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Wind turbine blades still happen to fail in the field nowadays and often already after some years in operation, although the blades have been tested and certified according to the latest standards and guidelines. From this it followsthat the actual reliability level of blades in field is lower than the aimed reliability level as estimated by the design model. Within the experimental validation, the mandatory full-scale test can only target a limited amount of failure modes. Hence,the representativity of the test is relatively low.In the static test, for example,the design load envelope is reduced to at least four load directions.Moreover, the fraction of valid testareas, which arefree of load framesalong the span,is limited. In the fatigue test, for example, representative stress ratios can only be realized in the inboard blade region while outboard stress ratios observed can be matched only at specific locations but not for the fullspan. The means of sub-component testing can overcome these drawbacks. This method enables to freely adjust the moment vector direction and distribution along a blade segment’s span while giving the same structural response as in a static full-scale test. In a fatigue test also the stress ratio can be freely adjusted enabling to increase the representativity compared to field conditions. Testing at more realistic and higher load ratios will not just improve the quality of the test but also decrease the testing time since less test cycles are required. Also the test frequency of a sub-component test is independent from the system’s natural frequency of a cyclic full-scale test. A more extensive reliability demonstration can be reached in a testing scenario wherein a full-scale test demonstrates the reliability for failure modes in the inboard area and an additional sub-component test demonstrates the reliability level for failure modes in outboard areas. A more extensive reliability demonstration improves the design model validation. Improved models can enhance the reliability estimation of blades in the field. |
