Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol–carbon nanotube fibers N.D. Alexopoulos a, * , C. Jaillet b , C. Zakri b , P. Poulin b , S.K. Kourkoulis c a University of the Aegean, Department of Financial Engineering, 821 00 Chios, Greece b Universite ´ de Bordeaux, Centre de Recherche Paul Pascal – CNRS, Avenue Schweitzer, 33600 Pessac, France c National Technical University of Athens, Department of Mechanics, Laboratoryof Testing and Materials, 9 Heroes Polytechniou Str., 157 80 Athens, Greece ARTICLE INFO Article history: Received 6 September 2012 Accepted 26 February 2013 Available online 6 March 2013 ABSTRACT Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers differing on their pre-stretching con- dition were embedded in glass fiber reinforced plastic (GFRP) composites and used as strain sensors for damage monitoring of the composite. Strain sensing of the composite was made by the in situ measurement of the embedded fiber’s electrical resistance change dur- ing the mechanical tests. Four glass fiber composite plates were manufactured; each one had embedded a different type of produced PVA–CNT fibers. The multi-functional materials were tested in monotonic tensile tests as well as in progressive damage accumulation tests. The electrical resistance readings of the PVA–CNT fibers were correlated with axial strain values, taking into account the induced damage of the composite. It has been demon- strated that increasing the fiber’s pre-stretching ratio, its electrical resistance response increases due to higher degree of the CNTs alignment in the PVA matrix. Higher fiber pre-stretching degree enables the better strain monitoring of the composite due to higher measured electrical resistance change values noticed for the same applied axial strain val- ues. To this end, it enables for the better monitoring of the progressive damage accumula- tion inside the composite. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Composites are continuously increasing their share in engi- neering applications where strength and light-weight proper- ties are of imperative importance. In aircraft applications, their share has increased from 30% of covered area to almost more than 60% in civil aircrafts during the last decade, while for the military aircrafts their exploitation exceeds 90%. De- spite their superior strength properties, the Achilles tendon of the composites is their monitoring ability; due to the external applied loadings (fatigue or even impact) developed non-visible damage can be accumulated without being iden- tified. Several monitoring/sensing techniques of the compos- ites have been proposed in the literature [1] over the last decades, but several of these techniques cannot be applied during the composite’s mechanical operation, e.g. the acou- sto-ultrasonics, while others use embedded sensors such as the active piezoelectric, fiber optical and acoustic emission sensors that can monitor the composite structures in real-time [2]. Every different monitoring technique presents specific advantages and disadvantages regarding the manu- facturing method of the composites, types of damage that 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.02.055 * Corresponding author. E-mail address: nalexop@aegean.gr (N.D. Alexopoulos). CARBON 59 (2013) 65 – 75 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon