離岸風機扣件於不同荷重頻率之腐蝕疲勞行為研究

Effect of Loading Frequency on Corrosion Fatigue of Bolts Using in Offshore Wind Turbine Tower

本研究主要針對鎖附風力發電機塔筒間之大型扣件，於離岸環境使用時之腐蝕疲勞行為研究。比較離岸式與陸域風機於操作時之差異，前者所承受的力源較複雜且環境亦更為嚴苛。離岸式風機受力主要來自風、海波及海流的交互作用；腐蝕源主要來自於海水飛沫區之鹽霧環境。利用拉伸試驗機以Tension-Tension週期式荷重模式，設定不同應力及荷重頻率參數，並加置鹽水噴霧系統進行其腐蝕疲勞行為解析。研究結果顯示，鹽霧環境相對於空氣環境會使扣件之疲勞壽命下降，原因為扣件表面因Cl-侵蝕發生局部腐蝕，導致應力集中誘發疲勞裂紋；荷重頻率對腐蝕疲勞壽命下降之影響則以低荷重頻率更為明顯，原因為低荷重頻率之疲勞裂紋前端之pump-effect較高荷重頻率更為明顯，故於單次荷重週期條件下，腐蝕源對裂紋前端之塑性變形區之腐蝕時間較長，腐蝕現象對應力之加成作用致使低荷重頻率之腐蝕疲勞壽命明顯下降。綜合研究結果顯示，扣件在腐蝕性環境中承受週期式應力作用下，低荷重頻率之環境腐蝕作用對疲勞影響較大，高荷重頻率則以應力作用相對明顯。

Offshore wind turbines suffer more aggressive corrosion environment than onshore types due to the salt-fog attack in splash zone. Besides, bolts connect offshore turbine tower bear more complicated loading than that of onshore types. In this study, corrosion fatigue behaviors of offshore using bolt, loading frequencies and salt-fog environment were set with different loading stresses in cyclic tension-tension mode fatigue were investigated. The results show bolt fatigue life downgraded obviously in salt-fog as compare with that in air, which was due to Cl- attack bolt surface and resulted in stress concentration sites. Corrosion fatigue life of bolt decreases evidently while lower loading frequency applied, the stress concentrated sites sustain corrosive attack. As a result, Clwould penetrate through micro-crack and corrode the plastic zone apparently at maximum loading especially. On the contrary, corrosive attack at high loading frequency corrodes the stress concentrated sites would not be as apparent as lower loading frequency under one cycle fatigue. Therefore, salt-fog would damage corrosion fatigue life of bolts, and environment factor would dominate fatigue at lower loading frequency while mechanical factor dominates corrosion fatigue at higher loading frequency.