Contents lists available at ScienceDirect Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng Experimental determination of Representative Volume Element (RVE) size in woven composites Behrad Koohbor, Suraj Ravindran, Addis Kidane ⁎ Department of Mechanical Engineering, University of South Carolina, 300 Main street, Columbia, SC 29208, USA ARTICLE INFO Keywords: Digital image correlation Strain averaging Optical methods, RVE Woven composite Meso-scale ABSTRACT A systematic approach is proposed to estimate the length scales of the representative volume element (RVE) in orthogonal plain woven composites. The approach is based on experimental full-field deformation measure- ments at mesoscopic scales. Stereovision digital image correlation (DIC) is conducted to determine the full-field strain distribution in on- and off-axis specimens loaded axially in tension. A sensitivity analysis is carried out to optimize the image correlation parameters. Using the optimized set of image correlation parameters, full-field strains are measured and used in conjunction with a simple strain averaging algorithm to identify the length scales at which globally applied and spatially-averaged local strains converge in values. The size of a virtual window containing local strain data, the average of which has the same value as the global strain, is identified as the RVE dimensions for the examined material. The smallest RVE sizes found in this work are shown to be both strain and angle dependent. The largest RVE dimension obtained is reported as a unique, strain and orientation insensitive RVE size for the woven composite examined. 1. Introduction Macroscale non-linear mechanical response of woven composites has been documented to be the result of complex fiber-matrix inter- actions; whereas, the degree of this nonlinearity itself is a function of the angle between the loading direction and the principal fiber axes [1– 3]. The Complex deformation mechanisms that govern such orienta- tion-dependent nonlinear response can be investigated by studying the deformation response of off-axis specimen at micro and meso-scales [4]. Although micro-scale studies can provide useful information on the response of individual components in a woven composite, deformation characterizations at this scale are basically incapable of providing any evidence on the prevailing fiber-matrix interaction mechanisms. On the other hand, meso-scale analyses have been established to overcome this challenge by allowing a more accurate examination of the deformation mechanisms at yarn scales [5–8]. Data extracted from meso-scale studies on woven composites not only reveals the governing deformation mechanisms, but can also be used to capture the local deformation response, in order to validate micromechanical and finite element simulations, the concepts currently of great interest in the area of composite research [9–12]. Traditionally, verification and validation of numerical approaches, particularly finite element analyses, is generally conducted at only one length scale. However, it has long been realized that the deformation response of fiber composites at smaller scales is clearly different from that of macroscales. Accordingly, researchers commonly attempt to take advantage of homogenization algorithms to determine bulk deformation behavior from the local response. Such homogenization algorithms are required to be conducted over a specifically selected volume of the material which is small enough to capture the local components’ response, while sufficiently large to encompass all in- dividual constituents and represent the material as a whole [13]. Therefore, the concept of representative volume element (RVE) has been introduced and successfully implemented as the underlying concept in homogenization techniques [14,15]. To date, several numerical studies have been carried out attempting to characterize the RVE and its length scales for different material systems and under various loading conditions [15–18]. On the other hand, there are not many experimental works to characterize RVE, particularly in the case of woven composites. Digital image correlation (DIC) has been proven to be a promising technique, enabling accurate deformation measurements at a wide range of length scales, thus allowing for experimental characterization of RVE [19,20]. However, certain challenges exist in the application of DIC, particularly at small scales and for highly heterogeneous deformation patterns, not to mention that both cases are present in meso-scale study of woven composites [21]. Application of a fine speckle pattern on millimeter and sub-millimeter sized fields of interests, as well as selection of http://dx.doi.org/10.1016/j.optlaseng.2016.10.001 Received 6 May 2016; Received in revised form 16 September 2016; Accepted 3 October 2016 ⁎ Correspondence to: Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Room A132, Columbia, South Carolina 29208, USA. E-mail address: kidanea@cec.sc.edu (A. Kidane). Optics and Lasers in Engineering 90 (2017) 59–71 0143-8166/ © 2016 Elsevier Ltd. All rights reserved. crossmark