English: Examination of the flat fracture region using a scanning electron microscope (SEM) showed that the region had an intergranular appearance near the crack initiation site (the aft face of the web) and a transgranular appearance farther away from the initiation site. (The disk material, made from a nickelbased [sic] superalloy, had an internal structure that consisted of microscopic crystals called grains. The planar intersection of two grains is called a grain boundary. Cracks that follow grain boundaries are intergranular and cracks that cut across grains are transgranular.) The transgranular regions exhibited striations (linear features on a fatigue fracture surface that indicate how far a crack advanced with each stress cycle) consistent with low-cycle fatigue crack growth. (In airplane engines, low-cycle fatigue is associated with one striation per flight cycle.) Figure 7 [Optical image of underside of HPC stage 8 disk rim] shows an image of the underside of the HPC stage 8 disk rim at the location shown in figure 6 [Location of crack in HPC stage 8 disk web].