Experimental evaluation of PVB/ITO nanocomposite sensors for bending crack monitoring in metallic structures

Fatigue cracks in thin metallic structures subjected to bending can nucleate at multiple surface locations and propagate rapidly through the thickness, resulting in a brief interval between damage initiation and final failure. This study experimentally evaluates a piezoresistive sensor comprising indium tin oxide (ITO) nanowires dispersed in a polyvinyl butyral (PVB) matrix for real-time qualitative monitoring of this bending fatigue damage. Six structural steel specimens with a geometric stress concentrator were excited near their first bending resonance at frequencies between 28.5 and 29.5 Hz. Two sensor configurations were examined: two narrow nanocomposite strips, tested on four specimens, and one wider sensor covering the critical region, tested on two specimens. Specimens with narrow sensors failed between approximately 48,000 and 58,000 cycles, whereas the wider-sensor configuration failed near 45,000 cycles. In all four narrow-sensor tests, changes in the electrical response were detected prior to final structural failure. The wider sensors produced more stable electrical signals but showed lower sensitivity to localized damage, with significant changes occurring only shortly before or during specimen failure. This difference is attributed to the fewer parallel conductive pathways in the narrow sensors, which increases their sensitivity to localized disruptions in the ITO percolation network. The results demonstrate the feasibility of PVB/ITO nanocomposite sensors for real-time qualitative detection of rapidly evolving bending fatigue damage and highlight sensor geometry as a key parameter that influences damage sensitivity.