Mechanics of bioinspired functionally graded soft-hard composites made by multi-material 3D printing

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Functional gradients are material transitions that are found in nature and are known to result in materials with superior properties and multiple functionalities. The emerging multi-material 3D printing (=additive manufacturing, AM) techniques provide a powerful tool for the design and fabrication of bioinspired functionally graded materials (FGMs). In particular, the spatial distribution of materials can be controlled at the level of individual volumetric pixels (voxels i.e., cubes with side lengths of 20–40 μm), thereby ensuring accuracy, reliability, and reproducibility of the obtained properties and allowing for systematic studies of how various design variables affect the deformation and fracture behaviors of FGMs. Here, we designed, 3D printed, and mechanically tested tensile and notched FGMs specimens with step-wise (i.e., 5-, 10-, and 15-steps) and continuous (sigmoid and linear) gradients. The deformation and fracture mechanisms of these FGM composites were studied using digital image correlation, digital microscopy, and scanning electron microscopy. We further characterized the chemical composition and local mechanical properties of FGM composites using XPS and nanoindentation measurements, respectively. Tensile test specimens with a continuous gradient (i.e., linear) exhibited much higher Young's moduli (≈3-folds) and ultimate strengths (≈2-folds) but lower elongations (≈2-folds drop) as compared to those with stepwise gradients (i.e., 5-steps). Similarly, notched specimens with linear gradients exhibited 2-folds higher values of the stiffness and fracture stress, but 1.5-folds lower fracture strains as compared to those with 5-steps gradients. Although we found non-uniform highly concentrated strain distributions in all specimens, FGMs with linear gradients showed a smoother strain distribution and smaller crack blunting zones as compared to those with stepwise gradients. Our results imply that for stiffness and strength linear-gradient perform better than abrupt hard-soft-hard specimens.