Strain sensing in single carbon fiber epoxy composites by simultaneous in-situ Raman and piezoresistance measurements Nataliya Kalashnyk a, b, * , Eric Faulques c , Jan Schjødt-Thomsen a , Lars R. Jensen a , Jens Chr. M. Rauhe a , Ryszard Pyrz a a Department of Mechanical and Manufacturing Engineering, Aalborg University, Fibigerstræde 16, 9220 Aalborg East, Denmark b Institut Jean Lamour, UMR 7198 CNRS e Universit e de Lorraine, Parc de Saurupt, 54011 Nancy, France c Institut des Mat eriaux Jean Rouxel (IMN), Universit e de Nantes, CNRS, 2 rue de la Houssini ere, BP 32229, 44322 Nantes Cedex 3, France article info Article history: Received 2 June 2016 Received in revised form 25 July 2016 Accepted 29 July 2016 Available online 3 August 2016 abstract The change of electrical resistance in continuous single carbon fibers and single carbon fiber/epoxy model composites with applied strain has been investigated with a new method combining simulta- neous in-situ Raman and electromechanical measurements. In all cases, a sudden increase to infinity of the relative electrical resistance corresponds to fiber fracture. The gage factors of the piezoresistance curves were determined. Reinforcement/matrix interface are compared for sized and unsized systems. It is shown that, in principle, it is possible to correlate the fiber strain and the variation of electrical fiber resistance as a function of the applied strain for a single fiber embedded in epoxy. This study indicates that carbon fibers embedded in epoxy matrix may serve as electrical strain sensors to detect both their own onset of damage and that of the composite under load, prior to specimen fracture. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Carbon fibers [1] and carbon nanotubes (CNT) [2] used as rein- forcement in various structural materials like polymers, cements and metals are electrically conductive and show drastic changes of their electrical resistance when there are strained. The piezore- sistance properties of single carbon fibers under tensile loading have been shown previously by several authors to provide rich information about the mechanical behavior of the fibers [3e8]. In this case piezoresistance effects can be used for precise strain sensing of bare carbon fibers alone, or in bundles, as well as of carbon fibers embedded in a matrix of composites. Since the pio- neering work of Conor and Owston [3], it is known that the change of electrical resistance of strained carbon fibers can be linked to the degree of misorientation of graphitic crystallites. However, there are relatively few studies devoted to the electromechanical prop- erties of single fibers, mainly because the measurement technique requires an appropriate method to connect the fiber to the elec- trical measurement setup. From the early works of Owston [4], Berg et al. [5], and later from DeTeresa et al. [6], it is nevertheless established that the electrical properties of the fibers depend strongly on their macroscopic mechanical and microstructural properties such as elastic modulus and contact resistance between carbon fibers crystallites. For instance, DeTeresa et al. [6] showed that piezoresistance in low and high modulus carbon fibers in- creases quasi-linearly with tensile strain while it decreases with compressive strain. Sudden increase of resistance to infinity on compression was linked to the failure of the fiber. Since then, much effort has been made to ameliorate single fiber specimen handling for reliable electrical testing [7,8]. Based on this, subsequent electrical resistance measurements focused on multiple carbon fibers embedded in various matrix materials with the aim to monitor load transfer, failure and damage properties. Up to now, these investigations concern mainly carbon fiber bundles without epoxy [9,10], laminates [11e 16], hybrid car- bon/glass fiber composites [17], and carbon fiber polymer- and cement-matrix composites [18]. Still, the electromechanical studies of a single carbon fiber itself, embedded in an epoxy matrix, have only been conducted by a few numbers of investigators [19e23]. As a matter of fact, piezoresistance techniques applied to such spec- imen type should yield valuable information concerning the indi- vidual damage behavior of fiber in composite. Influences of residual stress [20,21], sizing and diameter of the embedded fiber [23], and * Corresponding author. Institut Jean Lamour, UMR 7198 CNRS e Universit e de Lorraine, Parc de Saurupt, 54011 Nancy, France. E-mail address: n.kalashnyk@yahoo.com (N. Kalashnyk). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2016.07.064 0008-6223/© 2016 Elsevier Ltd. All rights reserved. Carbon 109 (2016) 124e130