What is the minimal acceptable defect size before crack propagation in nitrided and carburized steels?
Dear Dr. Jean-Baptiste Libot,
The data below may be helpful:
Threshold Stress Intensity Factor
Both plots show the threshold stress intensity factor (KTH) as a metric for hydrogen embrittlement, where KTH is a fracture mechanics property that characterizes material resistance to subcritical crack propagation.
https://www.sciencedirect.com/topics/engineering/threshold-stress-intensity-factor
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A novel fracture mechanics technique has been employed for the determination of threshold stress intensity factor (K Iscc) for stress corrosion cracking (SCC) using small circumferential notch tensile (CNT) specimens. The technique was applied successfully for testing SCC susceptibility of heat treated 4340 steel in 3.5% sodium chloride solutions at room temperature. The K Iscc has been determined to be 15 MPa.m 1/2 , which is very close to K Iscc values obtained by others using standard specimens. The fracture surfaces have been examined by scanning electron microscopy (SEM) in order to identify the evidence of chloride stress corrosion cracks.
https://www.researchgate.net/publication/329999993_DETERMINATION_OF_THE_THRESHOLD_STRESS_INTENSITY_FACTOR_K_Iscc_OF_4340_STEEL_USING_SMALL_CIRCUMFERENTIAL_NOTCHED_TENSILE_CNT_SPECIMENS
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https://en.wikipedia.org/wiki/Stress_intensity_factor
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Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} is the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is more » relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.
https://www.semanticscholar.org/paper/Measurement-and-interpretation-of-threshold-stress-Dadfarnia-Nibur/d0a575cc5aec31659813d63cef09e061e6b478ae
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