Dwell Crack Growth Research Articles

Modelling and predictions of time-dependent local stress distributions around cracks under dwell loading in a nickel-based superalloy at high temperatures

Finite element (FE) analysis modelling of crack configurations was used to provide insights into the role of time-dependent deformation on dwell fatigue crack growth (DFCG) in turbine disc alloys. In particular, the potential influence of time-dependent deformation on fatigue crack growth rates observed during dwell holding periods and the potential crack growth retardation phenomena that can occur for overload-dwell cycles were investigated. The FE model evaluated the mechanical stress state evolution in a two-dimensional cracked geometry subjected to standard dwell and overload-dwell stress waveforms for a given microstructural condition (based on its γ’ particle distribution). Crack-opening stress distributions as a function of dwell time and distance ahead of the crack-tip were interrogated, with the aim to investigate the potential influence of local crack-tip stresses on crack growth during dwell holding periods and the potential for crack growth retardation following overload cycles. Stress distribution profiles predicted by dwell-only simulations showed how quickly local crack-tip stresses relax due to time-dependent plasticity. Characteristic effects of overloads of dwell crack growth resistance observed in experiments were also reasonably well captured. A numerically-derived incubation time was successfully applied in predicting the general trends of achieving more pronounced retardation effects with larger overload factors and extended periods of overloading prior to periods of dwell. FE modelling predicted a significant difference in behaviour between an overload for a given overload factor with a hold time at peak load of 1 s and 10 s. This study emphasises the importance of studying local crack-tip stresses in regards to characterising DFCG behaviour of turbine disc alloys.

A numerical study on the influence of grain boundary oxides on dwell fatigue crack growth of a nickel-based superalloy

• A computational multi-physical modeling approach to oxide-assisted intergranular dwell crack growth of γ′ strengthened nickel-based superalloys is presented. The modeling framework explicitly couples mass transport of chemical species involved in oxide formation to the evolution of mechanical fields ahead of a crack tip. The viscoplastic response accounts for the influence of the γ′ dispersion. • Detailed Quantitative study of the interaction between the dynamic stress-assisted oxide formation ahead of the crack tip and the resulting near crack-tip stress fields was implemented. • A method for extracting crack growth rates based on failure of the oxide wedge ahead of a crack tip is presented • The predicted rates are shown to be in good agreement with available experimental data without any direct information or calibration of model parameters to crack growth rate data in advance. • Established model shows capability of reveal the effect of particle dispersion of nickel-based superalloy on the dwell fatigue crack growth rate. A theoretical treatment on the oxide-controlled dwell fatigue crack growth of a γ’ strengthened nickel-based superalloys is presented. In particular, this study investigates the influence of an externally applied load and variations in the γ’ dispersion on the grain boundary oxide growth kinetics. A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature. The viscoplastic model explicitly accounts for multimodal γ’ particle size distributions. A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip, where stress-assisted vacancy diffusion is assumed to operate. The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme. Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement. Simulations indicate that the presence of a fine γ’ size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge. It is shown that a unimodal dispersion leads to reduced oxide growth rates (parabolic behavior) when compared to a bimodal one. Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip, which become progressively negative as the oxide wedge develops. However, mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge, where failure of the oxide is predicted. The time taken to reach this critical stress for oxide failure has been calculated, from which dwell crack growth rates are computationally derived. The predicted rates are shown to be in good agreement with available experimental data.

A novel methodology for modeling dwell fatigue crack growth in Ni-based superalloys

A novel, empirically-based damage tolerance life prediction methodology has been developed to model dwell fatigue crack growth (DFCG) behavior in Ni-based disk superalloys. The methodology is based on a new crack driving force parameter, termed Ksrf, which was developed specifically to account for dwell crack growth. The approach taken in the development of this parameter was to account for the apparent contradiction between the brittle nature of the oxide induced intergranular crack growth process and the extensive creep deformation occurring simultaneously in the crack tip region’s visco-plastic zone. It was hypothesized that the rate of the brittle intergranular crack growth is controlled by the magnitude of the crack tip tensile stress field which varies depending on the constitutive visco-plastic behavior of the various microstructures evaluated. The new parameter accounts for these differences by measuring and normalizing the remaining tensile stresses generated by simulations of the crack tip visco-plastic behavior through simple stress relaxation tests. The Ksrf parameter was able to account for up to an order of magnitude of differences in DFCG rates in the LSHR and RR1000 alloys when correlated using standard linear elastic fracture mechanics (LEFM) K stress intensity parameter. The same Ksrf methodology was able to explain and account for the substantial effects that small overloads have on DFCG behavior.

Influence of Tertiary Gamma Prime (\u03b3\u2032) Size Evolution on Dwell Fatigue Crack Growth Behavior in CG RR1000

This work investigates coarsening behavior of strengthening precipitates (γ′) in coarse-grained RR1000 after isothermal exposure for various times (50 to 500 hours) at 700 °C and 750 °C. The impact of these isothermal treatments is then studied by carrying out dwell (1 hour) fatigue crack growth tests at 700 °C in air. Such dwell periods can increase crack growth rates by nearly two orders of magnitude compared with baseline (0.25 Hz trapezoidal waveform) fatigue crack growth tests. Predominantly, scanning electron microscopy was used for γ′ analysis. Transmission electron microscopy was also utilized when necessary. Overaging as a thermodynamically driven and diffusion-controlled process strongly affects tertiary γ′ precipitates for the temperatures and treatment times investigated here. No influence of overaging on baseline fatigue behavior is observed. However, after overaging at 750 °C for 500 hours (which increases the mean tertiary γ′ size by a factor of two from 17 to 37 nm), dwell crack growth rates at 700 °C are reduced by one order of magnitude. Increased dwell fatigue crack growth resistance is also measured after overaging for 100 hours at 700 °C. The potential influence of γ′ distributions on dwell fatigue crack growth resistance is discussed in terms of stress relaxation behavior during dwell periods.

Influence of continuum precipitates on intergranular fatigue crack growth of a P/M nickel-based superalloy

This paper examines the role of microstructure, in particular, the size and volume fraction of secondary (γ′s) and tertiary (γ′t) γ′ precipitates, on intergranular crack growth in P/M IN100. A series of different heat treatments were carried out on the as-received material to vary the γ′statistics through control of the different features of the solutioning, stabilization, and aging parts of the heat treatment cycles. Dwell fatigue crack growth experiments have been performed on specimens having as received as well as heat treated microstructures at 650°C and 700°C in air with a 0.5Hz loading frequency with and without a dwell time. Results of the as received material show that the dwell crack growth rate, while dependent on the temperature level, is independent of the hold time period. For the modified microstructures tested at 650°C, the dwell crack growth rate is shown to be sensitive to γ′ variations in the surrounding continuum. This sensitivity was illustrated by correlating the crack growth rate and the corresponding continuum yield which shows the dwell crack growth rate decreases with the increase in the yield strength. The influence of the continuum yield on dwell crack growth was first examined numerically by modelling an intergranular crack path surrounded by a continuum region represented by a coarse crystal plasticity model while the far field continuum is represented by an internal state variable model. The grain boundary path considers grain boundary sliding and a critical sliding length which is implemented as a fracture criterion. Results of this model show that the grain boundary sliding is influenced by the viscous strain developing in the surrounding continuum. An increase in the continuum yield strength accompanied with a lower viscous strain results in higher constraint on the grain boundary sliding and thus a decrease in the crack growth rate. This observation was further examined by calculating the individual contributions of the solid solution, γ′s, and γ′t to the total yield strength of the continuum.

Modern Machining of Advanced Aerospace Alloys - Enabler for Quality and Performance

Advanced Aerospace alloys are designed to withstand high loads at extreme temperature conditions in their specific applications. These advanced alloys are more demanding in terms of machinability as their structure and material properties are focused on improved resistance to dwell crack growth, environmental damage (hot corrosion and oxidation) and creep strain, as well as higher levels of microstructure stability and high temperature yield stress. Machining of these alloys therefore focuses on high stock removal rates in roughing processes as well as in high surface finishing rates and good surface finish and surface integrity in finish machining. High performance machining methods will enable the aero engine manufacturing industry to meet increasing future requirements supported by an improved understanding of processes and process control factors. In addition to the technical challenges a number of other elements have to be considered to meet future business requirements, which include robustness of machining solutions and control of boundary conditions to be met in order to ensure acceptable surface and subsurface conditions in machining. Modern machine tool concepts, advanced machining processes and methods are key enablers to achieve overall quality and productivity goals to meet future market requirements.

Fatigue resistance of the grain size transition zone in a dual microstructure superalloy disk

Mechanical property requirements vary with location in nickel-based superalloy disks. In order to maximize the associated mechanical properties, heat treatment methods have been developed for producing tailored microstructures. In this study, a specialized heat treatment method was applied to produce varying grain microstructures from the bore to the rim portions of a powder metallurgy processed nickel-based superalloy disk. The bore of the contoured disk consisted of fine grains to maximize strength and fatigue resistance at lower temperatures. The rim microstructure of the disk consisted of coarse grains for maximum resistance to creep and dwell crack growth at high temperatures up to 704 °C. However, the fatigue resistance of the grain size transition zone was unclear, and needed to be evaluated. This zone was located as a band in the disk web between the bore and rim. Specimens were extracted parallel and transverse to the transition zone, and multiple fatigue tests were performed at 427 and 704 °C. Mean fatigue lives were lower at 427 °C than 704 °C. Specimen failures often initiated at relatively large grains, which failed on crystallographic facets. Grain size distributions were characterized in the specimens, and related to the grains initiating failures as well as location within the transition zone. Fatigue life decreased with increasing maximum grain size. Correspondingly, mean fatigue resistance of the transition zone was slightly higher than that of the rim, but lower than that of the bore. The scatter in limited tests of replicates was comparable for all transition zone locations examined.

An investigation of the effects of microstructure on dwell fatigue crack growth in Ti-6242

This paper presents the results of a combined experimental and mechanistic modeling approach to the study of dwell fatigue in Ti-6242. Crack shape evolution, depth and surface crack growth rates are established using beachmarking, acoustic emission and scanning electron microscopy (SEM) techniques. The underlying crack nucleation and fatigue fracture modes are elucidated for three microstructures: equiaxed (microstructure 1), elongated (microstructure 2) and colony (microstructure 3) of Ti-6242. The dominant crack nucleation mode is shown to involve a Stroh-type dislocation mechanism, where sub-surface cracks are characterized by prominent facetted fracture modes in the near-threshold regime. Subsequent fatigue crack growth occurs by fatigue striations and ductile dimples or cleavage-like static modes at higher stress intensity factor ranges. The long crack growth data are similar for both dwell and pure fatigue. However, the dwell fatigue crack growth rates are shown to be much greater than those due to pure fatigue in the short crack growth regime. The differences between the dwell crack growth rates and the pure fatigue crack growth rates in the short regime are attributed to possible creep effects that give rise to a mean stress effect in the case of dwell fatigue. Subsequently, the measured crack growth rates are incorporated into a fracture mechanics framework for the estimation of fatigue life in the three microstructures. The implications of the predictions are discussed for the modeling of dwell fatigue.

Microstructural effects on fatigue and dwell-fatigue crack growth in α/β Ti-6Al-2Sn-4Zr-2Mo-0.1Si

This article presents the results of an experimental and theoretical study of the effects of microstructure on room-temperature fatigue and dwell-fatigue crack growth in Ti-6242. The crack growth rates and micromechanisms of long crack growth are elucidated for equiaxed, elongated, and colony microstructures. The article shows that dwell and pure fatigue crack growth rates are generally similar in the long crack growth regime. The underlying mechanisms of long crack growth are also generally similar under pure fatigue and dwell-crack growth conditions. However, differences in dwell and fatigue sensitivities are observed between the different microstructure in the mid-and high-ΔK regimes. These are explained by considering the possible interactions between fatigue and creep during dwell fatigue loading at room temperature. Linearized fracture mechanics crack growth laws are presented for the modeling of crack growth in the near-threshold, Paris, and high-ΔK regimes.

An investigation on fatigue and dwell-fatigue crack growth in Ti–6Al–2Sn–4Zr–2Mo–0.1Si

This paper presents the results of a combined experimental and analytical study of fatigue crack growth and dwell-fatigue crack growth in forged Ti–6Al–2Sn–4Zr–2Mo–0.2%Si (Ti-6242). Following an initial characterization of microstructures and basic mechanical properties, the micromechanisms of long fatigue crack growth are presented for three microstructures. These include: a duplex α/ β structure, an elongated α/ β structure, and a colony α/ β microstructure. The colony microstructure is shown to have the best resistance to fatigue crack growth. The elongated α structure has intermediate resistance, while the equiaxed α structure exhibits the fastest fatigue crack growth rates. The fatigue crack growth rates in the near-threshold, Paris and high Δ K regimes are then characterized with empirical crack growth laws that relate the crack growth rates to the stress intensity factor range and key parameters on the fatigue crack growth curve. Finally, the results of dwell-fatigue crack growth experiments are presented for the three microstructures. The dwell-fatigue crack growth rates are shown to be almost identical to the fatigue crack growth rates in the intermediate Δ K regime. However, the fatigue crack growth rates are faster at higher stress intensity factor ranges. The underlying mechanisms of dwell crack growth are compared with the mechanisms of fatigue crack growth before discussing the implications of the work for the prediction of dwell or fatigue crack growth in Ti-6242. The effects of cyclic frequency on fatigue crack growth are also explored.