The stress-life method works well in predicting fatigue life when the stress level in
the structure falls mostly in the elastic range.
Under such cyclical loading conditions, the structure typically can withstand a
large number of loading cycles; this is known as high-cycle fatigue.
When the cyclical strains extend into plastic strain range, the fatigue endurance of
the structure typically decreases significantly; this is characterized as low-cycle
fatigue, more details about it can be found in the next section.
The generally accepted transition point between high-cycle and low-cycle fatigue is
around 10,000 loading cycles.
SN Curve
The SN curve, first developed by Wöhler, defines a relationship between stress and
number of cycles to failure.
Typically, the SN curve (and other fatigue properties) of a material is obtained
from experiment through fully reversed rotating bending tests. Due to the large
amount of scatter that usually accompanies test results, statistical
characterization of the data should also be provided (certainty of survival is used
to modify the SN curve according to the standard error of the curve and a higher
reliability level requires a larger certainty of survival). Figure 1. SN data from testing
When SN testing data is presented in a log-log plot of alternating nominal stress
amplitude Sa versus cycles to failure N, the relationship between S and N
can be described by straight line segments. Normally, a one or two segment
idealization is used.
Note: Some SN curves use stress range
instead of amplitude. In the material database, you can set the material
property Stress Definition as Amplitude or Range. Amplitude is used by
default.
Figure 2. One segment SN curves in log-log scale
(1)(1)
for segment 1 (1)
Where:
S is the nominal stress amplitude
Nf are the fatigue cycles to failure
b1 is the first fatigue strength exponent
S1 is the fatigue strength coefficient
Nc1 is cyclic limit of endurance
The SN approach is based on elastic cyclic loading, inferring that the SN curve
should be confined, on the life axis, to numbers greater than 1000 cycles. This
ensures that no significant plasticity is occurring. This is commonly referred to as
high-cycle fatigue.
