Auxetic metamaterials exhibit a negative Poisson’s ratio (NPR) through geometry-controlled deformation and offer considerable potential for impact energy absorption. This review systematically examines the structural configurations, deformation mechanisms, enhancement strategies and engineering applications of auxetic energy absorbers. Six representative topology families, including perforated, re-entrant, chiral, rotationally rigid, origami and other specialized structures, are reviewed in terms of their geometry, NPR mechanisms, collapse behavior and energy dissipation performance. Five major enhancement strategies are then evaluated, including baseline enhancement, optimization algorithms, multi-material integration, multi-structure integration and gradient design, with emphasis on their effects on load paths, peak force, plateau stability, densification and failure modes. A distinctive contribution is the quantitative synthesis of 37 paired, within-study comparisons of specific energy absorption (SEA). Reported SEA enhancements range from 6% to 800%, with a median of 81.5%, demonstrating the potential of structural innovation while revealing substantial variability among designs. The analysis indicates that NPR magnitude alone cannot determine energy absorption performance, as geometry, constituent material, relative density, loading conditions, manufacturing precision, environmental conditions and boundary constraints are also critical. Applications in aerospace, transportation, personal protection, biomedical devices and civil infrastructure are reviewed in relation to their functional requirements. Finally, current limitations and future directions are discussed, emphasizing integrated design strategies, manufacturing accuracy, performance under complex loading and environmental conditions, and the transition from laboratory concepts to practical engineering applications.
Geometric heterogeneity (including the particle morphology and spatial distribution) of Al/SiCp composites complicates tool-particle-matrix interaction and surface damage state, making low-damage diamond cutting challenging. Therefore, a profound understanding of geometric heterogeneity-induced damage behaviors is essential for achieving ultra-precision machining of Al/SiCp composites. This study systematically explores the combined effects of mechanical and geometric heterogeneities using plunge-cut experiments and finite element (FE) simulations. Experimental results show that edge collapse caused by the loss of the soft matrix support inevitably occurs at the SiC-Al interface, and its severity is significantly influenced by interparticle spacing, interfacial morphology angles (i.e., the geometric heterogeneity discussed in this study) as well as the tool rake angle. Consequently, to reconstruct the evolution of the ductile-brittle transition and interfacial damage, a newly developed FE model incorporating geometric heterogeneity is proposed and thoroughly validated through qualitative and quantitative comparisons (including the second critical transition depth dc2 and cutting forces). Simulation results indicate that narrow interparticle spacings, obtuse interfacial morphology angles, and negative rake angles give rise to tensile stress concentration toward the front of the tool rake face. This stress redistribution promotes horizontal crack propagation, thereby delaying premature brittle failure and alleviating interfacial damage. For instance, as interparticle spacing decreased from 1.5 to 0.5 μm, the dc2, a quantitative indicator of brittle removal, increased from 188.50 nm to 196.75 nm. Ultimately, a mechanical analysis model is developed to intuitively explain the stress-driven mechanisms of heterogeneity-induced interfacial damage, offering theoretical guidance for ultra-precision machining of Al/SiCp composites.
Effective noise suppression in acoustic enclosures often relies on precisely tuned Helmholtz resonators (HRs). However, conventional design methodologies are hindered by the unavailability of accurate analytical models, especially for complex-shaped cavities, and the practical infeasibility of dense three-dimensional acoustic field measurements. To overcome these limitations, this paper proposes a hybrid Data-Physics-Informed Neural Network (Data-PINN) framework for reconstruction-driven HR design using sparse microphone measurements. First, a spatial-frequency decomposition strategy decouples the acoustic response: environmentally sensitive frequency-dependent characteristics are directly extracted from sparse microphone measurements, while the relatively stable spatial pressure distributions are reconstructed via a PINN constrained by a homogeneous Helmholtz equation. Second, a bi-level discrete optimization framework hierarchically determines optimal sensor placements within physically realizable discrete grids, maximizing both signal-to-noise ratio and spatial observability. Finally, leveraging the reconstructed full-field pressure, an analytical design methodology based on Excitation-Dependent Representative Basis (EDRB) decomposition and fixed-point theory is proposed to determine the optimal HR parameters. Numerical and experimental validations on both a canonical rectangular cavity and a complex helicopter cabin mockup demonstrate the high fidelity of the proposed framework in reconstructing the acoustic field, achieving a normalized mean squared error of less than 7 × 10−3. Consequently, this enables HR designs that achieve noise suppression performance within a 3 dB deviation from the ground-truth benchmark. (Code available upon acceptance)
During heatwaves or droughts, soft plant organs wilt by loosing stiffness. This softening corresponds to a loss of intracellular fluid. By controlling osmosis in vitro, the stiffness of soft plant tissues was shown to increase with turgor pressure. This increase was traditionally explained by considering an incompressible fluid enclosed in an elastic membrane and small deformations of the cell walls. However, we found an opposite trend when testing a synthetic cellular tissue. Here we show that the stiffness of pressurized cellular solids depends on the thermodynamic process of the fluid and the hyperelastic behavior of the cell wall. At low pressure, the Young’s modulus of pressurized cellular solids decreases when the pressure inside the cell is fixed (isobaric process). The opposite trend occurs when a liquid is enclosed in the membrane (isochoric process). When pressure is increased, the strain stiffening of the walls dominates and leads to an increase of stiffness regardless of the fluid process. We show that linear elastic models assuming osmotic process are incompatible with turgor-induced stiffening of plants. However, considering nonlinear strain stiffening of cell walls, the observed behavior is recovered. The actual turgor-dependent rigidity of plants may involve more complex mechanisms, possibly combining mechanical, chemical and biological processes. The idealized models of stiffness will be valuable tools for theoretical biology, from plant growth to impacts on soft animal tissues. Furthermore, we believe that the clarification of the fluid–structure interaction in pressurized cellular solids will allow to design active metamaterials with programmable stiffness.
With superior symmetry and scalability, polyhedral mechanisms are ideal building blocks for large-scale modular expansion in metamorphic mechanisms and reconfigurable metastructures. To leverage these advantages, this paper presents a tessellated metamorphic mechanism based on an origami- and kirigami-inspired polyhedral mechanism. First, through mechanism equivalent simplification, a streamlined degrees-of-freedom (DOF) analysis method is proposed, mapping complex spatial cells to planar 8R-2P mechanisms. This significantly reduces the analysis complexity for polyhedral cells, especially when tessellated into large-scale networks. Subsequently, the DOFs and configurational characteristics of the tessellation are analyzed. Next, a novel compliant constraint inspired by the principle of minimum magnetic field energy is introduced to construct a rigid- compliant coupled multivariate metamorphic mechanism. Finally, the combined effects of external forces, geometric boundaries, and internal compliant constraints support configuration maintenance and sequential transitions, yielding five motion modes with decoupled two-DOF and constraint-induced single-DOF characteristics. This paper provides a feasible theoretical framework for investigating the configuration regulation of modular metamorphic mechanisms.
