Capacitive Structural Health Monitoring of Hybrid Metal–Composite Joints

Hybrid metal–composite joints are increasingly used in lightweight structures for automotive and marine applications, yet their reliability is often compromised by interfacial damage such as delamination or adhesive failure. This study presents a multi-channel Structural Health Monitoring (SHM) approach for hybrid metal–GFRP joints based on capacitive self-sensing. The sensing principle relies on a capacitor-like configuration embedded in the joint: the metal adherend acts as one electrode, while thin metallic foils applied or co-cured on the composite surface form the second. An array of these foils enables localized monitoring by detecting capacitance variations driven by dielectric properties during damage propagation. A distinctive feature of this work is the synergistic interaction between the sensing system and additively manufactured lattice structures at the interface. These 3D architectures, originally designed to enhance mechanical interlocking and joint strength, act as electric field concentrators, significantly enhancing sensitivity to interfacial separation. During Mode I fracture testing (Double Cantilever Beam), the advancement of the crack front triggers the sequential fracture and instantaneous electrical disconnection of the metallic lattice struts. This transition from a grounded to a floating state produces discrete, localized capacitance drops, enabling real-time spatial tracking of the crack tip. The experimental campaign validates this approach through preliminary dielectric characterization and synchronized mechanical-capacitive testing, utilizing a custom high-resolution acquisition system. The results demonstrate that this capacitive technology provides a reliable, non-intrusive solution for monitoring damage evolution in high-performance hybrid joints, reducing the need for periodic inspections.