Ultrasonic Measurements of Pretreated Aluminium Joints Durability during Hot Cycles

Adhesive bonded joints have been investigated due to their growing importance in aeronautic, aerospace, and automotive industries. This article examines the durability of adhesive bonded 2024 T3 aluminium alloy joints by comparing the stiffnesses of the two interfaces before and after exposure to a hot atmosphere (70 degrees C) for several cycles each of 67 hours. The stiffnesses values are calculated by ultrasonic measurements via an equivalent rheological model where the two aluminium/adhesive interfaces are replaced by an uniform distribution of longitudinal and transversal springs without inertia. Before any ageing, and for all studied specimens, the adhesion was increased by pretreating the metal by phosphoric acid pickling prior to bonding. During the first cycle of ageing, the complete crosslinking of the adhesive is observed. This phenomenon involves an increase of Young modulus of the adhesive. For the three following cycles of ageing, a small degradation of the structural joint is observed, and several hypothesis are examined. These different observations are obtained by a theoretical and experimental study of guided Lamb modes of the structure.