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

Material tests were used before to determine the basic material properties, but could
perhaps also be taken from actual blades to reflect the actual material properties of a
blade, as opposed to just some generic material properties from coupons that are
produced entirely separately from the blades, sometimes by other companies and
hence may not fully reflect the actual properties of the material in the blade, such as
the misaligned fibres discussed previously in the paper.
The material properties are used to establish numerical models of the blade. Some
critical areas of the blade could be modelled by performing simple component tests,
other areas will require more complex component tests, such as the T-bolt “IKEA”
connections typically used to connect the blade to the hub.
If a component is thought of as a part of a blade, it is generally not trivial to obtain the
correct boundary conditions of a component as they would occur in the blade, which
requires either a complex test set-up or complex test specimens. However, via the
numerical model, it is possible to establish an “equivalent” test which would be as
severe a case, but with more basic boundary conditions. In other cases, scaled-down
models might be tested to see whether they concur with the numerical results.
Other problems that seem to lend themselves well to component testing would be
either potential buckling problems or bonded joints. The use of smaller specimens
instead of a full blade also allows for more extensive testing with for example fatigue
tests at different stress ratios, or tests at other temperatures and at raised humidity
levels. Moreover, rather than just testing a single test specimen, a full series can be
tested, revealing important statistical properties of the production. Conceivably
specimens could be produced along with the blades (comparable to concrete cubes
used in the construction business to check the quality of concrete structures), to
check the consistency of the production in time. If implemented successfully, the new
testing technology, which reflects long established practises in airplane construction,
could result in a raised level of reliability, at reduced overall testing costs.
Conclusion
Performing and analysing static and fatigue testing of rotor blades of wind turbines
poses major challenges for test centres, blade manufacturers and certification bodies
alike. With the increasing size of wind turbines, testing the rotor blades is becoming
increasingly costly and time-consuming. The compulsory fatigue testing of rotor
blades might become a hindrance for the further development of the industry. Therefore,
an alternative test method, based on static blade tests and component tests, as
well as more extensive material tests might offer a way to satisfy reliability demands
at considerably lower costs. Especially in case of several blades of one family with
minor variations, or in case of scaled-up blades, the need for dynamically testing the
full blade may be all but eliminated using this test philosophy. This requires the definition
of accepted component tests, which should occur in a body like the IEC, a
committee is being formed to set up these tests.
Acknowledgements
The authors are grateful to valuable discussions with the German Competence
Group Wind Energy „Rotor Blade Testing and Rotor Blade Materials“.
The work is supported by the State of Bremen, Senate of Civil Engineering, Environment
and Transportation and Bremerhaven Economic Development Company
Ltd, the Federal Ministry for the Environment, Nature Conservation and Nuclear