Life assessment of a critical aeroengine part using finite element crack modelling

Abstract

During the design of a Rolls-Royce military gas turbine aeroengine several years ago, the rear flange was identified as the life-limiting part of the outer casing of the combustion chamber (CCOC). An early finite element (FE) analysis identified the dowel pin hole in the flange as a potentially critical area and predicted a low cycle fatigue (LCF) safe life much lower than the design target. However, whole engine testing since then has proved those predictions to be too pessimistic, but LCF testpieces failed to deliver a better life estimate, possibly because of the difficulty of achieving representative testing conditions. In order to predict a more accurate life for the component, an alternative procedure incorporating FE static crack analysis, LCF testing, crack propagation tests and analytical calculations was used to life the component. FE submodelling techniques were applied to study the behaviour of a growing crack, with boundary conditions taken from FE global models of the component and the LCF testpiece. The FE analysis explained the discrepancies between earlier modelling attempts and experimental results, and provided the necessary information to declare an increased component life.