Likely causes of porosity in composite parts that use an autoclave

[daxman1]

Good Morning,
I have been working on and off with composites for some time now but have mainly been involved with repairing parts, not so much the root causes analyses. Can someone tell me the most likely causes of porosity in composite parts that use an autoclave? How about de-bonding issues?

Most of my work has been with carbon fiber pre-preg material but there is now a contract to use Kevlar which I have not worked with before. Is the process for working, forming, trimming...Kevlar the same as carbon fiber? What are some of the issues.

I know these are kind of general questions but any information would be helpful. As always thank you.

 

[berkshire]

daxman1,
You may get better replies if you move this post to the Composite engineering forum 327. Kevlar is not the same as carbon fiber, it is harder to trim, when ground it fuzzes. requiring you to trim off the fuzz with a sharp cutting tool , (at least it will cut without tearing up the tool), It requires special scissors to cut the fabric, because it is slippery, tough, and just slides out of conventional scissors. anyway you will get better answers in the Composite forum.
BE.

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

 

[WKTaylor]

daxman1

Have You tested the cured composite for mechanical properties?

Kevlar has good tensile and energy absorbing properties... but grossly-lacks the compression stiffness inherent in carbon-fiber reinforced composites.

Be wary about pure Kevlar composites... fibers have a propensity for absorbing moisture which can 'make' for poor quality structures with wicked-up moisture.

NOTE. When moisture-laden Kevlar pre-pregs and composites are heated above the boil-off temperature of water, there might be a tendency to out-gas steam... and most certainly will cause grief to embedded metals.

Regards, Wil Taylor

o Trust - But Verify!
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[Worldtraveller]

My experience with composites is fairly limited, but varied (solid laminates, honeycomb panels, of AL-Al, Carbon, Kevlar, and glass).

I assume when you talk about porosity, you mean in a solid laminate?

The number of plies (and total thickness of the part), the pressure used, the minimum radius of complex parts; all these things can affect product quality, and all these are specifically very likely to affect porosity in a stackup.

The process is very critical in any laminate stackup, and simply vacuum forming is not as effective as an autoclave that can achieve higher positive pressure. Also, thicker laminates usually need some outgassing time during the cure to allow for volatiles to escape.

Design is also important, as it can be very difficult to get sharp corners in a complex shape with some composite materials.

 

[MikeHalloran]

You did not mention the resin/matrix.

Most of my very limited experience is with phenolic, which requires a dwell at ~160F to release water, a normal reaction product.

I'm sure other resins have their own peculiarities.

Mike Halloran
Pembroke Pines, FL, USA

 

[blakmax]

One of the major causes of porosity in adhesive bonds and composites is exposure of the uncured resin to a humid environment. Epoxies absorb moisture which then turns to steam when heated above 100C (212F). Most people think that applying high vacuum during a cure cycle drags out the porosity, but in reality it makes it worse. The low vaccuum pressure causes the entrapped volatiles to expand making the bubbles larger. The resin flows out as the void size increases.

To reduce porosity:
1.Only expose uncured materials in a controlled environment.
2.Apply full vacuum until the resin starts to gel, then back off the vacuum to about 10 inches Hg to reduce th size of the porosity.

FAA guidance on environment controls is a bit vague to say the least. AC 20-107B para 6: Material and Fabrication Development states:
The environment and cleanliness of facilities used for bonding processes are controlled to a level validated by qualification and proof of structure testing
Whereas AC 21-26 para 8 Manufacturing Controls states:
Control humidity
Below 46% at 75F, (23C)
Below 63% at 65F, (18C)

I would strongly urge following AC 21-26 advice, especially for Nomex cores and Kevlar composites which in themselves absorb even more water than the resins.
I suggest you try this link:
http://www.adhesionassociates.com/papers/Managing Micro-Voiding of Adhesive Bonds.pdf

Regards

Blakmax

 

[berkshire]

Hey Max,
You forgot the link !
B.E.

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

 

[blakmax]

Thanks Berkshire. I did put it in but there appears to be a gremlin in what I did. SNAFU


Link

Blakmax

PS I just tried the link, It doesn't open the file automatically it just downloads the pdf file, Check your downloads to see if it there. Something about old dogs and new tricks.