RBF:
So either:
the IP overspeed happens near instantaneously on decoupling
the HP gas flow is sufficient to overspeed the decoupled IP turbine
An overspeed would be a transient event. The structure certification is required to show that an overspeed:
"Section 33.27(c)(2)(v) states that rotor structural integrity must be demonstrated at: “105% of the highest speed that would result from failure of the most critical component or system in a representative installation of the engine.” This single component may not necessarily be in the rotor itself; instead, it may be some part of another engine sub-system that, upon failure, could cause a rotor overspeed. Examples of such components might include certain key elements of the engine control system and its related sensors. A failure modes effect analysis (FMEA) or other similar engineering assessment would generally be required to identify such components". (AC 33.27-1, AC 33-2B, AC 33-3 )...
FDR