tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 13

High Cycle Fatigue: A Mechanics of Materials Perspective part 13. 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. | 106 Introduction and Background that random construction of a staircase sequence will provide similar results particularly for the mean value of the FLS. In preparing data for a publication Morrissey and Nicholas 45 used the bootstrap method to determine the FLS of Ti-6Al-4V for a cyclic life of 109 cycles based on available test data some of which were obtained from tests to only 108 cycles or even less. In this artificial staircase procedure using the bootstrap method every data point was treated in one of the following three manners and every data point was used once 1. If a specimen failed at a given stress level in less than 108 cycles for example then that data point provided a failure point for 109 cycles at the same stress level. 2. If a specimen failed within 109 cycles at a given stress level then that specimen provided a failure point at any of the higher stress levels used in the staircase sequence that is at any number of stress increments fixed above the stress level at which the original test was performed. 3. If a specimen survived for 109 cycles at a given stress level then that specimen provides a survival point for any stress level lower than the one at which it was actually conducted. Using this logic and using all of approximately 20 data points several different artificial staircase test sequences were constructed from the available data pool. Surprisingly the mean value of the FLS from each sequence used was reproducible to within 1 MPa for a FLS of approximately 510 MPa. There is no rigorous mathematical basis for applying the bootstrap method to the construction of an artificial staircase test sequence using the three rules stated above but the consistency of the values for the mean suggests that the method is both viable and reliable when testing time machine availability or limitation in number of specimens available becomes a practical limitation in conducting a real staircase test sequence. . OTHER METHODS In addition to step and .