Fiberglass (Owens-Corning WindStrand) reinforced laminates containing internal ply drops were fabricated using resin film infusion with Epon 862 epoxy as the matrix resin. In addition, in the region of the ply drop, the resin was enhanced using two types of additive: DuPont Kevlar pulp and Applied Sciences Inc PR-25 vapor grown carbon nanofiber. Laminate lay-ups of [0/0*/0]_(T) and [+45/-45/0*/-45/+45]_(T) (where * indicates the ply that was discontinued) were tested in both static and fatigue loading. Failure typically initiated at the resin pocket formed at the termination of the dropped ply, which was detected by both edge photomicrographs and a jump in longitudinal strain at the ply drop location. For the [0/0*/0]_(T) laminates, addition of the Kevlar pulp resulted (when compared to the non-modified epoxy-only resin) in a large increase in the quasi-static stress required to initiate damage, and a slight increase in laminate strength. Addition of the carbon nanofiber resulted in essentially no change in the stress required to initiate damage, and a reduction in the ultimate strength. The trend for the [+45/-45/0*/-45/+45]_(T) laminates showed an appreciable increase in the damage initiation quasi-static stress in both the Kevlar pulp and PR-25 laminates, appreciably no change in ultimate strength due to the Kevlar pulp, and a notable ultimate strength increase due to the inclusion of the PR-25. Results from fatigue testing showed a large fatigue penalty (in the thin laminates) due to the inclusion of the ply-drop, and no statistically significant difference in fatigue life due to the inclusion of the nano-scale reinforcements. Finite element stress analysis modeling of the crack growth indicates the effects of having increased matrix modulus in the resin pocket zone, as well as increased value of laminate critical energy release rate. The analysis indicates the sequence of failure to be, first, the development of a transverse crack in the resin pocket, followed by delamination along the dropped ply. This sequence was confirmed by photomicrographs taken at fixed load intervals.
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