Study on the stress wave control method of the Hopkinson bar used in the impact fatigue experiment

In the national defense and civilian fields, equipment and structures will inevitably be subjected to intermittent, high loading rates, and repetitive severe impact loads, which are the so-called repeated impacts or impact fatigue. To study the impact fatigue behavior of equipment or structures, it is necessary to first establish a reliable impact fatigue testing techniques or methodologies. This study modifies and enhances the conventional Hopkinson bar impact loading technique to meet this need. The stress wave propagation characteristics in the loading bar, specimen, and fixtures under successive impacts are analyzed in detail. The amplitude, pulse width, and waveform configuration of the impact loading pulses applied to the specimen are systematically analyzed and controlled. And a theoretical analysis on how to achieve single pulse loading in impact fatigue testing is provided. Effective control of the amplitude, pulse width, and the stress wave pulse configuration of the loading wave is realized by optimizing and modifying the impact velocity, length, and geometric shape of the projectile. This research primarily proposeds a simple and rapid single pulse loading method suitable for impact fatigue testing. The principle involves designing the length and material parameters of the loading bar such that the end surfaces of the specimen and the bar act in coordination and then separate, thereby preventing irregular and random secondary or multiple loadings caused by reflected stress waves. This design ensures that each individual impact in a continuous impact sequence results in a single loading on the specimen. The effectiveness and feasibility of the proposed impact fatigue testing technique have been verified through numerical simulations and experiments. Finally, a loading device for shear impact fatigue was established, and the shear impact fatigue stress-life curve of TC4 titanium alloy was obtained.