86
Chapter 2
28
26
24
22
20
18
~16
~ 14
o, 12
10
2
0
O.Ol
Figure 2.21
_
/~~
Polystyrene
_
_ ~lymethyl
methacrylate
5OO
4OO
200
- 100
-
Polyethylene
-
~
1.0
10
100
1000
Frequency (cps)
Dynamic data for three polymers at room temperature.
Here }c~r, the standard deviation needs development. Further, note, for
ductile materials Kf will be applied to the stress condition for or’ r and also
to a’m for brittle materials. In calculating ~’r a simple but important case
is a rotating beam subjected to a constant bending moment. For no stress
concentration the stress Fig. 2.25 is
Mrnc
at, = cr~a = -I(2.86)
I
for a reversal stress from a constant bending moment.