Sandwich tee joint sizing ? 1

[fcboat]

The domain is small yacht building. The tee joint is hull bulkhead joint. The hull and bulkhead are epoxy/fiberglass/linear PVC. Infused or at least vacuum. Marine rules allow me to correctly size hull plating and bulkheads. But are very short of hull bulkhead joint.
Current "rule of thumb" methods use charged epoxy fillet with tape reinforcing. Fillet radius is a function of bulkhead core thickness, and tape fiberglass weigth is 1.5 time bulkhead skin fiberglass weigth.

But it is a very time consuming operation.

So my questions are:
Are there better methods for sizing the fillet ? if a bigger fillet could save me all tape, or even one layer of tape.

Are there better joint methods that can save time ? heard of triangular foam fillets, but got nothing on sizing this.

Are there book for current cored composite structural design for 'dummies' ? I would prefer an engineering workbook than a do-it-yourself boatbuilding with very raw and crude assumptions.

NB : hull/ bulkhead junction is a big thing. a bulkhead is typically 7" * 10" (2m * 3m). The tee joint is not right angle. It may vary from near 90° in the bottom to less than 70 ° on topsides. Hull is all round from evrywhere, so it is very hard to have hull bulkhead gap less than 1/4' (5mm). Better count on a gap between from 0 to 1/2' (10mm).

 

[GBor]

FEA works fine for these situations so long as you properly orient your fibers in the analysis. You can generally use a 2-D analysis. Look at structural putty fillers and wedge blocks.

How thick are the bulkheads and how thick is the hull?

Structural Composites in Melborne, FL used to put out a product for this type of joint. Haven't spoken to them in quite some time...not sure if they continued the product line.

 

[fcboat]

Not so sure. I fear FEA can give you useful results when you CAD you metal piece, and the you CNC machine it. Ie the real piece is very similar to the computer description.

Here, the piece have at least 5mm tolerance (1/4'). That very time consuming to go better. So the joint will go from 0 to 5mm. And then the fillet and tape layup is hand made. So you must account for significant variation from computer design to real part.

Typical core thickness for hull and bulkhead is 15mm - 20mm (5/8' - 3/4'). Skin for hull are around 1.5mm 3mm(1/16' to 1/8'). More toward 1/16'. Skin for bulkheads are about half. Infusion makes very thin skins.

 

[GBor]

I typically err on the side of caution. I use skins with the highest resin content...lowest laminate modulus. Account for additional thickness around a radius. I've only done this in military applications, but with incredibly close comparison to testing. If your process is consistent enough, you can do it...and I've never CNC'd a composite part.

 

[fcboat]

Although one can find a bunch of engineering data on epoxy, glass fiber and core, which allow to do any sensed computation, when it comes to filler/putty, the only figures you get from vendor is low density, high density, suited for structural bonding, etc. Anything but a figure and a unit.

In the best case, you know the product is glass microsphere, phenolic microsphere, or microfibre based and you have the density.

I fear I will have to rely on my own tests and experiments or very crude rule of thumbs just to T join two composite parts.

 

[GBor]

There is a program available from Wolfram Research and/or Materials Sciences Corporation (

http://www.materials-sciences.com)

called SUBLAM. It does a great job for this type of joint. There is probably also a written paper available from them.

I will try to dig up some T-joint preliminary sizing that I used to do with hand calculations. It involved the moment that a joint could take, but it was fairly crude. Detailed work was usually done with FEA. Putty, if structural, would be tested for property data, or some generic number was put in and a sensitivity analysis was performed.

 

[fcboat]

Thanks for the link.

Next thing is to find what to use as putty/filler. Microsphere charged epoxy seems to be very brittle for that application.

 

[GBor]

Check for information on the USS Arthur Radford. We used a glass/vinyl-ester resin mixture for our putty. I don't recall the properties off the top of my head, but you could do a "law of mixtures" type of approximation:

Etot = Vresin * Eresin + Vglass * Eglass where "V" represent the voume fraction. Probably need to add something for voids.

I vaguely recall a 300ksi modulus with a poisson's of 0.3 and treating it as homogeneous, but I also remember a 10ksi modulus...we may have been bounding the problem...memory is fuzzy after 10 years.

 

[Compositepro]

A lot of work has been done on developing pi-preforms for T-joints in aerospace because there are many challenges with this type of joint. Cracking of the filler is a common problem. It is my opinion that a flexible rubber or foam filler should be used to create a uniform joint gap and fillet for the reinforcement splices. Under load, the joint will have to move a certain ammount. If you look at what the percent strain this ammounts to in the joint gap it will be large. If the gap is zero the percent strain in the gap will be theoretically infinite.

The stiffness and strength of the joint will depend on the fillet radius. With smaller radius the joint will be stiffer but not as strong.