Core materials foam sandwich

The most widely used applications reported in 1973 included rib structures in the fabrication of lightweight reinforced-plastic parts and as a core material in sandwich construction, bonded to metal, wood, or glass. Other applications are as reinforcements for aircraft-control surfaces, radome housings, fUler blocks under fuel cells, tank floats for indicating devices, and ribs, posts, and framing in houses and shelters. Due to its buoyancy characteristics cellulose acetate foam has been used in lifeboats, buoys, and other flotation devices (21). 

Densities of syntactic foams range from 10 to 40 Ib/ft (160 to 640 kg/m ). Syntactic foams are being used as core materials in sandwich structures in construction of aircraft and hulls and decks of boats, and are being investigated for use in roofing structmes (2). 

Core materials and sandwich plies must be placed on to an adequate layer of resin and then firmly and evenly weighted to ensure full contact with the outer skin until bonded. Care needs to be taken to provide escape routes for entrapped air. They may be held against the wet resin by using a vacuum or pressure bag, press or autoclave instead of weights—this is a very efficient method. In situ foaming of core material can be used instead of ready foamed slabs, especially on difficult curves or for gap filling. 

A foamed resin created by using a foaming agent -rigid foams are useful as core materials in sandwich panels between stiffer outer layers. 

This type of analysis could also be used for a sandwich structure with solid skins and a foamed core. It is simply a matter of using the appropriate values of E, G 2,Ei,v 2 for the skin and core material. This is illustrated in the following Example. 

Since the stiffness of a laminate varies as a cubic relationship with the thickness, there are alternate methods employed to achieve thickness - and hence stiffness - than by using multiple layers of fiberglass and resin. Lightweight core materials, such as end-grained balsa wood, high-density polyurethane foam, PVC foam, and honeycombed materials, are available. These materials are sandwiched in between layers of unsaturated polyester resin to achieve increased laminate stiffness. The common terminology used for this technique is sandwich construction . 

It is interesting to note that foamed composites are suitable sandwich core materials for FRP boats (8). Cross-ties (or sleepers) for railroads, especially for use in tunnels and subways, are very promising applications. Japanese National raUways carried out a long-term test in a tunnel, and the results were excellent. 

Core materials for composite sandwich components are foams such as those based on polyvinyl chloride (PVC). Those appropriate for the SO are high-quality PVC foams which show good long-term performance, are easy to work with, are very lightweight and show very good thermal insulation with low thermal conductivity. The core material selected will be from certified closed celT-type foam with technical characteristics as follows ... 

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. 

P(l) Core materials may be load bearing or simply used as formers for shaping FRP sections. Stmctural cores should be used for efficient sandwich constmction design. These may be foam, honeycomb or solid materials. 

Foams consist of open or closed cell materials made from many plastics and may be used in sandwich panel manufacture from slab stock or by foaming in situ. The latter presents more problems of attaining uniformity of properties. Commonly used core materials include polyurethanes and rigid polyvinyls and polymethacryUmides (PMl) available in lightweight and industrial grades. 

Solid materials may be used as cores but in sandwich construction will only be efficient if lightweight strong materials are used. End grain balsa-wood has been widely used in sandwich constructions and can offer advantages to foams in compression and shear critical loadings. 

Table 3.4 Properties of PVC foam cores (Divinycell H Sandwich Core Material, reference 3.6).

Wood is the most appropriate material for a solid sandwich core and has been used extensively for many applications, such as doors and partitions. For lightweight constructions flat or end grain balsa is still used, although this is being superseded by synthetic materials. Bonded microspheres of inorganic materials, glass, ceramics, etc. to form syntactic foams are useful core materials. 

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