Sandwich Panels

[SamB]

Hey Guys

After numerous searches I have concluded that there is nothing on sandwich panels here, except for some lovely CAD videos.

We are looking to use sandwich panels on this years car to improve stifness, has anyone ever used these before and can offer any advice? We should be using honeycomb but I am very open to suggestions...

Cheers guys

Sam

[benny41]

g'day sam

It depends on what shape you want your panels. and how thick they need to be? If you want curved panels you will have to run honeycomb panels. (http://www.hexcel.com/Products/Core+Materials/Honeycomb/) if they are flat panels id look at something like rohacell. (www.rohacell.com). The main thing i found out order this stuff very early because they dont make alot of some particular types.

[Brian Perry]

Sam, will your panels be replacing any critical frame tubes (i.e. side impact) or just reinforcing the structure?

Its hard to beat the stiffness/strength to weight ratios of honeycomb cores.

Don't limit your core material choice by geometry as many teams have demonstrated the thermoforming capabilities of foam cores. You will be more limited by your manufacturing process, for example prepreg or vacuum resin infusion.

[NRBaer]

Sam,

You can in fact get curved panels using some of the structural foams. DIAB and Rohacell both offer thermoformable foams.

That aside, the real question you need to answer is how you are going to manufacture and attach your sandwich panels. At Lehigh we vacuum infuse almost everything and wet-lay the rest. Most of the structural foams, if not all, cannot withstand the temperature of an autoclave. With that said you have two basic honeycombs available: Nomex and aluminum (to the best of my knowledge, we don't pre-preg at Lehigh).

Good Luck

[Rellis]

If you are looking to bond in panels in a steel space frame find a place that works on airliners there floors have some great chunks of sandwich panels.

Also your epoxy you choose and your bond prep are probably more important than your panel.

[SamB]

Cheers guys

We have quickstep machine so its prepreg all the way for the actual panels and then aluminium for the core. What I am mainly stuck on is the method of bonding the core to the panels
The panels are not replacing any tubes, but after last year we need a vastly improved stiffness for the chassis and this one of our ideas.

[Ppada]

Why don't you just change the design of the spaceframe....?

Helsinki has a good design and its really stiff.

[Rellis]

why not just layup the pre preg with the core use a film adhesive

[RiNaZ]

SamB, Rellis is right, you can lay up your first set of plies, cure it, and then lay up the film adhesive and core, and the final set of plies, and then cure it again. Film adhesive is basically resin filled with filler. It comes in sheets.

You can also lay up all your pre-preg and core all at once, w/out the film adhesive (only apply to our type of homebuilt project). Some ppl avoid doing everything in one shot becoz that the core might show some print-thru on the carbon. In our case, cosmetic isnt much of a problem. And sometimes, it is much easier, to lay up the first stack of plies, and cure it, then to do all in one shot.

But make sure that if you lay up the first stack of plies, and cure it, make sure you include your peel ply in there too. If not, you would have to sand the surface so that the film adhesive and the core bond to the carbon's surface.

[Rellis]

I would do the layup all at once and make sure you have enough bevel on your core so it dosent shift.

[A Richards]

Originally posted by RiNaZ:

make sure you include your peel ply in there too. If not, you would have to sand the surface so that the film adhesive and the core bond to the carbon's surface.

I've heard that a peel ply surface is terrible for bonding on. Apparently something along the lines of its leaves a waxy film deposited on the surface. Anyone know anything about this? Anyway whenever I've bonded anything to carbon I've sanded the surface right back.

[Mully]

It depends on the peel ply that you use, some do contain releases in them which apparently transfer onto the part.

For most things a peel ply surface is perfectly adequite for bonding, but apparently sand blasting is the best surface prep for bonding.

[RiNaZ]

Our main application is mainly homebuilt projects. So peel ply surface is good enough for bonding. If you were in production or in the aerospace business, obviously, you would use a different type of peel ply which is far better than what you can buy at a local fiberglass shop. And it's very common to see ppl sanding the surface again even when they use peel ply, especially, since most of your parts are small, and you have a lot of time on your hand. Use your judgement, sometimes, when you take the peel ply off, some of it might get stuck on the part. If you see this, sand it down till you wont see stain of peel ply.

I personally wont sand blast my surface for bonding. I feel that you cant clean it good enough to prep it for bonding. You'll leave all these tiny particles of your media on to your surface.

[A Richards]

Sand blasting is a dramatic improvement over anything else that I have seen. When we did our carbon driveshafts a couple of years back the bloke designing them did a lot of testing on bond lengths and surface prep. To cut long story short sand blasting the inserts and tubes made the difference between them breaking in the door of the workshop and finishing the 06 comp with quite a few hundred kilometers of hard testing. I suppose surface prep is probably the single most important factor in getting a good join and really requires a lot of testing of samples to get it right sometimes.

I've never used adhesive films to join cores to carbon after it's been cured but can think of quite a few instances where it would be very handy. How well does the core stick to the film when you're working with it? Am I right to assume that you cure it under vacuum?

[Rellis]

Grit blasting is used in the aviation industry you get the best bond prep with the least amount of risk of cutting fibers. you only need to knock off the shiny part.

also if you need to attach stuff to it look at click bonds they make alot of handy threaded attachments

[RiNaZ]

Mario, where do they use the grit blasting in the aviation industry? I mean, which part or which section. Also, i havent heard of click bonds before, do you have any sites where they sell this or more info on it?

A Richards, yes, cure under vacuum and in an oven. The core stick to the film just as easy as you would stick a carbon on a carbon using resin. It is after all, resin.

If you had test samples that suggest sand blasting for best bond prep, you should stick to that route. I still think that as a general rule, stick to sand paper, especially if you're new to composites.

[Rellis]

My name isnt mario its a quote by Mario Andretti. The compnmay I worked for grit blasted every bond joint including the fuse halves

click bonds http://www.clickbond.com/index.php
http://www.theflightshop.com/