With broadening of application of adhesive joints for structural elements, many studies have been carried out on fatigue behaviors of various types of adhesive joints. However, investigations of impact fatigue have been made only in a relatively low stress cycles range.
In the present study, a series of impact fatigue tests were conducted on the butt joint specimens bonded with epoxy-polyamide adhesive, where the run-out number of cycles was over 106. In addition, the ordinary reversed push-pull fatigue tests were carried out to obtain the fundamental fatigue data. From the strength behavior plotted on an S-N diagram for these two stress conditions, it was revealed that the impact fatigue strength was higher than the ordinary fatigue one in a relatively low stress cycles range but it decreased rapidly at a certain transitional number of stress cycles despite that the ordinary fatigue strength did not show such a strength trend in the range of this experiment.
Furthermore, the variations of total and inelastic strain ranges of the adhesive layer were determined under these two sorts of fatigue loading patterns to obtain a reasonable material information to interpret the characteristic impact fatigue behavior as mentioned above. It was found that the variations of these strain ranges in the impact fatigue were somewhat different from those in the ordinary fatigue. A detailed discussion was made on these trends. It was confirmed that the cumulative hysteresis energy could play a role of the fatigue failure criterion in the case of adhesive joints used in this experiment.
