Characterization of dynamic damage mechanisms in Palmetto wood as biological inspiration for impact resistant polymer composites

Palmetto wood has been previously identified as a potential biological template for inspiring the development of synthetically engineered materials with hierarchical microstructures that exhibit enhanced mechanical behavior. Previously, the multi-scale mechanical behavior has been studied under quasi-static loading in order to understand the relationship between the microstructure of Palmetto wood and its mechanical behavior. In this study, the mechanical behavior of dry Palmetto wood is investigated under dynamic loading using low velocity impact. The experimental results reveal that the macrofiber concentration of the Palmetto wood plays a key role in the dynamic failure mechanisms. Under low velocity impact, the dynamic damage was found to be dominated globally by axial loading induced by bending leading to localized, shear-dominated debonding at the macrofiber-porous cellulose matrix interface, as well as compressive loading induced by indentation under the projectile leading to local crushing of the porous cellulose matrix and shear cracking of the macrofibers and matrix. By increasing the macrofiber concentration, it was found that the dominant failure mechanism could be transformed from the former to the latter by increasing the energy absorbed by indentation in order to increase impact resistance. This explains why the macrofiber concentration gradually decreases radially towards the center of the wood stem, since the outer portion of the wood has a high indentation resistance while the inner portion absorbs more energy through bending. A new model was proposed for to better understand the variation in mechanical behavior with macrofiber concentration and loading rate consistent with the evolution of the observed damage mechanisms. It was found that the greatest effect of increasing loading rate and macrofiber concentration was to increase the elastic modulus by 450-600% and the yield stress associated with the pore collapse mechanism by 125-175%. There is also a coupling between the evolution of plastic strain and damage that depends more strongly on macrofiber concentration than loading rate. These two effects combine to cause a significant increase in the density of energy absorption by 75-133% with increasing macrofiber concentration and strain rate. Therefore, the structure of Palmetto wood can be used as a template to guide the development of more impact resistant polymer composites, such as inserting 12 to 20 vol.% pultruded carbon fibers into the foam core of sandwich composite structures. (C) 2012 Elsevier Ltd. All rights reserved.