Abstract
In order to clarify the influence of moisture in air on fatigue crack propagation characteristics at stage 2 of high strength steel (SNCM439) whose tensile strength was about 2000MPa, fatigue tests were conducted under stress ratio, R, of 0.1-0.8 in wet air (relative humidity, RH, of 20-80%) and dry air (RH=0%), and the fracture surfaces were observed by SEM.
The following results were obtained:
(1) The acceleration of crack growth with moisture in air (RH=20-80%) was recognized in the high ΔK region under R=0.1-0.8. This phenomenon was due to the hydrogen embrittlement.
(2) The degree of crack growth acceleration was influenced by stress ratio and depended on both ΔK and Kmax.
(3) The thickness effect of specimen was not recognized on the crack growth acceleration.
(4) The crack growth acceleration was different between two charges. This difference might be caused by the amount of penetrating hydrogen and the diffusivity of penetrating hydrogen.
(5) The fatigue fracture toughness was influenced by the pre-austenitic grain size.
