Glass transition is the change in state of a polymer or a polymerised composite material under the action of temperature leading to significant variations in its mechanical properties.
Glass transition is characterised by a glass transition temperature (Tg).
Below this temperature the polymer is said to be vitreous (solid state) and has the behaviour of a solid elastic body.
Above this temperature it has the behaviour of a solid plastic (viscoelastic state) following the weakening of intermolecular bonds (Van de Waals forces etc.).
Glass transition is reversible and is seen mainly in thermoplastic polymers all the more so because they are less crystallised and less cross linked. It also occurs in some thermosetting polymers (e.g. in epoxy resins); of which the cross linking density is not too high.
This change in state is always accompanied by a significant variation in physical properties, especially mechanical properties (e.g. a drop in elasticity modulus).
A structure should always be designed with resins that have a glass transition temperature considerably higher than the temperature of use.