PSI - Issue 75

Monisha Manjunatha et al. / Procedia Structural Integrity 75 (2025) 650–659 Monisha Manjunatha et al. / Structural Integrity Procedia (2025)

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3. Experimental Methodology

Fig 2. Block diagram explaining the test methodology The experimental methodology followed for the investigation is presented in the block diagram Figure 2. Tested grades of steels are widely used for heavy machinery and construction equipment therefore the aim is to understand their total fatigue behaviour by separately characterising crack initiation and crack propagation. The results provide essential input for a future total fatigue life assessment of these grades. The ultrasonic fatigue testing (UFT) specimens were designed in accordance with the hourglass geometry specified in WES-1112 (2017), ensuring a standard gauge diameter of 3 mm as shown in depicted 3(a). The specimen design was optimized using harmonic analysis in ANSYS Workbench, verifying longitudinal resonance at 20 kHz while eliminating any unwanted torsional or bending motions that could lead to inconsistent loading across the cross-section. Roughness was measured using the Mitutoyo SV-2000 shown in Figure 3 (b). Given the mechanical properties of the materials tested, the same specimen geometry was applicable for all the grades of steel tested. To achieve accurate non-contact temperature monitoring, the gauge section of the specimens was polished to a mirror finish before being coated with black matte Hycote spray paint. This coating significantly enhanced the emissivity of the specimen, improving its infrared temperature detection capabilities. The paint was chosen for its strong adhesion to metal surfaces and its resistance to high temperatures encountered during testing displayed in Figure 3 (c). Threads were used to attach the specimen to acoustic horns on both sides and the height of the top horn was adjusted to achieve zero mean stress. UFT were conducted in start – stop mode to avoid specimen heating. The loading sequence consisted of 0.11 s full-amplitude load blocks followed by cooling pauses of 0.5 – 5 s, with forced air convection provided by compressed dried air nozzles. The cooling interval was adapted during testing according to the measured heat generation. Surface temperature was monitored continuously with an infrared sensor, and the control system ensured that the temperature remained below 30 °C. Unlike conventional structural steels such as Q355B and S355JR, where the ferritic microstructure promotes significant heating under ultrasonic loading (Milne et al., 2022), the HSS investigated in this study Grades 1 and 2 exhibited comparatively stable thermal behaviour. The resonant frequency of the manufactured specimens was measured using a Shimadzu USF-2000A ultrasonic fatigue machine, confirming a natural frequency falls within the operational range of 19.5 – 20.5 kHz. This ensured that the specimens could efficiently resonate while allowing sufficient air cooling within the permissible displacement range of the ultrasonic horn as observed in Figure 3 (d).