Abstract
Fatigue crack growth (FCG) tests were carried out at ambient temperature in air for 316 L (N) stainless steel varying with nitrogen (0.08, 0.14 and 0.22 N (wt %)) designated as 8N, 14N and 22N. The crack growth was monitored with direct current potential drop technique (DCPD) and crack closure estimated by compliance technique. The threshold stress intensity factor range (Δ Kth ) for 8N, 14N and 22N steels were found to be 5.1, 10.5 and 5.78 MPam 1/2 respectively. The effective threshold stress intensity factor range (Δ Keff,th ), (i.e., after correcting for crack closure) were found to be 4.5, 7.7, and 4.4 MPam 1/2 respectively. Both the extrinsic and intrinsic thresholds at the 14N steel were found to be higher than that at other nitrogen contents viz, 8N and 22N. A model has been proposed for quantifying slip irreversibility using the fatigue fracture surface roughness parameters. The trend in variation of FCG thresholds with different nitrogen levels follows that of slip reversibility estimated using this model. The results imply that the lower slip irreversibility indicated higher fatigue thresholds at 14N.
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