Strength of repairs to composite surfaces

Gentlepeople;
This week I completed a course in aircraft advanced composite repair, and I asked the forbidden question. That being, does a properly done repair to a composite structure return that structure to 100 percent of it’s strength? The damage, lets say to a four layer carbon fiber sandwich 12 inches long by six inches wide, was caused by a hammer strike which put a one inch diameter hole in the top surface, damaged the 1/2 inch honeycomb but did not damage the lower surface of four layers of carbon fiber. Replacing the honeycomb properly, then scarf patching in five layers of carbon fiber according to the manual returns that area to as good as new. We were told that the resin bonds the original broken strands to the patch strands, allowing loads to transfer between them as if they were unbroken. My contention is that if that is true, that same piece of surface could be made by using six pieces of carbon fabric each three inches wide laid side by side (overlapping by 1/2 inch) to make up the 12 inch long piece, and be as strong as the first piece made with unbroken strands of carbon.
The resin (like the repair mentioned above) would allow a transfer of the loads between the overlapping pieces to transfer the load from one to the next.
Now, quite frankly, I don’t believe that is true. So by that same thinking, I don’t believe that a properly done repair as mentioned above returns that panel to it’s original strength. I say that I asked the forbidden question because the instructors of this business have to answer it with a half truth because the FAA requires that any repair to an aircraft structure must return that structure to 100 percent of it’s original strength.
My question to this forum is; Truthfully (the FAA be damned), does a properly done repair return the structure to 100 percent of it’s original strength?

Ive read a while ago that with a scarf of 1:20 you can achieve around 100 % of original strength. 1:10 (commonly used in boathull repairs) gives about 80%.

If done right a repair can be be more stronger than before,it a case of understanding where loads are applied and using the right reinforcement.

Agreed, although it is a case-by-case assessment. The shape and original schedule make or break how well a repair can replicate original strength while keeping outer appearances in line.
Not to mention post cure needs etc.There is a LOT to consider in such a claim to 100% without destructive follow-up testing…which rarely happens.

You almost have to over-build in order to supersede or match strength in post.

I would be really interested in a report on structural repairs.

I once had a manual by an aircraft company, which described various repairs. Lost it though. I believe the link to the download was somewhere on this forum, cannot find it though…

Every damaged structure can be repaired in order to recover its orginal capability to fully sustain design loads. The problem is that this will be achieved with a (repaired) structure that will have an increased weigth… and for an aircraft this is not a good news

Herman: try a look on the web to DOT/FAA/CT-87/9 titled “Engineering compendium on the manufacture and repair of fiber- reinforced composites”

I can’t offer any scientific study but some practical evidence. In 2011 I performed 109 structural repairs on carbon fiber bike frames (mountain and road). None of them have failed (knocking on wood). I perform static stiffness testing before and after each repair as well as having some frames tested externally to verify my results. At least with these structures I can confirm that it works well but I have no experience with core containing objects.

As mentioned above, the repair will be slightly heavier than the original since you need to overlap layers. The laminate will also be one layer thicker at the edges of the repair. The additional thickness can be compensated for with good planning and a thicker topcoat. I can see where it might still be a problem on the outer skin of aircraft. It’s also a problem on certain bike parts such as seat posts, handlebars, and forks, which makes these parts financially non-repairable.

Here are two documents I have related to composites repair. The second one is quite long and technical. Give it a read!

The problem isn’t getting 100% of the strength back, that’s easy.
The hard part is distributing the stress loads so you don’t create a stress concentration hard point.

It’s on a case per case basis! We have been known to exceed the original structural integrity on various train fronts that we have carried out structural repairs on. Materials (in many cases) are far superior today than they were years ago.