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Zero Poisson’s ratio structures are a new class of mechanical metamaterials wherein the absence of lateral deformations allows the structure to adapt and conform their geometries to desired shapes with minimal interventions. These structures have gained attention in large deformation applications where shape control is a key performance attribute, with examples including but not limited to shape morphing, soft robotics, and flexible electronics. The present study introduces an experimentally driven approach that leads to the design and development of (near) zero Poisson’s ratio structures with considerable load-bearing capacities through concurrent density and architecture gradations in hybrid honeycombs created from hexagonal and re-entrant cells. The strain-dependent Poisson’s ratios in hexagonal and re-entrant honeycombs with various cell wall thicknesses have been characterized experimentally. A mathematical approach is then proposed and utilized to create hybrid structures wherein the spatial distribution of different cell shapes and densities leads to the development of honeycombs with minimal lateral deformations under compressive strains as high as 0.7. Although not considered design criteria, the load-bearing and energy absorption capacities of the hybrid structures are shown to be comparable with those of uniform cell counterparts. Finally, the new hybrid structures indicate lesser degrees of instability (in the form of cell buckling and collapse) due to the self-constraining effects imposed internally by the adjacent cell rows in the structures.

 

Abstract The residual effect of thermally and mechanically loaded polyurea samples was investigated in this study using terahertz time-domain spectroscopy (THz-TDS). Samples of different thicknesses were submerged in liquid nitrogen and allowed to reach cryogenic isothermal conditions while another set of samples were extracted from quasi-statically loaded strips. All samples were interrogated using THz-TDS since terahertz waves exhibit non-ionizing, nondestructive interactions with polymers. The time-domain terahertz signals were used to extract the optical and electrical properties as a function of sample thickness and loading conditions. The residual effect was prominent in the mechanically loaded samples compared to a nearly negligible presence in thermally loaded polyurea. On average, the results of the thermally loaded samples were subtle when compared to the virgin samples, whereas samples that were mechanically stretched showed a considerable difference in the characteristics of the time-domain signals. For example, the peak amplitude in the time-domain signal of the stretched thick sample showed a 9% difference from that of the virgin sample, whereas the thermally loaded sample saw only a 4.9% difference. Spectral analysis reported the frequency-dependent, complex refractive index of virgin and loaded polyurea as a function of thickness and spectral peaks associated with fundamental vibrational modes of the polyurea structure. The disappearance of three spectral peaks, 0.56 THz, 0.76 THz, and 0.95 THz, elucidated the residual effect of the mechanically loaded samples. In general, terahertz spectroscopy was shown to be a promising tool for future in situ and in operando investigations of field-dependent polymer responses. 

Density‐graded elastomeric foams are emerging as effective protective structures to guard humans against mechanical loading. This research investigates the deformation of ungraded and graded foams under quasistatic and impact scenarios using digital image correlation (DIC). The graded samples are assembled using two interfacing strategies (seamless and adhered), leveraging the adhesiveness of the foam slurry and bulk polyurea, respectively. Deformation mechanisms, including the effect of the interface type on strain transduction and localization in density‐graded structures, are imperative for improving the impact efficacy of protective paddings. Cuboid foam plugs are subjected to quasistatic and impact loading while recording the corresponding deformation for DIC analysis. The DIC results are separated into three case studies based on the number of layers (1, 2, and 3). The interface effect on the overall mechanical performance of polyurea foam is revealed from the bilayer, monodensity samples, showing drastic differences between the deformations within each layer. Seamless interface samples exhibit greater compliance than their adhered counterparts in the bilayer density‐graded configurations. Trilayer‐graded foams broaden strain–time history, extend the impact duration, and reduce strains. This research substantiates the importance of interfacing and gradation strategies on the mechanical response of elastomeric foams as a function of strain rate.

 

The proliferation of ordered cellular structures in industrial and technological applications is justified by their superior mechanical performance, including tunable energy absorption strategies and potential multifunctionality. This research evaluates the mechanical response of composite lattice structures fabricated using vat photopolymerization additive manufacturing process and printable particulate composite materials. Several generations of modified printable resins are prepared by hybridizing flexible resin with varying glass microballoons reinforcement weight percentages. Multifaceted characterization regiments highlight the process–property–performance interrelationship by submitting printed composite structures to quasi‐static and impact‐loading scenarios combined with digital stills and high‐speed photography, respectively. Image analyses of optical and scanning electron micrographs quantify the dimensional accuracy of the composite lattice structures with cylindrical and hexagonal cellular geometries. The mechanical characterization uncovers the effect of cell geometry and reinforcement on the global structural behavior, eliciting differences in load‐bearing capacity, local strain developments, and structural densification. Exploratory digital image correlation supports the global structural deformations, revealing their relationship with the developed local strain state within the unit cells. The outcomes of this research elucidate the effect of strain rate, unit cell geometry, and reinforcing ratios on the structural performance of composite lattice structures at the macro‐ and microstructure levels.

 

Elastomers with segmental microstructure are a fascinating class of shock‐tolerant and impact‐resistant materials. However, their technological potential remains untapped due to a vague understanding of the molecular contributions to their superior mechanical behavior. Herein, in situ light‐matter interactions, to reveal the extent of microstructural mobility by temporally exploiting molecular processes during creep response, are leveraged. The segmental microstructure comprises aromatic hard domains embedded within an aliphatic soft matrix. High‐resolution digital image correlation reveals the development of strain striations, mild anisotropy, and the mechanisms responsible for domain mobility, where the rate of hard segment mobility is found to be 60% slower than that of the soft segment. Terahertz spectral analyses pinpoint the contributions of interchain hydrogen bonding of the hard segments and their significant conformational changes by observing spectral features at ≈1.2THz and ≈1.67THz. Moreover, the domain mobility is examined using experimental and computational light scattering approaches, uncovering dynamic scattering and elucidating the difference in the complex refractive index of the soft and hard segments. The study unlocks the pathway for quantitative measurements of elusive molecular mobility and conformational changes during mechanical loading and sheds light on the origin of the shock tolerance in some elastomeric polymers with segmental microstructure.