Fatigue Crack Growth Rates In Hydrogen Research Articles

The role of hydrogen in promoting differences in fatigue crack growth rates observed in Type 304/304L stainless steel at elevated temperature

Hydrogen has been postulated to affect fatigue crack growth rates (FCGRs) in Type 304/304L stainless steel during exposure to high purity water (HPW) environments at elevated temperature. To assess this mechanism, FCGR testing was performed at elevated temperature (260°C) in air and pressurized hydrogen gas. Initial results indicate the measured FCGRs in hydrogen are up to 8x lower than those measured in air under comparable fatigue loading conditions. Post-test evaluation of compact tension specimens indicates crack mouth opening displacements and fracture surface morphologies generated in hydrogen that are consistent with those generated in HPW environments. Observations of crack tip blunting suggest the reduced FCGRs in elevated temperature hydrogen occur due to a decrease in the effective ΔK. Finite element plasticity modeling, using a hydrogen-dependent nonlinear kinematic hardening approach, supports the hypothesis that crack tip blunting and retarded FCGRs are due to a hydrogen-based change in cyclic behavior (e.g. cyclic creep) occurring ahead of the fatigue crack tip.

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.

3511 SUS310S鋼の疲労き裂伝ぱ速度に及ぼす水素雰囲気の影響およびその周波数依存性(S20-3 水素ガス雰囲気での疲労特性,S20 材料強度特性に及ぼす水素の影響)

In order to investigate whether hydrogen enhance the fatigue crack growth (FCG) rate of stable austenitic stainless steels or not, FCG tests of a type 310 steel were performed in a hydrogen gas environment and in a nitrogen gas environment. The FCG rate tested at 5.0Hz in hydrogen is higher than that in nitrogen. It seems that the acceleration of FCG rate in hydrogen does not necessarily need the formation of strain-induced α' martensite, though it increases the degree of the acceleration. The cause of the acceleration of type 310 seems to be hydrogen enhanced concentration of slip at a crack tip. The degree of the acceleration became smaller in a frequency of 0.1Hz. It seems that the generation of heat at the crack tip concern. Further investigation is necessary.

Crack growth behaviour of heat-treated 4140 steel in air and gaseous hydrogen

The fatigue crack growth rates (FCGRs) of AISI 4140 steel specimens which were subjected to various heat treatment procedures were performed on compact-tension specimens both in air and gaseous hydrogen. Experimental results indicated that the acceleration of crack growth in hydrogen was more pronounced for QT (quenched and tempered) specimens tempered in the range of 230 to 370°C as compared to those specimens tested in air. Tempering in such a temperature range coincided with the `one-step temper embrittlement' range of the 4140 steel. For specimens tempered at a higher temperature, e.g. 550°C, the influence of hydrogen on FCGRs became insignificant. In case of specimens austempered (AT specimens) at 230–300°C, the yield strength could reach approximately 1500 MPa with reduced gaseous hydrogen embrittlement (GHE) in fatigue tests as compared to QT specimens of the same strength level. The extent of accelerated FCGRs in hydrogen or the susceptibility to GHE could be related to the fracture surface of specimens covered by brittle features. The change in fracture modes of several heat-treated specimens in hydrogen, relative to the fracture behaviour in air, was also discussed.