156
Molesworth’s pocket-book
Varying Stress in Ikon and Steel.
(T. W. Fowler.)
By Wohler’s law rupture may be caused not
only by a steady stress exceeding the breaking
strength of the material, but also by repea ted
applications of stresses, none of which are equal
to this stress. The greater the variation of
stress, the less is the stress necessary to produce
rupture. Call the stresses which would have
to be repeated an infinite number of times to
produce fracture; when the stress is steady,
i; when entirely removed and reapplied, so as
to leave the bar unstrained, w; when changed
from a tension to an equal compression, s; when
a minimum stress is left in the bar, a. Then
Hannhardt’s formulae, which give results agreeing
with Wohler’s and .Spaugenberg’s experiments,
min. stress /m\ . ...
are, when----------- I - ) is a positive expres-
max. stress \M/
sion, a = u + (t — u) ~ • When the expression
M
/n _ . z x m
— is negative, the formula is a — u + (u — s) — .
In using this, it is important to remember that
the latter part is a negative expression. The
values of these constants in tons per square inch,
deduced from Wohler’s experiments, are:—
Wrought iron i = 20’84 w = 13’94 s = 7'43
Krupp’s steel t — 46’6 u = 22'28 s = 13'02
These values are given in Weyrauch’s work
on ‘ The Structure of Iron and Steel ’ reduced