More cracks found in F-35B second life testing

Ground fatigue testing of the F-35B has been stopped until fourth quarter of this year, while a fix is developed for cracking found in the 496 bulkhead. The cracks were found at the 9,400 hour mark of testing, so it won't affect flight testing of the fleet. The 496 bulkhead is a carry through bulkhead for the wing section. In 2010 cracks were found at the 1500 hour mark, resulting in grounding the fleet.

The expected fix, which is under development will add about two pounds of weight to the aircraft. A new bulkhead will be ready by March, and the fix for the cracks will have to be installed on currently fielded aircraft. Lot 9 is expected to be the first fielded with the fix from the start, although they're looking at whether Lot 8 will be able to have it installed.

Meanwhile, the primary complaint from the testing crews at Eglin seems to be turnaround time. This results in repair and checkout missions taking longer than they should. The program is still in a test mentality for flightline maintenance, instead of an operational mentality.

Cracks on an F-35B’s primary support structure found last year are more extensive than previously thought, triggering a halt in ground-based durability testing until the fourth quarter of this year.

The initial cracks were found on section 496, a primary wing carrythrough bulkhead, last fall, prompting officials to stop the ground-based testing at hour 9,400 during the second life’s worth of use — or second 8,000 hr. of equivalent flight hours — to investigate the issue.

Since then, cracking also has been discovered on adjacent bulkheads, according to Joe Dellavedova, spokesman for the F-35 Joint Program Office. “Subsequent inspection of surrounding structures in light of this discovery found additional cracks in two of the other adjacent bulkheads,” he says.

Because the discoveries were found to occur beyond the first 8,000 hr. of use, the issue is not affecting flight operations for the young F-35B fleet. These issues are also, thus far, limited to the B model that employs a sophisticated lift-fan for vertical takeoff and landing operations for the U.S. Marine Corps. The U.K. and Italy also are expected to buy the F-35B.

F-35B cracks

Lotta torque on the ole duct fan from the vertical take off and landing phase...prolly not flight stresses....
heavier metal will fix.....

It's gonna replace the A-10 huh?

So just before that daft bint comes on here stealing oxygen (becky the troll?), this was in the second life, ie. after the first 8000 hour life?

Are bulkheads usually part of a mid life maintenance program?

Another thing, does the F35b test bed simulate the engagement and disengagement of the fan for an equivelant number of flying hours? For example, perhaps (like some jet engines) the F35b would (maybe it does) have 2 tests, starts and flying hours, so the maintenance routing for a F35B is either 8,000 flying hours or (for example) 2,000 fan engagements, whichever is first.

Surely some of the catapult aircraft are similar?

cavtrooper7
It's gonna replace the A-10 huh?

Can the A-10 tkae off land on a carrier?

SirDrinksalot
So just before that daft bint comes on here stealing oxygen (becky the troll?), this was in the second life, ie. after the first 8000 hour life?

Yes, this is to determine the ultimate life cycle of the aircraft. Generally fighter type aircraft have an 8,000 hour life cycle, but it frequently gets extended.

Are bulkheads usually part of a mid life maintenance program?

They're generally inspected during the PDM phase of maintenance.

Another thing, does the F35b test bed simulate the engagement and disengagement of the fan for an equivelant number of flying hours? For example, perhaps (like some jet engines) the F35b would (maybe it does) have 2 tests, starts and flying hours, so the maintenance routing for a F35B is either 8,000 flying hours or (for example) 2,000 fan engagements, whichever is first.

Surely some of the catapult aircraft are similar?

It simulates all aspects of flight, so in this case they engage and disengage the lift fan, etc. So yes, they test the lift fan as well.

reply to post by Zaphod58


This issue might not be an engineering issue, it very well could be a manufacturing issue.
Several well published episodes of airframe component failures, that in media were attributed to "fatigue" and or "out of spec" manufacturing, have actually been attributed the finish machining processes, for the most part in milling.
The issue shows itself mostly in high strength Al alloys such as 2024,7075 and to a lesser degree in 6061 and Ti alloys .
As machine tools and cnc controls evolved the ability to remove metal advanced exponentially in the 60's thru the 80's. The possible volume of metal removal increased to a degree that master aerospace machinist from the sixties would scarcely recognise the processes in use by the nineties.
In order to make more money , you have to remove more metal per minute.
In aeroepace parts a lot of metal is removed, in the case of the F35 the bulkhead that attaches the cockpit assembly to the main airframe, starts as 11,000 lb billet of forged 6Al/4V Ti, and spends nearly 30 days 24 hrs a day to machine down to a roughly 400lb piece.
So what was discovered in the late nineties, by a supplier looking back at failed part, is that even in a high pressure flood cooled cnc , with through the tool coolant, the temperature gradient between the cutting edge and work surface is so great that submicroscopic surface cracks form, that eventually will propogate into larger cracks.
This effect is very pronounced in heavily pocketed parts with small corner radii.
What happened in the machining world is that ultra high speed machining came about.
In UHS machining depth of cut and %age of tool engagment are substantially decreased and spindle speed and speed at which the tool moves across the work piece are increased.
An example is, lets say when the SR 71 was built, Ti was machined at a cutting speed of 120 ft/min. That means the the edge of the cutting tool is moving at a rotary rate of 120 feet per minute., with a tool engagement of 70% and a depth of cut of .5-1.00 x dia.
For a 3/4" endmill , that equates to 1/2" width of cut x 3/4" of depth of cut. This is a spindle speed of 611rpm and a table speed of 12.25 inches per minute.
In the modern world , I would program the same part at 1500 surface ft min, with a depth of cut of .06" and a tool engament 60% and chipload of .02 per tooth compard to .005 for the previous example.
This gives a spindle speed of 7,640rpm and a table speed of 611inches per minute. What happens is that the extreme speed of the spindle generates so much heat that the base metal softens beyond it's yield point and the tool is moving so fast across the surface that all the heat is ejected with the chip instead of being absorbed by the work piece.
So this brings me back to the F35 mention from before, in light of the advances in metal removal, I was surprised to see Lockheed using 50 yr old techniques to machine the bulkhead.

reply to post by punkinworks10


The F-15 fleet flew almost 20 years before they found over 300 (I think it was 360) that had poorly manufactured longerons. Something like 240 had to be retired. It was only found after an aircraft broke in two during a flight.

reply to post by Zaphod58


It wasn't until the nineties that the industry had the analytical tools available to discover the submicro cracks, that lead to ultimate failure.
The other incidents involved civilian aircraft.
Personally, I'm not buy the out of spec manufacturing excuse, I think it's just a way fir lawyers to lay blame where none should be laid. I have done a little aerospace work,
I don't see how a bad part could make it past QC, with the level of documentation required by the industry. I would lay odds that the "bad"parts were made when it was thought that statistical QC would suffice, as it does in the automobile industry.

reply to post by punkinworks10


In the case of the F-15 and so many others it was money. It was cheaper to build them just slightly too thin, and from testing they were strong enough to last the planned life cycle. Then the life cycle was extended, and now there is an issue.

Zaphod58
reply to post by punkinworks10

 

In the case of the F-15 and so many others it was money. It was cheaper to build them just slightly too thin, and from testing they were strong enough to last the planned life cycle. Then the life cycle was extended, and now there is an issue.

I will always defer to your knowledge in things aerospace, except for this.
When it comes to machined aircraft parts, or any part that is machined to final spec., less is never cheaper. It takes more time to remove more metal, and time is money, that's why the envelope is constantly pushed in metal removal, the faster you can remove metal the more money you make.

I just found this article,

The aft bulkhead of the F-35B BH-1 fatigue-test specimen has developed cracks after 1,500 hours of durability testing, Ares has learned. This is less than one-tenth of the planned fatigue test program, which is designed to prove an 8,000-hour airframe life with a safety factor of two.

The bulkhead design was modified in the course of the jet's weight-saving redesign in 2004-05, switching from forged titanium - proven on the F-22 - to a new aluminum forging process developed by Alcoa.
According to Lockheed Martin,"the cracks were discovered during a special inspection when a test engineer discovered an anomaly." The company says that flight-test aircraft have been inspected and found crack-free and that flight testing has not been affected.

Engineers are still investigating the failure and it is not yet known whether the cracks reflect a design fault, a test problem (for example, a condition on the rig that does not reproduce design conditions) or a faulty part.

If the bulkhead design is found to be at fault, it will be a serious setback for the F-35B program, potentially imposing flight restrictions on aircraft already in the pipeline or requiring expensive changes on the assembly line.

Six F-35Bs are included in the LRIP-2 contract, now in the mate or final assembly stage, and nine in the 17-aircraft LRIP-3 batch - which are intended to support initial Marine Corps training and operations. If a redesign is necessary it could also delay deliveries of LRIP-4 aircraft.

Bulkheads are a major structural component of the F-35, carrying the major spanwise bending loads on the aircraft. They are produced from forgings weighing thousands of pounds, which are machined into the final shape. They are among the longest lead-time items in the airframe, being built into mid-body sections produced by Northrop Grumman.

The F-35A and F-35C bulkheads are still made of titanium, as are similar bulkheads on the F-22.

Correction:  Northrop Grumman asks us to point out that the parts in question are built into the wing/centersection assembly made by Lockheed Martin.

In this case cost is for sure a reason to switch from To to Al for the the cost for Ti forgings is astronomical, and seeing as how the airframe was designed around Ti bulkheads, I don't see how they could accommodate the added bulk of the Al parts within the space. Because to have the same mechanical properties a Al part will be almost double the volume of a Ti part. Another thing to consider is in a part that sees cyclic flexing Ti is a better choice as it is more flexible for a given strength.
Also I bet that the "Al" bulkheads are not actually aluminum, but a MMMC (Multiple Metal Matrix Composite". A MMMC is a powdered metallic composite that is hot isostatically pressed into shape, forged but not forged in the sense forging as most people think.
In the nineties Alcoa was pushing mmmc's in aerospace, automotive and cycling industries. but they didn't take , as there were issues with cracking in high flex cycle parts, one major bicycle Mfg er jumped right in with mmmc's only to see a failure rate that approached 75%.

reply to post by punkinworks10


It's not the cost of making them, but the cost of replacing them with correct parts. The F-15 fleet was estimated at $15,000 a longeron, and $250,000 to replace them.

reply to post by Zaphod58


Im beginning to think one of the companies involved is really screwing up. They brought BAE Systems in on this because there experience with the Harrier im wondering if that wasnt a mistake. They had very few problems with the F22 except software and i can understand that. But maybe its just to many companies working on the F35 and they arent coordinating well. Somethings wrong though there taking orders on an aircraft and still redesigning the airframe thats bad. Bet the British feel stupid selling there Harriers to the marines. They have an aircraft carrier with no aircraft and doesnt look like theyll get replacements for years.

reply to post by dragonridr


I think you hit the nail on the head, too many cooks stiring the pot. Combine that with the fact that the f35 mission has been a moving target. I think it's a mistake to think you can build a swiss armyknife aircraft to suit everyone's needs, because by the time you tailor it to suit everybody it becomes a bastardized airframe.

@pumpkinworks and Dragonridr...

Sorry but are you trying to say that it is a British manufacturing / design problem that is causing delays to the project??

Whilst I do reign from BAE heritage, I can assure you without predjudice that the British manufacturing techniques are not the issue.

To be pro British for a moment":

In composites (to which we technology share with F1 and Airbus) we are at the cutting edge, as much as any US known technology or that on something they are willing to publicise and sell.

Also, a design is approved by engineers on both sides of the pond, making it, well thats easy once a design is done - so manufacturing isnt an issue, design is. Do you suggest America has better designers that Britain or that somehow, the F35 consortium would allow (backwards) UK to put at risk the entire project...

Finally in one last pro British - you bought the Harrier of us because you recognised we built a truly superb aircraft you couldnt match.

So to the next challenge, we (UK) are involved in Eurofighter, a 4 nation (was 5 but we couldnt agree with the french) aircraft build program. When you put multiple nations together, you come up with some unique problems, its in the mating of roles and requirements within the airframe (no gun, 2 guns, vtol, stovl, A2A, multi role....) If America wanted and could afford a purely US platform, they would not have asked (or allowed) the F35 partners an in, they would have built it themselves and sold it....what is that, they wouldnt sell it like the F22......wait, but the F22 was sucessful yeah...never been in combat, never sold aboad...define successfull...you cant sell it if its so secret..compromise.

You know we proved to the americans we could do stealth to get an in.

Sorry, outside of F22 which I would imagine you dont dare to send into offensive combat incase it lands in the enemys hands you have F/A 18 and F-15 - neither of which offer more than the Tiffy or Rafale. I am sure the US will design another few unique american weapons systems but F35 is a mass, Allied airframe unlike any before. Maybe it will prove the business model doesnt work??

Oh, and Airbus is as good as Boeing, even better in some cases.

Get over it.

reply to post by SirDrinksalot


Easy there SirDrinksalot,

I said nothing about BAE, and my reply to other poster was about too many companies involved in the program. In today's aerospace Mfg climate, it seems that collaboration is a severe hinderance. Boeing's experiencence with the dreamliner should be an example of how not to collaborate.
On CBS's 60 Minutes, two weekends ago, did a piece on theF35 program, and it is truely dysfunctional.

By the way the harrier is a truely fantastic aircraft

reply to post by punkinworks10


Yeah sorry, I read Drognridrs post as a critic of UK manufacturing and America is best.

I agree too many cooks is a major issue especially with my experience on Eurofighter Typhoon.

IamSirDrinksalot
reply to post by punkinworks10

 

Yeah sorry, I read Drognridrs post as a critic of UK manufacturing and America is best.

I agree too many cooks is a major issue especially with my experience on Eurofighter Typhoon.

It's all good Sir,
Not only are there too many cooks in the kitchen, but the dinner guests are in the kitchen as well

reply to post by Zaphod58


Where are these made? I have to know. S&F for the info.

reply to post by swanne


The main Lockheed line in Fort Worth. Also where the F-16 line is.