Environmental Engineering Reference
In-Depth Information
500
R
= - 0.5
R
= - 1.0
R
= 0.1
R
= - 2.0
400
10
0
Cycles
300
10
2
R
= 0.5
200
10
4
10
6
100
10
8
0
-500
-400-300
-200-100
0
100
200
300
400
500
600
700
Mean Stress,
S
m
(MPa)
Figure 12-18. Sample Goodman diagram for fiberglass composite material based on
fatigue tests at 5 R-ratios
[Southerland and Mandell 2005].
A fundamental concept of the fracture-mechanics method is that all structures contain
small flaws when they are first placed in service. The sizes of these
pre-existing
or
initial
flaws
are related to the inspection methods used and the specified
acceptance criteria
. Flaws
in critical areas can develop into cracks when fatigue loads are applied, and these cracks con-
tinue to grow in length or
propagate
during the service life of the structure. Crack length is
the measure of fatigue damage, and the
crack propagation rate
(expressed in length units per
cycle) is a highly non-linear function of crack length and elapsed time. At the start of service,
crack lengths and crack propagation rates are very low. As the crack grows the propagation
rate accelerates. Eventually a crack length is reached that is unstable, the propagation rate
becomes infinite, and fracture occurs.
Application of the fracture-mechanics method to the fatigue analysis of wind turbine
structures can be divided into the following steps:
1. Define a
load spectrum
for each major component, in terms of interface forces and
moments and their probability of exceedance.
2. Estimate section sizes (
e.g.
, skin thicknesses) and calculate a
stress spectrum
for se-
lected points around each critical section in the component, such as welds and joint
flanges.
3.
Normalize stresses
in each spectrum by its largest stress,
S
maxmax
, which serves as a
scale factor to be defined.
4. Specify a
design initial flaw size
for the selected material, fabrication method, and
inspection capability, and a
design failure crack size
.
5. Derive and verify a
crack-propagation model
for the selected material, expressed in
terms of the an incremental growth rate
vs
. stress parameters away from the crack and
the instantaneous crack length or depth.
6. Estimate
S
maxmax
and calculate
crack size vs time
by forward integration of the propa-
gation model over the design lifetime, at each selected point around each critical
Search WWH ::
Custom Search