Fibreglass; help needed with numbers

[Wynand N]

Being a steel boatbuilder for many years, I had done little straight forwarded GRP jobs in between eg.hatch covers, shower trays etc. that does not need a lot of finesse..
However, times are changing and I am moving over to fibreglass with a little production daysailer soon, and I have a few questions regarding manual hand lamination vs vacuum resin infusion vs strength.

To keep this simple, I will just quote the basic fibers I will use and has the following data based on the average laminated thickness from two reputable sources.

CSM 300g = 0.78mm
CSM 450g = 1.2mm
CSM 600g = 1.55mm
W.R 610g = 0.79mm
Coremat 2mm = 2mm

A couple of days ago I did a small resin infusion experiment and I used the following dry pack; 3 x 450g CSM with 1 x 610 WR - and after the test piece was removed it measure on average about 1.7mm thick! About a 30/70 resin to glass ratio achieved.
To check, we done exactly the same layup with hand laminating and the thickness was 3.4mm, much less than the "expected average" of 4.4mm which is cool.
Compared to even the better than average hand layup, the VRI method produced a laminate 50% thinner and what a saving on resin, or is it:(

I cut two strips, one each from the different methods, and compared the stiffness. Although the hand layup is twice as thick as the VRI, it also feels twice as stiff :?: I was under the impression that the VRI should be stiffer due to the fact of it 's better glass to resin ratio...

As an example; the hull sheer of the day sailor calls for a thickness of 4.4mm and the following layup should be OK according to yet another source, D. Gerr's book on boat strength.

CSM 300g = 0.81mm thick
Coremat2 = 2mm
WR 600g = .79mm
CSM 450g = 1.22mm

and all this layups add to a laminated thickness of about 4.79mm and is on the save side of the required thickness at that spot.
However, if I use the result based on my VRI comparison, the laminate thickness will be reduced by about 50% leaving it only about 2.4mm thick...:confused: I believe that thin layup will not stand up to the task at hand if I am to believe the strength based on my two sample strips explained above... More glass to get to the required "stiffness" will be defeating the object with VRI as the resin savings goes out the back door.

Am I missing something :?:

[Manie B]

This may or may not help ;)

what i have done with many samples is to cut pieces 1.0m x 0.2m and put it on top of two bricks at either end and put weights in the middle on it, until it breaks.

very interesting to learn how some bend and some just snap in half

i even jumped on one and fell on my ass :D :D :D

like you i have also worked in steel that just bends but i can assure you that my 9mm ply and one layer of 600 woven glass and epoxy is bloomin strong

so it will be very interesting to see the outcome of a simple test would be on your different thicknesses

from my readings the strength is supposed to come from the glass and not the resin - so the thin vacuum infusion is supposed the be right :?:

[marshmat]

Wynand,

The issue of stiffness versus strength is one area where composites differ greatly from steel.

The stiffness of a panel, as you know, is a function of the Young's modulus and the cube of the thickness.

When you do a vacuum infusion, the fibreglass layers are squished together before the resin is added. Thus, the overall panel is thinner, even though it has the same amount of glass. In your test, you found that it was almost exactly 1/2 the thickness of your hand-laid sample. Thus, the stiffness due to thickness would be 1/8 of that for the hand-laid sample. But the Young's modulus of your vacuum-infused piece is much higher than for the hand-laid one, because it has a much higher ratio of (stiff) fibre to (floppy) resin.

What you'll find is that on a stiffness to weight or ultimate strength to weight basis, the infused sample is both stiffer and stronger per kilogram (or pound) of material used. The hand-laid sample is appearing slightly stiffer because it is much thicker, and likely much heavier, than the infused one.

You can't get this effect in metal, where the properties of the steel itself are the same regardless of how thick it is. In a composite, small changes in the ratio of fibre to resin (and thus the thickness, for a given amount of fibre) make for significant changes in the intrinsic properties of the material. Switching from hand-laid fibreglass to vacuum infusion is roughly akin to switching from garden variety 350 MPa mild steel to heat-treated HSLA; changing the layup process effectively makes for a totally different material.

I'm curious why you'd be using so much CSM in an infusion layup. The benefits of infusion really become obvious only when you have continuous-fibre fabrics in the outermost layers.

[Wynand N]

Matt,
I assume by your explanation I must keep to the laminate schedule as mentioned above with the longitudinal stringers fitted as per plan, although the laminate "thickness" will only be about half of what is required with VRI. And it is safe to assume the overall weight of the hull would be around 30% less than with hand layup method.

As for the CSM - I have two full rolls of each of the weights and have to use them first as it represent a small fortune in monetary terms here. That said, when it is used, I'm switching over to multilayer/Multiaxial fabrics that is in fact cheaper in weight compared to loose WR and CSM for same from a new supplier Manie so kindly introduced to me awhile ago.

[Fanie]

WR 450g - 0.35mm
WR 700g - 0.5mm

Ratio is about 40 to 50% resin to glass weight, if the vacuum is as you said and no leaks you should come out at about 40%.

WR 1 streight 450g,
1 @ 45 deg 450 WR,
1 layers CS,
1 layer WR 600g @ 45 deg
1 layer CS
1 layer WR @ 45deg,
and 1 streight 450 g WR.

If you come this way again I'll give you a piece of foam to play with - you're going to love it ;) Un be friggin lievable stuff.

[marshmat]

Wynand,

You should see roughly comparable strength at failure for both samples. But going with a thinner, infused layup instead of a thick hand-laid one could lead to a more flexible hull, even if it is roughly the same strength. That may pose a problem if the hull skin tends to flex significantly, both from a mechanical fatigue standpoint and from a performance / hull shape standpoint. So it might prove necessary to build in a few stiffening ribs, or to use a bit more glass in the infused one, so that the overall laminate is still stiff enough.