Pratt & Whitney alerted the media mid-day Sunday that an F135 engine had been damaged during qualification ground-testing, and quickly held a press call with a "program expert". Here's essentially what he said.

The CTOL engine was 2,455 cycles into a 2,600-cycle durability test leading to initial service release when "sparks were noticed coming out of the jet pipe". The engine was still running and capable of producing thrust when it was shut down.

Inspection revealed tip damage to a "handful" of blades on the first and section fan stages, which are integrally bladed rotors (IBR). Downstream damage was confined to the compressor. There was no visible damage to the combustor and turbine.
 
Pratt is working to identify the root cause, which could be foreign object damage - something coming from outside the engine - or "domestic object damage" - something failing inside the engine, perhaps due to a manufacturing defect or durability issue.

The F135 was 5 hours into an 11-hour supersonic high-cycle fatigue test and was "being pushed very hard" through a sequence of throttle transients. For this test, the inlet-plenum hardware on the test stand had been changed to run at supersonic conditions. 

Pratt says inspection of the inlet hardware is underway, but no missing pieces such bolts or seals have yet been found. The engine is being torn down. "There is something at the bottom of the engine [between the fan and compressor] that we want to retrieve," says the program expert.

On the impact to the F-35 program, the expert says the Joint Program Office is estimating it will take Pratt five days identify the root cause and the corrective action. If it is a durability issue, the expert says there is plenty of time to design and retrofit a fix as the test engine had accumulated the equivalent of 8 years in service when the damage occured.

Pratt also says there should be no impact on F-35 flight testing, as the engine that failed has the "second-generation" IBR fan, which is lighter. The flight-test engines have the first-generation fan, which has already undergone durability testing, the company says. There is no halt to flight testing or engine ground testing, Pratt says.

Two other second-generation engines are on ground test and are being examined. The damaged engine should be repairable to retstart and complete qualification testing, the expert says. The fan will have to be replaced, but damage to the compressor blades is blendable and the rest of the turbomachinery looks undamaged.