Studies On Failure Of Soft Media Containing Hard Inhomogeneity In 2D Deformations

The composite systems of soft matrices and hard/rigid inhomogeneities have found wide applications in mechanical and electronic engineering,such as soft robots,and stretchable electronics.The failure analysis of these systems is very important for corresponding applications.The rupture behavior of soft materials,most of which are natural rubbers,has been a concern of engineers and scientists since 1950 s.Recently,interests in fracture of soft materials have been renewed by the developments of material science and stretchable electronics.These studies mainly focused on the effects of cracks on the fracture or the interfacial debonding between hard and soft materials.However,few researches refer to the conditions of soft materials with hard inclusions,which is a common situation in inorganic stretchable electronics.From the viewpoint of inhomogeneity in solids,the rigid inclusion(e.g.a rigid line)in soft material is in analogy with the crack in brittle material.Generally,crack is always treated as the inducement of fracture in brittle materials,for the extreme moduli mismatch.Philosophically speaking,hard inclusion can be treated as the fracture inducement in soft materials for the same reason.Although soft materials are not susceptible to cracking,they indeed may be punctured by sharp hard matter.In particular,for the structure of soft matrix and line hard inhomogeneities,the mismatch in modulus and thermal expansion coefficient may lead to various failure at the interface of the structure.In the past researches regarding the problems of hard-line inhomogeneity,the inhomogeneity was treated as a rigid body which only rotates about its center and the axial stretch of the inhomogeneity was neglected.In the thermo-mechanical problems of a soft matrix containing hard line inhomogeneity,however,the axial stretch of the inhomogeneity may not be neglected.In fact,a mechanically rigid inhomogeneity may possibly have a moderate or large stretch induced by the thermal expansion,and this stretch may increase the possibility of the puncture of the soft matrix.For example,the thermal expansion of hard inhomogeneity may induce interface failure for the electronic part consisting of copper or siliceous equipment and soft substrate.Consequently,in order to establish a modified model for the thermos-mechanical problem,the line inhomogeneity should be treated as a mechanically rigid but thermally stretchable body.The main content of this paper includes:1)A detailed study on the fracture behaviors of soft materials with hard inclusion is presented.Based on linear fracture theory,the fracture behaviors of soft materials with elliptical hard inclusion are investigated.Stress concentrations,consisting of tensile,hoop and compressive stress,are observed with changes of inclusion geometries and the modulus ratio of hard and soft materials.And their influences on the categories of principal stress concentration are shown in a phase diagram.Finite element analysis is carried out with consideration of the large deformation of soft material,which demonstrates the effectiveness of the theoretical predictions in a great scope of applied loading.the predictions based on theoretical and simulation results are validated by experiments.This work points out that the hard line inclusion is the source of danger in soft materials just like the crack in brittle materials.2)A theoretical analysis for the single-fiber pullout with unloading process is presented based on the energy-based debonding criterion.The relationship between the applied stress and the interfacial relative displacement is expressed as a function of the radial residual thermal stress,fiber pullout rate and volume content as well as the length of reverses frictional sliding.It provides an effective tool for the determination of interface friction coefficient.3)The effects of thermal stress on failure modes of soft matter with a sharp-hard inclusion is analyzed.Based on theoretical analysis,significant insight into the failure behavior of this soft-hard system is got.Finite element analysis is performed with consideration of the large deformation of the soft matter,and the results demonstrate the effectiveness of theoretical predictions within a large range of loads.4)The failure modes of the structure composed of a soft matrix and a hard line inhomogeneity under uniform remote thermo-mechanical loading is analyzed.5)The Green function for the problem of sharp hard inhomogeneity embedded in soft matrix under point heat source is presented.6)The viscoelastic effects of soft materials have been preliminarily studied.It is found that the hard and sharp inclusion is an important factor causing mechanical failure of soft media.In the soft and hard material system,there exists not only the failure mode of interface debonding,but also the failure mode of soft material puncture.This finding is verified by combining theory,experiment and finite element method.The thermos-elastic problems of soft media with rigid linear inclusions are studied systematically,and the classical theoretical model of rigid line is improved by taking the thermal deformation of rigid line into account.The Green function of the thermoelastic problem of line inclusion with expansion-induced stretch is given for the first time.the soft(hard)materials containing cylindrical inclusions are studied in depth.The loading and unloading processes are analyzed by using the model considering all strain components.It provides an effective tool for the determination of interface friction coefficient.