Viscoelasticity

Slow, time-dependent deformation of a material under sustained load. Polymers and adhesives creep far more readily than metals, particularly at elevated temperature or as the glass transition temperature is approached, so long-term loads must be assessed separately from short-term strength.

The gradual reduction in stress within a material held at constant strain, the counterpart of creep. It is important for gaskets, seals and bolted joints in polymers, where clamping loads can decay over time.

A thermal analysis technique that applies a small oscillating deformation to a specimen while temperature or frequency is varied, measuring storage modulus, loss modulus and tan delta. It is a sensitive method for determining the glass transition temperature and degree of cure of resins, adhesives and composites, and is also used on bituminous and polymeric roofing materials.

The temperature region over which an amorphous or partly amorphous polymer changes from a rigid, glassy state to a softer, rubbery one. It sets a practical upper service temperature for resins and adhesives and can be reduced by absorbed moisture or incomplete cure.