Page 161 - Plastics Engineering

144 Mechanical Behaviour of Plastics

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Mean stress (MN/m2)
Fig. 2.76 Relationships between stress amplitude and mean stress

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and
superimposed on this is an alternating axial load of fl00 N. If the fatigue limit
for the material is 13 MN/m2 and the creep rupture strength at the equivalent
time is 40 MN/m2, estimate a suitable diameter for the rod. Thermal effects
may be ignored and a fatigue strength reduction factor of 1.5 with a safety
factor of 2.5 should be used.

Solution The alternating stress, a, is given by
Alternating load 4 x 100
-
a, = -- (MN/m2)
area nd2
Also the mean stress, a,,,, is given by
Steady load 4 x 50
--
a,,, = - (MN/m2)
area nd2