طراحی سنسور فیبر نوری برای فرایند شیمیایی و پایش محیطی

“Curing” is a term that is used to describe the cross-linking reactions in a thermosetting resin system. Advanced fiber-reinforced composites are being used increasingly in a number of industrial sectors including aerospace, marine, sport, automotive and civil engineering. There is a general realization that the processing conditions that are used to manufacture the composite can have a major influence on its hot–wet mechanical properties. This paper is concerned with the design and demonstration of a number of sensor designs for in situ monitoring of the cross-linking reactions of a commercially available thermosetting resin system. Simple fixtures were constructed to enable a pair of cleaved optical fibers with a defined gap between the end-faces to be held in position. The resin system was introduced into this gap and the cure kinetics were followed by transmission infrared spectroscopy. A semi-empirical model was used to describe the cure process using the data obtained at different cure temperatures. The same sensor system was used to detect the ingress of moisture into the cured resin system.

Fiber-reinforced composites (FRC) consist of three primary components: (i) the fiber, (ii) the matrix and (iii) the interface between the fiber and the matrix. The properties of FRC are, in general, dictated by the nature of the reinforcing fibers and the interface. However, the matrix can influence the so-called hot–wet and compressive properties of FRC. A number of previous studies have shown that absorbed moisture and elevated temperatures can cause significant and detrimental changes in the mechanical properties of FRC. Absorption of moisture can bring about changes such as (i) lowering of the glass-transition temperature (Tg), (ii) reduction in the resin modulus over a wide temperature range, (iii) swelling stresses induced by absorbed moisture and (iv) degradation of the resin and leaching, especially at high temperatures and prolonged exposure. The response of resins to moisture can be influenced by several factors including (i) exposed area, thickness of the sample, (ii) temperature, relative humidity and (iii) the cross-link density, morphology, free-volume, functional groups present and the resin-hardener system used. A wide range of fiber-optic sensor systems have been used for monitoring the cross-linking process and these can be classified into qualitative and quantitative techniques [1]. The qualitative techniques include intensity-based sensor designs [2], [3] and [4]. Quantitative analysis of the cross-linking kinetics is obtained using sensor designs that enable UV–visible [5], infrared [6] and evanescent wave [7] spectra to be obtained. The development of a multi-functional sensor for monitoring various parameters of relevance to thermosetting resins and composites has also been reported recently [8]. A number of papers have also reported on the use of optical fiber-based sensor designs for sensing humidity [9], [10] and [11]. However, only a limited number of publications have reported the use of optical fiber sensors to study the diffusion of water in thermosetting resins [12], [13] and [14]. In general, optical fiber-based sensor designs for detecting moisture ingress tend to be based on polymer-coatings on a de-clad portion of the wave guide; the presence of chemicals are detected using evanescent wave spectroscopy [15], [16] and [17]. The versatility of the extrinsic fiber Fabry–Perot (FP) interferometric (EFPI) sensors was demonstrated in a previous publication [18]. This current paper exploits the basic EFPI sensor design and describes three simple sensor configurations to monitor the cross-linking reactions in a commercial thermosetting resin. One of the sensor designs was then used for monitoring the ingress of moisture in the cross-linked resin.