High Cycle Fatigue: A Mechanics of Materials Perspective part 20. 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. | 176 Effects of Damage on HCF Properties All dimensions in mm Figure . Double-notch-tension specimen geometry and crack shape nomenclature. surface Figure . Schematic of LCF-HCF loading history and nomenclature. LCF-HCF Interactions 177 400----------1-----1----1--1 i i r 300 A 200 O Rlcf a- 430 MPa A Rlcf a- 265 MPa -------Kmax MPaVm R a c Fit ------- R Fatigue limit stress 107cycles rhcf 0 1 100 -------------1-------1----1----1--1--1 1------------- 10 20 50 100 200 c gm Figure . HCF thresholds at R on a Kitagawa type diagram. c gm Figure . HCF thresholds at R on a Kitagawa type diagram. The horizontal line in each figure represents the experimentally determined endurance limit of the uncracked specimen corresponding to 107 cycles. It can be seen in both figures that the data obtained using LCF at R circles show what appears to be equivalent to an overload effect since the threshold values of stress are consistently above the extrapolated long crack threshold. Conversely data obtained using LCF at R triangles tend to fall slightly below the projected long crack threshold the type of effect being representative of what one would expect when a material sees an underload during prior cycling. These results seemed to indicate that an overload type condition LCF at R retards the subsequent HCF propagation while an underload LCF at R might tend to slightly accelerate the subsequent HCF propagation. The apparent overload effect from precracking at a higher stress than that required to propagate the crack 178 Effects of Damage on HCF Properties under HCF is the result of multiple overloads since constant amplitude loading was used to precrack the specimens. The apparent multiple overload effect is consistent with observations such as those of Frost 41 who noted that cracks formed by precracking at a higher alternating stress are all stronger than expected and that the propagation stress stress to obtain AKth is
