NDI for overtemp'd Titanium

[Somber]

We have an issue in which some panels/doors get exposed to extreme exhaust gas temperatures. The current guidance for judging the panels good/bad is based on color and distortion. The maintainers find this to be very subjective. I understand some things have to be subjective, but I'm interested in ideas for other more objective ways to make the judgement.

- some ground rules:

1. The inspection has to be Non-destructive.
2. Surface Conductivity testing equipment available is not the best, and they say their results on Ti have proven pretty useless in the past.
3. I'd rather not do traditional hardness testing on the panels, as we would be introducing new stress risers in an already suspect structure, but I'm willing to listen to arguments to the contrary.

Any help would be greatly appreciated.

Thanks,
Paul

 

[Timelord]

Paul,

Perhaps a over-temp monitor on the panels/doors could be used as an indicator. There are stick on devices that can record the temp that the piece to which it was stuck reached. There are also crayons that you can use to mark an object that will change color if the object exceeds a set temp.

Timelord

 

[Somber]

Excellent suggestion, but unfortunately an active temp monitoring system would not be possible in the area, and crayons don't give you a feel for how long the temperatures were maintained...

We know that there are times when the doors are getting too hot (part of the reason they are Ti), but the main factor that is in question is was it in the "too hot" range for a long enough time to have done damage beyond acceptable limits...

effectively, I can't control the input conditions more than we have already done, and I can't measure those conditions well enough in flight to make the determination...

so I'm still looking for options to evaluate the results...

 

[Sparweb]

How about sticking in a "dummy" component, made of the same Titanium, that you can subsequently remove and Rockwell test?

For example, glue on (use a hi-temp adhesive) a piece of Ti sheet over top of the sheet in question. Fly for a few typical cycles and then take it off. Maybe peeling it off would muck up the sample, but there are many ways you could mount it (some better than others).

Mounting it in the right spot might not be easy, and I'm wary of fastening extra bits inside an engine bay. If anything comes loose you could have given yourself a FOD situation.

Structures in the engine bay are complex, so I would get an engineer (DER/DAR) to look at it, in case your test requires a "dummy" substantial enough to affect material stresses. I don't even know if you're talking about a helicopter or fixed wing, so I can only spin out ideas.



STF

 

[avio]

I don't know which alloy of Ti you are dealing with; but talk to your metallurgist and ask him if you should have the overheated part checked for alfa case.

 

[Somber]

ok, let me take another attempt at this...

Dummy part - this is a good idea if I was trying to establish that we had a problem - or bound how big of a problem, but that isn't the case. We know this is an issue, and there isn't going to be a funded option to eliminate the problem any time soon. There are doors at all stages of damage already, and while I am comfortable and confident in my ability to "shoot them down" by visual inspection, others are not.

I don't need to speak to a metalurgist about my concerns for the door, unless they have a recommendation for judging condition beyond the color, surface, and distortion that I already have. And none have so far. I understand the problem, I'm just looking for a more objective way of assessing the level of damage.

So, any other ideas about how to evaluate the damage already done to these Ti doors? The skin is 6AL-6V-2SN Aircraft grade Ti at .090" thickness. The damage is coming from exposure to what is effectively jet exhaust.

Thanks for the input, I really appreciate it - please let me know if you have any more or different info.

 

[davidjh]

The design should preclude "damage" during normal operation. Are you sure the airplane is being operated per the airframer's approved manual/envelope? What is the experience of other operators? Has the airframe manufacturer been involved? Is it his maintenance "recomendation"/manual to decide the fate of these panels by visual inspection? If the parts have been "damaged" as stated, by definition don't they have to be repaired/replaced to maintain the airframe in its "airworthy" condition?

 

[Somber]

davidjh - I'll answer each of the questions individually...

The design should preclude "damage" during normal operation.
- I have made a career out of fixing/addressing problems that should have been precluded. There is a myriad of reasons why the designers either get it wrong, their designs aren't carried through properly, or the usage shifted so far out of plan that it induced problems never conceived of.

Are you sure the airplane is being operated per the airframer's approved manual/envelope?
- we own the A/C, and we decide the envelope, and this damage can occur from time to time despite design and efforts to minimize.

What is the experience of other operators?
- same as us, but so far they have lived with the guidelines, I am trying to help make the guidelines clear and less arguable.

Has the airframe manufacturer been involved?
- they feel this has been addressed sufficiently and are thus not much interested unless we send more money for them to "study" the problem, money that isn't available.

Is it his maintenance "recomendation"/manual to decide the fate of these panels by visual inspection?
- the maintenance guidance is clear, but the interpretation can be subjective because of the nature of the inspection. As I said in the first email, I would be satisfied that I could make the right call each time based on the guidance, but the regular maintainers and their bosses are not as confident (and I'm not always available to help) - hence why I would like to add an additional/alternative inspection method that would be less contentious.

If the parts have been "damaged" as stated, by definition don't they have to be repaired/replaced to maintain the airframe in its "airworthy" condition?
- almost everything has acceptable damage limits on a fielded A/C, this is just one of those acceptable damage limits. My input decides the "airworthiness" with respect to problems like these, so that is part of why I am trying to help make it so that they don't have to come to me scratching their heads. The more stuff I can sort out through the maintenance guidance, the more time I have to work on the bigger problems that can't be addressed through the manuals.

so again, I don't want to use anyones time worrying about any aspect of this problem other than:

Does anyone know of a more objective inspection for level of Ti heat damage than a visual inspection?

 

[Sparweb]

Define the damage more clearly. You originally mentioned distortion. Can you be more specific about what damage you are observing? I particularly want to know what kind of damage is termed "acceptable" according to the Maint. Manual which you find cause to reject. What kind of aircraft/rotorcraft are we talking about? Damage within the engine compartment has different ramifications than damage to the cowling around an engine pod, or its access doors.

I don't have a table relating hardness tests to strength of Titanium, but if you do, I would continue to pursue this avenue, by the way.


STF

 

[Somber]

jet fighter A/C, external skin panel/door, are exposed to hot exhaust from a jet fuel starter... The inspection gives color and distortion criteria... effectively, if the material is into the "brown" color range, and there is visually perceivable distortion (usually looks like oil-canning) - the panel is bad. The problem is that this panel takes some monster stresses and gets beat around sometimes. So you can get distortion for reasons other than overtemp. That is what is causing the maintainers to worry. If it isn't "overtemp" induced, they can perform some standard repairs to the doors and press on. So they start arguing "are you sure that is "brown", maybe it is "straw"?", or "that distortion is a dent, and the heat damage, while "brown" is over in that other section, so is it still ok?" It is simply a fairly subjective call, and their bosses hate that, especially on extremely expensive and hard to get parts.

I have tons of information about Ti, but so far the only additional options for evaluating are either destructive or require equipment or techniques not available at the locations where the damage is being evaluated.

I was just taking a shot in the dark here, I have people all over the world digging around for me. If you know of a good way to evaluate Ti for heat damage, please give me a post or a link... otherwise, don't worry about it...

please don't take offense if you ask for more clarification and don't get it... I'm totally swamped, and I'm not going to have time to post here - I'll just drop in and check for a "silver bullet" idea that I'm hoping one of you have out there...

thanks again...
Paul

 

[Somber]

FYI - Based on all the responses I got from the requests I shotgunned out, the following is the most promising lead:

Meandering Winding Magnetrometry (MWM) seems like the most promising option for evaluating this type of damage. Its basically an Eddy Current technique used to measure absolute condutivity. We've already discussed that normal volumetric electrical conductivity measurements are not useful in Ti, but the permeability of Ti may allow us to glean some useful data using MWM. However, doing so will require some testing and correlation on affected parts.

Whether or not I can get the funding to do the up-front testing and evaluation - I'm not sure. But at least I have a solid recommended option to provide to the field bosses - if they want to pursue it, they can dig up the money...

 

[Sparweb]

I've never heard of MWM before, and a quick search on the 'net shows why: detection of damage in metals is not what it's usually used for.

Hope the project doesn't "blow up" in your face!

(For those who don't get it, the technology is used, among other things, to detect buried landmines.)


STF

 

[jetmaker]

Somber,

Sorry for the late response, but have you tried thermal imaging equipment? I saw a demonstration with some equipment in which variations in thicknesses could be established by such a method. I'm thinking that maybe ther is a change in the thermal conductivity of the material if it has been overtemped.

Suggested it 'cause it might be worht a shot.

Good luck.

jetmaker

 

[Somber]

I'll try to keep the exploding to a minimum SparWeb... heh

jetmaker - very interesting idea, I'll have to look into that, we always have a few "toasted" doors around, if I can get the equipment together, I might be able to produce a "profile" that we could judge other panels with...