Add time:07/21/2019         Source:sciencedirect.com

The damage evolution behavior of a novel Sanicro 25 alloy was investigated based on low-cycle fatigue tests at 700 °C. The results show that at relatively low total strain amplitude (Δεt/2), the dispersed damage was dominant, which led to prolonged fatigue life. However, at relatively high Δεt/2, the concentrated damage was dominant, which led to reduced fatigue life. Based on the damage characteristics, a modified Coffin-Manson equation that accounts for microporosity (f) was derived. The curved surface of plastic strain amplitude (Δεp/2) - f - number of reversals to failure (2Nf) shifts upward with increasing fatigue ductility exponent c. However, this curved surface shifts downward when the cyclic strength coefficient K′ increases. A new model of micro-crack initiation considering the precipitates at the grain boundary was derived. The Gibbs free energy change of crack nucleation (ΔG) decreased with decreasing Δεt/2. Finally, based on the prediction model of energy-based fatigue life, the theoretical formulas of the fatigue toughness Wf and the fatigue cracking exponent β were derived. Wf increases and β decreases with increasing Δεp/2.

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