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High Cycle Fatigue: A Mechanics of Materials Perspective part 35. The nomenclature used in this book may differ somewhat from what is considered standard or common usage. In such instances, this has been noted in a footnote. Additionally, units of measurement are not standard in many cases. While technical publications typically adhere to SI units these days, much of the work published by the engine manufacturers in the United States is presented using English units (pounds, inches, for example), because these are the units used as standard practice in that industry. The graphs and calculations came in those units and no attempt was made to convert. | 326 Effects of Damage on HCF Properties While the maximum serviceable limits for FOD in some jet engines in operation today range from 0.36 to 0.76 mm 0.014 -0.030 the damage depths found in service vary significantly see Appendix G . Of major concern is if FOD as small as sometimes found in service provides a significant fatigue strength debit in simulated engine blade specimens. Whether the damage should be blended out or the component be removed from service and replaced is very difficult to ascertain in many cases. Another item of concern is the determination of whether damage depth is a good indicator of the actual remaining fatigue life of an FOD impacted blade. 7.4. BACKGROUND There has not been a substantial amount of work done relating FOD to fatigue strength. Peters et al. 2 researched the effects of FOD on HCF thresholds by shooting steel spheres onto the flat surface of modified Kb edge cracked specimens. They found that the overall effect of FOD markedly reduced the fatigue life compared to that obtained on undamaged smooth-bar specimens by providing preferred sites for the premature initiation of fatigue cracks. Mall et al. 3 tested both diamond cross section and uniform rectangular cross section samples to study the effects of FOD. The samples were either ballistically impacted or damaged by quasi-static indentation or shearing. They found that the different damage methods created distinctly different damage mechanisms. It was suggested that a total damage depth parameter could be utilized to allow the use of inexpensive and easily controlled methods of simulating FOD such as the quasi-static chisel indentations to replace more difficult and expensive means such as ballistic impacting. Due to the continuing concern of FOD studies characterizing the damage sites have been conducted in order to understand the effects of FOD on the HCF strength of engine blades. There are several ways of simulating FOD in a laboratory environment which do provide an .
