The design of a long, wide blade for a large-sized steam turbine required the development of new design methodology. The most important consideration to ensure the integrity of a blade attachment is to evaluate the stress at the local point of a blade root. The major difficulties in stress evaluation are the three-dimensional effects and the contact problem at the fastening point between the blade root and the disk steeple. Because of these complexities, the design method based on the linear elastic finite-element method (FEM) analysis has to be verified experimentally.
For this purpose, a new fatigue testing apparatus was developed that simulates the operating loads for the steam turbine, including centrifugal forces, untwisting moments, and bending moments. The results of the strain measurement and the low-cycle fatigue tests verified the use of the peak stress, which was calculated by three-dimensional elastic FEM analysis using linear elastic connection of blade root and disk steeple and Neuber's rule for the evaluation of the strain range. If the strain is evaluated correctly, the low-cycle fatigue life can be obtained using a Langer's-type equation.