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The Case of the Battered Blade
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, December 13, 2004
A jet aircraft engine ingests an awful lot of foreign matter, none of which does it any good. Engines in flight suck in birds, some as large as migrating geese. These anserine-level impacts are sometimes simulated by throwing frozen chickens into a running engine. On the ground all manner of windblown rubbish is ingested, as well as unfortunate low-flying birds. The cumulative effects of these foreign body impacts is to blunt the leading edges of the first stages of blades. The engine becomes less efficient and less powerful, both of which are undesirable.
Replacement blades would be horribly expensive, so a cheaper fix is needed. I learned of such a fix a dozen or so years ago when contacted by an aircraft claims adjustor who presented me with a failed, first-stage compressor blade from an engine on a Boeing 727 Jet. The blade was said to operate at near room temperature. The blade had fractured straight across, perpendicular to the long axis. The blade was a forging of a very popular titanium-based alloy with 6 percent aluminum and 4 percent vanadium.
My study of the blade showed that the dings had been repaired at least once. A long, slender sliver, tapering from 1/4-inch at the blade tip to a point near the blade base had been removed and a piece the same size had been electron-beam welded into place. I was struck by the audacity of the repair. How could a blade with such a welded-on sliver ever survive the tremendous centripetal forces during operation? The blades in fact survive very well as such repairs are very common.
In-Depth Blade Analysis
I called on the usual tools of scanning electron microscopy (SEM), optical metallography, and hardness testing. SEM study revealed fatigue striations from the leading edge to about two-thirds of the way across the fracture surface. The remainder of the fracture surface showed the dimples characteristic of a ductile failure. Something had initiated a fatigue crack at the leading edge of the blade. The crack had proceeded bit by bit over many power cycles until the strength of the blade was so reduced that the outboard portion simply tore away.
There was a heavy ding on the leading edge of the blade, where the crack had started, but, these blades are supposed to take a significant amount of impact damage without failing. What went wrong? Metallography and hardness testing gave the answer.
Most structural alloys are multiphase so as to obtain optimum mechanical properties. Specifications for the alloy at hand called for the microstructure to be an equiaxed mixture of two phases. In fact one phase was in the form of plates, which caused the material to be harder than the specified maximum of Rockwell C-39. A hardness traverse on a polished cross section of the blade gave readings between Rockwell C-40 and C-50, with the highest values near the weld. A hardness of C-50 indicates a material so hard that it is likely to be brittle. The difference between C-39 and C-50 is huge, in metallurgical terms.
The Welding Aftermath
So what happened? The repaired blade should have been annealed after the welding to give the specified microstructure and hardness, but was not. The excessive hardness made the blade susceptible to fatigue failure, which is just what happened.
There is an interesting footnote to this case. Most airlines hand over a certain number of damaged blades to the repair firm and get the same number of repaired blades of the same type back, without regard for origin. A certain major European air carrier numbers its blades, and insists on getting the same blades back. Such meticulousness is a major factor in this airline being my overseas carrier of choice.
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