K.A. Soady

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.

Effects of shot peening on short crack growth rate and resulting low cycle fatigue behaviour in low pressure turbine blade material

Abstract The effect of shot peening on subsequent low cycle fatigue behaviour of a representative low pressure steam turbine blade material has been investigated in bend test samples. An analysis of the short fatigue crack growth behaviour has been conducted. For samples with no stress concentration feature, shot peening was found to have a more evident beneficial effect at lower strain levels than at higher strain levels, whereas for samples with a stress concentration feature, the beneficial effect was retained even at higher strain levels. Preexisting cracks were observed on the shot peened surface, which started to grow at 10–25% of fatigue life in the low cycle fatigue regime. The crack propagation rate was slower than that observed in the ground sample, suggesting that the shot peening process delayed crack propagation. This improvement in fatigue life has been attributed to the significant slowing of small cracks while growing through surface regions of significant compressive residual stresses and local workhardening developed by the shot peening process. Once cracks in the notch root have penetrated this region in the depth direction, faster crack growth rates, similar to those observed in the ground case, were seen.

Oxidation behaviour of single crystal nickel-based superalloys: intermediate temperature effects at 450–550°C

ABSTRACT The oxidation behaviour of two single crystal Ni-based superalloys has been investigated at 450°C and 550°C. Isothermal oxidation was carried out for varying times and it was found that exposure resulted in a sub-micrometre thick oxide. The external and internal oxide kinetics were studied via high-resolution image analysis and both showed sub-parabolic growth rates. Thermogravimetric tests indicated that the overall oxidation growth obeys a near quartic power law while parabolic kinetics can describe the transient oxidation period. Characterisation of the resulting oxides was carried out using electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Results from thermodynamic modelling of the oxide formation are also presented to further assess the postulated mechanism of low-temperature oxidation in these Ni-based superalloys.

Short Fatigue Crack Growth Micromechanisms in a Cast Aluminium Piston Alloy

The short fatigue crack growth behaviour of a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post-mortem analysis of fracture surfaces and crack profiles. Crack-microstructure interactions have been clearly delineated using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and three dimensional (3D) X-ray microtomography (SRCT). Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy

Three‐Dimensional Analysis of Microstructure in Cast Aluminium Piston Alloys

The 3D architecture of intermetallics distribution in two model cast aluminium piston alloys is examined using synchrotron X-ray microtomography and advanced image analysis tools. The highly complex morphology and 3D interconnectivity of intermetallics is delineated using advanced 3D image analysis tools. A novel technique which circumvents quantification difficulties associated with the high interconnectivity is employed for quantifying intermetallic particles. The intermetallic particle size distribution is then analysed using extreme value statistics to predict the maximum particle size in a sample of S-N fatigue specimens and subsequently, the lower bound fatigue life in the given sample.