Abstract
In this paper, the sensitivity of the structural integrity of wind turbine blades to debonding of the shear web from the spar cap was investigated. In this regard, modal analysis, static and fatigue testing were performed on a 45.7 m blade for three states of the blade: (i) as received blade (ii) when a crack of 200 mm was introduced between the web and the spar cap and (iii) when the crack was extended to 1000 mm. Calibration pull-tests for all three states of the blade were performed to obtain the strain-bending moment relationship of the blade according to the estimated target bending moment (BM) which the blade is expected to experience in its service life. The resultant data was used to apply appropriate load in the fatigue tests. The blade natural frequencies in flapwise and edgewise directions over a range of frequency domain were found by modal testing for all three states of the blade. The blade first natural frequency for each state was used for the flapwise fatigue tests. These were performed in accordance with technical specification IEC TS 61400-23. The fatigue results showed that, for a 200 mm crack between the web and spar cap at 9 m from the blade root, the crack did not propagate at 50% of the target BM up to 62,110 cycles. However, when the load was increased to 70% of target BM, some damages were detected on the pressure side of the blade. When the 200 mm crack was extended to 1000 mm, the crack began to propagate when the applied load exceeded 100% of target BM and the blade experienced delaminations, adhesive joint failure, compression failure and sandwich core failure.
Highlights
One of the main recurring problems within wind turbine blades is fatigue of the blade, especially as the magnitude of the blade size is increasing
The target bending moment (BM) should be matched but due to existing damage in the blade the maximum estimated target BM for the first two states of the blade was limited to 50% of estimated target BM
Monitoring of fatigue tests was done by controlling SG2 strain gauge readings of 1478 με at 2500 kNm BM at the root, which is equivalent to 50% of target BM
Summary
One of the main recurring problems within wind turbine blades is fatigue of the blade, especially as the magnitude of the blade size is increasing. The blades are subjected to a highly irregular loading condition caused by turbulent wind flow, gravity, and inertial loading during accelerating or decelerating of the turbine. A wind turbine blade is predominantly loaded in the flapwise and edgewise directions. The source of flapwise loads are mainly aerodynamic, while the edgewise loads are mainly caused by gravity. The aerodynamic loading is at maximum when the blade position is at 12 O’clock due to higher wind velocity from wind shear, and a minimum when the blade is at 6 O’clock when the blade passes through stagnant air in front of the turbine tower. A modern wind turbine is designed to last 20–30 years almost unattended; a comprehensive testing of the Materials 2017, 10, 1152; doi:10.3390/ma10101152 www.mdpi.com/journal/materials
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