Fatigue Design of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 702

section. Adjust S maxmax at each selected point until the crack size at the end of the
design lifetime equals the design failure crack size around each critical section in the
component.
7. Specify a set of fatigue allowable stresses equal to the adjusted values of S maxmax from
Step 6 and an associated set of fatigue design loads equal to the maximum set of loads
in the spectrum defined in Step 3.

Steps 1, 2, and 3 were discussed earlier and result in spectrum data like that in Table
12-1. The remaining steps will be discussed here, drawing upon methods used for the struc-
tural design of the 2.5-MW Mod-2 and the 3.2-MW Mod-5B HAWTs [Boeing 1979, Boe-
ing 1988]. The major components in these two machines, including the skins and spars of
their 91.4-m and 97.5-m rotors, were fabricated from welded-steel plates, forgings, or rolled
sections. The fracture-mechanics method was used throughout both design efforts, in part
because of the extensive experience at the Boeing Company and its subsidiaries with its ap-
plication and verification.

Design Initial Flaw Size

The initial flaw sizes specified for fatigue design are based on the flaw size acceptance
criteria specified for manufacturing the component in question. The latter specification in-
cludes not only technical considerations (like material properties) but also cost and schedule
requirements. Thus, system-level decisions are required in order to set flaw size acceptance
criteria. After this has been accomplished, design flaws are defined which are (1) located
in the most fatigue-critical areas and in their highest-stress orientation, and (2) significantly
larger than the acceptance criteria. Both of these procedures provide conservatism ( i.e. , ad-
ditional reliability) to the design.
Table 12-7 illustrates the relationship between the sizes of acceptable flaws and design
flaws used for the Mod-5B HAWT. Here and in the equations and calculations to follow,
customary U.S. units will be used for compatibility with the literature of fracture mechanics
technology.

Table 12-7. Inspection and Design Flaw Sizes for ASTM A-6 Steel
in the Mod-5B HAWT [Boeing 1988]
Defect planes perpendicular to the surface and all dimensions in inches

Inspection Method Detection Capability Acceptance Criteria

Visual 0.005 wide x 0.060 long Linear indications: 0.060 long

Dye-Penetrant 0.005 wide x 0.030 long Rounded indications: 0.125 long

Radiological 2% of thickness deep x 0.040 long

Ultrasonic 0.030 deep x 0.090 long

Material Design Initial Flaw Sizes

Surface Flaws Internal Flaws

Base Metal 0.022 deep x 0.110 long 0.044 deep x 0.110 long

“B” Weld 1 0.050 deep x 0.250 long 0.100 deep x 0.250 long

1 Full penetration; constant thickness in base and weld metal; grinding parallel to major stresses