The Wöhler curve, also referred to as the S-N curve, describes the function
σ
a
(
N
f
)
{\displaystyle \sigma _{a}(N_{f})}
or
σ
m
(
N
f
)
{\displaystyle \sigma _{m}(N_{f})}
. It is based on empirical results and often represents the median of the data scatter.
A significant interval of the curve can be approximated by the Basquin relation
σ
a
m
N
=
C
{\displaystyle \sigma _{a}^{m}N=C}
Where
C
{\displaystyle C}
is a constant specific to the test case.
The Basquin relation is often presented in the form
Δ
σ
=
Δ
σ
C
(
N
C
N
)
1
m
{\displaystyle \Delta \sigma =\Delta \sigma _{C}({\frac {N_{C}}{N}})^{\frac {1}{m}}}
where
N
C
=
2
⋅
10
6
{\displaystyle N_{C}=2\cdot 10^{6}}
cycles
Δ
σ
C
=
{\displaystyle \Delta \sigma _{C}=}
is the so called detail category number.
== Detail Category Number ==
The fatigue detail number defines the Basquin relation and specifies a Wöhler curve. The property is often denoted FAT, C, or in mathematical expressions:
Δ
σ
C
{\displaystyle \Delta \sigma _{C}}
.
If there are more than one FAT tied to a geometry-type, they refer to different specifics such as weld quality, loading direction etc. Moreover, the FAT normally corresponds to a fatigue life of
N
=
2
⋅
10
6
{\displaystyle N=2\cdot 10^{6}}
cycles.
== Statistical Properties ==