tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 18

High Cycle Fatigue: A Mechanics of Materials Perspective part 18. 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. | 156 Effects of Damage on HCF Properties a b Figure . HCF limit strength with and without LCF pre-damage a Rhcf 0-5 b Rhcf 0-1 c rhcf -8- LCF-HCF Interactions 157 Figure . Continued . to be higher than the baseline. The observation that they were lower suggests that there might be an effect of pre-damage due to LCF but the magnitude is not significant. In Figures c results are shown where LCF was applied at a stress of 900 MPa on a titanium alloy that has a yield stress of 930 MPa. In this case strain ratcheting occurred during the LCF portion of the test but no ratcheting was observed in the HCF portion of the test under step-loading conditions. Morrissey et al. 16 have reported that timedependent creep or ratcheting may play an important role at high stress ratios where a portion of each cycle is spent at stresses near or above the static yield stress of the material. They observed measurable strain accumulation dependent on the number of cycles as opposed to time at the stress ratio of where the applied maximum stress was slightly above 900 MPa. The fracture surfaces for the cases where strain ratcheting was observed in the tests summarized in Figure did not show any indication of this phenomenon. This is in contrast to the observations of Morrissey et al. 16 where ductile dimpling was observed on fracture surfaces on specimens tested at R with maximum stresses above approximately 900 MPa. In fact over the range of conditions studied in 14 there appeared to be little or no effect of prior LCF on the subsequent HCF limit stress. If any cracks formed during LCF they were not observable through fractography and were not of sufficient size to cause any significant reduction of HCF limit stress under subsequent 158 Effects of Damage on HCF Properties testing. So for conditions that covered a range from overloads to underloads from LCF there was no observable effect on the subsequent HCF limit stress in a titanium alloy. A similar .