A NUMERICAL STUDY OF INDENTATION WITH SMALL SPHERICAL INDENTERS

The finite element method for the conventional theory of mechanism-based strain gradient plasticity is used to study the indentation size effect. For small indenters （e.g., radii on the order of 10μm）, the maximum allowable geometrically necessary dislocation （GND） density is introduced to cap the GND density such that the latter does not become unrealistically high. The numerical results agree well with the indentation hardness data of iridium. The GND density is much larger than the density of statistically stored dislocations （SSD） underneath the indenter, but this trend reverses away from the indenter. As the indentation depth （or equivalently, contact radius） increases, the GND density decreases but the SSD density increases.

Editorial: Mechanics of soft materials, structures and systems

The concept of soft matter was first introduced by P. G. de Gennes in his acceptance speech for the No-bel Physics Prize in 1991. In mechanics community, however, people usually prefer using soft material in-stead of soft matter to describe the material whose en-ergy associated with thermal motion is comparative to the interaction energy. Unlike in the conventional con-densed matter, entropy plays an important and even de-terminative role in soft materials.

A Review on the Mullins Effect in Tough Elastomers and Gels

Tough elastomers and gels have garnered broad research interest due to their wide-ranging potential applications.However,during the loading and unloading cycles,a clear stress softening behavior can be observed in many material systems,which is also named as the Mullins effect.In this work,we aim to provide a complete review of the Mullins effect in soft yet tough materials,specifically focusing on nanocomposite gels,double-network hydrogels,and multi-network elastomers.We first revisit the experimental observations for these soft materials.We then discuss the recent developments of constitutive models,emphasizing novel developments in the damage mechanisms or network representations.Some phenomenological models will also be briefly introduced.Particular attention is then placed on the anisotropic and multiaxial modeling aspects.It is demonstrated that most of the existing models fail to accurately predict the multiaxial data,posing a significant challenge for developing future anisotropic models tailored for tough gels and elastomers.

X-Mechanics——An endless frontier

In contrast to the conventional wisdom that mechanics is a relatively mature subject, the new manifestation of mechanics in an extended or crossed form is unfolding. Mechanics is now powering all subjects, from physical sciences, life sciences to social sciences. We name this new phase for the development of mechanics X-Mechanics. The present article outlines the contents of X-Mechanics from four aspects: cross media, cross scales, cross compliances, and cross cyber/physical spaces. X-Mechanics constitutes an endless frontier of science and technology.

Pneumatically Actuated Soft Robotic Arm for Adaptable Grasping

Developing robotic manipulators capable of performing effective physical interac- tion tasks is a challenging topic. In this study, we design a soft robotic arm （SRA） with multiple degrees of freedom inspired by the flexible structures and the unique motion mechanism of the octopus arm. The SRA is fabricated with elastomeric materials, which consists of four series of integrated pneumatic chambers that play similar roles as the muscles in the octopus arm can achieve large bending in various directions with variable stiffness. This SRA displays specified movements via controlling pressure and selecting channels. Moreover, utilizing parallel control, the SRA demonstrates complicated three-dimensional motions. The force response and motion of the SRA are determined both experimentally and computationally. The applications of the present SRA include tightly coiling around the objects because of its large bending deformation （nearly 360°）, grasping multiple objects, and adjusting the grabbing mode in accordance with the shape of objects.

RATE DEPENDENT STRESS-STRETCH RELATION OF DIELECTRIC ELASTOMERS SUBJECTED TO PURE SHEAR LIKE LOADING AND ELECTRIC FIELD

The performance of dielectric elastomer （DE） transducers is significantly affected by viscoelastic relaxation-induced electromechanical dissipations. This paper presents an experi- mental study to obtain the rate dependent stress-stretch relation of DE membranes （VHBTM9473） subjected to pure shear like loading and electric loading simultaneously. Stretching rate depen- dent behavior is observed. The results also show that the tensile force decreases as the voltage increases. The observations are compared with predictions by a viscoelastic model of DE. This experiment may be used for further studies of dynamic electromechanical coupling properties of DEs.

A FINITE ELEMENT METHOD FOR DIELECTRIC ELASTOMER TRANSDUCERS

We present a finite element method for dielectric elastomer （DE） transducers based on the nonlinear field theory of DE. The method is implemented in the commercial finite element software ABAQUS, which provides a large library functions to describe finite elasticity. This method can be used to solve electromechanical coupling problems of DE transducers with complex configurations and under inhomogeneous deformation.

Inclusion Size Effect on Mechanical Properties of Particle Hydrogel Composite

Particle hydrogel composite(PHC)combines the characteristics of at least two mat erials and has pot ential applications in many fields.Different functions require the particle size to range from nanometer to millimeter,which has a noticeable effect on the mechanical properties of the hydrogel composites.In this paper,the mechanical properties of silica-inlaid PAAM hydrogel are measured with various particle diameters from 75 nm to 50|im.Experimental results show no obvious size effect on the mechanical properties of PHC when the particle diameter falls in micron scale.However,as the particle size decreases to nanoscale,the modulus of the PHC begins to increase rapidly.The size-irrelevant moduli and stress fields of PHCs with random and uniform particle distributions under different loading conditions are obtained based on the finite element method.Meanwhile,the toughening mechanism and the failure of the PHC are investigated.The size-irrelevant modulus of the PHC is also predicted by the equivalent inclusion theory.Finally,the interaction between the hydrogel polymer chains and the particles is described from the microscopic point of view,requiring the nanoscale size-dependent theory and new experimental approach to further explore the mechanical proper ties of PHC.

Fatigue‑Resistant and Hysteresis‑Free Composite Fibers with a Heterogeneous Hierarchical Structure

Fatigue-resistant and hysteresis-free composite fibers hold great promise for the next generation of wearable electronic devices.In this study,a novel approach for the fabrication of composite fibers with outstanding elasticity and mechanical stability is proposed.The design incorporates a heterogeneous hierarchical structure(HHS),which mimics the structure of arteries,to achieve enhanced fatigue resistance and hysteresis-free performance.The composite fibers,Ecoflex-polyacrylamide fibers(EPFs),are created through the combination of heterogeneous elastomers and strong interfacial coupling.The results show that the EPFs exhibit exceptional fatigue resistance,being able to withstand up to 10,000 load–unload cycles at strains of 300%without any noticeable changes in their mechanical properties.The potential applications of these EPFs are demonstrated through their use as strain sensors for monitoring human motion in both air and water,as well as in energyharvesting e-textiles.

Probing the constitutive behavior of microcrystals by analyzing the dynamics of the micromechanical testing system

The constitutive behavior of microcrystals remains mysterious since very little,or no information regarding plastic deformation in the measured stress-strain curve is available due to plastic instability.Furthermore,the measured stress-strain curves vary greatly under different control modes,while constitutive behavior should remain unaffected by test methods.Beyond these reasons,probing the real constitutive behavior of microcrystals has long been a challenge because the nonlinear dynamical behaviors of micromechanical testing systems are unclear.Here,we perform and carefully analyze the experiments on singlecrystal aluminum micropillars under displacement control and load control.To interpret these experimental results,a lumpedparameter physical model based on the principle of micromechanical testing is developed,which can directly relate nonlinear dynamics of the micromechanical testing system to the constitutive behavior of microcrystals.This reveals that some stages of the measured stress-strain curve attributed to the control algorithm are not related to constitutive behavior.By solving the nonlinear dynamics of the micromechanical testing system,intense plastic instability(large strain burst)starting from the equilibrium state is attributed to the strain-softening stage of microcrystals.Parametric studies are also performed to reduce the influence of plastic instability on the measured responses.This study provides critical insights for developing various constitutive models and designing a reliable micromechanical testing system.

Mechanical Behaviors of Soft Elastomers Filled with LowMelting Alloys

Soft elastomers with mechanical properties similar to biological tissues have shown encouraging potential in applications of biomedical devices and stretchable electronics.With the ability to enhance electric and thermal conductivity,embedding low melting alloys into soft elastomer matrix has received considerable attention in recent years.However,the mechanical properties,especially the fatigue behaviors of these soft composites,have not been extensively investigated.Here,we fabricate two silicone elastomers filled with eutectic gallium-indium,a liquid metal alloy,which has a melting temperature around room temperature.The cyclic loading–unloading tests are first performed on the composites with low melting alloys in either solid or fluid state.The results show that the modulus and energy dissipation density of the composite increase with the ratio of the alloys when the deformation temperature is below the melting temperature,while these properties decrease with the ratio of alloys when they are in the fluid state.In contrast,the failure strain shows an opposite trend.Mechanical tests are further performed on specimens with a precut to measure the fracture energy and fatigue threshold.It is demonstrated that both fracture energy and fatigue threshold are significantly enhanced in the presence of low melting alloys regardless of their states.Finally,we apply a continuum damage model to describe the Mullins effect of the soft composites observed in the loading–unloading cycles,which further reveals the change of mechanical properties with deformation for different compositions of soft composites.

Preface:Mechanics of soft materials and lexible structures

Soft materials and lexible structures have become essential and hot topic,which is expected to signiicantly promote the research and development of current mechanics,physics,chemistry and life science.Furthermore,the soft materials and lexible structures have attracted enthusiastic attentions due to their great potential and practical applications in deployable structures,morphing structures,sensors,actuators,health-monitoring etc.

Preface to the"Soft Matter Mechanics"Special Issue of Acta Mechanica Solida Sinica

Different from ordinary solids,liquids and gases,soft matter is a subfield of condensed matter comprising a complexity of physical states and softness that are easily modulated by external stimuli and/or thermal flue tu at ions,with the featured sensitivity to tiny perturbations from the environment.The soft matter research is considered to be“the science of the 21st century,”which is rapidly evolving and growing at the interfaces of mechanics,physics,biology,material science,chemistry,etc.