Chemistry Reference
In-Depth Information
Modulus
G
*
is constant
G
*
=
f
(
γ
)
Linear region (LVR)
Non-linear region
Stress or strain
Fig. 2.4
Linear and non-linear stress-strain behaviour.
shear modulus (
G
∗
), shear storage modulus (
G
), shear loss modulus
(
G
), complex shear compliance (
J
∗
), shear storage compliance (
J
)
and shear loss compliance (
J
). The ones most commonly used are the
moduli terms; storage modulus is the elastic component, which indicates
the amount of energy stored, and loss modulus is the viscous component,
which indicates the amount of energy dissi
pated throu
gh a generated
heat. The complex modulus is defined as
√
G
2
G
2
and defines the
overall stiffness of the sample. Another popular material function used
to describe the viscoelastic behaviour is the tangent of the phase shift
or phase angle (called tan delta), which is a function of frequency:
+
G
G
tan
δ
=
(2.9)
) varies between 0 and 90
◦
; it is zero in case of a
purely elastic material and 90
◦
for a purely viscous fluid.
Critical to these studies is the identification of the linear viscoelastic
region (LVR), within which material properties do not depend on the
magnitude of the stress, the magnitude of the deforming strain or the
rate of application of the strain. Within the LVR, depicted in Fig. 2.4,
an applied stress will produce a proportional strain response; therefore,
for example, doubling the stress will double the strain response. The
linear range of testing is determined from experimental data. Testing
can easily enter the non-linear range if excessive strain (usually greater
than 1%) or high deformation rates are applied to the sample.
The phase angle (
δ
