Transverse Cracks at Ply Drops in Fiberglass Laminates

Steven L Donaldson,T Jared Stonecash,Sangwook Sihn

doi:10.5923/j.ijme.20120206.06

Steven L Donaldson, T Jared Stonecash + Show 1 more

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https://doi.org/10.5923/j.ijme.20120206.06

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Abstract

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 init iated 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 epo xy-only resin) in a large increase in the quasi-static stress required to init iate 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 ult imate strength.The trend for the(+45/-45/ 0*/-45/+45)T laminates showed an increase in the damage initiat ion 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 ult imate strength increase due to the inclusion of the VGCNF.Results fro m fat igue 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 rein forcements.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 initiat ion of failu re v ia transverse cracks in the resin pocket.This was confirmed by photomicrographs taken at fixed load intervals.

Highlights

The rapid growth of the global econo my as spurred a corresponding increase in the need for electric power
Donaldson et al.: Transverse Cracks at Ply Drops in Fiberglass Laminates specimens were removed at fixed load values, based on the micrographs only, we know the init ia l crack formed between 0 and 316 MPa.A lso note the further damage development due to a subsequent load increase, seen as mu ltiple cracks in the mic rograph taken at 765 MPa stress level.The fina l fra me in Figure 5 shows the laminate edge after final failure
Initial strain ju mp d id not occur until 757 M Pa.This value was confirmed by the micrographs, which did not show any transverse resin cracks when a 322 MPa stress was reached, but a crack did appear after a load of 778 MPa was reached.when co mpared to the control, the laminate with aramid pulp in the region of the resin pocket had an in itial failure (t ransverse crack in the ply drop resin pocket) that was 2.75 times greater.The value of the ultimate failu re, shown in Table 1, did not change significantly: fro m 924 MPa to 966 M Pa, an increase of less than 5%, and within the scatter of the test data

Summary

Introduction

The rapid growth of the global econo my as spurred a corresponding increase in the need for electric power. In response t o th ese needs , th e wind tu rb ine generato r ind ust ry (in clu d ing g ov ern men t and acad emic p art ners ) has stepped-up research to reduce the cost, increase the life, and improve the reliability of the wind generator systems. Key components of the turbines are the long, slender rotating blades. The highly-loaded blades are typically fabricated fro m fiber-reinforced co mposite materials to reduce weight and increase the blade fatigue life[1]. The laminate thickness (s kin, s par, and web) for thes e long, high output

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