Epoxy is one of the widely used polymeric resin systems in composite industries, which is a two-part resin system that requires the mixing of a hardening agent to trigger the polymerization process. The mixing ratio and pot-life are usually specified and provided by the manufacturer; however, the stirring time and the details of the mixing process were left to the fabricator to configure, based on some visual observations, which may vary from person to person. Although the mixing ratio is fixed by the resin manufacturer, errors may occur during the weighing and transfer of resin parts. The hardener concentration and stirring time are two of the most important factors that can affect the resin properties. In addition, air bubbles can be formed during the stirring process, which is inevitable. Design of experiment (DOE) is one of the widely used tools to design, control, and study the effects of multiple factors. In this research, DOE with a factorial design of 23−1 (2k−1) was used to study the effects of hardener concentration, stirring time, and air bubbles on the tensile strength of epoxy resin. Test specimens were fabricated, cut, and tested as per ASTM D638 standard (i.e., common test performed in the industry for plastics) in a randomized order and then the results were statistically analyzed using Design-Expert software. The test results showed that all three factors significantly affect the tensile strength of the epoxy, and they should be carefully optimized and used for the fabrication of composite materials with optimal properties.
<div class="section abstract"><div class="htmlview paragraph">Most composite assemblies and structures generally fail due to weak interlaminar properties and poor performance of their bonded joints that are assembled together with an adhesive layer. Adhesive failure and cohesive failure are among the most commonly observed failure modes in composite bonded joint assemblies. These failure modes occur due to the lack of reinforcement within the adhesive layer in transverse direction. In addition, the laminated composites fail due to the same reason that is the lack of reinforcement through the thickness direction between the laminae. The overall performance of any composite structures and assemblies largely depends on the interlaminar properties and the performance of its bonded joints. Various techniques and processes were developed in recent years to improve mechanical performance of the composite structures and assemblies, one of which includes the use of nanoscale reinforcements in between the laminae and within the adhesive layer. However, most prior research has been focused on use of straight carbon nanotubes (CNTs) and other nanomaterials in particle forms. The goal of this research was to improve the properties of the adhesive film and the interfacial bonding effectiveness between the laminae. Because CNTs are inert in nature, their interaction with the resin and adhesive polymer molecules is very weak. In this research we have used CNTs with various geometrical configuration (straight and helical geometries) and various weight percentages as additional reinforcements. The objective was to investigate the effectiveness of helical geometries of the CNTs to form interlocking mechanisms with the resin and the traditional microfiber reinforcements to improve the overall performance of the composite structures and assemblies. Single lap joint test specimens and flexural test specimens were prepared based on the ASTM standard D5868-01 and ASTM standard D790 and then tested and analyzed. The experimental results showed that the samples with CNT reinforcements performed considerably better than neat epoxy samples. Among the two different CNT geometries, helical CNTs performed better than the straight CNTs.</div></div>
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