tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 45

High Cycle Fatigue: A Mechanics of Materials Perspective part 45. 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. | 426 Applications FODed undamaged Figure . FOD HCF probability diagram. value for a normal distribution or some other measure of spread in the distribution curve is needed for a robust probabilistic design as indicated in the figure. The diagram also shows a hypothetical distribution of fatigue strength of a material subjected to FOD FODed strength . Again both the mean and the spread of the distribution of material strength and FODed strength are important for design purposes. This representation is purely conceptual so the amplitude and shape of each of the distribution functions are not intended to represent any real distribution nor to be related to each other in any manner. The schematic shows that there can be a region where the FODed strength is below the minimum or allowable fatigue strength of the undamaged material. Such a low strength of FODed material may be produced by FOD from stones metal pieces or hard particles as indicated in the figure. These damaging objects will generally produce more severe damage in terms of reduction of fatigue strength than smaller objects such as sand and dust. These latter objects may reduce the fatigue strength only slightly. The type and number of potential particle impacts will therefore provide the input for determining or estimating the overall shape and limits of the FOD strength distribution curve. As in the case of the undamaged material strength the spread of the distribution curve as well as the mean are of importance in a probabilistic design. Similar to the method for accounting for notches in fatigue the material allowable would have to be reduced by a factor kf so that the minimum fatigue strength of the FODed material point A in the figure is at or above the reduced design strength of the material in order to minimize the possibility of failure. This constitutes moving the undamaged fatigue strength distribution curve over by a factor of kf as shown in Figure even though the entire curve will be .

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