PSI - Issue 79

Déborah de Oliveira et al. / Procedia Structural Integrity 79 (2026) 248–258

252

Uniaxial fatigue tests were carried out under load control using a stress ratio (R) of 0.1, consistent with procedures established in prior studies (Biswal et al., 2019; Dirisu et al., 2020; Wang et al., 2021). This ratio, defined as the minimum load divided by the maximum load in a cycle (R = σ min/ σ max), ensures predominantly tensile loading throughout the fatigue cycle, which is typical in high-cycle fatigue (HCF) investigations. The test frequency was initially set to 5 Hz, based on literature values that ranged from 5 to 50 Hz. However, subsequent evaluations demonstrated that increasing the frequency to 20 Hz did not adversely affect the integrity or resolution of the strain and load signals captured by the testing system. As a result, 20 Hz was adopted as the standard frequency for all tests to reduce total testing time while maintaining signal fidelity. To establish the material’s S–N curve (stress versus number of cycles to failure), the initial applied stress amplitude was defined as 61% of the ultimate tensile strength (UTS), corresponding to a nominal stress of 350 MPa. This initial condition was selected to initiate failure within a feasible number of cycles while remaining within the high-cycle fatigue regime, where the material response is primarily elastic. Subsequent stress levels were incrementally reduced in order to distribute the resulting fatigue lives (N) over several decades, enabling the construction of a well characterized S–N curve. Following fatigue testing, the vertically oriented specimens were analyzed to investigate the surface characteristics and crack propagation features. The fractured surfaces were examined using a confocal laser scanning microscope to obtain high-resolution, three-dimensional topographical data. This technique enabled both imaging and non-contact surface profilometry, which are critical for identifying fatigue-related features such as crack initiation sites, propagation paths, and fracture surface morphology. An Olympus LEXT OLS4100 confocal microscope was employed for this purpose. This system allows for sub-micron vertical resolution and precise surface roughness measurements, making it well-suited for characterizing fine-scale features associated with fatigue damage. The scanning process involved capturing optical sections of the surface, which were then reconstructed into a 3D surface profile. 3. Results and Discussion Following the methodology established for this study, fatigue tests were carried out on the vertically oriented specimens using a load ratio of R = 0.1 and a frequency of 20 Hz, with the objective of generating the corresponding S–N curve. Table 4 presents the maximum applied stress and the number of cycles to failure for each of the tested vertical specimens.

Table 2. Maximum Stress and Number of Cycles for Tested Specimens. Direction Specimen ID Maximum Stress (MPa)

Cycles 263141 123160 810460 381800 821180 174105 1200000 423650 731704 275000 1200000 901

Stress Ratio

V1 V2 V3 V4 V5 V6 H1 H2 H3 H4 H5 H6

302.40 384.00 350.40 293.20 316.80 302.40 384.00 283.20 316.80 316.80 350.00 302.04

0.53Su 0.67Su 0.61Su 0.49Su 0.55Su 0.53Su 0.67Su 0.49Su 0.55Su 0.55Su 0.61Su

Vertical

Horizontal

0.53Su Based on the data from the table, the S-N curves for vertical and horizontal specimens are shown in Fig. 3 and Fig. 4, respectively. This curve clearly demonstrates the inverse relationship between applied stress and the number of cycles to failure. During the fatigue tests, it was not possible to identify a stress level at which the specimen could endure more than 2 × 10^6 cycles, often considered the threshold for "infinite life." Furthermore, specimen V2 exhibited a markedly different behavior compared to the others, failing after a significantly lower number of cycles.