PSI - Issue 68

Shanyavskiy A. et al. / Procedia Structural Integrity 68 (2025) 453–457 A. Shanyavskiy et al. / Structural Integrity Procedia 00 (2025) 000–000

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by 10% difference between steps to the SIF threshold value when the crack increment was not detected. Then the SIF value was increased by 10 % from one run to the next run. If the crack length increased, then the SIF value at the following run was decreased by 10 %. As the result the fracture surface of a one specimen has several areas with different crack growth rates and loading parameters. An example of such fracture surface with beach marks is shown in Fig. 2(b).

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Fig. 2. (a) Geometry of specimen for fatigue crack growth tests; (b) SEM image of fracture surface of a specimen with beach marks indicated by different colors.

After the FCG tests, all specimens were subjected to the SEM investigation of the regularities in fatigue crack propagation by Carl Zeiss EVO40 microscope. The shape of the crack front and local increments of crack size were established. Based on loading parameters and initial crack front shape the numerical simulations of crack growth were performed. To predict the crack front evolution under high-frequency loading the harmonic analysis of specimen was performed and local stress intensity factors were calculated. It was assumed that crack develops in the direction of maximum local tensile stress that leads to the VHCF crack ‘re-initiation’ ahead of the beach mark. The results of such simulations were compared to the experimental crack front shape. 3. Results and discussions The study on the FCG specimen fracture surface shows the morphology without fatigue striations (Fig. 3). The fretting products due to crack lips’ contact are detected at some local areas. The fracture of lamellar alpha-phase has a quasi-brittle character and forms a ‘staircases’ at some local zones (Fig. 3(a), (b)). Such features are formed within a large zone with similar orientated alpha-phase lamellas. The main crack propagates through these lamellas perpendicular to the lamellas elongated direction. These specific features of the relief reflect the lamellar microstructure of the material and has not any correlation with fatigue striations. The analysis on the fracture surface near to the arrested fatigue crack front (beach marks), in the vicinity of beach marks, shows the specific morphology of the pattern indicated the new crack initiation. This new crack initiation just ahead of crack tip takes place from the boundary junction of neighboring alpha-phase or cleaved single alpha-phase (Fig. 3(c), (d)). The observed fracture surface pattern allows to introduce scenario of the specific mechanism for fatigue crack advance under a new run of cyclic loading after a pronounced pause. During the new run of cyclic loading, the crack tip front does not move while the material ahead of crack tip experiencing the cyclic loading and the new crack origins are formed. The ligament between the main crack tip and new crack origins decreases that leads to the main crack advance toward the new crack origins. The discovered mechanism was used for numerical simulation of the crack front prediction in FCG specimens under high-frequency loading.