Comparison of Strain Measurement Techniques for the Characterization of Brittle, Cementitious Matrix Composites

Five strain measurement methodologies are described and experimentally compared based on their performance to measure axial strain of brittle matrix composite materials under uniaxial tensile loading, and setup requirements.A clip-on extensometer, electrical strain gages, a laser extensometer, and two Digital Image Correlation (DIC) techniques (edge detection of a painted rectangle and speckle interferometry), are the focus of this investigation. The composite is a novel, fabric-reinforced cementitious matrix (FRCM) system intended for strengthening concrete structures. Instrumentation is not standardized for testing of FRCM materials, so this study is particularly relevant for this field. Results indicate that for this class of composite, both the laser and clip-on extensometers arewell suited for strain measurement,while strain gage measurements are only valid prior to the onset of cracking. Poor results obtained using the DIC technologies are likely attributable to out-of-plane displacement due to specimen curvature and fiber straightening during testing.