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II.1.12 Moisture diffusion in laminates

The moisture conservation equation in a solid material is written as follows:

M t = -qM,

where M is the moisture content (for example in [kg/m3]) and vector qM is the massic flux of moisture (in [kg/m2s]). When several materials are present in a structure, as often in laminates, it is more practical to work with moisture percentage H (in [%w]). The moisture percentage is related to moisture content by the following expression:

M = c H,

where c is the moisture coefficient in [kg/(m3%w)].

Generally, the moisture flux is related to the gradient of moisture by Fick’s law, and the moisture diffusion equation can be written:

H t = λH H.

Here λH is the tensor of moisture conductivity coefficients. This expression is very similar to the equation of thermal diffusion:

As one works with moisture percentages, nothing equivalent to the laminate thermal capacity ρCph lam is defined for moisture.