Environmental Degradation Study of FRP Composites Through Evaluation of Mechanical Properties

Sethi, Sanghamitra (2014) Environmental Degradation Study of FRP Composites Through Evaluation of Mechanical Properties. PhD thesis.



The performance of fibre-reinforced composites is, to a large extent, controlled by the properties of fibre-matrix interfaces. The interface chemistry and character is vital to a composite material. Good interfacial properties are essential to ensure efficient load transfer from matrix to reinforcement, which helps to reduce stress concentrations and improves overall sustainability of mechanical properties. The strength of composite materials depends not only on the substrate but also on the interface strength. The interface here does not have unique fracture energy unlike homogeneous materials.Consequently, there is a great interest in developing new concepts for tailoring the strength of fibre-matrix interface. Some of researchers have been reported the mechanisms responsible for improved fibre-matrix interface adhesion is removing weak boundary layer, and thereby improving wettability. However, a high performance composite functions because a weaker interface or matrix stops a crack running continuously between the strong brittle reinforcements. Fibre reinforced composite materials do, however, suffer some serious environmental limitations. Environmental exposures include temperature, moisture, radiations, UV and other different alkali treatments, which cause deterioration in the mechanical and/or physical behaviour, adhesion between fibre/matrix interface regions of the composite material over a period of time. The aim of the current investigation is to present the variation of mechanical properties of glass fiber/epoxy composite under the synergistic effect of temperature and rate of loading. In case of temperature we performed 2 types of cases as above and below glass transition temperature (Tg) and in second case abobe and below-ambient temperature. Glass fibre reinforced polymer composites (GFRP), carbon fibre reinforced polymer composites (CFRP) and Kevlar fibre reinforced polymer composites were fabricated by hand-lay up method followed by compression molding press. The composite specimens were subjected to elevated and high temperatures as +60°C,+100°C,+150°C and +200°C temperatures. 3-Point short beam shear test and 4-point short beam shear test were conducted in order to characterize the mechanical behavior of laminated composite and to determine the influence of loading rate on interlaminar shear strength. To understand the interactions between various failure mechanisms in the fiber, matrix and fiber/matrix interface, microscopic analyses were conducted.

In second case we performed in-depth analysis of interlaminar shear test and failure mechanisms of glass fibre/epoxy, carbon fibre/epoxy and Kevlar fibre/epoxy composites under +50°C,-50°C,+100°C and-100°C temperatures and different crosshead velocity. Different high and low temperature conditioning were performed using Instron with environmental chamber providing additional information regarding in-situ failure of laminated composites. Following the test, the fracture surfaces of the samples were scanned under SEM to understand the dominating failure modes. Microstructural assessments can also reveal the response of each constituent viz. fibre, matrix resin and the interface/interphase; under temperature and mechanical loading. This section comprehensively presents the mechanical behaviour and structural changes in fibrous polymeric composite systems during the mechanical loading under high and low temperature service environment. We specifically tailored this potential to describe the contradiction and confusion at polymer composite interface which may not be underestimated by material scientists. Fibre/matrix adhesion involves very complex physical and chemical mechanisms. One of the most important physical aspects is the geometry of reinforcing fibres, which influences adhesion between fibre and matrix, stress transfer and local mechanisms of failure. In addition to chemical bonding, the fibre/matrix bond strength in shear is largely dependent on the roughness of the fibre surface and the fibre/matrix contact area.
At cryogenic temperatures, due to difference in coefficient of thermal expansion between the fibre and the matrix phase, microcracks initiate and propagate through the laminated composites. Therefore, knowledge of the resistance to different failure modes of woven fabric composites laminates at cryogenic temperatures is essential to the materials scientist and design analyst. The aim of this investigation was to study deformation and mechanical behaviour of glass fibre/epoxy composites subjected to 3-point short beam shear test at low and ultra-low temperature with different loading speeds. The laminates were tested at ambient (+27°C) temperature and at (-20°C,-40°C,-60°C) temperatures using liquid nitrogen in an environmental chamber installed on an Instron testing machine. Testing was carried out in different loading covering low to high medium speeds. Following the test the fracture surfaces were scanned under SEM microscope. A need probably exists for an assessment of mechanical performance of such potentially promising materials under the influence of changing environment and loading speed. Using fractography study to characterize the onset and growth of failure modes has become generally accepted method.
During thermal cycling differential coefficient of thermal expansions and residual stresses is a prime cause in fibre reinforced polymer composites (FRP) material. The behavior of the interfacial contact between fibre and matrix is strongly influenced by the presence and nature of residual stresses. GFRP and CFRP composite laminates are used to analyze the thermal cycle effect on the mechanical behavior with different loading rates. 3-point short beam shear test was performed for the analyze the mechanical behavior. To study the failure modes which have great impact on mechanical behavior, Scanning electorn microscope (SEM) was used.
The ensuing research revealed a number of key challenges regarding interface issues in producing polymer nanocomposites that exhibit a desired behavior. The greatest stumbling block to the large-scale production and commercialization of nanocomposites is the dearth of cost effective methods for controlling the dispersion of the nanoparticles in polymeric matrix. Current interest in alumina/epoxy nanocomposites, Cu nano particle and Multi walled carbon nanotube (MWCNT) has been generated and maintained because nanoparticles filled polymers exhibit unique combinations of properties not achievable with conventional composites. In the present study, glass fiber reinforced composites filled with nanoparticle have been prepared. 3-point short beam shear test was conducted to analyze the Interlaminar shear strength (ILSS) variation with different loading rate. Alumina nanoparticle was well dispersed in epoxy polymer matrix to achieved high mechanical performance. The results show that it is possible to improve the interlaminar shear strength with the loading rate variations. Clearly, no follow-up work in this area will be commendation for better understanding of effect of nanoparticle in FRP composites in assessment of loading rate sensitivity. Under these conditions, fibre reinforced polymer nanocomposites have been shown to exhibit two glass-transition temperatures, Tg: one associated with polymer chains far from the nanoparticles, and a second, larger Tg, associated with chains in the vicinity of the particles. To analyze different failure modes SEM analyses was conducted. Good interfacial properties are essential to ensure efficient load transfer from matrix to fillers, which helps to reduce stress concentrations and improves overall mechanical properties. Consequently, there is great interest in developing new concepts for improving the strength of fibre−matrix interface.

Item Type:Thesis (PhD)
Uncontrolled Keywords:Fibre reinforced polymer composites; environmental degradation; fibre/matrix interfacemechanical behavior, ; fractography; glass transition temperature; spectroscopy analysis; Al2O3 nano-filler.
Subjects:Engineering and Technology > Metallurgical and Materials Science > Physical Metallurgy
Divisions: Engineering and Technology > Department of Metallurgical and Materials Engineering
ID Code:6653
Deposited By:Hemanta Biswal
Deposited On:05 May 2015 11:45
Last Modified:05 May 2015 11:45
Supervisor(s):Ray, B C

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