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High Cycle Fatigue: A Mechanics of Materials Perspective part 29

High Cycle Fatigue: A Mechanics of Materials Perspective part 29. 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. | 266 Effects of Damage on HCF Properties the contacting surfaces. It was shown by Tomlinson et al. 4 as early as 1939 that if relative motion slip occurs even at a level as small as 10-6 in. 0.025 m fretting will result. One rather ironic example of a fretting-fatigue failure is that experienced while conducting fretting-fatigue experiments in the laboratory. Figure 6.6 is a schematic of the upper part of a dovetail fixture used to conduct fretting-fatigue experiments 5 . The fixture is free to rotate because it is held by a pin which in turn is held in a clevis which supports it as shown in the figure. Similar to the case of the riveted joint described above small relative motion can occur between the pin and the fixture in the region shown as a thick line in the figure. After numerous tests to typical cycle counts of 106 or 107 per test the very large number of cycles approaching one billion eventually produced failure of the grip as depicted in Figure 6.6. This ironic example is one where fretting-fatigue failure of the grip assembly halted the conduct of real fretting-fatigue experiments. Another scenario where fretting-fatigue failure can occur is at a bolted joint as depicted schematically in Figure 6.7. While such a failure mode is rare it is not nonexistent. In a set of rotating components bolted together excessive vibration of one of the components can lead to cyclic loads that produce a combination of normal shear and transverse loads in the contact region shown as the thick line in the figure. These loads combined with the initial static axial load in the bolts due to tightening can lead to fretting-fatigue crack initiation in the bolt and potential failure of the bolt as depicted by a crack initiating in Figure 6.7. In this example and in all of the prior examples the conditions that produce fretting-fatigue cracks are assumed to be ones where partial slip occurs in the contact region. This is in contrast to the condition of total slip that is produced .