92168.pdf

258 Rheology

For Hookean elastic solids the stress and strain are in phase, whereas

for purely viscous liquids the strain lags 90° behind the applied stress.

Time-dependent deformation and structural characteristics

The deformation of a material when subjected to a constant stress is,

as discussed, usually time-dependent. At times of c. 10~^6 s and less all

materials, including liquids, have shear compliances (i.e shear/shear

stress) of c. 10~~^11 to 10~~^9 m^2 N~*. This is because there is only

sufficient time available for an alteration of interatomic distances and

bending of bond angles to take place, and the response of all

materials is of the same order of magnitude in this respect. The time

required for the various structural units of a material to move into

new positions relative to one another depends on the size and shape

of the units and the strength of the bonds between them.

The molecules of a liquid start to move relative to one another and

the shear compliance increases rapidly after the shearing force has

been applied for only c. 10~^6 s. On the other hand, hard solids, such

as diamond, sodium chloride crystals and materials at a low enough

temperature to be in the glassy state, show only the above rapid

elastic deformation, even after the shearing stress has been applied

for a considerable time.

10

810-c
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Liquid

Crystalline or

glass-like solid

10-6 10*

Time/s

Figure 9.7 The time-dependence of deformations under constant stress