tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 19

High Cycle Fatigue: A Mechanics of Materials Perspective part 19. 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. | 166 Effects of Damage on HCF Properties process. The proposed answer to this question is based on the concept of a weakest link theory where the location of the fatigue failure initiation point is a random variable in space. However in precracking of the material in the case of C-shaped specimens and particularly in the case of notched specimens the process finds the location of the weakest link and initiates the crack in that location. Threshold testing then samples locations in a larger region but the weakest link has already been identified. Thus the crack continues to propagate at the precrack location once the threshold stress is reached. Further evidence of the relatively small effect of a short crack on the FLS can be found in the work of Lanning et al. 8 where prior LCF loading was followed by step testing to establish the HCF limit stress corresponding to a life of 106 cycles. Results for the FLS corresponding to 106 cycles as a function of the number of LCF cycles are shown in Figure for specimens with small notches having kt . The full LCF life was 10 000 cycles at the stress used in the precracking 609 MPa. Many specimens were heat tinted after LCF loading so the final fracture surfaces could be examined for any cracks formed during the initial LCF loading. One crack was found which was formed in a small notch specimen after 2500 LCF cycles at 25 of LCF life. The crack had a depth of approximately 25 m and a width of about 250 m. This crack corresponds to the lower of the two data points in Figure at 2500 LCF cycles. Since a crack was not observed in the specimen with the data point at a higher HCF fatigue limit which is only greater by 10 MPa it appears that a crack of this size may not be significantly detrimental to the HCF limit stress. Note however that a crack with lesser depth perhaps only half of that found would not be easily detectable using the heat-tinting method. 1000 B 800 CL S f 2 600 to E L 400 o ZE CD 2 200 i i i i i i ii i