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Additive manufacturing, also known as 3D printing, is acknowledged for its potential to support sustainable design. In this paper, we explore whether the opportunities that additive manufacturing offers for sustainable design are also useful when designing for a circular economy, and to what extent additive manufacturing can support design for a circular economy. We performed a literature review on the sustainability aspects of additive manufacturing and held a series of interviews with designers about their 3D printed design projects to obtain in-depth information. The interviews were analysed using annotated portfolios, a novel analysis method created specifically for this research. This resulted in a visual representation of the outcomes. We found that additive manufacturing supports circular design strategies by creating opportunities to extend a product's lifespan, for instance by enabling repair or upgrades, even if these products were not originally designed for ease of repair or upgrading. However, the use of monolithic structurally complex parts that support design for recyclability may hinder high value product recovery, like repair. Besides this, the current offer of 3D printable materials should be extended with materials developed for durable use, as well as highvalue reuse. Concluding, when accounting for these drawbacks, additive manufacturing is able to support multiple product life cycles and can provide valuable contributions to a circular economy.

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 lineargradient perform better than abrupt hard-soft-hard specimens.

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The functional gradient is a concept often occurring in nature. This concept can be implemented in the design and fabrication of advanced materials with specific functionalities and properties. Functionally graded materials (FGMs) can effectively eliminate the interface problems in extremely hard–soft connections, and, thus, have numerous and diverse applications in high-tech industries, such as those in biomedical and aerospace fields. Here, using voxel-based multi-material additive manufacturing (AM, = 3D printing) techniques, which works on the basis of material jetting, we studied the fracture behavior of functionally graded soft–hard composites with a pre-existing crack colinear with the gradient direction. We designed, additively manufactured, and mechanically tested the two main types of functionally graded composites, namely, composites with step-wise and continuous gradients. In addition, we changed the length of the transition zone between the hard and soft materials such that it covered 5%, 25%, 50%, or 100% of the width (W) of the specimens. The results showed that except for the fracture strain, the fracture properties of the graded specimens decreased as the length of the transition zone increased. Additionally, it was found that specimens with abrupt hard–soft transitions have significantly better fracture properties than those with continuous gradients. Among the composites with gradients, those with step-wise gradients showed a slightly better fracture resistance compared to those with continuous gradients. In contrast, FGMs with continuous gradients showed higher values of elastic stiffness and fracture energy, which makes each gradient function suitable for different loading scenarios. Moreover, regardless of the gradient function used in the design of the specimens, decreasing the length of the transition zone from 100%W to 5%W increased the fracture resistance of FGMs. We discuss the important underlying fracture mechanisms using data collected from digital image correlation (DIC), digital image microscopy, and scanning electron microscopy (SEM), which were used to analyze the fracture surface.