September 2006 edition

Quantifying methods for the evaluation of carbon-based composite surfaces for subsequent adhesive bonding

By Wichita State University experts William T.K. Stevenson, Irish Alcalen, Lamia Sa

The production of fibre-reinforced composite parts has revolutionised the aviation industry due to properties such as high specific stiffness and resistance to environmental deterioration, but composite parts can potentially be damaged both during fabrication and during use, and composite repair is still an expensive and delicate process.

If a defect is found after cure and during assembly, or if a defect is introduced through wear and tear, the defect must be cut out, replaced with prepreg and cured and bonded in place. In order for a repair to be effective, the prepreg must form a strong bond with the precured composite surface. This type of composite repair is both labour-intensive and expensive and directly impacts manufacturing and maintenance costs.

Fibre-reinforced composites absorb water into the resin phase, mostly during use or in storage under a humid atmosphere. Before a strong bond can be formed, the composite must be thoroughly dried. This can take hours of heating and vacuum pumping the composite before the bond is formed with new prepreg. There is a need for a simple nondestructive test that can be used to identify whether the composite is wet or dry, both on the shop floor and in the field.

Water dissolved in plastic produces a unique and reproducible signature in the near Infrared (IR) spectrum. Transmission near IR spectroscopy can be used to look into and anal....

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