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Diagnosing an Overly Stiff Carbon Fin Laminate

Context

A completed carbon fin felt too stiff. Epoxy hardness was initially suspected, followed by the possibility that mild vacuum bagging had left the laminate resin-rich.

No laminate weight, dry-fibre weight, thickness or layup schedule was supplied, so the cause was not established.

Information received

The response described the complementary roles of the laminate constituents:

  • Carbon fibres provide most of the directional strength and stiffness. Fibre quantity, orientation, blade thickness, taper, width and span therefore dominate bending behaviour.
  • Epoxy holds fibres in alignment, transfers loads between them, supports them against local movement and buckling, and binds and protects the laminate.
  • Epoxy properties affect load transfer, creep, temperature response, cracking and damping, but epoxy hardness alone is unlikely to explain a large increase in stiffness.
  • A resin-rich wet layup can cure thicker than intended. Because bending stiffness is highly sensitive to total thickness, the added thickness could make a blade harder even though neat epoxy is much less stiff than carbon fibre.

The suggested diagnostic measurements were:

  • dry fibre mass;
  • finished laminate mass;
  • finished thickness; and
  • resin-to-fibre mass ratio, estimated as:
(finished laminate mass - dry fibre mass) / dry fibre mass

The response suggested controlling subsequent layups by measuring resin before wetting the fabric, squeegeeing the fabric between plastic sheets, using peel ply with perforated release film and bleeder/breather, maintaining a continuous bleed path, and comparing thickness and mass between blades.

Editorial notes

  • Epoxy hardness, elastic modulus, glass-transition temperature and toughness are different properties; the response sometimes treated them as interchangeable.
  • A higher resin fraction does not automatically make a laminate stiffer. At equal thickness it usually reduces fibre fraction and effective modulus. It may increase bending stiffness if the additional resin increases total thickness sufficiently.
  • Vacuum does not remove resin by itself. Resin removal depends on compaction and a suitable perforated film and bleeder path.
  • The proposed resin-to-fibre target of 0.35-0.55 by weight was presented too confidently. A suitable value depends on fibre and resin densities, fabric architecture and achieved fibre-volume fraction.
  • The mass calculation is useful only after excluding rails, adhesives, coatings and other non-laminate material.
  • Shore D hardness above 80 and a glass-transition temperature of 60-80°C were generic suggestions, not requirements derived for this fin.
  • The claim that edge-sealed bagging works well was contradicted by later experience: edge sealing proved unreliable, while enclosed bagging was successful.
  • Feather-sanding the existing blade was proposed but not reported as an adopted action or tested outcome.

Unresolved measurements

  • Dry fibre and finished laminate masses.
  • Finished thickness and thickness profile.
  • Number, type and orientation of plies.
  • Epoxy system and cure schedule.
  • Actual vacuum level and duration.
  • Whether perforated film and bleeder were used.
  • Comparison with a blade having acceptable stiffness.