Manufacture of adhesive joints and bulk specimens with high-temperature adhesives

Abstract For high-temperature usage (200°C and above) such as in certain supersonic aircraft structures, the adhesives used are either bismaleimides or polyimides, generally supplied as films, with or without a carrier. Other adhesives, such as modified epoxies, can also be used up to 200°C. For simulation purposes (such as finite element analysis), the adhesive mechanical properties are needed. These are generally obtained by testing a bulk specimen in tension or by testing adhesive joints under pure shear. However, a problem associated with adhesive films is void formation during manufacture. In order to achieve the full potential of the adhesive, it is necessary to understand the origin of the voids and to develop techniques to eliminate or minimise their formation. In this investigation, a bismaleimide adhesive, Redux 326 (Hexcel Composites), was studied in two forms: supported (woven glass) film and paste. Different manufacturing methods such as the vacuum release technique and sheet moulding were tested in order to produce adhesive joints and bulk specimens free of defects. The merits of each technique are discussed in terms of efficiency and practicability. The most appropriate method was used to manufacture single lap joints with and without voids in the supported film and paste form. These were tested at low temperatures where the adhesive is brittle and at high temperatures where the adhesive is ductile, to check the influence of voids and the carrier in the joint strength.