tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 37

High Cycle Fatigue: A Mechanics of Materials Perspective part 37. 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. | 346 Effects of Damage on HCF Properties The energy plotted in the figure represents the input kinetic energy only. There was no information obtained experimentally about the residual velocity of the spheres. If it is assumed that the spheres did not rebound then the energy plotted can be considered to be an upper bound of what went into deforming or fracturing the target. At any given energy level the figure shows that there is a lot of scatter in the permanent depth. Careful examination of the individual points revealed that at the highest energy level obtained with the mm diameter spheres at 305 m s impact velocity most of the leading edge specimens were chipped or fractured and showed what has been termed loss of material LOM 9 13 . Such a condition is shown in the SEM photo Figure a . The points having such a fracture appearance lie to the right and below the trend line of the quasi-static data. Some of the data points to the left had only permanent deformation as seen in the SEM photo in Figure b . This shows that the energy required to produce a fracture is less than that needed to produce a crater of equivalent size through a b Figure . SEM photos of indents showing a loss of material and b indentation. Foreign Object Damage 347 plastic deformation. A similar observation was made for the data points to the right at an energy level of approximately J which represent the mm diameter sphere impacts at 305 m s. Here again chipping and local fracture led to the formation of the crater rather than extensive plastic deformation. Thus the depth of penetration of all of the impacts can be correlated well with impact energy as would be expected except for cases where fracture occurred locally. In that case the depth of penetration is generally higher and represents the worst case scenario regarding fatigue strength debit as noted also in previous research 9 13 . Of significance is the observation that the two types of permanent distortion .