Numerical modelling of the fatigue crack shape evolution in a shot-peened steam turbine material

Abstract

In this study, the short crack initiation and growth behaviour in a notched sample under low-cycle fatigue (LCF) was investigated in a low-pressure steam turbine material. Different crack initiation mechanisms and crack shape evolution processes were experimentally observed in samples subjected to different surface treatments: polished, T0 (industrially applied shot peening process) and T1 (a less intense shot peening process). To better understand the effects of shot peening on fatigue, a 3D finite element (FE) model was developed to investigate the interaction between crack growth and the effects induced by shot peening. Firstly, residual stress redistribution caused by both mechanical loading and the presence of a crack was numerically investigated. This model was also used to successfully predict the differences in crack shape evolution between varying surface conditions, and quantified the retardation of short crack growth behaviour resulting from shot peening. Finally, the 3D model introduced in this study was compared with a previously developed 2D model with plane strain assumptions to demonstrate the limitation of the 2D model in simulating the crack growth behaviour, and to emphasise the importance of taking the 3D crack shape into account when evaluating the short crack growth behaviour.

Physical-Mechanism Exploration of the Low-Cycle Unified Creep-Fatigue Formulation

“…This equation shows that the second power order in Equation (25) is reasonably replaced by a general exponent (n) due to the equivalent transformation, which is consistent with FEA result shown by You [53]. Since larger plastic zone gives more crack growth [52], the crack growth in one cycle can be related with Δ in a power-law relation.…”

The effects of surface pits and intermetallics on the competing failure modes in laser shock peened AA7075-T651: Experiments and modelling

“…8, the formation of the pit can be simulated by deactivating the BCs defined at the plug-base interface, separating them as two single parts without any contact. Similar sub-modelling methodology has been adopted in [28] for crack modelling, which validates that the defined BCs at the interface between parts effectively maintain the integrity of the whole model and have negligible effects on the overall stress distribution. In order to more appropriately simulate the effects of the pit, the shape of the cross section of the pit was assumed to be represented by two semi-ellipses, as presented in Fig.…”

Fatigue crack initiation and growth behavior within varying notch geometries in the low‐cycle fatigue regime for FV566 turbine blade material

“…Increased dislocation density may limit the movement of persistent slip bands (PSBs) inhibiting crack initiation, while the compressive residual stress produced by the plastic deformation at the surface reduces the effective mean stress, increasing resistance to fatigue crack propagation. 2,[5][6][7][8][9][10][11] Conversely, shot peening also increases surface roughness, which is considered detrimental to fatigue due to the presence of local stress concentration features which induce earlier crack initiation. 12 Additionally, for relatively extreme Low-Cycle Fatigue (LCF) conditions (<10,000 cycles), the application of relatively high strain ranges (>1%) can relax the compressive residual stress produced from shot peening within the first 1% of fatigue life for FV448 material.…”

“…12 Additionally, for relatively extreme Low-Cycle Fatigue (LCF) conditions (<10,000 cycles), the application of relatively high strain ranges (>1%) can relax the compressive residual stress produced from shot peening within the first 1% of fatigue life for FV448 material. 5,9,[13][14][15] With a diminished compressive residual stress field, the retardation effect from shot peening on short crack propagation is no longer apparent. Studies have shown that the benefits of shot peening outweigh the drawbacks for the majority of fatigue situations, resulting in an improved resistance to fatigue overall, including for in-service turbine blades in low-cycle fatigue conditions.…”

“…Studies have shown that the benefits of shot peening outweigh the drawbacks for the majority of fatigue situations, resulting in an improved resistance to fatigue overall, including for in-service turbine blades in low-cycle fatigue conditions. 2,5,14 This paper therefore uses a systematic finite element analysis study to investigate how changing the notch geometry may affect the stress and strain range distribution within the notch field after one loading cycle followed by the corresponding residual stress distribution after unloading. Additionally, the effect of changing the notch geometry on residual stress relaxation following shot peening was explored.…”