EFFECT OF THERMALLY EXPANDABLE PARTICLE ADDITIVES ON THE MECHANICAL PERFORMANCE OF MIXED MATERIAL ADHESIVE JOINTS

This research investigates the effect of the addition of Thermally Expandable Particles (TEP) to a commercially available adhesive on the mechanical performance of bonded mixed material Single Lap Joints (SLJs) in lap shear tests. Investigated variables include additive concentration by weight (from 5 to 20%) and particulate size (ranging between 6 and 16 micrometers), for their effect on joint static and fatigue strength, as well as reversibility performance, as compared to those of SLJs bonded with the baseline adhesive with no additives. Each test joint is made by one Aluminum 6061 substrate that is adhesively- bonded to a composite substrate made of woven carbon fiber reinforced plastic 0/90/0 with epoxy matrix. The test samples are bonded using a commercially available epoxy adhesive that has been modified using two types of polymeric TEP additives with gaseous core. Lap shear tests are conducted to evaluate the maximum static Load Transfer Capacity (LTC), and fatigue S-N curves for a selected level of mean stress are drawn. Results show that particle enrichment is detrimental to the baseline adhesive performance in both static and fatigue scenarios. Reversibility performance is assessed using a charged RF coil that heats the substrate bondline by induction, causing significant volumetric expansion of the additive particles (TEP's) that leads to complete debonding of the adhesive joint. The time to complete debonding is used for assessing adhesive reversibility performance as affected by the concentration and the particulate size of the additive.