Core materials for carbon fiber composites

If a laminate carbon composite with a thick section has to be used, it is common practice to achieve weight saving by using a core of lightweight material (e.g. foam) with a density less than the reinforcement and matrix materials and is generally sandwiched between two laminates. The laminates are ideally bonded to the core material with a suitable film adhesive. Surface skins on core materials should be roughened to improve adhesion of the film and all open cell structures must be sealed with resin. Some of the core materials are honeycombs, which can vary in cell shape, cell size and core thickness. The honeycomb sections can be machined to final configuration using 3-, 4- and 5- axis numerically controlled machines. It is important that the core has good moisture resistance and the open edges are sealed to prevent ingress of moisture. A sandwich construction with a core thickness equal to the total skin thickness will be, for only a 3% increase in weight, about 7 times more rigid and 3.5 times as strong. The laminate faces take up tensile and compressive forces, whilst the core transfers the shear forces. The core will also contribute some resistance to impact damage.

1. Closed cell PVC foam—is a chemical hybrid of PVC and polyurethane and there are basically three types:
   1. Crosslinked—are better mechanical properties and elevated temperature performance. PVC crosslinked foams must be resin sealed before use.
   2. Uncrosslinked—are tougher, more flexible and easier to heat form around curves.
   3. Toughened—addition of plasticizer as toughenting agent contributes to some loss of mechanical properties.
2. Polystyrene foam—are rarely used for high performance composites.
3. Polyurethane foam—has only moderate mechanical properties.
4. Polymethylmethylacrylamid (PMI) foam—are expensive but the foams have good high temperature properties, enabling their use with elevated temperature curing prepregs.
5. Styrene/acrylonitrile copolymer (SAN) foam—is thermoformable, with higher toughness and elongation than PVC foams, hence superior impact properties. They tend to replace linear PVC foams due to superior high temperature performance and improved properties, especially toughness.
6. Polytherimide/polyethersulphone (PEI) foam—is expensive but has excellent high temperature performance coupled with good fire resistance.
7. Polycarbononate (PC) foam—has good heat resistance, mechanical properties and impact properties.
8. Nomex honeycomb cores—are more than twice as expensive as a foam core, but have good mechanical properties with fire retardant properties.
9. Balsa honeycomb—is cheap but readily soaks up resin, incurring a weight penalty.
10. Al honeycomb—is not too expensive, has good mechanical properties, but can be corroded by sea water.

Akay and Hanna have compared foams core and honeycomb core epoxy sandwich panels.