High Cycle Fatigue: A Mechanics of Materials Perspective part 36. 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. | 336 Effects of Damage on HCF Properties . SCOPE OF THE FOD PROBLEM As noted in Appendix G FOD can even be of a size that can barely be detected by the unaided eye. A majority of FOD involves damage sizes that are less than 2 mm in depth. There have been instances in service usage where concerns over HCF failures originating at extremely small FOD sites have led to frequent inspections of leading edges of fan blades using both a finger nail inspection or running dental floss up and down to find small FOD nicks or dents. More important it is not yet known whether HCF failures which emanate from very small FOD are due to the degradation of material properties or are instead due to excessive vibratory stress amplitudes which just happen to choose the FOD location as the origin site based primarily on statistics of there being a very slight reduction in material capability at that site. Fan and compressor blades at the front end of jet engines are the components that receive the majority of damage particularly at the leading edge of the airfoil. Due to the high frequency vibratory stresses that can be present in the fan and compressor sections associated with normal engine operation it is not uncommon for fatigue cracking to initiate from FOD sites and grow catastrophically within minutes to hours of engine running time. The HCF caused by steady state or transient vibrations of the component is one of the leading causes of in-service failures of blades that have been subjected to FOD. It is important therefore to know the fatigue strength of materials and airfoil geometries that have been damaged by foreign objects of various types sizes velocities and incident impact angles. Evaluation of FOD resistance of materials and structural components has been conducted in a highly empirical manner over the years. Engine companies generally subject individual components such as fan and compressor blades to simple vibratory loading to determine fatigue resistance. FOD is
