Hopkinson bar pulse smoothing

The conventional split Hopkinson pressure bar system for materials testing at high strain rates produces loading pulses that have a high level of oscillation superimposed on their approximately trapezoidal shape. These oscillations, which are a consequence of the short risetime of the loading pulse produced by the impact of a projectile, cause difficulty in the interpretation of the stress-strain results obtained by such a system. A modification to the conventional system is described, which virtually eliminates these oscillations, resulting in a smooth loading pulse and a significant improvement in the interpretation of the stress-strain behaviour. The modification involves placing a pre-loading bar before the two main bars of the conventional system. By suitable choice of pre-loading bar material with lower strength than the main bars, the high-frequency oscillations resulting from the projectile impact are attenuated as the pulse travels along the pre-loading bar; the loading pulse entering the main loading bar is then smooth with virtually no oscillations. Results for copper and steel test specimens are presented, which illustrate the significant advantages that the three-bar technique has over the conventional method in producing unambiguous stress-strain data.