Instron 4400 Series Operator's Manual page 239

Glossary
Fatigue.
Permanent structural change that occurs in a material
subjected to fluctuating stress and strain. However, in
the case of glass, fatigue is determined by long-term
static testing and is analogous to stress rupture in other
materials. In general, fatigue failure can occur with
stress levels below the elastic limit.
Fatigue Life.
Number of cycles of fluctuating stress and strain of a
specified nature that a material will sustain before fail-
ure occurs. Fatigue life is a function of the magnitude of
the fluctuating stress, geometry of the specimen and test
conditions. An S-N diagram is a plot of the fatigue life
at various levels of fluctuating stress.
Fatigue Limit.
Maximum fluctuating stress a material can endure for an
infinite number of cycles. It is usually determined from
an S-N diagram and is equal to the stress corresponding
to the asymptote of the locus of points corresponding to
the fatigue life of a number of fatigue test specimens.
An alternate term is endurance limit.
Fatigue Limit
CYCLES (N)
B-13