PSI - Issue 58

M.R.A. Rahim et al. / Procedia Structural Integrity 58 (2024) 9–16 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The fatigue crack initiation within the inhomogeneous microstructure morphology M1 is illustrated in Fig. 4(a), which indicates that the nucleation process occurred scattered throughout the model. Fig. 4(b) focuses on a single grain, as in the blue ellipse from Fig. 4(a), demonstrating the instant right before crack nucleation. The shear stress distribution prior to crack nucleation varied from 195.4 MPa to 213.7 MPa, thus showing an 8.95% variation within this grain. Determining the stress concentration factor K t is critical for comprehending the consequences of this variation. The stress concentration factor ( K t ) defined as the ratio of maximum to minimum stress ( K t = σ max / σ min ), is employed to examine the impact of structural geometry on material fatigue behaviour. Stephens et al. (2001) applied this K t -concept to determine the influence of the stress concentration factor on the macroscale. Concerning the geometry of microstructure morphology, this factor is used to assess its effect on fatigue crack initiation. According to the shear stress distribution within a single grain in Fig. 4(b), the stress concentration factor there is 1.1. In Fig. 4(c), the grain starts to nucleate a first crack segment with a maximum shear stress of 231.6 MPa, and it continues the crack elongation as shown in Fig. 4(d).

Fig. 5. (a) Homogeneous microstructure with fatigue crack initiation end. (b) The situation of a grain before crack nucleation, (c) crack is nucleating, (d) crack elongates through the grain.

The homogeneous microstructure morphology typically produces a more uniform shear stress distribution than an inhomogeneous microstructure. Fig. 5(b), emphasizing the grain within the blue circle from Fig. 5(a), exhibits a very narrow shear stress distribution before crack initiation, ranging from 213.3 MPa to 213.6 MPa. Crack nucleation occurs in areas of highest shear stress, as seen in Fig. 5(c). As the microstructure is exposed to cyclic stress, the crack initiation extends across the grains, as in Fig. 5(d). The number of cycles required for a crack to initiate depends on the geometrical structure. The homogenous microstructure in Fig. 5(a) exhibits a decreased stress concentration factor (1.0) compared to the inhomogeneous microstructure. A comparison of the patterns of crack nucleation between the two microstructural morphologies after complete fatigue crack initiation reveals that the microstructure in Fig. 4(a) exhibits a rather scattered crack development. In contrast, the hexagonal structure shown in Fig. 5(a) has aligned cracks nucleating due to the homogeneity of the microstructure morphology and the same geometrical orientations of the grains. This grain arrangement promotes a rather uniform stress distribution, consequently increasing the fatigue life to the initiation of cracks, in contrast to the irregular microstructure morphology. 4. Conclusion The investigation into microstructure morphology plays a crucial role in determining the number of fatigue crack initiation cycles in steel 9Cr-1Mo (P91), as analyzed using the Tanaka-Mura model (TMM). Previous research, which