Electric Time Domain Reflectometry Sensors for Non-Invasive Structural Health Monitoring of Glass Fiber Composites

Time domain reflectometry (TDR) offers the advantage of distributed sensing using a single transmission line sensor. In the present study, a parallel plate type non-invasive TDR sensor for structural health monitoring (SHM) of composite has been designed, modeled and experimentally tested. Five layer unidirectional glass fiber/epoxy composite specimens are fabricated. Specimens included a damage initiator in form of a cut in the central ply. The TDR sensor detects sub-surface damage in the composite non-invasively as the effective dielectric constant of the composite decreases due to the presence of delamination cracks. Previous work done on dielectrostriction is used to model the TDR response to strain changes. Qualitative agreement between theory and experimental results for strain sensing are found.