Untwist Creep Analysis of Gas Turbine First Stage Blade

Abstract

Abstract During operation, turbine blades subjected to high temperatures will experience permanent geometric distortion due to creep. The principal form of this distortion is blade elongation due to the centrifugal load, but the blade can also straighten or ‘untwist’. The magnitude of the untwist can be used to track the accumulated creep strain damage in a blade, and so measurements of blade untwist over time are often recorded. If the relationship between untwist and creep strain damage can be expressed, then it will be possible to make more informed maintenance decisions. This study describes a finite element creep analysis of a first stage turbine blade using a calibrated creep material model, and compares the calculated untwist to records kept by the operator. The creep model used in this study is a physics based model using the microstructural properties of the blade alloy, and is calibrated using a single creep test at representative stress and temperature. An important objective of this study is to demonstrate that application of the creep model in a component with non-uniform stress and temperature will lead to representative results. The analysis was repeated with two materials that correspond to different blade versions. The material with superior high temperature properties exhibited less untwist, and the analysis of both variants were comparable to the recorded blade measurements.