| Summary: | 碩士 === 國立東華大學 === 材料科學與工程研究所 === 89 === Abaqus® is a widely used finite element analysis software in engineering applications. With aid of user’s subroutine, it can simulate very complicated problems in materials processing, e.g. plastic deformation and creep. With the rapid advancement of computer hardware, today’s high-end personal computer performs as well as workstation or mainframe before. In this study, residual thermal stresses are primarily resulted from volume change of the steel upon rapid heating and cooling cycle during localized high power CO2 laser heat-treating. The CO2 laser heat treated area are mainly comprised of tensile stress, and the tensile stress will be gradually converted into compressive stress as the distance away from the heat treated area. It is found that crack tip has a maximum compressive stress. Further increase the distance from heat treated area will result in decreasing the magnitude of compressive stress. The compressive S11 stress ahead of the crack tip will be increased as the crack tip grows outside the heat-treated area. Therefore, it is observed that the growth rate of crack tip outside the heat-treated area is decreased in the experiment. Thus, the used of laser surface treating technique can effectively prohibit the nucleation and growth of fatigue crack outside the heat treated area if there is no phase transformation in the steel. As the fatigue crack tip entering into the heat-treated area, S11 in the surface layer ahead of crack tip is converted into tensile stress, but the central layer still remains compressive stress. The stresses ahead of the crack tip are compensated each other. Consequently, it is observed that the growth rate of the crack tip is not increased in the experiment, and it is similar to that of base metal.
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