Constant maximum stress intensity (
Kmax) 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
Kmax 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 (Δ
Kth) obtained under hydrogen supply was equivalent substantially to that in room air with an exception of the region of
Kmax>
KISCC under hydrogen supply.
(2) Δ
Kth decreased linearly with an increase in stress ratio (
R). In this regard, the effective value of the threshold stress intensity factor range (Δ
Keffth) was constant, Δ
Keffth≈3MPa√m, in the region of
R<0.7. On the other hand, in the region of
R>0.7, Δ
Kth decreased with increasing
R. From these results, it is noted that the design based on Δ
Kth 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-
Kmax fatigue test is suitable for the determination of Δ
Kth.
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