## 2 Experimental Section
### Design of Specimens  
In total, seven specimens with the re-entrant honeycomb configuration illustrated in Figure 1 were designed and manufactured. All the specimens were cut from the same 16 mm thick steel plate made of Q235B. Three corresponding coupon tests were conducted to obtain the mechanical properties of the base metal. Long and narrow steel plate specimens were designed. The middle parts of the specimens consisted of staggered re-entrant honeycomb units, and the two end parts had no perforations and were employed to clamp the specimen to a testing machine. The total length of each specimen was 256 mm. The length of the perforated area had a fixed value of 76 mm, and the width \(b\) was a variable, which changed with the hole ratio of perforated part. The perforated area was a typical concave re-entrant honeycomb structure. For all seven specimens, five concave units were alternately embedded in the same horizontal direction. The geometrical configuration of the re-entrant honeycomb units was clear, and was determined by the following three critical parameters, as illustrated in Figures 1 and 2 in this study. 1. Re-entrant angle, \(\alpha\), defined as the angle between the horizontal and vertical edges of the re-entrant honeycomb unit. This parameter determines the bending degree of the whole configuration, and from mechanical common sense, it affects the stiffness and ductility of the tensile direction. 2. Hole ratio, \(h\), defined as the ratio of perforated area to the original one, can be calculated using the following equation. \[h=\frac{S_{1}}{S_{2}}\,\] (1) where \(S_{1}\) and \(S_{2}\) are the perforated area and original area of cross section, respectively. This parameter can have a great effect on the strength and stiffness of re-entrant honeycomb specimens. In this experiment, the length of the perforated area \(l_{y}\) was a fixed value, and the width of a re-entrant honeycomb specimen, \(b\), is a variable. In this study, specimens with different hole ratios had the same cross-sectional area to ensure that the experimental results were comparable, and the hole ratio was the single factor affecting performance of the three specimens with different hole ratios. Thus, the width \(b\) was affected by the hole ratio. 3. Chamfer radius, \(R\). The re-entrant configurations had sharp angles at the corners of each honeycomb unit, which could lead to stress and strain concentration under loading. Thus, chamfering was conducted to reduce the concentration effect, and the chamfering radius was selected as a design parameter in this study. Seven specimens were designed in order to study the aforementioned three critical parameters, each with three different values: hole ratio \(h\) = 30%, 40%, and 50%; concave angle \(\alpha\) = 75\({}^{\circ}\), 80\({}^{\circ}\), and 85\({}^{\circ}\); and chamfer radius \(R\) = 0, 0.5, 1 mm. In terms of specimen numbering, Specim H40A80R0.5 indicates a test specimen with a hole ratio of 40%, a re-entrant angle of 80\({}^{\circ}\), and a chamfer radius of 0.5 mm. Specim H40A80R0.5 was also a standard test specimen, and the other ones were compared with this specimen by fixing two parameters and changing the other one. Table 1 gives a breakdown of all dimensions of the seven tested specimens. Figure 2a shows the schematic configurations of all the tested specimens, and Figure 2b shows the actual manufactured ones. The sample design was as follows. Firstly, the lengths of the tested specimens were the same. Secondly, each parameter was investigated with three specimens while the other two parameters were kept the same. For example, the cross-sectional areas of the three specimens with different hole ratios had the same re-entrant angles and chamfering radii. The same rule was applied to the other two parameters. Thus, the experimental results for the specimens were comparable. The engineering definition of [PERSON]'s ratio was presented by [PERSON] et al.