PSI - Issue 60

M. Suresh Kumar et al. / Procedia Structural Integrity 60 (2024) 433–443 Suresh Kumar et al.,/ Structural Integrity Procedia 00 (2023) 000 – 000

439

Fig. 7. Stress-fatigue life (S-N) curves for AR, 200 HT and 300 HT samples

where N f is the fatigue life, σ f  is the fatigue strength coefficient, and ‘b’ is the fatigue strength exponent. T he (σ f  ) value reflects true tensile strength (neglecting necking) of the material. In general, σ f  value will be high for higher strength materials (Shukla and Mishra (2020)). The ‘b’ value explains the initial damage mechanism and microstructure stability during cyclic loading. The value of ‘b’ generally varies from -0.05 to -0.12 for most of the alloys. A smaller absolute value of ‘b’ is indicative of a lower initial damage degree and, hence, higher fatigue life if other parameters are kept constant (Kun et al. (2008)). According to the basquin equation fitting lines, as shown in Fig.7, the AR and 300 HT samples exhibited similar fatigue strength coefficient (σ f  ), i.e., 535 and 518 MPa, respectively. However, in the case of the 200 HT sample, σ f  value was relatively low i.e., ~412 MPa. The lower σ f  value indicates a decreased strength/ductility of the material as compared to the AR sample when exposed to 200° C. The lower value of σ f  in the 200 HT condition corroborates well with experimental data that shows a decrease in tensile elongation of the alloy at this temperature. The AR and 300 HT samples showed a fatigue strength exponent ‘b’ of - 0.07 compared to the value −0.04 for 200 HT samples. The higher ‘b’ value for the 200 HT sample suggests a possible reduction in fatigue damage. In general, lower fatigue strength/life is expected in 200 HT conditions due to low σ f  as compared to AR and 300 HT samples. But, the sample is able to achieve higher fatigue strength/life as a result of higher “b” (Liu et al. (2016)). 3.4. Effect of creep forming temperature on high cycle fatigue fracture The fracture surfaces of fatigue tested samples for AR, 200 HT, and 300 HT samples are shown in Figs.8a, 8b, and 8c, respectively. All the fracture surfaces exhibited clearly delineated crack initiation, propagation, and final overload regimes that are typical of fatigue fracture. Striations were observed at high magnification within the crack propagation region (Fig.9a). Delamination-type features were observed on the fracture surface at higher magnifications. The delamination was observed predominantly surrounding the crack origin region on planes perpendicular to the crack propagation direction (Fig.9b). These features were observed in all the samples exposed to simulated creep-forming temperatures as well. Fig.10 a, b, c depicts delaminations at the crack imitation region on the fracture surface for AR, 200 HT and 300 HT samples respectively.