Test method Monitoring Poisson’s ratio of glass fiber reinforced composites as damage index using biaxial Fiber Bragg Grating sensors C. Yilmaz a , C. Akalin a , E.S. Kocaman a , A. Suleman b , M. Yildiz a, * a Facultyof Engineering and Natural Sciences, Integrated Manufacturing Technologies Research and Application Center, Sabanci University, 34956, Istanbul, Turkey b Department of Mechanical Engineering, Center for Aerospace Research, University of Victoria, Victoria, BC, V8W 3P6, Canada article info Article history: Received 29 February 2016 Accepted 10 May 2016 Available online 19 May 2016 Keywords: Poisson’s ratio Transverse cracking Stress transfer Glass fibers Fiber Bragg Grating abstract Damage accumulation in Glass Fiber Reinforced Polymer (GFRP) composites is monitored based on Poisson’s ratio measurements for three different fiber stacking sequences subjected to both quasi-static and quasi-static cyclic tensile loadings. The sensor systems utilized include a dual-extensometer, a biaxial strain gage and a novel embedded-biaxial Fiber Bragg Grating (FBG) sensor. These sensors are used concurrently to measure biaxial strain whereby the evolution of Poisson’s ratio as a function of the applied axial strain is evaluated. It is observed that each sensor system indicates a non-constant Poisson’s ratio, which is a sign of damage accumulation under the applied tensile loading. As the number of off- axis plies increases, transverse strain indicates a notable deviation from linearity due to the formation of transverse cracking, thereby leading to a larger reduction in Poisson’s ratio as a function of applied axial strain. Here, it is demonstrated that biaxially embedded FBG sensors are reliable to monitor the evolution of Poisson’s ratio, unlike biaxial strain gages which record strain values that can be significantly influenced by the cracks formed on the surface of the specimen. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Glass or carbon fiber reinforced polymeric composites have received a great deal of attention due to their high specific strength and stiffness for structural applications. The relatively high specific strength of composite materials makes them suitable for applica- tions where the weight and operating costs are intimately coupled. In comparison to metallic materials, where the failure is usually triggered by a single crack during their service life, composite materials exhibit poorly characterized damage mechanisms due to the existence of multiple cracks, which makes the prediction of service life rather complex and difficult. Therefore, in literature, there are several approaches developed and investigated to un- derstand the accumulation of damage and damage state of com- posite structures under static and dynamic loading conditions. For example, Highsmith and Reifsnider studied stiffness reduction as a damage indicator in composite materials [1]. Several researchers examined the reduction of Poisson’s ratio as a damage indicator [2e5] since Poisson’s ratio (n xy ¼ε y /ε x ) embodies both axial and transversal strain (ε x and ε y , respectively) information and is affected by the transverse cracks formed as a result of applied longitudinal strain [6e8]. Paepegem et al. [6] investigated the evolution of Poisson’s ratio of composite materials as a function of applied longitudinal strain, and showed that Poisson’s ratio de- creases with the applied strain. Smith et al. [8] modelled the evo- lution of Poisson’s ratio using a shear-lag theory under static loading conditions, correlated their analytic model with experi- mental results, and indicated that Poisson’s ratio decreases as the transverse crack density increases. The measurement of Poisson’s ratio of composites subjected to dynamic and static loading has mainly been performed using strain gages and extensometers. Due to their size, these sensors cannot be embedded in composite structures, and thus measure the strain from the surface. Additionally, surface mounted strain gages detach easily under cyclic loading, even at relatively few cycles. Moreover, strain gages and extensometers are sensitive to electromagnetic interference, and hence cannot be used in environments with high electromagnetic interference. To circumvent the drawbacks of strain measurements with surface mounted electrical strain sensor systems, one promising approach is the utilization of embedded Fiber Bragg Grating (FBG) based sensor systems [9]. FBG is a section * Corresponding author. E-mail address: meyildiz@sabanciuniv.edu (M. Yildiz). Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest http://dx.doi.org/10.1016/j.polymertesting.2016.05.009 0142-9418/© 2016 Elsevier Ltd. All rights reserved. Polymer Testing 53 (2016) 98e107