High Cycle Fatigue: A Mechanics of Materials Perspective part 27. 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. | 246 Effects of Damage on HCF Properties Figure . Comparison of predictions and experimental data for notch fatigue data in Ti-6Al-4V 29 . distance ac was and mm for R -1 and respectively. What makes this approach work so well is the fact that GF turns out to be a constant for a large body of data. This is analogous to finding a critical distance which is a constant for a material. In this case the critical distance is computed from Equation and is dependent on both stress ratio R and notch root radius p. It is also shown in 29 that the fatigue notch factor can be obtained from the following equation. kf k A0 s A03 2 1 s . ANALYSIS METHODS In the work of Nisitani and Endo 16 the stress field ahead of an elliptical hole in an infinite plate subjected to remote tension as shown in Figure is reported as m4 3 m2 m2 - m - 3 m 1 W -------- m - 1 m2 - 1 ------- 5 31 Notch Fatigue 247 I I Ia- I I Figure . Schematic of elliptical hole in tension. O- a x y ap 2ax x2 m kt 1 2m The authors note that the branch point in Figure where a notch starts to behave like a crack has a constant root radius p independent of notch depth for a given material. This constancy is attributed to the fact that the relative stress distribution near a notch root is determined by the notch root radius p alone. From experimental results and the analysis of a crack at the root of an elliptical hole they demonstrate that the crack initiation limit curve and the crack propagation limit and their intersection at the branch point see Figure can be determined by using only ama at the notch root and the notch root radius p. The determination of these quantities however is based on fitting of experimental data to the theoretical curves. The unified treatment proposed in 16 can be summarized in a schematic of their approach shown in Figure . The limiting values of the nominal stress times kt for crack initiation and propagation are .
