Originally Posted by
DaveReidUK
It's hard to imagine any material or geometric reason why a turbine disc failure should necessarily involve separation into 3 neat 120° segments.
There is some very simple physics why a disc failure will result in three roughly equal fragments, with none substantially larger than 120° (the largest on your example is 130-140°). Failure originates from a single point, with a crack propagating through the entire cross-section. Once the crack is complete, the integrity of the ring is compromised, and centrifugal forces will start to bend it outwards. The maximum stress due to these forces is not at the side opposite to the initial failure, but at the two sides, roughly 1/3 the way, where two more cracks will form. Of course all this happens in less than a millisecond, and things like lost/damaged blades and speed of rotation will determine the exact outcome. Due to interaction with other components the initial pieces may or may not fracture any further, but you will always start out with three roughly similar sized fragments. This also means that only one of the three (or subsequent smaller fragments) will be on a trajectory towards the fuselage. Of course if the bearing fails, and the entire disk goes AWOL, that is a different story.