| dc.description.abstract | This study explores the influence of different fatigue load cases on the stresses in tower structures and assesses the fatigue crack growth for different sizes of coplanar cracks. The NREL 5MW OWT wind turbine is adopted, and the fatigue load cases defined in the IEC 61400-3 standard including DLC 2.4 (power production plus occurrence of fault), DLC 3.1 (start up), DLC 4.1 (normal shut down), DLC 6.4 (idling), and DLC 7.2 (idling and fault condition) are considered. The analysis method integrates GH-Bladed, ANSYS, and MATLAB software. Wind and sea conditions are based on the data from the Hsinchu coast. The first part of this study focuses on investigating the impact of the aforementioned fatigue load cases on the magnitude and distribution of stresses in the tower structure. The effects of wind speed, significant wave height (Hs), and peak period (Tp) on the tower stresses are explored. The results show that only DLC 2.4 causes greater stress to the tower. The crack growth in Mode I form is generated, and the other load cases do not make the crack growth. Based on the DLC 2.4 condition, it is found that the Z-axis stress will increase during the wind speed from the cut-in to the rated, while the Z-axis stress will decrease during the wind speed from the rated to the cut-out. The greater the Hs value, the greater the Z-axis stress will be. The effect of Tp is not obvious.
In the second part, this study examines the effect of different crack sizes and distances between coplanar cracks on the distribution of the stress intensity factor and interaction factor. The results show that the shorter the spacing of the coplanar cracks or the larger the size of the cracks, the stress intensity factor and the interaction factor will increase on the side where the two cracks are close to each other. In the analysis of fatigue crack growth for coplanar cracks, a comparison of treating coplanar cracks as an equivalent single crack is made among the recommendations provided by BS 7910, ASME XI, and PD 6493. From the failure assessment perspective, PD 6493 provides a better definition of the critical distance for crack equivalence. Under the distance defined by this standard, the phenomenon of coplanar fracture interaction has been almost reduced to none. However, from the viewpoint of view of fatigue crack growth, the equivalent crack size defined by BS 7910 is more appropriate, because the difference between the equivalent crack a/c value and the coplanar crack a/c value is small. Therefore, better fatigue life results can be obtained compared with the other two standards in fatigue crack growth analysis. | en_US |