Environmental fatigue crack growth tests of a high-tensile strength steel HT 55 have been conducted in dry air and vacuum, and crack closure behavior as well as fracture surface morphologies were investigated. The crack growth rate da/dN was significantly reduced in vacuum due to fracture surface rewelding. da/dN in vacuum at R=0.1 was equivalent to the one at R=0.5 in case of ΔKeff greater than 15MPa·m1/2, whilst the former became higher than the latter at ΔKeff less than 15MPa·m1/2. Roughness-induced unconformity of fracture surfaces is considered to prevent the rewelding of newly exposed surface at low R. At high R, on the other hand, no crack closure and no prevention of the rewelding were observed, thereby reducing the growth rate particularly near the threshold region. The crack opening value Kop rised immediately and the growth rate decreased rapidly with the environmental change from dry air to vacuum. On the contrary, a rapid increase of da/dN as well as a rapid decrease of Kop was observed with the environmental change from vacuum to dry air. Fracture surface morphology also varied abruptly with the environmental change. It is concluded that an absorption of gas molecules to newly created surface affects directly the crack closure behavior. Clay-like products induced by a remarkable plastic flow and rewelding were observed on the fracture surface in vaccum.
