Direction of stitched glass fiber laminate?

I am designing a 13 m semidesplacement power offshore catamaran. I am to provide sandwich construction advice and need some input on fiber direction.

In the hull sides it will be biaxial but which direction?
+-45 or 0/90 and why?

Direction in bulkheads, longitudals and transom if you have preferences.

I can see Mr Tennant is using 0/90 but don't understand why. I have lots of experience with high speed mono hulls but not with catamarans. In the mono hulls we used +-45 in the hull sides.

If you have thoughts around sandwich configurations in areas of the ship, feel free to advice.

Thanks

 

If you have to ask a forum of people with no obvious qualifications you need to not make a recommendation at all.
Consult with a professional engineer in the appropriate specialty.

 

IMO - it would be dependent on design and manufacturing methods to optimize the strands to resist the forces in the locations with the most loading. In an ideal world I'd probably use 0/45/45 triaxial for hull sides, but there are always manufacturing and costs to consider.

The real truth is that it probably doesn't make much difference for most applications. Where it does - you probably will be using uni anyway.

 

If it was me I would use 0-90 purely for the full length fibre at 0.
+45-45 are all short fibres, 0-90 can be used in direction of max load/ stress. But as mentioned for a vessel this size the laminate should be properly engineered. You may find bigger materials suppliers can do this in house.

 

If you actually want to know what is "best" meaning strong enough and as light as possible, the first thing is to decide what the loads are in every area of the boat.
Next is to decide the margin of safety you want. Highly engineered military aircraft use a factor of 1.5 based on very expensive FEM modeling and perhaps some subscale testing.
Then you determine the actual strength and stiffness characteristics of your fiber and resin system, based on how you will manufacture the boat.
Then you determine the fiber direction and thickness for the specific parts of the boat, not forgetting the stiffness requirements (buckling of the structure is one).
Then you figure how closely you are going to match the minimum thickness with a practical layup.
Be sure to reinforce the layup where ever you are going to require bolts, at least for the bolt bearing strength of the laminate in that area.

Or you just use a 0/90/+45/-45 laminate and vary the thickness as you see fit.

If you want to use the graphite most effectively you need some engineering.

Good luck

 

Basicly there are two common ways in boat building. For foam and endgrain cored boats you need longitudinal and transversal fibers. 0/90 or triaxial 0/+-45. Stripplanked needs only transversial fibers +-45 commonly used for this..

BR Teddy

 

I agree with upchurchmr, it seems weird that you are being paid to design a large catamaran but don't know how to do it so are asking the general public??

Structural design of powercats is easier than sailing multihulls because of the lack of rig loads and reduced torsion. But you still need to consider diagonal panel strengths

0/45/90/45 is stronger than 0/90/45/45

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

First of all thank you for responding mr Woods.

I might have understated my knowledge in Naval Engineering. It is not that I do not have the education or hands on experience in this field.

I do have a masters degree in Engineering Design with sandwich technology being one of the subjects. I also have 10 years of experience building high tech composite, high speed monohulls for pleasure and racing.

Some times it is easier to ask people with the knowledge in hand rather than figuring every thing out for your self. I do know that there are people in this forum with knowledge in hand and I do know how to filter the information given.

I only needed to know why to use 0/90 in hull sides rather than +-45 as I have not done any calculations on this issue. It is by the way to be 25 mm 80kg PVC foam in the sides so it is for inner and outer laminate.

I certainly can figure out this for my self. redreuben was the only one answering my question rather than criticizing why to ask on a public forum...

Regards
Roy B

 

Roy,
Sorry about your reception here but we often get very similar questions from people who have no knowledge whatsoever. To be honest i don't think anyone on here has a definative answer to your question, i certainly don't. As you have noted some designers use +/- 45, others use 0/90 and they both seem to work just fine. I personally think that an all uni laminate is better than either from a theoretical standpoint just because of your ability to build a quadraxial laminate but with continuity of fiber in all directions which you don't get with any of the stitched fabrics at 50" wide. But in reality just about any configuration seems to work because for the most part boat structures are fairly lightly loaded except for certain areas that can be easily dealt with, with extra laminate. An interesting example of how anything seems to work is the Australian catamarans built with duflex panels, with the panels joined together every 1220mm there is no continuity of fiber in any direction and yet they seem to work just fine.
I would be very interested in what you eventually decide on.

 

xdoctorx-

Richard woods is correct “0/45/90/45 is stronger than 0/90/45/45”

If the load is not in plane with the axis (0 degrees) there is a substantial loss in the strength/modulus of the laminate. The most commonly used is the Hart-Smith rule of 10%. That is if you load a UD fiber at 45 or 90 degrees, you get only 10% of the original strength, or nearly just the strength of the resin acting.

For example, if a UD laminate has a strength of 135 GPa, loading it at 45 degree plus will yield only 13.5 GPa.

The best layup is when the direction of the fibers do not vary much from the next ply such as 0/45/90/45/0. Cross plied laminate are not very efficient.

Laminates are inertia based, meaning it is the outermost skin that has the highest stress. The farther the outermost skin from the base (thicker laminate) the less stress it receives and is best shown by the flexure formula
Sigma=Mc/I where:
M=bending moment
c= distance to neutral axis
I= moment of inertia

This formula works well for rough sizing single skin laminate but the best formula to use is;
Sigma=M/EI x E x y where;
M= bending moment
E= tensile/compressive modulus of plate
I= moment of inertia of laminate
y=distance from the neutral axis or height (h) of laminate/2 or h/2.

You can find out using the any of the formula that with the outermost laminate, the Ultimate strength or the modulus is at highest at 0 degree and least at 45+ degrees.

Thus, if you place a 0 degree on the outermost, you get 135 GPa. Placing a 45 degree will give you only 13.5 GPa.

There are other formula available to find the strength/modulus of a laminate for every angle deviation of load such as the sine cos rule (ply engineering constant).

For sandwich laminate the second formula will come close but you have to do some tweaking as it uses the transfer of moment inertia ie: I=bh3/12 + Ad2. Sandwich structures are much more complex.

 

RX,
Very useful data as usual. Care to share the source of the data that shows the 0/45/90/45 vs o/90/45/45 strength difference. I'm not disputing it but would like to see the rest of the story. As you know the former can really only be achieved by using a quadraxial fabric or with Uni whereas the latter can be achieved using 0/90 and a +/-45.

 

Steve, there are no real data if that is what you are looking for. It is just an explanation from various sources/books.

What I have explained in the formula is that the outermost skin should have the highest strength/modulus in orientation as it receives the highest stress. One side is in compression. the other side in tension. Both must present the highest strength in the direction of the load applied. The mid portion of the laminate is in gradual reduction of stress but in in increasing shear as it nears the middle or neutral axis. That is why, in cored composites, the low strength mid portion is replaced by a core capable of meeting the shear criteria.

The real explanation is the coupling effect, in which the plies adjacent to each other should not vary much in ply angle orientation. It should be gradual. Attach is the formula which shows the degradation of the laminate at is bent, twisted, and sheared. I don't use it as I don't do much of high end composites.

That rule can be broken if you are using very thin laminates such as prepreg which has an individual ply thickness of about 6-10 mils. The WR (0/90) can be used and cross plied (a loose term) arranged as a quasi isotropic laminate. That is the WR is arranged 0/30/45/60/90/-60/-45/-30/0. This can be assumed to be a homogenous isotropic plate. If it is too thick, a UD can be used following the same procedure.

 

While it appears that rules are made to be broken, the Classical Lamination Theory (CLT) always has an explanation for it.

Take the case of the panel. +45-45 is used to increase twisting load. That is good for a square panel as the 45 degree fibers starts from one corner and ends in the opposite panel. The strength is optimized.

In panel design, square panels do not lend well. A rectangular or an aspect ratio of 2 is often used. A 0/30 or a 0/60 is a better compromise as the fibers will start from one node to the opposite node, more or less.

In long slender mast, the inner laminate is always 90 degree. Not much help in bending stress but preventing inner laminate buckling due to increased hoop or circumferential strength.

 

When you refer to biaxial are you referring to a two directional weave or multidirectional weave? Depending upon the weave would depend upon the engineering for maximum strength. In unidirectional fiber one would consider 0, +45, 90, -45, etc. If you are using a woven fiberglass such as a plain weave in which every other stand alternates between vertical and horizontal, one may want to consider 0, 45, 0, 45, etc. because at 0 you also have the 90 and at +45 you also have the -45. One may also add a layer at 30 and 60 degrees depending upon the weight of the cloth and the amount of layers one will be applying. Although not as strong as a woven cloth, using fiberglass mat on the outer layer will provide a smoother finish with less sanding. If you are using a biaxial weave with several directions woven in, you may not need to use anything other than 0 orientation because the fabric already has the different angles for the stresses woven in.

The other thing to consider is the forces that you will be encountering for the component. For a hull, I like to use a plain weave (reducing cost of woven fabric) and a 0, 45, 0, 45, etc. After considering the orientation, the polymer is critical for strength. If you are building you own boat, I would suggest going with the extra cost up front and using a good marine epoxy resin versus the less expensive polyester or vinyl ester resins. A good marine epoxy resin will have less delimitation and better strength polyester or vinyl ester resins.

My comments are on the hull--for a mast, RXComposite's comment of 0, 30, 0, 60 would be best. A unidirectional prepreg will often benefit a mast build rather than a weave.

Good luck on your project.

 

gfusch

A bi directional fabric (BD) can be a 0/90 or a +45-45 stitched fabric. To differentiate, biax is used for +45-45.

WR or woven (0/90) is different as the fibers goes one over under with no stitches. The type of weave is either close (basket weave) or loose (harness satin). Woven have lower strength than BD as the fibers undulate during the weave losing directional strength but it has greater strength than BD in the +45-45 direction. It does not rely on the adhesive layer itself or stitch but "tugs" on each other.