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A thermosetting epoxy-polymer was modified by incorporating 9 wt% of carboxyl-terminated butadiene-acrylonitrile rubber microparticles and 10 wt% of silica nanoparticles. The tensile fatigue behavior at a stress ratio, R = 0.1 for both the neat-epoxy polymer (i.e., unmodified) and the hybrid-epoxy polymer was first investigated. The fatigue life of the hybrid-epoxy polymer was about six to ten times higher than that of the neat-epoxy polymer. Secondly, the neat- and the hybrid-epoxy resins were infused into a quasi-isotropic lay-up, E-glass fiber fabric via a ‘Resin Infusion under Flexible Tooling’ set-up to fabricate glass-fiber reinforced plastic (GFRP) composite panels. The tensile fatigue tests at a stress ratio, R = 0.1 were performed on both of these GFRP composites during which the matrix cracking and stiffness degradation were routinely monitored. The fatigue life of the GFRP composite increased by about six to ten times due to employing the hybrid-epoxy matrix, compared to employing the neat-epoxy matrix. Suppressed matrix cracking and a reduced crack propagation rate were observed in the hybrid-epoxy matrix, which resulted from the various toughening micromechanisms induced by the presence of both the rubber microparticles and silica nanoparticles. These factors were considered to contribute towards the enhanced fatigue life which was observed for the GFRP composite employing the hybrid-epoxy matrix.

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A thermosetting epoxy resin was modified and four different types of bulk epoxy polymer sheets were prepared: (i) neat epoxy, (ii) epoxy with 9 wt.% rubber micro-particles, (iii) epoxy with 10 wt. % silica nano-particles, and (iv) epoxy with both 9 wt.% micron-rubber and 10 wt.% nano-silica particles. The tensile fatigue behaviour at a stress ratio R = 0.1 was investigated for all the materials. Addition of either the micron-rubber or nano-silica particles alone in the epoxy polymer had almost a similar beneficial effect and enhanced the fatigue life by about three to four times. The presence of both micron-rubber and nano-silica particles resulted in a further significant enhancement of the fatigue life, by about six to ten times. Fractographic studies suggested that the energy-dissipating mechanisms such as rubber cavitation and silica particle debonding, both being followed by plastic void growth of the epoxy, contributed to the enhanced fatigue lives which were observed in the modified epoxy polymers. Introduction:

A thermosetting epoxy polymer was modified by incorporating 9 wt% of a CTBN rubber microparticles. The stress-controlled CA tensile fatigue behavior at stress ratio, R = 0.1 for both the neat and the modified epoxy was investigated. The addition of rubber particles increased the epoxy fatigue life by a factor of about three to four times. The rubber particle cavitation and plastic deformation of the surrounding material was observed to contribute to the enhanced fatigue life of the epoxy polymer. Then, the neat and the rubber-modified epoxy resins were infused into a quasi-isotropic, lay-up E-glass fiber, non-crimp fabric in a RIFT set -up to fabricate GFRP composite panels. Further, the stress-controlled CA tensile fatigue tests at stress ratio, R = 0.1 were performed on both of these GFRP composites. Matrix cracking and stiffness degradation was continuously monitored during the fatigue tests. Similar to bulk epoxy fatigue behavior, the fatigue life of GFRP composites increased by a factor of about three times due to the presence of rubber particles in the epoxy matrix. The suppressed matrix cracking and the reduced crack propagation rates in the rubber-modified matrix contribute towards the enhanced fatigue life of GFRP composites employing a rubber-modified epoxy matrix.

Two types of glass fiber reinforced plastic (GFRP) composites were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with hybrid modified epoxy matrix (GFRP-hybrid) containing 9 wt. % of rubber microparticles and 10 wt. % of silica nanoparticles. Fatigue tests were conducted on both the composites under the WISPERX load sequence. The fatigue life of the GFRP-hybrid composite was about 4-5 times higher than that of the GFRP-neat composite. The underlying mechanisms for improved fatigue performance are discussed. A reasonably good correlation was observed between the experimental fatigue life and the fatigue life predicted under the spectrum loads.Keywords: glass fiber composite, silica nanoparticle, rubber particle, spectrum fatigue. * Corresponding author: Tel. +91-80-2508 6310; Fax: +91-80-2508 6301 E-mail address: manjucm@nal.res.in (CM Manjunatha) 2 INTRODUCTIONDue mainly to their high specific strength and stiffness, continuous fiber reinforced plastic (FRP) composites are widely used in various structural applications such as airframes, wind turbines, ship hulls, etc. Such composite structural components experience variable amplitude or spectrum fatigue loads in service. Hence, the fatiguedurability of the composite materials under spectrum loads is an important requirement in these applications.Engineering polymer matrix composite materials generally consist of continuous glass or carbon fibers embedded in a thermosetting epoxy polymer. The epoxy polymer, being an amorphous and highly cross-linked material, is relatively brittle and exhibits a relatively poor resistance to crack initiation and growth, thus affecting the overall mechanical properties, including the fatigue and fracture behavior of FRP composites.One of the ways to improve the mechanical properties of FRPs is to add a second phase of fillers into the epoxy matrix.Incorporation of various types of micro-and nano-sized spherical, fibrous and layered fillers into the epoxy has been shown to improve the mechanical properties of composites [1][2][3]. Considerable improvements in the strength and stiffness [4], and dramatic improvements in the fracture toughness [3][4][5] EXPERIMENTAL Materials and ProcessingThe materials used and the processing employed to manufacture GFRP composites are briefly explained in this section. However, a detailed description of the materials and processing can be found in [24]. The epoxy resin used was LY556 from Huntsman, which is a diglycidyl ether of bisphenol A (DGEBA) resin. The silica (SiO 2 )nanoparticles were obtained as a colloidal silica sol with a concentration of 40 wt.% in LY556 from Nanoresins. The reactive liquid rubber was a carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber, obtained from Nanoresins as a 40 wt.% CTBN-LY556 epoxy adduct. The curing agent was an accelerated methylhexahydrophthalic acid anhydride, HE600 from Nanoresins. The E-glass fiber cloth was a non-crimp-fabric (NCF) with an areal weight of 450 g/m 2 .The required quantity of the neat epoxy resin, th...

A thermosetting epoxy polymer was hybrid-modified by the addition of 9 wt.% of rubber microparticles and 10 wt.% of silica nano-particles. Glass-fiber reinforced plastic (GFRP) composite laminates employing the unmodified epoxy matrix (GFRP-neat), and the hybrid epoxy matrix (GFRP-hybrid), were produced by a resin infusion technique. The experimental fatigue lives of both GFRP composites under three different variable amplitude load sequences, namely (i) a three-step increasing block, (ii) a three-step decreasing block and, (iii) a random block load sequence derived from a three-step load block, were determined. The fatigue life of the GFRPhybrid composite was higher than that of the GFRP-neat composite under all the three load sequence blocks investigated, by about x2.6 to x4.0 times. The saturated matrix crack density and the stiffness reduction rate were both lower in the GFRP-hybrid composite compared to the GFRP-neat composite material. The suppressed matrix cracking and reduced delamination growth rates measured in the hybrid-modified epoxy matrix enhanced the fatigue life of the corresponding GFRP-hybrid composite. Using the constant amplitude fatigue data generated at various stress ratios, the fatigue lives under these variable amplitude load sequence blocks were predicted using empirical models. The predicted fatigue lives, although non-conservative, were in reasonably agreement with the experimental results.

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