Abstract
The increased numbers of bonded joints and bonded repairs of defective structures make a knowledge of adhesive bonding crucial to aircraft design and life extension. In this study, monotonic tensile and tension-tension fatigue tests were conducted on 2024-T3 Al adhesively bonded joints with and without rivet holes and 7075-T6 Al substrates with and without an adhesive. The former Al substrates were milled to a half-dogbone shape with and without 2-rivet holes, and the latter were machined to three types of pre-cracked specimens: Al substrates with a side-crack and a stop-drill hole, with 2-rivet holes, and with a V-type side-crack. Scrim cloth structural adhesive tape (3M AF-163-2K) was applied to the substrates. All the specimens were heat-cured at a temperature of 121 °C, with a heating rate of 2.8 °C/min and a hold time of 90 minutes at 121 °C. The curing pressure was kept at 50 kPa. It was found that the static behavior of both types of joints (without and with rivet holes) consisted of three stages: a linear stage followed by a region of increased non-linearity and then a ‘yield’ region. It is within this yield region that the rivet holes affected the strength of the joints. The presence of rivet holes was found to reduce the fatigue lifetime by about 40% compared with the joints without rivet holes. For the 7075-T6 Al substrate, adhesive appeared to make all of the three pre-cracked substrates slightly more plastic. The presence of a pre-crack significantly decreased the plastic displacement and caused a slightly higher stiffness compared with the un-cracked Al substrate. The specimens with 2-rivet holes gave the longest lifetime, about 6 and 27 times more than specimens with a side-crack and stop-drill hole, and V-type specimens, respectively. It was found that a life extension factor between 1.5 and 2.0 was obtained for the cracked specimens with an adhesive. The adhesive alone appeared to increase the fatigue life of the cracked structures.
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