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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1Mo (P91), involves accumulating the number of loading cycles required for the nucleation of individual microcracks. According to equation (1), the Tanaka-Mura model (TMM) is beneficial in estimating the minimum number of cycles sufficient to nucleate a crack in a single grain, N g . (1) where G represents the shear modulus, W c is the specific fracture energy per unit area, v is Poisson’s ratio, d is the slip band length,   is the average shear stress range on the slip band, and CRSS is the critical resolved shear stress, which represents the crucial value of the shear stress along the glide path required for the dislocation movement. When the resolved shear stress falls below the CRSS, the formation of dislocation pile-ups at grain boundaries is prevented. Conversely, the motion of dislocations is obstructed. The CRSS value of 134.5 MPa was derived experimentally by assuming the position of the activated glide plane occurred at an 45° angle with respect to the loading direction. The method of determining the CRSS value will be explained in more detail in a further publication (Rahim, unpublished results). The short crack initiation stress ( S ) against the number ( N ) of cycles ( S - N ) curve is determined numerically, and Rahim et al. (2023) thoroughly explain this subject. 3. Result and Discussion 3.1. Microstructure morphology influences the fatigue crack initiation The microstructure morphology is illustrated in Fig. 1(c) and presents itself in the following manner: Microstructures M1 through M3 exhibit inhomogeneous grain morphologies which match the material characteristics of P91. In contrast, Microstructure M4 presents itself in the form of a homogeneous microstructure with homogeneous grain morphology. Five different stress levels were used on these models: 200, 300, 425, 475, and 558 MPa. The short crack initiation S - N curve is depicted in Fig. 2. The light green coloration denotes the M4 microstructure, while the light blue coloration specifies the microstructures M1 to M3, and the red circle represents the experimental data. 2 8 (1 ) ( v d 2 ) CRSS   −  − c = g GW N

Fig. 2. The fatigue crack initiation diagram of four microstructures of material P91.

According to both the Tanaka-Mura model (TMM) and experimental investigations with full fracture, the fatigue endurance limit is attained at a stress amplitude of 300 MPa, where all microstructures demonstrate life cycles exceeding 1×10 6 . Below this endurance limit, crack initiation begins to retard, leading to a condition termed run-out, as shown with load 200 MPa. Fig. 2 shows that microstructures M4 have more fatigue crack initiation cycles than M1 to M3, indicating better fatigue resistance. The comparison of the accumulated number of cycles between microstructure morphologies at stress amplitude 425 MPa is presented in Fig. 3 for a more detailed observation. The curve representing the microstructures M1 to M3, shown in light blue, indicates a more rapid crack initiation compared