EFFECT OF HALLOYSITE NANOTUBE
ON THE FATIGUE LIFE OF GLASS
FIBER REINFORCED EPOXY
Department of Mechanical Engg , Sri Krishna College of Engg & Technology , Coimbatore, Tamilnadu, India .
SAMPATH P S
Professor, Department of Mechanical Engg, K S Rangasamy College of Technology, Thirchengode, Tamilnadu, India.
Glass fiber polymer composites have high strength, low cost but frequently suffer from poor performance in fatigue. This investigation shows that the addition of small fraction of halloysite nanotubes (HNTs) in the matrix results in a significant increase in high-cycle fatigue life. Thermosetting epoxy polymer was modified by incorporating 4wt% of well dispersed Halloysite nano tube(HNT). The neat and HNT modified epoxy resins were used to fabricate glass fiber reinforced plastic (GFRP) composite laminates by hand layup followed by hot compression moulding technique. The stress- controlled tensile fatigue behaviour at a were performed on these composites; the fatigue life of GFRP composite was increased by about two times due to HNT. Cyclic hysteresis measured over each cycle in real time during testing is used as a sensitive indicator of fatigue damage. It was observed that when HNTs are present hysteresis growth with cycling is suppressed.
Keywords - Epoxy , Fatigue , Glass fiber , HNT, Nanocomposite. 1. INTRODUCTION
Development of new composite materials or modification of existing composite material is the real challenge for most of the materials engineers. So there are enormous research efforts are emerging in the field of composites to develop new materials with enhanced mechanical, electrical and thermal properties. Among these, fiber reinforced polymer composites are the most adorable because of their ample usage in various applications which includes many mechanical, automotive and structural components. These components habitually experience different types of constant and variable amplitude fatigue loads in service. In this consequence, safe operation of the structure for the required technical lifetime demands that such composite materials, apart from their good static mechanical properties, also should possess relatively high fatigue strength. [Schwartz. (1992)]. Fatigue concerns the damage of materials when subjected to cyclic loading at the stress level which is less than their ultimate static strength. Fatigue damage is known to be a slow process of which the development depends on the microstructure of the materials. For homogeneous materials, the fatigue behaviour is often characterized by an early crack that dominates the damage development and lead to final fracture, for inhomogeneous materials, such as fiber-or particulate-reinforced polymers, the fatigue damage at an early stage is often diffuse in nature, as the crack can be initiated from multiple sites. In this case, the dominant crack may not be apparent until it is very close to the final fracture. So Fatigue tests are important to mechanical characterization of the material, since the maximum cyclic load that a material can sustain is a fraction of its tensile strength .
The majority of engineering composite materials consist of a thermosetting epoxy matrix reinforced by continuous glass or carbon fibres. The epoxy, when polymerised, is an amorphous and highly cross-linked material. This cross-linked microstructure results in many useful properties such as a high modulus and failure strength, low creep, etc [Manjunatha et al. (2009)]. However, it also leads to an undesirable property whereby the polymer is relatively brittle and has a poor resistance to crack initiation and growth which may affect the overall fatigue and fracture performance of FRP composite. Understanding the fatigue crack propagation behaviours of epoxy composites has been of great importance because such composites are often used for engineering components that are subject to cyclic loading.
attracted considerable attention due to the improved mechanical, thermal and tribological properties of the resulting composites. Because of the extremely high surface to volume ratios and the nanometer size dispersion of nanoclays in polymers, nanocomposites exhibit improved properties as compared to the pure polymers. Various types of particulate, fibrous and layered nano fillers have been employed in composites [Bernd et al. al. (2003), Haque et al. (2003) , Zhou et al. (2008) , Kinloch et al. (2008) Thiagarajan et al. (2012)]
Halloysite nanotubes (HNT) are newly invented nanofillers and which has recently become the subject of research attention as a new type of filler for improving the mechanical properties of polymers. HNT’s are derived from naturally deposited alumino-silicate (Al2Si2O5(OH)42H2O), Some of the researchers also justified that HNTs are the new type of additive for enhancing the mechanical, thermal and fire-retardant performance of polymers [Ning et al. (2007) , Ismail et al. (2008) , Marney et al. (2008), Rawtani and Agrawal (2012), Alhuthali and Low (2013)] Comparing with other nanosized inorganic fillers, naturally occurring HNTs are easily available and much cheaper. More importantly, the unique crystal structure of HNTs, such as rod-like geometry and low hydroxyl density on the surface, makes them readily dispersed in a polymer matrix and they are bio-compatible [Mingxian et al. (2007)]
A few researchers have attempted to uncover the fatigue behaviour of polymer nanocomposites. Researchers [Grimmer and Dharan (2008)] observed that the addition of 1 wt% of carbon nanotubes (CNTs) to the matrix of glass fiber-epoxy composite laminates improved their high-cycle fatigue life by a remarkable 60–250% . Even more impressively, the addition of 2 and 5 wt% multi-walled CNTs enhanced the fatigue performance of physiologically maintained methyl metha crylate–styrene copolymer (MMA-co-sty) by 565 and 593%, respectively [Marrs et al. (2007)]. Authors [Zhang et al. (2007)] demonstrated an order of magnitude reduction in fatigue crack propagation rate for an epoxy system with the addition of 0.5 wt% of CNTs. The crack-tip bridging and frictional pull-out mechanisms were responsible for the suppression of fatigue in the nanocomposite. Other types of nanofillers also gave rise to improved fracture properties. For example, the introduction of SiO2 particles increased both the initiation fracture toughness and the corresponding cyclic fatigue behavior of epoxy [Blackman et al. (2007)]. The Al2O3 and TIO2 nanoparticles improved the flexural strength, stiffness and fracture toughness as well as the fatigue crack propagation resistance of the epoxy [Wetzel et al. (2006)]. In a recent study, it was observed that the addition of 9wt% rubber microparticles in the epoxy matrix enhanced the fatigue life of a glass-fiber reinforced plastic (GFRP) composite by about three times [Manjunatha et al. (2009)]
Most of the fatigue studies on composites have concentrated on synthetic fibre/resin composites and fatigue studies on nanocomposites are relatively new. However, detailed studies on the fatigue behaviour of HNT toughened fiber reinforced nanocomposites are limited. Hence, the main aim of this investigation is to study the stress-controlled constant-amplitude tensile fatigue behaviour of a glass fibre reinforced plastic (GFRP) composite with HNT modified epoxy matrix.
The following sections summarises the experimental part of the research work.
2.1.Materials and Fabrication of Composite Laminates
Due to several advantages over other thermo set polymers, epoxy LY 556 resin is selected as the matrix material for this research work. Chemically it belongs to the ’epoxide’ family and its common name of epoxy is Diglycidyl-Ether of Bisphenol-A (DGEBA), and corresponding hardener is HY 951 which were supplied by Vantigo. The resin hardener ratio is 100:10 by weight as recommended by the supplier, Plain woven fabric type glass fabrics with an aerial weight of 200 g/m2 are used as major reinforcement which was supplied by Saint Gobain. The halloysite nano tube (HNT) was procured from Natural nano, NewYork.
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2.2.3. Scanning Electron Microscopy
The fracture surface of the tensile specimens was examined by using Field Emission Scanning Electron Microscope (FESEM). Before the SEM studies were performed, a thin section was cut from the fracture surfaces and it was coated with thin layer of gold. Then, SEM micrographs of tensile fractured surfaces were obtained with an accelerating voltage of 15kV.
3. RESULTS AND DISCUSSION 3.1.Fatigue Behaviour
Fatigue life data for glass-fiber composites with and without the addition of 4wt% of HNT are shown in Figure 3. In general, both unfilled and HNT filled composites exhibit a typical stiffness reduction trend and it may be seen that for the same level of maximum applied stress, the HNT modified GFRP composite exhibited much longer fatigue life than the unfilled GFRP composite. Therefore it is confirmed that the addition of HNT enhances the fatigue life of the composites over the entire range of stress levels investigated. These results show good agreement with the results obtained by others [Manjunatha.et al (2009)]. The experimental data for stress– life (S-N) curves of the composites shown in Fig.3 were fitted in Eq (1) which is specified by Basquin’s law [Buch (1988)].
f is the fatigue strength coefficient (FSC) and b is the fatigue strength exponent (FSE). The values of
FSC and FSE determined for both unfilled and HNT filled composites. The addition of HNT increased the FSC values increased by about 35% and FSE values decreased by 2.9%, due to the HNT filled epoxy matrix being employed as opposed to the unfilled-epoxy matrix addition in the composite.
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Fig.4.Sem Micrograph of tensile fractured specimens of a) Unfilled and b) HNT filled EP/EF Composites
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