Interfacial Characterization of S2 Glass-Epoxy Resin Using Single Fiber Pullout Tests at Varying Temperature and Moisture Level

Abstract

Due to their high specific strength, stiffness, and energy-absorbing properties, glass fiber epoxy composites are commonly used in automotive, aerospace, and other consumer applications. The Army uses these composites in ground vehicles for various structures, and they are often subjected to a wide range of environmental conditions, with varying temperature and moisture conditions. They may also be subjected to high strain rate loading. While the influence of environmental conditions on the properties of the reinforcing fibers is minimal, a fundamental understanding of its effect on the resin and interfacial adhesion is critical in evaluating the behavior of these composites. This research focuses on characterizing the effect of temperature and moisture on the interfacial bonding between S2 glass fibers (with 463 sizing) and epoxy resin using single fiber pullout testing. Epoxy resin SC-15, commonly used in these applications, is tested at room temperature, -55ºC and 76ºC. The range is chosen based on the operating temperature defined by MIL 810 standards. The embedded length is comparable for all tests, of the order of 55-60 μm. The embedded length and average diameter are measured using a scanning electron microscope image of the failed specimen to calculate the apparent IFSS. The IFSS shows a strong correlation with temperature; an increase in IFSS at -55ºC and a decrease at 76ºC. An 89% increase was measured at -55ºC and a 45% decrease at 76ºC when compared to room temperature tests. The IFSS measured is dependent on various factors such as the surface roughness of the fiber, chemical bonds between the resin and the fiber (sizing), and the residual compression forces that induce friction within debond regions due to the difference in CTE (coefficient of thermal expansion) between the fiber and the resin. Due to competing mechanisms of resin failure or interfacial failure, in some cases, a residue of the resin on the fiber may be present. A discussion of these mechanisms and a comparison with the resin properties at different temperatures is discussed.