Advances in Fracture and Materials Behavior

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Authors: C.N. Weng, K.T. Wang, T. Chen
Abstract: The linear compressibility of a solid is defined as the relative decrease in length of a line when the solid is subjected to unit hydrostatic pressure. Materials with a negative linear or area compressibility could have interesting technological applications. However, in the case of homogeneous materials only rare crystal phases exhibit this effect. In particular, for isotropic or cubic solids the linear compressibility is known to be isotropic and positive, namely a sphere of a cubic or isotropic crystal under hydrostatic pressure remains a sphere. For less symmetric solids, it generally varies with the direction n. Here we derive explicit expressions of the stationary values (maximum and minimum) of linear compressibility for single phase solids with monoclinic, orthotropic, tetragonal, trigonal, and hexagonal symmetry. A list of crystals that may exhibit negative linear compressibility in certain directions is outlined. Next, by assembling a two-component material, we propose microstructure networks to achieve such a property. Numerical simulations, based on a refined finite element method, are provided.
Authors: Akiyuki Takahashi, Mitsuru Kawanabe, Masanori Kikuchi
Abstract: Recent 3-Dimensional Atom Probe (3DAP) experimental observations showed the formation of spherical γ-precipitates at a central region of γ’ phases in nickel based superalloys. The γ precipitates should give a significant effect to the material strength. Whereat, in this work, we first developed a simulation technique for the investigation of the formation of γ-precipitates in γ’ phases, which is based on a combination of an Embedded Atom Method (EAM) interatomic potential designed for Molecular Dynamics (MD) and the kinetic lattice Monte Carlo (KLMC) method. As a demonstration of the KLMC simulation, the formation of the L12 structure was simulated from an initial state with randomly distributed aluminum atoms (25at.%Al). Finally, the γ’ phase with the L12 structure is formed in the entire simulation volume. To understand the fundamental mechanism of the formation of the L12 structures, the binding energies between a pair of aluminum atoms in a nickel single crystal were calculated using MD method. The result gave a clear picture of the mechanism. The stability of the γ-precipitate was also investigated using the MD method. Finally, the formation of γ-precipitates was simulated using the KLMC method. The dependence of the formation on the temperature condition is discussed.
Authors: Zheng Zhang, Geng Liu, Tian Xiang Liu
Abstract: An adaptive meshless element-free Galerkin-finite element (EFG-FE) coupling model for thermal elasto-plastic contact problems is developed to investigate the influences of the steady-state frictional heating on the contact performance of two contacting bodies. The thermal elasto-plastic contact problems using the initial stiffness method is presented. The local adaptive refinement strategy and the strain energy gradient-based error estimation for EFG-FE coupling method are combined. The adaptive meshless model takes into account the temperature variation, micro plastic flow, and the coupled thermo-elasto-plastic behavior of the materials, considering the strain-hardening property of the materials and temperature-dependent yield strength. The adaptive model is verified through the contact analysis of a cylinder with an elasto-plastic plane. The thermal effects on the contact pressure, stresses distributions with certain frictional heat inputs are studied. The results show that the accuracy of the solutions from the adaptive refinement model is satisfactory but the cost of the CPU time is much less than that for the uniform refinement calculation.
Authors: Yasuhiro Kanto
Abstract: In this paper, a general post processing program for J-integral calculation is developed to apply to arbitrary shaped cracks in a three dimensional body. Usually J-integral calculation programs are options for specific stress analysis programs and they are not applicable to results from different analysis programs. In most cases, there are many limitations in analysis models or shapes of cracks. This situation is not favorable for users. This paper will demonstrate a development of a post-processing program to calculate J-integral for arbitrary shaped cracks in a three dimensional body. This program requires only discrete data of displacements and stresses at nodal or numerical integration points. Users can use their own programs for stress analyses and calculate J-integral after that. In this paper, errors in approximation will be discussed as the first stage of the development.
Authors: Yoshitaka Wada, Takuji Hayashi, Masanori Kikuchi, Fei Xu
Abstract: Due to more complex and severe design restrictions, more effective and faster finite element analyses are demanded. There are several ways to compute FE analysis efficiently: parallel computing, fast iterative or direct solvers, adaptive analysis and so on. One of the most effective analysis ways is the combination of adaptive analysis and multigrid iterative solver, because an adaptive analysis requires several meshes with difference resolutions and multigrid solver utilizes such meshes to accelerate its computation. However, convergence of multigrid solver is largely affected by initial shape of each element. An effective mesh improvement method is proposed here. It is the combination of mesh coarsening and refinement. A good mesh can be obtained by the method to be applied to an initial mesh, and better convergence is achieved by the improved initial mesh.
Authors: Fei Xu, Jian She Chen, Qi Qing Huang
Abstract: In this paper, the numerical stability of the SPH method under both tension and compression conditions are discussed by using one dimensional impact models. The artificial viscosity and the conservation smoothed approach are tried to suppress both tension and compression instability. The result clearly shows that the occurrence possibility of tension instability is much larger than the compression case. The comparison of the controlling techniques of the artificial viscous and conservation smoothed approach shows that the artificial coefficients are independent on the numerical instability but can reduce some perturbation, while the conservation smoothed approach does well for both tension and compression cases. Considering the tension instability directly , a simple method of inserting particles to avoid the numerical instability is introduced.
Authors: Yan Zhao, Fei Xu, Yu Long Li, Yoshitaka Wada
Abstract: A non-triangulation inserting particle method is presented to prevent numerical fractures of SPH computations. The particles in those regions in which numerical fractures may occur are chosen at the beginning of SPH computations. And, the chosen particles are arrayed by the two different rules respectively according to the particle position relations. When the distance between each two chosen connected particles exceeds a certain value and neither of the corresponding two particles has fractured in SPH computations, a new particle is generated between those two ones and the position relation of particles is updated. The physical quantities that the new particle carries are obtained according to some given rules. The method and the same type of method in the reference are compared. The application of the method is analyzed. Conservation of mass and Conservation of momentum of the whole system are kept in this method. Several examples are given to validate the efficiency of the method to prevent numerical fractures.
Authors: Jun Liu, Yu Long Li, Fei Xu
Abstract: This paper is focused on the development of an effective numerical method to simulate bird-impact aircraft windshield events. A new Smooth Particle Hydrodynamics (SPH) which has been incorporated as a solver option into the explicit finite element program PAM-CRASH was used to model the bird. The deformation between the numerical results and the experimental results is in good agreement. Simulation results of a bird-impact process indicated that the SPH bird model is more suitable to model the break-up of the bird into particles. Failure of the windshield in the experiment was simulated, and the good agreement between the numerical and experiment indicates that the failure model established in the present paper is reasonable. Finally, the energy changes of the bird and the windshield were calculated.
Authors: Tomonari Furukawa, John G. Michopoulos
Abstract: This paper presents an information-theoretic approach for computational material modeling, which characterizes materials by effectively utilizing all the known information including prior and empirical information. The approach is built within the framework of recursive Bayesian estimation where various inverse analysis techniques, such as Singular Value Decomposition (SVD) and Kalman Filter (KF) can be implemented. Numerical examples first investigate the validity of the proposed approach via parametric studies. The proposed approach has been then successfully applied to the identification of a composite specimen using a triaxial testing machine.
Authors: Muhetaer Kelimu, Marc Thiriet
Abstract: Air and blood flow in a set of deformable conduits. Nowadays, computational models of biofluid flow are based on zoomed domains reconstructed from medical image processing. Such modeling is already very useful in medical practice. However it splits the domain of interest from the remaining parts of the network. Most often, crude boundary conditions are used (stress free outlet BCs). Moreover, the living system corresponds to a frosen state, although physiological flows interact with cell lining the interface between fluid and solid. Therefore, computational models of flow in normal and damaged bioconduits require couplings. The talk will illustrate cases for which the nanoscale must be incorporate for future research.

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