Separation of Al skin from Al honeycomb 4

[GGU]

Recently we have been asked to repair an aluminum honeycomb part that we suspect has corrosion inside on a large area.
Due to the size and function of the part we are forced to replace the whole external skin (0.5 mm thick), but we need to keep internal core if at all possible.

Does someone know any sound method that could be used to separate skin from core without damaging them?

 

[rb1957]

i think we use dry ice to freeze the bond adhesive, then it's easy to break. if you know the portion of the face you want to remove, you could cut that ahead of time, creating a thermal barrier to the rest of the face.

Quando Omni Flunkus Moritati

 

[Compositepro]

Cut though the skin at the perimeter of the removal area and peel the skin off. The bond is relatively very weak in peel. You could cut the skin into strips to make the process easier. Set the cutter blade to cut 0.49 mm deep.

 

[MintJulep]

You suspect some form of magic corrosion that has attacked the inner surface of the skin but has not attacked the core?

 

[berkshire]

Depending on the curvature of the part, sometimes a tool like a sardine can key , actually a tube with a slot in the end can help.
the action is similar to a climbing drum peel tester.
Depending on the resin used, a heat gun with heat applied right at the peel point, getting the resin above its glass transition temp can also help, if freezing will not make the resin brittle enough. You have to be careful not to get the other areas too hot doing this.

If you have had water in the core, you may find the skin already dis-bonded. Do not be surprised to find the corroded/ dis-bonded area to be much bigger than you thought it was.
B.E.

You are judged not by what you know, but by what you can do.

 

[blakmax]

I'm with MintJulep on this one. "You suspect some form of magic corrosion that has attacked the inner surface of the skin but has not attacked the core?"

Corrosion just does not "appear". It requires the presence of moisture (water entrapment is common in sandwich structure) and that water needs to have some path to get into the panel. I strongly urge you to look at the failure surface when you remove the skin to identify the type of failure which allowed the water in. Unless your repair addresses that, you will be revisiting this repair again in the near future. I suggest you read "Davis, M.J., Bond, D.A., The Importance of Failure Mode Identification in Adhesive Bonded Aircraft Structures and Repairs, The International Conference on Composite Materials – 12, Paris, 05-09 July 1999." at Link

and "Davis, M.J., Chester, R.J., Perl, D.R., Pomerleau, E., Vallerand, M., Honeycomb Bond and Core Durability Issues; Experiences within CREDP Nations, Aging Aircraft Conference, Williamsberg, VA, Aug 31-Sep 02 1998" at Link

My bet is that your "corrosion" will result in coincidental adhesion failure between the adhesive and the core (see the CREDP paper above), where you will find the core appearing to be intact, and the adhesive separating from the core easily. I would urge you not to re-bond to that core because the surface will be fully hydrated and you will not achieve a durable bond. Also, in my experience wherever you experience water in sandwich structure you can also expect to find core node-bond separation between the cells of the core. In such cases, the core itself has lost shear integrity and must be replaced.

Regards

Blakmax

 

[GGU]

Thanks for all your replies.

Of course we are expecting to have corrosion on both skin and core in the same areas, but as commented in my first post we need to remove and replace the whole skin. We expect to need core replacement in corroded areas as well, but reskinning will also affect large areas with no signs of corrosion, where we don't expect to find damage to the core. Anyway, we are working on the basis that the worst case scenario could be complete core replacement.

In relation to moisture ingestion this concrete part has a SB to prevent it by improving sealing of some areas; reskinning of both sides plus SB application will probably prevent moisture ingestion in the future.

Regards,
Gustavo

 

[rb1957]

"concrete" ??

if there's a SB to repair the panel, it should include methods to achieve what it says, ie ...
"remove skin by ..."

Quando Omni Flunkus Moritati

 

[GGU]

Uups!!!, "concrete" is Spanglish for "specific".

SB is preventive, it gives instruction to add sealing on some edges of the part but doesn't cover any repair once there is corrosion.

 

[tbuelna]

If you must replace the "entire external skin" of this aluminum honeycomb panel structure, it should not be much more effort to remove the corroded sections of honeycomb core, along with any corroded sections of the inner skin.

Of course if the panel OML/IML has some curvature, it could be difficult to re-bond the outer skin panel to the core without the proper tooling.