Interpretation of Results—When a material shows a
knee in the S-N or stress-cycle diagram such that the curve
gives clear indications that it becomes asymptotic to a horizontal
(constant-stress) line, it is sufficient to carry the number
of cycles far enough beyond the knee to indicate with a good
degree of accuracy that the curve becomes asymptotic to a
constant-stress line. If the curve does not become asymptotic to
a constant-stress line, continue the test until the number of
cycles reached is greater than the number of cycles that the
material will be expected to withstand in its life. Report the
value of the alternating stress amplitude corresponding to this
number of cycles as the“ fatigue” strength at the maximum
number of cycles for which the stress-cycle diagram has been
well defined by the tests.