Early analysis suggests the crack was likely caused by either a manufacturing defect during the casting of the blade or possibly a creep rupture. This latter failure occurs when deformation begins under constant load and high temperatures and over time reaches a point where the blade material abruptly fails or ruptures. Casting issues can also trigger such creep ruptures by introducing microstructural weakness in the blade.
As with the recently discovered blade crack, the failures 5-6 years ago also struck blades in the third LPT stage, forcing a redesign of the module. However, those failures cropped up on engines configured for the F-35B short-takeoff-and-vertical-landing (Stovl) variant, rather than the standard F-35A/C engine variant. The work contributed to delays of the F-35B's first flight as well as significant cost overruns to the overall F135 engine development program.
The first incident occurred in an early Stovl test engine, FX635, on Aug. 30, 2007, after a total run time of 121 hr. The second failure occurred on Feb. 4, 2008, on the sixth flight-test engine, which was being used for flight clearance “proof-test” ground runs prior to installation in the first Stovl F-35, BF-1. The tests were aimed at proving, on an engine-by-engine basis, that the specific unit was safe to use in BF-1, and that the engine did not exhibit the same combination of assembly characteristics, tolerances and other factors that led to a similar failure.
The second failure occurred after only 18 hr. of total run time, proving the fatigue was triggered by a resonant response to aerodynamic excitation caused by the upstream stator vanes between the second and third turbine stages. A root cause was the long length of the unshrouded blade, which the engine maker acknowledged at the time was outside its “family of experience.” Pratt said it “had neither the standard work nor the advanced analytical techniques to address this phenomenon.” In other words, the design should have been tested, but Pratt's “standard work” design rules did not call for it.
The original fracture, which occurred during high-stress, powered-lift runs for the Stovl engine on a test stand, was traced to an internal crossover cooling hole in the affected blade. The redesign involved reducing stress through changes to the internal configuration of the third-stage blades, and a revised asymmetric vane configuration, which broke up the resonance between the stages.
Although reassured that this latest problem is manageable, program officials will be focusing on whether high-cycle fatigue played a part in either the initiation or propagation of the crack, and what caused the low, steady stress at the failure location.
F-35 Program Executive Officer U.S. Air Force Lt. Gen. Christopher Bogdan is both optimistic in suspecting that the blade problem is not major and cautious in not seeing an immediate return to testing. “It is not a great idea to speculate before you do all the testing on a component that has failed,” he said. “I would caution my Pratt & Whitney brothers to take a conservative approach until they see what is going on. . . . It would be bold to predict a return to flight-testing [this] week. But it is not bold to expect to [have found out] the root cause of the problem this [past] weekend.”
Testing on the failed component, a cracked turbine blade, had included cold-flow, X-ray, white-light and ping tests. As Bogdan spoke, coating was to be stripped from the blade, after which there would be a grain etching test and a fractology test.
Bogdan's concerns about Pratt are not limited to engine performance; he also expresses continued frustration with the aircraft prime and engine maker over their approach to contract negotiations. “What I see Lockheed Martin and Pratt & Whitney doing today is behaving as if they are getting ready to sell the very last F-35 and the very last engine, and are trying to squeeze every nickel out of that last engine and that last airplane,” Bogdan told reporters at the Australian International Airshow near Melbourne last week. “The behavior I want to see is that they want to and intend on and are knowledgeable that they are going to sell 3,000 airplanes and 3,000 engines, and take the long view on this program.”