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High Cycle Fatigue: A Mechanics of Materials Perspective part 9

High Cycle Fatigue: A Mechanics of Materials Perspective part 9. 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. | 66 Introduction and Background a a a W W Initiation Nogrowth Growth Figure 2.40. Schematic of initiation growth dilemma for fatigue under negative R. total stress or strain range will occur at some critical value of Aatot as shown. At those same stresses the positive stress range may be below the threshold for crack propagation. It is only when the crack driving force due to positive stresses only Aopos exceeds the threshold that the crack will continue to grow. Thus there may be a condition where cracks initiate but do not propagate if the loading applied is incrementally increased such as in a step-loading sequence. An alternate and more plausible explanation without any direct evidence is that the stress intensity needed to propagate an existing crack becomes smaller as the amount of compression increases. Of some relevance is the observation by Moshier et al. 35 that LCF cycling at R 0.1 produced an overload type effect on the subsequent HCF threshold when the HCF peak stress was lower than that in the LCF precracking but under similar conditions there was no overload effect using R -1 for the LCF. Stephens et al. 36 found compression overloads to be either detrimental or have no effect on fatigue life meaning that lower loads would be necessary to produce the same crack growth rate after a compression overload. Although this deals with growth rates Lang and Huang 37 found that KPR the crack propagation stress intensity factor decreases with increasing level of compression overload. For the same maximum K this means that a lower stress range is needed to propagate the crack. Another similar finding is that of Lenets 38 who showed that a compressive overload allows resumption of crack growth under compression cycling of a previously arrested crack in an aluminum alloy. With these various observations and the present data it seems reasonable to deduce the reasons behind the shape of the Haigh diagram at negative R Figure 2.38 namely the relatively flat .