有限元素三維疲勞裂紋閉合分析

疲勞裂紋中的裂紋閉合現象對於裂紋的成長有許多重要的影響，過去許多學者對於疲勞裂紋閉合現象的分析最常使用的方法即為有限元素計算。本研究對於一個三維構件含中央直線裂紋建立有限元素模型，進行彈塑性裂紋閉合分析。分別考慮元素細化、應力比、構件厚度、節點釋放時機和單一過載重等因素對於裂紋閉合所造成的影響。結果顯示在裂紋前端塑性區內之元素數目仍以最少10個為佳；應力比R的值愈高，裂紋閉合效應愈小；試體表面狀態影響範圍在考慮的載重下約可達深度0.8mm處，且不受試體厚度的影響；至於模擬裂紋成長的裂紋前緣節點釋放時機的影響，於加載過程中釋放節點的開啟應力較小，且卸載過程中釋放節點的開啟應力則較大；當裂紋受到單一過載重時，施加過載重後開啟應力立刻減少，隨裂紋繼續成長開啟應力開始增加，且在成長一段距離之後達到最大，此現象在試體表面及內部均相同。

關鍵字

裂紋閉合；有限元素分析；金屬疲勞；彈塑性分析

並列摘要

Crack closure of fatigue cracks has many important implication for the growth of cracks. In the past, many researches analysis of crack closure used finite element technique, the most commonly used method. For this study, using finite element model of a three-dimensional specimens including a middle tension crack for elastic-plastic analysis of crack closure. Considering the effects of element refinement level, stress ratio, thickness of specimens, crack advance scheme, and a single overload for crack closure analysis. The finite element analysis result showed that the number of elements in the forward plastic zone of crack tip at least ten is better; that the stress ratio R is higher, then the crack closure effect is smaller; that at the considering load the plane-stress condition may influence up to the depth of about 0.8mm from the free face, and no matter the thickness of specimens; that the crack front nodes released at loading acquires smaller crack opening stress, and released at unloading acquires larger crack opening stress; that the crack after the overload the opening stress immediately reduce and the opening stress increasing after the crack continue to grow, and growing some distance to achieve the maximum, this phenomenon is the same in both the surface and the interior.