Embracing nonlinearity and geometry: a dimensional analysis guided design of shock absorbing materials.

Design of shock absorbers requires a delicate balance between mechanical properties and geometric design, allowing them to be compressible yet strong enough to withstand crushing loads. Here, we present a unified framework for designing compact and lightweight shock absorbers by employing a streamlined kinematic model and dimensional analysis. We derive geometric constraints on the thickness and cross-sectional area of a protective foam with a given stress-strain response to ensure that acceleration and compressive strain remain within critical limits.

Disrupting Density-Dependent Property Scaling in Hierarchically Architected Foams.

Creating lightweight architected foams as strong and stiff as their bulk constituent material has been a long-standing effort. Typically, the strength, stiffness, and energy dissipation capabilities of materials severely degrade with increasing porosity. We report nearly constant stiffness-to-density and energy dissipation-to-density ratios─a linear scaling with density─in hierarchical vertically aligned carbon nanotube (VACNT) foams with a mesoscale architecture of hexagonally close-packed thin concentric cylinders.

Exceptional-point-based accelerometers with enhanced signal-to-noise ratio.

Exceptional points (EP) are non-Hermitian degeneracies where eigenvalues and their corresponding eigenvectors coalesce. Recently, EPs have attracted attention as a means to enhance the responsivity of sensors, through the abrupt resonant detuning occurring in their proximity. In many cases, however, the EP implementation is accompanied by noise enhancement, leading to the degradation of the sensor's performance.

Extreme Dynamic Performance of Nanofiber Mats under Supersonic Impacts Mediated by Interfacial Hydrogen Bonds.

Achieving extreme dynamic performance in nanofibrous materials requires synergistic exploitation of intrinsic nanofiber properties and inter-fiber interactions. Regardless of the superior intrinsic stiffness and strength of carbon nanotubes (CNTs), the weak nature of van der Waals interactions limits the CNT mats from achieving greater performance. We present an efficient approach to augment the inter-fiber interactions by introducing aramid nanofiber (ANF) links between CNTs, which forms stronger and reconfigurable interfacial hydrogen bonds and π-π stacking interactions, leading to synergistic performance improvement with failure retardation.

Extreme Energy Dissipation via Material Evolution in Carbon Nanotube Mats.

Thin layered mats comprised of an interconnected meandering network of multiwall carbon nanotubes (MWCNT) are subjected to a hypersonic micro-projectile impact test. The mat morphology is highly compliant and while this leads to rather modest quasi-static mechanical properties, at the extreme strain rates and large strains resulting from ballistic impact, the MWCNT structure has the ability to reconfigure resulting in extraordinary kinetic energy (KE) absorption. The KE of the projectile is dissipated via frictional interactions, adiabatic heating, tube stretching, and ultimately fracture of taut tubes and the newly formed fibrils.

Superior Energy Dissipation by Ultrathin Semicrystalline Polymer Films Under Supersonic Microprojectile Impacts.

Distinct deformation mechanisms that emerge in nanoscale enable the nanostructured materials to exhibit outstanding specific mechanical properties. Here, we present superior microstructure- and strain-rate-dependent specific penetration energy (up to ∼3.8 MJ kg) in semicrystalline poly(vinylidene fluoride--trifluoroethylene) (P(VDF-TrFE)) thin films subjected to high-velocity (100 m s to 1 km s) microprojectile (diameter: 9.

Asymmetric acoustic energy transport in non-Hermitian metamaterials.

The ability to control and direct acoustic energy is essential for many engineering applications such as vibration and noise control, invisibility cloaking, acoustic sensing, energy harvesting, and phononic switching and rectification. The realization of acoustic regulators requires overcoming fundamental challenges inherent to the time-reversal nature of wave equations. Typically, this is achieved by utilizing either a parameter that is odd-symmetric under time-reversal or by introducing passive nonlinearities.

High-velocity projectile impact induced 9R phase in ultrafine-grained aluminium.

Aluminium typically deforms via full dislocations due to its high stacking fault energy. Twinning in aluminium, although difficult, may occur at low temperature and high strain rate. However, the 9R phase rarely occurs in aluminium simply because of its giant stacking fault energy.

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.

Dynamic creation and evolution of gradient nanostructure in single-crystal metallic microcubes.

We demonstrate the dynamic creation and subsequent static evolution of extreme gradient nanograined structures in initially near-defect-free single-crystal silver microcubes. Extreme nanostructural transformations are imposed by high strain rates, strain gradients, and recrystallization in high-velocity impacts of the microcubes against an impenetrable substrate. We synthesized the silver microcubes in a bottom-up seed-growth process and use an advanced laser-induced projectile impact testing apparatus to selectively launch them at supersonic velocities (~400 meters per second).