Unidirectional Fiberglass

I remember reading an article about biax unis in Composite Fabricators Association (CFA) magazine. I dont have the issue at hand but heres what i can remember.

Unis of the stiched variety tends to pack closely together when laminated than the woven fabric variety. The woven variety has undulations that increases the bulk of laminate and increases thickness. This in turn leads to potential voids during lamination.

Since you are very familiar with prepregs which has a very low resin content, Unis tend to have less resin and is the choice for high performance laminates.

A high packing density leads to a stronger laminate. Prepregs has about 28% to 30% resin content. Unis, in a normal lamination technique will have about 48% resin content. Less if you take time to roll it or even less when you vaccuum bag.

 

I am sure the engineers and certainly the long term boat builders on this forum can attest to getting use to certain things (i.e.) “I know that if I use some bolt, it’ll be more than adequate.”… “This thickness of ____ will be fine for _____”. Plug-in steel, aluminum, fiberglass, etc. and mast, boom, hull, etc. respectively… and you KNOW/FEEL it’ll be fine. You don’t even have to run the numbers. You’ve done it before!

I am having a great deal of trouble re-adjusting my mindset. From my experience, I “knew” I could make a fiberglass mast stronger, stiffer and lighter than an equivalent aluminum mast. In my mind, I “knew” that fiberglass was only 75% as stiff as, but considerably stronger than aluminum. Any first-year structural engineer “knows” they can vary the geometry up the mast and easily reclaim the bending stiffness with weight to spare. In “my mind” it was stronger, stiffer, lighter and one tenth of the cost.

Then a new reality slaps me up the face… NO, the materials you have to work with are less than 25% as stiff as aluminum. I can dig around in my bag of tricks all day long and I know I’m not going to be able to recover from that hit. I guess that’s why you don’t see any “useable” glass masts out there! Eh?!

You’re dead-on rxcomposite! All properties (modulus, strength, impact strength…) are 3, 4, ten times higher in prepreg MAINLY because of fiber density. In prepreg, the fibers are the same. The resin is better, but NOT that much better! The important thing is trying to approach that 78% prepreg fiber volume! Your observation about the undulations in cloth is quite poignant. At best, a prepreg/vacuum bagged/autoclave cured WOVEN CLOTH would still have less than 50% fiber volume JUST because of the weave undulations!

Which lead me to start this thread. Why don’t we use more unidirectional (stitched not woven) material in boat design and building? Why are there so few material offerings like: http://www.aircraftspruce.com/catalog/cmpages/s2tape.php? I was hoping someone on this forum has gone down this road and might point out the pot holes.

 

It comes down to cost..

G'day Inquistitor,
We use CSM/woven rovings/CSM then add CSM/woven rovings/CSM etc, etc, until we have our req'd laminate thickness. This is the standard layup with POLYESTER resin for boats built to suvey here. It's the cheapest laminate you can produce, probably 35-40% glass.
Stiched fabrics allow 50% just because of the way they are made, but the fabric is about twice the price of woven roving. But if you use the stiched fabric with polyester you MUST sandwich it with CSM!!! ...no adhesion.
But if you use epoxy you don't need the CSM.
Regards,
Rob.

 

Aerospace composites is the first road i took and i suspect that you did too.

I think you are more into advanced composite design/application than a boatbuilder as you are geared towards high performance use of composites.

I think high performance composite design is moving towards boat. We all hear of filament winding, fiber placement, using bands of unidirectional fibers to build up layers. It is used in rocket nozzles, fuel tanks, and even complex shaped fuselage structure. Resin infusion, vaccuum bagging is gaining popularity in the high end boat market.

I agree that aluminum has a better characterestics than composite. It is only now that with so many composite fabrication method available can the composite equal or better aluminum. Composites can be built up in layers, dialling the strength where it is needed, or combined with other low performance material to produce a high performance product.

For example, a very strong vacuum bagged laminate is thin. It also flexes readily, so it is combined with a foam/balsa/honeycomb core to provide stiffness. We see this in boats nowadays. Regulating bodies have also adopted this technology with some conservatism.

Others have stated that Woven rovings and mat have to be in exact proportion in the laminate. They are correct. It is stated in the construction method for single skin laminate. However, the manual does not state the sequence of the ply. I usually save a layer or two by careful placement of the fibers and or stiffener spacings.

I agree. Boatbuilders should look into the materials that enhances the mechanical properties of the laminate. The sales manufacturer should explain more of the high technology application rather than concentrate on what sells.

 

Unlike aluminum whose strength is equal in all direction, composites fibers are not.
But composites have higher ultimate strength than aluminum so that when fibers are aligned at the greatest direction of stress, it has more load bearing capacity.

Aluminum is stiff but stiffness is a function of thickness so building up fiber bulk will compensate for the lack of stiffness. The use of lightweight materials such as balsa, foam core or aluminum achieves this.

So by combining different composite materials where its mechanical properties are at its best, composite comes out the winner.

Attached is the general response of the Marine structures as published in “Marine Composites” by Eric Greene Associates. Attached also are my sketches of the subject as interpreted/coming from other sources.

The images are for general design guide and understanding of the stresses involved.