Testing of rotor blades of wind turbines Arno van Wingerde ...

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LOAD 3 LOAD 2 LOAD 1+3 c) Bending moment distribution along blade axis due to two concentrated loads at the blade Figure 6 bending moment distribution along the blade length Fatigue testing of rotor blades A fatigue test of a rotor blade is notoriously more complex than a static test. All the previously outlined problems, regarding the need for a clean representation of the actual stress states across the cross section and along the length of the blade are still there, but a number of additional problems occur. Required testing time Actual bending moment distribution Bending moment due to test load Wind turbines experience about 10 8 -10 9 cycles, much more than virtually any other known structure, see Figure 7. Also, the loadings vary more than other structures, making fatigue testing of rotor blades particularly difficult. Figure 7 Overview of fatigue loaded structures [9] Assuming a frequency of 1 Hz, 10 8 -10 9 cycles would take about 3 to 32 years, which is highly impractical. Therefore the tests are carried out at a raised load level, so that 10 6 or 2·10 6 cycles would suffice to reach the equivalent fatigue damage. However, many blades are tested at their natural frequency, which is typically below 1 Hz, especially for larger blades, in which case 0.3 Hz. and lower are possible. Testing outside of the natural frequency is possible, but requires vastly more force and
hence energy for testing, raising the cost of the blade test considerably. If a blade is tested at higher frequencies, the test may be harder to carry out due to higher harmonic excitations of the blade. An interesting option to increase the natural frequency of the blade is the removal of the tip part, which also helps to limit the maximum deflections of the actuators, which would otherwise exceed strokes of 10 m. Testing a blade for a million cycles at 0.3 Hz takes only 38.5 days, slightly over one month, and is therefore entirely feasible. Since the stresses are raised and the maximum stresses are already close to the static strength, the spectrum would be compressed anyway, making the step to a constant amplitude loading a small one. Required fatigue loading Many fatigue tests have been carried out as a single loading at a single cross section, so that the bending moment is only correct for a limited length of the blade. Typically the blade is first loaded in flap direction and then in edge direction, thus doubling the total test time. Carrying out the test at another cross section, as was suggested for static tests, doubles the total testing time yet again to the better part of a year. This would be a major cost factor for the manufacturer, who has invested large sums in the design and moulds of the new blade and wants to certify and produce and sell the blades as soon as possible. Furthermore some parts might actually be overloaded due to the subsequent fatigue tests. Instead, for the fatigue test it is suggested to load the blades at several cross sections simultaneously, so as to get the bending moment distribution right along the length of the blade, rather than just at a limited length. Moreover, the blade should be tested bi-axially, so as to get the whole cross section properly loaded. A bi-axial test is carried out with cylinders in two directions at a cross section, so as to make the blade move in an elliptical way, rather than a uni-axial movement. However, controlling the loading in two or more cross sections, while at the same time loading the blade in two directions, poses a major challenge for the controller of the test set-up. In spite of its shortcomings and difficult execution, fatigue tests have often exposed failures in blades which had already passed the static tests. Hence the fatigue tests must be considered a valuable check on the integrity of the blade. On the other hand, the effort and time spent is growing for larger blades and the realism of the fatigue test is often questionable, because of an unrealistic stress field in the blade. The aforementioned disadvantages are the reason for the search for establishing alternative test methods. New Testing Methodology for Rotor Blades Currently, a debate is raging worldwide concerning the need for compulsory fatigue testing in the certification process. Given the previously discussed large technical and economical problems associated with a fatigue test of a blade, the industry and research institutes are currently investigating possible alternatives. Although these alternatives may not totally eliminate the need for fatigue testing of rotor blades, they could reduce the absolute need for such tests, especially in case of modifications of existing blades or scaled-up version of an existing design. A possible new testing methodology has been suggested in an earlier publication [9]. In order to replace full size fatigue blade tests, emphasis is given to material tests and tests of simple components, supported by more extensive numerical analyses.
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Testing of rotor blades of wind turbines Arno van Wingerde ...

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