Process Monitoring of Fiber-Reinforced Polymer Composites
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Process Monitoring
of Fiber-Reinforced Polymer Composites B. Degamber and G.F. Fernando
Abstract This article presents a review of optical-fiber-based process-monitoring techniques that can be used to track the chemical reactions that take place during the processing of materials, with specific reference to thermosetting resins. The techniques covered include quantitative process-monitoring methods based on near- and mid-infrared, Raman, UV–visible, evanescent wave, and fluorescence spectroscopy. The basis for refractive-index-based process monitoring using optical fibers is also presented. The techniques described here can be readily applied to other classes of materials and other areas of interest such as aging and degradation. Recent advances in noncontact process monitoring are also presented.
Keywords: advanced fiber-reinforced composites, optical-fiber sensors, infrared spectroscopy, polymers.
Introduction Advanced fiber-reinforced composites (AFRCs) such as carbon, glass, and polyaramid-reinforced materials are used extensively in a number of industrial sectors, including aerospace, automotive, marine, recreation, and civil engineering. These composites offer a number of unique advantages, such as high specific strength and stiffness and relatively good fatigue resistance and impact resistance. The general properties of AFRCs are normally dictated by the properties of the reinforcing phase, including its volume fraction. The so-called hot-wet properties (evaluated for a sample at high temperature and/or in a humid environment) and compressive properties can be influenced by the properties and volume fraction of the matrix. In essence, AFRCs consist of a reinforcing agent, usually particulate, short, or continuous fibers, and a binding medium such as a thermosetting or thermoplastic resin. The thermosets are in general multicomponent liquids or semisolids that react upon the application of heat to yield a highly cross-linked solid. The transformation of the liquid or semisolid to a cross-
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linked structure is referred to as the cure reaction, or curing. Once cross-linked, thermosets have a high resistance to common solvents and when subjected to heat tend to degrade without exhibiting a melting point. On the other hand, thermoplastics can be reprocessed upon the application of heat, and this generally requires the material to be heated above its glasstransition temperature or melting point. Common classes of matrices used in the manufacture of AFRCs include epoxy, polyester, phenolic, cyanoacrylate, poly(ether ether ketone) (PEEK), polyamide (nylon), and polypropylene. The manufacturing method used in the production of AFRCs is generally dictated by the nature of the reinforcement, the chemical nature of the matrix, the shape and dimensions of the desired component, and the target production rate and cost.1 For example, particulate and short-fiber composites can be subjected to conventional polymer manufacturing techniques such as injection and compression molding. Continuous fibers can be obtained i
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