Environmental Engineering Reference
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rate of cracks along laminate and adhesive interfaces. If an interface is not prop-
erly designed for damage tolerance, even small initial cracks could potentially
grow to a critical size over the 20-year design life of a wind turbine blade. For
materials or interfaces containing cracks, the cyclic crack growth rate, d
a
/d
N
, can
be measured as a function of the applied energy release rate. Figure 13 shows a
Sa
m
ple: Optimat, UD2,
R=10, Fatigue test
R =
0
.10
R = 0.10 run ou
t
Pow
e
r (S
ind
e
p
e
n
d
e
n
t
)
Low
e
r 95
%
c
o
nf
i
.
95%
/
95% -lin
e
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
1000
10000
100000
1000000
10000000
Number of cycles to faliure
Figure 12: S-N curve for compression-compression fatigue of a glass fi ber/epoxy
composite [ 40 ].
Paris-Erdogan
Regime
log(
Δ
)
th
Ic
Figure 13: Schematics of a typical relationship between cyclic crack growth rate
(d
a
/d
N
) and maximum applied energy release rate,
G
max
for cyclic
crack growth. A threshold value,
G
t
h
, often exists, below which no
crack growth is assumed to occur. As
G
max
approaches the fracture
energy,
G
Ic
, the crack growth rate increases asymptotically. In between
these regimes, the crack growth rate can be described in terms of the
Paris-Erdogan relationship.
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