The S-N curve, first developed by Wöhler, defines a relationship between stress and
number of cycles to failure.
S-N Curve
Typically, the S-N 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, you should also provide
statistical characterization of the data (certainty of survival is used to modify
the S-N curve according to the standard error of the curve; a higher reliability
level requires a larger certainty of survival).
Figure 1. S-N data from testing
When S-N testing data is presented in a log-log plot of alternating nominal stress
amplitude Sa or range SR 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.
Figure 2. One segment S-N curves in log-log scale
(1)(1)
S=S1(Nf)b1
for segment 1 (1)
Where:
S is the nominal stress range
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 S-N approach is based on elastic cyclic loading, inferring that the S-N 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.
In Simsolid, the S-N is estimated based on the UTS of the materials as shown below:
