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Bicrystals of Al2 O3 were fabricated to study the effects of thermal expansion anisotropy on fracture in a model system containing one grain boundary. Fractures occurred perpendicular to the directions of maximum tensile stress in bicrystals with thermal expansion coefficient differences as low as 0.31×10−60 C−1 and originated at the boundary, probably in areas of high residual stress associated with pores. Numerical stress analysis of two-dimensional model bicrystal configurations showed that stresses induced by thermal expansion anisotropy are maximal in a region localized along the bicrystal boundary. 相似文献
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A. MAR M. A. HICKS 《International journal for numerical methods in engineering》1996,39(23):3969-3983
Benchmark solutions are presented for a simple linear elastic boundary value problem, as analysed using a range of finite element mesh configurations. For each configuration, various estimates of local (i.e. element) and global discretization error have been computed. These show that the optimal mesh corresponds not only to minimization of global energy (or L2) norms of the error, but also to equalization of element errors as well. Hence, this demonstrates why element error equalization proves successful as a criterion for guiding the process of mesh refinement in mesh adaptivity. The results also demonstrate the effectiveness of the stress projection method for smoothing discontinuous stress fields which, for this investigation, are more extreme as a consequence of the assumption of nearly incompressible material behaviour. In this case, lower order smoothing produces a continuous stress field which is in close agreement with the exact solution. 相似文献
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The widespread Mexican apple snail Pomacea flagellata (Say 1827) and the strictly endemic
"tegogolo" P. patula catemacensis (Baker 1922) (restricted to Lake Catemaco), are the only known American
Ampullariidae that have haploid complements n=13. Pomacea patula catemacensis has suffered a critical
reduction in abundance due to immoderate fishing for human consumption. Chromosome slides were obtained from colchicine-injected Pomacea snails collected from nine locations along the coastal zone of the
Gulf of Mexico, including Lake Catemaco, for use in principal component analysis (PCA). Total proteins in
foot homogenates were analyzed through isoelectric focusing (IEF) and native-PAGE electrophoresis on polyacrylamide gels. The chromosome number 2n=26 was confirmed for snails from all locations, with a uniform
9 m + 4 sm formula. However, P. patula catemacensis showed significantly larger chromosomes (absolute
size) than any population of P. flagellata. Pomacea patula catemacensis also differed from all populations of
P. flagellata in a PCA with standardized data, i.e., independently of the absolute size difference between
species. Proteins with an acid isoelectric point were dominant in the foot of both species. The electrophoresis
analysis showed that P. flagellata has 17 protein bands, with an upper bound at IEF=7.6, while P. patula
catemacensis has only 15 bands, with an upper bound at IEF=7 and a more evenly spaced band pattern.
Molecular weights ranged from 40 to approximately 130 kDa in both species. Proteins with high values (>94
kDa) were the most abundant. Pomacea patula catemacensis showed a band of 93 kDa, which was absent
from all specimens of P. flagellata. Samples of P. flagellata did not cluster according to any geographical
pattern in the statistical analyses, nor did they show any taxonomically useful differences in their electrophoretic patterns that merit sub-specific discrimination. 相似文献
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The goal of this study was to determine the effect of putrescine, added to the culture medium, on the in vitro development of stenospermic grape (Vitis vinifera L) embryos. The cross breedings of Perlón x G.C88552 and Perlón x Argentina were used. 0 (control), 2 and 4 mM of putrescine were added to the immature seed’s culture medium. In Perlón x Argentina, 2mM of putrescine statistically increased the percentage of total embryos, direct germination, polyembryos and normal plants. In Perlón x G.C88552, only 2 mM of putrescine increased all the variables considered, eventually tripling the percentage of normal plants obtained. The results suggest that the endogenous concentration of putrescine may be a growth limiting factor. Adding putrescine to the culture medium of immature grape seeds is a legitimate resource to significantly increase the results of this technique. 相似文献