| Summary: | 碩士 === 中正理工學院 === 兵器系統工程研究所 === 85 === Impression process and shot peening were employed for crack-tip buildup of residual stresses. Such a stress system enhances the crack closure for reduction of crack driving force and therefore for the retardation development. The compact tension specimens of AISI 304 stainless steel were utilized in the research. In impression process, the 6kN load was used for a better insight into the retardation effect of various impression systems at different crack length positions. Of current impression systems, the simultaneous impressions play a leading role, and the bi-ditch impressions is second to any other system. At similar crack size, the two-side impressed specimen didn''t fully show the expected superiority over the single-side impressed specimen in retardation performance. Better retardation effect occurred ahead of crack-tip when bi-impressions system was applied to both of the specimen sides, and the number, Nd, of delay cycles increased rapidly while the impressions are closer to each other. Values of Nd could be up to 2,000,000, much greater than that value of 30,000 with the crack-tip impression. In shot peening the intensity was kept constant at 12C, and the test specimens were peened on three regions. The retardation effect was minor in the ahead-tip region and reached its climax in the behind-tip region.
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