In carrying out a repair to an alum

[Nick Melton]

In carrying out a repair to an aluminium honeycomb panel, the AC43.13-1B, has the following (Chapter 3, Section 2, Figure 3-11):

"For the repair of larger holes in which it is inconvenient to use a face patch because of aerodynamic smoothness requirements in that area, both the core and facing are sometimes replaced with glass-fibre fabric discs and resin. Undercut the core, as shown in the figure below, in order to obtain a better bonding of the fill with the facing. Fill the core cavity with accurately shaped resin-saturated glass cloth discs and press each ply down to remove any air bubbles. Special care should be taken that the final plies fit well against the underside of the top facing. When the core cavity is filled, close the cut-out in the facing with resin-impregnated glass fibre fabric discs that have been pre-cut to size."

How is this possible to get the plies to fit tightly to the underside of the undercut without using access from the other side? An undercut repair would only be applicable if the cavity was potted?

 

[berkshire]

Nick Melton,
This technique relies upon the fact that fiberglass cloth has some spring back. You press down on the glass cloth with a tool while you insert the final pieces, then use the natural spring of the weave of the cloth to exert the pressure. You then need a caul plate to hold the surface flat, or the material will spring above the finished surface. To me it is an ugly system and heavy.
B.E.

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

 

[SWComposites]

Why would you not repair per the SRM?
How is this glass fabric "repair" structurally justified?

 

[blakmax]

I'm with you SWC. If you look at Figure 3-10 there are several examples of how to replace damage core, but it is not clear that there needs to be a doubler bonded to cover the cavity. I have seen many examples where core has been potted up and sanded flat and no external doubler applied. In many cases this results in extensive corrosion as water migrates in through the exposed cavity. Structurally such repairs may be "justified" on the basis of low skin loads. Eventually my organisation mandated that at the minimum a metallic foil was to be sealed over the "repair" to prevent moisture ingress.

If I had to perform a repair where the aerodynamic profile on the damaged sheet had to be preserved, I would attack it from the other side, cut out the damage and make an insert piece to suit, then bond an internal doubler plate over the INSIDE of the damaged face sheet and insert. Then make up a replacement core and bond a doubler over the other face sheet to restore integrity of both sheets. There are considerable weight and time penalties so I would need to be convinced that the aerodynamics was sufficiently critical to justify both.

Given that AC 43-13-1B also deals with wooden and fabric structures it suggests that this AC may be quite old. It has not been updated since 1998 and maybe a review is required. If so, I'd strongly recommend that the requirement for an external doubler is included in Figure 3-10.

Regards

Blakmax

 

[berkshire]

All,
I think the one point being missed here and AC 43-13b specifically says it . These techniques are for secondary structures. To me if the stress loads are low enough and the core is thick enough I see nothing wrong with using a countersunk or dimpled split ring doubler and making a flush wet patch held in place with Cherrymax or similar flush rivets. You could install the doubler , clean up and replace the core and pot around the edges like a standard honeycomb repair. then close the repair with the flush patch, dont forget to really clean the aluminum first or Max will haunt you. This might take a little more time, but unless the part has a strong curvature it should yield good aerodynamic results. The fiberglass plug shown in figure 3-10 looks good in the picture, but is messy to do and does not end up looking that good.
B.E.

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

 

[WKTaylor]

NM... be cautious... the repair You cited is not executed exactly as described.

1. CAUTION. This repair is best used for a where a fastener hole or a reinforcement is required. It creates a very high stiffness concentration; and will be very heavy. Great for drilling thru for a hole/csk, anchor, etc

The undercut is greatly exaggerated... only needs to be about 1/4 to 1/3 depth shown.

Brush-coat the exposed honeycomb core and skin/adhesive surfaces with laminating epoxy resin.

Make a 'stack' of glass-fiber cloth patches [plies], pre-sized to match the hole. Each one is [or a few are] then inserted 'wet' with laminating resin... a really messy build-up procedure. These plies are NOT pre-cured, then inserted. I also think that a very thin dense weave fiberglass ply should be laminated to the exterior... but that's another discussion.

2. Primarily I agree with blakmax RE: working thru from the not aero side.

3. WARNING. Fiberglass is notorious for wicking moisture along the fibers. For this reason aluminum patches are far more dependable and simple [on flat or single-curved surfaces, another issue/story]. When fiberglass [any form] is bonded to aluminum... especially external surfaces... ensure aluminum surface treatments and adhesive primer are applied ‘by the book’. After cure/NDI/flash removal, over-coat [smother/bury] the repair and surrounding surfaces with an un-broken coating of corrosion protective epoxy primer [MIL-PRF-23377 Ty I Class C1, C2 or N] to seal against moisture intrusion...otherwise premature aluminum corrosion is guaranteed.

3. A few HDBKs, books, etc exist for validated honeycomb repairs... although adhesives, primer and surface pre-treatments, etc may/may-not very 'dated' for specific applications... are as follows.

MIL-A-83376 [CX] Adhesive Bonded Metal Faced Sandwich Structures, Acceptance Criteria

MIL-HDBK-337 Adhesive Bonded Aerospace Structure Repair

MIL-HDBK-803 Glass Reinforced Plastics - Preventive Maintenance and Repair

MIL-HDBK-83377 Adhesive Bonding (Structural) for Aerospace and Other Systems, Requirements for

MIL-STD-768 [CX] Instructions for Repair of Aircraft and Weapons Reinforced Plastic and Sandwich Structures

DOT/FAA/AR-00/46 Repair of Composite Laminates

DOT/FAA/AR-08/54 Guidelines for the Development of a Critical Composite Maintenance and Repair Issues Awareness Course

DOT/FAA/AR-TN06/57 Best Practice in Adhesive-Bonded Structures and Repairs

AFFDL-TR-79-3129 PRIMARY ADHESIVELY BONDED STRUCTURE TECHNOLOGY (PABST) -
Design handbook for Adhesive Bonding

Advances in the Bonded Composite Repair of Metallic Aircraft Structure [Baker, et all]

SAE AIR4844 Composites and Metal Bonding Glossary Favorites

SAE AIR4938 Composite and Bonded Structure Technician/Specialist Training Document

SAE AIR5719 Teaching Points for an Awareness Class on "Critical Issues in Composite Maintenance and Repair

SAE AIR6291 Guidelines for Repair Process Evaluation of Aluminum Bonded Structure

SAE AMS3920 Acceptance Criteria - Adhesive-Bonded Metal-Faced Sandwich Structures

SAE AMS-STD-401 Sandwich Constructions and Core Materials; General Test Methods

SAE ARP1524 Surface Preparation and Priming of Aluminum Alloy Parts for High Durability Structural Adhesive Bonding

SAE ARP5256 Mixing Resins, Adhesives and Potting Compounds

ARP5219 Impregnation of Dry Fabric and Ply Lay-Up

SAE R-336 Care and Repair of Advanced Composites

And if You have access to a copy...

USAF T.O. 1-1-690 General Advanced Composite Repair Processes Manual

And for grins [really good basic knowledge]...

FAA-AIR-90-01 Repairs to Damage Tolerant Aircraft

DOT/FAA/CT-93/79 Effects of Repair on Structural Integrity



Regards, Wil Taylor

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