Foreign object damage (FOD) occurs when hard, millimeter-sized objects such as gravel or sand are ingested into aircraft jet engines. Particles impacting turbine blades at velocities up to about 300 m/s produce small indentation craters which can become sites for fatigue crack initiation, severely limiting the lifetime of the blade. A framework for analyzing FOD and its effect on fatigue cracking is established in this paper. Finite element analysis is used to determine the residual stresses and geometric stress concentration resulting from FOD. The roles of material rate sensitivity and inertia are delineated. The most important non-dimensional parameters governing impact indents are identified, significantly reducing the set of independent parameters. The second step in the analysis focuses on the potency of cracks emerging from critical locations at the indents. The results have been used to address the question: When and to what extent do the residual stresses and stress concentration caused by FOD reduce the critical crack size associated with threshold fatigue crack growth? For deep indents, it is found that elastic stress concentration is the dominant factor in reducing critical crack threshold when the applied cyclic load ratio, R, is large, otherwise the residual stresses are also important. Comparisons with a set of experiments conducted in parallel with the theory show that the numerical approach can account for various phenomena observed in practice. © 2002 Elsevier Science Ltd. All rights reserved.
Bibliographical noteThe authors are grateful for helpful discussions with Drs. Brad Boyce, Jan Peters, Jim McNaney and Professor Robert Ritchie at the University of California at Berkeley. This work was supported in part by the Multi-University Research Initiative on High Cycle Fatigue, which is funded at Harvard University by AFSOR under Grant No. SA1542-22500 PG, and in part by the Division of Engineering and Applied Sciences, Harvard University.
- A. Impact
- B. Critical crack size
- C. Finite element
- Foreign object damage