The fatigue and durability behaviour 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 substrates 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. The cyclic fatigue tests were conducted in a relatively dry environment, of 23 degrees C and 55% RH, and in a "wet" environment, namely immersion in distilled water at 28 degrees C. The "wet" fatigue tests clearly revealed the significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The present paper, Part II, discusses the modes and mechanisms of failure for the two adhesive systems in both the "dry" and "wet" environments. The failure surfaces of the joints tested in Part I have been examined using a variety of analytical techniques and the surface chemistry and morphology compared with that of the "as prepared" (i.e. non-bonded) 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 topography has been examined using scanning electron microscopy and atomic force microscopy. Also, cross-sections of the joints have been studied using the transmission electron microscope. The results reveal that for both the aluminium alloy and EG steel joints that the failure path is complex, and is associated with electrochemical activity (i.e. corrosion) in the case of the latter joints when tested in the "wet" environment. In part III [2], the results presented in the earlier papers will be used to predict the lifetime of single-overlap joints subjected to cyclic fatigue loading.