Design allowable value knockdown for wet layup

[wc.waldemar]

Dears,
I have some allowable values for prepreg carbon fiber, however I would like to estimate the allowable values for wet layup + cure at room temperature.
Is there any knockdown factor that I can use in this case?

 

[SWComposites]

Nope.
a) the prepreg material and wet layup material use different resin systems
b) room temp cure will likely have low hot or hot/wet strengths
c) a lot depends on what temperature / environment condition you want the allowables for
d) a lot depends on what property (tension, compression, ILS, ILT, etc) you want the allowables for
e) suggest purchasing a copy of CMH-17 Volume 2 Rev H at aae.org. That has a bunch of composite properties, mostly prepreg, but at least one wet layup material system

 

[Fibre and resin guy]

Hello Waldemar,
Wet lay-up Carbon Epoxy is still fairly used for marine structures. with our without vacuum bagging.
ISO 12115-5:2008 (and maybe later revisions ?) has tables and calculations for determining design allowables for such materials, based on fibre mass or volume content depending on parameters such as complexity of the surface, use of vacuum bagging, etc.
You can find similar rules in Bureau Veritas NR546, Sec 5 Art. 2. and I suspect DNV GL and ABS have something that offer similar approaches. You can find the BV one at erules.veristar.com.

Keep in mind your fibre volume fraction and thus mechanical properties will be highly dependent on the skill of your laminator (these rules are therefore fairly conservative). If a full mechanical testing campaign is out of budget, at least a fibre volume content determination on a representative laminate would be recommended.

As SWComposites mentionned, your wet/hot properties may be significantly lower : your resin will have a very low Tg compared to 120°C or 180°C cure prepreg, especially if not postcured. I haven't heard of epoxies going past 60°C, after several days of postcure at ambient, with hardener+resin systems designed for this (happy to be corrected if someone knows references that can go higher without postcure). Make sure your service temperature is well below that. Such low Tg is acceptable for outside parts if painted in bright white. Anything darker and you will likely enter the soft domain of the matrix.

Best regards.

 

[wc.waldemar]

Thank you very much, guys! Really appretiate your help!

 

[SWComposites]

Ack, typo above. Sb sae.org.