Localized functionally modified glass fibers with carbon nanotube networks for crack sensing in composites using time domain reflectometry

An electric time domain reflectometry (TDR) based sensing approach with an external parallel plate transmission line has been developed to evaluate high-frequency electromagnetic changes in composites due to applied load and internal damage. A model system of cross-ply glass fiber/vinyl ester composites with and without the selective integration of localized carbon nanotube (CNT) networks was studied where microcracking and delamination are introduced during tensile loading. A sizing technique has been used for localized functional modification using CNTs. The TDR sensing approach has been correlated with strain and acoustic emission (AE) measurements as well as micrographs of edge replicas capturing the damage state. Both the nanotube modified and baseline composites have similar mechanical properties and damage progression which is reflected in similar stress–strain plots, AE measurements and edge replica studies. However, the CNT introduced composites have enhanced strain and damage dependent TDR response. Hence, through localized functional modification of the composite electromagnetic properties using CNTs and the electromagnetic–mechanical property coupling of CNTs, it is possible to (1) increase TDR sensitivity to strain and (2) sense development of micro-scale cracks. This approach offers potential for use in existing composite structures or permanently integrated during the manufacturing process and is in situ and non-invasive.