Shear Strain Measurement Techniques in Composite V-Notch Shear Testing

Background: V-notch shear testing is a common method for measuring shear properties of fiber-reinforced polymers (FRP) composites. While bonded gauges have traditionally been used to measure shear strain in V-notch specimens (ASTM D5379), digital image correlation (DIC) is attracting interest due to its full-field, multiaxial measurement capability. Standard practices for using DIC to measure shear strain of a D5379 specimen have not been established. Objective: The objectives of the current investigation are to characterize shear moduli of unidirectional IM7/8552 carbon/epoxy composite and to explore best practices of using DIC to measure shear strain in D5379 specimens with different material orientations. Methods: Quasi-static experiments involving standard D5379 specimens were performed, and shear strain was measured using bonded, resistance-based strain gages as well as several different areas of interest in DIC. Finite Element (FE) modeling of the specimens was used to evaluate different areas of shear strain averaging and for comparison to the DIC measurements. Results: FE models confirmed that only considering a small area between the notch tips was the optimal method for strain averaging while other sub-optimal areas led to 3–18% error. DIC measurements produced comparable errors. Erroneous DIC data near the notch tips complicated post-processing, and two methodologies to remove the erroneous data were implemented using an automated statistical approach and a manual selection approach; the latter was slightly more accurate. Conclusions: The results of this investigation can be used in future shear test method standardization with DIC strain measurement by definitively showing the optimal procedures and the challenges associated with DIC measurements on D5379 specimens.