Introduction
Tensile testing is usually conducted to select a material for its application, quality control, and to forecast how the material is likely to react on deployment of different types of forces. The properties evaluated through the tensile test include the ultimate tensile strength, the maximum elongation, X-area reduction and so forth. These evaluations facilitate determination of such properties as Young’s Modulus, yield strength, Poisson’s ratio, and the characteristics of strain hardening [1. 2]. Tensile testing on glass fibre is, thus, performed to determine the quality of the material it contains, and to ascertain its suitability for various applications.
History of Fibre Glass
The first ever modern glass fibre was developed accidently by Dale Kleist .He had been trying to weld two glass blocks, and unexpectedly, a jet of compressed air hitting a stream of the molten glass created a shower of glass fiber. Through 1930s, certain developments in the field of fiberglass took place, and it was patented in 1938 [3].
Fiberglass, also referred as glass fibre and spelled as fiberglass, is a fibrous form of glass utilized mainly as insulators and reinforcing agents in plastics. Till 1930s, glass fibers were a little known entity, a novelty, and their thermal and electric insulating properties were being recognized. By that time, methods to produce continuous glass filaments had been developed. In modern times, the production of liquid glass has been undertaken with the help of glass melting furnace and from re-melting of performed glass marbles. For the production of continuous fibre, the liquid is filled into a bushing that is a receptacle pierced with several fine nozzles. It is through these nozzles that liquid ejects in fine streams. The streams solidifying are collected into a single strand wound onto a spool. Strands can conveniently be plied or twisted into yarns chopped into short pieces, woven into fabrics, and bonded into mats. On the other hand, discontinuous fibres are developed usually through a rotary process under which fine streams of glass are thrown out through the holes in the spinning dish. These are then broken and blown by a blast of steam or air [1, 4].
Fiberglass wool is an excellent thermal and sound insulator extensively used in appliances, buildings, and plumbing. Also, glass filaments and yarns enhance electrical resistivity and strength to the products of molded plastics. It includes automobile parts, pleasure boat hulls, and housings for various electronic consumer products. Glass fibres also have their utility as electrical insulators and reinforcing belts in tires of automobiles [4]. Nowadays, every family in the U.S. uses fibre glass like shower stall, a bath tub, a water faucet. It is used even in cars and boats and wall insulation. The list is exhaustive and ever increasing [3].
The ancient civilizations like Phoenicians and Egyptians and a few others made glass, glass into fibres, and thus, fiberglass. The small amount of glass fibres produced was a coarse and fine fibres that were for only decoration. People were unaware of the potential of glass fibers. This was first in 1870 that a person namely John Player discovered a process for mass production of glass strands with steam jet processes to produce so called mineral wool used effectively for insulation purposes. Fiberglass cloth was developed in 1880 for which Herman Hammesfahr was awarded [4].
Test on Glass Fibre
Tensile test on glass fibre involves testing of its elongation properties and tension, and it is usually performed through a machine called the Universal testing machine which has two crossheads. The first one is used to test the length of the material while the second tests the tension. The machine is of two types; one hydraulic powered and the other electromagnetically powered [3, 5].The tensile test on glass fibre is intended to determine the comparative tensile properties of glass fibre yarns, strands, and rovings. This is conducted in the shape of impregnated rod test of specimens having predetermined conditions of pretreatment, humidity, temperature, and the speed of tension testing machine. The test method is deployable to continuous glass fibre material; filament coated with resin compatible sizing. While in case of tensile testing of glass fiber, the main consideration should be accorded to the utilization of polymeric binder producing specimens yielding the highest consistent values for the glass material being tested. However, tensile properties differ with specimen preparation, speed and environment of testing, and resin impregnation. For precise comparative results, these factors must be taken into account carefully. Even then, this standard does not address all safety concerns with respect to its use. Hence, it is dependent on the user of this standard to focus proper health and safety practices determining the applicability of the regulatory limitations before use [6].
Properties of Glass Fibre
Glass fibres are thermal insulators and useful due to the high ratio of surface-area to weight. Nevertheless, the increase in surface area makes them more vulnerable to chemical attack. The trappings of air in them make blocks of glass fibre quality thermal insulator having thermal conductivity to the tune of 0.05 W/(m·K). The glass strength is commonly tested and published for pristine or “virgin” fibres that have been newly manufactured. The thinnest and freshest glass fibres are the stoutest or strongest due to the inner fibres being more ductile. The increase in the scratching on the surface leads to a reduction in tenacity due to the glass being an amorphous structure. The properties of glass are similar along as well as across the fibre. Humidity impacts the tensile strength immensely. The moisture being easily absorbed worsens microscopic cracks and defects on surfaces and reduces tenacity [7, 8 ].
Contrary to carbon fibre, the elongation in glass fibre is a lot more prior to it breaking. There is a close relation between the filament diameter and the bending diameter of the filament. The manufacturing success is dependent on viscosity of the molten glass to a very great extent. In the course of drawing or reduction of fibre circumference by the pulling of the glass, the viscosity should comparatively be low as it being too high, the fibre is likely to break while drawing. Nevertheless, the viscosity being too low, may lead to the glass forming droplets rather than fibre [7,8 ].
Many studies have been conducted to find out composition having properties that are desired. The right selection of materials and the right selection of the process of manufacture for various applications are the greatest challenge to engineers presently. The composite materials are commonly categorized on the basis of the matrix materials. The three important groups of composites include polymer matrix composite "PMC", ceramic matrix composite termed "CMC", and metal matrix composite (MMC). Out of these three, polymer matrix composite is the most common and is also referred as the fibre reinforcement polymer (FRP). It utilizes a polymer based resin as the matrix whereas the reinforcement commonly is fibres like glass, aramid, or carbon [9].
Young’s Modulus
In order to obtain such parameters of materials as modulus, E and residual stress, a value has to give to Poisson's ratio, v. The values determined for silicon-nitride ranges between 0.22 and 0.25 while the value for Poisson’s ratio is assumed as 0.25 and for comparison purposes; these values are commonly used.
Zhang, (2006) analyzed the characterization of PBT thin film, PBT standing for Pbx Ba1-x Tio3 that is a solid solution consisting of lead titanate and barium titnate. It is having similar ferroelectric behavior of barium titnate and lead titnate, and which can perform 90% domain switching within external stress state or electric field. The perovskite structure was selected for the tetragonal geometry at normal temperatures and the strain's linear variation (c/a) with its composition. Hence, the strain expected ranged between 1.1% and 6.5%. A least-squares fitting of the curves of the pressure-displacement with the analytical formula provides the estimated values for Young’s modulus E and the residual stress. The residual stresses detected were within the expected range. The Young’s modulus determined for the first film has been in the expected range between 100 GPa. The Young’s modulus deduced in the second film is a bit higher than expected [10].
In addition, the three layers of PBT thin films included PBT layer with thickness 190 nm, IBAD MgO layer with a thickness between 20 and 50 nm, and Si3N4 layer with thickness 75 nm. In a majority of the ferroelectric thin films, it is thick substrate on which ferroelectric layer is commonly grown. Thus, the mechanical behavior of the film is overwhelmed by the behavior of the substrate. The interface between the ferroelectric and substrate layers imposes geometrical constraint to the former. To the contrary, the PBT thin film is relatively free-standing as it is dominated by the ferroelectric layer [10].
Taniguchi et al. (2006) developed relationships between mechanical properties of glass fiber and strain rate based on data generated through experiment performed on tensile testing machine. The effect of strain rate on Young’s modulus was evaluated which revealed that the resultant Young’s modulus is independent of strain rate due to elasticity characteristics of e-glass fiber. The strain rate effect tensile strength of glass fiber. The impact of tensile strength is associated with fiber diameter due to dependency of the rate of strain on fiber diameter. This means that smaller diameter of glass fiber can create inclusive dependency on rate of strain [5].
Sadabadi and Ghasemi (2008) examined the tensile modulus of built short glass fiber / Polystyrene compounds by simulation and experimental investigations. They pointed out enhancing of tensile modulus of these composites in irregular form with increasing weight fraction of fiber material. This fiber weight fraction was much pronounced that was observed more than 40 % by weight of glass fibre [11]. The tensile properties of injection-molded PS composites enhanced with SGF and the results reveal that the tensile moduli of these compounds do not raise drastically especially for high values of fiber weight fraction. Simulation of fiber texture for numerous values of fiber material discloses that the combined effects of fiber weight fraction as well as fiber orientation establish the final tensile residential properties of compounds. The outcomes of tensile examinations show that the fiber to fiber contact has significant impacts on composite of short glass fiber and polystyrene. The industry must presume the design procedures of these composites for effective utilization in different sectors [11]
Kaushik, Singh and Kaushik (2006) evaluated the impact of chopped strand fiberglass fiber in the context of chemical and mechanical behavior of the composite polymers. They reported that the fracture parameters of reinforced glass fiber including Young’s modulus and modulus of rupture have tendency to increase gradually with increase fractional volume of fiber. They added that fracture parameters can also be improved due to mechanics associated with fibers like pull out, fiber bridging and so forth. The flexural modulus can also be increased having 10% of fiberglass up to the value of 1846 MPa. Moreover, micrographic electron scanning proved uniformity of glass fiber dispersal and excellent bonding [12].
Vacuum insulation panels are commonly used in different equipment and industries like refrigerators, astronautics, building industry and so forth. Fibreglass is a preferred material in the form of vacuum insulation due to its various characteristics like excellent thermal insulation, acoustic insulation, low density, and stable chemical properties. Li et al. (2013) evaluated the tensile strength, density and porosity of vacuum insulation panel made of glass fibre. The tensile strength of the material was observed as 503 and 576 N/m in transverse and longitudinal direction respectively. They reported excellent resistant of glass fibre against temperature based on drying characteristics of manufactured material. Moreover, vacuum insulation panels presented stable and low thermal conductivity by minimizing the drying temperature [13]
Sample specimen and Procedure
There are numerous materials and techniques available to place sample specimen to perform the tensile test. Epoxy material of different grades are commonly used at the ends. Creed (1993, P-22) used a high toughness epoxy termed "Hysol 9309.2” and rubber sheets to keep sample specimen in alignment. A relatively thick piece of reinforced fibreglass, vinyl ester, un-punched electrical vector board can also be placed on the sides [14]. Taniguchi et al. (2006) used aluminum tabs that placed at the end of the sample specimen in order to evaluate tensile characteristics of e-glass. The tensile measurement was carried out by two type of machines including hydraulic and electronic machines. They found it as excellent material to keep aligned fibreglass in the grabber. The sample specimen is shown in figure 1 having all dimensions in mm [5,14]
Figure 1: Geometric cross-section of fibre glass specimen [5, P 166)
Conclusion
Tensile testing of glass fibres is performed to ascertain its suitability for various applications. The evaluation of tensile strength, maximum elongation, and reduction in area is done to determine Young's Modulus, yield strength, Poisson ratio, and the characteristics of strain hardening. Due to the ability of great thermal and sound insulation, glass fibres as electrical insulators are extensively used in a variety of applications. Fibre glass is widely used as vacuum insulation panels in different equipment and industries like refrigerators, astronautics, and building industry.
References
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- Sadabadi ,H. and Ghasemi, M. 2008. Study on Fiber Weight Fraction Effect on Tensile Modulus of Polystyrene (PS) Composites Reinforced with Short Glass Fiber (SGF) Based on Their Fiber Orientation, Polymer-Plastics Technology and Engineering, 47(4),pp. 427-432
- Kaushik, A..Singh, P. and Kaushik, J.2006. The Mechanical Properties and Chemical Resistance of Short Glass-Fiber-Reinforced Epoxy Composites, International Journal of Polymeric Materials and Polymeric Biomaterials, 55(6), 425-440
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