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4 Materials and Methods
4.1 Materials for Fatigue Life Assessment of Multivariable
Amplitude Loadings
The investigated materials were two multidirectional laminates of E-glass/polyester,
typical materials used in wind turbine blade applications. The
first material had the
±
corresponding lay-up of [90/0/
45/0]
S
and is called as DD16 (Mandell and Sam-
borsky
2010
). The second material had the corresponding lay-up of [0/(
±
45)
2
/0]
T
and were cut by diamond saw wheel at on-axis (0
°
) and off-axis (15
°
,30
°
,45
°
,60
°
,
75
) orientations (Vassilopoulos and Philippidis
2002
). The corresponding
database containing fatigue data of various R values (R = 0.1, 0.5, 0.7, 0.8, 0.9,
−
°
and 90
°
2 and 10) makes it suitable for the study purpose.
From the fatigue data, stress ratio R, maximum stress (S
max
) and minimum stress
(S
min
) values were used as input set and the output was the corresponding fatigue
cycles (log N) for the input set. For each particular R value, mean fatigue life values
were used. Also, all the data were normalized so that they fall in the range of
0.5,
−
1,
−
−
1to
1 using the following formula:
2
x
x
max
x
min
x
max
x
min
x
n
¼
ð
9
Þ
where: x
n
is the normalized value of the input variables (R,
˃
min)
or the
output variable (log N), x is the un-normalized data and x
min
, x
max
are the minimum
and maximum values of the variables, respectively.
Table
1
summarizes the materials examined together with the orientation and the
training and testing sets employed. Note that stress ratio values R were arranged in
CCW direction according to the CLD, moving across from tensile-tensile sector to
compressive-compressive sector. In addition, fatigue data of R = 0.1 and 10 were
chosen as training set because the best relative positions of the R values in the CLD,
as shown in Fig.
1
. Moreover, the fatigue testing on the R values are also commonly
conducted. Note also that number of stress levels in each R value employed was 5.
˃
max
and
Table 1 Materials examined with the orientations and the training and testing sets employed for
fatigue life assessment of multivariable amplitude loadings
Material
Angle
orientation
Fatigue data as
training set
Fatigue data as
testing set
E-glass/polyester [90/0/
45/0]
S
(Mandell and Samborsky
2010
)
±
On-axis
R = 0.1 and 10
R = 0.9, 0.8, 0.7,
0.5,
−
0.5,
−
1 and
−
2
E-glass/polyester [0/(
45)
2
/0]
T
(Vassilopoulos and Philippidis
2002
)
±
On-axis
R = 0.1 and 10
R = 0.5 and
−
1
45
°
R = 0.1 and 10
R = 0.5 and
−
1
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