Moisture in composites: the slow, hot-wet knockdown
Leave a composite in humid air and it slowly drinks water — swelling, softening, and dropping its hot-wet strength. How moisture diffuses in, why it matters most with heat, and the knockdown engineers design around.
Composites drink water
Epoxy and most polymer matrices are slightly hydrophilic. Left in humid air or immersed, a composite slowly absorbs moisture — typically a percent or two by weight over weeks to months. It is slow and invisible, but it changes the part in three ways: the water plasticises the matrix (softens it), swells it (setting up internal stress), and lowers the temperature at which it softens.
None of this touches the glass or carbon fibres themselves, which do not absorb water. It is the matrix that drinks — and so it is the matrix-dominated properties that suffer.
Fickian diffusion
For most composites the uptake follows Fick's law: water is at its highest concentration at the surface and diffuses inward down the gradient. The mass-gain curve has a characteristic shape — rising steeply at first, in proportion to the square root of time, then levelling off as the part approaches saturation.
Two numbers describe it: the diffusion coefficient (how fast the water moves in) and the saturation content (how much it holds at equilibrium). The moisture-uptake simulator integrates exactly this diffusion, so you can watch the square-root-of-time rise and the plateau.
The hot-wet knockdown
Moisture matters most in combination with heat. Because absorbed water lowers the matrix glass-transition temperature, a wet composite softens at a lower temperature than a dry one. The worst case for any matrix-dominated property — compression, shear, hot-service stiffness — is therefore 'hot-wet': fully saturated and at the maximum service temperature at once.
Composite design allowables carry an explicit hot-wet knockdown for exactly this reason, and it is a major part of why aerospace qualification conditions its test specimens to saturation before pulling them. The number you design to is the wet one.
What it hurts, and what it doesn't
Moisture mainly attacks matrix-dominated properties: interlaminar shear, compression, transverse strength, and the softening temperature. Fibre-dominated properties — tensile strength along the fibres — are far less affected, because the load rides on the dry fibres.
So a wing skin loaded in tension along its fibres is relatively safe, while a bonded joint, or a part loaded in compression, is where hot-wet bites hardest. Knowing which of your loads are fibre-dominated and which are matrix-dominated tells you how much to worry.
Slow in, slow out
Because diffusion is slow, moisture effects lag the weather by months: a part equilibrates with an average humidity, not with today's. The effect is also largely reversible — dry the part and most of the water leaves and the properties recover — though repeated wet–dry cycling can cause micro-cracking that does not.
It is the quiet, long-timescale environmental factor sitting behind a 'hot-wet allowable', and a clean example of why provenance matters: a strength number is only meaningful once you know the moisture and temperature at which it was measured.
1. How does moisture typically diffuse into a composite?
2. Why is the 'hot-wet' condition the worst case?
3. Which properties does moisture mainly attack?