The effect of mean load on the fatigue crack growth behavior of a high strength steel has been investigated. The material used was SNCM 439 tempered at 200°C. The specimens were loaded in in-plane bending at a cyclic frequency of 2000 cycles per minute at various stress ratios (R) and mean stress intensity factors (Kmean) under hydrogen supply. The results of fatigue crack growth at various stress intensity ranges (ΔK) were arranged in terms of Kmean and maximum stress intensity (Kmax) in order to characterize the effect of mean load. At all ΔK levels, the acceleration of crack growth rate occurred beyond the level of Kmean=KISCC. In the region of Kmean<KISCC, the fatigue crack growth behavior depended on ΔK level, irrespective of Kmean level. On the other hand, in the region of Kmean>KISCC, crack growth was dominated by Kmax level. These results were also confirmed by fractographic observations.
