Theory and Calculation of the J-Integral for Coupled Chemo-Mechanical Fracture Mechanics

In this paper,by introducing a chemical field,the J-integral formulation is presented for the chemo-mechanical coupled medium based on the laws of thermodynamics.A finite element implementation of the J-integral was performed to study the mode I chemo-mechanical coupled fracture problem.For derivation of the coupled J-integral,the equivalent domain integral(EDI)method was applied to obtain the mode I J-integral,with expression of the area integrals based on constitutive relationships of a linear elastic small deformation for chemo-mechanical coupling,instead of the finite deformation problem.A finite element procedure is developed to compute the mode I J-integral,and numerical simulation of the y-direction stress field is studied by a subroutine UEL(User defined element)developed in ABAQUS software.Accuracy of the numerical results obtained using the mode I J-integral was verified by comparing them to a well-established model based on linear elastic fracture mechanics(LEFM).Furthermore,a numerical example was presented to illustrate path-independence of the formulated J-integral for a chemo-mechanical coupled specimen under different boundary conditions,showing a high accuracy and reliability of the present method.The variation laws of J-integral and the y-direction stress field with external chemical,mechanical loading and time are revealed.The J-integral value increases with larger external concentration loading in the same integral domain.The extent of diffusion is much greater with larger concentration,which leads to a stronger coupling effect due to the chemical field.This work provides new insights into the fracture mechanics for the chemo-mechanical coupled medium.

Micromechanics-Based Elastic Fields of Closed-Cell Porous Media

Fluid-filled closed-cell porous media could exhibit distinctive features which are influenced by initial fluid pressures inside the cavities.Based on the equivalent farfield method,micromechanics-based solutions for the local elastic fields of porous media saturated with pressurized fluid are formulated in this paper.In the present micromechanics model,three configurations are introduced to characterize the different state the closed-cell porous media.The fluid-filled cavity is assumed to be a compressible elastic solid with a zero shear modulus,and the pressures in closed pores are represented by eigenstrains introduced in fluid domains.With the assumption of spheroidal fluidfilled pores,the local stress and strain fields in solid matrix of porous media are derived by using the Exterior-Point Eshelby tensors,which are dependent of the Poisson’s ratio of solid matrix and the locations of the investigated material points outside the spheroidal fluid domain.The reliability and accuracy of the analytical elastic solutions are verified by a classical example.Moreover,for finite volume fraction of the fluid inclusions,the local elastic fields of the porous media subjected to the initial fluid pressure and external load are obtained.The results show that the present micromechanics model provides an effective approach to characterize the local elastic fields of the materials with closed-cell fluid-filled pores.

A Finite Element Procedure for Analysis of Chemo-Mechanical Coupling Behavior of Hydrogels

Chemo-mechanical coupling behavior of materials is a transformation process between mechanical and chemical energy.In this paper,based on the coupled chemo-mechanical constitutive equations and governing equations during isothermal process,the equivalent integral forms of chemo-mechanical coupling governing equations and corresponding finite element procedure are obtained by using Hamilton’s principle.An isoparametric plane element for chemo-mechanical coupling is associated into ABAQUS finite element package through user element subroutine UEL.The numerical examples exhibit that the ionic concentration variation can cause mechanical deformation and mechanical action can produce redistribution of ionic concentration for hydrogels.It is proved that the present developed chemo-mechanical coupling finite element procedure can be utilized to model the coupling behavior of hydrogels effectively.

智柔超材料及其力学性能的研究进展

力学超材料(或超结构)因其独特的微观结构设计而展现超常的物理和力学特性.将力学超材料设计思想与智能柔性(简称智柔)材料相结合,可以制备出具有自感知和自驱动功能的智柔力学超材料(简称智柔超材料).本文对近年来智柔超材料的研究现状和进展进行了评述,分析了此类材料的基本设计思想、变形机理及力学特性,重点关注了基于形状记忆聚合物和水凝胶智柔超材料的设计原理和性能分析方法;阐释了先进制造技术为智柔超材料发展带来的机遇,并讨论了此类材料在设计和开发方面面临的关键问题以及未来发展趋势.

The Influence of Entanglements of Net Chains on Phase Transition Temperature of Sensitive Hydrogels in Chemo-Mechanical Coupled Fields

Phase transition of hydrogel,which is polymerized by polymer network,can be regarded as the transition of polymer network stability.The stability of the polymer network might be changed when the external environment changed.This change will lead to the transformation of sensitive hydrogels stability,thus phase transition of hydrogel take place.Here,we present a new free density energy function,which considers the non-gaussianity of the polymer network,chains entanglement and functionality of junctions through adding Gent hyplastic model and Edwards-Vilgis slip-link model to Flory-Huggins theory.A program to calculate the phase transition temperature was written based on new free energy function.Taking PNIPAM hydrogel as an example,the effects of network entanglement on the phase transition temperature of hydrogel were studied by analyzing the microstructure parameters of the hydrogel networks.Analytical results suggest a significant relationship between phase transition temperature and entanglement network.

ANALYTICAL SOLUTIONS FOR A ONE-DIMENSIONAL CHEMO-MECHANICAL COUPLING PROBLEM

Chemo-mechanical coupling exists in a lot of intelligent materials including hy- drogels, biological tissues and other soft materials. These materials are able to respond to ex- ternal stimulus, such as temperature, chemical concentration, and pH value. In this paper, a one-dimensional theoretical model for chemo-mechanical coupling is proposed for analyzing the uniaxial stress/strain state of coupling materials. Based on the chemo-mechanical coupled gov- erning equation, the displacement function and concentration function are derived and the stress and chemical potential are obtained. It is shown that the present chemo-mechanical theory can characterize the chemo-mechanical coupling behavior of intelligent materials.

All-solid-state carbon-nanotube-fiber-based finger-muscle and robotic gripper

Carbon nanotube fibers(CNTFs)have many desirable properties such as lightweight,high strength,high conductivity,and long lifetimes.Coiled CNTF is an ideal material for preparing electrochemically driven artificial muscles.While previous studies focused mainly on the actuation performance of artificial muscles made of CNTF,this study focuses on an actuator that mimics human finger movements(flexion).More specifically,the preparation of CNTF muscles were optimized by twisting with weight.Then,actuators are designed and assembled by combining all-solid-state CNTF muscles with polypropylene(PP)sheets.Moreover,a dualelectrode system,which is infiltrated by a gel electrolyte,is built into the muscle actuator.In addition,a robotic gripper is fabricated,which uses these actuators.This study can help improve the design of CNTF-based muscle-actuators and future applications in robotics.