1. Core materials for sandwich panels John Summerscales

2. Outline of lecture Polymer foam cores Syntactic cores Wood cores Honeycomb and corrugated cores Alternative technologies

3. Polymer foam cores Wide variety of polymers: o Polyethylene terephthalate (PET), e.g. Airex o Polymethacrylimide (PMI) e.g. Rohacell o Polystyrene (PS) o Polyurethane (PU) e.g. Kapex o Polyvinylchloride (PVC)  linear/uncrosslinked e.g. Airex  rigid/crosslinked e.g. Divinycell or Herex o StyreneAcryloNitrile (SAN) e.g. Corecell Densities from 30 kg/m 3 to 400 kg/m 3

4. 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

5. 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.

6. 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.

7. 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.

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

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

10. Wood cores End Grain Balsa (EGB): ultra-light wood o 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. denser woods included to anchor attachments

11. Honeycomb/corrugated cores honeycomb cores o 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.

12. 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 DuPont aramid fibre) o used for aerospace quality honeycombs o be aware: aramid is hydrophilic

13. 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/chiralHny.gifhttp://silver.neep.wisc.edu/~lakes/chiralHny.gif

14. Alternative technology: Parabeam Parabeam ® 3D glass fabric o 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 images from http://www.parabeam.nl/http://www.parabeam.nl/

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

16. Alternative technology: Lantor Soric ^ < Coremat

17. Summary of lecture Polymer foam cores Syntactic cores Wood cores Honeycomb and corrugated cores Alternative technologies