FATIGUE LOADING IN WIND TURBINES

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1 FATIGUE LOADING IN WIND TURBINES M. Ragheb 5/6/2009 INTRODUCTION Because o the complex systems o variable loads that wind turbines are subjected to, they are particularly susceptible to atigue damage. The rotor blades are mostly at risk. Research eorts have been directed into assessing the potential or blade atigue damage during operation. Some drive train ailures have also been due to atigue, and eorts have devoted into alleviating the eects o variable loads on them Fig. 1: Inspection o rotor blade tips or possible cracks. They are subjected to wind speeds reaching 250 km/hr. Source: Nordex. FATIGUE LOADING AND FAILURE The cyclic loading o the structure o a wind turbine could cause ailure i some critical level o damage is exceeded. Once initiated, the damage will grow with the load cycling until ailure occurs. The ailure process would occur because o one o the ollowing reasons 1. The net section stress, accounting or the loss o section caused by the damage, exceeds the ultimate strength o the material. 2. A critical crack orms by the accumulation o damage. I the damage growth rate in a component depends on the cyclic stress range, the load ratio R, is deined as:

2 R = Load Ratio = Maximum applied cyclic load Material Tensile Strength (1) and the current value o damage D, then: where: dd = ( Δ σ, RD), (2) dn Δσ is the cyclic stress range R is the ratio o maximum applied cyclic stress to the material's tensile strength D is the current value o damage as a percent o section subject to racture N is the number o loading cycles The atigue lietime or the number o cycles at ailure N is deined as the number o load cycles that are necessary to raise the initial damage state D to the inal or critical i level o damage D, where ailure occurs. By integration the inverse o Eqn, 2, we can write: N = dd ( Δσ, RD, ) (3) It is necessary to deine the unction in order to quantiy N. This is carried out empirically by the use o the Stress versus Number o cycles or S-N curves. An alternating stress is applied to the material being tested and the number o cycles to ailure N is determined as a unction o the stress amplitude S. The slope o the S-N curve is a measure o the resistance o the material to atigue, and the actual shape o the curve varies rom one material to another.

3 Fig. 2: Shape o the S-N curve or a given material. The y axis is the cyclic stress range as a percent o the Ultimate Tensile Stregth (UTS) and the y axis is the logarithm o the number o cycles N. FORCES RELEVANT TO FATIGUE Even though the S-N curves provide an indication o the relative atigue properties, they do not take into account the complex eect o the large number o dierent cyclic orces which act on a wind turbine blade during operation. These orces arise due to the blade's own mass and the orce o the wind acting upon it and include: 1. The gravitational orce caused by the pull o the Earth on the mass o the blade. This leads to compression and tension in each cycle. 2. The centriugal orce due to the rotation o the rotor blade. 3. The wind thrust which is a orce that is perpendicular to the plane o the rotor blade. It varies relatively slowly. 4. Other rapidly varying orces arising rom wind turbulence which increase as the stall conditions are approached. Research indicates that the relatively low requency high amplitude wind thrust orces primarily contribute to atigue damage. ENVIRONMENTAL FACTORS AFFECTING FATIGUE For a chosen atigue resistant material, environmental attack can rapidly reduce its atigue strength. This could occur in two ways: a) Blade Topography:

4 The topography o the blade surace may be modiied by minute erosive and corrosive pits rom sand or rain impingement. These would act as stress concentrators during cyclic loading, causing localized cracking to be initiated. The corrosive attack along the grain boundaries in metals acts in a similar way; setting up crack initiation sites or intergranular racture. For larger Glass Reinorced Composites (GRP) and wood laminate blades, the erosion attack can occur near the blade tips where the rotational velocities can reach the equivalent o 100 m/sec. b) Bulk material properties: The bulk material properties may be altered, thus reducing atigue strength throughout the blade wall thickness or through the surace layers. An example would be a moisture intrusion into the wood laminate blades in poorly protected regions or at the trailing edge where a loss o bonding o the laminates can happen. The remedy is the use o protective coatings that are applied initially at the manuacturing stage and then regularly checked during the maintenance process. The wood laminate blades are usually coated with an epoxy skin or environmental protection, in a similar technique to that used in the manuacture o helicopter blades. The leading edge o wind turbine blades always requires a special inish and care. ROTOR BLADE DESIGN CONSIDERATIONS The rotor blade design should not exacerbate the eect o the cyclic orces it is subjected to. Some blade geometries should be avoided in order to reduce the prospect o early atigue ailure. Sharp changes in the blade proile at the blade root and hub junction can act as stress concentration regions, causing the yield stress o the blade material to be locally exceeded and leading to crack initiation. As an example o this process is the British Comet passenger plane disasters in the early 1960's which were due to such a design error. It was caused rom the use o a sharp radius o curvature at the corners o the uselage window openings. The design o the connecting joints is important in rotor blade construction. The veneer butt joints in wood laminate rotor blades are particularly susceptible to atigue ailure. Using a simple modiication by substituting scar or angled joints at the veneer junctions would signiicantly reduce the ailure rates. CHOICE OF ROTOR BLADE MATERIALS The S-N graphs or some turbine rotor blade materials are shown in Fig. 2. The data about the atigue lie o materials must be considered with care. In the case o a composite the materials dierences in the iber or mat type, the matrix and reinorcement bond strength or the construction methods can introduce signiicant uncertainties.

5 Fig. 3: S-N diagrams o some rotor blade materials. The turbine rotor blade materials may be classiied into the ollowing groups: METALS Some metals such as mild steel are relatively atigue resistant; provided it is subjected to a cyclic stress below its atigue threshold. This is usually less than 1/2 its Ultimate Tensile Strength (UTS). Mild steel can be used or long periods i the component is designed, abricated and maintained correctly. Some light alloys like aluminum have an S-N curve which alls continuously with time. Failure becomes increasingly likely i the material remains in service under cyclic stress conditions long enough. This suggests in the concept o a limited service lie or the component ater which it must be discarded, regardless o whether any damage is apparent. Such an approach is used in the maintenance o some civilian and most military aircrat. WOOD LAMINATES A number o wind rotors manuacturers produce wood laminate blades up to 25 meters in length. Wood laminate oers good strength to weight properties when compared to Glass Reinorced Composites (GRP). A avorite wood laminate material rom the perspective o atigue response is Khaya, or Arican Mahogany. COMPOSITE MATERIALS

6 The atigue properties o composite materials depend on the inherent strength and stiness o their component materials as well as on their structure. Composite materials are common such as in automobile tires, airplane wings and uselage and reinorced concrete. The steel reinorcement in concrete carries out the tensile loading, while the concrete matrix carries the compressive load. Glass Reinorced Composites (GRP) are the most commonly used turbine rotor blade material. Experimental ull scale simulations have been conducted and indicate a satisactory service lie under normal conditions. Laboratory testing data show a steadily decreasing S-N curve or GRP, indicating a inite service lie. A careul monitoring o GRP rotor blades under operational conditions should be undertaken. Composite materials containing higher modulus or stier ibers such as possess better atigue properties, i cyclic stress is applied parallel to the iber orientation. This is because the matrix epoxy material is constrained by the reinorcing ibers during cyclic loading, subjecting the composite to relatively low strains which do not approach the cracking strain o the matrix. The second phase, such as the glass iber in GRPs can also act as a crack arresting zone, eectively pinning the growth o atigue cracks. Carbon iber reinorced composites exhibit outstanding atigue perormance when compared with metals and other composites, especially when subjected to tension atigue in the iber direction. Their advantage is reduced when the matrix material becomes the predominant load bearer, but they nevertheless oer many times the perormance o other composites or metals. Cost constraints cause carbon iber composites to be rarely used in turbine rotor blade construction.