Fatigue blade tests are performed in order to determine the fatigue properties of a blade.
The reported data enables certifying bodies to compare measured data to the calculated data of the design criteria. Thus full-scale blade fatigue tests are often an essential part of the approval process.
Figur 1 Flapwise fatigue test
The DANAK accredited fatigue test performed by BLAEST includes a separate edgewise and flapwise test. A test includes the following:
- Strain gauges are applied to the blade skin and wiring is done.
- An eccentric mass exciter and dead loads are mounted onto the blade to obtain the required lengthwise bending moment distribution, and to approach R-ratios close to the required.
- A static calibration test is performed to obtain strain- and stiffness information for the blade.
- The blade is subjected to the required number of load cycles. A control system ensures that the root bending moment range is kept at a constant level during the test.
- At specified numbers of load cycles scans of all strain gauges are performed automatically.
As the blade interior is accessible through the blade root, design engineers can study movements of the internal blade structure while the fatigue test is running.
Stiffness information of a cross section is obtained from the calibration tests, which are carried out at regular intervals during the fatigue test.
Figur 2 Slope of strain/local bending moment. The strain ranges at all gauges are recorded to monitor and verify adequate loading of individual cross sections.
Figur 3 Strain distribution