High Cycle Fatigue: A Mechanics of Materials Perspective part 52. 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. | 496 Appendix B tools were again applied for redesigns and proved totally inadequate as the frequency of failure actually increased as the redesigns were incorporated. Only after the initial stages of the HCF Program were underway did it become known that the aerodynamic driver was the downstream stationary airfoils and an effective redesign occurred. By the late 1980s Headquarters Tactical Air Command became concerned enough with HCF failures and fleet impacts that the Director of Logistics sent a letter to the Air Force Systems Command s Aeronautical Systems Division ASD requesting a thorough review of the design development and qualification process that was not catching these issues before they occurred in the field. The ASD EN conducted a study that noted the issue and made modest recommendations for improvements. As the decade of the 90s opened HCF became a more notable problem as the B-1 engine experienced two spectacular failures one of which led to the engine separating from the aircraft. These failures demonstrated a high susceptibility of the fan to Foreign Object Damage FOD with stimulation by the inlet characteristics that had never been tested. The Assistant Secretary of the Air Force directed an Air Force Scientific Advisory Board SAB study of the issue which focused on the issue of the use of titanium alloys in jet engine compression systems fans and compressors . The 1992 SAB report entitled Air Force Jet Engine Manufacturing and Production Processes presciently made five recommendations that later would form the backbone of the HCF program and the technical partitioning of the program. These included extension of the application of fracture mechanics to HCF establishing a substantial research program to increase understanding of HCF and determining a procedure to demonstrate resonant frequencies and vibratory stress levels in qualification engine testing. HCF becomes a crisis By the mid 1990s HCF issues had become the dominant failure mode for .
