Theoretical-Experimental Model for Predicting Crack Growth Rate in Structural Alloys of Aircraft Gas Turbine Engine Disks with Regard to Fatigue and Creep

Hold times of 300 and 3600 s under trapezoidal cyclic loading are shown to result in an increase in crack growth rates by many factors of ten for EP962 alloy and several times for EP742 alloy at a temperature 973 K. It has been found that in case the creep crack growth diagram has a first segment where the crack growth rate decreases, the crack growth kinetics under trapezoidal cyclic loading can be predicted by means of the hypothesis of linear summation of fatigue and creep crack growth rates considering special features of the first segment of the creep crack growth diagram. The authors put forward empirical approaches to determining the mean rate on the first segment of the creep crack growth diagram. __________ Translated from Problemy Prochnosti, No. 6, pp. 15 – 25, November – December, 2005.     

Crack propagation in concrete subjected to flexuralcyclic loading

Fatigue tests were carried out on notched microconcrete beams subjected to three-point bending tests in a saturated atmosphere. The feature of the crack growth was studied by means of the replica technique associated with scanning electron microscopy, in conjunction with the measurement of the crack mouth opening displacement (CMOD). Two domains must be distinguished in which the fatigue can be considered, depending on the upper boundary of the cyclic load. Beyond a given threshold for the upper boundary of the cyclic load, the first cycle induces an actual crack at the tip of the artificial notch. Observations of fatigue crack propagation are presented in that case and compared with previous investigations in static tests. The influence of the load level on the crack growth rate is discussed.

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Résumé Des essais de fatigue ont été réalisés sur des éprouvettes encochées de microbéton, en flexion trois points sous atmosphère saturée. La topographie de la croissance de fissure a été observée au microsope électronique à balayage par la technique de la réplique, couplée avec la mesure de l'ouverture de la fissure. Deux domaines de comportement à la fatigue, dépendant du niveau maximal de charge doivent être distingués dans de tels essais de fatigue. Au delà d'un seuil de charge, le premier cycle provoque déjà l'ouverture d'une fissure réelle à la pointe de l'encoche artificielle. Les observations de la propagation de fissure par fatigue sont menées dans ce cas. L'influence du niveau de charge sur la vitesse de propagation est aussi étudiée.

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Editorial Note Prof. Alain Bascoul is a RILEM Senior Member and a member of Technical Committee 148-SSC, “Test Methods for the Strain Softening Response of Concrete”.     

On a reliable equation for the second portion of the fatigue crack growth diagram

The fatigue crack growth kinetics on the second portion of the fatigue crack growth diagram is described using an equation that takes into account both the current crack tip opening displacement and the instant when the crack closure in a cycle vanishes and does not appear until the specimen failure. This instant corresponds to the critical effective SIF range, ΔK_fcl. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 83–87, May–June, 1999.     

Stochastic model for the size distribution of dispersed cracks. Part 2. Unsteady-state crack growth

The stochastic model for describing the statistical length distribution of dispersed cracks over a given time interval was developed. The parameters of crack length and rate distributions at the beginning of the time internal and the intensity of crack initiation per unit surface area were used as the initial data for the numerical representation of this stochastic model. The model was used to obtain time-predicted crack size distributions and the distributions of remnant life to the initiation of the crack of a maximum admissible length for the initial exponential flaw length and growth rate distributions. Experimental investigations confirmed that small surface crack growth rate distributions under low-cycle loading of and éI698VD nickel-based alloy corresponded to the exponential one. Kiev International University of Civil Aviation, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 59–67, July–August, 1999.     

A fatigue-to-creep correlation in air for application to environmental stress cracking of polyethylene

The present study was undertaken to determine whether the correlation between fatigue and creep established for polyethylene in air could be extended to environmental liquids. Fatigue and creep tests under various conditions of stress, R-ratio (defined as the ratio of minimum to maximum load in the fatigue loading cycle), and frequency were performed in air and in Igepal solutions. The load–displacement curves indicated that stepwise fatigue crack growth in air was preserved in Igepal solutions at 50 °C, the temperature specified for the ASTM standard. In air, systematically decreasing the dynamic component of fatigue loading by increasing the R-ratio to R = 1 (creep) steadily increased the lifetime. In contrast, the lifetime in Igepal was affected to a much smaller extent. The fatigue to creep correlation in air was previously established primarily for tests at 21 °C. Before testing the correlation in Igepal, it was necessary to establish the correlation in air at 50 °C. Microscopic methods were used to verify stepwise crack growth by the sequential formation and breakdown of a craze zone, and to confirm the fatigue to creep correlation. The crack growth rate under various loading conditions was related to the maximum stress and R-ratio by a power law relationship. Alternatively, a strain rate approach, which considered a creep contribution and a fatigue acceleration factor that depended only on strain rate, reliably correlated fatigue and creep in air at 50 °C under most loading conditions of stress, R-ratio and frequency. The exceptions were fatigue loading under conditions of R = 0.1 and frequency less than 1 Hz. It was speculated that compression and bending of highly extended craze fibrils were responsible for unexpectedly high crack speeds.     

The variation of beta phase morphology after creep and negative creep for duplex titanium alloys

The creep resistance of SP700 (Ti–4.5Al–3V–2Mo–2Fe) is superior to Ti–6–4 (Ti–6Al–4V) at 500 °C under a constant load corresponding to an initial stress of 100 MPa. The β phase grains in the SP700 alloy prefer to orient along the loading axis in contrast to the Ti–6–4 alloy. The grain growth occurs during the stress drop incubation period. The observation of different amounts of negative creep/anelasticity upon loading is closely associated with the difference in the amount of grain/subgrain coarsening.     

Study of the applicability of C1 for correlating crack growth rates at elevated temperature in alloy 800H

Crack growth in Alloy 800H in a temperature range of 650 to 900°C for various loading conditions was examined in this study. Creep and hold time crack growth rates were shown to be correlated satisfactorily by C¹ even for the hold period as short as 1 minute at 650°C. The fatigue loading portion of the cycle in hold time tests apparently has little effect on crack growth mechanisms at 650°C. This conclusion can be verified by 1.) load-line deflection versus crack length plots, 2.) microhardness profiles ahead of the crack tip, and 3.) examination of the fracture surfaces. At higher temperatures, C¹ still provides adequate correlation of creep crack growth rates in this alloy.     

The equation of the second portion of the fatigue fracture kinetic diagram for various additional load ratios

The second portion of the fatigue fracture kinetic diagram is described by a reliable equation with additional load ratios R in the case of the crack-tip closure effect and by the Forman equation for the values of load ratio R^* in the case without the crack closure effect. We have obtained the analytical dependence of the effective range of the stress intensity factor ΔK_eff on the nominal range ΔK_R for the first part of the second portion of the fatigue fracture kinetic diagram (ΔK_1-2,R≤ΔK_R≤ΔK_fcl,R) with various load ratios. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 75–80, March–April, 2000.     

High-temperature salt and gas corrosion of refractory alloys based on nickel

We present the results of investigations of chloride corrosion and oxidation of refractory éP539 and éP99 nickel-base alloys. We established that the heat-resistance of these alloys is rather high up to a temperature of 1273 K, whereas, at a temperature of 973 K, the chloride corrosion proceeds intensively and has an intercrystalline character. Under the same kinetic conditions, the rate of chloride corrosion is greater than the rate of oxidation by two to three orders of magnitude. éP539 alloy is more resistant against chloride corrosion than éP99 alloy by a factor of 1.1–1.3, but the heat-resistance of the former is less than that of the latter by the same factor. Frantsevich Institute for Problems of Materials Technology, Ukrainian National Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 87–92. January–February, 2000.     

Modeling for fracture in materials under long-term static creep loading and neutron irradiation. Part 3. Crack growth rate prediction for austenitic materials

We propose an engineering method which permits predicting the creep crack growth rate under neutron irradiation conditions. Theoretical analysis of the creep crack-tip stress-strain state is carried out. Calculations are performed to determine the effect of neutron flux intensity (flux) and pre-irradiation dose (fluence) on the crack growth rate. __________ Translated from Problemy Prochnosti, No. 6, pp. 5–16, November–December, 2006.     

Grain size effects in a Ni-based turbine disc alloy in the time and cycle dependent crack growth regimes

The fatigue crack growth (FCG) behaviour in a Ni-based turbine disc alloy with two grain sized variants, in a low solvus high refractory (LSHR) superalloy has been investigated under a range of temperatures (650–725 °C) and environments (air and vacuum) with trapezoidal waveforms of 1:1:1:1 and 1:20:1:1 durations at an R = 0.1. The results indicate that a coarse grained structure possesses better FCG resistance due to the enhanced slip reversibility promoted by planar slip as well as the reduction in grain boundary area. The fatigue performance of the LSHR superalloy is significantly degraded by the synergistic oxidation effect brought about by high temperature, oxidising environment and dwell at the peak load, associated with increasingly intergranular fracture features and secondary grain boundary cracking. Secondary cracks are observed to be blocked or deflected around primary γ′, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation. The apparent activation energy technique provides a further insight into the underlying mechanism of the FCG under oxidation–creep–fatigue testing conditions, and confirms that oxidation fatigue is the dominant process contributing to the intergranular failure process. At high enough crack growth rates, at lower temperatures, cycle dependent crack growth processes can outstrip crack-tip oxidation processes.     

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.     

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

Abstract

The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m^1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.     

Stress Intensity Controlled Kinetic Crack Growth and Stress History Dependent Life Prediction with Statistical Variability

Consistent with viscoelastic behavior, a power law form in terms of the stress intensity factor is used to specify crack kinetics (growth rate) in the central crack problem under Mode I conditions. The crack growth rate is integrated to obtain the crack size and thereby the stress intensity factor as a function of time. The crack is allowed to grow in a controlled, load dependent manner until it reaches the size at which it becomes unstable. The corresponding time at which this occurs is taken as the lifetime of the material under the specified load history. The special cases of constant load (creep rupture) and constant strain rate to failure are found to have a very simple relationship with each other. This lifetime relationship is verified through the comparison with corresponding data upon a polymeric composite. Finally the creep rupture case is generalized to a probabilistic formalism. The theoretically predicted lifetime distribution functions are verified with data, also upon a polymeric composite. Possible extension of the entire formalism to cyclic fatigue in metals is discussed. Dedicated to Professor Z.P. Bažant for his many contributions.     

Acoustic emissions of fatigue crack growth

Acoustic emissions of fatigue crack growth have been monitored and quantitatively correlated with growth rate and the applied range of stress intensity for high cycle fatigue of 2024-T851 aluminum alloy. The data suggest a more cogent relationship for acoustic emissions and the applied range of stress intensity rather than between acoustic emissions and the average crack growth rate. Since nearly all crack growth is expected during the maximum load portion of the fatigue cycles, only the emissions from the acoustic events in the vicinity of the peak load were incorporated in correlations with $\text{d}\text{a/}\text{d}\text{n}$ and $\text{ΔK}$. Large amplitude emissions in the proximity of the minimum cyclic load were also detected. Because of their characteristics, these emissions are attributed to crack surface interference and, consequently, were not included in the correlation analyses.     

Numerical investigation of creep crack growth in cross‐weld CT specimens. Part II: influence of specimen size

A numerical investigation of the influence of specimen size on creep crack growth in cross‐weld CT specimens with material properties of 2.25Cr1Mo at 550 °C is performed. A three‐dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa √m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility‐based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross‐weld CT specimens.     

Hold-time effects on high temperature fatigue crack growth in Udimet 700

Crack growth behaviour under creep-fatigue conditions in Udimet 700 has been studied, and the crack growth data were analysed in terms of the stress intensity factor as well as theJ-integral parameter. Crack growth behaviour is shown to depend on the initial stress intensity level and the duration of hold-time at the peak load. For stress intensities that are lower than the threshold stress intensity for creep crack growth, the crack growth rate decreases with increase in hold time even on a cycle basis, da/dN, to the extent that complete crack arrest could occur at prolonged hold times. This beneficial creep-fatigue interaction is attributed to the stress relaxation due to creep. For stress intensities greater than the threshold stress intensity for creep crack growth, the growth rate on a cycle basis increases with increase in hold time. For the conditions where there is no crack arrest, the crack growth appears to be essentially cycle-dependent in the low stress intensity range and time-dependent in the high stress intensity range. Both the stress intensity factor and theJ-integral are shown to be valid only in a limited range of loads and hold-times where crack growth rate increases continuously.     

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep of a Fe₃Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873–1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms of the threshold stress concept. A rapid fall of the stress exponent and of the threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results of long-term creep tests in tension performed recently on the same alloy. __________ Translated from Problemy Prochnosti, No. 1, pp. 117–120, January–February, 2008.