# A Study on Shock Absorption Characteristics of Honeycomb-Inserted Bollards
## 2 Design and Tests of In-Plane Honeycomb
### In-Plane Honeycomb Compression Test  
Compression tests were done on the 3D-printed honeycomb unit specimen to select the one for actual vehicle-bollard collision experiments. All honeycomb units are designed to have 100 mm horizontal side length(x), 100 mm vertical side length(y), and 50 mm thickness(z). However, by varying \(l\) from 2.3 to 5.3 mm and \(t\) from 0.6 to 1.2 mm, five units with different honeycomb cellular structures were designed, with more detailed lengths shown in Figure 2a. Using PLA (Polylactic acid) materials and a 3D printer, three sets of each design were manufactured. Using a universal testing machine, all manufactured honeycomb units were compressed from their in-plane direction, as shown in Figure 2b, to collect their load-displacement data. All compression tests were done at 20 mm/min speed, which is commonly used for honeycomb tests [19, 20, 21], and for three-minute duration. Specimens were fixed at the bottom, the x-y plane in Figure 2a, by a jig as shown in Figure 2b, so that no deformation occurred in the x-y plane. Figure 2c shows the deformed honeycomb specimen and the load displacement curves are plotted in Figure 3 for various \(l\) and \(t\). The displacement values (x-axis values in Figure 3) are the displacements of the honeycomb vertical sides (the y-axis in Figure 2a).  
Figure 1: Cell deformations by cell wall bending and rotation in plastic collapses.  
Figure 2: Honeycomb Unit Designs and Compressions Tests Pictures: (**a**) Honeycomb modeling; (**b**) In-plane direction compression test; (**c**) Compressed specimen showing results.  
According to the compression test results, maximum load decreases as the honeycomb cell wall length increases and maximum stress increases as the cell wall thickness (t) increases. As indicated in Equation (4), yield stress increases proportional to t\({}^{2}\), while inversely proportional to \(l^{2}\). This, however, decreased to its 70% level if 0.2 offset (0.2% OYS, 0.2% proof stress, RP0.2, RP0,2) methods were used.  
The data collected from the honeycomb unit compression tests were used as inputs to the LS-DYNA simulations with the honeycomb property set as Mat 24 (Piecewise Linear Plasticity). The simulations allowed compressions along the y-axis only. With the honeycomb property in the simulations closely calibrated to the compression test specimen and cell failures simulated as well, both tests and simulations show qualitatively-similar results as presented in Figure 4. All compression simulations were run for a 90.0 s duration with the minimum mesh size and time steps set at 0.24 mm and 100 ns, respectively. Cells were considered to fail when the strain rate of the material exceed 10% since 5% to 10% criteria are typically adopted in PLA compression tests [22].  
Figure 4: Comparisons of Honeycomb Compression Experiment and Simulation: (**a**) Experimental results of a Honeycomb specimen; (**b**) Simulation results of a Honeycomb specimen.  
Figure 3: Load displacement curves for various honeycomb cell wall length (\(l\)) and thickness (\(t\)).