Elasto Plastic Stress Analysis of LP Steam Turbine Blades Under Centrifugal Loading

The low pressure last stage blades in a steam turbine are some of the most severely stressed machine elements under centrifugal loading. The stresses in the root region near a strain raiser are in the plastic range. The conventional analysis is based on elastic theories and any stresses found beyond the ultimate tensile strength in a small zone are ignored. In this paper an elasto-plastic analysis of a last stage low-pressure steam turbine blade is made by considering a finite element model of the blade with the root alone and also by including the disk effect. The results obtained by EMRC NISA finite element package are presented. The material is shown to yield in a small zone confined to a fraction of an elemental depth near the notch point of the root and in the vicinity of the ultimate tensile strength of the material.