High Cycle Fatigue: A Mechanics of Materials Perspective part 6. 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. | 36 Introduction and Background extrapolating data to the required life often 107 cycles. A schematic diagram of S-N data obtained at different values of R constant is presented as Figure . Here points A B and C represent the FLS either directly from the data or by extrapolation to 107 cycles in this case. In Figure the stress is plotted as the maximum stress although the stress range or alternating stress could also be used. Each of the data points can then be used in a constant life plot such as a Haigh diagram as depicted in Figure . For each data point the mean stress and alternating stress have to be computed for the given value of R. The Haigh diagram Figure represents the locus of points having a specified number of cycles to failure for example 107 as illustrated in the example. The number of cycles is typically taken to be those corresponding to a run-out condition perhaps 108 or even 109 but there are few data available to demonstrate that a true run-out condition ever exists for a material. Rather the cycles chosen can be either in a region where the S-N curves of Figure become nearly flat with increasing number of cycles or where the number exceeds the number of cycles that might be encountered in service. In some Figure . Schematic of S-N curves obtained at constant values of R. Figure . Schematic of Haigh diagram constructed from S-N data obtained at constant values of R and extrapolated to 107 cycles. Characterizing Fatigue Limits 31 cases neither condition may be satisfied. The points A B and C from Figure are shown in the Haigh diagram Figure corresponding to stress ratios R of and -1 respectively. In this plot alternating stress half of the stress range is plotted against mean stress. Lines corresponding to these stress ratios are also shown with the vertical axis being the R -1 line for fully reversed cycling. Although the Haigh diagram is often presented or used with a straight line to represent .
