed and the limit analysis solutions adopted.

Characterisation and quantification of defects in rotors and casings of steam turbines

The loading situation of components in modern fossil-fired power plants is characterised by higher cyclic service in comparison to older plants. This leads in combination with larger units to increased stresses in the relevant components. To evaluate the risk of failure due to natural defects, life time approaches have been developed on the basis of advanced methods of fracture mechanics. The assessment of natural defects using fracture mechanics is, however, based on the following preconditions:• knowledge and data on the behaviour of such defects in terms of damage mechanism taking into account crack initiation and crack propagation under loading situations comparable to those in service,• data on the real size and geometry of defects,• data on the exact location of the defects.To create such a database an extensive joint research programme was started in the early eighties by German turbine manufacturers, steel makers and research institutes. The paper provides an overview of the results of these efforts. Special emphasis is placed on describing the improvements in ultrasonic testing techniques. The results of feature tests are reported. They have been performed to obtain information and data on the behaviour of natural defects under creep, creep fatigue and fatigue load. Subsequent metallographic investigations determine the real size and location of the defects but also the relevant damage mechanisms. Comparison of metallographic data and NDT data prove the reliability of ultrasonic testing. Thus an improved rating of the permissible ultrasonic indications in turbines and castings was obtained.

LIFE PREDICTION FOR THE TITANIUM ALLOY IMI 834 UNDER HIGH TEMPERATURE CREEP–FATIGUE LOADINGS

In the present investigation the life behaviour of the titanium alloy IMI 834 under loading conditions which are close to real service was considered. Different types of creep–fatigue loadings were applied at 600°C, i.e. balanced and unbalanced cycles with different ramping times into tension and into compression and stress hold Strain Range Partitioning (SRP) cycles. The testing was performed in laboratory air. The results of the creep–fatigue tests showed that life was reduced by the different types of creep–fatigue cycles. In the stress hold tests the mean stresses had a pronounced influence on life. The observed life of the stress hold SRP cycles could be predicted within a factor of ±2 of actual life by frequency‐modified approaches. If mean stresses were considered in terms of maximum tensile stress or by a modified Morrow approach, life predictions remained within a factor of ±1.5 of the actual test results. A SRP approach which was frequency modified in the present study led to life predictions which fell within a band of a factor of ±1.35 of the actual test results.

低炭素・窒素添加ＳＵＳ３１６鋼に対するクリープ疲労寿命評価法の検討

Low-carbon/medium nitrogen 316 stainless steel called 316FR is a principal candidate for the high-temperature structural materials of a demonstration fast reactor plant. Because creep-fatigue damage is a dominant failure mechanism of the high-temperature materials subjected to thermal cycles, it is important to establish a reliable creep-fatigue life prediction method for this steel. Long-term creep tests and strain-controlled creep-fatigue tests have been conducted at various conditions for two different heats of the steel. In the constant load creep tests, both materials showed similar creep rupture strength but different ductility. The material with lower ductility exhibited shorter life under creep-fatigue loading conditions and correlation of creep-fatigue life with rupture ductility, rather than rupture strength, was made clear. Two kinds of creep-fatigue life prediction methods, i.e. time fraction rule and ductility exhaustion method were applied to predict the creep-fatigue life. Accurate description of stress relaxation behavior was achieved by an addition of “viscous” strain to conventional creep strain and only the latter of which was assumed to contribute to creep damage in the application of ductility exhaustion method. The current version of the ductility exhaustion method was found to have very good accuracy in creep-fatigue life prediction, while the time fraction rule overpredicted creep-fatigue life as large as a factor of 30. To make a reliable estimation of the creep damage in actual components, use of ductility exhaustion method is strongly recommended.

Modelling tail effects in creep-crack initiation and growth for 316L stainless steel

For complex loading history (creep and fatigue) applied to engineering components, assessment procedures generally estimate the crack initiation and growth by using the summation of continuous fatigue and pure creep crack growth rates. This text deals with the pure creep correlation established in laboratory tests and applied to components subjected to creep-fatigue loading. The trend of the creep opening displacement history superimposed onto the crack progress is sufficient to predict what kind of tail effect will occur when plotting ȧ vs. C*. The exponent of this correlation is demonstrated to be very close to unity, whatever creep stage is concerned. The contribution of either the material behaviour or the crack extension to the ȧ -C* correlation is discussed.

HIGH TEMPERATURE SHORT FATIGUE CRACK BEHAVIOUR IN A STAINLESS STEEL

Abstract— Continuous low cycle fatigue (LCF) tests with‐and without‐hold time in push‐pull and torsion loading modes and sequential LCF tests in push‐pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and creep‐fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.Crack density was higher on the surface of a CF tested specimen than that of a PF tested specimen. Mainly short cracks oriented at 45° to the specimen axis were observed on a torsion fatigue tested specimen surface. For push‐pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45° to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis.Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent CF loading and this produced a total damage summation significantly larger than one.

A Probabilistic Model for Creep-Fatigue Failure

We outline here a method for incorporating the scatter observed in creep rupture times and fatigue cycles-to-failure into a probabilistic model for creep-fatigue failure. We do this within the context of the well-known damage fraction summation rule. Various numerical methods for calculating the probability of failure for given creep-fatigue loading cycles are discussed.

Study on metallographic damage parameter in creep-damage-dominant condition under creep-fatigue loading

The microstructural changes of type 304 stainless steel in the creep-dominant condition under creep-fatigue loading were investigated and the representativeness of some metallographical damage parameters (MDPs) was studied. A series of creep-fatigue tests was conducted at low total strain range and long strain hold time. Some of these were interrupted during the periods of life cycle. Cross-section of all tests specimens were observed with a scanning electron microscope. Microstructures such as cavities, precipitate nucleation sites and internal microcrack initiation along grain boundaries were investigated. Cavity segregation phenomena which occur in an angular relationship between cavitated grain boundaries and the loading direction were also studied. Then the mechanism of cavity initiation and growth up to microcrack initiation was analysed. The number of cavities per unit area (NC), mean cavity area (MCA) and area fraction of cavity per unit area (AFC) were measured and studied in respect of their suitability to evaluate the damage under the present condition. Finally a more suitable MDP which should be possible to relate with both cavitation and grain boundary deformation quantitatively is discussed.

Creep-fatigue damage assessment by subsequent fatigue straining

A series of creep-fatigue tests has been conducted with modified 9Cr-1Mo steel at 873 K in a high vacuum environment of 0.1 mPa. In order to investigate the accumulation of creep-fatigue damage, the creep-fatigue test programme includes changes in strain waveform during the test: from creep-fatigue type to fatigue type and from fatigue type to creep-fatigue type. The conventional linear cumulative damage rule for fatigue and/or creep-fatigue damage fails in evaluating the creep-fatigue life under the present complicated strain wave history. The linear summation of the life fraction is smaller than unity when the prior loading is creep-fatigue type and larger than unity when the prior loading is fatigue type. Scanning electron microscope (SEM) observation of the fracture surface was also conducted. In the case where the strain waveform changes from prior creep-fatigue type to subsequent fatigue type, the crack mode changes from transgranular to intergranular with an increase in the prior creep-fatigue loading history. In the case where the strain waveform changes from prior fatigue type to subsequent creep-fatigue type, the primary crack mode is generally intergranular regardless of the prior fatigue loading history.

Life Prediction Model Based on the Creep-Fatigue Damage Mechanism.

A damage model based on micro mechanics was developed in order to assess creep-fatigue interaction in Modified 9Cr-1 Mo steel. The model assumes the following damage processes for each loading. The fatigue life consists of two stages ; damage initiation life and its growth life. On the other hand, creep life is a process of damage initiation only. In the case of creep-fatigue loading, the damage is initiated by creep loading and grows with the fatigue loading. It was shown that the damage model could accurately predict creep-fatigue life of Mod. 9Cr-1 Mo steel under complicated loading histories.

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.

Creep and creep-fatigue cracking behaviour of two structural steels

The present study deals with crack initiation and crack growth, not only under creep and creep-fatigue conditions but also under more complex thermomechanical cyclic loadings, in both 316L and 1Cr-1Mo-0.25V steel. In these creep ductile materials, most studies have focused on the creep crack growth rates, d a /d t and load-geometry parameter C∗ correlations. In this paper, the creep crack initiation time is defined as the time T i necessary for a defect to grow by a small critical distance X c ( X c ≈ 50 μ m for example). This initiation stage may represent a large part of the rupture life of a cracked component. The importance of such studies is discussed in the first part. In the second part, an attempt is made to present a simplified method based on the fracture mechanics of creeping solids to define the relevant load-geometry parameters for crack initiation and crack growth under creep-fatigue loadings. In particular, it is shown that d a /d N - ΔK correlations apply only when the hold time t h is smaller than the transition time t tr between small-scale and large-scale viscoplasticity. Conversely, for long hold times, it is suggested that the T i -C∗ correlation be used to predict the fatigue.

Creep-Fatigue Fracture Mechanism of Modified 9Cr-1Mo Steel and its Evaluation by the Damage Model Based on the Overstress Concept.

A series of creep-fatigue tests was conducted with Modified 9Cr-1Mo steel at 600°C in 0. 1 mPa vacuum to assess the accumulation of creep-fatigue damage. In these tests, each test specimen was subjected to prior creep-fatigue loading followed by subsequent fatigue loading or prior fatigue loading followed by subsequent creep-fatigue loading. The linear summation of cumulative damage of fatigue and creep life fraction was smaller than unity for the former case, and larger than unity for the latter case. The damage model based on the overstress concept was applied to the test results to verify its applicability. The accuracy of the life evaluation was not enough because of the nonlinear interaction between creep and fatigue.

Temporal Instabilities (Dissipative Structures) In Cyclically Deformed Metallic Alloys

AbstractThe existing models for the “classical” Portevin-Le-Chatelier effect have been analyzed, and the non-linear dynamical model has been proposed in order to quantify the nature of temporal instabilities in fatigued metallic alloys. The model employs the concept of a positive feed-back among the populations of mobile, immobile and Cottrell-type dislocations with atmospheres of point defects. Three major types of loading have been numerically simulated: pure sinusoidal, creep fatigue (“the Lorenzo-Laird bursts”) and ramp loading (“the Neumann bursts”, when the amplitude of otherwise cyclic loading grows linearly with time). Computer movies of the temporal evolution of stress and dislocation densities have been prepared as an aide for analysis and illustration. The model successfully reproduces stress serrations in terms of the underlying dislocation mechanisms and thus for the first time establishes a fundamental link between the micro-and macromechanics of cyclic deformation.

Creep-fatigue failure of SAF 2205 and 316 stainless steels in Ar+3%SO2 environment at 700 °C

The creep-fatigue behaviour of SAF 2205 duplex stainless steel and 316 austenitic stainless steel was studied under a sulphur-containing environment of an Ar + 3%SO2 atmosphere at 700°C. The cyclic loading employed creep-tension and plastic compression following the creep-plasticity (cp)-mode of the strain-range partitioning life-prediction method. The results showed that a premature failure was obtained with both SAF 2205 and 316 stainless steels under the combination of creep-fatigue loading and an SO2-bearing atmosphere. However, SAF 2205 was significantly more resistant than 316 as far as the number of cycles to failure and the sulphidizing attack at the external surface were concerned. In both materials, the creep-fatigue crack propagation was directly controlled by the environmental attack of the SO2-containing environment at the region ahead of the crack tip.

Deformation substructure in the nickel-base alloy In738LC under superimposed creep-fatigue loading

The nickel-base superalloy IN738LC containing a unimodal distribution of [gamma]' precipitates was deformed under superimposed creep-fatigue loading at elevated temperatures. The alloy shows a stable cyclic deformation behaviour under LCF portion of the loading and at the same time a cyclic softening effect in the periods of creep deformation. Transmission electron microscopic examination reveals different mechanisms of dislocation- [gamma]' precipitate interaction under various loading conditions. At high hold stress levels and strain amplitudes stacking faults were frequently observed, indicating shearing of [gamma]' precipitates by dislocations. At low hold stress levels and strain amplitudes the observed absence of dislocations in [gamma]' precipitates suggests that they are bypassed by dislocations. Dislocation networks at [gamma]/[gamma]' interfaces were found in most of the specimens. A directional coalescence of the [gamma]' precipitates was observed under certain testing conditions. The development of both these features with deformation could be followed. The softening effect is accompanied by both processes. (orig.)

Fatigue and Creep-Fatigue Damage of Austenitic Stainless Steels under Multiaxial Loading

The objectives of the present study are to observe and model physical damage induced by cyclic multiaxial (tension-torsion) loading of 316L stainless steel both at room temperature and at elevated temperature (600 °C). Four types of experiments were carried out on thin tubular specimens: (a) continuous pure fatigue (PF) tests; (b) PF sequential tests with different sequences of push-pull and torsional loading; (c) creep-fatigue (CF) tests with superimposed hold time at maximum tensile strain; and (d) sequential tests involving sequences of PF and CF loadings. Optical microscopy and scanning electron microscopy (SEM) were used to study quantitatively the damage, in particular, to determine the orientation of cracks and to measure the kinetics of crack nucleation and crack growth. It is shown that in pure fatigue at 600 °C, the classical crack initiation stage I is bypassed due to a strong interaction between cyclic plasticity, oxidation, and cracking. Intense slip bands act as diffusional short circuits, leading to the formation of external (Fe2O3) and internal ((FeCr)3O4) oxide scales. The orientation of the microcracks during initiation and propagation stages, which is strongly affected by oxidation effects, explains qualitatively the significant deviations observed in the sequential tests from the Miner linear damage cumulative rule. It is also shown that creep-fatigue damage, which involves intergranular damage, is a complex process rather than a simple superposition of fatigue and creep damage. A stochastic model based on a Monte-Carlo simulation is developed. This model, which accounts very well for the situations in which crack initiation and crack propagation are coplanar, includes damage equations based on quantitative metallographical observations. Damage is modeled as the continuous nucleation of a population of growing cracks which eventually coalesce to lead to final fracture. It is shown that this simulation is able to reproduce with a good accuracy the fatigue lives measured under multiaxial continuous and sequential tests.