Layup strategy advice

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#21

Thanks guys for all the replys.
Have a clearer idea of what to do now.

I have to balance several factors, with different relative weights:

Weight: 30%
Stiffness: 30%
Impact: 20%
Cost: 20%

That is, I have to have a lightweight structure that does not deform (or deforms not much) under load. It has to have some impact resistance and the cost can’t be prohibitive. That being said, could I have some comments on the following layup?

1 layer 0/90 200gsm twill carbon fabric
1 layer 0/45 170gsm twill kevlar fabric ( for impact resistance)
1 layer 0/90 200gsm twill carbon fabric
1 layer 0/90 245gsm twill carbon fabric
6mm pvc foam core (airex or divinycell)
1 layer 0/45 245gsm twill carbon fabric
1 layer 0/90 200gsm twill carbon fabric

In my original layup I stayed away from unbalanced laminates because as it is a case and it needs to close (i.e the edges have to be perfectly alligned in both case and lid) I was afraid that with resin shrinkage I had problems in that area.

I may consider inserting a longitudinal reinforcing inside brace of 1 core layer+ 1 carbon layer for added stiffness ( the case has a lenght of over 2m)

Another thing, is there any advantages in this fabric :
Carbon/Kevlar Hybrid

Thanks

#22

Just a litte tip: try to avoid sharp angles on the external shape. A 20/25 mm. radius could be considered a good compromise between the optimal section for bending stiffness and edge impact resistance.
Otherwise, you could use some L shaped metal extrusions to protect the composite from being damaged on the edges. They can be screwed in, so you can change them, if needed.
Let the hybrid use for aesthetics purposes only. The properties of one fiber weakens the properties of the other.

#23

Hi

In the drawing phase of this project of mine I’ve already ran away from sharp angles. Nevertheless, in the outside of the case I’m planning to put some rubber extrusions in strategic to protect both the case finish (I like the shiny carbon look) and the shell from potential cracks.

Another thing I remembered is that I can take a Kevlar tape and reinforce the layup just in the corners for added impact strength, without adding too much weight.

About the layup, would your reckon I can put a moderately high weight on top of the case without it bending? (at least not bending much)

Thanks

#24

Suspended at the tips, I assume you can put around 30kg on the center without any risk for what’s inside .
More, if the load is spread all over the surface.
Two centimeters of soft insulating material is required all around inside.
Soft neoprene.

#25

I would use the aramid in the most inner layer, or after it. It’s specifically strong in pulling loads, so with forces from the outside, in the inner laminate you use the properties from the material better.

#26

Suspended at the tips, I assume you can put around 30kg on the center without any risk for what’s inside .
More, if the load is spread all over the surface.
Two centimeters of soft insulating material is required all around inside.
Soft neoprene.

I’m not imagining the case being subjected to point loads but mostly a distributed load so I guess the layup would suffice. Interior foam is to be applied all over the case and on some strategic point of the double bass. Not sure quite yet of what kind of foam is it going to be as it will depend on the overall weight of the case ( I have a somewhat strict weight limit)

I would use the aramid in the most inner layer, or after it. It’s specifically strong in pulling loads, so with forces from the outside, in the inner laminate you use the properties from the material better.

The aramid is to improve impact characteristics of the layup. If I put it on the inside layers would that be better than on the outside layers?

#27

The inner layers will be subjected to more pulling loads, the outer more on compression. Foamcore will absorb a lot of energy, but if you worry about damage on the outer skin I would place the aramid on the foam core. Aramids are better on pulling forces, rather than compression forces, hence my choice to put them as far inside as practically possible.

#28

When creating lightest weight structures you have to consider 4 things:

  1. Use the right amount of material. Using more fiber, regardless of the type, than is neccessary increases the weight of the structure. How did you arrive at the chosen fiber volume? Maybe you have too much…or not enough.

  2. Use the right fiber format in the maximized position. The fiber ordering within the laminate can make a significant difference in the properties. Typically the fibers on the very outside and very inside do the most work. The farther you move to the middle of the laminate the less stress there is on the fibers. This is why sandwiches work so well.

  3. Use the correct fiber orientation to handle the forces involved. Fiber that are 45* to the loading axis can be up 1/20 of the strength of those aligned with the loading axis. Improperly oriented fibers require more fiber to accomplish the same task.

  4. The fibers need to oriented and positioned so the stability of the structure is maintained. Balanced laminates in large open surfaces yield the most stability. Unbalanced sandwiches can work depending on the shape. I’ve seen many flat surface unbalanced panels turn into potato chips.

There are a lot of variables that can the performance of the structure. If I was you, I would layup some test panels so you can feel, touch, and test the difference. Put the kevlar in different locations. Do unbalanced and balanced layups to feel the difference. Doing stuff like this is invaluable.

There are a lot of misunderstandings about fibers and structures. These often get passed on in internet forums. The easiest way to confront these misunderstandings is through testing.

I had a guy work for me for a year that came from a formula 1 racing team. They only used kevlar in the floor boards and head rests. He tried to convince me that using kevlar in a specific UAV application was foolish. I laid up some test nose cones and impact tested them. The kevlar sample was the only sample that was laid up that survived the test.

TEST. TEST. TEST.

#29

along with that Wyowindworks said, you don’t need to balance a part perfectly if there is a lot of shape to it. You might need to take the core out in some places (bolt hardpoints, etc), or add extra patches in some areas (handle carrying area, corners, etc) Start simple, see if it works. If you think you can add more in some areas, less in others, the second box will be better.

#30

Whatever the material if they are all the same weight cloths and used in the same amount of layers how can one be heavier than the other?

So which do you think is heavier…1 kilo of feathers or 1 kilo of rocks?

Surely you mean there are differences in rigidity between the fibre types, not weights in the example that you used? :confused:

#31

But the glass needs less resin… Which is why you end up with a lighter (but thinner) laminate.

If the cases are 2 meters long, and risk bending over the long end, I would probably start like this:

-CF 200 gr/m2 (5.7 oz) (for looks and overall stiffness)
-UD 300 gr/m2 (8 oz) (for lengthwise stiffness, and it blacks out the previous carbon layer)
-2x Carbon/Aramide 170 gr/m2 (for overall stiffness and impact resistance)
-foam Airex R63.80 10mm
-Carbon/Aramide 170 gr/m2 (for overall stiffness and impact resistance)
-UD 300 gr/m2 (8 oz) (for lengthwise stiffness)
-Carbon/Aramide 170 gr/m2 (for overall stiffness and impact resistance)

Perhaps even skip one of the C/A layers on the outside and inside, if integrety after impact is less important.

On corners replace the foam with glass/aramide or carbon/aramide tapes, for improved impact resistance.

Make shapes (ridges) in the surface, which further stiffen the structure.

I know I made a mess of the laminate scheme, but when ridges are present, I care less for balancing the laminate.

#32

Maybe I should ask you which laminate is thicker between an equal weight of fiberglass, carbon, or kevlar?

The density of fiberglass, carbon, and kevlar are not the same. Kevlar has the lowest density (1.44 g/cc) which means that it has the greatest volume of fiber for its weight. That greater volume will require MORE RESIN to saturate the greater volume of fiber. The fiber weight may be the same but the total thickness of the laminate is not the same…assuming fiber volume fraction is equal.

In composites it’s best to think about the volume of fiber you are using not the mass of fiber.

So, in conclusion I most assuredly wasn’t talking about rigidity. :smiley:

#33

Okay, I see where you are coming from now.

#34

The compound curves that are likely in a double bass case shouldn’t be left out of the stiffness questions. The shap, at least as I envision it, will be much stiffer than flat panels made of equal material.

Aramid has great tensile strength, orienting aramid properly with plain old glass in a sandwich type construction is what I would do…CF is exotic, cool, expensive, etc, but not superior to aramid/ glass when all factors are considered, in my opinion.