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The influence of specimen temperature on displacement efficiency was investigated for duplex stainless steel EN 1.4462 using a 20 kHz ultrasonic fatigue testing system. Resonance-controlled tests were conducted over incremental excitation levels defined by the function generator output voltage (VPP), with continuous phase control to maintain stable resonance. Axial displacement amplitude, resonant frequency, phase difference, and specimen temperature were continuously recorded. Displacement efficiency was defined as the ratio of displacement amplitude to excitation level (µm/VPP). During testing, specimen temperature increased from approximately 29 °C to 90-100 °C, depending on excitation level and cooling conditions. Peak temperatures exceeding 100 °C were observed in high-load tests above 1.5 VPP, corresponding to displacement amplitudes above 13 µm. Over this range, a consistent downward shift in resonant frequency of approximately 200-250 Hz (1-1.3%) was measured, indicating temperature-induced reductions in effective dynamic stiffness. Displacement efficiency showed a strong temperature dependence. Under well-cooled conditions, increases in excitation produced substantial displacement gains, while excessive cooling caused resonance instability, whereas at elevated temperatures progressively higher excitation was required to achieve equivalent displacement, indicating diminishing returns due to reduced elastic stiffness and increased dissipation. At fixed excitation levels, displacement amplitudes varied by up to 10% depending on phase, with stable resonance and maximum displacement confined to a narrow phase window of 0-2°. These findings demonstrate that temperature- and phase-dependent effects govern ultrasonic displacement response, underscoring the need for temperature- and phase-aware displacement calibration in ultrasonic VHCF testing of duplex stainless steels.
Keywords: Very-high-cycle fatigue (VHCF), Ultrasonic fatigue testing, Temperature effects, Resonance stability, Displacement efficiency, Duplex stainless steel EN 1.4462© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.