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pu panel
24/02/2011
09:28
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Sandwich panels are modern lightweight building components used to cover walls and roofs of buildings and to isolate spaces inside buildings. They are typically made of two thin metal sheets with an insulating core between the faces.
The use of sandwich panels is continuously increasing and new application areas are opened in industrial, residential and office buildings. The European Standard for sandwich panels EN 14509 has a lack of rules or requirements for many important areas like fastening of the panels, openings in panels, axially loaded panels and panels stabilizing frame structures. Despite there is a lot of knowledge in different countries on these subjects, no common rules have been developed, thus putting a hinder to the standardisation and leading to barriers for some of these applications.
The aim of this project is to overcome these problems and develop solutions and technical guidelines ready for implementation for the revision of the standard EN 14509 published in December 2008.
The second goal of the project is to implement the new information in the use in practice, which will be carried out through seminars and practical guidelines as well as e-learning modules. The subjects are of high industrial and user interest. The project introduces guidelines for topics not included in the present version of the standard but will on the base of this research be implemented in a later revision of the standard. The subjects are very important in practice. Practical guidelines and seminars will help and broaden the correct and safe use of sandwich panels in Europe and ICPC. The innovative parts of the project are new applications like the in-plane shear and axial resistance.
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pu panel
24/02/2011
09:28
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Sandwich Panel European collaborative project
Seventh Framework Programme
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pu panel
24/02/2011
09:28
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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'. |
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pu panel
24/02/2011
09:27
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As a note of interest; 'End-Grain Balsa wood' (Balsa is the stuff used for model airplanes) makes for the stiffest and most shear resistant cores known in boatbuilding. It also shows superior compressive and bonding strength to the skins. Of course, it doesn't mean that it is the best choice in all situations. Some cores are less stiff which makes for better impact absorption.
The point here is that despite advances in material science, the 'Good old wood' is still up there with the most sophisticated materials used today.
As far as cedar wood strip core is concerned, it has its own unique advantages that no other core has.
It is cheap (commercial cores are notoriously expensive)
It is self-supporting (NO other core is able to form fair compound surface when wrapped over a mold with 12" to 18" station spacing). All commercial cores need close spaced forms or solid surface plugs so that they don't sag.
No special skills or tools are required to build a professional looking kayak with wood strips.
For this reason, wood strips are the ultimate core material for prototyping and builders wanting 'cheap & quick' boats. The Wood Core Kayak page goes into more detail about its virtues.
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pu panel
24/02/2011
09:27
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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.
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pu panel
24/02/2011
09:27
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This picture illustrates the shear in a weak core such as the unglued deck of cards or a sheet of elastic material like rubber.
The skins experience very little stress because the core deforms easily. Such cores are said to have low 'Shear Modulus of Elasticity'
Materials with very low Shear Modulus are unsuitable as structural cores because they cannot withstand shear stress. Boats made with such cores would be weak, excessively flexible, and easily deformed. This would defy the whole point of this construction.
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pu panel
24/02/2011
09:27
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The Core
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.
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pu panel
24/02/2011
09:27
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Unlike the simple beam, which is designed to withstand stresses mostly along the x axis and bending about the y axis, the sandwich panel can be stressed along and about any axis laying in the x-y plane. The implication is that such panel can extend 'infinitely', forming a strong and continuous self-sustaining plate or shell such as a wood strip kayak. No reinforcing elements are needed because they are already built into the structure.
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pu panel
24/02/2011
09:27
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page is intended for those who do not have any engineering background but would still like to get a peek at the inner workings of sandwich cores.
Derivations of Flexure formulas, Moments of Inertia and other complexities are beyond the scope of this page. A lot of assumptions are made for the sake of simplicity. All cross sections are symmetrical about a neutral axis(centriod) and the material is subject to pure bending(flexure).
The best way to visualize the structure of a 'sandwich core panel' is to use the analogy of a simple "I" beam. (see above)
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.
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pu panel
24/02/2011
09:26
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Composite Sandwich Core
This page will attempt to shed some light on the mechanics of composite sandwich panels. The main points will illustrate:
the structure of a 'Composite sandwich panel'
the role of the skin and core in resisting loads
the distribution of forces in such panel (bending only)
why composite sandwich cores are stiffer and stronger than the same weight single skin panels.
examples of 'modes of failure' and buckling in composite core panels
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pu panel
24/02/2011
09:26
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•16.5 mil Embossed white acrylic-coated aluminum sheet facer laminated to 1 mil aluminum on one side, 1 mil aluminum facer on the other side
•Glass-fiber reinforced polyisocyanurate foam core for fire resistance
•Pressure-washable at 2000 psi
•Heavy impact resistance
•For wall applications only
THERMAX™ Heavy Duty Plus Insulation
•1.25 mil Embossed aluminum finish facers on both sides
•Glass-fiber reinforced polyisocyanurate foam core for fire resistance
•Pressure-washable at 1000 psi
•Light impact resistance
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pu panel
24/02/2011
09:26
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Distributed through select fabricators/laminators
•High-performance extruded polystyrene foam with planed surface
•Manufactured in lengths up to 96", widths up to 48" (122 cm) and thicknesses up to 5" (12.5 cm)
•R-5.0
•30 psi (207 kPa)
STYROFOAM™ Brand Panel Core 30
Distributed through select fabricators/laminators
•High-performance extruded polystyrene foam with planed surface
•Manufactured in lengths up to 96", widths up to 48" (122 cm) and thicknesses up to 5" (12.5 cm)
•R-5.0
•40 psi (280 kPa)
STYROFOAM™ Brand Panel Core 40
•Thermally efficient rigid foam insulation for tilt-up interior exposed walls
•Exceptional fire resistance
•High R-value
•Washable surface
•Excellent high temperature stability
•4 mil Embossed white acrylic-coated aluminum sheet facer on one side, 1.25 mil embossed aluminum facer on the other side
•Glass-fiber reinforced polyisocyanurate foam core for fire resistance
•Pressure-washable at 2000 psi
•Moderate impact resistance
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