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 modified by incorporating 10 wt. % of silica nanoparticles, which were well dispersed in the polymer. Two different glass-fiber-reinforced plastic (GFRP) composite laminates were prepared to give: (1) a GFRP composite with an unmodified epoxy matrix (GFRP neat), and (2) a GFRP composite with a silica-nanoparticle-modified epoxy matrix (GFRP nano). Fatigue tests were undertaken employing a standard wind-turbine spectrum-load sequence, WISPERX. The fatigue life of the GFRP nanocomposite was about four times longer than that of the GFRP neat composite. This was reflected in (1) the development of matrix cracking, and (2) the rate of degradation of the stiffness of the composite, both being more severe in the GFRP neat composite, compared to the GFRP nanocomposite. The underlying mechanisms for the observed improvement in the spectrum fatigue life of the GFRP nanocomposite are discussed. Further, constant amplitude fatigue tests were conducted at various stress ratios. Using the static and fatigue data, constant life diagrams (CLD) were constructed. The spectrum fatigue life was then predicted following a standard procedure using the CLD. Very good correlation was observed between the predicted and experimental fatigue life for both GFRP neat and GFRP nanocomposites.

The fatigue life of a glass fibre reinforced plastic (GFRP) hybrid composite containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles, under a standard helicopter rotor spectrum load sequence was determined and observed to be about three times higher than that of GFRP with unmodified epoxy matrix. The underlying mechanisms for the observed improvements in spectrum fatigue life of GFRP composite with the hybrid matrix are discussed.

Unidirectional carbon fiber-reinforced plastic (CFRP) IMA/M21E polymer composite was manufactured by standard autoclave curing method. This is a new aircraft-grade CFRP composite presently used in the manufacture of aircraft wing parts. Test specimens as per ASTM standards for weight gain, tensile and compression tests were obtained from this composite laminates. Some of the test specimens were subjected to hygrothermal (hot–wet) aging in an environmental chamber under three different conditions, that is, (i) 45°C/85% relative humidity (RH), (ii) 70°C/85% RH and (iii) 55°C/100% RH until reaching moisture absorption saturation. Matrix-dominated mechanical properties, that is, transverse tensile and longitudinal compression were determined for dry and hygrothermally aged test specimens. During mechanical testing in a servo-hydraulic test machine, the respective hygrothermal conditions were maintained while testing. It was noted that the rate of moisture absorption increases progressively and reaches saturation around 0.76–1.24 wt% depending on the aging conditions. Moisture absorption followed Fickian diffusion behavior in all the conditions of the study. Also, a software program was developed to predict the moisture content and time of saturation. The predicted results from this software program correlated with experimental results. It was observed that the presence of moisture reduced the tensile strength significantly by about 9–31% and compression strength by about 2–8%. Microscopic observation of tested samples was carried out using scanning electron microscope to study the failure behavior.

Two different E-glass fiber reinforced plastic (GFRP) composite laminates having quasi isotropic [(+45/-45/0/90)2]S layup sequence were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with modified epoxy matrix (GFRP-nano) containing 9 wt. % of CTBN rubber micro-particles and 10 wt.% of silica nanoparticles. Standard fatigue test specimens were machined from the laminates and end-tabbed. Spectrum fatigue tests under a standard fighter aircraft load spectrum, mini-FALSTAFF, were conducted on both the composites at various reference stress levels and the experimental fatigue life expressed as number of blocks to fail, were determined. The stiffness of the specimen was determined from the load-displacement data acquired at regular intervals during the fatigue test. The matrix cracks development in the test specimens with fatigue cycling was determined through optical photographic images. The fatigue life of GFRP-nanocomposite under mini-FALSTAFF load sequence was observed to be enhanced by about four times when compared to that of GFRP-neat composite due to presence of micro-and nanoparticles in the matrix. The stiffness degradation rate and matrix crack density was considerably lower in GFRP-nanocomposite when compared to that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance of GFRP-nanocomposite are discussed.