TAILIEUCHUNG - High Cycle Fatigue: A Mechanics of Materials Perspective part 22

High Cycle Fatigue: A Mechanics of Materials Perspective part 22. 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. | 196 Effects of Damage on HCF Properties . LCF-HCF nomenclature Combined LCF-HCF loading has caused some confusion over the years because of nomenclature and the differences that occur when dealing with actual usage versus mathematical formulations involving simple linear summation concepts. Consider the schematic of Figure that shows the major cycles LCF with a hold or dwell time in between them during which HCF cycles can occur. The problem we deal with is the superposition of the HCF cycles on the LCF behavior. The nomenclature is that of Powell and coworkers at University of Portsmouth formerly Portsmouth Polytechnic. LCF loading alone produces a stress intensity level AKm_l ol. and represents the contribution of the major throttle excursions in an engine. During dwell times at maximum K for the LCF cycle a vibratory loading occurs whose total amplitude is denoted by AKmilior. If the total contribution of the individual loading components is considered then a linear summation law would require that the total growth rate be the sum of AKtotal AKminor not AKmajor AKminor. This is because in the combined case the effective amplitude of the LCF cycles is AKtotal not AKmajor since the LCF cycle now goes from minimum to maximum through an amplitude AKtotal. This concept while fairly clear has not been followed consistently in the literature over the years and has led to some confusion when evaluating the applicability of a linear summation concept for crack growth under combined LCF-HCF loading. The confusion arises because the minor cycle amplitude also contributes to the major cycle amplitude as shown in the schematic of Figure . Another point to consider in discussing combined LCF-HCF referred to here as CCF is the manner in which the data are both recorded and discussed. While the stress or load ratio R is a common and well-understood quantity there are other parameters with which to describe the entire load sequence in CCF not including spectrum .

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