Institute of Mechanics

Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

We report significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and a procedure are developed to calculate back stress basing on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient. This induces strain gradient, which needs to be accommodated by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening to increase the ductility.Impact Statement: Gradient structure leads to high back stress hardening to increase strength and ductility. A physically sound equation is derived to calculate the back stress from an unloading/reloading hysteresis loop.

Heterostructured materials have been reported as a new class of materials with superior mechanical properties, which was attributed to the development of back stress. There are numerous reports on back stress theories and measurements with no consensus. Back stress is developed in soft domains to offset the applied stress, making them appear stronger, while forward stress is developed to make hard domains appear weaker. The extra hardening in heterostructured materials is resulted from interactions between back stresses and forward stresses, and should be described as hetero-deformation induced (HDI) hardening and the measured ‘back stress’ should be renamed HDI stress.

Heterostructured (HS) materials are a new class of materials that are composed of heterogeneous zones with dramatically different (>100 %) mechanical or physical properties. The interactive coupling between these heterogeneous zones produces a synergistic effect in which the integrated property exceeds the prediction by the rule-of-mixtures. HS materials possess superior mechanical or physical properties that are not achievable by their conventional homogenous counterparts. This review focuses primarily on structural HS materials, whose superior mechanical properties are enabled by a new scientific principle: hetero-deformation induced (HDI) strengthening and HDI work hardening. Geometrically necessary dislocations (GNDs) in the soft zones pile up and accumulate near the zone boundaries, producing back stress in the soft zones and forward stress in the hard zones, which collectively produces the HDI stress. HS materials have a unique deformation behavior: formation of dispersive microscopic strain bands, which helps to distribute plastic strain over the whole gauge length, increasing uniform elongation. They can be readily produced using conventional industrial technologies and facilities at large scale and low cost. The superior properties, new materials science and great application potentials are driving the fast development of the HS materials field. This review is meant to introduce students and researchers to this emerging field, and to serve as an authoritative reference on HS materials.

Here we present a perspective on heterogeneous materials, a new class of materials possessing superior combinations of strength and ductility that are not accessible to their homogeneous counterparts. Heterogeneous materials consist of domains with dramatic strength differences. The domain sizes may vary in the range of micrometers to millimeters. Large strain gradients near domain interfaces are produced during deformation, which produces a significant back-stress to strengthen the material and to produce high back-stress work hardening for good ductility. High interface density is required to maximize the back-stress, which is a new strengthening mechanism for improving mechanical properties.

Gradient structures have evolved over millions of years through natural selection and optimization in many biological systems such as bones and plant stems, where the structures change gradually from the surface to interior. The advantage of gradient structures is their maximization of physical and mechanical performance while minimizing material cost. Here we report that the gradient structure in engineering materials such as metals renders a unique extra strain hardening, which leads to high ductility. The grain-size gradient under uniaxial tension induces a macroscopic strain gradient and converts the applied uniaxial stress to multiaxial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby the accumulation and interaction of dislocations are promoted, resulting in an extra strain hardening and an obvious strain hardening rate up-turn. Such extraordinary strain hardening, which is inherent to gradient structures and does not exist in homogeneous materials, provides a hitherto unknown strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones with dramatic (>100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. The heterostructured materials field explores heterostructures to control defect distributions, long-range internal stresses, and nonlinear inter-zone interactions for unprecedented performances. This paper is aimed to provide perspectives on this novel field, describe the state-of-the-art of heterostructured materials, and identify and discuss key issues that deserve additional studies.

The strength-ductility trade-off has been a long-standing dilemma in materials science. This has limited the potential of many structural materials, steels in particular. Here we report a way of enhancing the strength of twinning-induced plasticity steel at no ductility trade-off. After applying torsion to cylindrical twinning-induced plasticity steel samples to generate a gradient nanotwinned structure along the radial direction, we find that the yielding strength of the material can be doubled at no reduction in ductility. It is shown that this evasion of strength-ductility trade-off is due to the formation of a gradient hierarchical nanotwinned structure during pre-torsion and subsequent tensile deformation. A series of finite element simulations based on crystal plasticity are performed to understand why the gradient twin structure can cause strengthening and ductility retention, and how sequential torsion and tension lead to the observed hierarchical nanotwinned structure through activation of different twinning systems.

The discovery of gravitational waves (GWs) by the LIGO collaboration [1] in 2016 has provided a direct test on the prediction made by Albert Einstein a century ago based on his general theory of relativity [2]. It has caused a significant influence worldwide on the basic research in science. Space-based GW detection has become the next interesting target for the further study on the gravitational universe as space-based GW detection would reach a wider range of gravitational radiation sources than the ground-based GW detection can [3]. After the LISA/eLISA strategic plan [4] on space-based GW detection was put forward in the 1990s, Chinese scientists also showed their interest and began to make proposals for space-based GW detection in the 2000s. Recently, the Chinese Academy of Sciences (CAS) has set up a strategic priority research program that includes the pre-study of space-based GW detection referred to as the ‘Taiji Program in Space’ [5]. GWs are expected to provide a new window to explore the evolution of early universe and the nature of gravity. Though space- and ground-based GW detectors adopt the same detecting principle of a Michelson interferometer, the required critical technology is quite different. Space-based GW detection needs to make use of gravitational reference sensors and a low-noise micro-thruster to implement drag-free performance as well as non-contacting discharging of test masses for free-flying test mass. It requires picometer optical assemblies for resolution ranging between the test masses and the spacecraft and high-stability monolithic precision ranging of test mass and spacecraft. The high-stability telescope, precision attitude control, a high-accuracy phase-meter and frequency stabilization are all needed for the precision ranging from spacecraft to spacecraft. To reach high thermomechanical stability, it is necessary to orbit the Sun. The Taiji program is proposed to detect GWs with frequencies covering the range of 0.1 mHz to 1.0 Hz with higher sensitivity around 0.01–1 Hz than eLISA (see Table 1). The Taiji program proposes to use a triangle of three spacecrafts in orbit around the Sun (see Fig. 1). Laser beams are sent both ways between each pair of spacecrafts, and the differences in the phase changes between the transmitted and received laser beams at each spacecraft are measured. The preliminary design for the Taiji mission is based on 3-million-kilometer separations between the spacecrafts, and the expected launch date is about 2033. The purpose of the Taiji program is to study the most challenging issues concerning massive black holes, such as how the intermediate mass seed black holes were formed in the early universe, whether dark matter could form a black hole, how seed black hole grows into a large or extremely large black holes and what is the nature of gravity. Taiji spacecraft in orbit around the Sun. Baseline design parameters of the Taiji preliminary mission proposal in comparison with the LISA/eLISA mission proposal. Baseline design parameters of the Taiji preliminary mission proposal in comparison with the LISA/eLISA mission proposal. Gravity was the first recognized basic force in the universe, while its nature is not as clear as for the three other basic forces. Single gravitons are far from being observable, although GWs are thought to be composed of gravitons. Understanding the nature of gravity and the unification of all basic forces are the most challenging problems in basic sciences in the twenty-first century. Dark matter and dark energy observed via gravitational effects are regarded as two big ‘puzzling clouds’ in the twenty-first century in physics and astronomy. Solving those problems requires us to develop quantum gravity and build a unification theory for all basic forces. As the early universe was filled with elementary particles at very high energy and temperature, exploring the intrinsic correlation between the very small elementary particles and the extremely large universe has been regarded as the frontier of particle physics and cosmology in basic sciences. The establishment of quantum gravity [6] is a necessity to understand the origin and evolution of the universe. This work was supported in part by the National Natural Science Foundation of China (11690022, 11475237 and 11121064), and by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB23030100), as well as the CAS Center for Excellence in Particle Physics (CCEPP).

Kinking of a plane strain crack out of the interface between two dissimilar isotropic elastic solids is analyzed. The focus is on the initiation of kinking and thus the segment of the crack leaving the interface is imagined to be short compared to the segment in the interface. Accordingly, the analysis provides the stress intensity factors and energy release rate of the kinked crack in terms of the corresponding quantities for the interface crack prior to kinking. Roughly speaking, the energy release rate is enhanced if the crack heads into the more compliant material and is diminished if it kinks into the stiff material. The results suggest a tendency for a crack to be trapped in the interface irrespective of the loading when the compliant material is tough and the stiff material is at least as tough as the interface.

Rational design of artificial micro-structured metamaterials with advanced mechanical and physical properties that are not accessible in nature materials is challenging and important. In our paper, mechanical designs of 2D and 3D chiral mechanical metamaterials are reviewed, and their mechanical behaviors and deformation mechanisms can be investigated through equilibrium principle, strain energy analysis, micropolar elasticity and homogenization theories. Afterwards, multifunctional properties of chiral mechanical metamaterials are elaborated, such as: vibration attenuation, impact energy absorption and negative coefficient of thermal expansion (CTE). Finally, several successful industrial applications of chiral mechanical metamaterials are demonstrated, such as: morphing airfoil smart deployable antenna and reconfigurable structures, auxetic stent, chiral flexible electronics and phase transforming metastructures, etc. Finally, perspectives and challenges on chiral mechanical metamaterials are discussed.

The work done during indentation is examined using dimensional analysis and finite element calculations for conical indentation in elastic-plastic solids with work hardening. An approximate relationship between the ratio of hardness to elastic modulus and the ratio of irreversible work to total work in indentation is found. Consequently, the ratio of hardness to elastic modulus may be obtained directly from measuring the work of indentation. Together with a well-known relationship between elastic modulus, initial unloading slope, and contact area, a new method is then suggested for estimating the hardness and modulus of solids using instrumented indentation with conical or pyramidal indenters.

Gradient structures are characterized with a systematic change in microstructures on a macroscopic scale. Here, we report that gradient structures in engineering materials such as metals produce an intrinsic synergetic strengthening, which is much higher than the sum of separate gradient layers. This is caused by macroscopic stress gradient and the bi-axial stress generated by mechanical incompatibility between different layers. This represents a new mechanism for strengthening that exploits the principles of both mechanics and materials science. It may provide for a novel strategy for designing material structures with superior properties.

A flexible and elastic carbon coil (see figure) has been fabricated using a continuous yarn of carbon nanotube arrays. The processed yarn is both elastic and pliable and can be freely manipulated and molded to any desired shape that is retained after heat treatment. Owing to their highly ordered macroscopic structures, the good electrical and thermal conductivity of the nanotube components, and their good mechanical properties, these carbon nanostructures may find extensive use in a wide range of applications.

Conventional alloys are usually based on a single host metal. Recent high-entropy alloys (HEAs), in contrast, employ multiple principal elements. The strength of HEAs is considerably higher than traditional solid solutions, as the many constituents lead to a rugged energy landscape that increases the resistance to dislocation motion, which can also be retarded by other heterogeneities. The wide variety of nanostructured heterogeneities in HEAs, including those generated on the fly during tensile straining, also offer elevated strain-hardening capability that promotes uniform tensile ductility. Citing recent examples, this review explores the multiple levels of heterogeneities in multi-principal-element alloys that contribute to lattice friction and back stress hardening, as a general strategy towards strength-ductility synergy beyond current benchmark ranges.

Rock failure phenomena, such as rockburst, slabbing (or spalling) and zonal disintegration, related to deep underground excavation of hard rocks are frequently reported and pose a great threat to deep mining. Currently, the explanation for these failure phenomena using existing dynamic or static rock mechanics theory is not straightforward. In this study, new theory and testing method for deep underground rock mass under coupled static-dynamic loading are introduced. Two types of coupled loading modes, i.e. “critical static stress + slight disturbance” and “elastic static stress + impact disturbance”, are proposed, and associated test devices are developed. Rockburst phenomena of hard rocks under coupled static-dynamic loading are successfully reproduced in the laboratory, and the rockburst mechanism and related criteria are demonstrated. The results of true triaxial unloading compression tests on granite and red sandstone indicate that the unloading can induce slabbing when the confining pressure exceeds a certain threshold, and the slabbing failure strength is lower than the shear failure strength according to the conventional Mohr-Column criterion. Numerical results indicate that the rock unloading failure response under different in situ stresses and unloading rates can be characterized by an equivalent strain energy density. In addition, we present a new microseismic source location method without premeasuring the sound wave velocity in rock mass, which can efficiently and accurately locate the rock failure in hard rock mines. Also, a new idea for deep hard rock mining using a non-explosive continuous mining method is briefly introduced.

Ductility, i.e., uniform strain achievable in uniaxial tension, diminishes for materials with very high yield strength. Even for the CrCoNi medium-entropy alloy (MEA), which has a simple face-centered cubic (FCC) structure that would bode well for high ductility, the fine grains processed to achieve gigapascal strength exhaust the strain hardening ability such that, after yielding, the uniform tensile strain is as low as ∼2%. Here we purposely deploy, in this MEA, a three-level heterogeneous grain structure (HGS) with grain sizes spanning the nanometer to micrometer range, imparting a high yield strength well in excess of 1 GPa. This heterogeneity results from this alloy's low stacking fault energy, which facilitates corner twins in recrystallization and stores deformation twins and stacking faults during tensile straining. After yielding, the elastoplastic transition through load transfer and strain partitioning among grains of different sizes leads to an upturn of the strain hardening rate, and, upon further tensile straining at room temperature, corner twins evolve into nanograins. This dynamically reinforced HGS leads to a sustainable strain hardening rate, a record-wide hysteresis loop in load-unload-reload stress-strain curve and hence high back stresses, and, consequently, a uniform tensile strain of 22%. As such, this HGS achieves, in a single-phase FCC alloy, a strength-ductility combination that would normally require heterogeneous microstructures such as in dual-phase steels.

Core-shell nanoparticles (NPs) with lipid shells and varying water content and rigidity but with the same chemical composition, size, and surface properties are assembled using a microfluidic platform. Rigidity can dramatically alter the cellular uptake efficiency, with more-rigid NPs able to pass more easily through cell membranes. The mechanism accounting for this rigidity-dependent cellular uptake is revealed through atomistic-level simulations.

Exosomes, molecular cargos secreted by almost all mammalian cells, are considered as promising biomarkers to identify many diseases including cancers. However, the small size of exosomes (30-200 nm) poses serious challenges in their isolation from complex media containing a variety of extracellular vesicles (EVs) of different sizes, especially in small sample volumes. Here we present a viscoelasticity-based microfluidic system to directly separate exosomes from cell culture media or serum in a continuous, size-dependent, and label-free manner. Using a small amount of biocompatible polymer as the additive in the media to control the viscoelastic forces exerted on EVs, we are able to achieve a high separation purity (>90%) and recovery (>80%) of exosomes. The proposed technique may serve as a versatile platform to facilitate exosome analyses in diverse biochemical applications.

Bending rigidity and Gaussian bending stiffness are the two key parameters that govern the rippling of suspended graphene-an unavoidable phenomenon of two-dimensional materials when subject to a thermal or mechanical field. A reliable determination about these two parameters is of significance for both the design and the manipulation of graphene morphology for engineering applications. By combining the density functional theory calculations of energies of fullerenes and single wall carbon nanotubes with the configurational energy of membranes determined by Helfrich Hamiltonian, we have designed a theoretical approach to accurately determine the bending rigidity and Gaussian bending stiffness of single-layered graphene. The bending rigidity and Gaussian bending stiffness of single-layered graphene are 1.44 eV (2.31 × 10(-19) N m) and -1.52 eV (2.43 × 10(-19) N m), respectively. The bending rigidity is close to the experimental result. Interestingly, the bending stiffness of graphene is close to that of lipid bilayers of cells about 1-2 eV, which might mechanically justify biological applications of graphene.