# Fiberglass layer strength

Discussion in 'Materials' started by rturbett, Jul 6, 2021.

1. Joined: Aug 2005
Posts: 193
Likes: 4, Points: 28, Legacy Rep: 35
Location: FINGER LAKES, NY

### rturbettSenior Member

we are building decks using divinycell- my friend had done good work putting two layers of 7 oz cloth on each side of the diviny.

Is there any advantage to doing multiple layers of glass instead of just a single layer? For simple math, would a single 10 0z layer on each side be as strong as two 5 oz layers?

FYI we are not vacuum bagging, but still getting good adhesion

Thanks for any thoughts or expeience.

Rob

2. Joined: Dec 2016
Posts: 7,877
Likes: 1,762, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

Turning the same glass in different directions is stronger, ceteris parabis than the same total weight cloth not turned.

Since the 7 oz is likely a woven, turning the follow on layer when possible, is stronger. For turning, you have to determine how many layers total and turn accordingly.

3. Joined: Oct 2010
Posts: 10,386
Likes: 1,048, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

You will need a little bit more glass on the outward facing side, to get a measure of puncture resistance if something heavy (and sharp edged) is dropped. 2mm minimum I would say

4. Joined: Dec 2005
Posts: 2,980
Likes: 614, Points: 113, Legacy Rep: 506
Location: Monroe WA

### ondarvrSenior Member

What type of resin?

5. Joined: Aug 2005
Posts: 193
Likes: 4, Points: 28, Legacy Rep: 35
Location: FINGER LAKES, NY

### rturbettSenior Member

Thanks- two layers it is- for the point that fallguy made. Using epoxy resin
Rob

6. Joined: Dec 2016
Posts: 7,877
Likes: 1,762, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

Just remember it needs to make sense visavie overlaps and such; it may not be worth turning 45 versus 10 degrees or going with a single layer if you are forced to overlap and butting glass is really not wise.

For my cockpit panels, I used triax and turned the stuff 90 degrees to get some 0 strands on the width and pretty sure it was wise. For my transom, turned each of four layers of biax about 20 degrees...etc. had to accept using overlaps on cockpit either way

plan ahead and you can also relief for overlaps and reduce fairing

7. Joined: Aug 2005
Posts: 193
Likes: 4, Points: 28, Legacy Rep: 35
Location: FINGER LAKES, NY

### rturbettSenior Member

Agree- butting glass would be pointless to the mission here. the construct is able to handle overlaps where needed

8. Joined: Dec 2016
Posts: 7,877
Likes: 1,762, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

Another thing to consider, the top side of a deck is not the side in tension and the bottom side does more work and may be invisible, so you can take advantage of that at times by making the bottom much stronger. A walkable deck with onlg a single layer of less than 10 oz is a bit light...

bajansailor likes this.
9. Joined: Aug 2022
Posts: 15
Likes: 0, Points: 1
Location: USA

### Ajg2199Junior Member

I understand the general point about turning, but I’m not following what fallguy said here.

I’m also still curious if it would it make a difference using two 5oz layers or one 10oz (independent of direction).

10. Joined: Dec 2016
Posts: 7,877
Likes: 1,762, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

So, first of all, a butt seam is not going to be great. Overlaps are fairly essential to avoid an area of profound weakness next to higher strength. A well done butt seam is not impossible, but rational behavior is needed, like not an inch apart, and not a single layer..etc. Let's put that behind us.

There is no such thing as regardless of direction! Direction matters; almost always. I suppose an exception for all csm layup; I digress, a bit.

If you are able to turn the fabric, without creating fairing nightmares (either by nature or relief in the Divinycel), then two layers of 5 ounce is better because you have tows going in four directions. And this is a bit oversimplified, but generally accurate. It really doesn't matter on an already stiff laminate or small part. So, say your part is 6"x6". The deflection difference between two 5 ounce laminations one turned 45 degrees and a 10 ounce is going to be negligible. But head on up to 24"x24" and it'll likely be notable, based on my limited experience. Or, a layup of ?5 pieces of 10 ounce glass is probably not going to vary against a layup of 10 pieces of 5 ounce.

But, when it matters, it is all about the tow directions each participating. This is an amateur answer.

Consider the simplest case of a uni. All the tows are in a single direction. Say, for and aft on a deck. There are no tows then in the athwart direction and athwart stiffness will be poor. As you begin to add tow directions, the entire panel stiffens in as many directions as you add...make sense?

When I built my deck panels, I ran the triaxial glass at 90s. Not perfect, perhaps, because the 45 degree fibers then aligned, but the zero fibers were at 90 degrees to each other, so that is what I did versus running the 2nd layer at 45s. Probably some debate as to which would have been best...since I was dealing with 6 tow directions. Maybe an expert can coin in and give us a better answer on the ideal triaxial layup for deck stiffness.

11. Joined: Aug 2022
Posts: 15
Likes: 0, Points: 1
Location: USA

### Ajg2199Junior Member

Thanks for the clarification.
What confused me was specifically this:
I still don’t quite understand it, but my main question is more broadly theoretical. That’s what I meant by “independent of direction.”
If using CSM or uni, for example, I’m guessing there shouldn’t be a difference in strength (in the one direction for uni) using 5 layers of 10oz or 10 of 5oz. Obviously the uni would be weaker in the other direction.

I apologize if these are stupid questions, but I’m new to fiberglassing.

When turning, does it matter if the layers are all turned the same direction, or should they alternate?
For example with the layers at 20 degrees: 0, 20, 40, 60, 80 clockwise or 0, +20, -40, +60, -80

And (not to question your methods), but why the triaxial at 90 degrees rather than biaxial at 45?

12. Joined: Dec 2016
Posts: 7,877
Likes: 1,762, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

If you think through the logic; alternating doesn't matter much.

As for the use of triax versus biax, more about available materials toward the end of a single build really. You'd not order biax if you had triax in stock is all it was...

13. Joined: Aug 2016
Posts: 242
Likes: 66, Points: 28, Legacy Rep: 10
Location: Massachusetts

### rnlockSenior Member

I'm not aware of any materials that are stronger in compression than tension. Unless the deck's structure works like a hammock, a drum head, or the trampoline on a multihull, there will be significant compression, in addition to tension. Even then, in a cored structure, there will be important, local compression loads around someone's foot, an anchor, etc. Most decks will have important bending loads, requiring both tension and compression. Since compression is likely to be the failure mode, making the bottom thicker than the top doesn't make a whole lot of sense.

14. Joined: Aug 2022
Posts: 15
Likes: 0, Points: 1
Location: USA

### Ajg2199Junior Member

Would there be any merit to building in mini ‘compression posts’ or ‘stringers’ within a cored layup?
For example, little blocks of g10 in the core where compression is expected on the face of the panel, or strips of g10 where the forces would be at the panel edge.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.