Durability studies for aerospace applications using peel and wedge tests

Measurements of peel strength are reported for some small, 2 mm wide, unstressed aluminium adherend peel specimens based on aerospace adhesives AF163-2KO6 and Redux 775 immersed in distilled water at 35C for over 7 years. Predictions of water distribution across the specimen width were made as a function of time using a measured diffusion coefficient and assuming Fickian diffusion. In the absence of primary corrosion, specimens after 7 years immersion showed no significant change in peel strength relative to strengths obtained in their initial dry state. This was after almost complete saturation of the adhesive. A brief review of the literature is given on the use of the Boeing wedge and DCB type tests for assessing the environmental durability of adhesive joints. Some of the assumptions used in the analysis of the wedge test have also been examined. An optical image correlation method is presented which permits accurate measurement of the adherend displacement. So long as sufficiently stable features can be identified on the adherend edge, then the position where the adherend displacement is zero may be used to give a well-defined effective crack length, and therefore fracture energy, even in the presence of extensive damage zones typical of those associated with tough structural adhesives. A beam on elastic foundation model is also given which demonstrates that apparent crack growth in the wedge test, associated with movement of the zero adherend displacement position, may be explained by relaxation of the adhesive. (C) 2004 Elsevier Ltd. All rights reserved.