FRP composites characterization by ftirimaging

Abstract

It is well known that the high-performance properties of glass-fiber-reinforced polymer composite materials are not simply the sum of the properties of their constituents. The properties of composites depend on the ability of the interface to transfer stress from the matrix to the reinforcement. In fact, it is at the interfacial region where stress concentrations develop because of differences in the thermal expansion coefficients between the reinforcement and the matrix phase due to loads applied to the structure, cure shrinkage (in thermosetting matrices), and crystallization (in some thermoplastic matrices). Coupling agents have two different functionalities that are designed to chemically bond with the reinforcement at one end and the organic matrix at the other. The most commonly used coupling agents are bifunctional organosilicon compounds named silanes. The silane coupling agents of most commercial glass fibers have three hydrolyzable alkoxy functional groups. These groups allow the silanes to react with each other and with the glass to form a multilayer network on the glass surface. In order to study the effect of moisture absorption by the glass-fiber surface to the bulk epoxy, we used FTIR imaging to investigate this problem. Furthermore, to the best of our knowledge, there are no studies of the epoxy/glass fiber interface using the “FTIR imaging” technique. This technique uses a focal-plane array detector (FPA) coupled with a step scan interferometer to improve FTIR microscopic measurements, yielding spatially resolved spectroscopic information in the infrared region. In addition, this technique allows one to obtain consecutive IR images of the microscopic region of interest in intervals of time less than 5 min. For this reason, the FTIR imaging can be used to study the evolution of several components at the same time in specific sites in the sample. The composite was fabricated using the conventional HAND LAYOUT method. The materials used are E-Glass Fiber and Epoxy resin (araldite LY556). The hardener used is HY951. A sixteen layered structure was formed as per the ASTM standards. The fiber and the matrix were taken in the ratio of 50:50. The sample was left for drying for 24 Hrs after the fabrication so that the matrix completely seeps in and become dry. The samples after cutting and oven drying were divided into 3 parts. One part is kept as such and was wrapped in Aluminium foil and stored in the Dessicator after weighing all the samples. Second and third part were first weighed and then given a hygrothermal treatment by placing the samples in the humidity chamber for 50 Hrs and 100 Hrs respectively at 50°C and 95% humidity. After ithe Hygrothermal treatment, the dimensions of all the samples were measured and then they were being tested for 3-Point Bend Test. Then the samples were characterized using FTIR- maging and he fractured surface was studied by SEM. The ILSS value increases with initial moisture absorption due to the relief of residual stresses but after a certain stage it decreases due to the loss of adhesion between matrix and fiber. The ILSS value increases with the strain rate but after a certain stage it decreases because the matrix is unable to transfer load properly i.e. ILSS value is low at low strain rate as well as high strain rate as at low strain the load is applied for more time and thus the specimen fails at low stress value and at high strain rate, the time available for transfer of load is insufficient and the load acts as an impact and thus specimen fails at low stress. Thus the rate of loading should be optimum. The FTIR-IMAGING Results Shows that the moisture absorption is more at the interface in low Hrs treatment as the components of composites have the property to absorb moisture and then moisture absorption is more in matrix due to more debonding leading to creation of voids at the interface and thus the water diffuses in easily through the interface to matrix. The SEM Images of the fractured surfaces shows that the initial moisture absorption results in the increase in the bond strength as the matrix gets squeezed but after a saturation stage, the moisture absorption results in the debonding of the matrix-interface bond and also matrix- matrix bond and thus ILSS value initially increases and then decreases.