Core materials for sandwich panels John Summerscales Outline

Core materials for sandwich panels John Summerscales

Outline of lecture • polymer foam cores • syntactic cores • wood cores • honeycomb and corrugated cores • alternative technologies

Polymer foam cores • Wide variety of polymers: o o o polyethylene terephthalate (PET) , e. g. Airex polymethacrylimide (PMI) e. g. Rohacell polystyrene (PS) polyurethane (PU) e. g. Kapex polyvinylchloride (PVC) linear/uncrosslinked e. g. Airex v rigid/crosslinked e. g. Divinycell or Herex v o Styrene. Acrylo. Nitrile (SAN) e. g. Corecell • densities from 30 kg/m 3 to 400 kg/m 3

PMI foam • high overall strength and stiffness for foam of a given density • high dimensional stability • high fatigue life • can be used at elevated temperatures • high cost limits use to aerospace components: o helicopter rotor blades, ailerons, stringer profiles in pressure bulkheads

PS foam • • light weight, from 40 kg/m 3 low cost easy to shape extensive use in surf board manufacture • mechanical properties are relatively poor • styrene in polyester resin acts as a solvent o hence, normally used with epoxy resins.

PU foam • available as thermoplastic or thermoset • isocyanate and polyol mixture “free foams” to fill hollow mouldings • generally inexpensive • poor mechanical properties • health and safety issues with isocyanate.

PVC foams I • • usually a hybrid of PVC and polyurethane balance of static and dynamic properties good resistance to water absorption wide operating temperature range typically -240°C to +80°C • resistant to styrene which permits their use with polyester resins.

PVC foams II • Uncrosslinked foams o o o flexible, tough easily thermoformed lower mechanical properties lower resistance to elevated temperatures lower resistance to styrene • Cross-linked PVC harder, relatively brittle o less susceptible to softening when hot. o

Syntactic cores • Syntactic foam: lightweight composite of hollow spheres in a resin matrix. spheres are normally polymeric or glass o typical diameter is 10 to 200 microns o • Core: densities ranging from 300 kg/m 3 to 550 kg/m 3 o high compressive strength-to-weight ratio o

Wood cores • End Grain Balsa (EGB): ultra-light wood high strength to weight ratio for a core o closed cell structure imparts high compressive (and hence high resistance to crushing) and shear stiffness and strength to a sandwich panel o in end grain configuration, balsa has the ability to handle dynamic loads with high resistance to fatigue. o good thermal and acoustic insulator. o • denser woods included to anchor attachments

Honeycomb/corrugated cores • honeycomb cores aluminium, paper or polymers o made by selectively bonding layers of scored material and then expanding the stack to produce a regular cellular structure. o alternative manufacture includes corrugation followed by bonding extrusion of thermoplastics. o

Honeycomb/corrugated cores • aluminium yields on expansion to give permanent deformation when expanded. o do remember that galvanic corrosion may occur if carbon fibres are in contact with aluminium in a moist environment. • short fibre paper cores are expanded and impregnated with resin to hold shape • Nomex (a Du. Pont aramid fibre) used for aerospace quality honeycombs o be aware: aramid is hydrophilic o

Honeycomb/corrugated cores • Auxetic (negative Poisson’s ratio) structures • hexagonal materials > anticlastic curvature • auxetic structures > synclastic curvature re-entrant cells chiral structure Right-hand image from Rod Lakes (University of Wisconsin) webpage http: //silver. neep. wisc. edu/~lakes/chiral. Hny. gif

Alternative technology: Parabeam • Parabeam® 3 D glass fabric woven E-glass yarn o two deck-layers bonded by vertical piles o when impregnated with a thermoset resin, fabric absorbs resin and the capillary forces cause fabric to rise to a preset height o o images from http: //www. parabeam. nl/

Alternative technology: Lantor • "bulker mats" lofty layer of synthetic fibre with embedded microspheres. o o o can build laminate thickness quickly reduced resin and glass usage hence, save both weight and labour. Lantor Coremat® random polyester fabric Lantor SORIC® for use in closed molding (resin infusion) processes, acting as both resin transfer medium and a core material.

Alternative technology: Lantor Soric ^ < Coremat

Starlight. TM foamed clay • Experimental product from English China Clays John Keay Laboratory in St Austell

Lattice webs • blocks of core separated by FRP webs 1 -D: one-way o 2 -D: orthogonal lattice o 3 -D: three-dimensional • ductility index increases with lattice web number o https: //ars. els-cdn. com/content/ image/1 -s 2. 0 -S 135983681400002 X-gr 1. jpg

Summary of lecture • polymer foam cores • syntactic cores • wood cores • honeycomb and corrugated cores • alternative technologies