Fatigue crack growth behavior of a coarse- and a fine-grained high manganese austenitic twin-induced plasticity steel

The fatigue crack growth behavior of a coarse-grained (CG) and a fine-grained (FG) high manganese austenitic twin-induced plasticity (TWIP) steel has been investigated at room temperature. Crack growth tests were performed at stress ratios of 0.1 and 0.6 under the control of stress intensity factor range using three-point bending specimens. The results indicate that at the two stress ratios, the CG steel exhibits a higher fatigue crack growth resistance than the FG steel in both the near threshold and Paris regimes. Furthermore, a decreased stress ratio and an increased grain size both lead to an increased fatigue crack growth threshold. Microstructural observations reveal that cracks propagate more tortuously in the CG steel than in the FG steel, accompanied by rougher fracture surfaces, which tends to generate more roughness-induced crack closure and thus a higher fatigue threshold value. Additionally, the CG steel shows much larger plastic zone sizes ahead of the crack tip than the FG steel, suggesting that plasticity-induced crack closure may also play an important role in decreasing the fatigue crack growth rate in the CG steel. By excluding the crack closure effects, the CG steel still demonstrates a higher effective crack growth threshold than the FG steel; this is considered to be due to the increased planarity of slip in the CG steel, as compared with that in the FG steel.