tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 7

High Cycle Fatigue: A Mechanics of Materials Perspective part 7. 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. | 46 Introduction and Background Static breaking stress ji z Tension Zero stress Compres sion 0 1 Figure . Goodman diagram as shown in Goodman 1899 20 on p. 455. Figure . L-W equation for R 0 plotted on a Haigh diagram. Characterizing Fatigue Limits 47 from the original Goodman equation at R 0 where allla . The curve is closer to a Gerber parabola than a straight line in that region but may not have any more physical significance than the other two representations of experimental data. It can be seen that the development of methods to represent fatigue limit data was based on a combination of mathematically simple forms as well as graphical representations. While there seemed to be a physical basis of some sort for all of the approaches combined with simplicity it should be pointed out that the choices of methods of representing data were scrutinized throughout their development. A particularly insightful example of such a critique was made in 1880 26 although similar words would be equally valid today. Commenting on the W-L formula used to fit Wohler s data Smith writes In fact the formula seems to be more the product of pure imagination than to be based on the experiments. He further points out that the formula can of course be made to agree with the extreme results of any particular set of experiments. What I and I dare say some others would like to know is whether this formula agrees with even a rough degree of approximation with the intermediate results. He goes on to question whether the parabolic curve. is the correct curve. and sums up I submit that the proper thing to do is not to coin rules out of imagination but to persevere in careful and patient experiments and to watch narrowly and to study and analyze closely the results until the true intimate laws of the stress and fatigue of metals are revealed. . HAIGH DIAGRAM AT ELEVATED TEMPERATURE In the final program on HCF material .