where material was removed, was less severe than that predicted by notch strength analysis. A similar trend was also observed in the flat samples that were quasi-statically indented. While the generally observed lower strength of the as-impacted samples can possibly be attributed to tensile residual stresses below the notch surface, the stress relieved samples exhibit fatigue strengths above which one would expect for the geometry of the observed notches. It appears that the notches produced from the ballistically impacted FOD have effective notch fatigue strengths higher than predicted by their geometries using conventional notch fatigue analysis; however, most work on notch fatigue deals with notches that are considerable larger than those being dealt with in this FOD investigation. It is concluded, therefore, that FOD that produces notch geometries involving the dimensions discussed herein is not as detrimental as FOD with larger dimensions of geometrically similar notches. This conclusion is based on the detrimental effects of the notch geometry and the calculated large notch fatigue factor, and does not consider the effects of residual stresses nor of irregularly shaped indents that may involve loss of material, tears, or other geometric discontinuities. The most detrimental effects of FOD appear to occur when irregularly shaped notches are produced from conditions such as tearing or material removal. Finally, the role of residual stresses in tension can be blamed for the low fatigue limit stress in many cases based on observations in nearly identical impact events where stress relief annealing improves the fatigue strength. It can only be concluded, in these cases, that tensile residual stresses led to the degradation of the fatigue behavior. REFERENCES 1. Peters, J.O., Roder, O., Boyce, B.L., Thompson, A.W., and Ritchie, R.O. (2000). Metallurgical and Metals Transactions, 31A, 1571. 2. Hamrick, J.L., Major, USAF (1999). Ph.D. Dissertation, Air Force Institute of Technology, USA. 3. Ruschau, J.J., Nicholas, T., and Thompson, S.R., (2001). Int. Jour. Impact Engineering, 25/3, 233. 4. Thompson, S.R., Ruschau, J.J., and Nicholas, T., (2001). Int. Jour. Fatigue, (in editing). 5. Maxwell, D., and Nicholas, T., (1999). In: Fatigue and Fracture Mechanics: 29th Vol., ASTM STP 1321, pp. 626-641, Panotin, T.L., and Sheppard, S.D. (Eds). American Society for Testing and Materials, USA. 6. Bellows, R.S., Muju, S., and Nicholas, T. (1999). Int. Jour. Fatigue, 21, 687. 7. Bellows, R.S., Bain, K.R., and Sheldon, J.W. (1998). In: Mechanical Behavior of Advanced Materials, MD-Vol. 84, pp. 27-32, Davis, D.C. et al (Eds). ASME, New York. 8. Haritos, G.K., Nicholas, T., and Lanning, D.B. (1999). Int. Jour. Fatigue, 21, 643. 9. Bannantine, J.A., Comer, J.J., and Handrock, J.L. (1990). Fundamentals of Metal Fatigue Analysis. Prentice Hall, Englewood Cliffs, New Jersey. 10. Kaufman, A., and Meyer, A.J. Jr. (1956). National Advisory Committee for Aeronautics, Technical Note 3275. USA. 11. Martinez, C.M., Capt., USAF (2000). M.S. Thesis, School of Engineering, University of Dayton, Dayton, OH, USA. 12. Ruschau, J.J., John, R., Thompson, S.R., and Nicholas, T. (1999). ASME J. Eng. Mat. Tech., 121, 321.
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