Composite Bolted Joints: Why Metal Design Rules Don't Apply

Carbon fiber reinforced polymer (CFRP) is increasingly used in aerospace and EVs, but CFRP bolted joints work completely differently from metal joints. NASA technical memorandum TM-100281 provides simplified design procedures for composite bolted joints.

Why CFRP Joints Are Harder

CFRP and metal have vastly different mechanical properties:

• Low interlaminar shear: CFRP has high in-plane strength but interlaminar shear strength is only 1/5 to 1/10 of metals. Excessive clamp-up pressure crushes the laminate.
• Hole-edge stress sensitivity: Composites are more sensitive to hole stress concentration because they can't redistribute stress through plastic deformation like metals.
• Galvanic corrosion: Carbon fiber is conductive. When in contact with aluminum or titanium in humid environments, galvanic corrosion occurs.
• Clamp-up limit: In metal joints, more preload is better. In CFRP, clamp-up pressure has a strict upper limit.

NASA Simplified Design Parameters

• Hole-to-width ratio (d/w): ≤0.5 (metals often 0.5–0.7)
• Edge distance ratio (e/d): ≥3.0 (metals typically 1.5–2.0)
• Pitch ratio (p/d): ≥4.0
• Clamp pressure: Must not exceed 50% of through-thickness compressive strength

Fastener Material Selection

• Titanium bolts (Ti-6Al-4V): Preferred. Galvanically compatible with CFRP, high strength.
• Stainless steel: Acceptable but requires insulating washers to prevent galvanic corrosion.
• Aluminum bolts: Not recommended — accelerates corrosion when in contact with CFRP.
• Always use washers under bolt heads and nuts to spread clamp pressure.

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