One of fundamental but difficult problems in damage mechanics isthe formulation of the ef- fective constitutive relation ofmicrocrack-weakened brittle o quasi-brittle materials under complexloading, especially when mi...One of fundamental but difficult problems in damage mechanics isthe formulation of the ef- fective constitutive relation ofmicrocrack-weakened brittle o quasi-brittle materials under complexloading, especially when microcrack interaction is taken intoaccount. The combination of phenomenological and mi- cromechanicaldamage mechanics is a promising approach to construction andapplicable damage model with a firm physical foundation.展开更多
The scattering of Love waves by an interface crack between apiezoelectric layers and an elas- tic substrate is investigated byusing the integral transform and singular integral equationtechniques. The dy- namic stress...The scattering of Love waves by an interface crack between apiezoelectric layers and an elas- tic substrate is investigated byusing the integral transform and singular integral equationtechniques. The dy- namic stress intensity factors of the left andthe right crack tips are determined. It is found from numericalcalculation that the dynamic response of the system dependssignificantly on the crack configuration, the ma- terial combinationand the propagating direction of the incident wave. It is expectedthat specifying an appro- priate material combination may retard thegrowth of the crack for a certain crack configuration.展开更多
Research on the propagation of elastic waves in piezoelectric nanostructures is very limited.The frequency dispersion of Love waves in layered piezoelectric nanostructures has not yet been reported when surface effect...Research on the propagation of elastic waves in piezoelectric nanostructures is very limited.The frequency dispersion of Love waves in layered piezoelectric nanostructures has not yet been reported when surface effects are taken into account.Based on the surface elasticity theory,the propagation of Love waves with surface effects in a structure consisting of a nanosized piezoelectric film and a semi-infinite elastic substrate is investigated focusing on the frequency dispersion curves of different modes.The results show that under the electrically-open conditions,surface effects give rise to the dependence of Love wave dispersion on the film thickness when the thickness of the piezoelectric film reduces to nanometers.For a given wave frequency,phase velocity of Love waves in all dispersion modes exhibit obvious toward shift as the film thickness decreases or the surface parameters increase.Moreover,there may exist a cut-off frequency in the first mode dispersion below which Love waves will be evanescent in the structure due to surface effects.The cut-off frequency depends on the film thickness,the surface parameters and the bulk material properties.展开更多
Tensile deformation and fracture characteristics of polyimide/montmorillonite nanocomposite films are investigated to enhance the particular mechanical properties and understand the effective factors in dominating the...Tensile deformation and fracture characteristics of polyimide/montmorillonite nanocomposite films are investigated to enhance the particular mechanical properties and understand the effective factors in dominating the mechanical properties of nanocomposites, such as the nanolayer, matrix and nanolayer/matrix interface. How to contribute to the mechanical properties of nanocomposite film is a very complex problem. In this paper, these factors are analyzed based on the addition amount and fracture mechanics. The results indicate that the specimen at 20 wt% MMT breaks prematurely with a fracture strength （σb = 78 MPa） much lower than that （σb = 128 MPa） at the 1 wt% MMT. However, the Young＇s modulus （3.2 GPa） of the former is higher than that （1.9 GPa） of the latter. Fractography also indicates that the brittle cracking formed in high content addition is the main cause of failure but microscopically ductile fracture morphology still exists locally. And for the trace element addition, the smaller threading slipping veins are evenly distributed on the entire fracture section of these films. Therefore, these characteristics would presumably be associated with both the concentration effects of size of nanocomposite sheets and the increasing deformation harmony in nanolayers.展开更多
The effects of dislocation configuration, crack blunting and free surfaces on the triggering load of dislocation sources in the vicinity of a crack or a wedge tip subjected to a tensile load in the far field are inves...The effects of dislocation configuration, crack blunting and free surfaces on the triggering load of dislocation sources in the vicinity of a crack or a wedge tip subjected to a tensile load in the far field are investigated. An appropriate triggering criterion for dislocation sources is proposed by considering the configurational forces acting on each dislocation. The triggering behaviors of dislocation sources near the tips of a crack and a wedge are compared. It is also found that the blunting of crack tip and the presence of free surfaces near the crack or the wedge have considerable influences on the triggering load of dislocation sources. This study might be of significance to gaining a deeper understanding of the brittle-to-ductile transition of materials.展开更多
In this paper,a phenomenological continuum theory of surface piezoelectricity accounting for the linear superficial interplay between electricity and elasticity is formulated primarily for elastic dielectric materials...In this paper,a phenomenological continuum theory of surface piezoelectricity accounting for the linear superficial interplay between electricity and elasticity is formulated primarily for elastic dielectric materials.This theory is inspired by the physical idea that once completely relaxed,an insulating free dielectric surface will sustain a nontrivial spontaneous surface polarization in the normal direction together with a tangential self-equilibrated residual surface stress field.Under external loadings,the surface Helmholtz free energy density is identified as the characteristic function of such surfaces,with the in-plane strain tensor of surface and the surface free charge density as the independent state variables.New boundary conditions governing the surface piezoelectricity are derived through the variational method.The resulting concepts of charge-dependent surface stress and deformationdependent surface electric field reflect the linear electromechanical coupling behavior of nanodielectric surfaces.As an illustrative example,an infinite radially polarizable piezoelectric nanotube with both inner and outer surfaces grounded is investigated.The novel phenomenon of possible surface-induced polarity inversion is predicted for thin enough nanotubes.展开更多
Surface wrinkling of materials holds promise for important applications in diverse fields such as multifunctional surfaces and biomedical engineering. For these applications, it is of interest to attain various surfac...Surface wrinkling of materials holds promise for important applications in diverse fields such as multifunctional surfaces and biomedical engineering. For these applications, it is of interest to attain various surface wrinkles with tunable wavelengths and amplitudes. Through a combination of experiments and numerical simulations, we here propose a method to regulate the wrinkling patterns in a film-substrate system by introducing periodic surface stiffness, which is generated through sequential specified ultraviolet-ozone(UVO) treatments. Both experiments and numerical simulations demonstrate that the proposed technique can produce various patterns with wide, tunable geometrical features and anisotropy. The effects of surface stiffness distribution, the exposure durations of UVO-treatments, and the loading biaxiality are examined on the generated surface patterns.展开更多
In this study,we theoretically investigate the dynamic indentation for measuring the loss (damping) factor of a linear viscoelastic material from its indentation response.A rigid indenter with arbitrary tip profile is...In this study,we theoretically investigate the dynamic indentation for measuring the loss (damping) factor of a linear viscoelastic material from its indentation response.A rigid indenter with arbitrary tip profile is assumed to indent into a viscoelastic substrate with arbitrary shape.We perform a theoretical analysis and identify the conditions under which the loss factor of the material can be determined from the phase angle between the applied harmonic indentation load and the corresponding harmonic displacement,a directly measurable quantity in a dynamic indentation test.To validate the conclusion drawn from our theoretical analysis,a series of numerical experiments are performed,including the spherical indentation of a soft layer with irregular surface morphology bonded to a rigid substrate,a conical indenter with tip defects indenting into a half-spherical particle,and the indentation of porous materials.This study may facilitate the use of the dynamic indentation technique to evaluate the damping properties of linear viscoelastic materials,including some advanced polymers and biological soft tissues.展开更多
Both chemical and mechanical determinants adapt and react throughout the process of tumor invasion. In this study, a cell-based model is used to uncover the growth and invasion of a three-dimensional solid tumor confi...Both chemical and mechanical determinants adapt and react throughout the process of tumor invasion. In this study, a cell-based model is used to uncover the growth and invasion of a three-dimensional solid tumor confined within normal cells. Each cell is treated as a spheroid that can deform, migrate, and proliferate. Some fundamental aspects of tumor development are considered,including normal tissue constraints, active cellular motility, homotypic and heterotypic intercellular interactions, and pressureregulated cell division as well. It is found that differential motility between cancerous and normal cells tends to break the spheroidal symmetry, leading to a finger instability at the tumor rim, while stiff normal cells inhibit tumor branching and favor uniform tumor expansion. The heterotypic cell-cell adhesion is revealed to affect the branching geometry. Our results explain many experimental observations, such as fingering invasion during tumor growth, stiffness inhibition of tumor invasion, and facilitation of tumor invasion through cancerous-normal cell adhesion. This study helps understand how cellular events are coordinated in tumor morphogenesis at the tissue level.展开更多
基金the National Natural Science Fouudation of China (19891180)
文摘One of fundamental but difficult problems in damage mechanics isthe formulation of the ef- fective constitutive relation ofmicrocrack-weakened brittle o quasi-brittle materials under complexloading, especially when microcrack interaction is taken intoaccount. The combination of phenomenological and mi- cromechanicaldamage mechanics is a promising approach to construction andapplicable damage model with a firm physical foundation.
基金the National Natural Science Foundation of China (No.19891180)the Fundamental Research Foundation of Tsinghua University (JZ 2000.007)the Fund of the Education Ministry of China.
文摘The scattering of Love waves by an interface crack between apiezoelectric layers and an elas- tic substrate is investigated byusing the integral transform and singular integral equationtechniques. The dy- namic stress intensity factors of the left andthe right crack tips are determined. It is found from numericalcalculation that the dynamic response of the system dependssignificantly on the crack configuration, the ma- terial combinationand the propagating direction of the incident wave. It is expectedthat specifying an appro- priate material combination may retard thegrowth of the crack for a certain crack configuration.
基金Supported by National Natural Science Foundation of China(Grant No.11372261)Excellent Young Scientists Supporting Project of Science and Technology Department of Sichuan Province,China(Grant No.2013JQ0030)+4 种基金Supporting Project of Department of Education of Sichuan Province,China(Grant No.2014zd3132)Opening Project of Key Laboratory of Testing Technology for Manufacturing ProcessSouthwest University of Science and Technology-Ministry of Education,China(Grant No.12zxzk02)Fund of Doctoral Research of Southwest University of Science and Technology,China(Grant No.12zx7106)Postgraduate Innovation Fund of Southwest University of Science and Technology,China(Grant No.15ycx128)
文摘Research on the propagation of elastic waves in piezoelectric nanostructures is very limited.The frequency dispersion of Love waves in layered piezoelectric nanostructures has not yet been reported when surface effects are taken into account.Based on the surface elasticity theory,the propagation of Love waves with surface effects in a structure consisting of a nanosized piezoelectric film and a semi-infinite elastic substrate is investigated focusing on the frequency dispersion curves of different modes.The results show that under the electrically-open conditions,surface effects give rise to the dependence of Love wave dispersion on the film thickness when the thickness of the piezoelectric film reduces to nanometers.For a given wave frequency,phase velocity of Love waves in all dispersion modes exhibit obvious toward shift as the film thickness decreases or the surface parameters increase.Moreover,there may exist a cut-off frequency in the first mode dispersion below which Love waves will be evanescent in the structure due to surface effects.The cut-off frequency depends on the film thickness,the surface parameters and the bulk material properties.
基金Project supported by the National Basic Research Program of China (No.2004CB619304)the Hundred TalentsProgram of Chinese Academy of Sciences, Key Research Programme of Beijing City Science and Technology Committee(No.H020420020230).
文摘Tensile deformation and fracture characteristics of polyimide/montmorillonite nanocomposite films are investigated to enhance the particular mechanical properties and understand the effective factors in dominating the mechanical properties of nanocomposites, such as the nanolayer, matrix and nanolayer/matrix interface. How to contribute to the mechanical properties of nanocomposite film is a very complex problem. In this paper, these factors are analyzed based on the addition amount and fracture mechanics. The results indicate that the specimen at 20 wt% MMT breaks prematurely with a fracture strength （σb = 78 MPa） much lower than that （σb = 128 MPa） at the 1 wt% MMT. However, the Young＇s modulus （3.2 GPa） of the former is higher than that （1.9 GPa） of the latter. Fractography also indicates that the brittle cracking formed in high content addition is the main cause of failure but microscopically ductile fracture morphology still exists locally. And for the trace element addition, the smaller threading slipping veins are evenly distributed on the entire fracture section of these films. Therefore, these characteristics would presumably be associated with both the concentration effects of size of nanocomposite sheets and the increasing deformation harmony in nanolayers.
基金Project supported by the National Nature Science Foundation of China(Nos.10572067,10525210 and 10121202).
文摘The effects of dislocation configuration, crack blunting and free surfaces on the triggering load of dislocation sources in the vicinity of a crack or a wedge tip subjected to a tensile load in the far field are investigated. An appropriate triggering criterion for dislocation sources is proposed by considering the configurational forces acting on each dislocation. The triggering behaviors of dislocation sources near the tips of a crack and a wedge are compared. It is also found that the blunting of crack tip and the presence of free surfaces near the crack or the wedge have considerable influences on the triggering load of dislocation sources. This study might be of significance to gaining a deeper understanding of the brittle-to-ductile transition of materials.
基金supports from the National Natural Science Foundation of China(Grant Nos. 10772093,10972121,and 10732050)the National Basic Research Program of China(Grant Nos. 2007CB936803 and 2010CB-631005)
文摘In this paper,a phenomenological continuum theory of surface piezoelectricity accounting for the linear superficial interplay between electricity and elasticity is formulated primarily for elastic dielectric materials.This theory is inspired by the physical idea that once completely relaxed,an insulating free dielectric surface will sustain a nontrivial spontaneous surface polarization in the normal direction together with a tangential self-equilibrated residual surface stress field.Under external loadings,the surface Helmholtz free energy density is identified as the characteristic function of such surfaces,with the in-plane strain tensor of surface and the surface free charge density as the independent state variables.New boundary conditions governing the surface piezoelectricity are derived through the variational method.The resulting concepts of charge-dependent surface stress and deformationdependent surface electric field reflect the linear electromechanical coupling behavior of nanodielectric surfaces.As an illustrative example,an infinite radially polarizable piezoelectric nanotube with both inner and outer surfaces grounded is investigated.The novel phenomenon of possible surface-induced polarity inversion is predicted for thin enough nanotubes.
基金supported by the National Natural Science Foundation of China(Grant Nos.11672161,11432008)
文摘Surface wrinkling of materials holds promise for important applications in diverse fields such as multifunctional surfaces and biomedical engineering. For these applications, it is of interest to attain various surface wrinkles with tunable wavelengths and amplitudes. Through a combination of experiments and numerical simulations, we here propose a method to regulate the wrinkling patterns in a film-substrate system by introducing periodic surface stiffness, which is generated through sequential specified ultraviolet-ozone(UVO) treatments. Both experiments and numerical simulations demonstrate that the proposed technique can produce various patterns with wide, tunable geometrical features and anisotropy. The effects of surface stiffness distribution, the exposure durations of UVO-treatments, and the loading biaxiality are examined on the generated surface patterns.
基金supported by the National Natural Science Foundation of China(Grant Nos. 10972112,10525210,and 10732050)the National Basic Research Program of China(Grant Nos. 2010CB631005)
文摘In this study,we theoretically investigate the dynamic indentation for measuring the loss (damping) factor of a linear viscoelastic material from its indentation response.A rigid indenter with arbitrary tip profile is assumed to indent into a viscoelastic substrate with arbitrary shape.We perform a theoretical analysis and identify the conditions under which the loss factor of the material can be determined from the phase angle between the applied harmonic indentation load and the corresponding harmonic displacement,a directly measurable quantity in a dynamic indentation test.To validate the conclusion drawn from our theoretical analysis,a series of numerical experiments are performed,including the spherical indentation of a soft layer with irregular surface morphology bonded to a rigid substrate,a conical indenter with tip defects indenting into a half-spherical particle,and the indentation of porous materials.This study may facilitate the use of the dynamic indentation technique to evaluate the damping properties of linear viscoelastic materials,including some advanced polymers and biological soft tissues.
基金supported by the National Natural Science Foundation of China(Grant Nos.11672161,11620101001)
文摘Both chemical and mechanical determinants adapt and react throughout the process of tumor invasion. In this study, a cell-based model is used to uncover the growth and invasion of a three-dimensional solid tumor confined within normal cells. Each cell is treated as a spheroid that can deform, migrate, and proliferate. Some fundamental aspects of tumor development are considered,including normal tissue constraints, active cellular motility, homotypic and heterotypic intercellular interactions, and pressureregulated cell division as well. It is found that differential motility between cancerous and normal cells tends to break the spheroidal symmetry, leading to a finger instability at the tumor rim, while stiff normal cells inhibit tumor branching and favor uniform tumor expansion. The heterotypic cell-cell adhesion is revealed to affect the branching geometry. Our results explain many experimental observations, such as fingering invasion during tumor growth, stiffness inhibition of tumor invasion, and facilitation of tumor invasion through cancerous-normal cell adhesion. This study helps understand how cellular events are coordinated in tumor morphogenesis at the tissue level.
