Testing Wind Turbine Rotor Blades
Fatigue Testing of Rotor Blades
Video Soren Krohn
© 1999 DWIA
Click on image to restart video
Teating Rotor Blades The video to the left (122 K) shows how a 32 m rotor blade is fatigue tested by being bent cyclically in a flapwise direction for 5 million full cycles. A full flapwise test thus takes about three months.
If you look closely to the left you can see another (shorter) rotor blade being bent cyclically in an edgewise (chordwise) direction.
In both cases the blades are bent using a cycle close to the natural frequency of the blade.
The natural frequency is the frequency with which the blade will oscillate back and forth, if you push it once in a certain direction and let go. The natural frequencies are different in the flapwise and edgewise direction: The blade tends to be much stiffer in the edgewise direction, thus it has a higher natural frequency for edgewise bending.
Each blade is set in motion by an electric motor mounted on the blade which swings a weight up and down. The foundations which carry the blade socket have to be very solid: The foundation for the large blade socket consists of 2,000 tonnes of concrete.
This video was shot at the rotor blade test facility of the Risoe National Laboratory Sparkær Test Centre in Jutland, Denmark. (Type approval requirements for rotor blades are very strict in Denmark, requiring physical testing of rotor blades for both fatigue properties (fatigue testing) and strength properties (static testing). Other countries usually have less stringent requirements for type approval of rotor blades).
Rotor Blade Materials
Rotor blades are usually made using a matrix of fibre glass mats which are impregnated with a material such as polyester (GRP = Glass fibre reinforced polyester). The polyester is hardened after it has impregnated the fibre glass. Epoxy may be used instead of polyester. Likewise the basic matrix may be made wholly or partially from carbon fibre, which is a lighter, but costlier material with high strength. Wood-epoxy laminates are also being used for large rotor blades.
The Purpose of Testing Rotor Blades
The purpose of rotor blade testing is to verify that laminations in the blade are, safe, i.e. that the layers of the rotor blade do not separate (delamination). Also, the test verifies that the fibres do not break under repeated stress.
Measuring Strains
Photograph Soren Krohn © 1999 DWIA
Strain Gauge Strain gauges, (i.e. flat electrical resistors which are glued on to the surface of the rotor blades being tested), are used to measure very accurately the bending and stretching of the rotor blades.
Monitoring Fatigue Testing
Photograph Soren Krohn © 1999 DWIA
omputer Monitor The measurement results from the strain gauges are continuously monitored on computers. Nonlinear variations in the pattern of bending may reveal a damage in the rotor blade structure.
Infrared Inspection (Thermography)
Infrared cameras are used to reveal local build-up of heat in the blade. This may either indicate an area with structural dampening, i.e. an area where the blade designer has deliberately laid out fibres which convert the bending energy into heat in order to stabilise the blade, or it may indicate an area of delamination or an area which is moving toward the breaking point for the fibres.
Modal Forms of Rotor Blade Vibrations
From the year 2000 blade testing (in Denmark) also includes a verification of the different modal forms of vibration of each blade. This is done using a special type of equipment which excites the blade vibrations at different frequencies and in different directions.
Different modal forms of oscillation are also known when building musical instruments: A string on a violin may oscillate with is basic tone, i.e. the centre of the string moving up and down, but it will usually also oscillate with the first overtone or first harmonic, with two centres of oscillation located at a distance of 1/4 from each end of the string, moving at twice the frequency of the basic tone or natural frequency.
The reason why manufacturers of wind turbines are interested in studying and verifying the various forms of vibration frequencies in rotor blades, is that they have to make sure that the turbine on which the blade is to be mounted does not have some of the same natural frequencies as the rotor blade. Otherwise, a resonance may occur in the whole structure of the turbine, leading to undampened vibrations which may eventually wreck the whole wind turbine. We will return to this issue on the page on structural dynamics in the design section later in this guided tour.
Static Testing of Rotor Blades
Rotor blades are also tested for strength (and thus their ability to withstand extreme loads) by being bent once with a very large force. This test is made after the blades has been subject to fatigue testing, in order to verify the strength for a blade which has been in operation for a substantial amount of time.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003
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