PSI - Issue 14

K. Chawla et al. / Procedia Structural Integrity 14 (2019) 571–576 K.Chawla et al. Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Continuous fibre reinforced polymer composites have a wide range of civil and defense applications. Due to their high specific strength and high specific modulus, they are particularly used in fabricating lightweight structures. Although the laminates have excellent in-plane properties, they are often susceptible to interfacial failure during common operating conditions. Therefore, to enhance the reliability of laminated structures, enhancement of interlayer characteristics has always been a topic of research. Among various failure modes, the opening mode (mode I) often dominates in a laminate failure. Several studies were performed in past to study the factors which improve interfacial fracture toughness of laminates. Johnson and Mangalgiri (1986) investigated the effect of fibre bridging on Mode I interlaminar fracture toughness. They minimized fiber bridging by avoiding nesting of fibres across the mid-layers of a laminate and showed that this resulted in the reduction in the fracture toughness value. Huang and Hull (1988) performed similar study where they eliminated fibre bridging by using the stress corrosion treatment. Sela and Ishai (1989) suggested that the damage tolerance of a composite could be increased by using a toughened delamination-resistant resin. Spearing and Evans (1992) observed significant enhancement in fracture toughness of reinforced composites which were fabricated by using a modified resin system inducing several additional fracture mechanisms. Ebeling et al. (1997) studied the effect of composite architecture on the fracture toughness values. They showed that the fracture toughness of fiber composites depends upon the weaving pattern of fibers. They demonstrated that with optimal fiber orientation energy dissipation during crack growth can be enhanced. Arai et al. (2008) investigated mode I and mode II fracture toughness of CFRP laminates, where the laminate layers were reinforced with the vapor grown carbon nanofibers. Similarly, Dong et al. (2014) dispersed short carbon fibres in the resin system prior to fabricating laminates. They observed that the fracture toughness of composites was increased with the addition of short fibers. Yesgat and Kitey (2016) studied the influence of filler geometry and volume fractions on the fracture mechanisms in glass particle filled epoxy composites. They demonstrated that the flexural stiffness and fracture toughness of composites increased when the slender fillers were used to prepare the specimens. Also, the composites failure properties were seen to have improved at higher filler volume fractions. Literature review indicates that the presence of fibre bridging increases interlaminar fracture toughness. In current investigation the glass fibre reinforced epoxy laminates are fabricated by embedding short fibres to enhance fiber bridging. Double cantilever beam (DCB) specimens are loaded in mode I and modified beam theory is considered to determine interlaminar fracture toughness ( G Ic ) of the laminates. 2.1. Material and methods The laminates are fabricated by using woven unidirectional glass fabric (E-glass) of 1250 GSM and epoxy matrix. First, the matrix material is prepared by mixing DGEBA (Bisphenol diglycidyl ether) resin of density 1.15 gm/cm 3 and TETA (triethylenetetramine) hardener of density 0.97 gm/cm 3 in 10:1 ratio. Prior to mixing, the resin is kept in the oven for degassing at 120°C temperature for about 30 minutes. This is followed by cooling the resin at room temperature and mixing it uniformly with hardener for about 30 minutes. The laminates are then fabricated by hand layup technique and vacuum oven cured under a calibrated compression in a temperature controlled environment. Another epoxy system is also prepared which is reinforced with short fibres at 2% volume fraction. The E-glass fillers of average length and diameter, 6.5 mm and 16 µm, respectively, are used in this investigation. During the mixing process sonication is performed for uniform dispersion of fillers into the matrix. The prepared filler epoxy system is then used as a matrix material for the laminate fabrication. 2. Experimental study