| 摘要: | 本研究針對半橢圓裂縫結構建立快速的疲勞裂縫成長分析方法。以NREL 5MW OWT風力機為對象，綜合考量IEC 61400-3之DLC 3.2啟動耦合陣風與DLC 1.2正常發電工況。先利用單位負載法與API 579規範提供之權重函數法，以針對塔架上半橢圓裂縫進行快速的動態應力與應力強度因子分析，再結合疲勞裂縫成長理論，求得半橢圓裂縫於疲勞負載下之形狀變化，以進行快速疲勞裂縫成長分析，並使用BS 7910規範提供之失效評估法，決定疲勞裂縫成長時的最終裂縫尺寸，以求出壽命。根據IEC 61400-3規範要求風力機之20年壽命，可求得臨界初始裂縫尺寸，即發現裂縫大於此值時，壽命將不符合規範要求，並進而探討半橢圓裂縫長短軸、裂縫位置、海水位高度與風浪方向對疲勞裂縫成長分析結果之影響。分析方法整合GH-Bladed、ANSYS及MATLAB，風況及海況採用新竹沿海資料。
於動態應力及應力強度因子分析中，發現在正常發電及啟動工況下，塔架底部沿著壁厚方向的正向應力幾乎為線性分布，且利用膜應力與最大彎曲應力描述此路徑之應力分布時，發現此兩應力有線性關係。此外，在不同塔架截面方位角及不同工況皆有相同結果。於裂縫在疲勞負載下的形狀變化探討中，發現不同初始長短軸比之半橢圓裂縫隨著裂縫成長，其長短軸比會趨於近似值，並得知影響半橢圓裂縫形狀變化的主要參數為初始裂縫尺寸(初始長短軸比)，而應力大小與應力比幾乎不影響。於塔架銲道裂縫失效評估中，發現於同樣裂縫深度下，較小之裂縫長短軸比會較早失效。在疲勞裂縫成長分析，考量裂縫位置及不同環境參數時，初始裂縫長短軸比較小者，其臨界初始裂縫深度較小，又發現塔架內壁裂縫比外壁裂縫之臨界初始裂縫深度小。正對風浪方向之塔架位置存在裂縫時之臨界初始裂縫深度較小，此時初始裂縫長短軸比每增加0.1，臨界初始裂縫深度上升趨勢為0.75 ~ 1.15 mm。而海水位高度較高或是風力機沒有對準風浪方向時，臨界初始裂縫深度亦較小，而當初始裂縫長短軸比每增加0.1，臨界初始裂縫深度上升趨勢分別為0.7 ~ 0.9 mm與0.5 ~ 0.9 mm。
;In this study, a fast fatigue crack growth analysis method was established for semi-elliptic crack structures. The NREL 5MW OWT wind turbine is taken as the object, and DLC 3.2 start-up coupling gust condition and DLC 1.2 normal power generation conditions in IEC 61400-3 are considered. First, unit-load method and weight function method provided by API 579 standard are used to analyze the dynamic stress and stress intensity factor of semi-elliptic cracks on the tower. Then combined with fatigue crack growth theory, the crack shape development of semi-elliptical crack under fatigue load is obtained for fast fatigue crack growth analysis. Failure assessment method provided by BS7910 standard is used to determine the final crack size in the fatigue crack growth analysis to obtain fatigue life. According to the requirement of 20-year service life of wind turbine from IEC 61400-3 standard, the critical initial crack size can be obtained, that is, if the crack size is larger than this value, the service life will not meet the standard requirement. After the calculation, the influence of aspect ratio of crack, crack position, sea level and wind-wave direction on the results of fatigue crack growth analysis are discussed. The analysis integrates GH-Bladed, ANSYS and MATLAB softwares. The wind and sea conditions are based on the data at Hsinchu coastal.
In the analysis of dynamic stress and stress intensity factor, it is found that under normal power generation and start-up conditions, the axial normal stress along the tower wall is almost linearly distributed at the tower bottom. When the stress distribution of the path was described by membrane stress and maximum bending stress, it is found that the two stresses have a linear relationship. In addition, the same results are obtained in different azimuthal angles of the tower section and various working conditions. With respective to the shape development of the crack under fatigue load, it is found that the aspect ratio of semi-elliptical crack with different initial aspect ratios will reach an approximate value under fatigue crack growth process. The main parameter affecting the shape change of the semi-elliptical crack is the initial crack size (initial aspect ratio), while stress level and stress ratio have little effect. In the failure assessment analysis of the crack on the tower weldment, it is found that under the same crack depth, the smaller aspect ratio of crack will fail earlier. In the fatigue crack growth analysis, considering the crack location and different environments, the critical initial crack depth is smaller when the initial crack aspect ratio is smaller. The critical initial crack depth of the crack at the inner wall of tower is smaller than the outer one. The critical initial crack depth is small when there is crack in the position directly facing wind and waves. The critical initial crack depth rises from 0.75 - 1.15 mm for every increase of 0.1 in the initial crack aspect ratio. When the seawater level is high or the wind turbine is not aligned with the direction of the wind and waves, the critical initial crack depth is also small. The critical initial crack depth rises from 0.7 - 0.9 mm and 0.5 - 0.9 mm for every increase of 0.1 in the initial crack aspect ratio, respectively. |
