Hi Guys

 

This was one fascinating and scary bond failure!  Has anyone found the top panel of the wing yet?  The thing that was readily apparent was that the failure was adhesive, not cohesive (the adhesive bond failed, there was no shearing of the adhesive in the middle).  The part separated cleanly which means a surface preparation problem, although possibly the adhesive could have started to gel but still been soft thus not wetting properly (not likely but possible).  Or maybe the surface was cleaned with a rag contaminated with wax.  Whatever the problem, this looks like a mold release was applied since the bond line is so perfectly clean.  Even a poor bond that fails is likely to have fiber strands and lots of fracturing around it which this showed no evidence of.  And what were the conditions at the time that the skins were bonded?  To me it looks like the peel ply may have been left on but that's a guess. 

 

There are several aspects that are important.

 

1.    What adhesive was used?  Hysol 9360 is a blue color but this appears to be tan.  I am assuming that the Legacy specifies Hysol for bonding the skins or is this incorrect? The color looks more like a Jeffco adhesive which has much poorer bond strength (but still probably more than adequate).

 

2.    Was the whole aircraft prepared like this?

 

3.    Is this similar to other aircraft prepared at the time?

 

I lost the original photo's that were sent but going from memory and some guess work, assuming the inside wing panel was 24" x 36" and the bond area was 2", the total bond area would be around 1728 sq. inches, plus there was structure in the middle, say another 24" x 2", so total bond area would be around 1776 sq. inches.  Doing a very crude calculation of the low pressure side of the wing of 27 pounds a sq. ft. wing load (2200 lbs gross/82.5 sq. ft.) for a 6 sq. ft. section of wing, the load would be only 162 lbs. 

 

You sticklers for accuracy will point out this is not how it works since a real wing load is not linear across the surface front to back and that really, you need to adjust the wing area by adding the negative pressure of the upper surface of the wing plus the positive pressure of the underside thus the stated wing area of 82.5 sq. ft. in this case is actually a much smaller area thus lowering the actual wing load per sq. ft.

 

But I digress.

 

So even doubling that number by reasoning that with the wheels down, you can add the positive pressure from the underside of the wing to lower pressure of the upper wing, you still have only 324 pounds pressure on the wing panel.  324 pounds/1776 = .182 lbs./sq. in.  The tensile strength of Hysol is over 3000 psi for any normal temperature so that clearly isn't even close to the limits of strength in tension.  Pure tensile load is unlikely regardless so looking at another scenario, if air was forced against the adhesion line, like if there was a crack in the leading edge forcing air against the bond line, it would be acting more like a wedge so a peel test would be more appropriate.  The Hysol has at least 30 pounds lineal inch adhesion, so if it were 24" across the front of the panel, the bond should be good for 720 lbs if it were possible to perfectly load the bond line alone.

 

The real load would tend to be a mix of peel and tensile strength with some bond line peel and a tensile load behind that based on the stiffness of the cored panel.  The greater the stiffness of the panel, the more of a pure tensile load to the bond.  Anyway, the point is that just doing some very crude calculations and by way of observation, (how clean the bond parted) there isn't any way that the bond of that part of the panel should have even been close to failing if the bond prep had been done correctly. 

 

Can someone send those photo's to me again?  I lost mine when my AOL cut out.

 

Thanks!

 

Dan Newland