Auxetic and Hybrid Structure Designs and Advanced Manufacturing Study for Energy Absorption Improvements

One of the major concerns for many athletes in todays' sports is mild Traumatic Brain Injury (mTBI), which is commonly known as concussion. Researchers and manufacturers of sport helmets are constantly trying to develop new designs and technologies to better prevent mTBI. The objective of this research is to study the effective designs for sport helmets that can potentially absorb and dispel both linear and rotational forces acting on the head during impact. Inspiration by the different types of working mechanisms and structures existing in nature that can absorb energy from different types of impacts, new designs were explored. Honeycomb structures have been used extensively in lightweight sandwich structure and impact energy absorption applications. Recently, Auxetic structures are attractive for various engineering applications because of their unique mechanical properties, volume change control and excellent impact energy absorption performance. In this study, novel designs and performance improvement of new auxetic-strut structures were presented. A comparative study of in-plane and out-of-plane uniaxial compression loading behavior of regular honeycomb, re-entrant auxetic honeycomb, locally reinforced auxetic-strut structure and a hybrid structure of combining regular honeycomb and auxetic-strut structure was conducted. Finite element modelling was carried out to reveal their structure-property relationships. The deformation and failure modes of the different designs were studied and their performance was also discussed. The new auxetic-strut structure showed better mechanical properties than the honeycomb and auxetic structures with a small density increase. For in-plane performance, the compressive strength of the auxetic-strut design is ~300% more than that of honeycomb structure and ~65% more than that of auxetic structure. With lower values of the Poisson’s ratio, the new design can absorb more energy when compared to the other structures. The out-of-plane properties of auxetic-strut design showed an increase of ~68% in the compressive strength, ~63% in Young’s modulus and ~32% in the total energy absorbed when compared with the honeycomb structure. The hybrid structures also showed excellent out-of-plane properties. With better in-plane and out-of-plane properties, auxetic-strut design can be used in various energy absorption applications. Hybrid designs allow us to tailor properties of the structures with their specific in-plane and out-of-plane deformation and failure modes A comparative study of dynamic crushing behavior of the structures was also carried out. Finite element modelling was conducted to compare the dynamic crushing behavior of these structures at different impact velocities. Deformation mechanisms of these structures were studied, that provided the new insights on how to control the deformation of the structure and tailor the properties. For in-plane impact tests the energy absorbed by auxetic- strut and hybrid structures was half when compared with honeycomb and re-entrant auxetic structures at lower strain levels. But at higher strain levels, the new structures performed twice as that of the later. In contrary for out-of-plane crushing, the energy absorbed by the auxetic-strut and hybrid structures was higher than the honeycomb and re-entrant auxetic structures at lower strain levels and vice-versa. Advanced manufacturing or 3D printing method were employed to produce samples of the new designs. The results of the sample tests are in good agreement with the modeling predictions. These results are valuable to provide new fundamental understanding of structure-property relationships for new auxetic-strut and their hybrid honeycomb structures for potential aerospace and sporting product applications, especially in football helmets.