Experimental measurement of value and associated uncertainty of wave speed in Hopkinson bars with an accurate and robust method

摘要

Post processing the strain waves in the measurement bars of Split Hopkinson Bars (SHPB) to get the stress-strain in the sample requires the knowledge of the longitudinal wave velocity c_0. In the context of metrology, we need to assess the measurement uncertainty in the value of c_0. The most intuitive way of getting the velocity c_0 relies on the measurement of the length of the bar and of the time shift between multiple reflections of a stress pulse in the bar. This method however faces some difficulties which increase the uncertainty in c_0. In this paper, we rather use a method based on the frequency spectrum of an impact test on a single bar and on the expression of longitudinal resonance frequencies. The relative uncertainty on the obtained velocity co is also assessed and is lower than 0.04%: it is mainly related to the uncertainty in the length of the bar. A precise value of the velocity c_0 and of the associated measurement uncertainty is an important step in the context of SHPB to precisely assess the final uncertainty in SHPB test results, which is scarcely ever done.