Viscoelasticity and Damping

We have now reviewed three types of behaviour the enthalpic elasticity of rigid polymers at temperatures below Tg, the entropic elasticity of polymers at temperatures above Tg- -40°C and the viscous response of uncrosslinked polymers in the molten state. When examining the various states of polymer materials, we emphasised analogies between temperature and time behaviour. This naturally leads us to consider the viscoelastic character exhibited by polymers under certain conditions, particularly in the region of the glass transition. 

A material has viscoelastic behaviour when its response to excitation or load varies with time. For example, under constant stress, its strain e = t) increases with time. This is called creep. Under constant strain, the stress a = a i) decreases with time. This is called stress relaxation. 

The viscoelastic response also appears under dynamic excitation. Consider a sinusoidal excitation of frequency u  

Under this excitation, elastic behaviour responds in phase a t) = E u))eosmut,  

Another way of describing the out-of-phase response is to decompose it into a part in phase and a part in quadrature  

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Niederstadt, G.,et al. Viscoelastic and Damping Behavior of Composites Made from Carbon Fiber Reinforced Piepregs. Kunststoffe, pp. 27-29, Jan. 1990. 

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