PSI - Issue 7

Dalila Dimaggio et al. / Procedia Structural Integrity 7 (2017) 198–205 D. Dimaggio et al. / Structural Integrity Procedia 00 (2017) 000–000

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Keywords: fatigue; highly stressed volume; local stress; high cycle fatigue

1. Introduction Steam turbine blades are subjected to a wide range of load spectra and extreme conditions during their lifetime resulting in a combination of Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) [1], which needs to be properly addressed during the blade design in order to avoid component failure. The investigations in this paper have been focused on the assessment of mechanical load cases in the LCF- and HCF -regime; LCF-regime is characterized by the centrifugal force due to turbine rotation, while in the HCF-regime vibrations due to the presence of oscillating excitation forces linked to rotor unbalance and/or the presence of upstream vanes and stator parts. The rapid change of operational requirements in the energy sector and the frequent explorating of lifetime extension requires an appropriate numerical approach to be applied for blades life evaluation. Accounting for the larger number of start-ups required by the new flexible energy market, the experimental campaign aimed to develop a representative methodology for large number of cycles (for example: N = 1•10 11 cycles, i.e. 10000 runs-up/down for a life of 25 years) that the turbine would experience during the entire lifetime using most conservative assumptions. Experimental tests have been performed at the temperature of 120°C. This paper illustrates the local stress methodology aimed to evaluate the component lifetime accounting for stress gradient support via Highly Stressed Volume approach for both LCF and HCF damage. Moreover, the experimental campaign was the basis to obtain results for the method, which is described here, both in its procedure and results. The methodology is finally applied to a real industrial case (a steam turbine low pressure rotor blade).

Nomenclature HSV 90%

Highly Stressed Volume Slope of the S-N-curve Slope after the knee point Mean stress sensitivity Number of load cycles Number of cycles at knee point Theoretical stress concentration factor

k

k* K t M

N

N k

n HSV R ε /R σ

Support factor, Highly Stressed Volume approach

Strain ratio / Stress ratio

σ a

Stress amplitude

σ a,k σ va

Normalized stress amplitude at knee point

Supportable stress amplitude

T P s

Testing temperature Probability of survival Finite Element Analysis

FEA

2. Material, specimen, geometries and testing 2.1. Investigated Material

The development of a reliable fatigue lifetime approach for cyclic mechanically loaded turbine blades was carried out on the basis of a precipitation-hardening steel commonly used for low pressure rotor blades due to its high corrosion resistance, strength and toughness. The specimens were cut-out from turbine blades. 2.2. Specimen Geometries