EFFECT OF ABSORBED WATER IN CFRP COMPOSITES ON ADHESIVE BONDING

This paper reports the results of wedge testing adhesive-bonded joints made from dry, and water-immersed and dried, carbon fibre-reinforced plastic (CFRP) laminates based on Ciba Fibredux 914. Five groups of sp ecimens were tested. Those bonded with Hysol EA 9394 adhesive were tes ted on dry, and immersed and dried, CFRP adherends using mechanical ab rasion as the surface preparation. Hysol EA 9390 repair resin was test ed on abraded dry samples and on dry, and immersed and dried, peel ply surfaces. Only the initially dry specimens that were also dried befor e bonding and given the mechanical abrasion treatment gave cohesive fa ilure (now considered to be matrix failure) after 1388 days' immersion . All the peel ply surfaces - whether initially dry, or immersed and t hen dried - gave interfacial failure. Unfortunately, in the case of th e immersed and dried specimens, the two lowest temperature drying cycl es were both given to the specimens which were prepared by mechanical abrasion and the two highest temperature drying cycles were given to t he peel ply samples. All the peel ply samples gave interfacial failure but the immersed and dried peel ply surfaces gave the same result, in terms of fracture energy and durability, as the initially dry ones, i ndicating that the higher temperature drying cycles were adequate. One strange and unexpected result was that the fracture energy obtained w ith Hysol EA 9394 on the immersed and dried surfaces was higher than o n the dry surfaces by a large amount. This suggests that, for this par ticular adhesive, a small amount of moisture actually improves the cur e and the fracture toughness, even though a cohesive failure was not o btained. In view of the extensive use of peel plies in industry it wou ld seem that further research is required to achieve more durable bond s. From a repair point of view, where mechanical abrasion is the most common method of surface preparation, it was encouraging to find that in some cases repair bonds could actually be more durable than those m ade by the manufacturer if peel plies are used.