Emergence of Exceptional Points in Periodic Metastructures with Hidden PT-symmetric Defects

We study the elastodynamics of a periodic metastructure incorporating a defect pair that enforces a parity-time (PT) symmetry due to a judiciously engineered imaginary impedance elements - one having energy amplification (gain) and the other having an equivalent attenuation (loss) mechanism. We show that their presence affects the initial band structure of the periodic Hermitian metastructure and leads to the formation of numerous exceptional points (EPs) which are mainly located at the band edges where the local density of modes is higher. The spatial location of the PT-symmetric defect serves as an additional control over the number of emerging EPs in the corresponding spectra as well as the critical non-Hermitian (gain/loss) strength required to create the first EP - a specific defect location minimizes the critical non-Hermitian strength. We use both finite element and coupled-mode-theory-based models to investigate these metastructures, and use a time-independent second-order perturbation theory to further demonstrate the influence of the size of the metastructure and the PT-symmetric defect location on the minimum non-Hermitian strength required to create the first EP in a band. Our findings motivate feasible designs for the experimental realization of EPs in elastodynamic metastructures

Shock formation and rate effects in impacted carbon nanotube foams

We investigate rate-effects in the dynamic response of vertically aligned carbon nanotube (VACNT) foams excited by impacts at controlled velocities. They exhibit a complex rate-dependent loading–unloading response at low impact velocities and they support shock formation beyond a critical velocity. The measured critical velocities are ∼10 times lower than in other foams of similar densities—a desirable characteristic in impact protective applications. In-situ high-speed microscopy reveals strain localization and progressive buckling at low velocities and a crush-front propagation during shock compression. We correlate these responses to quantitative measurements of the density gradient and fiber morphology, obtained with spatially resolved X-ray scattering and mass attenuation

Dynamic Hardness Evolution in Metals from Impact Induced Gradient Dislocation Density

A clear understanding of the dynamic behavior of metals is critical for developing superior structural materials as well as for improving material processing techniques such as cold spray and shot peening. Using a high-velocity (from 120 m/s to 550 m/s; strain rates >10^7 1/s) micro-projectile impact testing and quasistatic (strain rates: 10^-2 1/s) nanoindentation, we investigate the strain-rate-dependent mechanical behavior of single-crystal aluminum substrates with (001), (011), and (111) crystal orientations. Independent of crystal orientation, the dynamic hardness initially increases with increasing impact velocity and reaches a plateau regime at hardness 5 times higher than that of at quasistatic indentations. Based on coefficient of restitution and post-mortem transmission Kikuchi diffraction analyses, we show that distinct plastic deformation mechanisms with a gradient dislocation density evolution govern the dynamic behavior. We also discover a distinct deformation regime-stable plastic regime-that emerge beyond the deeply plastic regime with unique strain rate insensitive microstructure evolution and dynamic hardness. Our work additionally demonstrates an effective approach to introduce strong spatial gradient in dislocation density in metals by high-velocity projectile impacts to enhance surface mechanical properties, as it can be employed in material processing techniques such as shot peening and surface mechanical attrition treatment

Rate-sensitive strain localization and impact response of carbon nanotube foams with microscale heterogeneous bands

We describe the deformation mechanisms and the bulk dynamic response of vertically aligned carbon nanotube (VACNT) foams comprised of bands of different densities. The densities of the bands are controlled during synthesis by varying the flow-rate of gas feedstock in discrete steps. We show that the impact response of VACNT foams can be distinctively tailored by introducing heterogeneous bands. For example, we demonstrate that this approach can be used to maintain the stress plateau at low stresses over a broad range of strains and to disrupt the expected progressive deformation of the sample. These are desirable characteristics for impact and energy absorption applications. The banded VACNT foams exhibit different deformation mechanisms in dynamics compared to those in quasistatic compression, as observed through in-situ high-speed microscopy

Anomalous impact and strain responses in helical carbon nanotube foams

We describe the quasistatic and dynamic response of helical carbon nanotube (HCNT) foams in compression. Similarly to other CNT foams, HCNT foams exhibit preconditioning effects in response to cyclic loading; however, their fundamental deformation mechanisms are unique. In quasistatic compression, HCNT foams exhibit strain localization and collective structural buckling, nucleating at different weak sections throughout their thickness. In dynamic compression, they undergo progressive crushing, governed by the intrinsic density gradient along the thickness of the sample. HCNT micro-bundles often undergo brittle fracture that originates from nanoscale defects. Regardless of this microstructural damage, bulk HCNT foams exhibit super-compressibility and recover more than 90% of large compressive strains (up to 80%). When subjected to striker impacts, HCNT foams mitigate impact stresses more effectively compared to other CNT foams comprised of non-helical CNTs ([similar]50% improvement). The unique mechanical properties we revealed demonstrate that the HCNT foams are ideally suited for applications in packaging, impact protection, and vibration mitigation

Impact absorption properties of carbon fiber reinforced bucky sponges

We describe the super compressible and highly recoverable response of bucky sponges as they are struck by a heavy flat-punch striker. The bucky sponges studied here are structurally stable, self-assembled mixtures of multiwalled carbon nanotubes (MWCNTs) and carbon fibers (CFs). We engineered the microstructure of the sponges by controlling their porosity using different CF contents. Their mechanical properties and energy dissipation characteristics during impact loading are presented as a function of their composition. The inclusion of CFs improves the impact force damping by up to 50% and the specific damping capacity by up to 7% compared to bucky sponges without CFs. The sponges also exhibit significantly better stress mitigation characteristics compared to vertically aligned carbon nanotube foams of similar densities. We show that delamination on the MWCNT-CF interfaces occurs during unloading, and arises from the heterogeneous fibrous microstructure of the bucky sponges

Self-Assembled Recyclable Hierarchical Bucky Aerogels

We describe a simple method for the scalable synthesis of three-dimensional, elastic, and recyclable multi-wall carbon nanotube (MWCNT) based light weight (density <1 g cm^(−3)) bucky-aerogels (BAGs) that are capable of efficiently absorbing non-polar solvents and separating oil-in-water emulsions. Our facile synthesis involves the self-assembly of MWCNTs and carbon fibers into a multi-layered, highly porous, and hierarchical structure that can be easily flexed, compressed, or burnt without any noticeable changes in its structure and absorption capacity. The BAG surface absorbs non-polar solvents efficiently up to ≈20 times its own weight due to its superhydrophobic nature arising from the presence of MWCNTs. Furthermore, BAGs exhibit excellent resilience to impact by recovering more than 70% of the deformation. The energy dissipated by BAGs at 80% compressive strain is in the order of 500 kJ m^(−3), which is nearly 50 times more than the energy dissipated by commercial foams with similar densities