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To investigate the fatigue characteristics and the frecture feature of Ti-6Al-4V in very high cycle region, uniaxial tension fatigue tests were carried out. The S-N diagram showed a complex folding shape, which corresponded to the locations of the fracture origins; i. e., surface-originating fractures in high stress level and interior-originating fractures in low stress level. In this study, observed also were interference type fractures in which the main crack was formed by the coalescence of a surface-originating crack and interior-originating cracks in low stress level.

A new surface modification technique, scanning cyclic press (SCP), was developed. SCP scans a metal surface with a vibrating indenter under precise loading control based on servo fatigue testing machine and can apply a variable cyclically compressive load. This study applied SCP to magnesium alloy AZ31 to investigate the effect on fatigue properties. After applying SCP, the surfaces of specimens were observed by using a laser scanning microscope and uniaxial push-pull fatigue tests were conducted. As a result, surface roughness of SCP-treated specimens slightly increased; however, the fatigue life became longer than that of untreated specimens. To clarify the reason for the improvement effect, fracture surfaces of the specimens were investigated in detail on the basis of SEM observation. SEM observation showed differences between the fracture surfaces of the untreated and SCP-treated specimens. The origin of fracture was at the surface in the untreated specimen. In the SCP-treated specimen, however, the fracture origins were sub-surface and a band-like layer just beneath the surface was observed on fracture surface, whereas the layer did not exist on that of untreated specimen. The surface hardness of SCP-treated specimen increased to almost twice as much after applying SCP. The result suggests that SCP modified surface layer of specimen and this layer suppressed crack initiation from surface.

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