# A Study on Shock Absorption Characteristics of Honeycomb-Inserted Bollards
## 1 Introduction  
Bollard is a road safety structure that separates sidewalks from streets to prevent vehicles from entering into sidewalks [1; 2; 3]. In South Korea, stone or steel bollards were widely used until the \"Act on Promotion of the Transportation Convenience of Mobility Disadvantaged Persons\" was passed in 2019 [4]. This act mandated all bollards to be manufactured with either steel pipes or stainless coated with materials such as polyurethane for the improved safety of the physically-challenged (especially the blinded). Either stone or steel bollards are being used throughout the world as well. The recently increased use of anti-ram bollards to counter the increased vehicle-ramming events, suicide bombings, and other terrorist activities prompted research on their shock absorption and safety protection capabilities. These research papers, however, were mostly focused on reducing or distributing collision shocks to the bollards, instead of to the vehicle [5; 6]. This paper, thus, not only focused on the reduced shocks to the bollards but also to the colliding vehicles.  
Vehicle-bollard collisions cause significant damages on the colliding vehicles and consequent repair costs. Automobile manufacturers of today are either fully or partially replacing steel materials with lighter ones such as ultra-high-strength steel sheets, aluminum, magnesium, and/or carbon fibers.  
Additionally, the increased use of various sensors and electronic devices yields higher automobile prices, thus increased repair cost from vehicle-bolland collisions [7; 8; 9; 10; 11].  
As one of diverse shock absorption schemes, cellular honeycomb structure has higher shock absorption capability due to its higher-strength hexagonal shape when compared to the foam or tube structure [11; 12; 13; 14; 15]. It is known to absorb impact energy with its gradual plastic collapsing and high load-carrying abilities and to allow diverse structural designs by adjusting geometric parameters. Its lower prices compared to hydraulic or electromagnetic energy absorption devices further motivates its broader applications in aerospace, mechanical, and civil engineering [16; 17].  
Therefore, this research started with designing and testing of the honeycomb structure specimen to be inserted into bollards, simulating vehicle-bolland collisions with LS-DYNA, a general-purpose finite element program capable of simulating real world problems, to calculate the shock absorption characteristics of vehicle front bumper frame when colliding into both steel and honeycomb-inserted bollards.  
Thus, the LS-DYNA numerical simulations provided data for the honeycomb structure designs, which were three-dimensionally (3D)-printed and inserted inside the bollard cover (\"in-plane\" insertion). These \"in-plane\" honeycomb-inserted bollards, along with a conventional steel bollard, were used in the actual vehicle-bolland collision experiments. These experiments allowed a comparison of simulated and measured acceleration characteristics of vehicle front bumper frames during collisions and qualitative observations of the damage done to the vehicle by both steel and honeycomb-inserted bollards.