High-order asymptotic field of tensile plane-strain nonlinear crack problems

In this paper, the governing equations and the analytical method of the secondorder asymptotic field for the plane-straln crack problems of mode I have been presented. The numerical calculation has been carried out. The amplitude coefficients k2 of the second term of the asymptotic field have been determined after comparing the present results with some fine results of the finite element calculation. The variation of coefficients k2 with changes of specimen geometry and developments of plastic zone have been analysed. It is shown that the second term of the asymptotic field has significant influence on the near-crack-tip field. Therefore, we may reasonably argue that both the J-integral and the coefficient k2 can beeome two characterizing parameters for crack initiation.

Response: Discussion of “Mixed Mode ductile fracture using the strain energy density criterion,” by E.E. Gdoutos

thermal conduction, and acoustic wave propagation are included. This

Application of the strain energy density criterion to ductile fracture

The strain energy density criterion due to Sih is used to predict fracture loads of two thin plates subjected to large elastic-plastic deformation. The prediction is achieved with a finite element analysis which is based on Hill's variational principle for incremental deformations capable of solving gross yielding problems involving arbitrary amounts of deformation. The computed results are in excellent agreement with those obtained in Sih's earlier analysis and with an experimental observation.

Ductile crack propagation with the strain-energy density criterion

The strain energy density criterion is used to characterize subcritical crack growth in a thin aluminum alloy sheet undergoing general yielding. A finite element analysis which incorporates both material and geometrical nonlinear behaviors of the cracked sheets is developed to predict fracture loads at varying crack growth increments. The predicted results are in excellent agreement with those measured experimentally, thus confirming the validity of the strain energy density criterion for characterizing ductile crack propagation.

The effect of strain softening in the matrix material during void growth

Furthermore, the compressed flow driven by the piston is discussed. The consistent solution of gasdynamical equations including solar gravity is obtained for the unsteady and two-dimensional configuration, which is applied to the region between the piston and shock wave. This solution may satisfy the jump conditions of shock wave, which separates the region of compressed flow and quiet corona.

Substrate Strain and Proliferation & Differentiation of Bone Marrow Mesenchymal Stem Cells

New methods of surface modification of transparent silicone substrate were developed, and a new set of cell culture devices that provide homogeneous substrate strain was designed. Using the new device, effects of cyclic substrate strain on bone marrow mesenchymal stem cells(MSCs) were studied. It was found that cyclic strain influenced proliferation and differentiation of bone marrow MSCs in different ways.

Transient Flows in Low Permeability Porous Media with Composite Start-Up Pressure Gradient

A new mathematical model for the transient flow in the composite low permeability is established. It is solved by FEM with different boundary conditions such as infinite, circular closed and constant pressure boundary conditions. The typical curves for transient wellbore pressure have been presented. It is shown that the pressure and pressure derivative curves with composite start-up pressure gradients have different slopes which are depended on the start-up pressure gradients and the mobility radios in different regions. The boundary effects are the same as the normal reservoirs without start-up pressure gradients. The study provides a new tool to analyze the transient pressure test data in the low permeability reservoir.

The Displacement and Strain Field of Three-Dimensional Rheologic Model of Earthquake Preparation

Based on the three-dimensional elastic inclusion model proposed by Dobrovolskii, we developed a rheological inclusion model to study earthquake preparation processes. By using the Corresponding Principle in the theory of rheologic mechanics, we derived the analytic expressions of viscoelastic displacement U(r, t) , V(r, t) and W(r, t), normal strains epsilon(xx) (r, t), epsilon(yy) (r, t) and epsilon(zz) (r, t) and the bulk strain theta (r, t) at an arbitrary point (x, y, z) in three directions of X axis, Y axis and Z axis produced by a three-dimensional inclusion in the semi-infinite rheologic medium defined by the standard linear rheologic model. Subsequent to the spatial-temporal variation of bulk strain being computed on the ground produced by such a spherical rheologic inclusion, interesting results are obtained, suggesting that the bulk strain produced by a hard inclusion change with time according to three stages (alpha, beta, gamma) with different characteristics, similar to that of geodetic deformation observations, but different with the results of a soft inclusion. These theoretical results can be used to explain the characteristics of spatial-temporal evolution, patterns, quadrant-distribution of earthquake precursors, the changeability, spontaneity and complexity of short-term and imminent-term precursors. It offers a theoretical base to build physical models for earthquake precursors and to predict the earthquakes.

CHARACTERIZATION OF SHEAR BANDS IN TWO BULK METALLIC GLASSES WITH DIFFERENT INHERENT PLASTICITY

Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

Torsion fracture behavior of drawn pearlitic steel wires with different heat treatments

In this paper, torsion fracture behavior of drawn pearlitic steel wires with different heat treatments was investigated. Samples with different heat treatments was investigated. Samples with different heat treatment conditions were subjected to torsion and tensile tests. The shear strain along the torsion sample after fracture was measured. Fracture surface of wires was examined by Scanning Electron Microscopy. In addition, the method of Differential Scanning Calorimetry was used to characterize the thermodynamic process in the heat treatment. A numerical simulation via finite element method on temperature field evolution for the wire during heat treatment process was performed. The results show that both strain aging and recovery process occur in the material within the temperature range between room temperature and 435 degrees C. It was shown that the ductility measured by the number of twists drops at short heating times and recovers after further heating in the lead bath of 435 degrees C. On the other hand, the strenght of the wire increases at short heating times and decreases after further heating. The microstructure inhomogeneity due to short period of heat treatment, coupled with the gradient characteristics of shear deformation during torsion results in localized shear deformation of the wire. In this situation, shear cracks nucleate between lamella and the wire breaks with low number of twists.

Characterization Of Shear Bands In Two Bulk Metallic Glasses With Different Inherent Plasticity

Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

Strong Strain Hardening in Nanocrystalline Nickel

Low strain hardening has hitherto been considered an intrinsic behavior for most nanocrystalline (NC) metals, due to their perceived inability to accumulate dislocations. In this Letter, we show strong strain hardening in NC nickel with a grain size of 20 nm under large plastic strains. Contrary to common belief, we have observed significant dislocation accumulation in the grain interior. This is enabled primarily by Lomer-Cottrell locks, which pin the lock-forming dislocations and obstruct islocation. motion. These observations may help with developing strong and ductile NC metals and alloys.

Short-range-order effects on intrinsic plasticity of metallic glasses

Through a systematical analysis of the elastic moduli for 137 metallic glasses (MGs) and 56 polycrystalline metals, we use a simple model developed by Knuyt et al. [J. Phys. F: Met. Phys. 16 (1986) p.1989; Phil. Mag. B 64 (1991) p.299] based on a Gaussian distribution for the first-neighbor distance to reveal the short-range-order (SRO) structural conditions for plasticity of MGs. It is found that the SRO structure with dense atomic packing, large packing dispersion and a significant anharmonicity of atomic interaction within an MG is favorable for its global plasticity. Although these conditions seem paradoxical, their perfect matching is believed to be a key for designing large plastic bulk MGs not only in compression but also in tension.

A surface energy model and application to mechanical behavior analysis of single crystals at sub-micron scale

Size effects of mechanical behaviors of materials are referred to the variation of the mechanical behavior due to the sample sizes changing from macroscale to micro-/nanoscales. At the micro-/nanoscale, since sample has a relatively high specific surface area (SSA) (ratio of surface area to volume), the surface although it is often neglected at the macroscale, becomes prominent in governing the energy effect, although it is often neglected at the macroscale, becomes prominent in governing the mechanical behavior. In the present research, a continuum model considering the surface energy effect is developed through introducing the surface energy to total potential energy. Simultaneously, a corresponding finite element method is developed. The model is used to analyze the axial equilibrium strain problem for a Cu nanowire at the external loading-free state. As another application of the model, from dimensional analysis, the size effects of uniform compression tests on the microscale cylinder specimens for Ni and Au single crystals are analyzed and compared with experiments in literatures. (C) 2009 Elsevier B.V. All rights reserved.