Energy Absorbing Cellular Structures for Crashworthiness Applications

Abstract

Energy absorbing materials are utilized in many applications. Aircraft, automobiles, and helmets all use energy absorbing materials to ensure the safety of the individual during an impact event. The seats in aircraft are made from a material that can minimize the force that is transferred from the impact to the occupant. In a similar manner, the material in the front of an automobile is designed to absorb the energy from an impact event and redistribute it in a manner that minimizesthe amount of energy experienced by the main cabin. Helmets perform in the same way: by taking the impact and distributing the load to protect the wearer. The materials used in these applications were tailored to meet the needs of the application, particularly the density and strength of the material. Using cellular structures allow for more control of the design for energy absorbing applications, particularly when looking to increase the performance of the material. There are three options for increasing the energy absorption in materials for crashworthiness applications: decrease the force with a constant mean crush stress, increase the mean crush stress with a constant force, or decrease the force while increasing the mean crush stress. In a force- displacement diagram, the area under the curve is the amount of energy that a material can absorb during an impact. By decreasing that initial force, the initial peak force will begin to equilibrate with the mean crush, resulting in a higher energy absorption. The structures that have been relied on throughout history for these applications are cellular structures. Cellular structures are described as any structure that is made of one phase composed of either air or fluid. As Lakes describes in his work, foams, honeycombs, and lattices are categorized as such; the voids allow the materials to reach physical limits beyond their previous. With the improvements of technology, it is important to re-asses these structures to determine whether they too can be manufactured and remain as effective in their original crashworthiness applications as before. Throughout this work, different methods of additive manufacturing are used to create honeycomb structures specifically for energy absorption applications. Each of these studies focuses on a different attribute that additive manufacturing can help improve in energy absorption materials. In this dissertation, four case studies involving the out-of-plane compression of additively manufactured honeycomb will be discussed. The first chapter will center on the applications of visco-elastic theromplastic polyurethane (TPU) as a potential material of choice for energy absorption materials. TPU is a material that has the ability to achieve significant deformation and return to its original shape within a matter of minutes. This material is of interest due to the need to re-use helmet liners and other safety mechanisms before buying a new one. This work also focuses on the impact that adding buckling initiators will have to the structure in terms of energy absorption during quasi-static conditions. The next chapter is centered on the applications of these TPU honeycomb undergoing dynamic testing. Crashworthiness materials experience impact velocities bordering on 10- 15 m/s (22- 35 mph). These tests differ from the previous due to the velocity no longer being constant. As the impactor falls, the velocity changes, while the quasi-static tests were completed under a constant velocity. This set of dynamic tests is most representative of long term applications, however the performance of these materials change drastically as discussed. In some applications, a visco-elastic plastic is not going to be able to absorb the energy from the impact. In these situations, a stiffer material would be necessary. To provide an alternative for these applications, acrylonitrile butadiene styrene (ABS) was studied since it is a commonly used plastic when additively manufacturing. Once again, honeycomb were manufactured and tested under out of plane, uni-axial quasi static compression. The samples were studied to determine the effects of buckling initiator location as well as the effect of the inscribed diameter. For this, samples were manufactured with an internal diameter of 10, 15, or 20 mm. The buckling initiators were located either 1/2, 3/4, or at the top of the samples to determine the design which enables the best energy absorption. The final study recognizes that traditional honeycomb has been manufactured using metals like aluminum and steel. By moving towards an additively manufactured honeycomb, this work has been focusing on polymeric honeycomb instead. The metallic additive manufacturing methods require drastic safety precautions be taken. A safer alternative is proposed in this last study: combining stereolithography and electroplating. Here, an isotropic material can be the core of the structure, with a thin layer (about 150 μm) of metal creating the ductile layer. These samples demonstrate a ductile failure as opposed to their plastic only counterparts who experience a brittle failure. The energy absorption performance is then characterized as a function of buckling initiator height as well.