tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 16

High Cycle Fatigue: A Mechanics of Materials Perspective part 16. 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. | 136 Introduction and Background and carbon normalized. While most of the materials show an increase of fatigue limit strength as a function of frequency carbon steel and to a lesser extent Armco iron show a maximum stress at a frequency of approximately 10 kHz. The early results of Jenkin 72 were obtained using a resonance technique on simply supported beams excited electromagnetically but the specimen dimensions were much smaller than those used by Lomas et al. thereby producing a range of resonant frequencies that were much higher. Later Jenkin and Lehmann 73 further extended the frequency range by using even smaller bend specimens excited pneumatically with pressure pulses. The first experiments of Jenkin 72 on copper Armco iron and mild steel were able to easily cover a frequency range up to 1 kHz. Their results showed that materials . similar to those tested gain slightly in their strength to resist fatigue as the speed goes up but for most practical speeds the gain is insignificant. Jenkin and Lehmann 73 achieved frequencies up to nearly 20 kHz and showed significant frequency effects. They question however their own mathematical analysis of the strains based on beam deflection contributed by Prof. Love and the purely elastic behavior of the materials which might affect the smaller bar higher frequency tests in a systematic manner. The results of the tests by Lomas et al. 71 shown in Figure were obtained from resonance tests on cantilever beams of different dimensions in order to achieve a wide range of natural frequencies. Of note is the characteristic shape of the curves where the fatigue limit strength corresponding to 108 cycles increases with frequency up to a maximum and then decreases with further increase in frequency for almost all of the ferrous alloys tested. As pointed out by the authors the grave difference in the results with those of Jenkin is that the peak is obtained at a very different frequency. In Figure the peak is .