Some observations on the evolution of transversal hygroscopic stresses in laminated composites plates: effect of anisotropy

The transient and non-uniform moisture concentration distributions in laminated composite plates give rise to transient and non-uniform stress fields [Composite Structures 30 (1995) 201; Communication au 12 journées nationales sur les composites, JNC12 (in French), 2000, 969; Composite Structures 55 (4) (2002) 393]. It was shown [Composite Structures 30 (1995) 201; Communication au 12 journées nationales sur les composites, JNC12 (in French), 2000, 969; Composite Structures 55 (4) (2002) 393] that the heterogeneity and the anisotropy of such plate, have an influence on the distribution of transient hygroscopic stresses through the thickness of composite plates. The purpose of this paper is to investigate the effect of anisotropy on the transverse residual stresses and strains in polymer composite systems under long term exposure to hygrothermal environments. The anisotropy is evaluated using the degree of anisotropy introduced from polar representation method of tensors [Communication au 12 journées nationales sur les composites, JNC12 (in French), 2000, p. 969; Composite Structures 55 (4) (2002) 393; Mechanical Behaviour, Design and Application, 1990, p. 29; Computer Aided Design in Composite Material Technology, 1988, p. 243; Proceeding of the seventh International Conference on Composite Materials, 1989, p. 358; Proceedings of the Fifth French Conference on Composite Materials, 1986, p. 267; Eighth International Conference on Composite Materials (ICCM-8), Honolulu (USA), 1991, p. 1; 12th International Conference on Composite Materials (ICCM-12), Paris, 1999; 12th International Conference on Composite Materials (ICCM-12), Paris, 1999; 12th International Conference on Composite Materials (ICCM-12), Paris, 1999; Découplage et quasi-homogénéité pour les stratifiés renforcés par tissus équilibrés”, JNC12, (in French), 2000, p. 265]. The principal objective of this study is to contribute to the understanding and predicting the behaviour of transversal hygroscopic stresses and strains in the studied composites as a function of the degree of anisotropy. The analysis has indicated that: (i) the reduction of degree of anisotropy favours the transversals hygroscopic stresses to be in tensile state, contrary to the thermal stresses where they become in compressive state; (ii) the values of the transverse residual stresses in laminates having the same degree of anisotropy are nearly identical; (iii) from certain value of in-plane degree of anisotropy which is about 38.5% for the used material in this study, the sensibility to the anisotropy is more felt. For inferior values, the transversal residual stresses are nearly identical to those of quasi-isotropic laminates.