International Journal of Fatigue, Volume 54:Evaluating surface deformation and near surface strain hardening resulting from shot peening a tempered martensitic steel and application to low cycle fatigue

The plastic deformation resulting from shot peening treatments applied to the ferritic heat resistant steelFV448 has been investigated. Two important effects have been quantified: surface roughness and strainhardening. 2D and 3D tactile and optical techniques for determining surface roughness amplitude parametershave been investigated; it was found that whilst Ra and Sa were consistent, Sz was generally higherthan Rz due to the increased probability of finding the worst case surface feature. Three different methodsfor evaluating the plastic strain profile have been evaluated with a view to establishing the variation inyield strength near the surface of a shot peened component. Microhardness, X-ray diffraction (XRD) linebroadening and electron backscatter diffraction (EBSD) local misorientation techniques were applied toboth uniaxially deformed calibration samples of known plastic strain and samples shot peened at intensitiesvarying from 4A to 18A to establish the variation in plastic strain and hence the variation in yieldstrength. The results from the three methods were compared; XRD and EBSD profiles were found to bethe most similar with microhardness profiles extending much deeper into the sample. Changes in themeasured plastic strain profile after exposure to low cycle fatigue and the correlation of these changeswith the cyclic stress–strain behaviour of the material are also discussed with a view to assessing theimportance of the dislocation profile in component life assessment procedures. 2013 Elsevier Ltd. All rights reserved

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725oC under three-point loading. Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650oC to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10-2?10-3Pa. Cracks mainly initiate from grain boundaries or ?/?? interfaces due to the formation and subsequent cracking of Cr-rich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature

Reducing conservatism in life assessment approaches: industrial steam turbine blade to disc interfaces and the shot peening process

Damage tolerant life assessment and the resulting component repair and replacement scheduling is crucial to the continued operation of UK power plant. Typically a major outage on a conventional power station takes 8 -12 weeks; there is a drive to minimise the scope of remedial works undertaken allowing the length of the outage to be reduced. Recent trends in life assessment methods are towards approaches that can be used to justify deferring invasive inspections and reducing the scope of the maintenance works. This thesis reports the development of a life assessment protocol for industrial steam turbine blade to disc interfaces which explicitly accounts for the residual stresses and strain hardening resulting from shot peening with a view to improving life cycle efficiency and reducing operator cost. A modelling approach has been developed which allows for the effect of shot peening to be explicitly accounted for in the damage tolerant life assessment process. The approach requires the input of experimentally determined residual stress, surface roughness and strain hardening profiles. Whilst the methods for determining residual stresses are well standardised, the optimum means of determining both the surface roughness characteristics (two and three dimensional) and the strain hardening profile (microhardness, X-ray diffraction and electron backscatter diffraction) were investigated as a first step towards standardising these measurements: It was found that a three dimensional analysis of the surface topography is most likely to capture the worst case defect. X-ray diffraction and electron backscatter diffraction measurements of cumulative plastic strain were shown to be most similar, with microhardness measurements appearing to overestimate the strain hardened region. Furthermore, the retained benefit of the shot peening process in terms of total life even under very high strain range LCF conditions has been clarified in terms of the applied stress distribution, specific material cyclic characteristics and residual stress relaxation. Under high strain range three point bend fatigue loading, the residual stresses and strain hardening in notched samples of the tempered martensitic steel under investigation did not change significantly. This allows the shot peening process to be specified for notched components under high strain range cyclic bend load, such as low pressure turbine disc blade interfaces, with an improved understanding of the resulting benefits

Stress relaxation in shot–peened geometric features subjected to fatigue: experiments and modelling

This paper investigates experimentally and numerically the degree to which the stress relaxation in simple shot-peened geometries can be modelled by using a knowledge of simplified eigenstrain distributions. The residual stress generated due to shot peening is determined as the elastic response of the whole component when the plastic strains caused by the process are incorporated as an eigenstrain in an appropriate finite element (FE) model. The application of a subsequent live load is then modelled as an additional load step in the FE model which will superpose the effect of this loading on the residual stress field. The results show that the hybrid eigenstrain/FE approach is particularly useful in predicting residual stress relaxation in shot-peened components

Fatigue crack growth behaviour in the LCF regime in a shot peened steam turbine blade material

In this study, short fatigue crack initiation and early growth behaviour under low cycle fatigue conditions was investigated in a shot peened low pressure steam turbine blade material. Four different surface conditions of notched samples have been considered: polished, ground, T0 (industry applied shot peened process) and T1 (a less intense shot peened process). Fatigue crack aspect ratio (a/c) evolution in the early stages of crack growth in polished and shot peened cases was found to be quite different: the former was more microstructure dependent (e.g. stringer initiation) while the crack morphology in the shot peened cases was more related to the shot peening process (i.e. surface roughness, position with respect to the compressive stress and strain hardening profiles). Under similar strain range conditions, the beneficial effect of shot peening (in the T0 condition) was retained even at a high strain level (??11=0.68%), Nf, ground< Nf, T1 < Nf, polished < Nf, T0. The a/c evolution effects were incorporated in K-evaluations and used in calculating da/dN from surface replica data. Apparent residual stress (based on crack driving force ?K difference) was applied to describe the benefit of shot peening and was seen to extend significantly below the measured residual stress profile, indicating the importance of the strain hardening layer and stress redistribution effects during crack growth

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

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

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 work hardening 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

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.</p