2. Constitutive laws and basic equations for simple shear
In the constitutive law (1), the elastic second Piola–Kirchhoff stress Pe under the integral is assumed hyperelastic, i.e. the
instantaneous response can be derived from an energy potential W (the strain energy function). The deformation gradient
tensor F is defined by
FðsÞ ¼
I; s 2 ð1; 0Þ;
@x
@XðsÞ; s 2 ½0; t;
8<
:
ð2Þ
with xðsÞ denoting the position of a generic particle P at time s 2 ½0; t, and X its position at the initial reference time. Note
that the start time of the motion, and any imposed tractions, will be taken as t ¼ 0. The quantity J ¼ det F, expressing the
local volume change, is a constant J ¼ 1 when the deformation is isochoric. Let us assume that the material is isotropic
and therefore from the deformation gradient tensor F we obtain the right Cauchy-Green tensor C ¼ FTF, and its principal
invariants
I1 ¼ trC; I2 ¼
1
2 ðtrCÞ2 trC2 h i
¼ ðdet CÞtrðC1Þ; I3 ¼ det C ¼ J2: ð3Þ
Given isotropy, the viscoelastic Cauchy stress is (referring to De Pascalis et al., 2014)
TðtÞ ¼ J1FðtÞ Pe
DðtÞ þ
Z t
0
D0ðt sÞPe
DðsÞ ds
FT ðtÞ þ J1FðtÞ Pe
HðtÞ þ
Z t
0
H0ðt sÞPe
HðsÞ ds
FT ðtÞ; ð4Þ