Influence of low temperature cycles on the integrity of pretreated 2024 T3 aluminium joints by an ultrasonic method

Abstract

Many authors have considered the problem of adhesive bond durability over a long period of time or under special environmental conditions, and all have conclued that surface preparation is of paramount importance. Here, 2024 T3 aluminium alloy bonded joints, whose metallic plates underwent phosphoric acid pretreatment prior to bonding, are investigated before and after exposure to -40 °C for 67 h. This procedure of ageing is repeated several times and the samples are examined by ultrasonic Lamb waves after each cycle. In particular, the position of the cutoff-frequencies of ultrasonic guided waves which propagate into the bonded aluminum/epoxy/aluminium structure allows us to calculate the stiffnesses of the metal/adhesive interfaces with a rheological model. Thus, after exposure, a progressive decrease in the value of the stiffnesses is observed even for pretreated surfaces. Two complementary mechanical explanations are proposed. First, the cold exposure of the sample and the difference between thermal dilatation coefficients of aluminum and epoxy produce a damage of the interfacial zone. Second, the cooling introduces microcracks in the epoxy then caracterized by a smaller Young modulus.