| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 101 === 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 µm, 10 µm, and 100 µm) 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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