Page 30 - Mechanical Engineer's Data Handbook

STRENGTHS OF MATERIALS 19

Tensile strength, Q, Fatigue stress, om
Material (Nmm-’) (Nmm-’), (5 x lo7 cycles) .Jam
~ ~~~
N3 non-heat-treated 110 48 0.44
130 55 0.42
175 70 0.40
H9 heat treated 155 80 0.52
240 85 0.35

Plastics Efect of surface finish on endurance limit

Plastics are very subject to fatigue failure, but the data The values of endurance limits and fatigue stress given
on fatigue stress are complex. A working value varies are based on tests on highly polished small specimens.
between 0.18 and 0.43 times the tensile strength. For other types of surface the endurance limit must be
Curves are given for some plastics. multiplied by a suitable factor which varies with tensile
strength. Values are given for a tensile strength of
1400 N mm - ’.

Surface Surface factor

Polished 1 .o
Ground 0.90
Machined, cold drawn 0.65
Hot rolled 0.37
As-forged 0.25

There are also factors which depend upon size, tem-
perature, etc.

1.3.3 Causes of fatigue failure in welds
Under fatigue loading, discontinuities lead to stress
concentration and possible failure. Great care must be
taken in welds subject to fluctuating loads to prevent
unnecessary stress concentration. Some examples are
given below of bad cases.

penetration