Schematic diagram of the energy harvesting and ENF experimental setup.
Schematic diagram of the energy harvesting and ENF experimental setup.

Smart composite materials promise the ability to reinforce structural components while monitoring structural health. Now researchers have created a new composite material that acts as a self-powered sensor to detect stress and damage without an external power source [Yu et al., Composites: Part A 172 (2023) 107587, ].

The new composites developed by the team from Tohoku University in Japan, University of Chester and Aston University in the UK combines a carbon fiber reinforced polymer (CFPR) with a sodium potassium niobate nanoparticle-filled epoxy (KNN-EP). Together the materials offer improved mechanical properties and piezoelectric performance, with the ability to convert mechanical energy from bending stresses into electrical signals, which can be harnessed as a force sensor for structural damage detection.

“Composite materials often need to strike a balance between mechanical strength and other functional properties,” explains Fumio Narita of Tohoku University, who led the study. “[We] sought to create a composite that does not compromise the piezoelectric properties of KNN-EP while enhancing its mechanical strength with a CFRP.”

The team found that a KNN content of 30 vol.% provides the most advantageous combination of mechanical and piezoelectric properties. In combination with CFPR electrodes, the composite’s voltage output is 600% higher than the piezoelectric alone. Crucially, different patterns in the voltage signal indicate different crack growth patterns and directions, so the composite can be used to detect stress and damage within the material.

“There is a need in materials science to better understand how different materials respond to damage, specifically in terms of crack propagation,” points out Narita. “Understanding [how] different signal patterns correspond to different types of damage could provide crucial insights into improving the damage resistance and durability of materials.”

Monitoring the electrical signals generated by the CFRP/KNN-EP composite could enable the tracking of structural health in real-time and the prediction of imminent failures. The composite could be integrated into buildings, bridges, or other structural infrastructure to provide both repair and reinforcement of components, as well as early warnings of the need for repair.

“CFRP is also crucial in aerospace and automotive applications,” says Narita, “[so our] combination strategy involving lead-free piezoelectric composites with CFRP… could enable early detection of structural damage in applications like aircraft fuselages, spacecraft structures, or automotive bodies.”

Although CFRP/KNN-EP production and integration costs are high, the composite could also be a vital part of the Internet of Things as an energy harvester.