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High Cycle Fatigue: A Mechanics of Materials Perspective part 24

High Cycle Fatigue: A Mechanics of Materials Perspective part 24. 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. | 216 Effects of Damage on HCF Properties r d Figure 5.3. Values of kt for U-shaped notch. 5.4. FATIGUE NOTCH FACTOR The fatigue behavior of a notched component is not necessarily governed solely by the maximum stress at the notch root. In general the fatigue life or fatigue strength is greater in a notched component based solely on the stress at the notch root compared to a smooth bar with that same stress distributed uniformly over the entire cross section. Many researchers over the years have attributed this observation to the concept that fatigue behavior depends on the stresses or strains over a critical volume of material. Thus as the size of a notch becomes very small the fatigue behavior tends to be governed more by the far-field stresses or average stresses. The notch root radius also plays a significant role in determining the fatigue limit. In the limiting case as the notch root radius decreases to zero the notch will behave like a crack as discussed later. On the other hand as the notch root radius becomes very large the fatigue behavior is governed more by the peak stress at the notch. In the latter case in the limit the stress is given by the far field stress multiplied by kt. From these observations and from experimental data developed over many years equations have been developed for predicting the fatigue behavior in a notched component based on fatigue behavior in smooth bars where the majority of databases can be found. The fatigue notch factor kf is defined as the ratio of the fatigue strength in a smooth bar to the fatigue strength in a notched bar that is unnotched fatigue limit stress kf 7 - . 7 notched fatigue limit stress 5.1 This equation is defined in the strictest sense for fully reversed loading R -1 as well as for a specific number of cycles and a specific specimen geometry. The fatigue Notch Fatigue 217 notch factor generally covers the range 1 kf kt indicating that use of kt rather than kf can be overly conservative to describe the .