While I was working on a fiberglass fuselage, I found a slight deflection in the exterior surface. This damage was not evident at first glance, but after performing a tap test, I identified a delaminated area.
I chose a 3-inch, 90° die grinder to carefully grind the perimeter and start revealing the damage. Once I opened a small area, I saw that the glass had released from the resin layer on the foam. The resin was imprinted with the glass pattern, but all the glass had achieved a clean release.
Once it was open, I could easily see a gap between the glass skin and the foam around the perimeter. Using a flat probe and a razor blade knife, I easily found the terminal edges of the delamination, or at least where it continued out of reach. I continued opening the area until I reached where the skin was still firmly attached to the foam.
I needed to carefully bevel the glass skin to the foam to lay the new glass and keep the repair as close to grade as possible. Once I identified the size and shape of the area, I used a plastic sheet to cover it and draw the first and last layers (marking not shown). I cut the three layers of glass using this plastic template. After cutting out the outer shape of the plastic, I used it to cut the first piece of glass, which would be the last layer I applied. This preserved the inner mark (the first layer to be applied) on the remaining piece of plastic, which I then laid on the glass to cut around with a rotary cutter. I drew a new mark on the plastic halfway between the inner line and the outer edge of the plastic. Cutting to that line allowed the plastic to lie flat so I could cut around it with a roller cutter. I cut out the final mark and cut the glass for the first layer.
With three layers cut, my smallest layer was just large enough to cover the foam to the edge of the bevel, the next layer was slightly larger, and the last extended just to the top edge of the bevel. Prior to applying the first layer, I sanded the foam/resin layer down to bare foam. I treated the foam with a skim coat of glass bubbles and resin to take up any inconsistencies and seal it. I applied Layers 1–3 back-to-back and allowed them to gel, at which time I made a glass microballoon putty to fill the surface for sanding and fairing. Once I sanded it, I circled the low spots with a pencil and filled them for final sanding.
I performed this particular repair with epoxy. I always wait until the gel stage of the resin before applying putty to assure a good chemical bond to the glass. If I do not have time to putty at the proper time, I use peel ply, which is especially important with epoxy. If I use a vinyl ester, the glassed surface remains receptive to good bonding for about 24–48 hours, but peel ply can be used in any instance. If I suspect the laminate repair will be high anywhere, I know that filling and fairing out around the perimeter will be needed.
I have been asked what caused the damage. There was no evidence on the outside of any impact or abuse. No heat was used in this area, and no glass laminate delaminated from the resin layer. I suspect the vacuum bagging process had a leak or breach, causing this area to release at some point. I have seen this kind of clean release from the resin/foam boundary layer when a laminate is peeled away, but there was no peeling in this area.
I had no idea how much precision goes into repairing fiberglass delamination. Articles like this make technical topics much easier to understand for those of us who aren’t experts. Thanks for breaking it down so well.
In some cases delamination can be repaired without having to do a patch. At least one common, large, turbine, composite propeller allows delaminations on the blades to be repaired by drilling small (up to 0.125 inch) holes at the edges/ends of the delam and then using syringes to inject resin.
yes it can , this delamination was of unknown origin and given it’s foam core, and not knowing if it was damaged from impact, opening it up for inspection was prudent. If it had been found immediately after de molding from the vacuum bagging process, odds would be a delamination caused by a bag leak and injection would be a safe bet. I’m curious what the repair process would be on the blades if a delamination is found after installation and use , injection or an invasive repair. I’m sure all manufactures have a protocol.
For the specific repair in the CMM I was pulling from, it’s down to location (not over the spar) and size (25 square inches or less); there is a different method for single delaminated areas up to 150 square inches, as long as the total area to be repaired is under 300 square inches. It is not uncommon to see blades from the 80’s covered in stippling where this type of repair has been done repeatedly for new damage over time (my best customers fly off of gravel runways).
inject and inject until when? is there an end game that becomes an issue. Now that you are mentioning the stippling, I saw a relatively new MT blade at near the hub that has whet I could call stippling/ ribbing/modeling and thought it was suspect… maybe it was a factory repair
The repairs end up being slightly heavier than OEM blade material, so eventually you reach the point where even with the paint on minimum thickness and no lead in the taper bore it outweighs the balance master and is unrepairable. The attached is what I meant by stippling, this blade has an 87 serial number and has been through my shop ~10 times for different reasons and is still serviceable.
Will all due respect, the bond line does not look sufficient around the perimeter. Minium 1/2″ per ply. That molded joggle needs to be cut into as well.
the laminate is about .050 which would bring the scarf to about 1.5 inches in a structural application at a generous 30X scarf. A straight .5″ per laminate is not an actual number that is used in composite repairs. That said, it certainly could be used. 4 laminates at .5″ would be 2″. this repair was closer to 20x at 1″. Yes cutting into the joggle was done, but as you said, could be cut further than is was. a judgement call was made because of the non structural location.
This is excellent info Mr.Chase, you make it look easy. Thank you.
Perhaps a dirty or oily hand or other object touched the foam before the initial lamination.
Good thought… the glass could certainly have been contaminated before production. My understanding is the Glastar was vacuum bagged which is a wet process, so gloves were certainly worn. That said, many don’t glove up when cutting raw glass which could have been the issue, like you said.
…Or when cutting and shaping the foam prior to lamination. This was just another guess at what could have gone wrong. Or maybe the glass was dirty at that location. It was a good fix none the less.