A novel rotor blade fatigue test setup with elliptical biaxial resonant excitation

Abstract

Abstract. The full-scale fatigue test of rotor blades is an important and complex part of the development of new wind turbines. It is often done for certification according to the current IEC (2014) and DNV GL AS (2015) standards. Typically, a new blade design is tested by separate uniaxial fatigue tests in both main directions of the blade, i.e. flapwise and lead-lag. These tests are time consuming and rather expensive due to a high number of required load cycles, up to 5 million. Therefore, it is important to run the test as efficiently as possible. During fatigue testing, the rotor blade is excited at or near its resonant frequency. The trend for new rotor blade designs is toward longer blades, leading to a significant drop in their natural frequencies, and a corresponding increase in test time. In order to reduce the total test time, a novel test method aims to combine the two consecutive uniaxial fatigue tests into one biaxial test. The biaxial test excites the blade in both directions at the same time and at the same frequency, resulting in an elliptical deflection path of the blade axis. Using elliptical loading, counting of damage equivalent load cycles is simplified in comparison to biaxial tests with multiple frequencies. In addition, the maximum loads in both main directions remain separated, while off axis loading is introduced. To achieve such a test, specific load elements need to be arranged to equalize the natural frequencies of the test setup for both test directions. This is accomplished by adding stiffness or inertial effects in a specific direction.

Vibration Fatigue Testing Procedure of High Strength MARS 600 Steel Fillet Welds Using Stainless Steel Consumable Electrode

“…A random, harmonic or custom input signal is selected according to project interest. The method is significantly less limited by specimen shape and is often suitable for full-scale in-situ welded product test [29].…”

Optimized method for multi-axial fatigue testing of wind turbine blades

“…A few papers have suggested optimization approaches to generate test setups that either maximize the accuracy level of the test loads respect to the target loads or minimize the total test time [19,20]. Lee and Park [19], for example, found an optimal flapwise test setup in which the exciter location was defined in a way that the actuating force and the maximum error between the test loads and the target loads were minimized in a 48.3-meters blade.…”

“…Whereas, Melcher et.al. [20] suggested an optimization scheme using virtual masses and spring elements to minimize the total test time, which can be applied to both uni-axial and bi-axial test setups. By applying this method, they found that 30% of the total test time was saved in comparison with current standard tests.…”