LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC-ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep-fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.     

INFLUENCE OF CREEP-FATIGUE INTERACTION ON HIGH TEMPERATURE FATIGUE CRACK GROWTH BEHAVIOR of Ti-1100

Abstract— A series of crack growth experiments has been preformed on the near alpha titanium alloy, Ti-1100, to determine the mechanism of the creep-fatigue interaction. Based on pure creep crack growth results, the increase in the creep-fatigue crack growth rate is not amenable to separate contributions of creep crack growth and fatigue crack growth.
A mechanism has been proposed to account for the increase in creep-fatigue crack growth rate that is based on the planar slip of titanium alloys which results in the formation of dislocation pileups at the prior beta grain boundaries and leads to intergranular fracture. This mechanism has been validated through crack growth experiments preformed on a Ti-1100 that has been microstructurally modified through the precipitation of internal slip barriers. These show that the intergranular fracture and increase in crack growth rate are absent.     

GH33A合金在蠕变-疲劳交互作用下的裂纹扩展特性

通过对 GH33A 合金在蠕变与疲劳复合加载条件下的系列试验,发现拉伸保时使蠕变与疲劳发生了交互作用,加快了疲劳裂纹扩展速率,加速裂纹早期进入失稳扩展,大大降低了疲劳寿命。GH33A 合金具有良好的抗蠕变裂纹扩展能力,但疲劳裂纹扩展阻力较低。由此讨论了拉伸保时对裂纹扩展的影响,并对在蠕变-疲劳交互作用下的裂纹扩展模型作了探讨。     

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 ? 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 ? -C* correlation is discussed.     

Thermomechanical fatigue—damage mechanisms and mechanism-based life prediction methods

An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJ_eff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δ_CF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.     

A cohesive zone model for fatigue and creep–fatigue crack growth in single crystal superalloys

A numerical analysis using cohesive zone model under cyclic loading is proposed to develop a coupled predictive approach of crack growth in single crystal. The process of material damage during fatigue crack growth is described using an irreversible cohesive zone model, which governs the separation of the crack flanks and eventually leads to the formation of free surfaces. The cohesive zone element is modeled to accumulate fatigue damage during loadings and no damage during unloadings. This paper presents the damage model and its application in the study of the crack growth for precracked specimens. The use of cohesive zone approach is validated through a convergence study. Then, a general procedure of parameters calibration is presented in pure fatigue crack growth. In the last section, an extension of the cohesive zone model is presented in the case of creep–fatigue regime at high temperature. The model showed its capability to predict with a good agreement the crack growth in the case of complex loading and complex specimen geometries.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

FATIGUE, CREEP AND CREEP/FATIGUE BEHAVIOUR OF A NICKEL BASE SUPERALLOY AT 700°C

Abstract— This paper presents the results of an experimental testing programme to examine the uniaxial creep, low cycle fatigue and creep/fatigue interaction behaviour of a Ni-base superalloy at 700°C. The material is used in the manufacture of aeroengine turbine discs. A creep continuum damage mechanics model is shown to be capable of accurately predicting the creep and creep rupture behaviour of the material. A healing term has been incorporated into the damage mechanics model to allow the behaviour under creep/fatigue conditions to be described.     

P92钢蠕变-疲劳交互作用下的裂纹扩展行为EI北大核心CSCD

在630℃下,对P92钢进行应力控制下的蠕变-疲劳交互作用实验,研究P92钢高温蠕变-疲劳交互作用下的裂纹扩展行为,并结合断口形貌分析蠕变-疲劳裂纹扩展的机理以及a-N曲线的转折点含义。结果表明:P92钢在蠕变-疲劳交互作用下的断裂属于蠕变韧性断裂,应该用(C_t)_(avg)作为裂纹扩展的断裂参量;P92钢在蠕变-疲劳交互条件下,试样的断口主要表现为蠕变孔洞以及微裂纹。此外,发现a-lg(N_i/N_f)曲线以及(da-dN)-N曲线中的拐点,分别对应蠕变-疲劳裂纹萌生区向扩展区转变周次以及扩展区向瞬断区转变的周次。     

Studies on crack growth rate under high temperature creep,fatigue and creep-fatigue interaction—I: On the experimental studies on crack growth rate as affected by aσg,σg and temperature

Many experimental studies have been reported on the measurements of crack growth rate and the observation of crack growth behaviour under high temperature creep, fatigue and creep-fatigue interaction in literatures. However, many of them have been done in air atmosphere. Furthermore, in many of them the measurements of the crack growth rate have been carried out by interrupting intermittently the running of the testing machine. In such experiments the complex effects due to the atmosphere, the interruption period and the corresponding unloading operation for the crack length measurement might have been involved.In the present paper in order to eliminate such effects, series of experimental studies on the crack growth behaviour under creep, fatigue and creep-fatigue interaction conditions on 304 stainless steel have been carried out by using high temperature microscope and observing the crack length continuously during running the test without interruption in vacuum of 10^?5mm Hg.Among the results, it was found that crack growth rates on a time basis, $\text{da/dt}$, under high temperature creep and creep-fatigue interaction conditions can not be described in terms of solely elastic stress intensity factor $\text{k}{}_{\mathrm{i}}$ or only net section stress σ_net, both independent of gross section stress σ_g. The relation between crack growth rate and stress intensity factor under high temperature fatigue condition changes with some trend according to gross section stress at lower $\text{K}{}_{\mathrm{I}}$ level and it can be approximately described in terms of stress intensity factor $\text{K}{}_{\mathrm{I}}$ only, at higher $\text{K}{}_{\mathrm{I}}$ level. The threshold stress intensity factor and the threshold net section stress under high temperature creep, fatigue and creep-fatigue interaction conditions appears to be almost independent of temperature.     

CREEP, FATIGUE AND CREEP-FATIGUE CRACK GROWTH RATES IN PARENT AND SIMULATED HAZ TYPE 321 STAINLESS STEEL

Abstract— Crack growth rate data are presented from a range of fully reversed displacement-controlled fatigue and creep-Fatigue tests and from static load-controlled creep crack growth tests on aged 321 stainless steel (parent and simulated HAZ) at 650 ° C. In the creep fatigue tests, constant displacement tensile hold periods of 12–192 h were used. Crack growth rates comprised both cyclic and dwell period contributions. Cyclic growth contributions are described by a Paris-type law and give faster crack growth rates than those associated with pure fatigue tests. Dwell period contributions are described by the C* parameter. The total cyclic crack growth rates are given by summing the cyclic and dwell period contributions. Estimates of C* using a reference stress approach together with the appropriate stress relaxation creep data are shown to correlate well with experimentally measured C* values. Crack growth rates during static load-controlled tests correlate well with C* . Good agreement is obtained between crack growth rates during the static tests and those produced during the hold period of the creep-fatigue tests.     

PREDICTION AND EVALUATION OF LONG-TERM CREEP-FATIGUE LIFE

Ordinary short-term creep fatigue tests were carried out on a servo-hydraulic testing machine for various heat resistant steels. From these results a life prediction method in the long-term creep-fatigue regime is proposed using creep rupture ductility data up to 10⁵ h. To verify the predicted life the authors have developed a new type of testing machine driven by thermo-actuator which can evaluate the long-term creep-fatigue behaviour of materials beyond 10⁴ h.     

High-temperature fatigue: Example of creep lifetime prediction for grade 2 alloy 800 at 550°C

Summary Experimental data on the material characteristics of structures subjected to thermal and mechanical cycling are needed for designing structural parts for creep and creep-fatigue interaction. Moreover, high-temperature low-cycle fatigue data are not sufficient to predict the fatigue creep lifetime.In order to check the reliability of steam generators, tests on pipe materials are conducted under cyclic thermal loading. The tests have been performed on an iron-nickel chromium alloy (alloy 800) in grade I condition. Isothermal low-cycle fatigue tests have been conducted at 350°C. For creep-fatigue tests, the strain was held constant during a variable relaxation period. To be representative of the service conditions, the duration of the hold time was equal to 168 hours with a total strain amplitude of 1.5%. Two damage processes are examined: fatigue and creep. The data are combined in three different models given by Cailletaud, Chaboche and Bul-Quoc. These models are used to predict fatigue-creep lifetime for very long hold periods. They were verified with the hold times of 1 week. It appears that the quality of the fatigue-creep lifetime predictions depends strongly on the quality of the creep relaxation phenomenon model. Comparison between prediction and experimental data Is quite satisfactory.Translated from Published in Problemy Prochnosti, No. 4, pp. 36–42, April, 1994.     

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.     

Fatigue behaviour and lifing of two single crystal superalloys

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.     

Creep fatigue crack growth in 0·5Cr–Mo–V steel

Abstract

An investigation is presented of crack growth in a normalised and tempered 0·5Cr–Mo–V steel under cyclic displacement controlled loading conditions at 565–600°C. A transition from fatigue to creep dominated behaviour was observed as the duration of the tensile dwell period in the cycle was increased. This change was a result of a progressive increase in the extent of crack tip grain boundary damage accumulation which, in the long dwell tests, was sufficient to give rise to crack extension directly. Time dependent crack propagation rates during the dwells of the long dwell tests were found to approach those determined for static load conditions. No evidence was found for a significant creep-fatigue interaction and it appears that overall crack growth rates are determined by crack tip oxidation and damage accumulation processes.

MST/756     

CRACK GROWTH RATES DURING CREEP, FATIGUE AND CREEP-FATIGUE IN AN AUSTENITIC FEATURE WELD SPECIMEN

Abstract— Static creep crack growth tests and displacement controlled fatigue and creep-fatigue crack growth tests have been performed on austenitic feature weld specimens at 650°C. The creep-fatigue tests incorporated hold times of up to 96 h. During these tests, crack growth appeared to comprise cyclic and dwell components. Cyclic crack growth components were characterised by the fracture mechanics parameter K whilst creep crack growth contributions were correlated with C *. In order to determine K and C * for the non-standard feature weld specimen, elastic and elastic-plastic creep finite element analyses were conducted. Good correspondence is shown between the feature weld data and comparable data from compact tension specimen tests on similar materials. Equations obtained from the compact tension specimen results, which describe total crack growth rates as the sum of the cyclic and dwell contributions, are shown to adequately describe the features test results also. Furthermore, it is demonstrated that a reference stress approach can be used to estimate C * for the features specimens.     

Damage propagation mechanism in low-cycle creep fatigue of Cu–Cr–Zr alloy

This paper describes a characteristic damage propagation mechanism in low-cycle creep–fatigue of Cu–0.7Cr–0.09Zr (mass%), as investigated by creep–fatigue tests including strain controlled fatigue and stress-holding type creep, and following microstructural observations by scanning electron microscopy (SEM). The total stress-holding time until rupture in the creep–fatigue test was shorter than one-tenth of the rupture life in the simple creep test, and the rupture life of the specimen in the creep–fatigue test was shorter than half of that in the simple fatigue test. The SEM images suggest that the connection between fatigue crack propagating along grain boundaries and intergranular creep voids rapidly accelerates crack propagation.     

Crack initiation life at notch root under the transition of creep condition

Acceleration of creep fracture is posible in a high-strength material such as a superalloy under the transition from the small scale creep (SSC) condition to the large scale creep (LSC) condition. In this study, an analytical method of predicting the creep crack initiation life for a notched body was presented. In order to assess the validity of this method, the crack initiation at a notch root was also experimentally observed on a high-strength Ni-base superalloy through load-controlled creep-fatigue tests at 923 K. As a result, this method was found to be sufficiently applicable to the crack initiation life prediction for a notched body under the transition from SSC to LSC.