Al. vs Steel gouge repair

[daxman1]

I think this will be a simple one that I should already know but I don't. Most of the time we work with aluminum (2024, 6061, 7075...) and for nicks, gouges, scrapes...the disposition is to smooth and blend unless the discrepancy is greater than 10% of the thickness of the material then it is scrapped if a doulber is not an option. What I'm hoping to find out are there similar rules of thumb for steel and composites? Thanks.

 

[rb1957]

1) think of a better thread title (it defaults to the first few words of your post ... not very helpful).

2) 10% thickness may be a typical SRM allowance. I think you can go further if you know enough.

3) 10% is probably good for steel, though I'd worry about this. Steel fittings are usually more highly stressed than s/m Al. Also finishing processes (that Will will probably add).

4) I wouldn't blend composites much at all ... maybe blend and repair with RT repair.

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

For composites, blend "repairs" on the surface are generally not acceptable.

 

[daxman1]

For composites with minor nicks, gouges, scrapes, after NDI for the most part a "Blend" repair consists filling the defect (resin, chopped glass, EA9394 or others...)and smooth using 180-grit, 220-grit abrasive paper or scotchbrite equivalent. Do you have other information for these cases?

 

[SWComposites]

Dax - what you descibe is generally ok. What is NOT ok is sanding the original plies and either leaving it or just filling it with resin.

 

[Sparweb]

Daxman1,
The blend you describe only sounds OK for non-structural fiberglass cloth structure, because the 10% scratch you mention is deep enough to score the outer layer of fabric. I would not expect that to be sufficient for either structural fiberglass, or any form of carbon structure, fabric or tape. I'm basing this on my familiarity with composite repair schemes in the DA-42 structural repair manual, though it has been quite a while since I had that document in my hand. Most of that aircraft is composite, and most of the external skins are fiberglass cloth and carry structural load. In many places the skin is only 3 layers thick, so the scratch/gouge you are talking about eliminates 33% of the local strength even if it penetrates 10% deep. By damaging the outer layer of fabric, one assumes that this layer is completely severed.

The scratch removal must go down by one complete layer in the damaged area. No half-layers if you understand what I mean. Blending required is specified by a ratio of depth to width, and in many places on that aircraft's structure it's 1:30 or more, IIRC. Into the blended area is bonded a fresh layer of fabric matching the structure in the area, using resin that also matches. Then lots of sanding and filling and sanding and filling ensues... That's all I really remember.

The other thing I remember is "don't drop your screwdriver".
(No, it wasn't me!)

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[berkshire]

I will second Sparweb's remarks, I have spent most of my life repairing fiberglass and advanced composite sailplanes which the DA-42 evolved from. For example the outer plies of a wing skin are frequently only two layers of glass or carbon-fiber. The third layer ,if it is there, is usually a sacrificial layer of very light cloth. Depending on the maker your scarf ratios are minimum 30-1 usually 50-1 and on carbon sometimes 100-1.
If you do not go all the way through the sacrificial layer you are home free, blend out and putty. If you go into the structural plies you end up with a difficult repair trying to get a scarf joint onto a material which wants to yield just a little as you are grinding it.
Some of the German manufacturers have a technique for dealing with thin skins on PVC foam cores called Hammer Scharfen, where instead of grinding they gently tap the skin in the repair area to slightly compress the foam, then roughen the surface of the laminate to apply new layers. This enables the repairman to get a full thickness of carbon fiber into the repaired area, and you do need instruction on this before trying it yourself.
B.E.

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

 

[blakmax]

Back to the original question. Aluminium vs steel. You really need an assessment of the risk of depleting the fracture toughness below an acceptable level. Many steels have a relatively low fracture toughness so you need to be careful.

I have seen data that suggests that blending notches in composites actually reduces static strength after fatigue because fatigue results in crazing at the tip of the notch and that reduces the stress concentration.

Blakmax