A B S T R A C TThe effects of hydrogen charge on cyclic stress-strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS-S45C) were studied. The total strain range of the stress-strain hysteresis loop in the hydrogen-charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen-charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen-charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen-slip band interaction may play an important role in the hydrogen-influenced metal fatigue.Keywords fatigue crack; hydrogen; hysteresis loop; localization; slip bands.
N O M E N C L A T U R E2a = crack length C H = hydrogen content f = test frequency N = number of cycles N f = number of cycles to failure R = stress ratio t = time after hydrogen charge ε t = total strain range ε = strain σ = stress σ a = stress amplitude
I N T R O D U C T I O NIt is known that fatigue failure can occur in the ultra long life regime of N > 10 7 in high strength steels and that the fracture origin is at a non-metallic inclusion contained in the material. Murakami et al. [1][2][3][4] pointed out that a typical morphology, named the optically dark area (ODA), exists around the non-metallic inclusion at the fracture origin and they have shown evidence that the formation of the Correspondence: H. Uyama.ODA is related to the hydrogen trapped by non-metallic inclusions. However, the exact mechanism of the formation of the ODA and the role of hydrogen has not been made clear.Recently, the demonstration and commercialization of fuel cell (FC) systems has led to the realization of their potential for solving the global warming problem. Many metallic components used in FC systems are directly exposed to hydrogen environments. The so-called 'hydrogen embrittlement' has been used to express the degradation of metals due to hydrogen. However, the
Effects of hydrogen charge on fatigue behaviour of two carbon steels, JIS-S10C (SAE1010) and JIS-S45C (SAE1045) were investigated. There was no hydrogen effect in the cyclic stress-strain hysteresis loops of S10C hydrogen-charged with 0.2 ppm. On the other hand, the strain amplitude was decreased in S45C hydrogen-charged with 0.8 ppm. The delayed yielding and the decrease in the saturated value of the strain amplitude were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C under the constant stress amplitude tests. It is supposed that the degree of influence of hydrogen on cyclic stress-strain properties depends or material structure and/or hydrogen content. The effect of hydrogen charge (H : 0.5 ppm) on the fatigue life, the fatigue limit and the crack growth curves of S45C were not remarkable, while there was a distinct difference in the morphology of the slip bands between the hydrogen-charged and uncharged specimens. The localized slip bands were observed in the hydrogen-charged specimen of S45C. Therefore, it is presumed that the decrease in the strain amplitude in hysteresis loop by hydrogen charge is caused by the localization of slip bands. More crack initiations from ferrite grains were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C. This phenomenon also corresponds to the localization and the formation of slip bands.
An ignition point of a non-combustible Mg alloy is about 300K higher than that of a Mg alloy currently in use. It is expected that the non-combustible Mg alloy can be used as a substitute for Al alloys as structure components. In this paper, the fundamentals, the notch effect and the long or small crack effect on fatigue strength of the non-combustible Mg alloy, are investigated. Moreover, the fatigue strength characteristics of the non-combustible Mg alloy are compared with that of an Al alloy to clarify the differences in the fatigue strength characteristics between the non-combustible Mg alloy and Al alloy.
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