Multifunctional MEN-Doped Adhesives: Strengthening, Bond Quality Evaluation, and Variations in Magnetic Signal with Environmental Exposure

Featured Application Non-destructive evaluation of bond quality using magneto-electric nanoparticles. Adhesive bonding of polymer matrix composites offers various advantages over traditional fasteners, such as a uniform stress state, reduced weight, and delay of composite delamination. However, adhesive bonding has limited implementation due to challenges in the prediction of durability. This work introduces a new method to monitor an adhesively bonded composite joint by dispersing magneto-electric nanoparticles (MENs) into the polymer precursor and monitoring changes in their surface charge density by evaluating the output magnetic signal under an applied magnetic field. Real-time monitoring of the curing process of a polymer adhesive was performed and corroborated via thermal analysis and mechanical testing. Lap shear and end notch flexure testing showed that adding 1 vol% MENs led to a similar to 23% increase in shear strength and a similar to 12% increase in mode II critical energy release rates compared to the undoped adhesive. Adding 5 vol% MENs also increased the adhesive's peak tensile stress by similar to 8%. Strengthening mechanisms of the doped adhesive were monitored using in situ electron microscopy. A correlation between water ingression and a change in the magnetic moment was observed. Results show the MENs' potential as a structural health-monitoring tool for a wide range of materials and applications.