THE FATIGUE AND DURABILITY BEHAVIOR OF AUTOMOTIVE ADHESIVES - PART I - FRACTURE-MECHANICS TESTS

A fracture mechanics approach has been successfully used to examine th e cyclic fatigue behaviour of adhesively-bonded joints, which consiste d of aluminium-alloy or electro-galvanised (EG) steel substrates bonde d using toughened-epoxy structural paste-adhesives. The adhesive syste ms are typical of those being considered for use, or in use, for bondi ng load-bearing components in the automobile industry. The results wer e plotted in the form of the rate of crack growth per cycle, da/dN, ve rsus the maximum strain-energy release rate, G(max) applied in the fat igue cycle, using logarithmic axes. Of particular interest was the pre sence of a threshold value of the strain-energy release rate, G(th), a pplied in the fatigue cycle, below which fatigue crack growth was not observed to occur. The cyclic fatigue tests conducted in a relatively dry environment of 23 degrees C, and 55%; RH were shown to cause crack propagation at far lower values of G(max) compared with the value of the adhesive fracture energies, G(c), which were determined from monot onically-loaded fracture tests. Cyclic fatigue tests were also conduct ed in a ''wet'' environment, namely immersion in distilled water at 28 degrees C. The ''wet'' fatigue tests clearly revealed the further sig nificant effect an aggressive, hostile environment may have upon the m echanical performance of adhesive joints, and highlighted the importan t influence that the surface pretreatment, used for the substrates pri or to bonding, has upon joint durability. The development and standard isation of ''wet'' fatigue tests may provide the basis for a very effe ctive accelerated-ageing test.