THE FATIGUE AND DURABILITY BEHAVIOR OF AUTOMOTIVE ADHESIVES - PART II- FAILURE MECHANISMS

In part I [1] a fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel su bstrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. T he cyclic fatigue tests were conducted in a relatively dry environment , of 23 degrees C and 55% RH, and in a ''wet'' environment, namely imm ersion in distilled water at 28 degrees C. The ''wet'' fatigue tests c learly revealed the significant effect an aggressive, hostile environm ent may have upon the mechanical performance of adhesive joints, and h ighlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The p resent paper, Part II, discusses the modes and mechanisms of failure f or the two adhesive systems in both the ''dry'' and ''wet'' environmen ts. The failure surfaces of the joints tested in Part I have been exam ined using a variety of analytical techniques and the surface chemistr y and morphology compared with that of the ''as prepared'' (i.e. non-b onded) metal surfaces and cured adhesive. In the present investigation use has been made of an elemental mapping form of X-ray photoelectron spectroscopy (EM-XPS) along with conventional XPS. The surface topogr aphy has been examined using scanning electron microscopy and atomic f orce microscopy. Also, cross-sections of the joints have been studied using the transmission electron microscope. The results reveal that fo r both the aluminium alloy and EG steel joints that the failure path i s complex, and is associated with electrochemical activity (i.e. corro sion) in the case of the latter joints when tested in the ''wet'' envi ronment. In part III [2], the results presented in the earlier papers will be used to predict the lifetime of single-overlap joints subjecte d to cyclic fatigue loading.