Transverse Cracks at Ply Drops in Fiberglass Laminates

Glassfiber reinforced epoxy resin laminates containing internal ply drops were fabricated using resin film infusion.The resin in the region of the ply drops was enhanced using two types of additive: aramid pulp and vapor grown carbon nanofiber (VGCNF).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.Failures 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]Tlaminates, addition of the aramid 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 increase in the damage initiation quasi-static stress in both the aramid pulp and VGCNF laminates, appreciably no change in ultimate strength due to the aramid pulp, and a notable ultimate strength increase due to the inclusion of the VGCNF.Results from fatigue testing showed a large fatigue penalty due to the inclusion of the ply-drop, and no 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 initiation of failure via transverse cracks in the resin pocket.This was confirmed by photomicrographs taken at fixed load intervals.