ACOUSTIC
EMISSION & NDT SCIENTIFIC PUBLICATIONS
Full-Scale Blade Testing Enhanced By Acoustic Emission
Monitoring
To appear in the proceedings of European Wind Energy Conference
- EWEC 2003
D. J.
Lekou, P. Vionis, P. A. Joosse, D. R. V. van Delft, D.
Kouroussis, A. Anastasopoulos, M. J. Blanch, A. G. Dutton, A.
Proust
Current wind turbine blade
certification procedure requires static and fatigue tests on the
full-scale blade, to ensure that the blade can sustain design
loads and to assess its behavior during service life. During
these tests sudden audible cracking sounds from somewhere within
the blade are often generated, without the operators being able
to locate the noise source or to evaluate the existence or the
extent of any damage. As blade structures continue to grow in
size the detection of any damage, whether through visible
inspection or through the location of the sound source becomes
an indomitable task for the test engineer. Moreover, since
applied loading increases along with the size growth of the
blade, it is also important to have a forewarning of any
impending failure, since the energy released by such an event
becomes enormous due to the high forces. Both these issues can
be improved by the use of acoustic emission (AE) monitoring
methodology in parallel with standard testing procedures. Within
the framework of an EU-funded project named AEGIS
(JOR3-CT98-0283), a comprehensive assessment of the application
of AE monitoring complementing structural testing of composite
material blades was conducted and respective methodologies were
developed.
A previously developed testing
procedure for small blades incorporating AE monitoring was applied
to full-scale static and fatigue tests of commercial quality and
size blades. Two 16m blades were driven to failure following current
certification testing procedures, enhanced, however, through the
quasi on-line analysis of the AE data. On-line analysis of AE data
included damage location and criticality assessment by use of
pattern recognition software previously calibrated through tests on
small blades. The methodology was assessed investigating the
possibility of including AE monitoring during standard blade
certification tests.
Results from a static and a fatigue
test of the 16m blades following the developed methodology, which
were carried out within AEGIS project, are presented revealing that
the applied methodology can be successfully applied to large
commercial blades. Although some size and material related issues
might need some refinement, when passing the pattern recognition
software from smaller to larger blades, results from both tests show
that critical areas can be effectively identified and assessed with
respect to the blade's structural integrity. Especially when
combining standard measuring methods during blade testing, e.g.
strain reading, with AE monitoring, the test operator is fortified
against untraceable damage.