On the Transient Hygroscopic Stresses in Polymer Matrix Laminated Composites Plates with Cyclic and Unsymmetric Environmental Conditions

The use of aircraft structural parts made of polymer matrix composites subjected to severe environment conditions calls for better knowledge of their long – term behaviour, with an emphasis on their ability to withstand important cyclic variations of moisture and temperature. The influence of temperature and moisture on such structures is receiving special attention, because it induces transient residual stresses within the plies. Such stresses must be taken into account in the design of composite materials, particularly aerospace structures, e.g. aircraft. In the present paper the transient hygroscopic stresses induced by cyclic and unsymmetric environmental loadings which simulate an aeronautical application are investigated. It is shown that the heterogeneity and anisotropy of such plates generally result in transient stress distributions which are very different from the equilibrium stress distribution. Some stacking sequences exhibit important stresses within the plies. These stresses have to be taken into account in the design of composite structures submitted to a moist environment.

Boundary Layer Transient Hygroscopic Stresses in Orthotropic Thick Shells Under External Pressure

An exact elasticity solution is obtained for the stresses and displacements in an orthotropic cylindrical shell loaded by an external pressure under imposed constant moisture concentrations on the inner and outer surfaces. The material properties are assumed moisture independent and a displacement approach is used. Since the moisture diffusion process is relatively slow, the hygroscopic stresses are confined for practical time values to a boundary layer region near the surfaces. Illustrative results are presented for graphite-epoxy material regarding the boundary layer hygroscopic effect on the stress field with respect to time and the coupling of mechanical loading (external pressure) and moisture diffusion. For this material, it is shown that this effect is more pronounced for the axial component of stress.