P.A.S. Reed

Short crack initiation and growth at 600 °C in notched specimens of Inconel718

The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.

Fatigue crack initiation and short crack growth in nickel-base turbine disc alloys: the effects of microstructure and operating parameters

Abstract An assessment of the effects of microstructure and operating parameters on both crack initiation and propagation of short fatigue cracks is presented. The assessment was carried out on RR1000, U720Li and microstructural variants of U720Li. Fatigue tests were carried out at room temperature (20 Hz sinusoidal cycling) and at 650 °C (1–1–1–1 trapezoidal cycling). Comparisons of the performance of the different microstructures revealed that initiation occurred predominantly at pores at both temperatures. At room temperature, stage I crack growth predominated and the presence of large primary γ′ precipitates on the grain boundaries, larger grains and larger coherent γ′ sizes gave improved fatigue crack growth resistance, whereas at 650 °C, larger grains gave the most significant performance benefits.

Elevated temperature short crack fatigue behaviour in near eutectic Al-Si alloys

This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent short crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by cracking or decohesion (dependent on test temperature) and are also sought out preferentially during short crack growth.

An example of the use of neural computing techniques in materials science: the modelling of fatigue thresholds in Ni-base superalloys

Two adaptive numerical modelling techniques have been applied to prediction of fatigue thresholds in Ni-base superalloys. A Bayesian neural network and a neurofuzzy network have been compared, both of which have the ability to automatically adjust the network’s complexity to the current dataset. In both cases, despite inevitable data restrictions, threshold values have been modelled with some degree of success. However, it is argued in this paper that the neurofuzzy modelling approach offers real benefits over the use of a classical neural network as the mathematical complexity of the relationships can be restricted to allow for the paucity of data, and the linguistic fuzzy rules produced allow assessment of the model without extensive interrogation and examination using a hypothetical dataset. The additive neurofuzzy network structure means that redundant inputs can be excluded from the model and simple sub-networks produced which represent global output trends. Both of these aspects are important for final verification and validation of the information extracted from the numerical data. In some situations neurofuzzy networks may require less data to produce a stable solution, and may be easier to verify in the light of existing physical understanding because of the production of transparent linguistic rules.

The mechanisms of long fatigue crack growth behaviour in Al–Si casting alloys at room and elevated temperature

Abstract Pistons are commonly made from multicomponent Al–Si casting alloys, which have complex, interconnecting three-dimensional (3D) networks of secondary phase particles. They are non-serviceable parts and so must be able to withstand high cycle fatigue while operating at temperatures between 30 and 80% of T m. Long fatigue crack growth tests were performed at room temperature (RT) and 350°C to assess the micromechanisms of fatigue. The fracture profiles at low and high da/dN were analysed; at low crack growth rates at both temperatures there is no crack path preferentiality with respect to the microstructure. At high da/dN in the RT sample the crack growth occurs preferentially via hard particles, while at 350°C there is a change in mechanism and the crack appears to avoid hard particles. X-ray tomography has been used to image the crack tips and gain a detailed insight into the mechanisms of fatigue in these complex 3D microstructures.

A comparison of high temperature fatigue crack propagation in various subsolvus heat treated turbine disc alloys

Abstract The microstructure and fatigue performance of three subsolvus heat treated nickel based superalloys for turbine disc applications are reported. The alloy variants studied are RR1000, N18 and Udimet 720 low interstitial (U720Li), with the latter tested both in a standard and large grain variant (LG). Their microstructures are examined in terms of grain and gamma prime size (γ′). Fatigue crack growth (FCG) rates for all materials at 650°C show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also important.

Life assessment methodologies incoroporating shot peening process effects: mechanistic consideration of residual stresses and strain hardening Part 2 – approaches to fatigue lifing after shot peening

Abstract Shot peening is a well known process applied to components in order to improve their fatigue resistance. In recent years, there has been an increasing interest in including the effects of the shot peening process in life assessment models since this would allow a reduction in conservatism compared to those in current application. Part 1 of this review dealt with the effects of the shot peening process (surface roughening, strain hardening and compressive residual stresses) and the resulting effect on component fatigue life. This part of the review considers how this effect on component fatigue life can be incorporated into life assessment modelling approaches with discussion of the relative merits of each approach. The paper concludes with a flow chart demonstrating a possible route for the inclusion of shot peening effects within industrial component life assessment frameworks.

Microstructural Analysis of Fatigue Initiation in Al-Si Casting Alloys

Fatigue initiation behaviour in three multi-component Al-Si casting alloys with varying Si content is compared using a range of microscopy and analytical techniques. A higher proportion of stiffer secondary phases leads to load transfer effects reducing particle cracking and particle/matrix debonding. Si appears stronger than the Al9FeNi phase, which cracks and debonds to form initiation sites preferentially over Si. Reducing Si content results in clusters of intermetallics forming, and increased porosity. The effect of porosity, combined with mesoscopic load transfer effects to the high volume fraction intermetallic regions make these potent crack initiation sites in low silicon alloys.

The effect of environment and orientation on fatigue crack growth behaviour of CMSX-4 nickel base single crystal at 650 °C

Abstract The fatigue behaviour of nickel base single crystal CMSX4 at 650 °C in air and vacuum has been investigated for two orientations with differing nominal crack propagation orientations but the same 〈001〉 tensile axis. The orientation containing a 〈110〉 crack growth direction shows better fatigue crack propagation resistance. This has been linked to differing orientations and amounts of interdendritic porosity (giving increased crack path roughness and shielding) increased stiffness along the crack growth direction (with implications for shear band decohesion) and lower resolved shear stresses along active slip systems, limiting faster Stage I crack growth. Faster crack growth rates are generally seen in vacuum compared with air, indicating the homogeneising effect of oxidation on slip, suppressing faster Stage I crack growth and possibly increasing oxidation-induced closure. The results of this study indicate that the effect of secondary orientation and environment in single crystal components will affect both failure mode and fatigue crack propagation rates.

Designing committees of models through deliberate weighting of data points

In the adaptive derivation of mathematical models from data, each data point should contribute with a weight reflecting the amount of confidence one has in it. When no additional information for data confidence is available, all the data points should be considered equal, and are also generally given the same weight. In the formation of committees of models, however, this is often not the case and the data points may exercise unequal, even random, influence over the committee formation. In this paper, a principled approach to committee design is presented. The construction of a committee design matrix is detailed through which each data point will contribute to the committee formation with a fixed weight, while contributing with different individual weights to the derivation of the different constituent models, thus encouraging model diversity whilst not biasing the committee inadvertently towards any particular data points. Not distinctly an algorithm, it is instead a framework within which several different committee approaches may be realised. Whereas the focus in the paper lies entirely on regression, the principles discussed extend readily to classification.