Investigation of the Interlaminar Shear Properties of Fiber-Reinforced Polymers via Flexural Testing Using Digital Image Correlation

The determination of the interlaminar properties of laminate composites to date has included a few direct measurement techniques: the standardized short-beam test via three-point flexural testing and the double beam shear test via five-point flexural testing. These test methods are limited to a determination of the apparent interlaminar strength, whereas the full stress-strain behavior cannot be determined. The double beam shear test allows for a calculation of the shear modulus, provided that four additional elastic, in-plane properties have been determined in advance via three additional types of tests. The present work includes a comparison of the three-point short-beam test, the nonstandardized four-point flexural test, and the double beam shear test (requiring small span-length-to-thickness ratios for the shear loading to become dominant). The tests are carried out on unidirectional carbon fiber-reinforced composite (CFRP-UD) with a symmetric layup. The experiments are accompanied by photomechanical investigations using digital image correlation (DIC) to visualize the phenomenology of shear deformation. The novelty of this approach includes the use of the DIC-determined shear strains, which show a parabolic distribution across the specimen's thickness. The DIC-calculated maximum shear strain across the thickness is used for a full description of the stress-strain behavior with a focus on the elastic loading range to evaluate the shear modulus. Interlaminar shear strengths and shear moduli determined from all three types of tests are compared with the intralaminar shear properties.