tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 21

High Cycle Fatigue: A Mechanics of Materials Perspective part 21. 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. | 186 Effects of Damage on HCF Properties A load-history-free material threshold is equivalent to precracking at threshold. This load-history independent threshold can be calculated from the model by setting AKpc equal to the AKth. This assumes that the load-history-free threshold would be measured if the LCF cracking could occur at threshold. This substitution gives AKtn 1 - R h AKeff 0 - P - PRth Experimentally the history-free threshold is measured using stress-relief-annealed specimens. AK thresholds calculated from the model Equation are MPaVm for R and for R for Ti-6-4 and for R for Ti-17. These compare favorably with the experimentally measured thresholds of and for Ti-6-4 at R and R respectively and at R for Ti-17. To further investigate the load-history effect on threshold outlined above from the work of Moshier et al. 4 6 Golden and Nicholas 43 followed the same procedure but precracked at an even lower stress ratio of R . They examined LCF-HCF interactions at negative values of R under both smooth bar conditions where LCF generated cracks are difficult to detect and may not even exist and under notch fatigue where cracks were deliberately introduced and detected. Their investigation explored the nature of the crack initiation threshold crack propagation and any associated load-history effects when a specimen is initially subjected to loading at negative stress ratios. Two types of experiments were conducted. In the first series of tests smooth bars were loaded at R in order to see if this would initiate cracks which might affect the subsequent threshold at a higher value of R. In the second series of tests LCF cracks were deliberately introduced with loading at R -3 and the subsequent threshold was determined experimentally. The effect of loading history was assessed for both test procedures using Ti-6Al-4V as the test material. The smooth bar specimens were tested .