Cellular forms are omnipresent in nature and are found in wood, cork, beehives, sponge, and bone where the performance or function is governed by the intricate arrangement of matter and pores that lead to excellent mechanical and/or functional properties. Nature-inspired cellular materials have a periodic or stochastic arrangement of building blocks at different length scales. Perio
dic cellular solids, usually referred to as lattices, possess a desired combination of properties such as strength and toughness and are widely used in aerospace, automotive, biomedical, energy storage, and construction sectors, as they outperform foams. In the pursuit of lightweight engineering, there is a constant urge for developing low-density lattices with excellent mass-specific properties.[11, 5] An important aspect of such lattices is that their properties can be tuned by carefully controlling the architectural parameters such as unit-cell geometry, unit-cell size, and ligament size5, 11, 12 for a given choice of constituent material(s). Custom-tailored materials for specific application requirements are common, and the emergence of additive manufacturing (AM) technologies enables fabrication of cellular structures with intricate 3D architectures at different length scales from a variety of materials such as metals, polymers, ceramics, and composites.11–13 AM is particularly suitable for the fabrication and design of complex 3D cellular structures as it eliminates the need for expensive tooling and dies.
Self-sensing lattices are capable of monitoring environments in addition to performing the intended primary function(s) in service. Such a sensing ability is useful for monitoring the in situ deformation state and/or damage state of the structure. For instance, these self-sensing lattices can also be used as smart rehabilitation assistive devices, and material architecture for robotics where sensing, control, and actuation are essential for increasing the efficiency of the robot’s function. Self-sensing can be engineered either by embedding sensing elements into the material or by creating multifunctional materials, which exhibit intrinsic sensing ability in response to external stimuli.14, 16 In this study, we focus on the latter approach and demonstrate the self-sensing performance of AM-enabled 3D cellular composites. Such multifunctional composites enable transduction of mechanical stimuli into electric signals based on piezoresistive, piezocapacitive, piezoelectric, supercapacitive iontronic, and triboelectric mechanisms.14, 16, 18 If sufficient amount of electrically conductive fillers is incorporated into a nonconductive matrix, conductive fillers form an electrically percolated conductive network within the matrix. The resulting electrically conductive composites exhibit a change in electrical resistance under external stimuli such as strain, referred to as piezoresistivity. The piezoresistive behavior of such smart composites can be leveraged to monitor the in situ environment. For example, Yang et al. developed 3D-printed lightweight smart armor with aligned graphene nano-platelets that can sense damage via its piezoresistive behavior.
Polypropylene (PP) and its composites are widely used for various engineering applications such as automotive, aerospace, marine, biomedical, piping, and construction industry due to their favorable properties such as excellent strength to weight ratio, high-energy absorption, corrosion resistance, environmental stress-cracking resistance, less water absorption, and weldability. Cellular structures made of PP and its composites are used as core of sandwich structures, prosthetic mesh for biomedical applications, and energy-absorbing foams in automotive bumpers, seating, and door panels. They can be potentially used in areas where spatially varying properties are desired, for instance, custom-made orthoses for scoliosis.23, 24 Cellular PP structures nano-engineered with electrically conductive fillers such as graphene, MXene,16 and metallic nanoparticles (such as gold and silver, etc.) could be useful for a multitude of applications. For instance, detection of damage initiation in cores of sandwich structures used in aerospace industry is of utmost importance to avoid catastrophic failure of the structure.
As the fabrication of cellular structures with intricate architectures is either cumbersome or impossible with traditional manufacturing methods, fused filament fabrication (FFF) AM was used to realize lattice structures, utilizing in-house nano-engineered polypropylene random copolymer/multiwall carbon nanotube (PPR/MWCNT) composite filaments. A range of lattice structures with varying constituent material (MWCNT content in the PPR matrix) and also architectural parameters (unit-cell geometry and the relative density) were realized. Lattices of three different architectures, namely body-centered cubic (BCC) plate–lattice, open-cell Kelvin foam, and gyroid–lattice with varying relative density (1). BCC plate–lattice is a closed-cell cellular structure comprising plates or shells11 placed in closest packed planes as in BCC crystals. Closed-cell structures are found to be capable of achieving close to Hashin–Shtrikman upper bounds on isotropic elastic stiffness due to the material constraints in two directions.11 Open-cell Kelvin foam is a bending-dominated structure composed of struts. It comprises tetrakaidecahedral unit-cells whose faces contain 8 hexagons and 6 squares. The sheet-based gyroid–lattices belong to the class of cellular structures that are made from triply periodic minimal surface geometries and are found to exhibit stretch-dominated deformation behavior, enabling them to outperform most of the strut-based lattices in terms of mechanical performance. We engineer piezoresistive PPR/MWCNT nanocomposites incorporating MWCNTs into PPR matrix to monitor the in situ strain state and/or damage state of lattice structures. Both self-sensing and mechanical performance of FFF AM–enabled PPR/MWCNT composite lattice structures under quasi-static compressive loading are demonstrated. To the best of authors’ knowledge, studies on AM-enabled self-sensing cellular structures have not thus far been reported in the literature. The results indicate that our AM-enabled 3D lattices exhibit both piezoresistive sensitivity and energy absorption characteristics superior to those of extant works.where ρ is the density of cellular strcture and is the density of constituent solid material), were additively manufactured at mesoscale. Each lattice has 2 × 2 × 2 unit cells (see Figure
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