| 摘要: | 本研究探討不同疲勞工況對於塔架應力所造成的影響，以及不同尺寸的共平面裂縫之疲勞裂縫成長評估。以 NREL 5MW OWT 風力機為模型，並考量IEC 61400-3之疲勞工況，包括DLC 2.4 正常發電加上故障、DLC 3.1 啟動、DLC 4.1 關機、DLC 6.4 惰轉及DLC 7.2惰轉加上故障等工況。分析方法整合了 GH-Bladed、ANSYS及MATLAB軟體，風況及海況採用新竹沿岸資料。第一步部分為探討前述疲勞工況對於塔架應力大小及分布的影響，接著再以造成較大受力條件之工況，進一步探討風速、Hs及Tp對塔架應力之影響。第二部分先探討不同的裂縫尺寸及裂縫相距距離對於共平面裂縫之交互因子分布影響，然後以應力分析之結果為基礎，根據BS 7910所制定的失效評估方法，取得失效裂縫尺寸，結合疲勞裂縫成長理論，以得到結構疲勞壽命。
在第一步部分的分析中，在各疲勞工況中，切入(Cut-in)、額定(Rated)及切出(Cut-out)等三種風速的應力分布都顯示，在塔架方位角180度的地方有最大Z軸應力，而其中僅DLC 2.4對塔架造成較大應力，產生Mode I 形式的裂縫成長，其餘工況則都不會使裂縫成長。
而基於DLC 2.4工況所討論的風速、Hs及Tp對Z軸應力的影響，結果發現從切入風速至額定風速的過程中，Z軸應力會越大，而從額定風速至切出風速的過程中，Z軸應力會越小，Hs是值越大，Z軸應力會越大。Tp則是沒有表現出明顯趨勢。
第二部分的分析中，得知共平面裂縫的間距以及裂縫大小，會影響裂縫前緣的應力強度因子及交互因子。裂縫的間距越短或裂縫的尺寸越大，兩個裂縫互相靠近的那側應力強度因子及交互因子會上升。在共平面裂縫之疲勞裂縫成長分析中，對於BS 7910、ASME XI 及 PD 6493三種規範所提出的將共平面裂縫等效為單一裂縫的建議進行比較。結果顯示，以失效評估的觀點來看，PD 6493對於裂縫等效的臨界距離之定義較好，在該規範定義的距離下，共平面裂縫互相影響的現象已幾乎降至沒有。但以疲勞裂縫成長的觀點來看，BS 7910所定義的裂縫等效尺寸較為合適，因為該規範所等效後的裂縫a/c值與共平面裂縫a/c值的差距較小，故在疲勞裂縫成長分析上，會相較於另外兩個規範獲得更佳疲勞壽命結果。
;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. |
