Environmental Engineering Reference
In-Depth Information
Figure 12-22 illustrates an acceptable fatigue design of a cross-sectional weld in a steel
rotor blade, in this case for an inboard blade section of the Mod-5B HAWT. The margin of
safety is zero wherever the design stress line (multiplied by the applicable factors of safety)
intersects the allowable stress lines. In this design most of the upwind-surface stresses are
within the “B” weld allowable. However, in a small area near the leading edge (where the
most critical combination of in-plane and out-of-plane loads often occurs) it was found to be
more economical to locally increase the weld quality and with it the fatigue allowable stress
than to increase the plate thickness. This so-called “B+” weld category has an initial crack
size smaller than that of the “B” welds, which requires additional inspection. This trade-off
between component size and the cost and time of additional inspection illustrates how the
specification of a design allowable stress is a system-level decision.
Figure 12-22. Example of fatigue design stresses in a HAWT blade cross-sectional weld
compared with fatigue allowable stresses.
[based on Boeing 1988]
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