High Cycle Fatigue: A Mechanics of Materials Perspective part 44. 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. | 416 Applications a value of AKth MPa m at R was found. For the constant Kmax increasing Kmin test a value of AKth MPa m at a final load ratio of was obtained. They conclude that this latter value corresponds to a lower bound threshold for small crack propagation even though the test is conducted on a long crack specimen. By comparison for the identical material Ti-6Al-4V small crack growth behavior from naturally initiated and FOD-induced small cracks was not observed below a AK of . . Compression precracking The concern for history effects in determining a threshold that can be used for engineering purposes in design has led to considerations of alternate techniques. Forth et al. 24 present data using the conventional load-shedding technique recommended by ASTM at constant R and constant Kmax on 7075-T7351 aluminum. In addition they tested using a compression precracking scheme to avoid development of plasticity-induced closure during the precracking. From the results for this particular material and from other reported observations they conclude that the threshold and near threshold growth rates obtained during the K-shedding portion of a test are not necessarily the same as those obtained under constant load increasing K tests. In their work constant load levels were maintained after reaching threshold. Although these tests are generally conducted at the load level and K level where the decreasing K tests were terminated at the defined threshold of 10-10m cycle there are many cases where the load has to be increased in order to start the crack growing again. In the work of Forth et al. 24 however the crack grew immediately. In their case they conclude that remote crack closure may still be present in decreasing K tests dependent on the load history thus producing an artificially high threshold. Further oxide buildup is also observed when plane stress conditions occur at the specimen surface. By comparison the authors introduced a
