RESUMEN
A room-temperature self-healing silicone elastomer was prepared based on the synergistic effect of multiple H-bonding and dynamic covalent bond. The multiple bonds constructed by inserting thiourea into the polyurea network can inhibit the crystallization of hard urea H-bonds segments and activate the diffusion movement of polymer chains. Dynamic imine endows materials with a strong connection for the fracture interface by imine metathesis. The effect of thiourea on urea H-bonds was confirmed by the Fourier transform infrared spectrum, which showed obvious changes of H-bond density according to peak revolution of CâO and N-H. Differential scanning calorimetry demonstrated the transition from the crystalline to amorphous state after the introduction of thiourea. Tensile tests and scratch-healing tests showed that this design method can improve the self-healing property without sacrificing the mechanical strength. Finally, the optimized self-healing process was analyzed from the perspectives of the contact process, the interpenetration diffusion of the polymer chain, and rebuilding of crosslinking points between the two interfaces, which would build an avenue for constructing a fast, self-healing, and tough material.
RESUMEN
Architected 2D structures are of growing interest due to their unique mechanical and physical properties for applications in stretchable electronics, controllable phononic/photonic modulators, and switchable optical/electrical devices; however, the underpinning theory of understanding their elastic properties and enabling principles in search of emerging structures with well-defined arrangements and/or bonding connections of assembled elements has yet to be established. Here, we present two theoretical frameworks in mechanics-strain energy-based theory and displacement continuity-based theory-to predict the elastic properties of 2D structures and demonstrate their application in a search for novel architected 2D structures that are composed of heterogeneously arranged, arbitrarily shaped lattice cell structures with regulatory adjacent bonding connections of cells, referred to as heterogeneously architected 2D structures (HASs). By patterning lattice cell structures and tailoring their connections, the elastic properties of HASs can span a very broad range from nearly zero to beyond those of individual lattice cells by orders of magnitude. Interface indices that represent both the pattern arrangements of basic lattice cells and local bonding disconnections in HASs are also proposed and incorporated to intelligently design HASs with on-demand Young's modulus and geometric features. This study offers a theoretical foundation toward future architected structures by design with unprecedented properties and functions.
RESUMEN
Engineering Janus structures that possess anisotropic features in functions have attracted growing attention for a wide range of applications in sensors, catalysis, and biomedicine, and are yet usually designed at the nanoscale with distinct physical or chemical functionalities in their opposite sides. Inspired by the seamless integration of soft and hard materials in biological structures, here a mechanical Janus structure composed of soft and hard materials with a dramatic difference in mechanical properties at an additively manufacturable macroscale is presented. In the combination of extensive experimental, theoretical, and computational studies, the design principle of soft-hard materials integrated mechanical Janus structures is established and their unique rotation mechanism is addressed. The systematic studies of assembling the Janus structure units into superstructures with well-ordered organizations by programming the local rotations are further shown, providing a direct route of designing superstructures by leveraging mechanical Janus structures with unique soft-hard material integration. Applications are conducted to demonstrate the features and functionalities of assembled superstructures with local ordered organizations in regulating and filtering acoustic wave propagations, thereby providing exemplification applications of mechanical Janus design in functional structures and devices.
RESUMEN
Although a wide range of self-healing materials have been reported by researchers, it is still a challenge to endow exceptional mechanical properties and shape memory characteristics simultaneously in a single material. Inspired by the structure of natural silk, herein, we have adopted a simple synthetic method to prepare a kind of elastomer (HM-PUs) with stiff, healable and shape memory capabilities assisted by multiple hydrogen bonds. The self-healing elastomer exhibits a maximum tensile strength of 39 MPa, toughness of 111.65 MJ m-3 and self-healing efficiency of 96%. Moreover, the recuperative efficiency of shape memory could reach 100%. The fundamental study of HM-PUs will facilitate the development of flexible electronics and medical materials.
RESUMEN
To develop the full application potential of composite materials, research on the post-buckling behavior of composite stiffened panels is of great significance. In this paper, the impact and compression after impact (CAI) behaviors of four different types of composite stiffened panels were studied by numerical simulation and experimental methods. The low-velocity impact damage simulated dynamically was introduced as the initial state in the compression simulation, and a two-dimensional shell model with Hashin failure criteria and stiffness degradation was adopted to estimate the failure load of composite stiffened panels under impact and CAI. The error between simulation results and test results was less than 10%, showing that the method used in this study achieved considerable accuracy in experimental results. Analysis of the impact test results revealed that the extent of damage is related to many factors, including the cross-sectional size of stiffeners, the spacing of stiffeners, and the material and thickness of the skin. In addition, the influence of fatigue damage on residual strength after impact was also studied experimentally, with results showing that the buckling and failure loads decreased by about 5% under 106 flight fatigue loads. However, there were obvious fluctuations in the load-displacement curves, which may have been caused by debonding between the stiffeners and the skin. Experimental results and the simulation matrix show that the post-buckling ratio increased with the increase of the stiffness ratio, then was stable after 2.0. Furthermore, the thinner the skin, the greater the post-buckling ratio. The experimental and simulation results provide an important reference for the structural design and failure-mechanism analysis of composite stiffened panels.
RESUMEN
An auxetic design is proposed by soft-hard material integration and demonstrate negative Poisson's ratio (NPR) can be achieved by leveraging unique rotation features of non-connected hard particles in a soft matrix. A theoretical mechanics framework that describes rotation of hard particles in a soft matrix under a mechanical loading is incorporated with overall Poisson's ratio of the soft-hard integrated metamaterials. The theoretical analysis shows that the auxetic behaviour of the soft-hard integrated structures not only relies critically on geometry of particles, but also depends on their periodic arrangements in the soft matrix. Extensive finite-element analyses (FEA) are performed and validate the theoretical predictions of hard-particle rotation and overall Poisson's ratio of soft-hard integrated structures. Furthermore, uniaxial tensile tests are carried out on three-dimensional printed soft-hard integrated structures and confirm auxetic behaviour of soft-hard integrated structures enabled by the rotation of hard particles. Besides, Poisson's ratio varies nonlinearly with the thickness of specimens and reaches a maximum NPR far out of the bounds of plane stress and plane strain situations, which agrees well with FEA. This work provides a theoretical foundation for the design of mechanical metamaterials enabled by soft-hard material integration with auxetic deformation behaviour.
RESUMEN
Carbon is one of the most important materials extensively used in industry and our daily life. Crystalline carbon materials such as carbon nanotubes and graphene possess ultrahigh strength and toughness. In contrast, amorphous carbon is known to be very brittle and can sustain little compressive deformation. Inspired by biological shells and honeycomb-like cellular structures in nature, we introduce a class of hybrid structural designs and demonstrate that amorphous porous carbon nanospheres with a thin outer shell can simultaneously achieve high strength and sustain large deformation. The amorphous carbon nanospheres were synthesized via a low-cost, scalable and structure-controllable ultrasonic spray pyrolysis approach using energetic carbon precursors. In situ compression experiments on individual nanospheres show that the amorphous carbon nanospheres with an optimized structure can sustain beyond 50% compressive strain. Both experiments and finite element analyses reveal that the buckling deformation of the outer spherical shell dominates the improvement of strength while the collapse of inner nanoscale pores driven by twisting, rotation, buckling and bending of pore walls contributes to the large deformation.