Posted on 03/06/2021 7:31:38 AM PST by G Larry
My SIL works for Rolls Royce.
Those blade manufacturing processes are their most closely guarded secret.
Material fatigue cracks by their very nature are of course caused by cyclical movement (mechanical fluctuations). While I understand why the blades may be hollow, one would assume that this design feature adds another layer to how inspections are done both in the factory when the blades are made and once in operation…..I mean there are inspections (visual) INSPECTIONS (NDT techniques such as dye penetrant and eddy current) and INSPECTIONS (thermal acoustic imaging for which the component is removed and shipped back to the factory since it can’t be done in situ).
The ‘Origin Area’ shown in Posts 11 and 12 is on the inside of the blade and this would lead one to assume that none of the regular inspection techniques currently used during regularly scheduled maintenance outages would have caught this, right? I suspect that since this is not on the outside of the blade on either the convex or concave side, the finite element stress analysis would show that this an area of the blade that has lower stresses and as such, it’s just acknowledged that this won’t get inspected period until the entire fan is shipped back to the factory. Is there anyone on FR here today that knows enough about airline maintenance and inspection practices that can confirm this?
Fatigue is a very tricky thing to deal with at best…. Things basically are good for a certain number of cycles and then cracks develop…. just think about the idle times you’ve had at your desk and you’ve picked up a paper clip and bent it back and forth until it cracked. This is actually the downfall of assuming that inspections will catch everything… something may be very near the end of its fatigue life and even a thorough inspection might show nothing that could be deemed as being problematic. Don’t get me wrong… inspections are important and no doubt many failures have been caught in time because cracks were noticed and a component was changed out with a new one. However, on highly stressed parts, there’s also the possibility that by the time the crack has become large enough to be identified in an inspection, there is virtually no time between that and catastrophic failure from occurring. Fatigue is all about understanding probability and knowing the application well enough to know when something is at the point where it must be changed out….. for fans in aircraft where human life is on the line, that point in time is assumed to leave plenty of margin to account for a variety of modifying factors that can occur during the manufacturing process i.e. factors that could result in fatigue life being lower than calculated.
I can imagine. It has to have taken a hugh and series investment to perfect the process.
Inspections should have been done based upon flight hours. Inspection costs suffered with cost cuts because of the impact of the *scam*demic.
not to mention the impossibility of VISUAL inspection INSIDE the hollow blades, plus those many feet of concave corners inside the blades just waiting to stress fracture because they had minor manufacturing deviations from true concavity ... it really blows my mind that anyone ever thought hollow turbine blades was a great idea ...
Inspections be damned!
United has surpassed all expectations on more important issues like diversity & inclusion & seating discomfort...
Some of the blades (on some engines) in hot zones have cooling passages, but this is not one of those.
United flight 328 engine failure: FAA orders scans of fan blades
The Federal Aviation Administration issued an Emergency Airworthiness Directive Tuesday night, ordering U.S. airlines to inspect any aircraft with certain Pratt & Whitney PW4000 engines. United Airlines is the only commercial carrier still using that engine, which is mated to Boeing 777s. ...The failure is described as being one of the early stages, a long blade near the front of the engine, part of the air compressor section.FAA Administrator Steve Dickson identified the focus on the stepped-up inspections as hollow fan blades unique to the Pratt & Whitney PW4000 engine model and used solely on Boeing 777s.
I couldn't agree more. It looks like United was caught cutting corners. It will not go well for them.
It depends. There is such a thing as "fatigue limit," a stress that is low enough that cracks "never" develop. Rule of thumb for steels is 50% of yield. Many parts normally stressed well above fatigue limit with number of cycles to failure (under load) being millions or billions.
Aluminum has a low fatigue limit, from memory (fallible), is about 10% of yield. I would guess fatigue limit of the somewhat exotic alloy in this application is closer to that of steel. It seems the life protocol is "inspect then return to service." The bearings however are replaced, regardless, at a certain number of hours. Roller bearing failure is very well understood.
All that said, it is as you say - tricky in practice. Plenty of empirical test data.
Scandalous. I rarely fly, and when I do, it’s almost always American. Heads should roll over no inspections of any particular part, for 4 years.
Aluminum has a low fatigue limit, from memory (fallible), is about 10% of yield. I would guess fatigue limit of the somewhat exotic alloy in this application is closer to that of steel.
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I suspect you are correct. We know that the fan won’t be made of aluminium…. It will be made from titanium (probably something like a 6AL-4V alloy or something similar since that is common for aircraft engines). Regardless, titanium has fatigue characteristics that are more like steel than aluminium. http://przyrbwn.icm.edu.pl/APP/PDF/128/a128z4p06.pdf If you take a look at Figure 2 at this link, you can see that the endurance limit for the 6AL-4V alloy is approximately 50% of its ultimate tensile strength. In fact, it might be higher than this…. who knows what it might kind of post-production thermomechanical and thermochemical treatments might have been used.
Recycled ME262 compressor fan blades? Uh-oh!
Spot-on.
Where is that from? Did I miss it?
Wish I'd posted this response earlier......
It’s from World War 2 history. The 262 was the first operational jet fighter. It was 100 mph faster than the P-51 Mustang, but the compressor blades were only able to last for about 20-some flight hours, due to their primitive manufacturing process. Then they came apart, with the expected drastic result.
Oh...I didn’t even stop to think about what the ME262 was. Good joke! Ya got me.
“I don’t think their steel.”
Their not.
Not so much Iron in those blade puppies.
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