Testing is mandatory for certification. It is necessary to validate the assumptions made in the design of a blade and to boosts the confidence of customers of wind turbines.
A Full Scale Blade Testing usually involves a static and a fatigue test.
Static blade tests are performed in order to verify the structural properties of a blade including stiffness data, stress and strain distributions. The reported data enables certifying bodies to compare measured data to the calculated data obtained from design simulations. Static tests may be carried out as a multipoint load application or number of consecutive point load application. In the early |
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blade testing, loads were applied on the blade in flapwise direction only but later also the edgewise direction was included in the test.
Fatigue tests are conducted to verify the ability of a blade to sustain operating loads over a long period. A one dimensional resonance fatigue test is traditional. In a one dimensional fatigue test, dead loads are mounted onto the blade to obtain a specific lengthwise bending moment distribution and an oscillating mass oscillates at the resonance frequency of the system in either the flap or edge direction. Main advantage of this test setup is that it is simple and economical. However as blade's stiffness decreases during testing, testing parameters are not constant and need to be controlled. In addition to this, the strain distributions achievable are mainly limited to either flapwise or edgewise directions and not necessarily represent critical fatigue loading of a blade in operation. For this reason, test centers are more and more changing to two dimensional testing which can expose the blade to more realistic maneuvers.
A two dimension fatigue test can be done in different ways. A recent publication describes a two dimensional fatigue resonance test in which blade is resonated simultaneously in flap and edge wise direction at the system resonance frequencies in both directions. The advantage of this method over one dimension fatigue test is that this fatigue test can put blade structures under strain patterns which are closer to operational situation. The resonance frequencies however slightly change in the course of the test (due to the decrease in stiffness of the blade) and need to be controlled/reviewed during the test. A number of test centers practice two dimensional, single resonance fatigue tests in which the phase angle between the displacements in flap and edge directions can be accurately controlled. A hydraulically powered actuation system is used to apply load on the blade through two actuators in horizontal and vertical direction simultaneously. The advantage of this test is that realistic strain patterns can be achieved. This test furthermore can easily simulate different fatigue 'events' during the lifetime of a blade (e.g. emergency stop) during the test by changing the loads and maximum deflection.
The developments in the structural design of modern rotor blades go hand in hand with material developments and increasingly powerful computer simulations. This demands improvements in testing techniques to simulate blade loads that are as close as possible to practical field condition. There is a strong need for advanced computer simulation techniques to compare both test conditions with practical field condition in order to continue to provide certification bodies and customers the confidence they seek.
Pavan Sinha
Blade Test Center Gujarat
Note: The expert opinion is a free opinion of an expert and not necessarily expresses the views / opinion of the BTCG organization. |
