Environmental Engineering Reference
In-Depth Information
In this chapter we describe the types of fatigue loads common to wind turbines, typical
statistical distributions of fatigue loads, empirical equations for estimating fatigue loads dur-
ing preliminary design, and typical fatigue design procedures . The sequence of tasks to be
performed in a typical fatigue life analysis is as follows:
1. Define the system configuration , from requirements and trade-off studies .
2. Define the external environment , including wind loadings, and operating conditions.
3. Determine system dynamic loads , both average and cyclic, at subsystem interfaces.
4. Calculate local average and cyclic stresses at critical locations such as joints.
5. Calculate local fatigue lives using material test data and a selected damage theory.
Structural Design Drivers
It is a common engineering practice to divide structural design requirements into three
general classes, as follows:
-- Limit strength requirements, to resist the infrequent application of the highest
loads expected during the useful lifetime of the structure.
-- Fatigue life requirements, to resist the repeated application of lesser loads for
a speciied lifetime, with or without repairs.
-- Stiffness requirements, to control deflections and place vibration frequencies
within specified ranges to avoid resonances.
All structural and mechanical components are subject to these three requirements, at least to
some degree. The requirement that governs the final selection of the materials, dimensions,
manufacturing processes, and inspection procedures is referred to as the design driver of the
component. The design-driving requirement is often followed closely in importance by a
second type of requirement, in which case the design may be referred to as balanced . When
one type of requirement strongly dominates the design of a major component, changes in the
system design may be in order, as discussed in Chapter 9.
Figure 12-1 illustrates a typical pattern of design drivers for a horizontal-axis wind tur-
bine (HAWT) [Boeing 1988]. In general, fatigue is a design driver for at least one-half of the
primary structure of a HAWT above the foundation. This still leaves large portions of a wind
turbine that are designed by limit loads and by stiffness requirements for proper placement
of natural frequencies.
Fatigue Life Requirements in Wind Turbines
The reason that fatigue requirements often drive the design of the majority of the pri-
mary structure of a wind turbine is that wind turbines must achieve very long operating lives
in order to be cost-effective. Fatigue life is usually expressed by the term cycles to failure ,
which is the number of repetitions of significant loads that can be sustained before cracks
begin (the initiation phase) and grow to an allowable length (the propagation phase). As
shown schematically in Figure 12-2 by a curve of allowable cyclic stress vs . the number of
cycles to failure at that cyclic stress (called an S-N curve), fatigue life is very sensitive to the
amplitude of stress variations. This is emphasized by plotting lifetime on a logarithmic scale.
Design lifetimes of wind turbines contain more load cycles than those of airplanes, bridges,
hydrofoils, and helicopters, so cyclic stresses in a wind turbine must be lower than those al-
lowable in other structures.
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