Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 425

All tests should be done in fl apwise direction towards both the downwind (suction)
and upwind (pressure) sides and in edgewise directions towards both the leading
and trailing edges. If it is important for the design, also a torsion test is needed
in order to determine the torsional stiffness distribution. The tests are undertaken
to obtain two separate types of information. One set of information relates to the
blade's ability to resist the loads that the blade has been designed for. The second
set of information relates to blade properties, strains and defl ections arising from
the applied loads.
All tests in a given direction and in a given area of a blade shall be performed on
the same blade part. The fl ap- and edgewise sequence of testing may be performed
on two separate blades. However, if an area of the blade is critical due to the
combination of fl ap- and edgewise loading, then the entire test sequence shall be
performed on one blade.
3.3 Examples of full-scale tests used to determine deformation
and failure modes
In the following, the focus is on the ultimate strength of the rotor blades. Results
and fi ndings from recent full-scale tests to failure are studied.
A 25 m blade was tested to failure in three sections in the study by Sørensen
et al . [2]. The blade was loaded in the fl apwise direction. The purpose of the test
was to gain detailed information about failure mechanisms in a wind turbine blade
especially with focus on failures in the compression side (downwind side) of the
blade. Prior to the tests the blade was inspected by ultrasonic scanning to get an
overview if any imperfections and damages were present already before starting
the test. The supports and loading of the blade was changed during the test such
that it was possible to use the same blade in three tests, i.e. having independent
failures in three different sections of the blade. During each of the tests the behav-
ior of the blade was recorded by means of video and photos, strain gauges, acous-
tic emission and defl ection sensors. Two different types of defl ection sensors were
mounted on the blade, one giving the total defl ection of the blade and another
giving skin and main spar displacements, locally. The identifi ed failure modes are
presented in the next section (Section 4).
In a study by Jensen et al . [8], results from a full-scale test of a 34 m blade were
compared with fi nite element (FE) analysis. The blade was loaded to catastrophic
failure. Measurements supported by FE results show that detachment (delamina-
tion) of the outer skin from the box girder was the initial failure mechanism
followed by delamination of the load-carrying laminate, leading to collapse.
4 Failure modes of wind turbine blades
4.1 Defi nition of blade failure modes
Wind turbine blades can fail by a number of failure and damage modes. Obviously,
the details of damage evolution will differ from one blade design to another. How-
ever, experience shows that, irrespective of specifi c blade design, several types of