Task 2.2 – Identification of most suitable face-sheets and optimization of panel properties

Duration: month 1 to month 12Partners involved: MOTULAB (WP leader), CETMA, TRE, PANDORA

In the building industry, the faces (skins) of sandwich panel are generally used in three forms:

flat

lightly profiled

profiled

The faces of sandwich panels serve various purposes. They provvide architectural appearance, structural stiffness, and protect the relatively vulnerable core material against damage or weathering.Tensile and compressive forces are supported almost entirely by faces.

Flat Lightly profiled Profiled

Local buckling of plate elements in flat or fully profiled sandwich panels is significantly improved by the presence of the core.However, these sandwich panels are always susceptible to local buckling failure under the action of compression, bending or a combination of these loading actions.

Flat Plates

Steel Face Foam coreLocal Buckle

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 panels 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 skins so that they don’t buckle (deform) and stay fixed relative to each other.

Core compression

TensionCore sheer

Compression

Applied bending forces

The sandwich panel is considered similar with a double T beam, where the flanges are similar in function with the sandwich facings and the web takes the place of the core.

The only difference is that the sandwich panel has the material in the web different from the material in the facings and it usually fills completely the space between the external layers.

Flanges

Web

Flanges

Simple I beam Infinite I beam

X

Y X

Y

Layers slide without resistance

Materials with very low shear modulus are unsustanaible as structural cores because they cannot withstand shear stress.Structures made with such cores would be weak, excessively flexible, and easily deformed.

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.

Layers slide past each other

Core weak in shear

Bonded layers resist shearing

Core strong in shear

The bending causes the sandwich to stretch above the “neutral axis” and to compress below the axis. As the panel bends, both the core and the skin elongate and shrink linearly from the neutral axis.

Skin 1 (flanges)

Skin 2

Core (web)

Neutral Axis

L

Core and skin stretching

No strectch or compressionat Neutral Axis

Core and skin compressing

The force acting on the skin are far larger than on the core. This is because the skin has a large modulus of elasticity while the core is made with recycled plastic.

The discontinuity of the stress at the skin/core boundary is a clear identification that the skins are assorbing far more tension and compression then the core.

Skin

Core

Skin

Core

The applied bending force (moment) produces internal reaction forces in the panel (the blue and red arrows) that counteract such bending.

In static problem like this, it just happens that the sum of the internal forces (moments) must be equal to the applied bending force (moments). In engineering

terms, the forces are in “equilibrium” or balance.

The following equations for the stresses in the core and skins of the sandwich panels are based on the assumptions that the skins are much thinner than the core and the modulus of the skins is much greater than that of the core. Perfect bonding is assumed.The longitudinal stress in the face sheet is given by:

σ = Ms / e * Af

The result of these assumption is that the skins carry the bending moment as longitudinale tensile and compressive stresses and the core the transverse shear force.

Nf1

e

Nf2

MsVs

Af1

Af2

The EcoPlasBrick Project considers sandwich panel was made of various sheets with two flat faces and plastmix core

Plastmix core

Skin Section of sandwich panel

The panel was considered simply supported with a midspan concentrated load. This load was a downward force applied on the panel to simulate the “Interaction between bending moment and support force”

For the simply supported panel with a point load in middlespan having faces whose stiffness cannot be neglected the bending moment is given by:

L/2 L/2

L

LT

P

Compression or tensile failure of the skin occurs when the axial stress in the sandwich face attains the maximum strenght of the skin material.For a symmetrical composite sandwich, the peack strength for this failure mode under three points bending can be predicted by:

In the analysis of sandwich structures it is usually assumed that the core only supports the shear and the skins carry the tensile and compressive loads under flexure.

The longitudinal stress of design in the face sheet is given by:

Taking into account a security reduction factors the skin must have a thickness of 3mm and tensile/compressive resistance above 9.3MPa.

An average skin-resistance above 9.3MPa needs for raised floors with a max dimension of 500mm.

commercial gres

polyester resin (only tensile)

alluminum

stainless sheets

marine playwood

The most suitable types of face-sheets or skins has been identified as follow:

The characteristics analyzed were:

the surface finish

panel’s dimensionsfor laboratory test

technical data sheets

Oukoumé

Next half year will be carried out following main tests:

Loading configurations

Sandwich Panel Thickness Dimensions

Our background in Mamas project