Sharp Impact Damages 1

[mrbliaison]

The problem concerns a 0.200 thick 7475-T7351 lower wing skin panel with a sharp-bottomed v-shaped impact damage with approximate 0.050 depth in the outer surface along a skin joint. This damage was apparently caused by an HST11AG6 Hilite head being embedded into the skin panel. Aside from evaluating the rudimentary details of this condition e.g. position of the damage relative to adjacent fasteners, determining the critical stress levels on the skin joint, resulting section reduction due to subsequent blend rework, etc. - should the nature of damage be a concern? Basic mechanical damage will require the usual 'smooth blend' and NDT. Are there any other details to consider e.g. blending deeper than the physical extent of the damage to ensure complete removal of any grain distortion (if any)? Should this condition be reproduced on some metallographic specimens for further analysis? Any experiences with this type of damage or can anyone suggest some references? Thanks in advance.

 

[SuperStress]

You don't state whether the damage is along a spanwise or chordwise skin joint which may have some bearing on how you want to treat the damage, but in general, no, you don't have to do anything beyond the cleanup of the visible damage as long as NDT gives you an okay.

As a disclaimer, let me say that I'm not that familiar with 7475-T7351 or how it performs in a tension-dominated stress spectrum like a wing lower surface.

Out of curiosity, what make and model of airplane uses this material for wing lower skins?

SuperStress

 

[mrbliaison]

SuperStress:

Really?! I seem to recall in a previous life as a rookie liaison engineer that a similar problem had occurred. The aircraft in question was much larger (250-400 passengers) although the problem was located on the upper wing surface (7075-T6). The stress engineer at the time had serious concerns with the condition – something about shock waves originating from the apex, shear planes through the grain boundaries, etc. Unfortunately the eyes glazed over at that point (I can only attribute that to immaturity although I've heard that some stress engineers have that sort of effect on liaison engineers ... nothing personal). In retrospect – I should've paid closer attention. I do remember that some tests were performed and the eventual rework was to blend out the damage an additional 0.020 beyond the maximum depth with subsequent NDT and saturation shot peening.

I probably shouldn't mention the actual make and model (proprietary information and all that) but I will say that it's used on a regional jet powered by two GE turbofan engines with a 24,000 kg MTOW carrying 50 passengers over a range of 2,000 nm.

Thanks for the input though.

 

[SuperStress]

It may be my lack of experience too (~15 yrs), but I've never heard of such things being considered in a liaison situation.

The restoration of surface treatments (shot peening) is a given, but it sounds to me like the stress engineer you dealt with in your past problem was overly cautious, to put a kind spin on it. I think he was probably trying to impress you with his (real or imagined) mastery of microscopic metallurgy.

Consider too, the limited amount of damage you are talking about; the impact from the edge of a fastener head. Is this detail worse than say, a double-drilled hole? Probably not, depending on the orientation, and there are plenty of standard repairs for that kind of damage that don't entail metallurgical analysis or coupon testing.

In the example of the upper wing skin of the 250-400 seat airliner, it fails the "giggle test" (sanity check, or whatever you want to call it) to think that damage on such a small scale requires that kind of expensive engineering intervention.

Lending a small amount to my credibility on the issue, I've been through the Boeing Structural Repair class (the first one) and nothing they teach in the class or say in the SRMs even comes close to what you described. Damage is blended out with the minimum material loss, and no more.

I'd be interested in anyone else's experience that differs from my own.....

SuperStress

 

[mrbliaison]

I can't argue with anything you've said.

I've been at this liaison business for about 20 years now. I had that first encounter with this type of damage at the onset of my so-called career and never again until recently ... now I've had the opportunity to re-live that first experience. I tried to convey what I had come across those many years ago and I was rewarded with the usual look reserved for the 'big fish that got away' story. That's when I thought about posting this problem.

I'm feeling kind of embarrassed now – maybe he was just having me on. I don't mean to imply anything but he was a veteran Boeing stress engineer as well. I think I'll try to locate this fellow and get some details about it (hope he's still in the game).

Thanks for everthing.

 

[jetmaker]

mrbliaison, SuperStress,

I recently had the pleasure of dealing with a similar issue... a csk Hi-Lok driven sideways into a skin panel at window belt level. The issue got more interesting as the maek was near a fastener location. The repair procedure was simply to blend (approx 0.047" deep) and then NDI the region. As this was a clad skin, I can not remember if I called out a shot peen requirement of the reworked area or not. The blend was then HVOF'd to restore appearance.

For my dispositions, it is customary to peen blends to restore the fatigue capability (due to higher net section stress and stress concentration effects), and close any exposed grain boundaries due to the blending operation.

Regards,

jetmaker

 

[Sparweb]

By the time you're done blending that down by 0.070", you're removed about 40% of the material. Blending and then shot peening is a fairly complex process, when all you're trying to do is remove a very small area of damage.

The damage presumably could be removed by drilling through the panel and driving a solid rivet through the skin. No load transfers through the rivet, so you don't have a big fatigue problem, and use some sealant to close everything up neatly. You could even use a csk rivet to keep things flush, meanwhile allowing you to use a smaller rivet while still taking away the same area at the surface. The cladding remains intact, too. This has been done and approved on pressurized fuselage skins, to my knowledge.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[jetmaker]

Drilling the hole through as SparWeb suggested is the prefered option for me too. However, there are situations where doing so would make the fastener a load bearing one, or the diameter could be considerably large. It is important to do the analysis on each option and select the one that is best for the airplane.

jetmaker

 

[Sparweb]

Indeed, jetmaker,

The repair only worked on some nicks that occured from prop shedding. We limited things with a D/t<4, and even at that, any holes bigger than a 5/32" rivet needed a little doubler anyway. Since our guy is dealing with a scratch that can be cleaned out with a 3/16" fastener, through 0.20 material, I think he's okay.

Perhaps I also should emphasize that the rivet passes through one sheet of material. If you connect two you will inevitably cause a load transfer through the supposedly "unloaded" rivet.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout