Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650 °C. The main load cycle for such components is typically defined by the start-up and shut-down of the engine. It generally includes hold times at high temperatures, which have been found to have a potential for greatly increasing the fatigue crack growth rate with respect to the number of load cycles. However, these effects may be totally or partly cancelled by other load features, such as overloads or blocks of continuous cyclic loading, and the actual crack propagation rate will therefore depend on the totality of features encompassed by the load cycle. It has previously been shown that the increased crack growth rate found in hold time experiments can be associated with a damage evolution, where the latter is not only responsible for the rapid intergranular crack propagation during the actual hold times, but also for the increased crack growth during the load reversals. In this paper, modelling of the hold time fatigue crack growth behaviour of Inconel 718 has been carried out, using the concept of a damaged zone as the basis for the treatment. With this conceptually simple and partly novel approach, it is shown that good agreement with experimental results can be found.     

Effects of temperature and hold time on dynamic strain aging in a nickel based superalloy

Abstract

Serrations known as Portevin Le-Chatelier effect have been observed in a coarse grained nickel based superalloy RR1000 in the temperature range from 300 to 750°C. These serrations may be due to dynamic strain aging (DSA), which affects the stress relaxation behaviour of the material. Further experiments have been carried out over a wide range of temperature and under selected loading conditions to interrogate the DSA phenomenon. The results show that, in addition to serrations, the alloy also exhibits unusual strain rate insensitivity and increased strain hardening in the DSA temperature regime. Possible mechanisms of DSA have been explored using transmission electron microscopy (TEM) to examine typical samples tested under selected and well defined loading conditions.     

Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T = 100–750 °C and optional dwells of 20 min at 750 °C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {1 1 1}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.     

Fatigue and creep crack growth behaviour of Inconel MA 6000

Crack growth in MA 6000 under cyclic loading was studied at 24, 760, and 1000°C and under static loading at 1000°C in two matenal onentatwns. Correlatwns of fattgue crack growth rate with parameters ?K and ?J were examined. Also comparisons were made of experimental and predicted growth rates.

The rate of growth was influenced by temperature and onentatwn m addttwn to the loading mode. Fatigue crack growth rate generally increased with temperature. However in the L-T orientation at 1000°C secondary cracks developed perpendtcular to the primary crack and significantly altered its behaviour. Creep crack growth at 1000°C was strongly orientation dependent, mainly due to secondary crackmg m the L-T oriented specimen in the direction perpendicular to the main crack.

Fracture surfaces were examined by scanning electron microscopy. Also, comparisons were made between crack growth behaviour of MA 6000, MA 754 and MA 956.     

Long fatigue crack growth in Inconel 718 produced by selective laser melting

Growth of long fatigue cracks is investigated in Inconel 718 superalloy produced by selective laser melting (SLM). The fatigue crack growth curve and the threshold value of the stress intensity factor are experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters.The crack propagation curve and the crack propagation threshold of this SLM material are compared with literature data describing the behavior of conventionally manufactured Inconel 718. The fatigue crack growth is discussed in terms of the specific microstructure and residual stresses produced by selective laser melting.     

Oxidation assisted fatigue crack growth under complex non-isothermal loading conditions in a nickel base superalloy

This paper deals with the prediction of fatigue crack growth at high temperatures in the N18 nickel base superalloy, which is employed by Snecma for turbine disc applications. This material and other nickel base superalloys were widely studied in the past under isothermal conditions and constant amplitude fatigue. Dwell time effects are observed which are attributed, in this material, to grain boundary oxidation. The main objective of this research is to use this knowledge to model the fatigue crack growth rate in the N18 nickel base superalloy when complex “missions” are encountered. This implies variable amplitude and non-isothermal loading conditions (450–650 °C). For this purpose, an incremental fatigue crack growth model which was originally developed for isothermal variable amplitude loading conditions was extended so as to be applicable to non-isothermal loading conditions. In addition, the incremental form of the fatigue crack growth law in this model is very useful to account for the coupling effect between fatigue and time-dependent phenomena such as creep or oxidation. In the present case, the effect of the environment was modelled as a competition between two phenomena: a detrimental effect of grain boundary oxidation ahead of the crack tip and a beneficial effect of the growth of a passivation layer of oxides on the freshly created crack surfaces. The model was used to simulate fatigue crack growth under complex cycles at high temperature and the comparisons with experimental results are satisfactory.     

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

Prediction of oxidation-assisted crack growth has been carried out for a nickel-based superalloy at elevated temperature based on finite element analyses of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. The material constitutive behaviour, implemented in the finite element code ABAQUS via a user-defined material subroutine (UMAT), was described by a unified viscoplastic model with non-linear kinematic and isotropic hardening rules. Diffusion of oxygen was assumed to be controlled by two parameters, the oxygen diffusivity and deformation-assisted oxygen mobility. Low frequencies and superimposed hold periods at peak loads significantly enhanced oxygen concentration near the crack tip. Evaluations of near-tip deformation and oxygen concentration were performed, which led to the construction of a failure envelop for crack growth based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure envelop was then utilised to predict crack growth rates in a compact tension (CT) specimen under fatigue-oxidation conditions for selected loading ranges, frequencies and dwell periods. The predictions from the fatigue-oxidation failure envelop compared well with the experimental results for triangular and dwell loading waveforms, with marked improvements achieved over those predicted from the viscoplastic model alone. The fatigue-oxidation predictions also agree well with the experimental results for slow-fast loading waveforms, but not for fast-slow waveforms where the effect of oxidation is much reduced.     

Load and temperature interaction modeling of fatigue crack growth in a Ni-base superalloy

An approach was developed to predict the thermo-mechanical fatigue crack growth rates under typical gas turbine engine spectrum loading conditions. The material studied in the development of this model was a polycrystalline superalloy, Inconel 100. Load interaction effects were determined to have a major effect on the crack growth life. A yield zone load interaction life prediction model was modified to include temperature dependent properties. Multiple overload effects were included in the model to incorporate enhanced retardation compared to single overload retardation behavior. Temperature interaction effects were included and proved to be very important because of the wide temperature ranges to which turbine engine components are subjected. The effects of oxidation and temperature changes were accounted for in the model by accelerating crack growth in regions that had been previously affected by elevated temperatures. Experimental data of isolated, first order effects were used to calibrate and verify the model. Temperature dependent mechanical properties were determined and were essential in the model’s development. Parametric studies were performed using this model to assess the sensitivity of specific crack growth variables on life predictions.     

Random load sequences and stochastic crack growth based on measured load data

This paper addresses the modeling of random fatigue load sequences based on real measured loads, which represents a key step in stochastic damage tolerance. Discrete-time Markov chains and hidden Markov chains are investigated to model sequences of max–min stresses. The parameters of these models are inferred from in-flight measured loads of a given fleet of fighter aircrafts. The accuracy of the two models is assessed based on the statistical properties of a parameter representative of the crack growth under variable amplitude loads, which accounts for load interactions and retardation/acceleration effects. This parameter obtained with the PREFFAS crack closure model represents the maximum stress of a constant amplitude load sequence supposedly equivalent in terms of crack extension to the variable amplitude sequence. The statistical properties of this parameter corresponding to variable amplitude load sequences generated with these models are compared to those of sequences randomly sampled from the measured loads. The Hidden Markov Model appears to be the most appropriate and accurate model. It is then used in a stochastic analysis of a crack initiating at a hole of a skin-stringer panel in a fighter aircraft. This illustrative example explores the effects of the scatter in both loads and material properties (here through the crack growth rate) on the stochastic crack growth.     

Application of crack tip plasticity based fatigue model on high temperature alloy HAYNES® 282®

Confined crack tip plasticity model is employed to predict time independent fatigue crack growth rate (FCGR) behavior of HAYNES® 282® alloy at temperatures 1200F and 1400F. Crack growth tests were done in lab air, vacuum and steam environments at load ratios R = K_min/K_max ranging from 0.05 to 0.5. Calibrated model predicts average cyclic crack growth rate behavior of the material reasonably well. Predictions do not capture the accelerated fatigue crack growth rates observed in the data at low load levels. Such effects are believed to be caused by environmentally driven factors, which are not expected to be predicted by plasticity based models.     

Influence of turning parameters on the high‐temperature fatigue performance of Inconel 718 superalloy

The safety‐critical rotating parts of aircraft engines are mainly designed using experimental material data, based on standard specimens and procedures, while few data are available on the effect of manufacturing anomalies on fatigue life. In this context, the paper investigates the effects of different machining parameters on the high‐temperature fatigue resistance of Inconel 718 superalloy specimens, cut from engine disk forgings, machined by turning on a vertical lathe. An unconventional specimen was designed in order to have the machining marks aligned with the fatigue loading axis, so to reproduce the hoop stresses in engine disks. For the test campaign, three machining parameters were chosen (depth of cut, cutting speed and insert wear) that typically may generate non‐geometrical anomalies. A correlation has been found between the machining parameters, the residual stresses, the surface roughness, and the distorted and amorphous layer thicknesses. Correlations of such data with fatigue life are also presented and discussed.     

Simulation of crack growth under low cycle fatigue at high temperature in a single crystal superalloy

Crack growth tests have been performed at 950 °C with Single Edge Notch specimens of the Ni-based single crystal superalloy PWA1483. In particular, several orientations and frequencies have been investigated, thus allowing the assessment of the influence of these parameters on the crack growth rate. In addition, oxidation experiments have been carried out to characterize the kinetics of the outer oxide scale growth at the same temperature.On the other side, crack growth has been simulated with the Finite Element program ABAQUS in real test conditions by the node release technique. The nodes are released according to the measured crack growth rate.The simulation results are compared with the test results on the basis of the computed Crack Tip Opening Displacement (CTOD). For this purpose, the crack is propagated until a stabilized value of the CTOD is obtained. This is usually the case when the crack has crossed the initial plastic zone. The procedure provides an evaluation of the effects of cycle frequency, crystal orientation, plasticity and oxide induced crack closure.     

Mode I fatigue crack growth reduction mechanisms after a single Mode II load cycle

A single Mode II load cycle, large enough to create residual displacements, decreases the subsequent Mode I crack growth rate. The distance for Mode I crack growth rate to fully recover, i.e., revert to the same da/dN as before Mode II load, is much longer than Mode II plastic zone size. The higher Mode II load, the larger is the reduction in growth rate and the longer the recovery distance. Higher Mode I R-ratio means smaller reduction in growth rate. Above a certain R-ratio, no reduction occurs at all. In the present study it is found that the reduction in growth rate is solely caused by crack closure due to tangential displacement of crack-surface irregularities that induce a surface mismatch between the upper and lower crack faces. The mechanism is called Mode II-induced crack closure. A model based on both analytical and experimental results is developed in order to estimate the degree of Mode II-induced crack closure after a Mode II load.     

Fatigue life time prediction of PoAF Epsilon TB-30 aircraft – Implementation of automatic crack growth based on 3D finite element method

The objective of this work is to predict the fatigue lifetime (TVF) of the Portuguese Air Force (PoAF) Epsilon aircraft based on the computational fatigue crack growth modelling.The spectra of loads were used in experimental tests of two specimen series (designed to simulate in the laboratory the critical area of the aircraft) to assess experimentally the difference between the PoAF and manufacture spectra.In order to predict the TVF by a generic spectrum was computationally implemented a methodology for automatic crack propagation. Through the development of a interface between ANSYS and MATLAB was possible to determine the stress intensity factors and hence the geometric factor for the specimen geometry which was designed by PoAF in previous works. The stress intensity factors were validated with the methods available in the literature: Pickard, Pommier and Newman.The spectra of charges and the geometric factor allowed the computer implementation of the following propagation laws: Paris, NASGRO, Walker, Forman and Wheeler.Finally, it was established for the PoAF operation the new inspections plans according to the manufacturer methodology by making an extrapolation of real scale test results obtained with the manufacturer spectrum.At the end of the article the authors concluded that the TVF of Epsilon aircraft is 24,500 flight hours (FH), the first inspection should be done when the aircraft reaches 10,000 FH and the flowing inspections should be done with a periodicity of 3000 FH until the crack reaches a critical dimension of 1.5 mm.     

Influence from geometrical features on the growth rate of a micro-structurally short fatigue crack

The influence on the crack growth rate on a micro-structurally short edge crack subjected to fatigue loading from changes in crack length, distance to grain boundaries and applied load has been investigated. The crack is assumed to grow in a single shear mechanism due to nucleation, glide and annihilation of dislocations along preferred slip planes in the material. The external geometry is modelled by distributed dislocation dipole elements in a boundary element approach under quasi-static and plane strain conditions. The evolving plasticity is described by individual discrete dislocations along a slip plane emanating from the crack in the crack direction. The crack growth rate is shown to be controlled by the plasticity, which in turn is controlled by geometrical parameters in combination with the external load.     

Temperature effects on fatigue thresholds and structure sensitive crack growth in a nickel-base superalloy

Fatigue thresholds and slow crack growth rates have been measured in a powder formed nickel-base superalloy from room temperature to 600°C. Two grain sizes were investigated: 5–12 μm and 50 μm. It is shown that the threshold increases with grain size, and the difference is most pronounced at room temperature. Although crack growth rates increase with temperature in both microstructures, the threshold is only temperature dependent in the material with the larger grain size. It is also only in the latter that the room temperature threshold falls when the load ratio is increased from 0.1 to 0.5. At 600°C the higher load ratio causes a 20% reduction in the threshold irrespective of grain size.The results are discussed in terms of surface roughness and oxide-induced crack closure, the former being critically related to the type of crystallographic crack growth, which is in turn shown to be both temperature and stress intensity dependent.     

Effect of load ratio and hydrogen concentration on the crack growth rate in Zr–2.5Nb tubes

Crack growth rates (CGR's) were determined under sustained and cyclic loads using 17 mm compact tension and cantilever beam specimens taken from Zr–2.5Nb tubes charged to 6–100 ppm H. The cyclic load effect on the CGR was investigated at 250 °C where load ratios, R were varied from 0.13 to 1 with a constant K_max. Under sustained loads, the CGR of the Zr–2.5Nb tube increased with supersaturation of hydrogen, ΔC and leveled off above 20–35 ppm H of the ΔC. Under cyclic loads with 1 cycle/min, the CGR at 250 °C decreased with decreasing R: 3.2 × 10⁻⁸ m/s at R = 1 and 4.8 × 10⁻⁹ m/s at R = 0.13. The striation spacing, corresponding to the critical hydride length, decreased with decreasing R, indicating easier cracking of the hydrides under cyclic loads. The decreased CGR under cyclic loads and its dependence on the ΔC are discussed using Kim's delayed hydride cracking model.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

SEM study of overload effects during fatigue crack growth using an image analysing technique and potential drop measures

To study the mechanisms affecting the crack propagation rate for fatigue cracks exposed to an overload, an in situ scanning electron microscope technique was used, together with potential drop measurements. High‐resolution images were analysed with an image analysis program to measure the displacements along the crack, and the potential drop technique was employed to measure the electrical contact between the fatigue crack surfaces. The crack closure level could, by image analysis, be determined as close as 1 μm from the crack tip. The indications from the image analysis pointed towards a somewhat lower closure load as compared to the potential drop technique. The effect of an overload on the crack propagation rate was found to depend on the magnitude of the overload in combination with the steady‐state conditions. Both overload induced crack retardation and crack acceleration was noticed to occur.