Yes, Composites Can Deteriorate Too

Brian Martinez

Originally published June 1998

(The following e-mail was received here at Chapter 1000. Chapter 1000 Composites Expert Brian Martinez authors the reply)

I read your article on composite construction on the web. After researching several designs, I decided to build a Vision, which is a composite aircraft. This aircraft is a plans built composite aircraft constructed using foam which is not gasoline soluble, and mostly bidirectional glass, unidirectional glass and epoxy. Everything is post cured at 150 degrees for 4 hours after construction.

I would be curions as to what generation this design would fall into.

Being an engineer, I chose composite construction, over the others because of resistance to the elements. After all wood rots, aluminum fatigues and corrodes, and steel rusts. The only material that I know that gets stronger with age is concrete, but then you have to keep it under water, and it is just a bit heavy.

Enjoyed your article

Robert Mardis, P.E.


Robert:

Not knowing the design that you mentioned I don't know all of your specific details. Based upon what you describe, however, I'll just say its in my 3rd Gen. I should point out that my generations are an arbitrary grouping in order to more easily describe the progression of largely fiber reinforced composite aircraft to folks who have no clue. So it's loose, but it gets the point across; gets folks thinking in a paradigm.

Metals oxidize and wood and other natural organic materials can degrade due to biological actions. Composite materials also degrade with age. Even though your aircraft may be post cured, that post curing in no way comes close to the high temperature/pressure autoclave processing that most aerospace industry grade airframe parts receive. Those process times can be over 24 hours and at temperatures over 300 deg F. As a result, your parts will have a higher incidence of microcracking due to voids and brittle fracturing as your parts age. Your aircraft will be more prone to creep under heat loading which over time will cycle your structure. Your aircraft will last a long time (especially if you keep it in a hangar), but not as long as the mil spec, carbon fibre/toughened high temperature epoxy resin, and industrially manufactured part. Make no mistake, composites for amateur construction are not a panacea in design. Remember, from the design engineering perspective, the material chosen comes as the result of a tradeoff. What you are getting is a performance improvement because you can achieve better surface smoothness more easily with composites and you will generally need no more than woodworking tooling to prosecute basic construction. You can also get closer to a true monocoque structure without excessive fasteners.

Most of us are not capable of getting the maximum allowable performance out of a composite structure, even when that part is largely prefabricated. The best that most of us will accomplish in our hangars and garages is at or just a little bit better than ordinary aluminum...and aluminum is good stuff.

Hope this clears up the picture a bit. Remember, all aircraft are design tradeoffs. Stay within your limits. Good luck with your project...and keep EAA Chapter 1000 in your technological heart and mind.


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Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 14 March 1999