Reversible energy absorbing meta-sandwiches by FDM 4D printing

Highlights

• Introducing dual-material auxetic meta-sandwiches by 4D printing technology.

• Arranging soft hyper-elastic polymers and elasto-plastic hard shape memory polymers.

• Proposing reversible energy absorbers by leveraging instability and elasto-plasticity.

• Implementing finite element methods to predict experimental observations.

• Showing high capability of reversible meta-sandwiches for dissipating energy.

Abstract

The aim of this paper is to introduce dual-material auxetic meta-sandwiches by four-dimensional (4D) printing technology for reversible energy absorption applications. The meta-sandwiches are developed based on an understanding of hyper-elastic feature of soft polymers and elasto-plastic behaviors of shape memory polymers and cold programming derived from theory and experiments. Dual-material lattice-based meta-structures with different combinations of soft and hard components are fabricated by 4D printing fused deposition modelling technology. The feasibility and performance of reversible dual-material meta-structures are assessed experimentally and numerically. Computational models for the meta-structures are developed and verified by the experiments. Research trials show that the dual-material auxetic designs are capable of generating a range of non-linear stiffness as per the requirement of energy absorbing applications. It is found that the meta-structures with hyper-elastic and/or elasto-plastic features dissipate energy and exhibit mechanical hysteresis characterized by non-coincident compressive loading-unloading curves. Mechanical hysteresis can be achieved by leveraging elasto-plasticity and snap-through-like mechanical instability through compression. Experiments also reveal that the mechanically induced plastic deformation and dissipation processes are fully reversible by simply heating. The material-structural model, concepts and results provided in this paper are expected to be instrumental towards 4D printing tunable meta-sandwiches for reversible energy absorption applications.

Introduction

Meta-materials are man-made materials shaped from repeating unit cells that are designed to achieve unique multi-functional properties that cannot be found in nature [1]. These structures gain the extraordinary thermo-mechanical properties from their architecture and programmed structures, rather than the bulk behavior of their elements. Meta-materials exhibit interesting features such as zero/negative Poisson's ratio (auxetic behaviors) [2], variable softening/hardening [3], effective thermal properties [4,5], and unusual dynamical behaviors [6,7].

Energy absorbing structures can be found in the nature. Bones, teeth, woods, antlers, teeth, horns and hooves are some examples of natural energy absorbers [8]. Meta-materials and bio-inspired cellular structures are extensively designed for energy absorption applications such as crash mitigation in vehicles and airplanes, protective packaging of sensitive elements and personal/sportive protection equipment. They can experience large compressive strains at almost constant stress level absorbing a large amount of energy without inducing high stress level. The principle of energy absorption in meta-materials and lattice-based structures provides the capability to convert kinetic energy into other types of energies through elastic and/or plastic deformations, mechanical instability and structural collapse [8], [9], [10]. Among various energy absorption mechanisms, plastic deformation in ductile materials like metals and polymers has been the most effective classical method to absorb energy [8,10]. Tan et al. [10] proposed stainless steel meta-materials that dissipate energy through instability and plastic deformation. Cyclic loading-unloading compression experiments were conducted to test the structure's repeatability, and annealing treatment was carried out to improve it. The results showed that the structure is repeatable but its repeatability declines as its dimensions increase. The disadvantage of these kinds of energy absorbers is that failure by fracture may occur during loading-unloading and plastic deformation growth. Shape memory alloys such as nickel-titanium (NiTi) have been employed to introduce reversible energy absorbers with shape memory effect and super-elasticity feature [11,12]. Their energy absorption mechanism is based on the recoverable macroscopic austenitic-martensitic phase transformations. Yuan et al. [13] proposed a thermo-mechanically triggered two-stage pattern switching approach, where an amorphous polymer and a flexible elastomer were incorporated in periodic lattices ranging from square mesh, re-entrant honeycomb to tetrachiral lattice.

In recent years, additive manufacturing technologies, well-known as three-dimensional (3D) printing, have enabled fabrication of meta-materials and lattice-based structures for elastic and elasto-plastic deformations [14]. For instance, Bates et al. [15] studied energy absorption capacity of hyper-elastic thermoplastic polyurethanes honeycombs fabricated by fused deposition modeling (FDM). The honeycomb density was graded by varying cell wall thickness through the structures to absorb a wide range of compression energies. Habib et al. [16] studied in-plane static compressive crushing behavior and energy absorption capacity of polymeric Nylon 12 honeycomb structures of different unit cell thicknesses fabricated by FDM. It was observed that the plastic deformation of hexagonal honeycombs is different in their two main in-plane directions. Taheri Andani et al. [17] fabricated NiTi shape memory alloys in dense and designed porous forms by selective laser melting (SLM). 3D printed NiTi showed good shape memory properties promising for lightweight structures and energy absorbers. Mirzaali et al. [18] used 3D printing based on the PolyJet technology to rationally design and fabricate multi-material cellular solids for which the elastic modulus and Poisson's ratio could be independently tailored in different directions. Yazdani Sarvestani et al. [19] investigated energy absorption and structural performance of lightweight sandwich polylactic acid (PLA) panels with architected cellular cores of various six-sided cells fabricated by FDM 3D printing. The results showed that the auxetic sandwich panel is an appropriate candidate for energy absorption applications. Al-Saedi et al. [20] investigated mechanical properties and energy absorption capability of functionally graded and uniform F2BCC lattice structures made of Al-12Si Aluminum alloy and manufactured by SLM process. Hedayati et al. [21] conducted analytical, numerical and experimental studies to investigate the elastic modulus and yield strength of PLA meta-materials based on the diamond- and cube-shaped unit cells with variable cross sections fabricated by FDM technology. Yang et al. [22] improved energy absorption properties of self-locked systems by designing the shape and geometry of tubes made of 3D printed soft-photopolymer resin and hard materials like stainless steel. Alomarah et al. [23] investigated compressive properties of polyamide12 auxetic structures 3D printed by Multi Jet Fusion (MJF) method. Xu et al. [24] investigated in-plane uniaxial compressive response and energy absorption capacity of a novel hybrid configuration of auxetic and hexagonal honeycomb cells fabricated with nylon material by Object 350 3D printer. The developed meta-structures exhibited superior Young's modulus, collapse strength and energy absorption than traditional honeycomb structures. Geng et al. [25] investigated mechanical response, damage initiation and failure evolution of metallic meta-materials fabricated by SLM 3D printing under quasi-static compression. The results showed that the failure behavior of the reentrant lattice is governed not only by its topology, but also by the geometric defects and surface defects. Using 3D printing based on the PolyJet technology, Zhao et al. [26] demonstrated the design and validation of the zigzagged periodic lattice-based metamaterials with thermally tunable deformation modes and the thermally switchable Poisson's ratio. By regulating the deformation mode with ambient temperature, the effective Poisson's ratio of the lattice could be intentionally switched between negative values and positive values. In parallel with the growth of conventional 3D printing technologies, 4D printing has been creeping up in the additive manufacturing field. [27]. The 4^th dimension associates to “shape change” that 3D printed objects evolve over the time, directly off the print tray. For instance, Li et al. [28] introduced 4D printed biodegradable, remotely controllable and personalized shape memory polymer (SMP) occlusion devices and exemplified atrial septal defect occluders. By incorporating Fe₃O₄ magnetic particles into the SMP matrix, the deployment of the occluders could be controlled remotely after implantation. Xin et al. [29] presented a comprehensive review of the mechanical models of SMPs, SMP composites, SMP nanocomposites and their applications in space-deployable structures and 4D printing techniques. Using 4D printing technology, a few researches [30,31] have been conducted to introduce reconfigurable mechanical single material meta-structures with impact-mitigation capabilities.

Literature review reveals that most of the previous experimental and numerical studies have been directed to investigate energy absorption of single material 3D/4D printed meta-structures under monotonic compressive loading. This paper aims at developing dual-material (soft-hard) auxetic meta-sandwiches by 4D printing FDM technology for reversible energy absorption applications. The conceptual design is based on arranging soft hyper-elastic polymers and elasto-plastic hard SMPs along with cold programming. Dual-material auxetics with different combinations of soft and hard components are 4D printed by FDM technology. It is revealed that the 4D printed meta-structures have great potentials in dissipating kinetic energy due to mechanical hysteresis by leveraging snap-through-like instability and elasto-plasticity. Numerical simulations implementing ANSYS finite element method (FEM) are performed to predict experimental observations on mechanical loading and unloading process. It is found that the non-linear compressive hardening-softening behavior, plateau stress and energy absorption capacity of meta-structures can be replicated accurately by the simulations. The material-structural model, concepts and results provided in this paper are expected to open an avenue for the design and implementation of reversible energy dissipation devices by harnessing shape memory effect (SME), elasto-plasticity, and mechanical instability.