tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 49

High Cycle Fatigue: A Mechanics of Materials Perspective part 49. 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. | 466 Applications It is clear then that if the pressure is sufficient to cause yielding of the cylinder due to hoop stresses beyond the elastic limit this yielding will take place first at the inner diameter. The residual stress state that results from plastic straining will depend on the cylinder geometry material strain-hardening characteristics unloading behavior of the material and the level of strain hardening imposed. A typical circumferential stress state near the inner diameter see 70 for example is depicted schematically in Figure where the tensile stresses due to loading are added to the residual stresses the latter being compressive. The net stresses can then be used to calculate the resultant value of K the stress intensity using methods such as weight functions as done in 70 . In that article the authors point out that the computed stress intensity is the sum of the contributions due to the internal pressure loading the residual stresses and the internal pressure acting on the crack faces. The resulting values of K can be as depicted schematically in Figure where the value of K decreases with increase in crack length near the inner diameter. Thus it is possible for a crack to initiate but arrest due to the decrease in K with crack length as depicted. As long as the stress intensity is below threshold there should be no further crack extension. In determining the threshold for crack propagation two issues should be considered. First it should be established whether the applied cyclic loading is in the LCF or HCF regimes. If it is in HCF involving very large cycle counts then the use of AKth is warranted. On the other hand if the loading is LCF involving a low number of cycles such as start up and shut down of a mechanical system then the use of AKth can be conservative because AKth typically corresponds to a growth rate of 10-10m cycles see Section . A value of AK corresponding to a higher growth rate would suffice provided that the total .