Our Publications

  • Phase Transitions of Carbon Materials under High Pressure. Wanlin Guo, Yitao Dai and Bin Zhang. Solid Mechanics and Its Applications, 2007, 144(6), 239-249.

    Wanlin Guo, Yitao Dai and Bin Zhang

    Solid Mechanics and Its Applications, 2007, 144(6), 239-249.

    Paradoxical experimental observations are explained by studying the high-pressure physical mechanics processes of graphite and carbon nanotubes (CNTs) and the macroscopic mechanics behaviors in the experiments of diamond anvil cells (DAC). The stress concentration on the graphite sample under non-hydrostatics compression in DAC experiments can produce a new phase that is hard enough to crack the superhard diamond. Those soft to hard phase transitions occur at the pressure of about 17 GPa for both graphite and CNTs, independent of the shape and the size of the indenter and the amount of the graphite layers. And a theoretical route is provided to industrially produce diamond and high strength CNTs-bundles composite at room temperature by using of high-pressure technology. Physical mechanics of nanomaterials in particular environment is also discussed.

    Key words: graphite - carbon nanotubes - ultrahigh pressure - bond switching - physical mechanics

  • Modifying atomic-scale friction between two graphene sheets: A molecular-force-field study. Yufeng Guo, Wanlin Guo, and Changfeng Chen. Phys. Rev. B, 2007, 76, 155429.

    Yufeng Guo, Wanlin Guo, and Changfeng Chen

    Phys. Rev. B, 2007, 76, 155429.

    Recently discovered ultralow friction (superlubricity) between incommensurate graphitic layers has raised great interest in understanding the interlayer interaction between graphene sheets under various physical conditions. In this work, we have studied the effects of interlayer distance change and in-sheet defects in modifying the interlayer friction in graphene sheets by extensive molecular-force-field statics calculations. The interlayer friction between graphene sheets with commensurate or incommensurate interlayer stacking increases with decreasing interlayer distance, but in the case of incommensurate stacking, ultralow friction can exist in a significantly expanded range of interlayer distance. The ultralow interlayer friction in the incommensurate stacking sheets is insensitive to the in-sheet defect of vacancy at a certain orientation. These results provide knowledge for possibly controlling friction between graphene sheets and offer insight into their applications.

  • Sonochemical preparation of nickel alumina nanotubes templated by anionic surfactant assemblies. Guoan Tai, and Wanlin Guo. J Mater Sci 2007, 42, 10245–10249.

    Guoan Tai, and Wanlin Guo

    J Mater Sci 2007, 42, 10245–10249.

    Nickel alumina nanotubes templated by dodecylsulfate assemblies have been successfully synthesized for the first time using a sonochemical process. These nanotubes were characterized by scanning electron microscope (SEM), a transmission electron microscope (TEM), X-ray diffraction (XRD). The formation mechanism of these nanotubes is also discussed. They were also calcined to study the change of the nanostructure morphology with the temperature. It was found that the nanotubes transformed from short nanotubes into dendritic structures of aggregations of nanoparticles into monodisperse nanoparticles, and these nanostructures hold high specific surface area.

  • The out-of-plane constraint of mixed-mode cracks in thin elastic plates. Chongmin She, and Wanlin Guo. International Journal of Solids and Structures, 2007, 44, 3021–3034.

    Chongmin She,1, and Wanlin Guo,1

    1. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    Received 7 April 2006; revised 28 June 2006. Available online 12 September 2006.

    The out-of-plane constraint factor, Tz, around I–II mixed-mode cracks in a thin elastic plate has been investigated systematically and a K–Tz two-parameter description of the three-dimensional (3D) stress field near the crack front has been evaluated using the boundary layer model and the finite element method. The Tz factor for I–II mixed-mode cracks depends not only on TzI for the pure tensile mode and TzII for the pure shear mode loading but also on the ratio of the thickness functions for the local stress intensity factors of both loading modes. Tz increases gradually with decreasing crack angle φ from mode I (φ = 90°) to mode II (φ = 0°). At the crack plane (θ = 0°), the shear loading has no contribution to Tz, which equals the value for the pure mode I. By fitting the numerical results, two empirical formulae were obtained to describe the 3D distribution of Tz around the crack front for the pure tensile mode and pure shear mode, respectively. Next, the formula describing the 3D distribution of Tz around the I–II mixed-mode crack front was obtained. Two-parameter description of the 3D stress field (K–Tz) for I–II mixed-mode cracks was proposed.

    Keywords: Three-dimensional stress field; Mixed mode; Out-of-plane constraint; Stress intensity factor

  • The in-plane and out-of-plane stress constraint factors and K − T − Tz description of stress field near the border of a semi-elliptical surface crack. Junhua Zhao, Wanlin Guo, and Chongmin She. International Journal of Fatigue, 2007, 29, 435–443.

    Junhua Zhao,1, Wanlin Guo,1, and Chongmin She,1

    1. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    International Journal of Fatigue, 2007, 29, 435–443.

    Received 15 December 2005; revised 17 May 2006; accepted 21 May 2006. Available online 26 July 2006.

    The elastic T-stress and stress intensity factor K for semi-elliptical surface cracks have been investigated in elastic plates by detailed three-dimensional finite element calculations. The distributions of normalized K and T-stress have been obtained along the crack font with aspect ratios (a/c) of 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, and far-field tension and the effect of Poisson’s ratio have also been considered. The normalized K increases and the normalized T-stress decreases with the increase of Poisson’s ratio v. For v = 0.3, the normalized K gradually increases in the range of crack face angle 22.5° and decreases in the range of 22.5° with increasing a/c. When rises to 90°, the K values tend to maximum for various a/c. The normalized T-stress increases in the beginning and then decreases with the increase of except for a/c = 1.0. By fitting the numerical results with the least squares method, empirical formulae have been given for the convenience of engineering applications. Combining with the corresponding out-of-plane constraint factor Tz, the three-parameter K − T − Tz approach has been provided, which can accurately describe the stress field around the crack front.

    Keywords: Elastic T-stress; Out-of-plane constraint Tz; Stress intensity factor; Semi-elliptical surface crack; Three-dimensional finite element

  • First-principles study of the dependence of ground-state structural properties on the dimensionality and size of ZnO nanostructures. Chun Li, Wanlin Guo, Yong Kong, and Huajian Gao. Phys. Rev. B, 2007, 76, 035322.

    Chun Li1,2, Wanlin Guo1,*, Yong Kong2, and Huajian Gao3
    1. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
    2. Max-Planck Institute for Metals Research, Heisenbergstrasse, 3, D-70569 Stuttgart, Germany
    3. Division of Engineering, Brown University, 182 Hope Street, Providence, Rhode Island 02912, USA

    Phys. Rev. B, 2007, 76, 035322.

    Received 20 November 2006; revised 2 April 2007; published 18 July 2007

    The dimension- and size-dependent ground-state properties of ZnO systems are investigated through first-principles density-functional theory calculations. It is found that the effective elastic constants C33 of polar surface terminated ZnO nanofilms and [0001] oriented ZnO nanowires increase with structural size. For nanofilms, the effective C33 quickly approaches the bulk value and becomes almost stable when the film contains over six Zn-O double layers. For nanowires, the effective C33 is as small as around 16% of the corresponding bulk value when the diameter is very small, and it increases almost linearly to 88% of the bulk value when the diameter reaches about 2.4 nm. For ZnO nanoclusters with hexagonal prism structure, the original Zn-O double layers merge into single layers after relaxation. A shape-driven phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure is found in a ZnO cluster with 48 atoms. Finally, a systematic energy analysis of all the above structures shows that the cohesive energies of the ZnO structures increase with both dimension and size.

  • Reassembly of single-walled carbon nanotubes into hybrid multilayered nanostructures inside nanotube extruders. Yufeng Guo, and Wanlin Guo. Phys. Rev. B, 2007, 76, 045404.

    Yufeng Guo, and Wanlin Guo

    Phys. Rev. B, 2007, 76, 045404.

    High-pressure induced structural reassembly of single-walled carbon nanotubes (SWNTs) inside carbon nanotube extruders is investigated by reactive molecular dynamics simulations. It is found that the encapsulated SWNTs undergo a series of structural transformations depending on tube diameters with increasing internal pressure, and finally reassemble into hybrid sp2-sp3 cross-linked multilayered tubular nanostructures inside the nanotube extruders. Density-functional calculations show that strong compression causes a semiconducting-to-metallic transition in the encapsulated tubes. The metallic properties of the formed nanostructures are mainly attributed to the shortening of carbon bonds and strong interlayer correlation.

  • First-principles study on ZnO nanoclusters with hexagonal prism structures. Chun Li, Wanlin Guo, Yong Kong, and Huajian Gao. Appl. Phys. Lett., 2007, 90, 223102.

    Chun Li, Wanlin Guo, Yong Kong, and Huajian Gao

    Appl. Phys. Lett., 2007, 90, 223102.

    Size-dependent ground state energies and electronic structures of ZnO nanoclusters with wurtzite structures are investigated using density-functional theory calculations. After structural optimizations, the initial Zn–O double layers merge into single layers. Particularly, a phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure is found in the ZnO cluster with 48 atoms, which indicates that clusters with too many dangling bonds will significantly change their configurations during the relaxations. The calculations also show that on the whole both the energy gap and the binding energy approach to the corresponding bulk values with increasing cluster size.

  • Three-dimensional stress concentrations at elliptic holes in elastic isotropic plates subjected to tensile stress. Chongmin She, and Wanlin Guo. International Journal of Fatigue, 2007, 29, 330–335.

    Chongmin She,1, and Wanlin Guo,1

    1. Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    Received 12 July 2005; revised 4 December 2005; accepted 2 March 2006. Available online 11 May 2006.

    The through-thickness variations of stress concentration factors along the wall of elliptic holes in finite thickness plates of isotropic materials subjected to remote tensile stress have been systematically analyzed using the finite element method. The three-dimensional stress concentration factor Kt is found to be a function of the thickness to root radius ratio B/ρ and the aspect ratio t (short to long axial length) of the elliptic hole under tensile loading. It is found that the maximum stress concentration factor through the thickness, (Kt)max, is 24–123% higher than the value on the free surface (Kt)surf when t changes from 1 to 0.01, and the ratio of the surface value (Kt)surf to the corresponding planar solution (Kt)p−σ at the root is only 0.82–0.48 when t ranges from 1 to 0.01 if B/ρ is large enough. When B/ρ is smaller than 1, both of the ratios will approach 1. Simple and efficient empirical formulae for the relationships between the three values were obtained by fitting the numerical results with good engineering accuracy for large range of B/ρ (from 1 to 100,000) and t (from 0.01 to 1). Combining with the plentiful known two-dimensional stress concentration factors in handbooks, the proposed formulae can be used to solve the maximum three-dimensional stress concentration factors of holes, notches and be useful for strength and fatigue designs of engineering structures with similar defects.

    Keywords: Stress concentration; Elliptic hole; Three-dimensional; Finite thickness; Finite element method

  • Three-dimensional stress state around quarter-elliptical corner cracks in elastic plates subjected to uniform tension loading. Bin Zhang, and Wanlin Guo. Engineering Fracture Mechanics, 2007, 74, 386–398.

    Bin Zhang,1,2, and Wanlin Guo,2

    1. College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Box 1001, 29 Yudao Street, Nanjing, Jiangsu 210016, China
    2. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, China

    Engineering Fracture Mechanics, 2007, 74, 386–398.

    Received9 December 2005; revised 10 May 2006; accepted 10 May 2006. Available online 21 June 2006.

    Detailed full-field three-dimensional (3D) finite element analyses have been conducted to study the out-of-plane stress constraint factor Tz around a quarter-elliptical corner crack embedded in an isotropic elastic plate subjected to uniform tension loading. The distributions of Tz are studied in the forward section (0° θ 90°) of the corner cracks with aspect ratios a/c of 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. In the normal plane of the crack front line, Tz drops radially from Poisson’s ratio at the crack tip to zero beyond certain radial distances. Strong 3D zones (Tz > 0) exist within a radial distance r/a of about 4.6–0.7 for a/c = 0.2–1.0 along the crack front, despite the stress-free boundary conditions far away. At the same radial distance along the crack front in the 3D zones, Tz increases from zero on one free surface to a peak value in the interior, and then decreases to zero on another free surface. The distributions of Tz near the corner points are also discussed. Empirical formulae describing the 3D distributions of Tz are obtained by fitting the numerical results, which prevail with a sufficient accuracy in the valid range of 0.2 a/c 1.0 and 0° θ 90° except very near the free surfaces where Tz is extremely low. Combined with the K–T solution, the transition of approximate plane-stress state near the surfaces to plane-strain state in the interior can be characterized more accurately.

    Keywords: Corner crack; Out-of-plane stress constraint factor Tz; Three-dimensional finite element

  • An efficient method for evaluating the nanohardness of layer-configured materials by atomistic simulation. Chunzhang Zhu, Wanlin Guo, and T.X. Yu. Nanotechnology, 2007, 18, 295704.

    Chunzhang Zhu, Wanlin Guo, and T.X. Yu

    Nanotechnology, 2007, 18, 295704.

    A new method for evaluating contact area and hardness is proposed for atomistic simulation of nanoindentation. The indenter is designed as a spherical virtual potential and atoms that enter the indenter are counted to evaluate the contact area. It is found that the nanohardness determined with the present method can be affected by the thermal activity of contact atoms, as well as the rigidity and dimensions of the virtual indenter. With the influencing factors carefully considered, molecular dynamics simulations employing the developed method are performed to investigate the indentation of carbon nanotubes (CNTs) along the radial direction and graphite sheets along the c-direction. It is found that the simulation results coincide well with available experimental findings, which demonstrates that the method is efficient for layer-configured nanomaterials such as CNTs and graphite crystal.

  • On the study of the creep damage development in circumferential notch specimens. Y.P. Jiang, W.L. Guo, and Z.F. Yue. Computational Materials Science, 2007, 38, 653–659.

    Y.P. Jiang,1, W.L. Guo,1, and Z.F. Yue,2

    1. Institute of Nano-science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
    2. Department of Engineering Mechanics, Northwestern Polytechnic University, Xian 710072, PR China

    Computational Materials Science, 2007, 38, 653–659.

    Received 22 February 2006; revised 13 April 2006; accepted 24 April 2006. Available online 27 June 2006.

    Numerical calculations with K–R damage law have been performed to study the creep damage development in circular notch specimens under constant loading. The emphasis was placed on the roles of notch radius, material constant-α and applied stress. The results show that the distributions of stresses under creep conditions are different from those of previous studies. Creep damage development and life are different for different notch specimens, and the distributions of the maximum creep damage in the minimum cross-section vary with the notch radius. The creep damage is remarkably affected by the applied stress, material parameter-α and notch radius. Higher stress and tri-axial stress state parameter-α can cause the creep damage to develop faster.

    Keywords: Creep; Damage; Stress; Finite element analysis; Circular notch specimen

  • Coaxial nanocable: Carbon nanotube core sheathed with boron nitride nanotube. Zhuhua Zhang, Wanlin Guo, and Guo’an Tai. APPLIED PHYSICS LETTERS, 2007, 90, 133103.

    Zhuhua Zhang, Wanlin Guo, and Guo’an Tai

    APPLIED PHYSICS LETTERS, 2007, 90, 133103.

    A coaxial nanocable model consisting of conductive carbon nanotube core and boron nitride nanotube sheath is proposed by ab initio calculations. The conduction electron density is mainly concentrated on the inner carbon shell at the optimal interwall distance about 0.35 nm. The conductivity of the core carbon nanotube and the insulation of the boron nitride nanotube sheath are found to be rather tolerant to mechanical deformation.

  • Energy Optimum Chiralities of Multiwalled Carbon Nanotubes. Wanlin Guo and Yufeng Guo. J. Am. Chem. Soc., 2007, 129(10), 2730–2731.

    Wanlin Guo, and Yufeng Guo

    J. Am. Chem. Soc., 2007, 129 (10), pp 2730–2731

    The energy optimum chiralities of neighboring layers of a multiwalled carbon nanotube (MWNT) are found to correlate with each other. Nearly identical chiralities are energy optimum in MWNTs with a large diameter. For a small tube, the chiral angle ratio of the outer to inner tubes decreases monotonically with decreasing tube diameter, and nearly identical zigzag chiralities become dominated for a very small tube. It is expected that low temperature control is necessary for achieving the optimum chiralities. The theory can explain most existing chirality characterizations of MWNTs synthesized at low temperature.

  • Synthesis and characterization of layered and hexagonal nickel–aluminum oxides. Guoan Tai, and Wanlin Guo. Materials Chemistry and Physics, 2007, 103, 201–205.

    Guoan Tai, and Wanlin Guo

    Materials Chemistry and Physics, 2007, 103, 201–205.

    Layered and hexagonal mesostructures of nickel–aluminum oxides templated by alkyl sulfate assemblies were synthesized for the first time by the homogeneous precipitation method using urea. X-ray diffraction patterns and transmission electron microscopy (TEM) proved that there was a transformation from layered to hexagonal mesostructure within a prolonged time of 20 h. The Brunauer–Emmett–Teller surface area was reported after solvent extraction and calcinations at a large range of temperatures. The mesostructured hexagonal solid by the anion-exchange with acetate species results in a mesoporous material with a specific surface area of as large as 277 m2 g−1. The specific surface area decreased from 299 to 71 m2 g−1 as the temperatures of calcination increasing from 523 to 1033 K.

    Keywords: Layered; Hexagonal; Nickel–aluminum oxides; Mesoporous materials

  • Magnetic properties of strained single-walled carbon nanotubes. Zhuhua Zhang, and Wanlin Guo. Appl. Phys. Lett., 2007, 90, 053114.

    Zhuhua Zhang, and Wanlin Guo

    Appl. Phys. Lett., 2007, 90, 053114.

    Strong effects of uniaxial and torsional strains on the magnetic properties of single-walled carbon nanotubes have been investigated by tight binding calculations and the Green function analyses. The strain-induced peaks of susceptibility are found in the carbon nanotubes, and paramagnetic-diamagnetic transition takes place at certain strains. The critical magnetic flux for semiconductor-metal transition changes linearly with strains depending on the chiralities of the tubes, mainly due to the tuning of the Van Hove singularities by the coupling of strains and magnetic flux. The positive and negative strain effects are asymmetrical in chiral tubes.

  • Size-dependent piezoelectricity in zinc oxide nanofilms from first-principles calculations. Chun Li, Wanlin Guo, Yong Kong, and Huajian Gao. Appl. Phys. Lett., 2007, 90, 033108.

    Chun Li1, Wanlin Guo2, Yong Kong3, and Huajian Gao3
    1. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China and Max-Planck Institute for Metals Research, Heisenbergstrasse, 3, D-70569 Stuttgart, Germany
    2. Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
    3. Max-Planck Institute for Metals Research, Heisenbergstrasse, 3, D-70569 Stuttgart, Germany

    Received 9 October 2006; accepted 4 December 2006; published online 17 January 2007

    The size dependence of the piezoelectricity in ZnO nanofilms is investigated using ab initio density-functional theory calculations. The effective piezoelectric constant of ZnO nanofilms increases monotonically with increasing film thickness in the nanoscale simulated in the present work, and surprisingly, exceeds that of the bulk ZnO when the thickness is greater than 2.4 nm. The enhancement over the bulk value reaches 11% when the film thickness is 2.9 nm.

  • The in-plane and out-of-plane stress constraint factors and K-T-Tz description of stress fields near the border of a quarter-elliptical corner crack. Junhua Zhao, Wanlin Guo, and Chongmin She. Fatigue Fract Engng Mater Struct, 2007, 30, 673–681.

    J. ZHAO, W. GUO and C. SHE
    Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    Received in final form 12 November 2006

    The elastic T-stress and stress intensity factor K for quarter-elliptical corner cracks have been investigated in elastic plates by detailed three-dimensional finite-element calculations. The distributions of normalized K and T-stress have been obtained along the crack front with aspect ratios (a/c) of 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0, and far-field tension and the effect of Poisson’s ratio have also been considered. The normalized K increases and the normalized T-stress decreases with the increase of Poisson’s ratio v. For v = 0.3, the normalized K gradually increases in the range of crack-face angle φ ≥ 22.5◦ and decreases in the range of φ ≤ 22.5◦ with the increase of a/c. The normalized T-stress increases in the beginning and then decreases with increasing φ except for a/c = 0.2 and a/c = 0.3. By fitting the numerical results with the least squares method, empirical formulae have been given for the convenience of engineering applications. Combining with the corresponding out-of-plane constraint factor Tz, the three-parameter K-T-Tz approach has been provided, which can accurately describe the stress field around the crack front.

    Keywords elastic T-stress; out-of-plane constraint Tz; quarter-elliptical corner crack; stress intensity factor; three-dimensional finite element.

  • On the study of the effects of notch shape on the creep damage under cyclic loading. Y.P. Jiang, W.L. Guo, and X.J. Shao. International Journal of Fatigue, 2007, 29, 836–842.

    Y.P. Jiang,1, W.L. Guo,1, and X.J. Shao,2

    1. Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, Institute of Nano-Science, Nanjing 210016, PR China
    2. Department of Engineering Mechanics, Northwestern Polytechnic University, Xian 710072, PR China

    International Journal of Fatigue, 2007, 29, 836–842.

    Received 24 February 2006; revised 3 September 2006; accepted 10 September 2006. Available online 2 November 2006.

    Numerical calculations with the K-R damage law have been performed to study the creep damage development of different notch specimens (Circumferential, U-type and V-type notch) under cyclic loading. Emphasis was placed on the roles of notch geometry, multi-axial stress rupture parameter α, frequency and stress ratio. Notch shape has a notable influence on the creep damage and fatigue life. Different notch specimens have different creep damage distributions. The development of creep damage in the minimum cross-section varies with notch shape and notch radius. For the Circumferential notch specimen, the maximum creep damage lies at the point in from notch tip; For the U-type and V-type notch specimens, the maximum creep damage always happens at the notch tip. Creep damage is greatly affected by the amplitude of load, stress ratio, multi-axial stress rupture parameter α. Larger mean stress, stress ratio and material parameter α cause faster development of the creep damage.

    Keywords: Creep; User subroutine; Damage; Finite element method; Notch specimen; Cyclic loading