High Cycle Fatigue: A Mechanics of Materials Perspective part 59. 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. | 566 Appendix G a b c d Figure . Damage to RB199 fan blades. concentration factors kt. The distribution of kt is given in Figure 5 . The average kt is about 4 however values of up to 10 can occur for the more severe FOD notches. Microscopic features of FOD Metallographic and fractographic examinations were performed on selected samples to determine the extent of local deformation and the possible existence of cracking. It was found that FOD notches encompass a wide range of microscopic features. The impact event leading to the damage site often resulted in non-propagating cracks as shown in Figure . Non-propagating cracks have been found in laboratory experiments and can have surprisingly little effect on the path of final failure 2 6 . Appendix G 567 FOD depth mm top In. bottom Figure . Distribution of service-induced FOD from two different surveys. Southwest Research Institute SwRI conducted study under a USAF contract. FOD root radius mm Figure . Distribution of FOD notch root radii. In addition to non-propagating cracks several notches were found with extensive local deformation but no cracking. Several damage sites were found with little localized damage and some were impacted with enough energy to cause brittle failure and generate debris. In short post-event inspection revealed that there had been a wide range of impact energies. 568 Appendix G 10 9 1 Frequency Q . Cumulative 6 -- 5 -- 3 -- 2 -- 1 -- 100 90 Q0 70 60 50 40 30 20 10 0 7 4 - 0 1 234567Q9 10 Elastic stress concentration factor kx Figure . Histogram and cumulative distribution function for FOD notch kt. Figure . Micrograph showing FOD site with non-propagating crack. The biggest benefit of the microscopic investigation was the identification of impact surface details that indicated the angle of incidence. These features indicated that the impactor tended to strike the blade at angles of 30 to 60 relative to the blade leading edge centerline which correlates well with .
