Failure analysis of wind blade with passive pitch control mechanism
To enhance the efficiency of power generation and the safety of wind blades, a technique for designing a passive pitch angle control mechanism for a small blade was developed. This technique will make the wind load on the blade with pitch control capability different from that of the blade without. The purpose of this paper is to evaluate the safety of the wind blade with pitch control capability. First of all, the test results of a composite wind blade subjected to stroke-control testing are used to validate the finite element model of the blade. We construct the relationship between the force and the tip displacement and study the failure modes such as buckling and material failure of the blade. The finite element analysis program ANSYS is used to analyze the linear and nonlinear deformations of the blade. It has been shown that the nonlinear finite element method can produce more accurate results than the linear one when compared to the experimental results. Secondly, the pitch control mechanism is designed to achieve a pitch angel of 9.2 degree when the rotor speed reaches 200rpm. To understand the differences of wind forces on the blade with and without pitch control, strains on the skin of the rotating blade with/without pitch control capability are measured using a wireless transmission system for wind loads identification. In the case with pitch control mechanism, it has been shown the theoretical and experimental strains are different. A method is proposed to correct the wind load via the minimization of the sum of the squares of the differences of the theoretical and experimental strains. As for the case with 0 degree pitch angle, the experimental and theoretical results are similar. Next, the failure indices based on the Tsai-Wu criterion of the blade with pitch angles of 0 degree and 9.2 degree at wind speed of 8m/s are determined through the finite element analysis of the blade. It has been found that the failure index of the blade with 9.2 degree pitch angle is much lower than that of the case with zero pitch angle. The results show that at high wind speed, pitch control can enhance the safety of the blade and the pitch control mechanism does not fail. On the other hand, with the consideration of only one failure mode, it has been found that the first-ply failure of the blade with 0 degree pitch angle occurs at 23m/s. Finally, the main conclusions of this study are as follows. A method of correcting the force of blade is proposed for the case with pitch angle. Through the experiment and analysis, it has been demonstrated that the change of pitch angle can not only improve the power generation efficiency but also effectively reduce the failure index of the blade. In the case of zero degree pitch angle, the blade element theory can be used to evaluate the blade force.
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