calculating laminate thickness

Hi all,

I have repositioned this question from "materials" to here. I was not getting any bites.

I need to determine laminate thickness as a function of number of layers, ratio of fibre to matrix, areal weight of fibre and the type of fibre. So far, I have two formulae, but they are giving different results. Anyone got a reliable method of calculating the thickness? It is for hand layup.

 

You need to know the density of the fibre and the resin matrix. You need to know whether your ratio of fibre to matrix is a volume ratio or a weight ratio (weight ratio is usually specified). Then if you consider unit area of laminate you have a certain weight of fibre and a certain weight of resin. Applying the material densities gives you a volume for each of these consituents and hence a total volume per unit area which is the thickness.
John

 

Thanks John, I get the idea.

 

The formula goes like this:

t = wf*[1/df + ((1-Wf)/Wf)/dm]*1000

where
t is the thickness [mm]
wf is the weight per unit area of the fibres [g/sq.m]
df is the density of the fibres [t/cubic m]
Wf is the fibre weight fraction [-]
dm is the density of the resin matrix [t/cubic m]

Some values:
df: E-glass: 2.55, HS Carbon: 1.78, HM Carbon: 2.1, Kevlar 49: 1.45
Wf: CSM: 0.25-0.35, WR: 0.45-0.55
dm: Isophtalic polyester: 1.21, orthophtalic polyester: 1.23, vinylester: 1.12, epoxy: 1.2

The formula ignores the effect of voids, the amount of which will usually be less than 5%.

A more simple approach is to use this rule of thump: The thickness is 1 mm per 600 g/sq.m of reinforcement.

 

the way I'm using is to calculate thickness using Gerr method,from that calculate the number of layers and weight per unit area of the fibres, after that I calculate resin weight starting from the assumption that glass is 30-35 % of total weight in a hand lay up using vinylester resin.
So if I have 7 kg of reinforcment i assume the total weight with resin should be: 7/35*100=20kg for sq. meter.

20kg for the area of the part of the hull I'm working on should give me the weight of the finished fiberglass for that part.
Am I doing wrong?

 

Sounds OK to me (if you're using CSM!)

 

Well,actually a mix of CSM and WR...does that makes a difference?
Example:

If I have to laminate a part 12 mm thick, I will go for a 450 mat right after the gel (1.2 mm) and a combo of wr 800 + a CSM 450 repeated for 5 times.
1.2 + (2.2 * 5) =12.2 mm

Now, to calculate the weight for a square meter laminated like that:

450+(450*5)+(800*5) = 6700 g/m2

6.7 kg m2

assuming a 65 % of resin and a 35 of glass:

6.7/35*100= 19.1 kg/sq.m

Let's say I have to laminate like that the upper part of a topside that is 20 sq.m :

19.1*20= 382 kg

My topside's upper part will weight 382 kg.

Please tell me if I'm correct since I'm a novice at scantling rules-

 

To understand better composites

To understand better composites and how it works.
http://callisto.my.mtu.edu/MY472/index.html
A course about composites engineering.
You do not need to make the math exercises...

More than 2% of bubbles or voids makes a carbon laminate useless. 5% is a max admissible in glass/polyester.

 

You can calculate a weighed average fiber content and use this in the formula given above.

You have 5 layers of 450 g/sq.m CSM = 2250 g/sq.m with a fiber content of 30%.
You have 5 layers of 800 g/sq.m WR = 4000 g/sq.m with a fiber content of 50%.
Weighed average fiber content is (2250/(2250+4000))*0.3 + (4000/(2250+4000))*0.5 = 0.428 or 42.8%.

I haven't included the 450 mat as this might be regarded as a surface mat, and as such shouldn't be included in these calculations. I don't remember what Gerr says about this, though, so if he says that it's OK to include the mat, then by all means do it - the fiber content will only drop to 41.9%.

 

Thanks for your help guys.

Ilan Voyager,

For carbonfibre laminate this tells me you just have to go for vacume bagging if you want to ensure you have less than 2% voids.

 

First, thanks to all who gave very useful information. It's the usefulness of forums.

*********Info avalaible for carbon fibers************************
-Glass are more tolerant and hand layups work if well done-

**It's important to mix the resin/hardener (generally epoxy) without introducing air**

When using a "thin resin", for Unidirectionnal (UD) not too heavy a manual good light rolling or squeegeeing (do not use a foam roll) is enough. For very thin cloth (like the 4 or 6 oz) a careful hand impregnation works.

It's the most common case in home boat building where carbon fibers are used in local reinforcements. The best is to use a "thin" resin. To impregnate with just enough resin that the weaver stays apparent; too much resin forbids the entrapped air to escape.

When using a "thick" resin, or heavier UD and clothes, a vacuum bag is needed.

With biaxials, triaxials and preimpregnateds, vacuum bag is mandatory.
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It was not the subject of the thread, but...

Another link if you want to calculate composites properties, after having understood the course on composites given in my precedent thread :

http://www.eng.hawaii.edu/~nejhad/

Professor Nejhad has developped online CGI softwares permitting to calculate the properties of composite and to predict failure using two criteria. I thank the Professor Nejhad for giving the free use of calculation tools which were found in very expensive softwares a few years ago. That makes the true interest and beauty of internet...It's very complete.

For some people internet acces is costly or difficult or you may want to have the soft in your hard disk. A very good shareware (29 US$) Laminator v 3.5 at http://www.thelaminator.net/

A trial is worth, this soft is good. 20 years ago a such soft under DOS and ANSI ugly design would cost a few thousands bucks plus a annual maintenance fee...

As always with engineering, calculations are good only if you use good data...
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Absolutely totally out of the subject of this thread.
Saving on the hard disk online manuals is a pain, long and the results deceiptive. A good website copier is a blessing.
the freeware HTTRACK is very easy to use.
http://www.httrack.com/page/1/en/index.html

 

Thanks for the Callisto link Ilan V. It is very helpful. "The Laminator" looks like a good program too