Near-Threshold Characteristics in Fatigue Crack Growth of High Strength Steel under Hydrogen Supply

Abstract

Constant maximum stress intensity (K_max) fatigue tests were carried out to obtain the near-threshold characteristics in fatigue crack growth of high strength steel. Specimens were loaded in in-plane bending at various K_max under hydrogen supply and in room air. Fractographic observations were made on the fracture surfaces at the near-threshold regions.
The results obtained are as follows;
(1) The threshold stress intensity factor range (ΔK_th) obtained under hydrogen supply was equivalent substantially to that in room air with an exception of the region of K_max>K_ISCC under hydrogen supply.
(2) ΔK_th decreased linearly with an increase in stress ratio (R). In this regard, the effective value of the threshold stress intensity factor range (ΔK_effth) was constant, ΔK_effth≈3MPa√m, in the region of R<0.7. On the other hand, in the region of R>0.7, ΔK_th decreased with increasing R. From these results, it is noted that the design based on ΔK_th obtained at lower R is dangerous even in room air.
(3) On the fractographic observations at the near-threshold region, the striation-like pattern occupied the major portion of the fracture surfaces in the region of R<0.7. On the other hand, in R>0.7, the proportion of intergranular facet increased with R.
(4) It is confirmed that the constant-K_max fatigue test is suitable for the determination of ΔK_th.