English: SEM images from the aft wall to the forward wall of the web were used to measure striation spacing as the crack progressed in low-cycle fatigue. Striation density measurement data were used to calculate the estimated number of flight cycles that elapsed in the low-cycle fatigue region beyond the point of detectable crack initiation, which is the size of a crack that can be detected using a nondestructive inspection method. FAA Advisory Circular 33.70-1, "Guidance Material for Aircraft Engine Life-Limited Parts Requirements," stated that this crack size has a depth of 0.015 inch and a surface length of 0.030 inch. The NTSB assumed that one striation correlated with one flight cycle and that the forces during takeoff, when the stresses on the web would be at their highest, were advancing the crack. The calculation showed that the crack progressed by low-cycle fatigue for about 5,400 cycles beyond the point of detectable crack initiation. (This cycle count did not include those cycles that accumulated in the transgranular region up to the 0.015-inch crack depth.)