# Improving the Impact Resistance of Anti-Ram Bollards Using Auxetic and Honeycomb Cellular Cores
## 1 Introduction
Between those standards, there are some minor differences relating to the tested mass of the vehicles, the speed at which the vehicle strikes the VSB and the possible angles at which it does so. These factors affect how well the VSB performs. Datum points on the VSB and the vehicle are used to measure the distance of vehicle penetration beyond the VSB during the impact test. Hollow steel tube bollards have been used as structural elements in highway barriers and construction systems, and they absorb impact through plastic dissipation energy. They are primarily built of thin-walled sections because such sections are effective and adaptable. Steel sections are affordable and have good mechanical qualities for use in industry; however, because of their thin-walled elements, they are vulnerable to local failure [5]. Due to the plastic deformation of the cross-section walls, thin-walled sections fail when subjected to lateral impact loads. One can estimate the member strength and energy absorption capacity via yield line mechanism (YLM) analysis of the collapse mechanism, which provides a bending moment-rotation relationship. The amount of energy that the bollards absorb due to global deformation is not dramatically altered by increasing their size. Hence, extra energy absorbers (damping systems) might be necessary for anti-ram bollards. Protective structures may utilize cellular metals to absorb impact energy via plastic deformation [10]. In their detailed review, [PERSON] et al. [11] confirmed that cellular materials are commonly used as cushions and are loaded dynamically in real-life applications. These lightweight cellular metals have honeycomb or auxetic topologies. Honeycombs are 2-D cellular materials consisting of regular periodic microstructures inspired by biological structures such as bee hives, bamboo cross-sections, and abalone shells [12; 13]. Honeycombs provide ideal solutions in terms of balancing high strength, high stiffness, and maintaining a light weight. In addition, honeycombs with regular hexagonal cells have perfect in-plane isotropy, considerably reducing the effort required for modeling and analysis [14]. The initial interest in honeycomb materials started with the German aircraft engineer, [PERSON], who patented the first weight-saving sandwich panel designed for aircraft wing boxes in 1915. Since that time, lightweight sandwich beams and shells with honeycomb cores have been the most commonly manufactured cellular material products [15; 16; 17; 18]. In recent years, applications of honeycomb designs as energy-absorbing structures for dynamic crushing [19; 20; 21], morphingwings [22; 23], and non-pneumatic tires [24] have attracted attention. In addition, the capabilities of honeycomb structures in terms of heat dissipation [25; 26], fire resistance [27], and noise insulation [28; 29] have also been studied. A study looked at the effect of ballard energy absorption on vehicle damage [30], showing that as steel bollards cannot absorb shock, they seriously damage vehicles, leading to high repair costs. Therefore, this study suggests the use of honeycomb structures made of polylactic acid (PLA) to reduce vehicle damage. Numerical simulations with LS-DYNA were performed to create a honeycomb-inserted bollard that was intended to be used in actual vehicle-bollard collision tests. Auxetic materials are receiving special interest in technical sectors due to their attractive mechanical behaviors that lead to improved mechanical properties, such as improved energy absorption, fracture toughness, indentation resistance, sound absorption, and shear modulus [31; 32; 33; 34]. Poisson's ratio is defined as the ratio between lateral strain to the longitudinal strain for a material undergoing tension in the longitudinal direction. Commonly, all materials possess a positive Poisson's ratio. For example, materials shrink laterally under tensile loading and expand transversely when subjected to compressive loading. However, in auxetic materials, the phenomenon is reversed.