Simulation of Cracking Behavior in Solid Oxide Fuel Cells;固態氧化物燃料電池破裂行為模擬研究

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題名:	Simulation of Cracking Behavior in Solid Oxide Fuel Cells;固態氧化物燃料電池破裂行為模擬研究
作者:	凱魯南;Anam,Khairul
貢獻者:	機械工程學系
關鍵詞:	ABAQUS;固態氧化物燃料電池;應力強度因子;熱應力;破裂;ABAQUS;Crack;Solid oxide fuel cell;Stress intensity factor;Thermal stress
日期:	2013-01-25
上傳時間:	2013-03-25 16:20:14 (UTC+8)
出版者:	國立中央大學
摘要:	在較高的工作溫度下，固態氧化物燃料電池(SOFC)可增加其能源轉換效率，然而在高溫下，各零組件間的交互反應可能導致材料的強度減弱，除此之外，SOFC在高溫下運作會因熱膨脹係數不匹配與溫度梯度產生熱應力，過大的熱應力將造成結構上的破壞，此二因素都將影響SOFC的可靠度。本研究主旨是計算電池片(陽極-電解質-陰極)在不同工作階段的應力強度因子，計算時將考慮裂縫形態，並考慮實際使用時，週期運轉對應力強度因子的影響。利用ABAQUS專業級有限分析軟體分別計算在常溫與工作溫度下，不同的表面裂縫形狀(半圓形、半橢圓形)與裂縫大小(1 μm、10 μm、100 μm)時的應力強度因子，並計算20次熱循環以了解運轉週期對應力強度因子的影響。根據最大主應力、最大x方向正應力σ_xx、最大y方向正應力σ_yy選定臨界應力區，最大主應力發生在常溫下，其值為53.16 MPa，x與y方向的最大正應力發生在工作溫度下，分別為28.88 MPa、16.76 MPa。應力強度因子計算結果顯示，對於較尖銳的半橢圓形裂縫，裂縫成長趨勢為向表面旁邊擴展，而較淺的半橢圓裂縫，裂縫成長趨勢為向中間深處延伸。常溫下，隨著循環數增加，應力強度因子逐漸減小，在高溫時，應力強度因子則會小幅度的增加，此乃應力場隨著熱循環作用變化所致。在本研究中所計算的應力強度因子皆小於其對應材料的破裂韌性，預估將不會發生破壞。The high operating temperature enables solid oxide fuel cells (SOFCs) to obtain a high efficiency of energy conversion while accompanying concerns such as degradation of materials resulting from undesirable reactions between components. Structural durability of an SOFC is affected by the thermal stress caused by considerable CTE mismatch between components and thermal gradient. Excessive thermal stresses may lead to fracture of components endangering the mechanical integrity of an SOFC stack. The main objective of this study is to calculate the stress intensity factor of positive electrode-electrolyte-negative electrode (PEN) at various stages. The effects of crack size, crack type, and thermal cycles are considered in calculating the stress intensity factors at the highly stressed regions in PENs.A commercial finite element analysis (FEA) code, ABAQUS, was used to find the highly stressed regions in PENs and calculate the stress intensity factors. The stress distributions are calculated at uniform room temperature and at steady stage with a non-uniform temperature profile. The stress intensity factors for various combinations of surface crack type (semicircular and semi-elliptical) and size (1 ?m, 10 ?m, and 100 ?m) are calculated at room temperature and steady stage. For thermal cycling, the stress intensity factor is repeatedly calculated for twenty cycles at the location having the greatest maximum principal stress and for five cycles at the location having the greatest maximum normal stresses in the x and y directions, σxx and σyy, respectively. Three critical stress regions are identified based on the maximum principal stress, maximum σxx, and maximum σyy. The maximum principal stress is of 53.16 MPa in principal direction of -44.04o at room temperature. The maximum σxx is of 28.88 MPa and σyy is of 16.76 MPa both at steady stage. The CTE mismatch between components and the temperature difference between room temperature and steady stage are the two major factors in generation of thermal stresses in the planar SOFC stack. Thermal stress fields induced by these two factors play a major role in determining the stress intensity factors for various surface cracks placed at selected locations. The principal direction also has an effect on the stress intensity factor for the critical region with the greatest maximum principal stress. For a sharp, semi-elliptical surface crack, it tends to grow wider rather than deeper. On the contrary, for a shallow one, the crack tends to grow deeper rather than wider. Variations of stress field during thermal cycling dominantly determine the variations of KI. The stress intensity factor for the critical region of maximum principal stress is significantly decreased at room temperature and is slightly increased at steady stage with increasing number of thermal cycle. All the calculated stress intensity factors in the present study are less than the corresponding fracture toughness given in the literature, ensuring the structural integrity for the given planar SOFC stack.
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