How to calculate shrinkage with FRP

[MerDeNomsX12]

Any help would be greatly appreciated. I need to caluclate shrinkage and shrinkage allowances for various resins, methods, and resin to glass ratios. Does anyone know how i can get this information besides trial and error?

 

[Compositepro]

It is very difficult to calculate. It is usually a trial and error process. Epoxy resins will have a volumetric cure shrinkage of about 2%. Polyester resins may be 2 or 3 times that. Fiber reinforcement will dramatically affect how and where shinkage occurs. Cure cycle will also affect results.
Shrinkage that occurs before the resin gels often does not cause observable effects. CTE of liquid resin is significantly higher that that of solid crosslinked resin. CTE effects are a major part of what is called cure shrinkage. Closed cavity molding is affected much more by shrinkage issues than open face molding processes like vacuum-bagging.

 

[RPstress]

It's probably worth contacting various suppliers to see what they can tell you; just phoning a large company's local country contact number and asking for technical support can help.

However, I should think that you're likely to need to do some experimental work yourself as well. Your post is a bit bare and general.

Simple resin-only shrinkage tests and simple uni- and bidirectional flat sheet laminate tests can begin to reveal fundamental material and lamainate behaviour. However, as 'Pro says, putting it together to try and predict real-world laminate cure-behaviour, maybe using arbitrary combinations of materials and ply directions, is very hard.

Depending on how your research and analysis skills stack up, you might find simple work plus 'micro-mechanics' methods profitable; or, you might find a more trial and error approach, perhaps guided by simple results, is better suited. (Or maybe you're embarking on a major piece of work which will involve charactisation of materials in hybrid liquid-gel-solid states at the molecular level...)

What you might get from suppliers (and maybe from googling research papers, etc.) is likely to be limited simple shrinkage data as described above.

When looking at the Internet for info, I've found that simple CTE data for carbon and glass is quite hard to come by (or, more accurately, there's quite a bit out there, but it tends to be variable and self-contradictory). Putting together basic resin CTE without any practical work is sort of doable, but establishing basic stuff like how much shrinkage occurs at what stage of gelation and how this interacts with adhesion to reinforcement is trickier.

Putting that sort of stuff together with macro laminate considerations and mold thermal characteristics is hard, hence the possibility of a more practical approach and a limited set of materials and manufacturing methods.

 

[PanelGuy]

Chawla, Broughtman and Chamis all put forth equations on how to calculate CTE based on material properties. They are not too complex but apply in a certain range of conditions. Great for theory, kind of applicable to analysis, unrepeatable in manufacturing.

The equations could be useful at least for understanding the base relationships.

AS well, they are largely generated by working equations back empirically based on the data.

I agree with everyone above, there are calcs, but you are better off making some sheets and measurings the change. Pretty easy to do, but be careful with the orientation issues and timing of cure.

Let me know if you need more.