Fluidity and solidity

The simple definitions given of solids and liquids are not always appropriate when the latter are subjected to applied forces. Pitch, for example, behaves like a solid, in that it shows a definite and measureable rigidity, when subjected to applied forces for short times, but flows like a liquid when it is subjected to applied forces for very long times. 

If hysteresis effects are ignored, the shear strain rate 7 of a material that shows flow behaviour may be written  

Using the Maxwellian idea of fugitive elasticity, let a shear stress 7 produce both a shear strain 7 and a Newtonian shear strain rate 7j in a given material. When a shear stress Tq is applied abruptly to the material the shear strain instantly becomes 7g = Tq/G, where G is the rigidity modulus. However, since the material is capable of Newtonian viscous flow, the stress immediately starts to relax, with the elastic strain being replaced by a viscous strain that is given by 7 = If h is assumed that the mechanical device producing the initial deformation does not move thereafter, Tg applied strain rate is 

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