Abstract
In order to examine the feasibility of applying synchrotron radiation μCT imaging to alloy steels for non-destructive inspection of inclusions for potential origins of internal fatigue damage in the very high cycle region, synchrotron radiation μCT imaging was utilized for repeated non-destructive observation of Cr-Mo steel. An ultrasonic fatigue testing machine was used in aid of the repeated observation. As a result, it was found that the synchrotron radiation μCT imaging with 70 keV was useful for non-destructive observation of inclusions of more than 10 μm, one of which may be an internal fatigue origin. No identifiable damage was observed around every inclusion, and in the base metal, at least up to 70% of fatigue life was observed in the imaging volume.
Highlights
Alloy steels may suffer from a fatigue fracture called a “Giga cycle fatigue” or a “Very high cycle fatigue,” which is a fatigue phenomenon as a result of cyclic loading in excess of 107 cycles
As mentioned in Introduction, it is thought that most of the fatigue life is consumed by the formation of optical dark area (ODA) and the crack growth rate in ODA is very slow, whereas penny-shaped microcracks around many inclusions nucleate in the early stage of very high cycle fatigue [4,5,6]
The second possible reason is that fatigue damage occurs just before the final break, and so there is no damage around inclusions in the μCT images
Summary
Alloy steels may suffer from a fatigue fracture called a “Giga cycle fatigue” or a “Very high cycle fatigue,” which is a fatigue phenomenon as a result of cyclic loading in excess of 107 cycles. The fatigue fracture in the very high cycle region often originates from an inclusion such as metallic oxide. A small, dark area, whose diameter is a few 10 μm, is observed around the inclusion when optical microscopy is used, and a fine granular fracture surface was observed in the dark area if scanning electron microscopy is used. The circular area is referred to as a “fish eye” [1], and the dark area in the center of a fish eye is referred to as “optical dark area (ODA)” [2] or “Fine granular area (FGA)” [3]. The diameter of ODA is a few tens μm
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