Some observations on the evolution of transversal hygroscopic stresses in laminated composites plates: effect of anisotropy A. Tounsi * , E.A. Adda Bedia Laboratoire des Mat eriaux et Hydrologie, Universit e de Sidi Bel Abbes, BP 89 Cit e Ben M’hidi 22000 Sidi Bel Abbes, Algeria Abstract 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  eme journ ees 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  eme journ ees 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  eme journ ees natio- nales 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 Ma- terials, 1986, p. 267; Eighth International Conference on Composite Materials (ICCM-8), Honolulu (USA), 1991, p. 1; 12th In- ternational 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; D ecouplage et quasi- homog en eit e pour les stratifi es renforc es par tissus  equilibr es’’, 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 lami- nates. Ó 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The environmental problem is of first importance for long term behaviour of this composite materials and above all those made of polymer matrix. In references [13–21] this problem has been extensively investigated, especially for aeronautical structures, to calculate the moisture concentration in composite materials exposed to environmental conditions. Another studies, such as presented in references [1–3] have been shown the im- portance of the non-mechanical residual stresses within the laminates. This stresses which are induced by envi- ronment in terms of temperature and moisture can be separated in two stresses: thermal and hygroscopic stresses. This can be generally made because of two circumstances. First, in polymer materials, both phe- nomena moisture sorption and heat diffusion have dif- ferent characteristic times: their ratio, which is, for a given geometry, the inverse of the diffusivities, can be as high as six orders of magnitude [15]. Secondly, for thicknesses of some millimetres, the variations for ex- ternal temperature and moisture are slow compared to * Corresponding author. E-mail address: touabdel@yahoo.fr (A. Tounsi). 0263-8223/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0263-8223(02)00119-8 Composite Structures 59 (2003) 445–454 www.elsevier.com/locate/compstruct