Impact Resistance of Single-Layer and Multi-Layered Materials

Use of impact resisting materials to prevent head injury and concussion is the subject of much study in protective equipment for sports and other activities. Understanding the mechanical response of impact resistant materials and how this response changes with geometric and material parameters is important when designing and optimizing new materials. This thesis summarizes the impact resistance of various material combinations using a twin wire drop tower. A database of the response of numerous samples subject to a step impact drop test was created. The maximum acceleration versus drop height, impact force versus displacement and time history of the impact impulse are presented for each sample tested. At a given impact height, the most optimal material response should have a stiffness allowing for maximum energy absorption which will decrease the forces due to the impact. The variation in material properties and geometry can be used to create a design criterion that can achieve a certain performance requirement. In this study, there were two types of impact resistant materials, urethane honeycomb and polymeric foam materials that were tested in various combinations including layer height, cell structure and material properties. The foam material which is classified mathematically as a hyperfoam material is categorized according to the material stiffness as P09, P15 and P25, based upon density parameters provided by the manufacturer. The honeycomb material is classified mathematically as hyperelastic and has a varying cellular structure where the cell wall shape and dimensions can be modified. The honeycomb material is classified according to material durometer (hardness) as H561, H781, H1036 and H1056 and according to cell geometric structure. Regular hexagonal shapes and irregular shapes were tested. One layer results of the foam material showed that a stiffer material is generally more optimal when the impact height increases. Stiffness in the materials tested is directly related to the density. Increasing the thickness and accordingly the deformation of the energy absorbing material allows the use of softer materials resulting in lesser impact forces and acceleration. Regarding the results of one layer of urethane honeycomb, increasing of the material thickness has the same net effect as in the foam. Overall stiffness of the honeycomb material is controlled by the material durometer and the solids ratio. In addition, the response is influenced by buckling of the cell walls which tend to limit forces imparted until the material is consolidated. Modification of the cell wall thickness or the cell size leads to changes in response that can be used to optimize the structure under impact. In addition, multilayered structures may be used to mitigate impact over a wider range of input energy than could a single layer material. Accordingly, the impact attenuation of several multi-layer samples was explored. Those samples consist of combinations from soft, moderate and stiff material to get reasonable values of the acceleration and displacement at every impact height