78 2. RELIABILITY OF A COMPONENT UNDER CYCLIC LOAD
11.6
11.5
11.4
11.3
11.2
11.1
11
10.9
10.8
10.7
10.6
3.5 3.55 3.6 3.65 3.7 3.75 3.8
y = -3.8812x + 25.272
R
2
= 0.99139
ln (N)
ln (sa)
Figure 2.17: e histogram of K
0
with a sampling size 195.
Table 2.35: K
0
with three distribution parameters
Material: 6061-T6
10-Gauge Sheet
Sample Size:
195
Test Conditions: Cyclic Axial Stress, Milled Machined
Sheet-type Flat Specimen
The Slope of the
Traditional S-N Curve: m
The Log-normally Distributed K
0
The Log Mean: μ
lnK
0
The Log Standard Deviation: σ
lnK
0
3.8812 25.3014 0.245451
2.9.3 THE COMPONENT FATIGUE STRENGTH INDEX K
As we have discussed in Section 2.4, we will use the Marin modification factors to consider
the differences between material fatigue specimen and component. After the differences are
considered, we can obtain the component fatigue strength index K.
e component fatigue strength index K is a mechanical property of a component and is ob-
tained through modifying the material fatigue strength index K
0
by the Martin modification
factors per Equation (2.79), and can be used to indirectly represent the component fatigue resis-
tance to the fatigue damage or the component fatigue strength. e component fatigue strength
index K can be determined by the following equation:
K D N
i
.
k
a
k
b
k
c
ai
/
m
D
.
k
a
k
b
k
c
/
m
N
i
.
ai
/
m
D
.
k
a
k
b
k
c
/
m
K
0
; (2.79)
where k
a
is the surface finish modification factor. k
b
is the size modification factor. k
c
is the
loading modification factor. ese Martin modification factors and its calculations have been
discussed in Section 2.3. m is the slope of the traditional S-N curve and can be determined
by Equation (2.74). K
0
is the material fatigue strength index and can be determined by Equa-
tion (2.73).