Voxel-based additive manufacturing of biomimetic hard-soft materials

Abstract

The fracture properties of natural materials often outperform artificial material properties. This could be explained by their complex multi-scale (anisotropic) hard-soft material structure, starting at a nano-scale. The interest in the fabrication of complex biomimetic hard-soft materials and interfaces is growing due to their high fracture properties, including stiffness, strength and fracture toughness. Additive manufacturing, in particular voxel-based additive manufacturing, enables the precise mimicking of complex natural three-dimensional (3D) structures at a micro-scale. Various studies have been performed on the fabrication of biomimetic materials with an Objet500 Connex3 polyjet printer which is able to print multiple materials at the same time. However, fundamental research on the influence of multiple biomimetic design parameters on the fracture properties of hard-soft materials was missing.
In this study the isolated and modulated effects of three different biomimetic design parameters on the fracture properties of hard-soft material samples were analysed. The samples differed in length scale (40, 240, 480 and 960 µm), hard material ratio (0, 25, 38, 50, 63, 75 and 100 %) and hard-soft material arrangement (semi-ordered, random and ordered). These design parameters could be found among different natural materials and interfaces. The fracture properties of the single edge notched samples, including stiffness, fracture strength, final strain and fracture toughness, were obtained with a tensile test. The effect of the different design parameters on the fracture properties was further analysed with digital image correlation (DIC) measurements and a digital microscope.
The results show that the biomimetic design parameters had a modulated effect on the fracture properties. The fracture properties were mostly affected by the hard material ratio and length scale compared to the hard-soft material arrangement. A higher amount of hard material resulted in a higher stiffness and fracture strength, but in a smaller final strain. A smaller length scale improved the fracture properties if a significant amount of soft material was present. An important result is that the fracture toughness of the hard-soft material samples could be increased by nearly two times compared to the toughness of the individual constituents, supporting the potential of using biomimetic design parameters. The effect of the biomimetic design parameters on the fracture properties can be explained by the DIC measurements and digital microscope images. The microscope images show that the fracture path tended to propagate through the soft material phases, which was more difficult at a smaller length scale. DIC measurements showed that the plasticity zone in front of the crack tip was larger for the samples printed at native resolution, resulting in a higher fracture toughness.
This study shows that a wide range of fracture properties can be achieved by regulating the length scale, hard material ratio and hard-soft material arrangement of hard-soft materials. The fracture toughness of hard-soft materials can be improved in comparison to the fracture toughness of the individual constituents, which supports the potential of using biomimetic design parameters in improving the fracture properties of hard-soft materials.