An Analytical Solution for Transient Heat and Moisture Diffusion in a Double-Layer Plate

In most materials, there exists a coupling effect between heat and moisture during their transient diffusion. In particular, the coupling effect gives a significant change in the distributions of temperature and moisture concentration in some porous materials and resin composites (Chang, et al., 1991). Moreover, it is known that the absorption of moisture by hygroscopic materials under high-temperature environments causes considerable hygrothermal stresses, and meanwhile their mechanical stiffness and strength are degraded a great deal (Komai, et al., 1991). Therefore, it is important to predict accurately the coupled heat and moisture diffusion behaviour within the materials in assessing the life of moistureconditioning building materials and resin-based structural materials such as CFRP and GFRP in hygrothermal environments. With regard to the transient heat and moisture diffusion problems, some researchers conducted theoretical analyses using analytical (mathematical) or numerical techniques. For example, Sih et al. presented analytical or numerical solutions for the coupled heat and moisture diffusion and resulting hygrothermal stress problems (Hartranft and Sih, 1980 a) (Hartranft and Sih, 1980 b, Sih, 1983, Sih, et al., 1980) (Sih and Ogawa, 1982) (Hartranft and Sih, 1981) (Sih, 1983, Sih, et al., 1981). Chang et al. used a decoupling technique to obtain analytical solutions for the heat and moisture diffusion occurring in a hollow cylinder (Chang, et al., 1991) and a solid cylinder (Chang, 1994) subjected to hygrothermal loadings. Subsequently, using the same technique, Sugano et al. (Sugano and Chuuman, 1993 a, Sugano and Chuuman, 1993 b) obtained analytical solutions for a hollow cylinder subjected to nonaxisymmetric hygrothermal loadings. All the above-mentioned papers, however, focus on/ target a single material body. Studies that address the coupled heat and moisture diffusion problem for composite regions (e.g., layered bodies) are limited. Chen et al. (Chen, et al., 1992) analysed the coupled diffusion problem in a double-layered cylinder using the FEM, which leads to timeconsuming computation. In order to improve this disadvantage, Chang et al. (Chang and Weng, 1997) later proposed an analytical technique including Hankel and Laplace transforms, and significantly reduced the computational time compared to the FEM analysis. However, the exact continuity of moisture flux was not fulfilled at the layer interface although the coupling terms were included in the governing equations. In this chapter, under the exact continuity condition the one-dimensional transient coupled heat and moisture diffusion problem is analytically solved for a double-layer plate subjected