Titanium alloys are equipped with impressive high strength and low density, along with other notable mechanical properties. Often the choice for low to intermediate temperature mechanical applications, titanium alloys are well utilised within the aerospace industry, making up 40% of the aero-engine. Within the gas turbine engine, the high transient thermal stresses developed due to variations in power requirements during a typical flight cycle give rise to the phenomenon of thermo-mechanical fatigue (TMF). The lifing models utilised within this research focus on damage tolerance approaches. TMF crack growth test techniques have been developed for high performance titanium alloys, in which diverse phasing (φ) between mechanical loading and temperature have been investigated. In addition, factors affecting the TMF behaviour of Ti-6246, including peak temperature, minimum temperature and temperature range have also been explored.

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.

In this work, we have extended our earlier work on the concept of ratchetting strain as a crack driving force (Tong et al., 2013), to examine the crack growth of a nickel-based superalloy at selected temperatures in vacuum under both fatigue and fatigue-creep loading conditions. The parameters of a unified constitutive model were calibrated against the material data obtained at selected temperatures from 550 to 775°C, and a finite element model was developed to simulate the near-tip stress–strain responses under fatigue and creep-fatigue loading conditions at the experimental temperatures. Both ratchetting strain and accumulated inelastic strain near the crack tip were utilised in the prediction of the crack growth rates collected in vacuum. It seems that, although both ratchetting strain and accumulated inelastic strain correlate with the crack growth rates obtained under fatigue and fatigue-creep loading conditions, the predictions based on accumulated plastic strain are particularly close to the experimental results at all temperatures and loading conditions examined.This is the first time the concept of ratchetting strain has been used to predict the crack growth rates of an engineering alloy at elevated temperature in vacuum, where the influence of oxidation on crack growth is removed.

The creep behaviour of the nickel superalloy RR1000 is studied through a number of constant-load creep tests. It is often assumed that creep data generated by constant-load testing are unsuitable for building a generalised creep model due to the non-constant stresses incurred. Analysis of existing models shows that significant errors may occur in many approaches, which attempt to recreate the strain evolution with time. A model is presented which is not reliant on time as a parameter and is therefore able to utilise constant-load creep data without enforcing the assumption of a constant stress. This model is demonstrated through numerical analyses to replicate the creep behaviour across a large range of stresses accurately. The proposed model is then adapted as an Abaqus user-subroutine to demonstrate capability within finite element analysis.

As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces.The temperature calibration method to achieve the desired thermal cycle will be briefly described, along with some considerations for ensuring reproducibility in the thermal cycles applied during tests. The measurement of crack growth under TMF conditions will also be considered. Such measurements are challenging because of the changing thermal conditions and the effect on conventional potential difference (PD) electrical methods. These effects make it difficult to continuously monitor crack size during the thermal/load cycles. Thus, convenient dwells within the TMF cycles, where both the load and temperature are held constant for a brief period, have been utilised to make time-averaged crack size measurements using PD methods. The performance of these experimental methods has been demonstrated with some trials on an advanced nickel base superalloy, RR1000.

The increasing performance requirements of gas turbines are driving higher operating temperatures in critical components, leading to greater creep and fatigue interactions. There is thus a requirement for thermomechanical fatigue (TMF) test data, including TMF crack growth rates. The test equipment required to meet this challenge is briefly discussed, along with the development of the test methodology. Thus, the accurate measurement and maintenance of thermal performance during TMF are considered. The determination of fatigue crack growth rates by conventional electrical potential difference measurements has been employed, utilising convenient dwells within the TMF cycles, where both load and temperature are held constant for a brief period. These experimental methods have been demonstrated with trials on the advanced nickel base superalloy RR1000 used for turbine discs. This work has also highlighted significant microstructural effects on the crack growth rates, with the coarser grain size offering a reduced crack growth rate.

Rolls-Royce has developed a hyperbolic tangent representation of creep, referred to as CT07 [1], which explicitly relates the accumulated creep strain to the current time, stress and temperature. Although, such an approach has been successfully applied to a number of nickel-based superalloys, it will be shown that it does not adequately capture the shapes of the creep curves and rupture times of the single crystal CMSX4 at temperatures below 800 degrees C and above 1000 degrees C. This result arises from the implicit assumption made in CT07 that the generic shape of the creep curve is tertiary dominated, which is not observed in CMSX4. This paper presents a new hyperbolic tangent formulation that accounts for sigmoidal creep, the large primary creep strains observed at low temperatures, the tertiary dominated creep behaviour at high temperatures (850-1000 degrees C) and the secondary creep dominated response above 1000 degrees C. A microstructure-explicit creep model has been used to derive parameters, such as the incubation time at low temperatures, which feed into the new hyperbolic tangent formulation. It will be shown that the new hyperbolic tangent model predictions of the CMSX4 creep response are in good agreement with the experimental data over a wide range of stresses and temperatures.

Modern gas turbines engines concentrate high power into a relatively small machine, e.g. more than 50MW in each engine suspended from the aircraft wing in a large civil transport application. The rotational speed of the shafts reaches as high as 13,000rpm in large engines and even higher in smaller engines as used in helicopters. Hence there is both a concentration of thermal energy in the combustion process and kinetic energy in the rotating parts that presents issues for structural integrity. This paper describes the regulatory requirements that must be achieved to allow operation of engines in civil applications and at how these requirements are satisfied in practice. It concentrates on those parts whose failure can directly threaten the integrity of the airframe through the generation of hazardous effects. The main issues are associated with fatigue through cyclic operation of the engine and the ability of the engine to survive abnormal conditions in such a way that the aircraft can safely land and be brought to rest.

The paper reports the results of a comprehensive research programme on two different compressor disc alloys: titanium alloy Ti 6246 and the nickel based superalloy Udimet 720Li. Both alloys are used for disc applications in gas turbine engines under conditions where the rims are exposed to fatigue, creep and environmental damage. The titanium alloy was investigated at temperatures of 80, 450, 500 and 550°C, whilst the nickel at 650 and 700°C. The paper presents the strain-life response of plain specimens and relates these data to notch fatigue behaviour. It also explores fatigue crack propagation behaviour in air and under hard vacuum (10−6Torr), in order to characterise the creep/environment/fatigue interactions that occur at the crack tip. The growth data encompass R values of −3 to 0.5 for cyclic and dwell waveforms. The information obtained allows the effects of environment and creep at these temperatures to be partitioned. The observed R value dependence is related to both empirical models and closure measurements. The experimental data are supported by detailed metallographic and fractographic studies using optical and SEM techniques.

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.

The stress rupture behaviour of single crystal alloy CMSX4 has been modelled using a damage mechanics technique. By ensuring failure is bound by the material UTS it is shown how the equations can be used effectively over the entire stress range of blade operation without the need for a bilinear approximation to the rupture curve. The temperature dependence of rupture can be modelled using a time-temperature parameter if the progressive weakening of the material with increasing temperature is accounted for.High temperature steady state and tertiary creep has been described by conventional damage mechanics equations. At intermediate operating temperatures of 750-800 degrees C the material exhibits pronounced incubation and primary creep of up to 4% which is usually neglected in creep modelling of blade alloys. This behaviour has been rationalised on the basis of dislocation relief of misfit stresses in the gamma channels. The observed creep curves have therefore been described by including an internal stress term in the creep equations which evolves as a function of the state of damage of the material.