Extruded Polystyrene (XPS)

What's the difference if you make a galley cabinet out of core cell, xps, paper mache or elbow macaroni?

What, will it give out from the load of the dinner plates?

What load is there on a cabinet or a settee that requires incredible sheer strength? I suppose really fat people on a settee, but on a cabinet?

What material would you make boat cabinets out of, then?

As I'm in eating lunch, I am looking at a piece of polystyrene I have in my rv. There is a counter with a sink in it and the under sink area was left exposed to a sliding door mechanism that is not as well insulated as the rest of the rv since it slides. The counter and support are made from 1x2 firring strips and dirt cheap door skin ply from Home Depot. The back wall of the under counter area (which is exposed to the less insulated sliding door mechanism) was left open. To increase insulation properties, I duct taped a sheet of xps to the open wall over 2 years and over 50,000 miles ago. No skins, no plywood, just naked insulation, duct taped in place. Still fine after 2+ years and 50,000+ miles. Why did it last?

It's not *structural*! It's just along for the ride.

 

Lets say you're in a beam sea and the boat is rolling a bit, you get tossed from one side of the cabin to the other and slam with much more then your own weight (mass x velocity . . .) against a mash potato cored, thin skinned, with questionable shear strength laminate. Boat interiors serve much more then holding up dinnerware. I'm not trying to be hard, but if you can't jump up and down on it, this is insufficient strength for accommodation partitions, furniture, lockers, cabinets etc. If you don't plan for this, you can rest assured you will need it, but if you do, you'll likely not. It's an unfair world.

 

Ill cut and paste from another site, if there are inaccuracies in the content, please point them out so we may learn something... the highest density XPS has compressive strength of 70 tonnes per m^2, its resistance to compression in the middle of a sandwich core in my unqualified opinion should be adequate for use in even medium sized yachts - for example a 50' sailing catamaran...
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The best way to visualize the structure of a 'sandwich core panel' is to use the analogy of a simple "I" beam.
Like the 'I' beam, a sandwich core panel consists of strong skins (flanges) bonded to a core (web). The skins are subject to tension/compression and are largely responsible for the strength of the 'sandwich'. The function of the core is to support the thin skins so that they don't buckle (deform) and stay fixed relative to each other. The core experiences mostly shear stresses (sliding) as well as some degree of vertical tension and compression. Its material properties and thickness determine the stiffness of such a panel.





The easiest way to illustrate how the core supports shear stresses is to take a deck of cards or a telephone book and bend it. You will notice how the individual layers slide or 'shear' past each other.

Now, suppose that the sheets were all glued together. The pages are no longer free to move and the deck becomes very stiff. At this point, the only way the deck could bend is if the layers on the 'tension' side of the 'neutral axis' (red dashed line) stretched and the 'compressed' side squeezed together.


The core in this illustration would be the equivalent of the deck of cards glued together. The material resists shear (high Shear Modulus) very well. Note that the sections throughout the core are perpendicular to the neutral axis (dashed red line).
This means that the 'layers' in the core resists sliding (shear deformation) and the core and skins are forced to stretch and compress.

Skins made of material of high 'Modulus of Elasticity' are best used in conjunction with cores of high 'Shear Modulus'. This balance is important so that neither material fails long before the other is stressed to acceptable level.
For instance, strong Graphite or Kevlar skins bonded to a 'Styrofoam insulation' core would be a complete waste because such 'Low Shear Modulus' core would always fail long before the skin could be stressed to 1% of its potential strength. Of course, for this reason Styrofoam is not considered a structural core material.


Now back to the cards. The deck of cards is only made of weak paper and given enough bending force it would crack and break. The cracking would begin at the surface and on the side that experienced tension. This failure is an indication that the stresses at the surface exceed the breaking (tensile) strength of the paper. Intuitively, it also shows that the largest stresses are confined to the surface of the deck.

To relieve the high stresses at the surface, a stronger skin must be bonded to the paper. The result is a panel 'composed' of different materials, each with its unique physical properties, thus a 'composite sandwich core'.

The Skin

This drawing illustrates a profile cross section through a sandwich core panel. The bending causes the sandwich to stretch above the 'Neutral Axis' and to compress below the axis. The neutral axis or neutral 'plane' in real material experiences zero stress and strain.
The original length of the relaxed panel is "L".

As the panel bends, both the core and the skin elongate and shrink linearly (for simplicity) from the neutral axis. The thick black line represents the new section of the panel after bending. (very exaggerated)

Since the skins are firmly glued to the core, both the core and skin will stretch the same amount where they bond together.

Now, the important thing to keep in mind is that even though the materials stretch equally at the skin/core boundary, they both have completely different physical properties and therefore will react differently to this elongation.

In engineering terms the ratio of the elongation to the original length is expressed as 'strain'.

The equation means


Knowing the strain, it is now possible to find the stresses in both the core and the skins.

It is important to realize that the 'stress' in a three dimensional panel applies to the entire face of the section. The drawings here represent only 2D 'side view'.
This is OK since stress can also be defined as 'force per area', which means that we can think of the arrows in the drawing below as force applied to individual strands or a slice of the sandwich.

The equation below shows that Strain multiplied by the material's Modulus of Elasticity equals Stress.

The equation means

Now, you will notice that the force (arrows) acting on the skin are far larger than on the core. This is because the fiberglass skin has a large Modulus (E =10,000,000 psi) but the core such as wood has Modulus roughly six times smaller (E = 1,700,000 psi). So, equal strain at the boundary multiplied by larger Modulus will produce larger stress in the skin.
The discontinuity of the stress at the skin/core boundary is a clear indication that the fiberglass is absorbing far more tension and compression then the core. The same applies to the simple 'I' beam.

So far, it has been only shown that bonding a strong, tensile material to the core will relieve it of a lot of stress yet make the entire sandwich core stronger.

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Its a simplfied explaination i know, however it clearly tries to show that the shear stress is greater the further from the center of the sandwich - is this not accurate?

 

My fathers 43' catamaran (sadly missed) was built back in the 70`s from plywood, spruce and glass and sailed around the planet, it weighed 4 tonnes empty... the new owners report its still going well today (structurally).

The main bulkheads were constructed using plywood skins sandwiched between spruce sticks that were arranged in a zigzag fashion between the ply skins, kinda like a roofing frame truss... around the spruce sticks there was nothing but an air gap...

What im suggesting for this XPS foam, is merely a means of seperating stronger core materials for use in a structural sandwich panel to save on cost and weight.... of course it goes without saying, non structural uses are endless... its cheap and light...

It would seem many people are now using XPS to build stringerless surfboards now... they say they are stronger, lighter and more resistant to damage...some guys prefer this to all other types of foam now, but this guy does recommend scoring the surface of it roughly for a better bond with epoxy - http://www.surfersteve.com/polystyrene.htm

if you can build a surfboard from it, take it into the surf and stand on it then wipe out on it repeatedly - im sure its fine for light structural work, and certainly interior furniture etc...

 

The length of the arrows in the illustration are indicative of the tensile/compressive stresses and strains, not the shear stresses and strains.

The shear stresses in the core will arise from two sources. One is the stresses applied by the skins at the faces of the core, and my recollection is the shear stress from from the face stresses will be will be constant through the core for a "balanced" skin-core-skin construction . The other is from the bending of the core itself which will be maximum in the center of the core.

The shear stresses will be zero at the exposed outer faces of the skins.

By the way, what was the source of what you copied?

 

Ok, I understand this argument. Definitely. Sorry if it came across that I was giving you a hard time. Looking back, the post was a little over the top, but made some points.

But I am wondering... what would you build a cabinet out of, then?

Also, for the others on the thread, anyone care to venture a numerical answer to how many LBS we could put on the end of a beam of the following before failure, owing to knowing the sheer strength of each (which we do)?

1) 1" thick XPS with 17oz, biaxial skins in epoxy, bonded by roughing it up so the interlaminate bond isn't what fails

2) 1" thick, 4lbs CoreCell with 17oz, biaxial skins in epoxy

We have the numbers on all of this stuff... sheer for foams, tension and compression for glass/epoxy. So, the answer to this thread should be evident by just doing the analysis. Of course, I'm exhausted from boat building today and need to go to sleep, but I'm sure anyone who is at a desk doing boat designs could whip this out in less than 10 minutes.

 

Ok it looks like you are correct Dcocky, the shear loads are constant throughout the core of a std sandwich panel in bending - a better resource on the subject is here - http://www.diabgroup.com/europe/literature/e_pdf_files/man_pdf/sandwich_hb.pdf

What we need to remember here is that the high density XPS has similar properties to H60 divinycell with *almost* the same shear strength 0.7MPA vs 0.9MPA. Using XPS would not be suitable for a hull panel of a large vessel - but it was never suggested for use in this manner...

The above resource has information pertaining to Box Beam Theory... It explains that the core properties are not important in this structure as all the loads are carried in the skins - hence a RHS section of steel has no core at all and behaves much differently to an I-Beam of RHS. Using a thick piece of XPS in a bulkhead laminated with glass on all sides, would behave in a similar fashion to a box beam no? the bending moment is not perpendicular to the thickness of the panel and so the loading is not the same as a hull panel in bending... Therefore isnt the slightly lower shear strength is not important when used in this manner? Wouldnt it be more important to select a material that doesnt absorb water or rot, has a long lifetime and is cheap and easy to shape etc?

With this consideration, do you stil beleive there are no uses for this material in certain structural parts of a yacht such as laminated bulkheads and stiffeners? Ive owned small powerboats that were built with entirely polyurethane stringers and stiffeners that were glassed over before the floor was laid on top. It was a very strong boat. Schionning designs catamarans in which the bottom of the bridgedeck is stiffened by laying PVC conduit then filleting the edges and laminating over it with UD and DB glass to form little hat sections etc... the core, well its hollow...

 

I also want to point out to anyone following this, that XPS (extruded polystyrene) is a totally different material with vastly different properties to EPS (expanded polystyrene), in case anyone has missed that...

 

Have you had a look and compared the numbers on any of the Australian XPS?

XPS Suppliers Australia

 

Yes mate, i already have a source of the 650Kpa rated stuff for the same price as the typical low density stuff (250-300Kpa is most commonly available).... in the 650kpa, they only have 100mm thick sheets @ $180aud in stock... It can be cut with hot wire - theres a local guy here that can cut it down to whatever thickness i need... Its hard to find the high density stuff here in oz and they dont like ordering it especially for small projects it seems... Ive only sopken to 2 suppliers, there are others i havnt bothered calling yet. I would do my own destructive testing right now if i could find it locally- which i cant - so this will have to wait...

 

Care to share who and where?
Once its been hot knifed will that seal the skin making bonding not as good?

 

And here we have it... at the end of this product broshure, it lists a product theyve branded 2XR TECH... its 400Kpa XPS and they list the product uses including;

• Insulated Doors
• Sandwich Panels
• Refrigerated vehicles
• Container Insulation
• Leisure vehicles (caravans, camping car)
• Cold stores, industrial floors
• Pipe insulation
• Surfboards

http://www.choicesolutions.net.au/downloads/CHOICE 2XR E-BROCHURE.pdf

Sabah, PM me for more details, there is a small quantity of the high density stuff leftover from another project and i dont wanna miss out on it by sharing publicly.... As for sealing it, you may be correct, might need a scuff up after hot knifing it... thats why i wanna get some and see how it behaves...

 

Ive just spoken to the supplier of the above linked product... they are selling quantities of the VIB 400Kpa (40kg/m3) to a manufacturer of refrigerated truck bodies. He explained that they are bonding fibreglass skins to it, not sheet metal, so they are in fact making large composite sandwich panels from it already. After speaking with the rep, it sounds as tho the surface has an infusion grid score pattern, 2mm wide grooves a few mm deep to form 40mm square grid pattern. He didnt know anything about resin infusion, but simply thought the surface helped bonding... they have stock, and its price is excellent - about $10 per m2 for 25mm thickness. They can also obtain the higher densities, but need a firm order and wait approx 8weeks...

I have little doubt that this material could be used as a core material, at the very least,to build furniture inside a yacht... possibly much more...

The leftover supply of 650KPA XPS, was from a project in Antarctica, apparently a hydraulically lifted floor of some type...

 

More info when you can, Grouper! Thank you! Very interesting.

 

i just bought 25m^2 of it... when it gets here ill make some various panels and do some tests