Complex sentence: The damage was not enough to cause an immediate failure; a fatigue crack developed and got worse with each takeoff and landing over eight weeks, until Flight 191, when the damaged rear pylon mount reached the breaking point and failed.
Split into simpler sentences: The damage was not enough to cause an immediate failure. However, a fatigue crack developed and expanded slightly with each takeoff and landing over eight weeks, until Flight 191, when the damaged rear pylon mount reached the breaking point and failed.