Fatigue Tests

: The Working Of Steel

It has been known for fifty years that a beam or rod would fail

at a relatively low stress if only repeated often enough. It has

been found, however, that each material possesses a limiting stress,

or endurance limit, within which it is safe, no matter how often

the loading occurs. That limiting stress for all steels so far

investigated causes fracture below 10 million reversals. In other

words, a steel which will not
reak before 10,000,000 reversals

can confidently be expected to endure 100,000,000, and doubtless

into the billions.

About the only way to test one piece such a large number of times

is to fashion it into a beam, load it, and then turn the beam in

its supports. Thus the stress in the outer fibers of the bar varies

from a maximum stretch through zero to a maximum compression, and

back again. A simple machine of this sort is shown in Fig. 10,

where B and E are bearings, A the test piece, turned slightly

down in the center, C and D ball bearings supporting a load

W. K is a pulley for driving the machine and N is a counter.