tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 11

High Cycle Fatigue: A Mechanics of Materials Perspective part 11. 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. | 86 Introduction and Background Figure . Comparison of number of cycles in last loading block of step test for HCF with and without LCF precrack. uncracked specimen. This observation is similar to the speculation in Chapter 2 where tests at negative R are thought to initiate cracks at stresses below the failure stress when using step loading. Additional data are available on the number of cycles on the last loading block in the step-loading tests when a crack is present. The data of Figure are supplemented with additional data on precracked notch specimens and replotted in Figure as a function of crack size. While very small crack lengths did not show a very well Figure . Number of cycles in last block for notched bars with LCF cracks. Accelerated Test Techniques 87 2 x 106 z a n 0 x 100 0 x 106 1 x 106 5 x 105 Ti-6Al-4V 2 x 106 cycle blocks C specimen . - - - 1 1 1 1 1 1 1 1 1_ _l 1 1 1 1 1 1 l_ 50 100 150 Crack depth microns 200 Figure . Number of cycles in last block for C-specimens with LCF cracks. defined threshold the longer crack lengths showed that specimens typically failed early in the block within a million cycles indicating that the precrack eliminated some of the initiation life. A similar observation can be made for the data in Figure which are obtained on a C-shaped specimen which was originally a pad in a fretting fatigue experiment where cracks were formed on the pad in the contact region 31 . From earlier observations the HCF load block was reduced from 107 to 2 x 106 cycles. Here again most of the failures occur within a million cycles indicating that initiation life in the step tests is reduced or eliminated. Of significance is the observation that in both series of tests uncracked specimens failed at a random number of cycles up to 107 which was the loading block used in the step tests in both cases for uncracked specimens. Another example of the small number of cycles in the last block of step testing can be seen