The response of aerospace composites to temperature and humidity

Pt ymer Cmnposites in the Aerospace Industry. lit //dx.d .orgA0.1016ffi978-0-85709-523-7.00012-8 

Most structural composites are highly anisotropic, consisting of unidirectional materials where the mechanical properties in the longitudinal (or 0°) direction are fibre dominated, while in the transverse (or 90°) direction they are matrix dominated. 

Therefore, moisture absorption has a larger effect on the transverse properties of a typical composite system. Despite this, the strength of a 0° composite is also affected by moisture ingress since the reloading of a broken fibre occurs through shear stress transfer Ifom the interphasal matrix. To achieve isotropy, unidirectional plies are stacked at a set of angles such as 0°, 45° and 90° to form a laminate. In this situation, moisture ingress will modify the residual stress state in the individual laminae. 

Whereas the thermodynamic properties of the resin and environment will determine the maximum extent of moisture absorption, diffusion is a kinetic property which is temperature dependent. Normally, the latter is considered to be Fickian in nature. For highly polar polymers, other diffusion models need to be considered, but for most structural composite materials, the Fickian laws are applicable. 

The distribution of water throughout the composite will arise from an averaging of a series of moisture profiles over time. The solution to Fick s second law, which defines the diffusion constant (D) for moisture uptake, is given by Eqn (12.1)  

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