Foreign-object damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue (HCF) life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations. However, recent experimental studies have shown the unexpected propensity for impact-induced fatigue crack formation at locations of compressive residual stress in the vicinity of the impact site. To address this issue, in situ and ex situ spatially-resolved X-ray diffraction and numerical modeling are utilized to show that the initial residual stress state can be strongly relaxed during the fatigue loading process. The magnitude and rate of relaxation is strongly dependent on the applied loads. For a Ti-6Al-4V turbine blade alloy, little relaxation was observed for an applied maximum stress of 325 MPa (0.35σy, where σy is the yield stress), and cracks tended to form in subsurface zones of tensile residual stress away from the damage sites. In contrast, at an applied maximum stress of 500 MPa (0.54σy), equal to the smooth-bar 107- cycle endurance strength, cracks tended to form at the damage sites in zones of high stress concentration that had initially been in strong compression, but had relaxed during the fatigue loading. © 2002 Elsevier Science B.V. All rights reserved.
|Number of pages||10|
|Publication status||Published - 2003|
Bibliographical noteThis work was supported by the US Air Force Office of Scientific Research under Grant No. F49620-96-1-0478 under the auspices of the Multidisciplinary University Research Initiative on High Cycle Fatigue to the University of California (for numerical modeling and fatigue analysis), the Office of Science, US Department of Energy under contract #DE-AC03-76SF00098 (for experimental diffraction results), and the Stanford Synchrotron Radiation Laboratory, operated by the Department of Energy, Office of Basic Energy Sciences (for X-ray beamtime). B.L. Boyce would like to thank the Hertz Foundation for their generous support in the form of a Hertz fellowship. Thanks are due to Dr J.M. McNaney for many helpful discussions.
- Foreign-object damage
- Residual stress
- X-ray diffraction