Composite Aircraft Structure

[MNLiaison]

Analyzing repairs to metallic aircraft structure by reverse engineering applied loads,having material allowables,and knowing failure modes is a routine action performed by non-OEM engineers in the industry.

Analyzing composite structure is a different animal all together. Material 'A' & 'B' allowables are hard to find / non existant, applied loads by reverse engineering joints is difficult / impossible with out lots of testing, and laminate failure modes are difficult due to unknown applied loads.

So my question. For those of you out there who have anlayzed composite repairs on aircraft structure what process did you follow, where did you find allowables, and have you applied it to primary structure?

With the 787 / A350 coming, is there going to be anybody capable of analyzing repairs other then Boeing/Airbus? Will this then make airline, MRO, or other engineers essentially into total paper pushers?

Any thoughts are welcome.

Regards,
MNLiaison

 

[A380istoobig]

I attended a course by Alteon Training in Seattle called "Composite Repair Repair of Advanced Composite Structures For Engineers" not too long ago.

http://www.alteontraining.com/training/coursedetails.aspx?id=1948

The same question posed to the instructor. The answer was that a lot of the composite material properties are proprietary and not available to aircraft engineers.

There are checks that an outside engineer (not working for an OEM) can do such as the thickness x stiffness (production) must be equal or greater than the thickness x stiffness (repair).

The Alteon course offers guidance how to accomplish these calculations. But the reality is that we outside aircraft engineers have to reply on the OEM’s to provide repair approvals.

I wished they taught paper pushing in college.

 

[berkshire]

MN Liaison
Your comment” Analyzing composite structure is a different animal all together. Material 'A' & 'B' allowables are hard to find / non existent, applied loads by reverse engineering joints is difficult / impossible with out lots of testing, and laminate failure modes are difficult due to unknown applied loads."
This has always been a problem when repairing advanced composites. Early on when most advanced composites were fiberglass reinforced plastic. It was sufficient to design for rigidity; you knew that if the laminate was stiff enough it was more than strong enough.
With the greater stiffness of carbon this is no longer the case. The other wild card in this equation is the bond line on the repair. You are always doing an adhesive joint; you are adding new material to an existing structure, very often in conditions where you cannot match the methods used to produce the factory part. Example Vacuum lay-up with heat blankets on a part that was autoclaved. You may not be able to debulk to the same extent that the factory did with the result that your resin ratios may be higher than the original part. Peel strength at the bond line is also critical. Not all resins that are good for laminating have high peel strengths. On some repairs it may be better to accept a resin with a lower structural strength and higher bond line/peel strength.
In some cases you may have to do test coupons before you do the actual repair to check your analysis .
B.E.