Cycles brøndsted et al 2005 fatigue loading of the

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cycles (Brøndsted et al.2005). Fatigue loading of theturbine blade and the gearbox can be a serious problem. One should point out amajor difference between the fatigue loading of the wing of an aircraft and the bladeof a wind turbine; the loading conditions in a windmill are more strenuous.An aircraft at its cruising speed at high altitudes is not subjected to the same forcesas a wind turbine blade on earth. It is important to realize that if, during high speedspinning of the rotor blades, one of the blades came loose and smashed into thecentral tower, it could cause the whole structure to collapse. Such incidents havebeen captured on video.1905Polymer Matrix Composites
5.4.1.11Static TestingStatic loading is carried out in a traction rig attached to the blade by steel wires.The blade is loaded in tension to maximum load. According to the practice used atLMGlasfiber, a large manufacturer of wind mills, the rotor blade must stay underload for at least 10 seconds without breaking. The tensile test is repeated twiceflapwise and then the extreme load is tested in all directions: leading edge, trailingedge, suction side and pressure side. It can take about a week to do the static testing,and the test is repeated after the blade has been subjected to the dynamic test (seethe description of the dynamic test below). This is done to make sure that thedynamic or fatigue loading of the blade has not introduced a subcritical damage.5.4.1.12Dynamic TestingDynamic testing of the rotor blades is very important because they must be able tosustain fatigue loads corresponding to 20 years of life. Every materials has a naturalfrequency of oscillation,f, given byf¼12pffiffiffikmrwherekis the stiffness (N/m) andmis the mass. The blade sets into oscillationcorresponding to its natural frequency. The larger or more massive the blade, thelower the natural frequency. It is not uncommon to subject the blade to five millionoscillations edgewise followed by five million oscillations flapwise. A 60 m longblade could easily be fatigue tested for a year. During fatigue testing, nondestruc-tive techniques are use to detect any damage in the blades, e.g., an infrared cameramay be used to detect cracks in the laminate. Concurrently, many strain gages,attached to the surface of the blade, are used to track the deformation anddeflections.5.4.1.13Protection Against LightningAs is well known, lightning strikes the highest point in a given area. This makeswind turbines a very likely target, because of their height and elevated location.Lightning strikes involve currents of very large amperage (can be up to 200 kA) andthey release very large amount of energy in a very short time period (milliseconds).Besides the obvious electrical and magnetic properties, lightning strikes involveX-rays, thunder which is nothing but an acoustic shock wave, and intense heat.

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Term
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