I'll be using 3/4 " MDF for the long straight's and epoxy resin. Along with some Bondo Matting. I was going to reinforce the purely fiberglass areas with some metal inserts. Similar to rebar in concrete. I've done some small stuff before and gotten up to 3/8" and it was pretty solid. Just looking for opinions.
With proper bracing and materials, 1/8" would be fine.
With poor bracing and materials, 3/8" or thicker can be very bad.
Use woven mat and brace at close intervals and you can get away with thinner walls.
"thick enough" is the same question as "stiff enough", and is totally subjective, and up to you.
That said, The sub enclosure in my car is 1/4 to 3/8 inch thick depending on where it is, lots of compound curves, with some parts up to 20 layers woven mat. Knocking on it sounds like knocking on a big old tree. Dead solid.
With poor bracing and materials, 3/8" or thicker can be very bad.
Use woven mat and brace at close intervals and you can get away with thinner walls.
"thick enough" is the same question as "stiff enough", and is totally subjective, and up to you.
That said, The sub enclosure in my car is 1/4 to 3/8 inch thick depending on where it is, lots of compound curves, with some parts up to 20 layers woven mat. Knocking on it sounds like knocking on a big old tree. Dead solid.
I would ask your retailer to ask their supplier for it. It's not as though it's a difficult to find item. You might even check in the yellow pages for people who do fiberglass. Also, you might ask them if you need to add some non woven matt over the top to help smooth it or if the roving will be OK.
Also ask about getting some talc to mix with your resin to act as filler if you are looking for a really smooth and finished look.
Roving is good stuff. We used it as the sandwiched layer in the middle when we were making little row boats in a mold. It seems as though you use one heck of a lot of resin. The stuff is really thirsty. Not as much resin as multi-lamming though and the bulk of your work is done in one step.
Cal
Also ask about getting some talc to mix with your resin to act as filler if you are looking for a really smooth and finished look.
Roving is good stuff. We used it as the sandwiched layer in the middle when we were making little row boats in a mold. It seems as though you use one heck of a lot of resin. The stuff is really thirsty. Not as much resin as multi-lamming though and the bulk of your work is done in one step.
Cal
Unless I am very greatly mistaken, non-woven glass will be less strong due to the unaligned, short fibers and the larger amount of resin it will suck up (without a vacuum bag).
It seems beyond reason to me that multiple layers of woven glass with the bias at varying angles will be weaker than unwoven glass.
It seems beyond reason to me that multiple layers of woven glass with the bias at varying angles will be weaker than unwoven glass.
I can't really tell exactly what it is you guys are trying to say, but theoretically non-woven glass is stronger. If you know the load path of the panel you can optimize the orthotropic properties of the laminate to be stronger in that direction, giving higher strength for the weight or lower weight for a given stiffness. Also, if you want a quasi-isotropic laminate you will get superior results by using non-woven fibers with a symmetric staggered layup. This is because the crimp in fibers from weaving reduces the fiber strength... straight runs in uni provides for maximum strength, and also less room for excess resin. The crimp reduction in strength is fairly insignificant for glass... much more important for carbon, especially if used with vinylester resins.
The primary advantage of woven cloths is convinience: easier to form the cloth around curves (especially compound curves, and especially if using a loose weave like crow's feet), uses fewer layers to achieve a desired layup so less time and effort is required, more robust during handling, available in wider cloths, etc. If you notice, stress critical parts for aerospace and automotive racing applications typically use unidirectional fibers to optimize strength/weight. Less stress critical areas, and areas that benefit from the usability characteristics of wider cloths, tend to use weaves (such as body panels).
A good compromise is biaxial or triaxial stitched cloths. Quick buildup, available in wide sizes, relatively rugged during handling, no crimp, high fiber volume ratio potential.
Of course, for a sub enclosure the strength is not nearly as important as the stiffness... so using uni doesn't make much sense anyway.
So long as you have good consolidation (decent fiber ratio), all you really need is a certain thickness... and since the load condition is that of a uniformly distributed pressure the layup should be quasi-isotropic. So I'd recommend as heavy of a woven roving cloth (or stitched biaxial if you can get it economically) as you can tolerate for the curvature of your enclosure.
The primary advantage of woven cloths is convinience: easier to form the cloth around curves (especially compound curves, and especially if using a loose weave like crow's feet), uses fewer layers to achieve a desired layup so less time and effort is required, more robust during handling, available in wider cloths, etc. If you notice, stress critical parts for aerospace and automotive racing applications typically use unidirectional fibers to optimize strength/weight. Less stress critical areas, and areas that benefit from the usability characteristics of wider cloths, tend to use weaves (such as body panels).
A good compromise is biaxial or triaxial stitched cloths. Quick buildup, available in wide sizes, relatively rugged during handling, no crimp, high fiber volume ratio potential.
Of course, for a sub enclosure the strength is not nearly as important as the stiffness... so using uni doesn't make much sense anyway.
So long as you have good consolidation (decent fiber ratio), all you really need is a certain thickness... and since the load condition is that of a uniformly distributed pressure the layup should be quasi-isotropic. So I'd recommend as heavy of a woven roving cloth (or stitched biaxial if you can get it economically) as you can tolerate for the curvature of your enclosure.
Ya, what he said. 
RH, thank you for putting the time into that post.
I think the only comment I would have after that is that multi layering of non-woven will not be stronger than roving due to the introduction of the independant resin layers. If you could get a 20 ounce non woven mat then perhaps that would be the best solution. I am not familiar with it though, and laying down 10 of the 2 ounce cloth (or W.H.Y.) and attempting to saturate it is not reasonable, and still deals with the individual layers of the cloth.
Very good reading on the physics of working with the different materials though,
Even if you did use big words.
Cal
RH, thank you for putting the time into that post.
I think the only comment I would have after that is that multi layering of non-woven will not be stronger than roving due to the introduction of the independant resin layers. If you could get a 20 ounce non woven mat then perhaps that would be the best solution. I am not familiar with it though, and laying down 10 of the 2 ounce cloth (or W.H.Y.) and attempting to saturate it is not reasonable, and still deals with the individual layers of the cloth.
Very good reading on the physics of working with the different materials though,
Even if you did use big words.
Cal
Cal Weldon said:
The only thing I can think of is that you must be referring to a single layer of cloth/fiber. Are you comparing a hypothetical very heavy woven vs. a layered uni? If so, you are partially correct (warning, big words approaching!) - the layered uni will have superior in-plane tension, compression, and shear strength, but the single woven layer will have slightly increased interlaminar shear and interlaminar tension strength.
I always think of composites as multi-ply layups anyway since that is what I deal with. And when you are layering, you have the same problems with interlaminar tension regardless of whether it is woven or uni, as ILT is a completely matrix dependent failure. And while it might seem that the weave improves interlaminar shear in multi-ply composites, real world measurements tell us that is not the case... probably because of the crimp reduction in strength.
As for wetting and laying up several layers, I agree that it can be difficult to do well with wet hand layup techniques, but it can be done. If you have the advantage of a vacuum bag, then wet layup and bagging can consolidate dozens of layers (with that many layers without bagging you will get poor fiber volume).
With vacuum assisted resin transfer methods, the number of layers can be quite high. We regularly produce good wetout and high fiber ratios in solid laminates several inches thick. It's not a big deal to specify a laminate schedule consisting of over a hundred layers using any combination of stitched, woven, and uni cloths.
RHosch said:
Yes, I believe so.
I would have thought it was more than slightly better.
I was not aware of that. I would need to witness some testing to help me swallow that.
Yes, very difficult
No vacuum bag, just a bucket and an assorment of rollers. All by hand.
That is absolutely amazing to me, I simply wouldn't have thought it could be done. Very interesting, I would love to see it.
Thanks again for your very informative posts. Very much appreciated.
Cal
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