Fracture mechanics studies on delayed fracture have been done extensively, but they are mainly concerned in a through crack in CT or WOL specimens and little concerned in a three dimensional surface crack. The surface cracks are often seen in actual structures. In the present work, surface crack growth in delayed fracture in tap water with cathodic charging has been examined by using the bend type specimens of AISI 4340 steels (JIS SNCM 439) tempered at 200°C and 400°C. The results obtained are summarized as follows;
(1) The shape of growing surface crack is semi-elliptic. Here, the crack length takes a maximum value at a little inside of the specimen surface. Comparing the shape of the delayed crack with the one of the fatigue crack, both are nearly the same for the material tempered at 400°C, but the shape of the delayed crack becomes flatter than the fatigue crack for the material tempered at 200°C. The reason should be attributed to the fact that the material tempered at 200°C is more sensitive to hydrogen than the one tempered at 400°C.
(2) Reflecting the result of (1), for the material tempered at 200°C, the crack growth rate in the direction of surface, db/dt, shows a higher value than the one in the direction of depth, da/dt.
(3) The values of delayed fracture toughness, Kdc, in the region of fast fracture and of threshold stress intensity factor in crack growth, KSCC, are lower in the case of surface crack than in the case of through crack.
