Sustained load tests were conducted on a high-strength steel SNCM 439 sensitive to hydrogen embrittlement with or without vibratory stresses of small amplitude, superimposed. Furthermore, varying load tests (stress ratio R being near zero) were carried out to compare with those mentioned above. The relation among static SCC under sustained load (R=1.0), cyclic SCC under varying load (R being near zero) and dynamic SCC under high-cycle vibratory stresses superimposed (R being near unity) were discussed, in contrast with those in 7N01 A1 alloy8) sensitive to active path corrosion type SCC. Similarly as those in 7N01 alloy, the threshold stress intensity factor Kmax=KDSCC considerably decreased from KISCC at R=1.0. However, KDSCC at a stress ratio of 0.95 hardly decreased from KISCC. On the other hand, in the Region II, where a subcritical crack growth appears, the crack growth rate in 7N01 alloy increased with a decrease of R. However, in SNCM 439 steel the smaller the stress ratio was, the slower the crack growth rate was. In SNCM 439 steel static SCC was dominant in the Region II just the same as cyclic SCC for the material sensitive to hydrogen embrittlement5).
