Black Hawk tail rotor washer stress-corrosion failure

Customer bought aluminum in T651 (stress-relieved) condition, machined washers, reheat-treated them, and did not re-relieve the residual stresses. Resulting 8 ksi residual stress exceeded the 5 ksi SCC threshold for the alloy; washers cracked. Resolved by going to overage temper with higher SCC tolerance.

That washer I showed you — they bought it in the T651 condition. It had been stress relieved, and then they cut a piece off — washer shaped like this — and they reheat-treated it. When they reheat-treated it they reintroduced the residual stresses, and they never did anything to relieve them. They said, oh well we bought it stress relieved. Yeah, but then you reintroduced residual stresses, and when they actually shipped it, it had 8 ksi residual stresses. Stress corrosion cracking for that alloy is 5 ksi, so they got cracking of the washers. They changed the heat treatment to go to an overage condition, which now you have 20 or 25 ksi, and so you can tolerate 8 ksi residual stress.

Stress-corrosion cracking of 6061-T6 aluminum washer holding the tail rotor of an Army Sikorsky Blackhawk; helicopter lost rotor and crashed in Arkansas. Tom traces the design history: the Coast Guard / Jayhawk team had already switched to T73 and not told the Blackhawk team. Same washers found pre-cracked in Navy supply depots from residual stresses alone (measured 8 ksi vs. 5 ksi SCC threshold).

Another example I have here is this washer. [Tom holds up an aluminum washer and a steel nut.] It's got a gray edge which means it came from a Seahawk helicopter. This is a steel nut that goes on top of it. The holes don't line up because of the design — instead of having a cotter pin, you put the washer on, put the nut on, tighten to the proper torque, and there's one more hole in one than in the other, so there'll always be one that lines up and you put a screw through it. This holds the tail rotor of a Blackhawk or Seahawk helicopter — one's an Army part, one's a Navy part. This particular alloy was 6061, which I told you yesterday was the workhorse aluminum alloy.

Stress corrosion cracking case study. Arkansas night-vision-goggle training crash; 7075-T6 washer should have been T7351 (compression-stress-relieved as T7352). Government contractor defense; Sikorsky settled for $10M two weeks after the chief of rotors testified.

This is another story of stress corrosion cracking, from an Army Black Hawk helicopter. This one is painted gray — so what type of Black Hawk is this? A Navy one. The Coast Guard has Jayhawks. The presidential Hawks — if you go down to Sikorsky with me, they actually have a separate engineering group for the presidential Hawks. They don't make that many, and it's the last Black Hawk you'd expect special attention on.

Tom's central forensic case for the lecture. 6061-T6 aluminum washer cracking on the shelf due to residual stresses (8 ksi) exceeding the alloy's stress corrosion cracking threshold (5 ksi) in marine atmosphere. Sikorsky had previously seen this on CH-53/MH-53 and switched to T7 over-aged heat treatment but failed to apply the fix to the Blackhawk part. Bars purchased as T-651 (stretched, stress relieved) were sliced, machined, and reheat-treated, wiping out the stretch stress relief. Government contractor defense raised by Sikorsky in subsequent litigation.

The fatal-failure realization of the same washer defect Tom was investigating for the Navy. Night vision goggle training exercise, cracked washer with T6 heat treatment, eight soldiers killed and one in a coma. Triggered the litigation in which Sikorsky raised the government contractor defense.

Recap from prior session. Tom brings the physical washers (Seahawk and Army variants, both steel and aluminum), explains the saw-cut residual stress measurement method (8 ksi measured), and the resolution: switch from a heat treatment with 5 ksi SCC threshold to an overaged T73 with 25 ksi threshold.

We talked about the Blackhawk washer two days ago, and I brought a couple of my little pieces left over from fifteen years ago. [Tom produces the washers.] This is the washer, and fifteen years ago it cost $350. This one's painted gray — that means it's a Seahawk. This one's green, it's an army part. This one's steel, this one's aluminum, don't ask me why. It's a very clever design. In order to keep it tight, there's a different number of holes in this than in here, and so no matter how you turn it, within about two or three degrees you always get one hole to line up, and they put a screw in there to keep it from loosening. They don't use lock wire or friction; they have a positive thing, because if you lose your tail rotor you just spin to the ground, which is exactly what happened.

$350 aluminum washer holding the tail rotor. Was supposed to have been stress-relieved and was not — residual stresses → stress corrosion cracking. Failure caused Arkansas night-vision training crash, killing 6 Army soldiers and 2 pilots. First cracks found by the Navy on Seahawks (corrosion leader in the service); first fatality on an Army Blackhawk. Same airframe as presidential helicopters.

Here's Backofen just showing you a rolling operation, and they call this alligatoring — splitting down the center. He's pointing out you get very bad residual stresses in your sheet. I tell the story in some of my other lectures — I've got a washer from a Navy helicopter. This is an aluminum washer that 15 or 20 years ago cost $350 for this little washer. It's about this thick, about this big around. It's the washer that holds the tail rotor for the Blackhawk helicopter. One of them failed on an Army helicopter. If it's a Navy helicopter it's called a Seahawk, if it's the Army it's a Blackhawk, the Coast Guard it's the Jayhawk, and the Air Force, Carl — you call them Blackhawks too. That's why I couldn't think of a different name for the Air Force. Jeremy knows all about these things, he did his thesis down in that plant, or he is doing his thesis down in that plant — he spent the last six months there.