Page 62 - Plastics Engineering

Mechanical Behaviour of Plastics 45
Hence,
W
stress, (J = -
A
6
strain, E = -
Lo
0 WLo
modulus, E = - = -
E A6
In flexure (bending) situations, these equations do not apply. For the three-
point loading shown in Fig. 2.3, the relevant equations are
MY
stress, 0 = -
I
where M = bending moment at loading point (= WL/4)
y = half depth of beam (= d/2)
I = second moment of area (bd3/12)

3WL
hence stress, rJ=-
2bd2
Also,
w L3
deflection 6 = - (see Benham et al.)
48EI
+) 6Ed =E(;)

66d
hence strain, E =
LL
WL3
(J
modulus, E = - = - (2.7)
E 4bd36
Note that these stress, strain and modulus equations are given for illustration
purposes. They apply to three-point bending as shown in Fig. 2.3. Other types
of bending can occur (e.g. four-point bending, cantilever, etc.) and different
equations will apply. Some of these are illustrated in the Worked Examples
later in this chapter and the reader is referred to Benham et al. for a greater
variety of bending equations.

2.4 Long-Term Testing of Plastics
Since the tensile test has disadvantages when used for plastics, creep tests
have evolved as the best method of measuring the deformation behaviour of