Residual Stress Measurement and Its Application to Achieve Predicted Full Life Potential of Low Cycle Fatigue Limited Engine Disks

摘要

Many of the engine families owned by the US Air Force have disks that are projected to not reach their low cycle fatigue design lives. Recent data suggest a relationship between residual compressive stress and remaining disk low cycle fatigue life. However, there has been no practical nondestructive way to determine whether a disk is about to crack or if a new disk was adequately and uniformly shot peened. The occasional disk at risk due to low residual stress can be identified using X-ray diffraction residual stress measurement techniques and retired. The measurement and tracking of residual stress of each disk and rotor stage in an engine population shows potential for identifying those disks at risk of initiating a crack, thereby extending the use of the remaining population to reach their full life potential. Paramount to this philosophy is that a threshold residual stress value exists below which the component is at risk. This threshold must be identified for each part configuration and application. In addition, residual stress mapping can be used to validate finite element analysis of critical local stress locations that are subjected to plastic strain and subsequent low cycle fatigue damage. It is recommended that residual stress of individual disks be measured and tracked in an effort to identify the residual stress threshold for crack initiation and that these data be included as part of a more comprehensive part tracking database. The US Air Force has supported development of this technology through Small Business Innovative Research programs and additional opportunistic tasks with Fatigue Management Associates over the past five years (Berkley, 1996; Coate, 1999). Products of these contracts are presented and discussed.