Page 189 - Trenchless Technology Piping Installation and Inspection

154 Cha pte r F o u r

4.7.1 Properties of Viscoelastic Pipe Materials
PVC and PE are thermoplastics, and thus, viscoelastic materials.
Viscoelastic materials exhibit elastic as well as viscous-like charac-
teristics. A material that deforms under stress, but regains its origi-
nal shape and size when the load is removed is classified as elastic.
Viscous materials, on the other hand, after being subjected to a
deforming load, do not recover their original shape and size once
the load is removed. In reality, all materials deviate from the linear
relationship between stress and strain (Hooke’s law) at some point
in various ways.
Defining the direct relationship between stress and strain when
a load is applied to a material is the most common way to evaluate
the strength and stiffness of that material. Graph A in Fig. 4.25 illus-
trate the linear relationship between stress-strain in elastic materials.
In an ideal elastic material, strain returns to zero as soon as the mate-
rial is unloaded, and the linear relationship is not typically time-
dependent. But it should be noted that in all materials, this behavior
is valid only up to a certain stress point, called the yield point, after
which the strain in the material will increase dramatically by creep,
before finally failing.
In the set of curves, B, in Fig. 4.25, it can be seen that the stress-
strain relationship is somewhat different for viscoelastic materials
than it is for elastic materials. Clearly, we no longer see a directly
linear relationship between stress-strain, and the gradients of the
curves depend on the loading time. In other words, for a given stress
level, the longer the loading time, the larger the strain reached.
Creep is defined as continuing deformation (increasing strain)
with time when the material is subjected to constant stress. As a

σ Stress (MPa)
A Elastic material behavior
B Viscoelastic material behavior
Yield point
Loading time

σ 0 Creep

Relaxation

ε 0 ε Strain (%)
FIGURE 4.25 Stress-strain relationship in elastic and viscoelastic materials.
(Janson, 1999.)