Chapter 18 Imaging Strain Localization in Fiber Reinforced Materials Yilmaz Akkaya, Masoud Ghandehari and Surendra P. Shah Abstract Composite materials such as Fiber Reinforced Cement or Concrete (FRC) are designed to prevent brittle fracture. FRC is characterized by enhanced ductility and distributed crack formation prior to failure. Mitigation of progressive crack propagation and localization of cracks and stresses are result of the even distribution of fibers throughout the matrix. This chapter presents the utility of full field optical methodology for tracking the progressive development of cracking in cement based FRC. A full field imaging approach is used to better understand the link between microstructural properties, and sequence of cracking. We present the application of Electronic Speckle Pattern phase measurement interferometry (ESPI) for the identification of crack locations, size and progression, and how these fea- tures are linked to the uniformity of fiber distribution in the matrix. 18.1 Introduction In this chapter we review the results of studies on high resolution mapping of surface strain fields, one which is particularly applicable to studying strain localization. The aspect ratio of the specimens used result in a plain strain field distribution, hence the surface measurements are a good approximation of the bulk response. This study highlights the utility of the ESP measurement technique to study the effect of fiber dispersion on crack formation in extruded FRC composites. The mechanical per- formance of an FRC composite depends on the fiber parameters including volume, Y. Akkaya Istanbul Technical University, Maslak, 34467 Sarıyer/İstanbul, Turkey e-mail: akkayayil1@itu.edu.tr M. Ghandehari (&) New York University, Six Metrotech Center, Brooklyn, NY 11201, USA e-mail: masoud@nyu.edu S. P. Shah Northwestern University, 633 Clark St, Evanston, IL 60208, USA e-mail: s-shah@northwestern.edu © Springer International Publishing AG, part of Springer Nature 2018 M. Ghandehari, Optical Phenomenology and Applications, Smart Sensors, Measurement and Instrumentation 28, https://doi.org/10.1007/978-3-319-70715-0_18 223