As an introduction, this is the first post that I am going to write on my new website. I've been writing posts on LinkedIn for over a year at this point, so I'm going to slowly transition to this site and write most of my posts here.
I talked about composites repair some months ago on LinkedIn, but I thought that I would get back to this subject and take up where I left off. I want to expand this discussion a bit, so my next three posts will all be about repair, how to find out what needs to be repaired, and how to design composite structures that are what is called fault tolerant – which means that they still perform their function even though they are compromised in some way.
The first thing we need to understand is what most commonly happens to a composite that caused it to have to be repaired. Some of this is obvious. Like for an airplane, it they get a bird strike on some part of the airplane that is made out of composites, and the composite comes out almost as bad as what happens to the bird. Or, if there is something on the runway and a tire kicks it up against a composite panel on the underside of the airplane. Mostly this is the accidental damage sort of thing, and whoever is responsible for the aircraft or boat or car knows when it happens.
Another sort of damage that also needs to be repaired is the insidious or invisible damage that can sometimes happen to composites. Fatigue stresses causing a hidden delamination or failure at a joint that is partially hidden can happen. If these sorts of things go unnoticed, they can lead to major structural failure – which is never a good thing. Other insidious damage or hidden damage can happen around areas where the composite is bonded and/or bolted to something else, like a metal frame for instance. For these kinds of damage, it is a bit more difficult to know what actually needs to be repaired. I’m going to go into this particular subject in some depth next week, so lets just say that you found some hidden damage and you need to fix it.
Where do we start? First we have to find the extent of the damage – is it just local, or is it spread out beneath the surface from an impact of some sort. The repair approach for each of these situations is somewhat different. If the damage is local, and away from any critical part of the structure, most of the time it can be sanded out, and a patch bonded in to make the part as good as new.
That is what is shown in this graphic, where the damaged material is removed and the composite is sanded back to expose each layer. Then a patch is made up to fit in the hole made by the repair prep, and co-cured with a good film adhesive to bond it to the original part. This sort of repair is done in the aircraft industry quite a bit because composite parts on airplanes get damaged just as much as their metal cousins do. And these sorts of repairs, if the adhesive that bonds in the patch has a good enough shear strength that it can shear any bending load into the fiber layers in the patch, it works out quite well.
This even works when there is a delamination that is discovered that doesn’t extend too far from any location. What is done then is to find the edges of the delamination, cut out the upper part of the material above the delamination, then sand down the edges so that a patch can be made that will bond to the original base material and make the part as good as new.
If the structure that is damaged has a foam or honeycomb core, which is quite common in large light weight composite panels, commonly the damage is not just to the outer layer of composite, it also extends down into the core as well. In this situation, a somewhat larger area of the outer plies of the part or panel need to be removed, and the core that is damage also needs to be removed back to where it is still undamaged.
Then, depending on the type of core (foam, honeycomb, etc.) it needs to be made whole again by bonding in some new core material, making sure that its top surface is flat so that when a composite patch is applied to the top surface of the replaced core. The graphic here shows a honeycomb core, but if the core is either a polyurethane or syntactic foam, the repair is much the same.
And, finally, if the surface finish doesn’t matter that much, you can always fall back on the brute force method of putting doubler plates on both sides of the part to fix it.
It is always best to repair the composite first, then add doubler plates, but that is not necessarily required, depending on the use of the part. The pic shown here uses just a plug to replace the damaged material, and then doubler plates are added to the inside and outside of the part to carry the load across the damage. If this works aesthetically, or if this is not a lifting or control syrface for an aircraft or submersible, then this is the easiest and quickest method of repairing a damaged composite.
That’s about enough for this week. Next week I’m going to be talking about non-destructive evaluation of composites – essentially finding out if there is a problem to begin with, and if there is one how bad it is. There are several techniques that work really well with metals that are much harder to apply to composites. But I’m getting ahead of myself a little here.
See you all next week, both here and on LinkedIn.
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