Comprehensive testing of NedWind 12-meter wind turbine blades at NREL

Abstract

This paper describes the structural testing of two NedWind 25, 12-m blades at the National Renewable Energy Laboratory (NREL). The tests were conducted under the Standards, Measurement and Testing (SMT) Program in conjunction with tests conducted by four European laboratories to develop a common database of blade testing methods. All of the laboratories tested duplicate copies of blades taken from series production. Blade properties, including weight, center of gravity, natural frequencies, and damping were determined. Static load tests were performed at 110% of the extreme design load for strain verification. NREL performed single-axis and two-axis fatigue tests using "businessas-usual" testing practices.The single-axis test combined equivalent life loading for the edge and flap spectra into a single resultant load. The two-axis test applied the edge and flap components independently at a phase angle of 90°. Damage areas were observed at (1) the trailing edge, which cracked near the maximum chord; (2) between the steel root collar and the composite, where circumferential cracking was noted; and (3) along the top of the spar between the 2500-mm and 4200-mm stations, where a notable increase in acoustic emissions was detected. NREL observed that the onset of visible damage occurred earlier in the single-axis test.

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“…In this study, the damping ratio was set as 0.3%, which is based on damping characteristics identified from comprehensive experiments carried out by Larwood etc. [15]. However aerodynamic damping was not considered in this analysis.…”

“…The orthotropic mechanical properties of glass-reinforced composite utilised for the blade structure in the simulated model are given in Table 1. The density of this composite material was r ¼ 1860 kg/m 3 and its damping coefficient approximately z ¼ 0.003 [15].…”

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“…There are two types of fatigue testing machines for full-scale wind turbine blades: forced displacement and resonance excitation. The strain and deflection of the blade is measured during tests [10,13]. Strain measurement in these tests is a challenging experimental task.…”

“…This is due to the fact that a complicated time load history and small changes in the previously mentioned conditions may strongly affect fatigue life. Thus, designers are willing to rely on full-scale blade tests in similar conditions as those in the real operation [10,13]. In addition, there are globally accepted standards and certification procedures for fullscale rotor blade tests, such as IEC-61400-23 (2001) [14] and GL-2010 [15].…”

“…The equivalent fatigue test load is obtained by matching the damage caused by 20-year-design load spectrum [7]. The number of load cycles is estimated up to 10 8 or 10 9 cycles for 20 years of service life [10]. These loads are always of variable amplitudes as depicted in Fig.…”