, Volume 44, Issue 4, pp 1714-1721
Date: 18 Nov 2012

Bilinear Coffin–Manson Relationship in Thin Sheets of Interstitial-Free Steel

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Bilinear Coffin–Manson (C-M) as well as cyclic stress–strain (CSS) relationship is observed during low-cycle fatigue study (strain amplitude ∆ε t/2 = 0.0015 to 0.004) of an interstitial-free (IF) steel sheet unlike some of the earlier reports. In this work, an attempt has been made to correlate the observed bilinearity with the evolution of dislocation substructure and the nature of cyclic strain hardening in the selected steel. To achieve this goal, some of the low-cycle fatigue (LCF) tests were interrupted after the elapse of 2, 5, 10, 20, and 50 pct of fatigue life, and the microstructures at various stages were examined using TEM. Cyclic hardening at low-strain amplitudes (∆ε t/2 ≤ 0.0020) is predominantly due to dislocation–dislocation and dislocation–precipitation interaction. On the other hand, at high-strain amplitudes (∆ε t/2 > 0.0020), subgrains start forming much earlier in fatigue life, and there is an additional contribution of subgrains toward the total hardening. The above phenomenon leads to a difference in the values of cyclic strain-hardening exponents, e.g., 0.24 at low (∆ε t/2 ≤ 0.0020) and 0.45 at high ∆ε t/2, respectively. The above difference is reflected in the bilinear C-M plot around the transition ∆ε t/2 of 0.0020 as also observed in the CSS plot.

Manuscript submitted September 26, 2012.