Detection of damage in steam turbine blades caused by low cycle and strain cycling fatigue

The blades of steam turbines are exposed to low cycle fatigue, as well as to strain cycling fatigue, which may cause their structural deterioration after a low number of cycles. This represents the blade damage in steady condition of operation, which may lead to failure. In this work, the blades of the last stage (L-0) from a 110 MW output steam turbine were analyzed. Samples of material AISI 410 were investigated to define the number of cycles against the stress to failure. As a result, the magnitude of the constants of a numerical model were obtained. In addition, numerical and analytical models predicted the blade damage based on mean stress correction, Strain-life, Morrow and Smith Watson Topper (SWT). The results showed that centrifugal load caused damage in blades and likely, it transforms into crack initiation due to low cycles fatigue. Steam load produced stresses low level in magnitude. However these can be dangerous for blade structure subject to non-linear and resonance conditions, which mainly occur during transient periods. Important differences in prediction of life estimation upon the method emerged for high levels of stress magnitude, which should be taken into account during blade design.