複合材料風力葉片之有限單元衝擊分析

Abstract

複合材料本身對於垂直纖維面上之衝擊的抵抗能力相當微弱，很容易受動態與衝擊負荷而在內部產生無法察覺的破壞，進一步將影響結構整體的安全性，故本研究目的將利用有限元素分析軟體ABAQUS搭配材料用戶子程序VUMAT，利用快速且有效的分析方法分析衝擊作用，預估結構受衝擊後的破壞行為，並利用衝擊實驗做驗證。 本研究先對複合材料平板進行衝擊分析，為了觀察三個方向的應力來檢視纖維破壞與脫層現象，將平板模型利用solid元素建模，其中，利用一套材料剛性退化程序，當材料應力超過極限強度時，降低該破壞元素92%的剛性，來模擬實際材料破壞後沒有足夠能力來承受負載，並經過衝擊實驗，結果驗證分析的正確性。由於solid元素在分析軟體的運算量較大，所以為了簡化分析軟體的運算量，本研究提出solid與shell兩種元素的結合來建立平板模型，將接觸面與可能破壞之區域建立solid元素，而其他區域建立shell元素來簡化分析軟體運算量，並利用相同的材料剛性退化程序，經分析與實驗結果顯示此方法的可行性。 確認了此方法的可行性後，將此方法運用於風力葉片的建模並進行衝擊分析，結果顯示蒙皮的應變變化和破壞行為與實驗情形吻合。驗證了分析的準確性。此研究將利用分析，探討風力葉片對不同位置進行衝擊的結果，分別對六層區域與六層三層疊層交界區域兩處進行比較，結果發現在相同衝擊速度下，對疊層交界改變處之蒙皮應變速率較快、承受的應力較大，導致疊層交界處較容易損壞，其中，環向破壞發生在六層區域的最內側，而軸向破壞是由三層區域之內側往六層區域第三層逐漸破壞。 不管是應用在複合材料平板或是風力葉片，利用本研究solid與shell元素結合的分析方法可以有效率且準確地預估受衝擊後的破壞行為。主要貢獻為利用此分析方法可以提升分析效率，未來可以應用在其他模型，也提供未來在風力葉片的設計，包括葉片蒙皮疊層、蒙皮尺寸、腹板位置與數量來改善葉片之耐衝擊性，提高風力葉片的安全性。

A composite structure is very weak to against the impact normal to the structure surface. The damage induced by a dynamic impact is not easy to be detected and thus may affect the overall safety of the structure. The purpose of the study is to propose a simple and yet effective finite element method via the software ABAQUS with subroutine VUMAT to analyze the impact behavior of a composite wind blade and predict the damage in the blade. An experimental study is performed to validate the feasibility and accuracy of the proposed method. First, the composite plate will be analyzed under impact using the finite element method in the study. The solid elements will be used to model the plate for determining the stress components in three directions to detect the failure induced in the plate. In the analysis, the progressive degradation of material stiffness will be considered, reducing 92% of the original stiffness of the damaged element when any of the stresses exceeds the corresponding ultimate strength. The results of the impact experiment have validated the correctness of the analysis. In order to simplify the finite element analysis, both solid and shell elements will be used to model the plate. The impact region of the plate which is in the vicinity of the impact area will be modeled using solid elements while the rest of the plate modeled using the shell elements. Using the same degradation method of material stiffness, the small differences between the theoretical and experimental results also validate the feasibility of the simplified method. After confirming the feasibility of the proposed method, the impact of a 2.5m wind blade is analyzed using the method. In the impact test, the blade was fixed at the root like a cantilever beam. The theoretical and experimental strains are in closed agreement. The effects of impact location are then studied via both experimental and theoretical approaches. The impact locations at the six-layer portion and the intersection of the six- and three-layer of the blade are considered in the damage analysis. Under the same impact velocity, it has been shown that damage is more likely to occur at the intersection of the six- and three-layer. The bottom layer of the three-layer portion will likely to fail first before delamination at the interface between the first and second layers. It has been shown that for both composite plate and wind blade, the proposed method of using both solid and shell elements can efficiently and accurately predict damage after impact. The proposed method can be extended to the impact analysis of other composite structures such as composite sandwich plates, composite tubes/tanks, large composite wind blades, etc.