Materials Science Forum Vols. 631-632

Abstract: The objective of this study is to determine stress intensity factors (SIFs) for a crack in a functionally graded layer bonded to a homogeneous substrate. Functionally graded coating contains an edge crack perpendicular to the interface. It is assumed that plane strain conditions prevail and the crack is subjected to mode I loading. By introducing an elastic foundation underneath the homogeneous layer, the plane strain problem under consideration is used as an approximate model for an FGM coating with radial grading on a thin walled cylinder. The plane elasticity problem is reduced to the solution of a singular integral equation. Constant strain loading is considered. Stress intensity factors are obtained as a function of crack length, strip thicknesses, foundation modulus, and inhomogeneity parameter.

Abstract: This paper addresses the numerical simulation of mixed-mode crack propagation in Functionally Graded Materials (FGMs) by means of eXtended Finite Element Method (XFEM), endowed with elastic and toughness properties which gradually vary in space. The method allows to follow crack paths independently of the finite element mesh, this feature is especially important for FGMs, since the gradation of the mechanical properties may lead to complex propagation paths also in simple symmetric tests. Each step of crack growth simulation consists of the calculation of the mixed-mode stress intensity factor by means of a non-equilibrium formulation of the interaction integral method, determination of the crack growth direction based on a specific fracture criterion. A specific fracture criterion is tailored for FGMs based on the assumption of local homogenization of asymptotic crack-tip fields in FGMs. The present approach uses a user-defined crack increment at the beginning of the simulation. Crack trajectories obtained by the present numerical simulation agree well with available experimental results for FGMs. The computational scheme developed here serve as a guideline for fracture experiments on FGM specimens (e.g. initiation toughness and R-curve behavior).

Abstract: This paper studies the effective thermal conductivity of multiphase composite in which a thermal boundary resistance exists at constituent interfaces. Based on the theoretical framework of conductivity for binary system composites in the presence of a thermal contact resistance between matrix and inclusion given by Y. Benveniste and T. Miloh (1986), the fundamental concept is generalized for the case of multiphase composites with imperfect contact which permits a temperature discontinuity between matrix and inclusions of different phases. A micromechanics model, the “generalized self-consistent scheme (GSCS)” based on a particle-matrix embedding in the effective medium, is generalized to evaluate the effective conductivity of multiphase medium with imperfect thermal contact at constituent interfaces. Numerical results are given for three-phase particulate composites with spherical particles to illustrate the effect of imperfect interfaces on the effective thermal conductivity of multiphase composites.

Abstract: Functionally Graded Materials Database (hereinafter referred to as FGMs Database) has been run by Japan Aerospace Exploration Agency (JAXA) since October, 2002. So far, the database contains 1,815 research paper information including 12,790 figures and tables, 2,632 author information, 904 PDFs, and 563 university, research institution and company information. The database had been used by 34,968 users from 113 countries in FY2007, where the effective total visits counted to 73,072. Approx. 20% of users had repeatedly visited the website. In FY 2007 users from Japan were significantly increased. Additionally, the interest of Europe and Middle east/Africa region was remarkable.

Abstract: Functionally graded materials for load bearing implants have a long history of academic development and an already high degree of maturity. Efforts are now undertaken to improve the biocompatibility and even induce bioactivity within the implant-bone interface by optimised surface nanostructure of porous ceramic and metalic layers grown or sintered on a metallic implant, in order to arrive at cementless implants capable of fast osteointegration and high interface strength. Several new methods for surface structure and composition modification are presented for Ti-alloy based implants: a nano-structuring of the surface by re-deposition of TiO2 using an ECR-Microwave Plasma treatment combined with ion bombardment on a sintered TiO2 ceramic surface, a multiscale modification of porous Ti-coatings by means of Micro-Arc-Oxidation, MAO, and a meso-structuring of the surface by means of a laser treatment. The goal is to establish a multi-functionality in such materials by formation of a morphological and compositional gradient spanning many dimensions. The applicability of these methods to real implants is discussed for a dental implant.

Abstract: Today, there is a strong push to improve the thermal management of electronic components in order to increase the performance and the reliability of electronic devices. Up to now, most of the heat sinks are mainly made of Copper that presents a good thermal conductivity (TC) but a coefficient of thermal expansion (CTE) much higher than the ceramic of the DBC (direct bonding Copper). It induces interfacial thermal stresses and indeed it decreases the reliability of the global electronic system. Therefore, there is a strong need for the development of novel heat dissipation material having low CTE combined with high TC. Carbon fibres reinforced copper matrix offers a good compromise between thermo mechanical properties (i.e. CTE) and medium TC. In order to increase surface TC, pure Copper can be added on the top surface and/or on the bottom one of the composite heat sink playing the role of heat spreader for hot spots linked with the Si components. The fabrication technique of these materials is based on powder metallurgy technique. The thermal properties of adaptive materials, TC and CTE, have been measured for different Copper thicknesses and architectures ([C/Cu], [Cu – C/Cu] and [Cu – C/Cu – Cu]). Simulation of the TC and CTE have been performed and compared to the experimental results.

Abstract: The size effect on particles reinforced metal matrix composites (MMCp) was investigated by a numerical method. A numerical multi-particles unit cell model has been constructed to carry out numerical analysis. In this model, circle particles were randomly placed in matrix according to uniform distribution, the sizes of particles in the paper were classified into 4 groups: 6μm, 13.5 μm ,50μm, and 100μm respectively. For investigating the effect of particles’ size on the plastic behavior of MMCp, the Griffith fracture criterion and the damage of ceramic particles were considered. Result showed that there was a close relationship between the particle size and the deformation behavior of the composites. Yield strength and plastic work hardening rate of the composites increase with decreasing particle size. The predicted stress–strain behaviors of the composites were in agreement with the experimental results.

Abstract: Functionally Graded Material (FGM) is a characteristic material having many functions and possibilities. The improvement of the long term stability is strongly necessitated by railway polymeric materials. To carry out the purpose, FGM is an expected material that enables to improve the certain properties and affinities. The authors tried to produce a graded compound of the magnetic particles in the polymer matrix in the magnetic field. From the results, the graded compound of the magnetic particles was produced by the control of magnetic flux. In particular, the control of time during which the magnetic field was charged was effective to improve the dispersion state of the magnetic particles in polymer matrix.

Abstract: The oxynitrides with composition A1-xBx(OyNz) (A: Nb and Ta, B: Al) were studied in preparation through citrate route and NH3 nitridation. In the case of Nb, the product of x = 0.5 prepared at 1000 °C was a new compound of a = 0.435 nm with rock salt structure. In this structure, Al and Nb atoms were distributed randomly in the cation sites. Anion sites were also randomly occupied by oxide and nitride ions with some amount of vacancy. The chemical composition was represented as Nb0.56Al0.44O0.38N0.37□0.25 from Rietveld refinement and oxygen/ nitrogen measurement. In the case of Ta, monoclinic Ta3N5 crystallized with a small amount of Ta4N5 impurity at x = 0.5. Aluminum compound was co-present as amorphous impurity.

Abstract: Nickel-titanium is a functional alloy currently used in various clinical applications, especially in vascular stents. There is an increased interest in the orthopaedic use of NiTi-based implants. The alloy enables the manufacture of applications of constant load, controllable motion, and minimal invasiveness. NiTi is considered biocompatible and it possesses mechanical properties that make it an especially good candidate for bone tissue surroundings. In our studies, we have investigated the effects of surface properties of NiTi on its biocompatibility. The martensitic phase was shown to have lower biocompatibility of material in comparison with austenitic NiTi. Cellular cytotoxicity increased and cell adhesion diminished on martensite phase. This was observed with both osteoblasts and osteoclasts. Our studies showed that the thickness of the oxide layer does not necessarily enhance the biocompatibility. The surface state of NiTi is strongly affected by thermal oxidation. Surface properties affect the initial adsorption of proteins and other macromolecules onto the biomaterial surface; this in turn impacts the following cellular responses, such as proliferation and differentiation, which are dictated by adhesion to the extracellular matrix components. Since adhesive force is connected to the interaction with the adsorbed molecules, it might be an important factor in the biocompatibility. Sol-gel derived titania-silica surface treatment was observed to increase the bone-implant coating of functional intramedullary NiTi nails. Sol-gel treatment together with the bending force increased the fixation of the implant (osseointegration). These studies indicate that the surface properties of NiTi are important for its biocompatibility.