Electrical monitoring of structural health of multiscale hierarchical composites using fibers modified by graphenic sheets or carbon nanotubes

Fiber reinforced composite structures are susceptible to complex damage, and current nondestructive evaluation methods are challenging to implement for health monitoring. To address these problems, multilayer graphenic sheets and multiwall carbon nanotubes were deposited over the surface of glass fiber weaves and infused with a vinyl ester resin to fabricate electrically conductive laminated polymer composites capable of self-monitoring structural damage. An array of 42 electrodes was placed at the top layer of the glass/vinyl ester laminated composite beams, which were subjected to monotonic and cyclic four-point bending tests. An artificial debond was induced at the center of selected bending specimens to deliberately control the expected critical damage location. In situ measurements of electrical resistance revealed presence of damage around the debond zone. For specimens without debond, damage was mainly located near the supports and load introduction elements. The regions with the largest electrical resistance changes also experienced the highest strain levels according to the strain fields obtained by digital image correlation, and showed remarkable correlation with X-ray tomography regarding damage location. As further observed by post-mortem X-ray tomography, major damage in the bending specimens occurred by fiber buckling in the compression surface and delamination. The graphenic sheet-modified laminated composites exhibited slightly higher electrical sensitivity than those modified with carbon nanotubes. However, the carbon nanotube-modified fibers achieved comparable electrical sensitivity using only one-fourth the weight concentration of graphenic sheets.