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Research Papers

Effect of Deep Penetration of Interleaf on Delamination Resistance in GFRP

[+] Author and Article Information
Dakai Bian

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: db2875@columbia.edu

Tizian Bucher, Y. Lawrence Yao

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027

D. J. Shim, Marshall Jones

GE Global Research,
Niskayuna, NY 12309

1Corresponding author.

Manuscript received May 30, 2015; final manuscript received November 7, 2015; published online March 10, 2016. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 138(7), 071011 (Mar 10, 2016) (10 pages) Paper No: MANU-15-1264; doi: 10.1115/1.4032566 History: Received May 30, 2015; Revised November 07, 2015

The problem of improving the delamination resistance and toughness of laminate fiber-reinforced composites especially for the drop-off structure is receiving considerable attention with the increasing need and application in industries. A hot melt-bonding process is developed to bond glass fabric laminates and the thermoplastic (TP) polysulfone (PSU) interleaf prior to the vacuum assisted resin transfer molding (VARTM) of laminate composites. The TP interleaf is heated above the glass transition temperature to reduce the viscosity when penetrating deeply into the glass fiber fabric. Mechanical tensile testing is performed to quantify the effects of the penetration depth on composite delamination resistance and composite toughness under different melt-bonding temperatures. Crack paths are observed by optical microscopy to characterize the crack propagation and arrest mechanism. Postmortem high-resolution imaging of the fracture surfaces is used to characterize the toughening mechanism of the TP interleaf reinforcements by using scanning electron microscopy (SEM). With deep penetration of the interleaf into the fiber bundles, cracks arrested within the penetration region improve the toughness by avoiding the cracks to reach the weak interface between interleaf and epoxy.

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References

Figures

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Fig. 3

(a) Schematic of the crack path under low penetration condition: crack initiates near the boundary of the pure interleaf region and the large plastic zone ahead of the crack tip reaches the interface between interleaf/fiber and epoxy/fiber, indicating that the crack migrates to this weak interface from point 1 to point 2 with the decreasing plastic zone size due to the rigid fiber. (b) Schematic of the crack path under high penetration condition: After crack initiates, the plastic zone only reaches the mixed region of interleaf and fiber (point 1). The decreasing plastic zone size arrests the crack propagating in the middle of the mixed region (point 2), where the delamination resistance is much larger than the interface between interleaf/fiber and epoxy/fiber.

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Fig. 1

(a) MTS System Corporation used their special testing system to test the wind turbine blade to failure under the flapwise loads. When the blade is in operation, the centrifugal forces will occur due to turbine rotation [16]. (b) Schematic of the specimen under external loads: interleaf is inserted between the external drop-off layer and the core layers. (c) Free-body diagrams of the drop-off region and interleaf: applied force and moment equilibrium about point O, both normal and shear stresses exist in the drop-off region and interleaf.

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Fig. 5

Three-dimensional schematic diagram describing the mechanical test setup. The strain gauge is placed on the top surface of the external drop-off layer and 0.4 in. away from the edge of the drop-off layer. A 2 in. × 1 in. interleaf is inserted underneath the external drop-off layer and hot melt-bonded before VARTM. Far-field load is in-plane tensile load. Note the coordinate system.

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Fig. 4

PSU viscosity versus temperature [23]

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Fig. 8

Experiment results for 254 μm thick PSU interleaf penetration depths from 280 °C to 380 °C. The error bars represent standard errors. The trend of the penetration correlates well with the decreasing viscosity temperature.

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Fig. 6

Y–Z cross section (parallel to the drop-off edge) of the specimen with melt-bonded 254 μm thick PSU at 280 °C. Xylene-etched surface shows low penetration of the PSU interleaf into the fiber bundles. Penetration boundary is indicated by the white lines. The average penetration depth is 32.5 μm, and the average remaining interleaf thickness is 168 μm. Due to the low melt-bonding temperature and high viscosity, a limited amount of the interleaf penetrates into the fiber bundles.

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Fig. 7

Y–Z cross section of the specimen with melt-bonded 254 μm thick PSU at 320 °C. Xylene-etched surface shows high penetration of the PSU interlayer. The average penetration depth is 103.6 μm, and the average interleaf thickness after melt bonding is 83 μm. Larger penetration depths form a thick mixed region of interlayer and fiber bundles.

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Fig. 17

Toughness of the specimens versus interleaf thickness under different melt-bonding temperatures. The error bars represent standard errors. The sharp increasing toughness from 25 μm to 127 μm is due to more participated TP interleaf in plastic deformation. After the peak point, the toughness decreases with interleaf thickness. It is because the influence of the mismatch of the drop-off structure and materials is magnified and induces adhesive failure, which reduces the toughness. For the specimens melt bonded at the higher temperature, which has a higher penetration depth and thus the better bonding quality, some toughness is preserved even the interleaf gets overly thick.

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Fig. 14

Y–Z cross section of the fractured specimen with melt-bonded 254 μm thick PSU at 320 °C. The white line indicates the penetration boundary. At this melt-bonding temperature, the average penetration depth is 106 μm. The crack propagates through the middle of the penetration region without reaching the weak interface between interleaf and epoxy.

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Fig. 2

(a) Illustration of the interlaminar shear and normal stress distribution along X-direction on the upper surface of the interleaf: the trends of the shear stress and normal stress are based on the analysis of the force and moment equilibrium for the external drop-off layer [18]. For the lower surface of the interleaf, the stresses are of the same magnitudes but opposite directions. Peak stresses occur close to the terminated surface. (b) Considering both normal and shear interlaminar stresses near or on the terminated surface of the interleaf, a crack is more likely to initiate near points A and C.

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Fig. 9

Predicted and experimental penetration depths increase with the melt-bonding temperature. The trend is primarily due to the deceasing viscosity of the PSU interleaf material with temperature.

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Fig. 10

Representative strain–stress curves obtained from the strain gauge mounted on the drop-off layer in uniaxial tensile tests: reference specimen (without interleaf), interleaved specimens under low/high melt-bonding temperatures. The reference specimen shows the highest stiffness while specimens with interleaf are much tougher.

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Fig. 11

Toughness of 254 μm thick PSU interleaved specimens from 280 °C to 380 °C. The error bars represent standard errors. The horizontal line represents the average toughness of the specimen without interlayer. The trend correlates well with the trend of the penetration depth (Fig. 8), indicating that the penetration depth directly influences the toughness.

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Fig. 12

Y–Z cross section (parallel to the drop-off edge) of the fractured specimen with melt-bonded 254 μm thick PSU at 280 °C. The white line indicates the penetration boundary. There are two kinds of the crack locations due to the thin penetration depth. Region A shows that the crack is near the interface between the pure interleaf and the matrix. Region B shows that the crack is near the interface between the interleaf/fiber and epoxy/fiber. The crack reaches the weak interface between interleaf and epoxy under low penetration condition.

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Fig. 13

X–Z cross section (perpendicular to the drop-off edge) of the fractured specimen with melt-bonded 254 μm thick PSU at 280 °C. It shows the crack locates at the interface between interleaf and epoxy/fiber or at the interface between interleaf/fiber and epoxy/fiber. Due to the low penetration depth, the crack always propagates along the weak interface between interleaf and epoxy. Less broken fiber, less bridged fiber, and bridged PSU are found compared to the specimen under the high melt-bonding temperature.

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Fig. 15

X–Z cross section of the fractured specimen with melt-bonded 254 μm thick PSU at 320 °C. The thickness of the interleaf after the melt bonding reduced to about 63 μm. The crack propagates through the penetration region of interleaf and fibers. The zone is about 106 μm wide as per Fig. 14 under this melt-bonding temperature. More broken fibers and bridged fibers result in high toughness.

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Fig. 16

(a) SEM image of the delamination fracture surface of the specimen with melt-bonded 254 μm thick PSU at 280 °C. The large clean region indicates brittle adhesion failure, which is caused by the crack propagating through the interface between interleaf and epoxy. The rough region indicates the crack goes through the penetration region, where part of the interleaf underwent plastic shear deformation. Fewer broken fibers and pulled-out fibers are found for this condition. (b) SEM image for the delamination fracture surface of the specimen with melt-bonded 254 μm thick PSU at 320 °C. The much rougher surface indicates more ductile fracture has taken place. Plastic shear deformation of the interleaf can be seen around fiber beams. More broken fibers and pulled-out fibers are seen for this condition due to the increased delamination resistance.

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