Curing Monitoring of Composite Material Using Embedded Fiber Bragg Grating Sensors

Abstract

The composite materials with high specific stiffness and strength have been widely applied in various fields such as aerospace or industry. Simultaneously, curing methods for joining composite materials have also gone through development intensively, for examples Hotpress, Pultrusion, Resin Transfer Molding (RTM) and Vacuum Molding. During the curing process, internal damages and residual strain are the most considerable relevance to quality of product, and hence demanded careful treatment. Commonly, the internal damage of composite materials could be detected by using ultrasound scan and X-ray, but these methods, however, are significantly high cost and not on-line monitoring. It is not suitable for smart structure application. Since several recent decades, optical fiber sensors have been utilized in composite material field popularly for their predominating advantages such as small size, low cost, and capability of avoiding electromagnetic influence. In 1988, Afromowitz proposed the polymer fiber embedded into composite materials to monitor the refractive index changes in the composite materials during curing process [Afromowitz, 1988]. And one year later the authors presented Fiber Optic Fresnel Reflection Technique for supervising the curing process [Afromowitz & Lam, 1990]. In late 1980s, Fiber Bragg Grating (FBG) sensor, one kind of optical fiber sensors, has attracted considerable attentions to the applications in aerospace, structural, medical and chemical spheres. FBG sensors are small and compatible with common polymeric materials, and thereby being easily embedded close to the internal sensing site in a composite structure without introducing significant defects. In 1990, Dunphy et al. employed the Fiber Bragg Grating embedded into composite materials to monitor the vitrification during curing process [Dunphy et al., 1990]. Similarly, FBG sensors were also applied to measure strain and residual stress after curing [DewynterMarty et al., 1998 & Okabe et al., 2002a]. On the other hand, Kuang and collegues improved detecting effect of the sensors by embedding FBG into composite materials in different layers [Kuang et al., 2001b]. Alternatively, in 2002, Okabe et al. utilized small-diameter FBG to study residual stress with micro damage of inner structure of the composite [Okabe et al., 2002b]. Furthermore, FBG has been also used to monitor the epoxy curing, and found the glass transition temperature with intensity changes [Giordano et al., 2004 & Wang et al., 2007].