Sandwich panels skins

Composites need not be made of fibres. Plywood is a lamellar composite, giving a material with uniform properties in the plane of the sheet (unlike the wood from which it is made). Sheets of GFRP or of CFRP are laminated together, for the same reason. And sandwich panels - composites made of stiff skins with a low-density core - achieve special properties by combining, in a sheet, the best features of two very different components. 

Foamed polyurethane. The basic chemicals are mixed in the liquid state with foaming agents, and swell into a low-density foam which sets by polymerization into a rigid mass. As the swelling material will expand into any shape required, it is ideal for the core of sandwich panels, and the sheet material skins may be flat or profiled. When the panels are manufactured the mixture is injected between the inner and outer skins and expands to the thickness required, adhering to the lining materials. 

The plastic insulants are rigid, homogeneous materials, suitable as the core of sandwich panels. Such a method of fahrication is facilitated when using foamed rigid polyurethane, since the liquids can he made to foam between the inner and outer panel skins and have a good natural adhesion, so making a stiff structural component [40]. 

Resistance to puncture is another type of loading. It is of particular interest in applications involving sheet and film as well as thin-walled tubing or molding and other membrane type loaded structures. Hie surface skins of sandwich panels are another area where it is important. A localized force is applied by a relatively sharp object perpendicular to the plane of the sheet of material being stressed. If the material is thick compared to the area of application of the stress, it is effectively a localized compression stress with some shear effects as the material is deformed below the surface of the sheet. 

The European Parliament has adopted phase-out dates for the use of HCFCs in rigid foam applications which are basically in line with those in the USA and Japan. From 1 January 2000, the use of HCFCs for integral skin PU and PE foams is prohibited. From 1 January 2002, the use of HCFCs in expanded PS foams is prohibited. From 1 January 2003, the use of HCFCs in flexible-faced PU foam laminates, appliances and sandwich panels is prohibited. EUROPEAN PARLIAMENT... 

Results are presented of experiments undertaken by Gaiker in the manufacture of sandwich panels containing foam cores based on PETP recycled by a solid state polyaddition process developed by M G Ricerche. Panels were produced with glass fibre-reinforced unsaturated polyester and epoxy resin skins, and allthermoplastic panels with PE, PP, PS and glass fibre-reinforced PETP skins were also produced. EVA hot melt adhesives and thermoset adhesives were evaluated in bonding glass fibre-reinforced PETP skins to the foam cores. Data are presented for the mechanical properties of the structures studied. 

Sheets of hard foam can be covered at both sides by a thin skin, e.g. by metal or glass-reinforced plastic, to form light and stiff sandwich panels, to be used as building panels. 

In some cases, it is not possible to evaluate a material or product (combination of materials) in a bench-scale test in a manner that is representative of its end-use. For example, it is difficult to use a bench-scale test method to evaluate the effect of joints on the fire performance of a thick sandwich panel that consists of a plastic foam core and metal skins. In this case, a room test is used to assess the reaction to fire of the materials. It is also very difficult to assess the fire performance of complex objects such as upholstered furniture based on the reaction-to-hre characteristics of the object s components. Large-scale reaction-to-hre tests have been developed to evaluate these complex objects. 

A typical layered structure consists of two thin, glass-fibre-reinforced polymer skins bonded to a thick, lightweight honeycomb core (Fig. 4.5a). Such sandwich panels are used in railway carriages and aircraft there are similar structures inside many skins. Other examples are less obvious the space between the outer container and the toughened polystyrene liner of a refrigerator is filled with rigid polyurethane foam. 

In these composites, the layers are bonded together. A sandwich panel beam is symmetrical if the skins have equal thickness, and are made of the same material. The neutral surface is at the mid-thickness, so the analysis of Appendix C can be used. Figure 4.6 shows the stress variation through a sandwich beam, calculated using Eq. (C.4) separately for the skins with high Young s modulus E, and the core with low modulus Eq. 

Figure 9.3 Residual deformation of a sandwich panel after impact, (a) Stiff skin, weak bond, (b) flexible skin, strong bond.

Graner and della Rocca [83] conducted a condition survey of 74 US Navy boats of 21 different classes from 5-15 years old (the median hull, fabricated in 1958, was 10 years old). Defects were classified into 17 categories, most of which were the result of impact damage or abuse. The laminates were generally in excellent condition, regardless of the age of the boats, with material degradation limited to minor, surface skin delaminations, skin/ core separation in sandwich panels and deterioration of the paper honey-... 

Sandwich panel with 1-mm skin of black-finished trapezoidal steel sheet with core of polyurethane thermal insulation covered with trapezoidal aluminium sheet of 1 mm in thickness 50 0.7... 

There was no clearance between the core and skin in the tested sandwich pieces, which is an important factor in upgrading the flammability behaviour. Any material used in the manufacture of sandwich panels must be at least as fire resistant as the skin and core materials. For example, when phenol/formaldehyde foam cored sandwich elements were mounted on firwood frames, the flames were conducted by the frame, thus enhancing the fire risk. 

The possibilities for making bonded structural sandwich elements in a variety of materials are very real. However, whilst there exist structural examples such as aluminium honeycomb panels (used in aircraft and transport applications) and metal skinned foam sandwich panels (used as the monocoque chassis in refrigerated transport applications), these composite constructions are normally utilised in non- or semi-structural ways. Typical skin materials are steel, aluminium, GRP and plywood, and common core materials are rigid foam polystyrene, polyurethane, polyisocyanurate, PVC, and honeycombed aluminium. In some instances the foam core is injected between the skins and adheres to them in others, adhesives are used to bond the separate components together. The nature of the manufaeturing process depends on the type of structure to be made, and the degree of investment in produetion maehinery. Both flat and eomplex eurved forms ean be made by a hand lay-up process as well as in an automated way. 

It is reported that sandwich panels are being produced in the USA by pultrusion. In this process a plywood core is completely encased in a 3 mm thick glass polyester skin, resin penetrating the plywood during production to give increased bond strength and moisture resistance. 

Sandwich panel—An assembly composed of metal skins (facings) bonded to both sides of a lightweight core. 

Mallite sandwich panels were tested in the VC 10 and proved satisfactory after 4000 h. However, the panels lacked sufficient flexural rigidity - excessive deflections caused matrix crazing under the skin (breaking up of the PF resin between fibres). Surprisingly, a deflection specification was not included in Armstrong s original report. When it was later added to the report it was found that Mallite was unable to meet the specification. 

---