Stress wave control method for the Hopkinson bar used in the impact fatigue experiment

In both national defense and civilian applications, various equipment and structural components are frequently subjected to intermittent, high loading rates, and repetitive severe impact loads, which are referred to as repeated impacts or impact fatigue. To study the impact fatigue behavior of equipment or structures, it is necessary to first establish reliable impact fatigue testing techniques or methodologies. Therefore, the conventional Hopkinson bar impact loading system was modified and enhanced, and the stress wave propagation characteristics in the loading bar, specimen, and associated fixtures under successive impacts were analyzed in detail. The method for controlling the amplitude, width, and waveform configuration of the impact loading pulse applied to the specimen was systematically analyzed. In addition, a theoretical analysis was conducted on the principle of achieving single pulse loading in impact fatigue testing. 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. Consequently, a simple and efficient single pulse loading method suitable for impact fatigue testing was proposed. The core principle involves designing the length and material parameters of the loading bar such that the end surfaces of the specimen and the bar coordinate 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 a combination of numerical simulations and experimental investigations. Additionally, a dedicated loading fixture for shear-type impact fatigue was developed, enabling the acquisition of the shear impact fatigue stress-life curve of TC4 titanium alloy, thus demonstrating the method’s applicability to complex loading modes.