The fatigue and durability behaviour of automotive adhesives. Part III: Predicting the service life

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 results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release-rate, G(max) applied in the fatigue cycle, using logarithmic axes. In Part II [2] the mechanisms of failure were considered, particularly the mechanisms of environmental attack. The present paper, Part III, discusses the use of the relationship between da/dN and G(max), which can be obtained in a relatively short timescale, to predict the fatigue lifetime of (uncracked) single-overlap joints cyclically loaded in tension. An analytical and a finite-element model have been derived to predict the number of cycles of failure, N-f for lap joints and, particularly when the latter model was used to deduce the value of the strain-energy release-rate, G, in the lap joints, the agreement between the theoretical predictions and the experimental results is found to be very good.