PSI - Issue 20

Valeriy Lepov et al. / Procedia Structural Integrity 20 (2019) 57–62 Valeriy Lepov et al. / Structural Integrity Procedia 00 (2019) 000–000

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a

b

Fig. 3. Fracture patterns characterized the fatigue process: (a) numerous branching; (b) radial near the notch.

Almost on all samples near the notch the thin relief of the radial center of fracture (Fig. 3b) characteristic of low temperatures is observed. The fractography analysis has shown that the source of the radial fracture pattern in this case is the globular particle located in grain structure of steel. It is obvious that the spheroidization of non-metal imperfection has happen under the thermal influence during the variable cyclic loading. Most of the samples have the brittle rocky transgranular fracture, revealing the numerous steps and grooving patterns, splitting on facets and a set of secondary microcracks (Fig. 4a and 4b) in the grain body is observed. Nevertheless at the prevalence of brittle fracture in the majority of the studied samples the mixed type of fracture is observed. This type of fracture has along with the chips patching and finely porous structure in the crest form or a chipping wall has been found. Also the intergranular pore at cavities formation typical for the accelerated stage of a fatigue crack growth (Fig. 5a) is revealed.

b

a

Fig. 4. Brittle fracture and microcracks: (a) transgranular in the grain body; (b) typical for fatigue fracture.