The main objective of this article is to introduce novel 3D bio‐inspired auxetic meta‐structures printed with soft/hard polymers for energy absorption/dissipation applications under single and cyclic loading–unloading. Meta‐structures are developed based on understanding the hyper‐elastic feature of thermoplastic polyurethane (TPU) polymers, elastoplastic behavior of polyamide 12 (PA 12), and snowflake inspired design, derived from theory and experiments. The 3D meta‐structures are fabricated by multi‐jet fusion 3D printing technology. The feasibility and mechanical performance of different meta‐structures are assessed experimentally and numerically. Computational finite element models (FEMs) for the meta‐structures are developed and verified by the experiments. Mechanical compression tests on TPU auxetics show unique features like large recoverable deformations, stress softening, mechanical hysteresis characterized by non‐coincident compressive loading–unloading curve, Mullins effect, cyclic stress softening, and high energy absorption/dissipation capacity. Mechanical testing on PA 12 meta‐structures also reveals their elastoplastic behavior with residual strains and high energy absorption/dissipation performance. It is shown that the developed FEMs can replicate the main features observed in the experiments with a high accuracy. The material‐structural model, conceptual design, and results are expected to be instrumental in 3D printing tunable soft and hard meta‐devices with high energy absorption/dissipation features for applications like lightweight drones and unmanned aerial vehicles (UAVs).

Both 2D and 3D re-entrant designs are among the well-known prevalent auxetic structures exhibiting negative Poisson's ratio. The present study introduces novel analytical relationships for 2D re-entrant hexagonal honeycombs for both negative and positive ranges of the cell interior angle θ (θ<0 showing a negative Poisson's ratio). The derived analytical solutions are validated against finite element method (FEM) and experimental results. The results show that, compared to the analytical solutions available in the literature, the analytical relationships presented in this study provide the most accurate results for elastic modulus, Poisson's ratio, and yield stress. The analytical/computational tools are then implemented for designing Kinesio taping (KT) structures applicable to treatment of Achilles tendon injuries. One of the main features of the Achilles tendon is a natural auxetic behavior. Poisson's ratio distribution of an Achilles tendon is obtained using longitudinal and transverse strains and are then used to design and 3D print thermoplastic polyurethane (TPU) KT structures with non-uniform distribution of auxetic unit cells. The presented novel KT shows that it is capable of replicating the deformation and global and local Poisson's ratio distributions, similar to those of the Achilles tendon. Due to the absence of similar formulations and procedures in the literature, the results are expected to be instrumental for designing and 3D printing of flexible implants with unusual auxeticity.

Additive manufacturing has provided a unique opportunity to fabricate highly complex structures as well as sandwich structures with various out-of-plane cores. The application of intelligent materials, such as shape memory polymers, gives an additional dimension to the three-dimensional (3D) printing process, known as four-dimensional (4D) printing, so that the deformed structures can return to their initial shape by the influence of an external stimulus like temperature. In this study, 4D printing of smart sandwich structures with the potential of energy absorption is investigated. The samples were fabricated considering various process parameters (i.e. layer height, nozzle temperature, printing velocity, and wall thickness) and tested mechanically. The experimental work reveals that the deformed sandwiches can fully recover their initial form by applying simple heating. Besides, a reliable finite element model (FEM) was developed to predict the functional behavior of the horseshoe sandwich structures in compression analysis. The experimental and simulation results show that among process parameters, wall thickness, layer height, and nozzle temperature are the most significant parameters to increase the compressive load and, consequently, the energy absorption rate. The concept, results, and modeling provided in this study are expected to be used in the design and fabrication of 4D printed sandwich structures for energy absorption applications.

In maritime transportation, a fender acts like a bumper to absorb the kinetic energy of a boat berthing against a jetty, pier wall, or other boats. They have high energy absorption and low reaction forces, preventing damage to boats and berthing structures. The aim of this paper is to introduce a novel conceptual design for a new class of lightweight boat-fendering systems with superior energy absorption/dissipation and shape recovery features. Different metamaterials with honeycomb, re-entrant, and re-entrant chiral auxetic patterns are designed in the form of boat fender panels, and their thermo-mechanical behaviors are analyzed experimentally and numerically. A finite element modeling (FEM) is developed to investigate the compressive behaviors of boat fenders. Some of designs are 4D printed by fused filament fabrication of shape memory polylactic acid polymers and then tested thermo-mechanically. A good correlation is observed between numerical and experimental results, supporting the FEM accuracy. Results reveal that proposed boat fenders have considerable energy absorption/dissipation along with the capability to fully recover plastic deformations by simply heating up. The excellent mechanical property recovery of the proposed boat-fendering system is also shown under cycling loadings. Due to the absence of similar conceptual designs, models, and results in the specialized literature, this paper is expected to be instrumental towards 4D printing novel boat fenders with supreme energy absorption/dissipation and shape recovery properties promoting sustainability.

In this study, numerical modeling is used to investigate the performance of a single-sided composite patch with different scale fillers, as reinforcement of a cracked aluminum plate under static tension. The main concerns of previous studies are about the geometry of patches, composite layups, and failure of adhesive. In this research, the effect of patch properties such as size and fiber volume fraction, the thickness of patch, and thickness of adhesive on the overall performance of the cracked aluminum plate are investigated numerically. Indeed, first, a 3 D representative volume element (RVE) is adopted to calculate the mechanical properties of carbon nanotube (CNT)/epoxy and carbon fiber (CF)/epoxy composite patch at each specified volume fraction for investigating the effect of patch properties on the performance of a single-sided patch for crack repairing. In this regard, the cohesive zone model is adopted to analyze the debonding between the epoxy matrix and reinforcement to characterize the mechanical properties of composite patches. Finally, a linear 3 D finite element analysis is performed to calculate the stress intensity factor (SIF) for cracked aluminum plate repaired by a single-sided composite patch at each specified reinforcement volume fraction for different thickness of patch and different thickness of adhesive. The results demonstrated that the stress intensity factor highly depends on the patch properties (patch stiffness) in addition to patch thickness and adhesive thickness.

Dielectric actuators (DEAs), because of their exceptional properties, are well-suited for soft actuators (or robotics) applications. This article studies a multi-stimuli thermo-dielectric-based soft actuator under large bending conditions. In order to determine the stress components and induced moment (or stretches), a nominal Helmholtz free energy density function with two types of hyperelastic models are employed. Non-linear electro-elasticity theory is adopted to derive the governing equations of the actuator. Total deformation gradient tensor is multiplicatively decomposed into electro-mechanical and thermal parts. The problem is solved using the second-order Runge-Kutta method. Then, the numerical results under thermo-mechanical loadings are validated against the finite element method (FEM) outcomes by developing a user-defined subroutine, UHYPER in a commercial FEM software. The effect of electric field and thermal stimulus are investigated on the mean radius of curvature and stresses distribution of the actuator. Results reveal that in the presence of electric field, the required moment to actuate the actuator is smaller. Finally, due to simplicity and accuracy of the present boundary problem, the proposed thermally-electrically actuator is expected to be used in future studies and 4D printing of artificial thermo-dielectric-based beam muscles.