Abstract :
[en] Joining materials with dissimilar properties is a challenging but ubiquitous task for load-bearing engineering and biological materials. Bimaterial interfaces are prone to stress concentration, which may trigger failure upon loading. Nature’s materials feature several strategies to mitigate stresses at bimaterial junctions, making soft-to-hard attachments long lasting. Some biological materials, such as squid beak and byssus thread, display a gradual change in mechanical properties over a region spanning up to several millimeters, which is a length scale much larger than the soft and hard elementary constituents. Other biological interfaces, like the bone-tendon junction, lack long-range property gradients and the transition in mechanical behavior occurs over a micrometer-sized region. Here, failure resistance is obtained by introducing local interface patterning. Advances in multimaterial manufacturing are opening unprecedented possibilities to prototype new bioinspired solutions for bimaterial attachments. Using 3D printing is nowadays possible to assign location-specific material properties to individual voxels within a macroscopic object, enabling the fabrication of complex centimeter-sized heterogeneous structures, referred to as voxel-based materials, with properties tuned at the micrometer level. In this work, we combined voxel 3D printing and computer simulations to prototype bimaterial attachments based on local interface patterning at the mesoscale. We used a polyjet printer (Object 260, Stratasys), which deposits and UV-cures photopolymer droplets in a layer-by-layer fashion. Using different inks, the printer allows fabricating structures composed of individual stiff or compliant cuboid voxels having minimum dimensions of 40 x 80 x 30 µm. In a previous work we have shown that, owing to the printing process, the spatial transition in elastic properties across bimaterial interfaces can be fairly broad (around 150 μm) and larger than voxel size [1]. Here, we manufactured centimeter-sized bimaterial samples featuring voxels having side length of 420 µm. This dimension, higher than printer resolution, is chosen to minimize the impact of the interface, such that a voxel can retain its mechanical character. Samples were fabricated by assigning to the stiff voxels a rigid glassy polymer (Young’s modulus of 2 GPa) and to the soft voxels a rubber-like material (Young’s modulus of 40 MPa). We architectured bimaterial interfaces based on the general idea of introducing minimal interface patterning, i.e. the width of the perturbed region at the interface between stiff and complaint materials (mesoscale) was at least one order of magnitude smaller than sample length (macroscale). Stiff and compliant voxels were rearranged to form either geometrical patterns or compositional gradients. Starting from a flat interface, we considered chessboard, rectangular and triangular designs as well as gradients with different slopes. We performed failure tests to measure strength and fracture toughness. Our results indicated that minimal interface perturbation in the form of triangular patterns outperformed compositional gradients to enhance failure resistance. Finite element simulations were done to characterize stress concentration and spatial distribution of strain energy density (SED). Firstly, a mesh analysis underlined that at least 4 hexahedral elements were necessary to model individual voxels in case the two materials were assigned very dissimilar properties. More relevant, simulations highlighted that while a flat interface led to the smallest stress concentration in a defect-free case, in the presence of a locally damaged interface triangular patterns minimized stress localization and helped to redistribute SED away from the damaged region. Our results shall provide guidelines to improve failure resistance of bimaterial junctions using voxel-based interface patterning.
