A systematic experimental study was conducted to examine the influence of fiber surface treatment and sizing on the formation of fiber-matrix interphase and its effects on the fatigue performance of (0,90₃)_s cross-ply laminates. Three material systems having the same Apollo graphite fibers and HC 9106-3 toughened epoxy matrix, but with different fiber surface treatments and sizings, were used in this study. The presence of different interphases in these materials was confirmed using dynamic mechanical analysis techniques. Fatigue tests at R = 0.1 and 10-Hz frequency were conducted at various load levels to characterize the long-term behavior. Stiffness reduction, penetrant enhanced X-ray radiography, and scanning electron microscopy techniques were used to monitor damage progression.

Results indicate that the presence of different interphase regions in these materials can be identified using the dynamic mechanical analyzer. The nature of the interphase region has significant effect on the long-term behavior of cross-ply laminates. The polyvinylpyrrolidone (PVP) sized, 100% surface-treated, 810 O laminates have longer fatigue lives at higher load levels and shorter fatigue lives at lower load levels compared to the epoxy-sized, 100% surface-treated, 810 A laminates. The 200% surface-treated, epoxy-sized, 820 A laminates have longer life compared to the other two materials systems, at all three load levels. The damage mechanisms and failure modes in cross-ply laminates under long-term loading are significantly influenced by the nature of the fiber-matrix interphase. The 810 O material exhibits greater damage and stiffness reduction than the other two materials. The epoxy-sized 810 A and 820 A systems exhibit a brittle stress concentration controlled failure, while the PVP-sized 810 O system exhibits a global strain controlled failure.