NANO-SCALE RESIN MODIFICATIONS IN FIBERGLASSLAMINATES CONTAINING PLY DROPS

摘要

Fiberglass (Owens-Corning WindStrand) reinforced laminates containing internal ply drops werefabricated using resin film infusion with Epon 862 epoxy as the matrix resin. In addition, in theregion of the ply drop, the resin was enhanced using two types of additive: DuPont Kevlar pulpand Applied Sciences Inc PR-25 vapor grown carbon nanofiber. Laminate lay-ups of [0/0~*/0]_Tand [+45/-45/0~*/-45/+45]_T (where * indicates the ply that was discontinued) were tested in bothstatic and fatigue loading. Failure typically initiated at the resin pocket formed at the terminationof the dropped ply, which was detected by both edge photomicrographs and a jump inlongitudinal strain at the ply drop location. For the [0/0~*/0]_T laminates, addition of the Kevlarpulp resulted (when compared to the non-modified epoxy-only resin) in a large increase in thequasi-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 toinitiate 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 inboth the Kevlar pulp and PR-25 laminates, appreciably no change in ultimate strength due to theKevlar pulp, and a notable ultimate strength increase due to the inclusion of the PR-25. Resultsfrom fatigue testing showed a large fatigue penalty (in the thin laminates) due to the inclusion ofthe ply-drop, and no statistically significant difference in fatigue life due to the inclusion of thenano-scale reinforcements. Finite element stress analysis modeling of the crack growth indicatesthe effects of having increased matrix modulus in the resin pocket zone, as well as increasedvalue of laminate critical energy release rate. The analysis indicates the sequence of failure tobe, first, the development of a transverse crack in the resin pocket, followed by delaminationalong the dropped ply. This sequence was confirmed by photomicrographs taken at fixed loadintervals.