## 1 Introduction  
With advantages of low weight, large specific stiffness, and high specific strength, the metal honeycomb sandwich structure is extensively applied in the field of aerospace engineering. For example, the high-bypass turbofan engine casing is made from the honeycomb sandwich structure in order to improve the impact resistance since the impact on the casing is a typical nonlinear physical process with the high strain rate up to \(10^{4}\) s\({}^{-1}\) during the fan blade out events. The high strain rate has a great influence on the mechanical properties of the structure. The different honeycomb structures also possess different cell structure characteristics and mechanical properties, therefore, it is crucial to understand the dynamic compression mechanical properties of the aluminum honeycomb structures at a high strain rate. The dynamic compression test is an effective way to obtain the dynamic mechanical properties of honeycomb materials. Equipment of dynamic compression tests mainly include dynamic material testing machine, the drop hammer impact table, Hopkinson bar, Taylor impact table and the high-speed impact test system, etc. Hopkinson bar is widely used at the medium and high strain rates due to the fewer restrictions. As early as 1914, [PERSON] [1] developed the Hopkinson compression bar test device. [PERSON] [2] proposed a separated Hopkinson pressure bar setup on the basis of Hopkinson compression bar. [PERSON] and [PERSON] [3] studied out-of-plane crushing properties of the honeycomb structures with a similar experimental device and obtained an increase of 74% of the crush strength under dynamical conditions compared to those under the quasi-static condition. The Split Hopkinson Pressure Bar (SHPB) method is widely used in dynamic compression tests of metal materials to obtain the dynamic mechanical properties. However, there are still many difficulties when it is applied to the honeycomb structure, especially at high strain rates due to several reasons. For example, the honeycomb specimen possesses the size effect [4] because of the diameter limit. The wave impedance of the honeycomb structure is lower than that of a conventional material bar which makes it difficult to form a strong signal in the transmission bar. On the other hand, the deformation of the honeycomb structure is large, and the nonuniform stresses may occur inside the specimen and undergo the multiple loading-unloading process [5]. The assumption of stress uniformity in the SHPB test for the honeycomb structure also needs to be further verified. Due to the difficulties mentioned above, the SHPB test method cannot always provide satisfactory precision of the honeycomb structures under the impact loading. In recent years, viscoelastic bar has been used as a solution in the SHPB test method [6]. [PERSON] et al. [7; 8] conducted viscoelastic bars and a two-strain measurement method on the SHPB test to improve the signal/noise ratio and to host larger samples containing a sufficient number of cells. They investigated the out-plane impact dynamic response of aluminum honeycomb materials at the medium (\(600\) s\({}^{-1}\)) and the low strain rates (\(10^{-4}\) s\({}^{-1}\)). The results indicated that the enhancement of the crushing strength occurred at the medium strain rate. [PERSON] et al. [9] adopted the Nylon Hopkinson bar with a large diameter to investigate the existence of a shock front of the honeycomb and the Cymat foam at the low impact loading. It was found that no significant shock enhancement was observed for aluminum honeycomb structures and the sensitivity of the corresponding rate was not responsible for the strength enhancement. On the basis of viscoelastic bar, the combined shear-compression impact test was presented [10]. [PERSON] et al. [11] and [PERSON] et al. [12] introduced the large diameter Nylon Split Hopkinson Pressure Bar system (NSHPB) with beveled ends of different angles to study the behavior of honeycomb structures under the multiaxial impact loadings.