## 1 Introduction  
The split Hopkinson pressure bar (SHPB) technique is currently the most popular measurement technique for the determination of material properties at a high-strain rate [1].  
The SHPB method is applicable when a specimen is deformed nearly uniformly at a constant strain rate under dynamically equilibrated stresses, and the propagation of the elastic waves through the input and output bars is described by one-dimensional wave theory [1]. The nearly uniform sample deformation at a constant strain rate under dynamically equilibrated stresses is realized by: a pulse-shaping technique [2, 3], the conical (tapered) strikers method [4, 5, 6], or a three-bar technique with a dummy specimen [7]. The pulse-shaping technique and the three-bar technique with a dummy specimen usually cause minor changes in the incident wave, which are limited to increasing the rise and the fall time [8, 9], and damping the Pochhammer-Chree high-frequency oscillations [10, 11]. However, ensuring the system behavior in accordance with one-dimensional wave theory is possible only by a proper axisymmetric arrangement of the experimental set-up. The non-axisymmetric position of the bars and the striker in the SHPB arrangement results in disturbances in the useful signals that are propagating in the bars and measured by strain gauges.  
Based on numerical analyses, [PERSON] et al. showed that the resulting signal disturbances in the Hopkinson split bar method are caused by flexural modes of vibration from an imperfect condition in the SHPB experimental set-up [12]. The flexural modes of vibration create a three-dimensional pulse propagation that is inconsistent with the previously described requirements of the SHPB method. The results of the numerical investigations were confirmed experimentally by [PERSON] et al. using the photonic Doppler velocimetry (PDV) technique [13]. They suggest using a quarter-Wheatstone bridge to measure deformations in the SHPB technique, which allows for easy detection of signal disorders. However, in practice [14], it was shown that the disturbance of the measurement signals caused by imperfections in the SHPB arrangement is dependent on the relative position of the imperfection (signal disruption) and strain gauges with respect to the symmetry axis of the bars and the striker. Disorder of the measurement signals is the highest when a relative position of these components coincides (Figure 1 strain gauge position 1), while the disorder of the signals is the lowest when the relative position differs by 90\({}^{\circ}\) (Figure 1, strain gauge position 2). The influence of the imperfection is particularly visible in the reflected signal. To evaluate the proper axisymmetric arrangement of the experimental set-up, at least two gauges should be used in a quarter-Wheatstone arrangement placed on the bar at an angle of 90\({}^{\circ}\) to each.  
The goal of this work is to demonstrate the effect of using a pulse shaper technique, such as rounding a striker or applying a pulse shaper on the signals recorded in the SHPB technique, when the striker and the input bar are in an imperfect position. Two of the most common cases, which can only be limited but not eliminated, have been analyzed. The first case consists of an offset of the symmetry axes of the striker and the input bar by a value of \(\delta\) (Figure 2a). In the second case, the striker hits the input bar at an inclination angle, \(\alpha\) (Figure 2b). To achieve the goal of this work, the author carried out numerical analyses using the commercial finite element method (FEM) software LS-Dyna [15].  
Figure 1: Disturbances of wave signals (**a**) depending on the relative position of the pulse shaper and the strain gauges (**b**) [14].  
Figure 2: Striker and bar misalignment; neutral axis offset \(\delta\) (**a**), striker hitting the input bar at inclination angle \(\alpha\) (**b**), (**a**), and (**c**) with use of the pulse shaper; (**b**,**d**)—with rounded striker.