What are some of the guides for designing in composite materials?

Many organisations develop their own guidelines for designing with composite materials. The guidelines given are non-exhaustive and taken from several sources:

The are intended as a guide and may or may not apply to certain designs,

• Eliminate/minimise potential galvanic corrosion and/or thermal expansion problems by selecting compatible materials.
• The supplier's prepreg material should be closely monitored (i.e., acceptance testing) to assure incoming material consistency and conformance to design values.
• Ensure consistant manufacturing process, variations in curing and machining can be responsible for a range of part strengths thus influencing reliability.
• Design parts for high stiffness in the fibre direction but ensure design incorporates an understanding of stress transverse to the fibre.
• Composites have a tendency to shrink during or after moulding and are prone to cracking at points of internal stress This should be accounted for in geometric designs.
• Avoid sharp changes in section to avoid moulding problems and stress concentrations.
• Sharp corners and edges, which can concentrate stresses, should be avoided. The minimum recommended corner radius is 1.5mm.
• Ensure laminated structures are balance and symmetric wherever possible.
• Keep the change in fibre angle between laminae to a minimum.
• Consider design of holes and cut-outs carefully. Holes can be moulded into components or machined out after moulding. Moulding requires complex tooling and distorts continuous fibres. Machining severs load bearing fibres. Areas around holes can be strengthened by increasing the plies in the local region.
• Improved reliability can be obtained by avoiding anomalies such as wrinkling and porosity in the laminate.
• Ribs and integral stiffeners help stabilise large flat surfaces, and produce overall rigidity without the need to increase laminate thickness. Rib thickness should be 60-70% of wall thickness to prevent sinking into the laminate.
• Good electrical contact between all metallic and carbon/epoxy structural components must provide for the dissipation of static and lightning-induced electrical currents.
• Use titanium alloy fasteners or other materials that are compatible with carbon/epoxy to prevent galvanic corrosion.
• Designs that enhance access for inspection tend to promote reliability.
• Avoid damaging additional plies during patch or repair operations.