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High Cycle Fatigue: A Mechanics of Materials Perspective part 60

High Cycle Fatigue: A Mechanics of Materials Perspective part 60. 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. | 576 Appendix G Figure G.22. Level 2 front surface. Figure G.23. Simulated FOD using light gas gun impact. several velocities created the impact sites. The microstructural changes aside from the scale of damage were consistent for all specimens. The primary microstructural features due to impact damage were a compressed microstructural zone and the formation of adiabatic shear bands. The presence of adiabatic shear bands is an important indication that ballistic FOD simulation accurately represents the high rate deformation process seen during actual FOD. Adiabatic shear bands form only during high rate deformation by effectively melting and re-solidifying the metal resulting in a different grain structure. The presence of shear bands around the impact of a spherical projectile has been noted by some studies 7 8 . Roder et al. 8 examined the damage caused by the impact of hardened steel spheres fired at 309 m s at a flat plate and mapped the distribution of shear Appendix G 577 bands. As can be seen from Figure G.24 their orientation is such that they could increase the susceptibility to fatigue crack growth. Figure G.25 is a back-scattered electron image showing an area of the edge of a V-notch produced by firing a cube projectile at Ti-6AL-4V plate. The damage strongly resembles that seen on RB199 fan blades displaying shear bands with evidence of Figure G.25. Edge of ballistic damage on plate. 578 Appendix G void opening. The microstructure is similar to the schematic representation shown on Figure G.24. The final type of simulation is engine debris ingestion. This method is mentioned here only for the sake of completeness. Engine debris ingestion is prohibitively expensive and of little scientific value. However it is the only method that accurately simulates the effect of FOD on an entire engine. Specimen design Once the appropriate impact procedure is selected the next step is to determine which specimen geometry will be used. In the process of specimen design