The exceptional physical properties and unique layered structure of two-dimensional (2D) materials have made this class of materials great candidates for applications in electronics, energy conversion/storage devices, nanocomposites, and multifunctional coatings, among others. At the center of this application space, mechanical properties play a vital role in materials design, manufacturing, integration and performance. The emergence of 2D materials has also sparked broad scientific inquiry, with new understanding of mechanical interactions between 2D structures and interfaces being of great interest to the community. Building on the dramatic expansion of recent research activities, here we review significant advances in the understanding of the elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behavior of 2D materials. In this article, special emphasis is placed on some new 2D materials, novel characterization techniques and computational methods, as well as insights into deformation and failure mechanisms. A deep understanding of the intrinsic and extrinsic factors that govern 2D material mechanics is further provided, in the hopes that the community may draw design strategies for structural and interfacial engineering of 2D material systems. We end this review article with a discussion of our perspective on the state of the field and outlook on areas for future research directions.

Recent advances in the mechanics of 2D materials

Straining techniques for strain engineering of 2D materials towards flexible straintronic applications

The emerging mechanical functional metamaterials reported with promising mechanoelectrical characteristics bring increasing attention to structurally functional materials. It is essential to deploy mechanical metamaterials in energy materials for effective triggering and controllable mechanoelectrical response. This study reports origami tribo-metamaterials (OTMs) that design triboelectric materials in the origami-enabled, tubular metamaterials. The octagonal, hexagonal, and conical origami units are deployed as the metamaterial substrates to trigger the triboelectric pairs for mechanoelectrical multistability. For the octagonal OTM configuration with the triboelectric pair of fluorinated ethylene propylene-paper, the peak open-circuit voltage, short-circuit current, and transferred charge are obtained as 206.4 V, 4.66 μA, and 0.38 μC, respectively, and the maximum instantaneous output power density is 0.96 μW/cm2 with the load resistance of 20 MΩ. The OTM takes advantage of the origami metamaterials to obtain the multistable force-displacement response as effective stimuli for the triboelectric materials, which leads to tunable mechanoelectrical performance for speed and weight sensing and energy harvesting. The proposed OTM not only offers a strategy to structurally design energy materials to achieve desirable mechanoelectrical response, but also provides a guideline for the applications of mechanical functional metamaterials in practice.

Origami Tribo-Metamaterials with Mechanoelectrical Multistability.

Anisotropic mechanics of van der Waals (vdWs) materials offers opportunity to peel off individual atomic layers, initiating a 2D revolution in the fields of materials science, physics, and chemistry. The elasticity, bending, and fracture strength of most of their 2D derivatives are also orientation‐dependent, which not only determines the reliability of devices based on 2D materials but also offers a vast playground for atomic manufacturing with tunable functions. Therefore, a comprehensive understanding of the anisotropic mechanical properties of 2D materials is imminent. In this review, the anisotropic mechanical properties of 2D materials are summarized in attempt to capture the current progress in this field, as well as the route toward their applications. Following a brief discussion of the anisotropic lattice structures of 2D materials, unique experimental methodologies that have been developed to characterize their anisotropic mechanics are discussed. Then, the review pivots on recent processes in anisotropic elastic, fracture, friction, and bending properties of 2D materials. Unique applications of these anisotropic properties, such as mechanical fabrication of atomic precision, as well as anisotropic strain‐induced piezoelectric and band modulation, are further highlighted. Finally, besides emphasizing the need for breakthrough in anisotropic mechanics, prospects for the developments of this field are suggested.

Anisotropic Mechanics of 2D Materials

Characterized by periodic cellular unit cells, microlattices offer exceptional potential as lightweight and robust materials. However, their inherent periodicity poses the risk of catastrophic global failure. To address this limitation, a novel approach, that is to introduce microlattices composed of aperiodic unit cells inspired by Einstein's tile, where the orientation of cells never repeats in the same orientation is proposed. Experiments and simulations are conducted to validate the concept by comparing compressive responses of the aperiodic microlattices with those of common periodic microlattices. Indeed, the microlattices exhibit stable and progressive compressive deformation, contrasting with catastrophic fracture of periodic structures. At the same relative density, the microlattices outperform the periodic ones, exhibiting fracture strain, energy absorption, crushing stress efficiency, and smoothness coefficients at least 830%, 300%, 130%, and 160% higher, respectively. These improvements can be attributed to aperiodicity, where diverse failure thresholds exist locally due to varying strut angles and contact modes during compression. This effectively prevents both global fracture and abrupt stress drops. Furthermore, the aperiodic microlattice exhibits good damage tolerance with excellent deformation recoverability, retaining 76% ultimate stress post-recovery at 30% compressive strain. Overall, a novel concept of adopting aperiodic cell arrangements to achieve damage-tolerant microlattice metamaterials is presented.

Superior Strength, Toughness, and Damage-Tolerance Observed in Microlattices of Aperiodic Unit Cells.

Biological materials such as conch shells with crossed-lamellar textures hold impressive mechanical properties due to their capability to realize effective crack control and energy dissipation through the structural synergy of interfacial modulus mismatch and lamellar orientation disparity. Integrating this mechanism with mechanical metamaterial design can not only avoid the catastrophic post-yield stress drop found in traditional architectural materials with uniform lattice structures but also effectively maintain the stress level and improve the energy absorption ability. Herein, a novel bioinspired design strategy that combines regional particularity and overall cyclicity is proposed to innovate the connotation of long-range periodicity inside the metamaterial, in which the node constraint gradient and crossed-lamellar struts corresponding to the core features of conch shells are able to guide the deformation sequence with a self-strengthening response during compression. Detailed in situ experiments and finite element analysis confirm that the rotated broad layer stacking can shorten and impede the shear bands, further transforming the deformation of bioinspired metamaterial into a progressive, hierarchical way, highlighted by the cross-layer hysteresis. Even based on a brittle polymeric resin, excellent specific energy absorption capacity [4544 kJ/kg] has been achieved in this architecture, which far exceeds the reported metal-based syntactic foams for two orders of magnitude. These results offer new opportunities for the bioinspired metamaterials to substitute the widespread syntactic foams in specific applications required for both lightweight and energy absorption.

Three Dimensional Printing of Bioinspired Crossed-Lamellar Metamaterials with Superior Toughness for Syntactic Foam Substitution.

Efficient toughening of the interlaminar fracture toughness for CFRP composites without sacrificing in-plane mechanical performance remained an unresolved challenge. Here, we present a synergistic toughening strategy by construction of hierarchical architecture, within which carbon nanotubes (CNTs) in-depth stitched into the nano-channel between neighboring carbon fibers at the interlaminar region. Mode I fracture test revealed that, even at low concentration of CNTs (0.3 wt%), a considerable improvement (50%) on mode I fracture energy GIc of composites can be realized, from 1098.6 to 1647.8 J/m2 which is nearly two times greater than that of most aerospace CFRP laminates (∼500 J/m2). The excellent fracture toughness is predominantly attributed to the desired hierarchical architecture, which simultaneously triggered the intrinsic toughening by CNTs nano-bridging and extrinsic toughening mechanisms due to carbon fiber bridging, as was demonstrated by SEM images of fracture surfaces and further verified by finite element simulations. These findings offer significant guidelines for designing CFRP composites with high fracture toughness by application of low content CNTs using cost-effective resin mixing process. 

Synergistic toughening on CFRP via in-depth stitched CNTs

Gradient evolution in graphene reinforced carbon/carbon composites

In-situ interface characterization of mesophase pitch-based carbon fiber/epoxy composites

Conventionally, tuning materials' properties can be done through strategies such as alloying, doping, defect engineering, and phase engineering, while in fact mechanical straining can be another effective approach. In particular, elastic strain engineering (ESE), unlike conventional strain engineering mainly based on epitaxial growth, allows for continuous and reversible modulation of material properties by mechanical loading/unloading. The exceptional intrinsic mechanical properties (including elasticity and strength) of two-dimensional (2D) materials make them naturally attractive candidates for potential ESE applications. Here, we demonstrated that using the strain effect to modulate the physical and chemical properties toward novel functional device applications, which could be a general strategy for various 2D materials and their heterostructures. We then show how ultralarge, uniform elastic strain in free-standing 2D monolayers can permit deep elastic strain engineering (DESE), which can result in fundamentally changed electronic and optoelectronic properties for unconventional device applications. In addition to monolayers and van der Waals (vdW) heterostructures, we propose that DESE can be also applied to twisted bilayer graphene and other emerging twisted vdW structures, allowing for unprecedented functional 2D material applications.

Deep Elastic Strain Engineering of 2D Materials and Their Twisted Bilayers.

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