## 1 Introduction  
Polymer matrix composites (PMCs) have shown improved properties over traditional structural materials, especially weight-normalized properties, such as specific strength and stiffness [1]. While known for some time, the implementation of PMCs as structural materials is limited by complications in joining complex geometries [2]. Currently, the bonding of composite panels is completed primarily through the use of mechanical fasteners [3]. However, while their use can create a robust joint with the potential to compartmentalize crack growth, the process of drilling holes in a laminate remains a consistent challenge. For example, drilling in a PMC can create edge effects, leading to delamination and interlaminar failure, and also create locations of stress concentrations [3]. Furthermore, traditional mechanical fasteners are mainly manufactured from high-density metals, adding significant weight and complexity (possible issues with corrosion, etc.) [4]. Over-design of the composite panel is often necessary to compensate for these factors, leading to a loss in the overall efficiency [4]. One solution that could mitigate the issues associated with mechanical fasteners is to use adhesively bonded composite joints (ABCJs). Adhesive bonds offer the potential to join complex geometries at a fraction of the weight, simplifying a challenging problem. The implementation of this type of bond also allows for an improved distribution, eliminates the corrosion occurring in metallic fasteners, and reduces the number of parts of the structure. While the potential is vast, the current Federal Aviation Administration certification requires a demonstration that every bond used on a primary structure maintains integrity over the expected aircraft life cycle [5]. The motivation behind this restriction is that unlike mechanical fasteners, adhesive bonds cannot act to delay damage propagation, potentially leading to large-scale catastrophic failure [6]. Therefore, to increase the scope of PMC usage, a robust and exhaustive structural health-monitoring (SHM) tool must be developed to evaluate the quality of ABCJs without reliance on the initiation of damage in the bond [6]. The method should advance beyond the current structural health-monitoring methods (such as an E/M impedance and wave propagation approach [7], acoustic and ultrasonic methods [8], and terrestrial laser scanning [9]) that are limited in resolution and scope and rely on damage initiation. In this work, the dispersal and integration of magneto-electric nanoparticles (MENs) into an adhesive resin was used to monitor the quality of a polymer adhesive when exposed to an accelerated temperature and a moisture environment. Monitoring was achieved through a coupling of magnetostriction and piezoelectricity in the MENs [10]. This consists in having an electric polarization when a magnetic field is applied, while obtaining magnetic polarization when an electric field is applied. A schematic of this relationship is shown in Figure 1. As a result, the dipole surface charge density of the particle induced an electric field, which caused a change in magnetization that was detected using standard magnetometry techniques [11]. The variation in the obtained magnetic moment was associated with the history of the environmental exposure of the bond. The integration of MENs also provided a multifunctional effect, leading to improved shear, tensile, and flexural performance over the unfilled adhesive. Additionally, the strengthening of the adhesive due to doping was investigated using in situ microscopy and a nanocomposite toughening model. The changes in the surface charge density of MENs were also used to non-destructively evaluate the curing process of the adhesive. In the past, MENs have been used for optical and magnetic applications [12; 13] and as materials for biomedicine and drug delivery [14; 15; 16], but this is the first study to investigate their potential as a multifunctional SHM tool.