Synergistic Energy Absorption Mechanisms of Architected Liquid Crystal Elastomers.

A unique rate-dependent energy absorption behavior of liquid crystal elastomer (LCE)-based architected materials is reported. The architected materials consist of repeating unit cells of bistable tilted LCE beams sandwiched between stiff supports. The viscoelastic behavior of the LCE causes the energy absorption to increase with strain rate according to a power-law relationship, which can be modulated by changing the degree of mesogen alignment and the loading direction relative to the director. For a strain rate of 600 s-1 , the unit cell exhibits up to a 5 MJ m-3 energy absorption density, which is two orders of magnitude higher than the same structure fabricated from poly(dimethylsiloxane) elastomer and is comparable to the dissipation from irreversible plastic deformation exhibited by denser metals. For a multilayered structure of unit cells, nonuniform buckling of the different layers produces additional viscoelastic dissipation. This synergistic interaction between viscoelastic dissipation and snap-through buckling causes the energy absorption density to increase with the number of layers. The sequence of cell collapse can be controlled by grading the beam thickness to further promote viscous dissipation and enhance the energy absorption density. It is envisioned that the study can contribute to the development of lightweight extreme energy-absorbing metamaterials.

Estimations of the effective Young's modulus of specimens prepared by fused filament fabrication.

The elastic response of homogeneous isotropic materials is most commonly represented by their Young's modulus (E), but geometric variability associated with additive manufacturing results in materials that are neither homogeneous nor isotropic. Here we investigated methods to estimate the effective elastic modulus (Eeff) of samples fabricated by fused filament fabrication. We conducted finite element analysis (FEA) on printed samples based on material properties and CT-scanned geometries. The analysis revealed how the layer structure of a specimen altered the internal stress distribution and the resulting Eeff. We also investigated different empirical methods to estimate Eeff as guides. We envision the findings from our study can provide guidelines for modulus estimation of as-printed specimens, with the potential of applying to other extrusion-based additive manufacturing technologies.

Effects of Environmental Temperature and Humidity on the Geometry and Strength of Polycarbonate Specimens Prepared by Fused Filament Fabrication.

It is widely known that the printing quality of fused filament fabrication (FFF) is heavily affected by environmental temperature and humidity, taking the form of warping and porosity. However, there is little understanding about the quantitative relations between environmental conditions, geometry, and the mechanical properties of printed parts. In this study, we systematically investigated those relations using bisphenol A polycarbonate as a model material system. For the environmental temperature, an in-situ infrared imaging analysis revealed the presence of an up to 5.4 °C/mm thermal gradient when printing using an open-chamber printer and a heated build plate. For the environmental humidity, an analysis of X-ray micro-computed tomography (micro-CT) scans showed an up to 11.7% porosity that was brought by polymer water content absorbed from environmental moisture. Meanwhile, tensile tests showed a mechanical performance loss associated with those defects, but, surprisingly, the transverse direction ductility had the potential to increase at a higher porosity. Furthermore, the experimental results were combined with analytical and parametrical studies to elucidate quantitative relations between environmental conditions and printing quality. Based on the results, quantitative guidelines for the estimation of printing quality based on environmental conditions are provided that would also help users to obtain desired printing results with a better understanding of the effects of environmental conditions.

Analytical, numerical, and experimental studies of viscoelastic effects on the performance of soft piezoelectric nanocomposites.

Piezoelectric composite (p-NC) made of a polymeric matrix and piezoelectric nanoparticles with conductive additives is an attractive material for many applications. As the matrix of p-NC is made of viscoelastic materials, both elastic and viscous characteristics of the matrix are expected to contribute to the piezoelectric response of p-NC. However, there is limited understanding of how viscoelasticity influences the piezoelectric performance of p-NC. Here we combined analytical and numerical analyses with experimental studies to investigate effects of viscoelasticity on piezoelectric performance of p-NC. The viscoelastic properties of synthesized p-NCs were controlled by changing the ratio between monomer and cross-linker of the polymer matrix. We found good agreement between our analytical models and experimental results for both quasi-static and dynamic loadings. It is found that, under quasi-static loading conditions, the piezoelectric coefficients (d33) of the specimen with the lowest Young's modulus (∼0.45 MPa at 5% strain) were ∼120 pC N-1, while the one with the highest Young's modulus (∼1.3 MPa at 5% strain) were ∼62 pC N-1. The results suggest that softer matrices enhance the energy harvesting performance because they can result in larger deformation for a given load. Moreover, from our theoretical analysis and experiments under dynamic loading conditions, we found the viscous modulus of a matrix is also important for piezoelectric performance. For instance, at 40 Hz and 50 Hz the storage moduli of the softest specimen were ∼0.625 MPa and ∼0.485 MPa, while the loss moduli were ∼0.108 MPa and ∼0.151 MPa, respectively. As piezocomposites with less viscous loss can transfer mechanical energy to piezoelectric particles more efficiently, the dynamic piezoelectric coefficient (d'33) measured at 40 Hz (∼53 pC N-1) was larger than that at 50 Hz (∼47 pC N-1) though it has a larger storage modulus. As an application of our findings, we fabricated 3D piezo-shells with different viscoelastic properties and compared the charging time. The results showed a good agreement with the predicted trend that the composition with the smallest elastic and viscous moduli showed the fastest charging rate. Our findings can open new opportunities for optimizing the performance of polymer-based multifunctional materials by harnessing viscoelasticity.

Multistable Architected Materials for Trapping Elastic Strain Energy.

3D printing and numerical analysis are combined to design a new class of architected materials that contain bistable beam elements and exhibit controlled trapping of elastic energy. The proposed energy-absorbing structures are reusable. Moreover, the mechanism of energy absorption stems solely from the structural geometry of the printed beam elements, and is therefore both material- and loading-rate independent.