Kyohei Kawamoto

Fatigue crack growth characteristics and effects of testing frequency on fatigue crack growth rate in a hydrogen gas environment in a few alloys

In order to investigate the hydrogen effect on fatigue crack growth (FCG) behavior in a few kinds of practical alloys; austenitic stainless steels (solution-treated metastable type 304 and stable type 316L), an aluminum alloy (age-hardened 6061) and a low carbon steel (annealed 0.13%C-Fe), FCG tests were carried out in hydrogen gas and in nitrogen gas. The FCG rates of these materials are enhanced by hydrogen, though the acceleration degrees are different. A crack grows across grains by slip-off in 316L stainless steel and in age-hardened 6061 aluminum alloys even in hydrogen. Faceted area increases in 304 stainless steel and in low carbon steel in hydrogen. In 304 stainless steel, the ratio of facets to the entire fracture surface was not so large. Thus, the FCG rate is not significantly affected through the facets in 304 stainless steel. In low carbon steel, facets were increased considerably, though a crack grows step by step or after a large number of loading cycles even along grain boundaries. Anyhow hydrogen enhances the FCG rate of these materials through the influence on slip behavior. Based on above-mentioned results, the effect of loading frequency on FCG rate in hydrogen of the age-hardened 6061 aluminum alloy was also investigated. The FCG rate increases as the testing frequency decreases, though the FCG rate in hydrogen shows the tendency to saturate.

Laser welding conditions to improve tensile strength and fatigue strength of a lap joint in aluminum alloys

This research shows a procedure for seeking a domain of conditions under which the tensile strength and fatigue strength are improved in lap joints of aluminum alloys made by laser welding. Laser welding parameters involve the following three factors: gap width, welding speed, and laser power. The following conditions were obtained in this research. (a) When searching for favorable welding conditions, the required work involves only observing the states around the weld-zone. (b) In order to improved the fatigue strength, it is necessary to prevent the generation of lack of fusion by the shortage of heat input and a crack near the toe of the weld due to the excess heat input. Using the results, the above-mentioned simple method is shown.

Effects of Hydrogen Environment on Fatigue Characteristics of 18Cr-8Ni Stainless Steel

In order to clarify the effects of hydrogen on the fatigue characteristics of an austenitic stainless steel, bending fatigue tests were conducted in air, in a hydrogen gas and in a nitrogen gas. Main results obtained are as follows. Effects of hydrogen gas environment are not clearly seen on the strain range - fatigue life diagram, because there are opposite effects to crack propagation and to crack initiation; accelerates crack propagation, but retards crack initiation. Striation spacing or in-situ observation confirms the acceleration. The retardation seems to be attributed to the absence of oxygen or water vapor in the hydrogen gas.