Closed-Loop Strain Control for Combined LCF/HCF Ultrasonic Fatigue Testing: System Design and Experimental Demonstration

Over the past two decades, Ultrasonic Fatigue Testing (UFT) at 20 kHz has become a widely used methodology for investigating the fatigue behavior of critical metallic components, such as aircraft engine blades, under combined low-cycle and high-cycle fatigue (LCF/HCF) conditions. A critical limitation of conventional open-loop UFT systems is their inability to maintain a constant strain amplitude throughout the specimen’s lifespan. As microstructural damage accumulates and LCF loading induces specimen elongation, the effective resonance length deviates, leading to a progressive reduction in the actual strain amplitude, even when the ultrasonic generator output remains constant. To address this limitation, the present study develops and experimentally demonstrates a novel, fully automated closed-loop strain-control platform for combined LCF/HCF ultrasonic fatigue testing. The system integrates a 20 kHz ultrasonic fatigue cell, a servo-hydraulic testing machine for LCF superposition, continuous strain gauge monitoring via a high-speed data acquisition system, and a custom LabVIEW-based feedback algorithm. The measured strain is monitored throughout the loading history, and the ultrasonic driving amplitude is adjusted automatically when the strain deviates from the prescribed target. Experimental validation on CK45 steel specimens showed a pronounced progressive strain reduction during open-loop testing at constant ultrasonic generator amplitude. In contrast, the closed-loop control strategy compensated for these changes and maintained the measured strain within approximately ±5% of the target value throughout tests extending to 1.4 × 107 cycles. These results demonstrate the feasibility of direct strain-based feedback control for maintaining stable loading conditions during combined ultrasonic and low-frequency fatigue testing. The proposed methodology therefore provides a reproducible framework for further investigation of LCF/HCF interaction and VHCF behavior in metallic materials.