High Cycle Fatigue: A Mechanics of Materials Perspective part 23. 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. | 206 Effects of Damage on HCF Properties the LCF cycles can be considered to be an underload on the baseline HCF cycles. On top of this simple spectrum a single overload is added to the baseline LCF cycle. Defining the overload ratio as OLR Kmax Kss experiments were carried out to see when the onset of HCF activity occurred. The results based on the use of a number of values of OLR are presented in Figure . The curves shown are the best fit to the actual data points which are not shown for clarity. OLR 1 represents the case where there is no overload in the baseline LCF-HCF cycle. The pure LCF curve is also shown. The results show that as OLR increases the retardation effect of the single overload diminishes the growth rate until the minor cycles have almost no influence on the baseline LCF cycle at a value of OLR . This work conducted on Ti-6Al-4V also shows that the apparent onset of HCF activity is delayed by the overload cycle. In this case there is both an underload in the baseline combined cycle as well as a superimposed overload. While the behavior of the baseline cycle defined by OLR is predictable with linear summation of the LCF and HCF cycles the additional effect of the overload is both to reduce the minor cycle contribution to the growth rate and to reduce the threshold where minor cycle activity begins. The natural conclusion arising from these studies is that growth rates and thresholds from constant amplitude loading cannot always be used directly in spectrum loading without consideration of interaction effects. Both retardation and acceleration effects have been noted in various studies with overloads usually observed to retard crack-growth rates while underloads are found to accelerate the growth rates. While these observations are common the exceptions prove that a single rule cannot be applied in all cases. In 52 HCF and LCF tests were used to establish baseline material properties and simple mission tests were used to assess .
