Dongsun Lee

Observation of small fatigue crack growth behavior in the extremely low growth rate region of low carbon steel in a hydrogen gas environment

To investigate the effects of hydrogen on crack propagation in the extremely low growth rate range, fully reversed bending fatigue tests were performed on low carbon steel (JIS S10C) in hydrogen and in nitrogen gas environments at a low pressure. A crack showed almost the same non-propagation behavior in nitrogen as that in air. However, a crack in hydrogen continued to propagate even near

Effects of gaseous hydrogen on fatigue crack growth behavior of low carbon steel

10^{7}

Initiation and propagation behavior of fatigue cracks in 5056 aluminum alloy studied by rotation-bending tests with smooth specimen

107 cycles in the same testing strain range as that in nitrogen. In hydrogen gas, a crack grew intermittently by coalescing with a new micro-crack generated by slip behavior. This implies that hydrogen could inhibit the action of any factor affecting non-propagation.

Effects of Hydrogen Gas Environment on the Extremely Low Rate Fatigue-Crack Growth Behavior from a Small Hole in JIS S10C

In recent years, increasing attention has been paid to the effect of a hydrogen environment on the fatigue limit of hydrogen-power systems and infrastructure. In carbon steel, strain aging is one of the important factors influencing non-propagating crack behavior, which is related to the fatigue limit. In the present study, to investigate the effects of hydrogen on the strain aging of low-carbon steel (0.13 % carbon steel), Vickers hardness tests were carried out on the carbon steel. A couple of 0.13 % carbon steel specimens with a large-scale plastic zone were aging heat-treated; one was a hydrogen-charged specimen, whereas the other was an uncharged specimen. The Vickers hardness of the hydrogen-charged specimen was found to be lower than that of the uncharged specimen. This observation implies that hydrogen inhibits strain-aging hardening of low-carbon steel. The observation also suggests that hydrogen could affect the non-propagating crack behavior through the inhibition of strain aging.

Fatigue limit in 5056 aluminum alloy extrusion studied by rotating-bending tests

In order to investigate the effects of hydrogen on the fatigue crack growth behavior of low carbon steel JIS S10C, bending fatigue tests were carried out using a specimen with a small blind artificial hole in a low pressure pure hydrogen gas atmosphere. The results show that the fatigue crack growth rate in hydrogen gas is higher than that in nitrogen gas, moreover, the degree of acceleration is greater in the high strain range. In fractography, intergranular facets mixed with ductile fracture and quasi-cleavage fracture with brittle striations appear in a hydrogen gas environment, while only ductile fracture mainly appears in nitrogen gas. In the low growth rate range, many intergranular facets are seen on the ductile fracture surface, and in the higher growth rate range, quasi-cleavage facets increase as the growth rate increases. The growth rate of a small crack in nitrogen gas can be expressed by dl/dN ∝ Δep n l in the wide range of applied total strain range Δet . The same type equation is also satisfied in hydrogen gas, but in the narrow range roughly from Δet = 0.25% to Δet = 0.37%. The fracture surface in this range shows only intergranular facets and a ductile morphology, but no quasi-cleavage fracture. Although the crack growth mechanism in hydrogen is different from that in nitrogen, observation of the mechanism of intergranular facet formation shows a similarity to the mechanism in nitrogen in which the slip-off mechanism of crack growth is valid. The formation of intergranular facets is also closely related to the slip behavior influenced by hydrogen. This means that there exists a high possibility for the application of the small crack growth law inhydrogen to not only S10C, but also to other carbon steels in which the intergranular facet appears.Copyright