Durability of Composites and Adhesive Bonds

Any consideration of the durability of the high performance composite materials and structural adhesives used in aerospace construction must recognize that these are brittle materials and that their failure mode is characterized by flaw growth and propagation. One can easily anticipate a variety of flaws and defects; surface cuts, internal cracks due to stress relief and deliberate holes cut for fasteners. In this presentation we wish to discuss yet another type of flaw in fibrous composites and adhesive bonds that is inherently present because of the processing methods used to fabricate composites or bonded joints. These flaws are microvoids created by air entrapment that usually occurs when a viscous liquid is forcibly spread over a solid surface. Such forced spreading is characteristic of both adhesive bonding and composite fabrication.