5.2. Dissipation of energy
We now consider the influence of the imposed deformation (or traction) and material response on the dissipation of
energy due to the viscous nature of the material. Define the velocity gradient tensor by
D ¼
1
2
F_ F1 þ ðF_ F1Þ
T
; ð26Þ
where the superposed dot represents the material time derivative. Since the deformation (9) is isochoric then trD 0. The
internal rate of working of the stress per unit current volume is TðtÞ : DðtÞ trðTðtÞDðtÞÞ ¼ T12
_ k. The (
time-averaged) dissipated
energy over a period t for the deformation imposed in (25) is therefore
Ed ¼
1
t
Z t
0
T12ðtÞ_kðtÞ dt: ð27Þ
We can obtain a slightly modified version of the dissipated energy, suitable for determining the energy dissipated in oscillatory
deformations over a period t. Taking the start of the period as t0, the time averaged dissipated energy is
Ed ¼
1
t
Z t0þt
t0
T12ðtÞ_kðtÞ dt: ð28Þ
Clearly, if t0 is chosen sufficiently large such that the initial transients have died out, then the deformation will have reached
a steady state and Ed will be independent of t0.
5.2.1. Single strain cycle
First, we consider the simple shear deformation as defined in (25). The important non-dimensional parameters are A0 and
t=sd which are respectively the rate of deformation and total time-scale of deformation, relative to the deviatoric relaxation
time-scale. Note that we must be careful as to the magnitude chosen for A0 since we have assumed from the outset that the
deformation can be described without inclusion of inertial terms in the equation of motion. Fig. 2 offers the nondimensionalised
energy dissipation as a function of A0, for three different values of t=sd. Unsurprisingly, the larger the deformation, the
larger the normalised dissipation. Note, as can be seen, the dissipation appears insensitive to the chosen material, except for
large A0 and t=sd ¼ 5, when the curve of the compressible material with s ¼ 1 deviates from the other curves.
5.2.2. Periodic cycling superposed on a large deformation
The second case is a linear shear deformation, growing in time, with a superposed oscillatory shear, i.e.