Abstract: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 (Δε 11 =0.68%), N f, ground < N f, T1 < N f, polished < N f, 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 ΔK 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.2

No abstract

The effect of shot peening on subsequent low cycle fatigue behaviour of a representative low pressure (LP) 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.Pre-existing 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.

The objective of this study was to establish the stress-strain empirical mode of oriented strand board (OSB) with random surfaces and oriented core 0° pattern (R/0°/R). The OSB specimens were loaded along the longitudinal (0°), diagonal (45°), and transverse (90°) directions of plates. The loading direction had a significant effect on the behavior of OSB. The OSB in compression exhibited high non-linear elastic behavior up to failure, while it expressed linear behavior when loaded in tension. Four types of failure modes under compression were included: end cracks between flakes, central cracks between flakes, diagonal shear failure, and surface folding. Most of the specimens in tension failed in tensile failure suddenly without plastic deformation. A refined empirical model was suggested and found to be in good agreement with the experimental data. The results provided useful information for modeling various structures containing OSB.

This paper investigates experimentally and numerically the degree to which the stress relaxation in shot-peened simple geometric features of steel turbine materials can be modelled by using an understanding of simple eigenstrain distributions. The residual stress is determined as the elastic response of whole component when the plastic strains caused by shot peening is 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 eigenstrain approach is particularly useful in predicting residual stress relaxation in shot-peened components.