High Cycle Fatigue: A Mechanics of Materials Perspective part 53. 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. | 506 Appendix C to reduce the risk of failure to an acceptable level. Blend limits for FOD and criteria for removal or repair should be based on the FOD analysis required in the preceding paragraph. Material allowables for material which has been repaired such as by blending or welding should be adjusted as necessary to account for any degradation of the fatigue limit due to the blend or repair operation. The effect of any redistribution of internal stress whether induced by FOD or by prior processing should be considered in the development of safe blending limits. Other Damage States Material allowables should be established based on any other damage state as described in section under the High Cycle Fatigue heading. Analytical studies have shown that not every part location will be limited by a crack growing to a calculated critical stress intensity equal to the material s fracture toughness. Some part locations will in fact be life-limited by cracks growing to a predicted vibratory threshold AKlh HCF where and and Kmax allowable LCF AK HCF f 1 - R r steady vibratory steady vibratory CTsteady maximum operating stress neglecting vibratory stress vibratory 1 2 peak to peak vibratory stress AA HCF f R temp. Overspeed residual strength requirements need not be considered for those part locations limited by cracks reaching a calculated vibratory threshold. One overspeed cycle occurring at a crack size equal to the vibratory threshold creates less damage change in crack size than additional LCF-HCF crack growth from the vibratory threshold to a maximum stress intensity KCRIT defined by the material fracture toughness. For those locations not limited by vibratory stress concerns the part s maximum allowable crack size should be limited to a size that will survive the maximum design stress that occurs on the last cycle of the calculated safety limit. REQUIREMENT GUIDANCE The flaw growth interval is also known as the safety limit. It is recommended that the .
