Localized functionally modified glass fibers with carbon nanotube networks for crack sensing in composites using time domain reflectometry Gaurav Pandey a,b , Mitchell Wolters d , Erik T. Thostenson a,b, * , Dirk Heider a,c a Center for Composite Materials, University of Delaware, Newark, DE 19716, United States b Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, United States c Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, United States d Department of Composite Materials Engineering, Winona State University, Winona, MN 55987, United States ARTICLE INFO Article history: Received 12 August 2011 Accepted 3 April 2012 Available online 13 April 2012 ABSTRACT An electric time domain reflectometry (TDR) based sensing approach with an external par- allel plate transmission line has been developed to evaluate high-frequency electromag- netic 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 delam- ination are introduced during tensile loading. A sizing technique has been used for local- ized functional modification using CNTs. The TDR sensing approach has been correlated with strain and acoustic emission (AE) measurements as well as micrographs of edge rep- licas 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 intro- duced 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Composite materials exhibit multiple damage mechanisms at different scales such as micro and transverse cracking, fiber– matrix debonding, delamination and fiber breakage. These various damage mechanisms interact and affect the ultimate performance of the material at the structural level [1]. Failure, durability, and damage tolerance are a critical part of any high-performance structural design process but current fail- ure theories suffer from large variations between theoretical predictions and experimental data [2]. As a consequence there is a critical need to develop robust and versatile struc- tural health monitoring (SHM) techniques to evaluate the per- formance of composites in long-term structural applications. An ideal SHM system would be capable of measuring the var- ious micro and macro damage modes that occur over the life- span of the composite structure. Recent advances in detecting damage have been made using X-ray radiography [3], fiber Bragg grating [4,5], acoustic emission (AE) [6,7], ultrasonic [3,8], eddy current [9,10] and 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.008 * Corresponding author at: Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, United States. E-mail address: thosten@udel.edu (E.T. Thostenson). CARBON 50 (2012) 3816 – 3825 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon