Comparative study of interlaminar fracture toughness of GFRP with different fiber surface treatments

This investigation is focused on the influence of glass fiber surface treat ment on the interlaminar fracture toughness of unidirectional laminates. Th ree different fiber surface treatments were considered: polyethylene treate d fibers to get poor adhesion, silane treated fibers to get good bond stren gth, and industrial fibers without special treatments with the coupling age nts. The interlaminar fracture behavior of unidirectional glass fiber reinf orced composites with different fiber surface treatments has been investiga ted in mode I, mode II, and for the fixed mixed mode I/II ratio 1.33. Doubl e cantilever beam (DCB), end notched flexure (ENF), and mixed mode flexure (MMF) specimens were used. The data obtained from these tests were analyzed by using different analytical approaches and the finite element method. Fo r the fibers treated with the silane coupling agent, a value about 2.5 time s higher of mode II interlaminar fracture toughness for crack initiation wa s obtained in comparison with the polyethylene sized composite. For the com posite made from the industrial fibers, a value about 2 times higher was ob tained. Because of extensive fiber bridging and pullout in the composites w ith poor fiber/matrix adhesion, the results of mode I and mixed mode I/II t ests did not characterize the interphase quality. In order to determine the interphase quality, the mode II tests are recommended.