Whether you are building wind turbines or helicopters, you have to take the strength, the dynamic behaviour and the fatigue properties of your materials and the entire assembly into consideration.
        The interplay of the forces from the external environment, primarily due to the wind and the various components of the wind turbine, result not only in the desired energy production from the turbine, but also in stresses in constituent materials. For the turbine designer, these stresses are of primary concern, because they directly affect the strength of the turbine and how long it will last. The turbine should be structurally sound so that it can withstand the loads it experiences, and the costs to make it structurally sound must be commensurate with the value of energy it produces.
        The categories of loads the wind turbine must withstand include:
- Static loads (not associated with rotation)
- Steady loads (associated with rotation, such as centrifugal force)
- Cyclic loads (due to wind shear, blade weight, yaw motion)
- Impulsive loads (short duration loads, such as blades passing through tower shadow
- Stochastic loads (due to turbulence)
- Transient loads (due to starting and stopping)
- Resonance induced-loads (due to excitations near the natural frequency of the structure)

Fatigue loads (forces)
        Wind turbines are continuously subjected to varying loads. Because of this, fatigue analyses is an important feature in designing wind turbines. In other words, to understand how wind turbine components would be expected to withstand a lifetime of continuously varying loads, fatigue properties need to be examined. Many materials which can withstand a load when applied once, can not survive if that same load is applied and removed a number of times, achieving a 'cycled' pattern. The inability to withstand cyclic applied loads is called fatigue damage. The most simple explanation of the underlying cause of fatigue damage is the growth of tiny cracks in the material until it fails. Wind turbines are subject to fluctuating winds, and hence fluctuating forces. This is particularly the case if they are located in a very turbulent wind climate. These forces can thus be the cause of fatigue damage on the wind turbine.
        Components which are subject to repeated bending, such as rotor blades, may eventually develop into the above mentioned tiny cracks which ultimately may make the component break. A historical example is the huge German Growian machine of 100 m rotor diameter which had to be taken out of service after less than three weeks of operation. Metal fatigue is a well known problem in many industries. Metal is therefore generally not favoured as a material for rotor blades.

Sources of wind turbine fatigue loads
        The actual loads that contribute to fatigue of a wind turbine originate from a variety of sources. These include steady loads from high winds; periodic loads from wind shear, yaw error, yaw motion, and gravity; stochastic loads from turbulence; transient loads from such events as gusts, starts and stops, etc.; and resonance-induced loads from vibration of the structure.

Structural Dynamics
        When designing a wind turbine it is extremely important to calculate in advance how the different components will vibrate, both individually and jointly. It is also important to calculate the forces involved in each bending or stretching of a component.
        This is the subject of structural dynamics, where physicists have developed mathematical computer models that analyze the behaviour of an entire wind turbine. These models are used by wind turbine manufacturers to design their machines safely.

        A 50 m tall wind turbine tower will have a tendency to swing back and forth, say, every three seconds. The frequency with which the tower oscillates back and forth is also known as the eigenfrequency of the tower. The eigenfrequency depends on both the height of the tower, the thickness of its walls, the type of steel and the weight of the nacelle and rotor.
        Each time a rotor blade passes the wind shade of the tower, the rotor will push slightly less against the tower. If the rotor turns with a rotational speed such that a rotor blade passes the tower each time the tower is in one of its extreme positions, then the rotor blade may either dampen or amplify (reinforce) the oscillations of the tower.
        The rotor blades themselves are also flexible and may have a tendency to vibrate, say, once per second. As you can see, it is very important to know the eigenfreqencies of each component in order to design a safe turbine that does not oscillate out of control.

Video frames
- Structural design drivers: overview of important issues
- Fatigue: general explanation
- Fatigue, why so important: graph for fatigue for airplanes
- Fatigue loads in wind turbines: examples with simple graphs
- Fatigue analysis (time domain): procedure, general explanation
- Fatigue analysis: some notes
- Fatigue calculation: general graph
- Rain flow counting of fatigue cycles: explanation
- Variable amplitude loading: explanation
- Fatigue: Wohler curve
- Design strength: safety factors

- A Summary of the Fatigue Properties of Wind Turbine Materials