Sensing of damage and healing in three-dimensional braided composites with vascular channels Amanda S. Wu a , Anthony M. Coppola a , Matthew J. Sinnott a , Tsu-Wei Chou a,⇑ , Erik T. Thostenson a , Joon-Hyung Byun b , Byung-Sun Kim b a Department of Mechanical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, USA b Korea Institute for Materials Science, Changwon, Gyeongnam 641-831, Republic of Korea article info Article history: Received 9 January 2012 Received in revised form 18 May 2012 Accepted 17 June 2012 Available online 23 June 2012 Keywords: A. Functional composites A. Nano-composites A. Smart materials D. Infrared (IR) spectroscopy E. Braiding abstract With the rise of composite materials as replacements for traditional monolithic materials comes an increase in demand for multifunctionality. Prior studies have demonstrated the ability of an embedded, electrically percolating carbon nanotube network to respond electrically to the onset and progression of damage in composite structures. We build upon this work by incorporating healing functionality into braided composites through the use of a hollow channel resin delivery system. This study demonstrates the ability of a carbon nanotube network to sense crack filling during resin injection, thus providing the scientific basis required for sensing healing in advanced composites. With practical application in mind, a two-part healant system is employed in this study. Two methods for qualitatively assessing healing are employed and compared; these include elastic modulus/strain energy recovery and FTIR spectroscopy. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Composite materials face many challenges associated with replacing monolithic materials in conventional structures. Fore- most among these are materials and processing costs and expendi- tures associated with testing and proofing new composite structures. To remain competitive against traditional monolithic materials, it is necessary for composites to fulfill multiple functions in addition to weight saving/fuel efficiency, including EMI shield- ing [1], damage sensing [2], self-healing [3], energy storage [4], etc. With this demand in mind, the present research focuses on the development of composite materials with the ability to: (1) sense the onset and accumulation of damage, (2) repair damage to the composite matrix and (3) sense that repair has occurred. There have been extensive efforts into composite materials damage sens- ing [5–8] and self-repair of polymers [9] and polymer-based com- posites [10,11]; however, little effort has been made to unite the two functionalities [12]. 1.1. Self-healing background Self healing materials, specifically those involving mechanical delivery of a healant to a damaged region, have undergone tremen- dous development over the previous decade [13]. This article will focus primarily on efforts into polymer and composite self healing involving mechanical delivery of a healant to a damaged region. Encapsulation of the healant provides a robust method of storing uncured resin within a part [9]; several studies have demonstrated the ability of these capsules to rupture upon fracture [14] and thus deliver healant to the site of damage. Typical encapsulated healant systems are an epoxy resin and amine curing agent; while encapsulating the former has become a well-established science [15], processing and scale-up of the lat- ter is only recently being pursued with some success [16]. An inter- esting alternative approach has been presented by Mookhoek et al. [17] in which binary capsules are developed, thus establishing the potential to encapsulate a two-part healant system [18] within the same region. Encapsulated self-repair is poorly suited to composite repair due to processing difficulties arising from low permeability and yarn-yarn/interstitial spacing of the fabric preform. For example, the 140 lm binary capsules developed by Mookhoek et al. [17] are too large to incorporate even into the interstitials of a woven fabric preform. While this can be overcome by incorporating the healant capsules into an composite lamina as an interstitial layer [19], a major drawback of the encapsulation approach remains: the inability to repair a site more than once due to a limited amount of healant stored in a part. Nevertheless, this approach is of great interest in the area of self-healing coatings [20]. Recent trends have moved from encapsulated healant delivery to vascular delivery systems (e.g., Fig. 1), due to the ability of vas- 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.06.012 ⇑ Corresponding author. Tel.: +1 302 831 1550; fax: +1 302 831 3619. E-mail address: chou@udel.edu (T.-W. Chou). Composites Science and Technology 72 (2012) 1618–1626 Contents lists available at SciVerse ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech