WM_Su2014_28

Back of a Blackhawk, you have similar things on Jayhawk okay, and the Air Force has some other Hawk right. Uh, they saw the same uh Pratt Whitney [Pratt & Whitney / Sikorsky] aircraft. Uh so this one we cut. We cut it.

If I see — yep, you can see the saw cut right down there. We cut this at the time that I was working on this, which was like early 1990s, so over twenty years ago. This was a three or four hundred dollar part okay. But it's a pretty sophisticated part, and it matches up with this part, which if you — uh, maybe I'll pass around tomorrow, well I can't probably pass around now. The number of holes in this are different than the holes here. They didn't want to use a keyway because keyways cause stre— stress concentration and fatigue. So the tail rotor of a Blackhawk or a Jayhawk or a Seahawk helicopter is just a conical fit, with no keyway, and then you have the rotor. This is what holds the rotor blade on. It's just a friction fit, and that creates the downward force to create the friction. Friction.

And it turns out uh, um, they started seeing the cracking. The Navy, you're the corrosion leaders in the service okay, you were the number one corroders in the service. If something's gonna corrode you will see it first okay. Maybe — there maybe the Coast Guard, but you'll see it first. The gray ones — that one hadn't cracked, but they found them still in their plastic wrappers from Sikorsky with cracks. Stress corrosion cracks.

We measured the residual stress by cutting that hole and measuring the thing very precisely before and after, and see how much the hole opens to expand. We estimated the residual stress of that of 8 ksi. That particular alloy, which was 6061 alloy, and the T6 optimum strength condition, has a stress corrosion cracking toughness — or his toughness — of 5 ksi. Which means 8 ksi residual stress, marine atmosphere, 5 ksi susceptibility, and we're right there in the middle of that little Venn diagram, and they would crack on the shelf without ever having been put in service.

Well, the Navy noticed this, they told Sikorsky, and so of course he says oh, you know, of course he keeps their Blackhawk program separate from their CH-53 or MH-53 program, which is a different helicopter, and different from some other helicopter. They said oh, we saw the same problem in corrosion in one of our other helicopters on the same washer okay. And so we went to — instead of the T6 heat treatment, we went to the T7 heat treatment, which is over-aged.

What do I mean by over-aged? Overaged. If I do precipitation heat treatment on aluminum alloy — I might as well do everything here, it's kind of the next stuff anyway. To heat treat an aluminum alloy, I — this is aluminum copper alloy here — I take it up into the solid solution range, which is around a thousand degrees Fahrenheit. I should say Fahrenheit yet. And then I quench it down to room temperature, in water typically. And I can't do air hardening like I do with steels, I usually have to quench it in water. And then I bring it back up to a tempering temperature.

And the schedule might look like this. In fact, they quench in cold weather, so the solution heat for one hour at 940. They quench in cold water, they age for ten hours at 340 degrees Fahrenheit. And they increase the strength — they can double the strength okay. And the doubling in strength looks like this. Depending on whether they're in — this is centigrade, but I have another one, I'll put up the other one that's in Fahrenheit since we just had Fahrenheit. Same curves, just someone likes to use Fahrenheit.

And so it was 340. They basically — we're going to do ten hours. Well ten hours at around 340, you can see you're going to get relatively high strength in ten hours. Uh, you probably — they're probably using — well, I don't know what they're using, anyway. They're gonna — they can go from a — this is hardness so it's related to strength — from 65 up to 105 or 110 or something. Anyway, you can go even higher if you age for longer times, but who wants to do a heat treatment for a thousand hours okay, that's a few months. So ten hours is reasonable.

So they solutionize these things. If you go to the peak temperature, if I'm at 374 [340], I'm doing my tempering, I can get the peak. That's a T6 heat treatment. I can lose ten percent and go over — we call over-aging — and lose ten percent of my strength, but gain about a factor of five in corrosion okay.

So what they'd found is, of course — oh, we did the T6 heat treatment on this other helicopter, we noticed problems with the cracking of the washers. You lose your tail rotor, and you can have a bad day. In fact, the Army one killed six — uh, killed eight soldiers and put one of them in a coma, in Arkansas. And uh, um, that was actually later okay. First they went — the Navy went, said we have a problem, these are cracking on the shelves. Of course he says, oh, we had fixed that on the other one, we went from T6 to C7 [T7]. And they went out and they ordered three thousand of those — not three thousand, thirteen hundred of those — and they put them on order. It had been a year, they were going to be coming in three months, and they're going to change out the whole fleet okay, all the services of the Blackhawk okay.

It turns out — they — the Army had this failure in Arkansas, night vision goggle exercises and stuff, crashed, killed eight people and put another one in a coma. And it was a cracked washer, due to stress corrosion cracking with a T6 heat treatment okay.

Now it turns out — how do I get back to why do they get off on this — it turns out, I've mentioned to you the government contractors defense, that military hardware has been determined by the Supreme Court to be cutting edge technology, or some people call it bleeding edge technology. And you can relieve the stresses by stretching. We talked about the great big Alcoa plates, and they pull on the whole six inch thick plate to stress relieve it. So it's mechanical stress relief in tension. That's a T-51 heat treatment okay. You can also stress relieve in compression. That's a — in this case it should have been a T-62 heat treatment okay, that they should have done.

And it turns out, if you — and Sikorsky's defense, when the people who had lost their lives in Arkansas were suing them, Sikorsky says oh we have the government contractors defense. The government approved the design of the Blackhawk helicopter, and therefore you can't sue us. We had to find a manufacturing defect. Well, the manufacturing defect is, Sikorsky bought bars that were about three or three and a half inch diameter that had been stretched, so it was T-651. So it's a 6061 T6, which is the peak of the precipitation hardening curve, and the 51 on the end means they had stretched it. And so that three and a half inch diameter bar had been put in a great big machine and stretched, and there are no residual stresses from the heat treatment okay.

Well then, once of course you got it, they sliced it up like a bologna, and they machined the thing, and they reheat-treated it. Well when they reheat-treated it, they wiped out the stretching that had relieved the residual stresses. They introduced new residual stresses on quenching, and they didn't do anything to relieve the residual stresses. Well they said, well we purchased it with stress relieved. Yeah, but then you reheat-treated it and you reintroduced residual stresses. This is sort of like the guys who said uh we preheated that steel on Friday and they're welding on Monday right. Uh, or it's like the guys at uh the jet engine company — oh well we put it into the furnace to diffuse out the hydrogen within four hours of getting the test report back three days later from the test lab okay. Oh great okay. They didn't understand the physics or chemistry of what they were dealing with.

And they had never stress relieved it. So I get the drawing that had been approved by the government, and it says that these washers are made out of T-651. Yes, they ordered T-651, they sliced it up, they reheat-treated it, it was no longer T-651, it had been T6. But they never did — how do you do a 5-1, how do you pull to stretch that washer to relieve residual stresses? Kind of hard to grip that thickness, or you know, something that thick. What they could have done was a compression — they could have done a T-62. So what they should have had was a T662. What the government approved was a precipitation hardened and stress relieved part, but what they had produced was not stress relieved, and that's why it cracked.

The other news in all this is, you need to know that the Blackhawk at the time was not only part of the Air Force, the Navy, the Coast Guard, and the Army, it was also the presidential helicopter okay. And so, could have lost the president's helicopter because of this little engineering fiasco.

Anyway, to show you the stretching and stress relief — if you go to alloy 7050, which is actually the alloy which I believe they make the wings out of nowadays. Not seven — not 7075, it's 7050 now, if I'm correct, if I'm not mistaken. And this is a little cut sheet from Alcoa, saying "supplying the world's best." Comes out of their plant in Davenport, Iowa, which has the world's largest rolling mill. And I'm actually — I can show you this page first, at the bottom. Um, you can buy this as T7651, 7050, or 7075, which was what they used to use. You can buy it two to three inches thick — I'm sorry, that's, yeah, that's in inches — and you'll get slightly better properties than the old alloy, but it's at 651 heat treatment. And elongation is actually a little bit better in this new alloy.

And if we go to 7050 T7451, which is different heat treatment, which they use for even thicker plate, which is most of the wings — five to six inches thick — you don't get quite the strength, but it's still significantly better than 7075. Still got reasonable elongation and whatnot. Um, so they are constantly tweaking the alloys, because um, that extra ten percent in strength means the ten percent lighter structure, and that's important on aircraft okay.

Um, so I think the next topic — well yeah okay, why don't we cut it there. We got to go to 3.42. Who's presenting to—