High Cycle Fatigue: A Mechanics of Materials Perspective part 28. 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. | 256 Effects of Damage on HCF Properties where aA is the fatigue strength at an arbitrary mean stress and depends not only on temperature but on time in order to account for the time-dependent behavior. This formula is based on the assumption that the fatigue ratio is a function of the creep rupture strength only. The formulation goes on to use a universal empirical relation between the fatigue ratio Vr and the normalized creep rupture strength r Dt T r --- F 20 C The empirical relationship is of the form Vr Arr-aR By applying this relation to data at two values of stress ratio R -1 fully reversed loading at zero mean stress and R 0 pulsating tension the coefficients Ar and aR can be obtained. From these the fatigue strength of a smooth bar at R -1 and R 0 can be determined at any temperature in the creep regime. At that temperature the Haigh diagram is plotted as shown in Figure where the two quantities representing the fatigue strengths at two values of R are connected by a straight line. Using the recommendation of Forrest 20 the diagram is cut by a vertical line through the creep rupture strength indicating a weak interaction between fatigue and creep damage. To apply this diagram to a notched component consideration has to be given to the fact that stresses at the notch root ktSm where the geometry can be defined by an elastic stress concentration factor kt will relax with time toward the nominal fictitious mean stress level Sm. The fatigue loading is then considered to be allowable by comparing the nominal mean stress level Sm and the alternating stress at the notch with the FLS based on smooth bar tests as illustrated in Figure . The lower curve is that shown in Figure when the value of kt is not known. According to Forrest 20 it is reasonable to assume that the mean stress component will not be affected by a notch and that the alternating stress component will be reduced by the fatigue notch factor kf. If kt is known kt can be .
