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

• Fatigue crack aspect ratio evolution in early fatigue stage is characterized.
• The aspect ratio in the shot peened cases relates to shot peening affected layers.
• Microstructure plays an important role in early crack initiation and propagation.
• Crack growth rates in the shot peened case are significantly delayed.
• The a/c evolution effects are incorporated in K-evaluations and assumptions.

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 (Δε11 = 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 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.