| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 94 === Abstract
The application of the strain distribution analysis plays an important role at fracture mechanics, especially in plasticity and stress concentration. It’s known that the technique of strain measurement can be used to observe strain change at interested positions. Raman-Spectroscopy can also be achieved non-contact to determine the strain field over the surface. However, it can not be generally used in areas of research and engineering, because of it’s high cost. In contrast to Raman-Spectroscopy, the digital optic measurement provides a cheap and precise measurement method to analysis strain distribution. The digital optic method is based on the matching algorithm. The matching algorithm by using the digital-image-correlation technique tries to find, within defined search boundaries, the best position of the corresponding window in the second image by comparing the matrices of the gray scale.
In this study a novel image processing is used to analysis the plastic deformation zone of a pure aluminum single crystal after indentation test. In order to investigate the precision of the DIC software in terms of strain, a series of experiments was systematically conducted. These experiments were the rigid body displacement in the X and Y direction, the rigid body rotation, the spatial resolution of the DIC software and the influence of the mark covering area. After that, this DIC software were used to investigate the deformation pattern after indentation test.
The experiments of the precision of the DIC software show the following results. Firstly, the strain errors of the rigid body displacement in the X and Y direction can be from the order of 10-4 to 10-5. Secondly, when the rigid body rotation angle is changed from 0° to 32°, the strain error is in the order of 10-4. When the rigid body rotation angle is between 32° to 90°, the strain error is in the order of 10-3. Thirdly, the resolution of the average strain remains constant, and it is not influenced by changing the facet size. The standard deviation associated with each facet size is however increased with the reduced facet size in both X and Y directions. Lastly, the appreciable mark covering area in terms of the black/white ratio lies between 35% to 65%.
In addition to the investigation of the software’s precision, full strain field measurement was successfully applied to demonstrate indentation-induced plastic heterogeneity around a spherical indenter. There exists a different strain distribution observed from Von Mises strain mapping around the spherical indenter. The strain distribution of the ploycrystall shows a homogenous deformation zone with a constant width around the indenter, i.e., regarded as isotropic deformation. In contrast to the polycrystalline, the single crystal does not reveal a circular form of the strain distribution. In contrast, it shows an irregular form of the strain distribution which indicates an anisotropic deformation pattern. In both cases of single crystal and polycrystal, a positive strain of εx and εy is observed in the central region and a negative strain of εx and εy is observed around this center. This strain gradient can be explained due to the effects of projection and real strains.
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