Showing papers on "Material properties published in 2004"

Designing for ultrahigh-temperature applications: The mechanical and thermal properties of HfB2, HfcX, HfNX and αHf(N)

Abstract: The thermal conductivity, thermal expansion, Young's Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB2, HfC0.98, HfC0.67, and HfN0.92 ceramics. The mechanical behavior of αHf(N) solid solutions was also studied. The thermal conductivity of modified HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0.98 to 1100°C for HfC0.67 ceramics. The transition temperature of HfB2 was 1100°C. Pure HfB2 was found to have a strength of 340 MPa in 4 point bending, that was constant from room temperature to 1600°C, while a HfB2 + 10% HfCx had a higher room temperature bend strength of 440 MPa, but that dropped to 200 MPa at 1600°C. The data generated by this effort was inputted into finite element models to predict material response in internally heated nozzle tests. The theoretical model required accurate material properties, realistic thermal boundary conditions, transient heat transfer analysis, and a good understanding of the displacement constraints. The results of the modeling suggest that HfB2 should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity. The comparison the theoretical failure calculations to the observed response in actual test conditions show quite good agreement implying that the behavior of the design is well understood.

Measurements of the material properties of metal nanoparticles by time-resolved spectroscopy

Abstract: An important aim of nanoparticle research is to understand how the properties of materials depend on their size and shape. In this Invited Article I describe how time-resolved spectroscopy can be used to measure physical properties of nanometre sized objects such as the characteristic time scales for electron–phonon coupling and heat dissipation, and their elastic moduli. The electron–phonon coupling and heat dissipation measurements are important for applications of particles that involve conduction of heat or electricity. On the other hand, the elastic moduli studies provide fundamental information about the properties of nanomaterials. The results of these experiments show that nanometre sized particles can have very different properties compared to the corresponding bulk material. For example, we have recently shown that gold nanorods produced by wet chemical methods have much smaller elastic moduli (an 18% decrease in Young’s modulus) compared to bulk gold.

Three-dimensional thermal buckling analysis of functionally graded materials

Abstract: Three-dimensional thermal buckling analysis is performed for functionally graded materials. Material properties are assumed to be temperature dependent, and varied continuously in the thickness direction according to a simple power law distribution in terms of the volume fraction of a ceramic and metal. The finite element model is adopted by using an 18-node solid element to analyze more accurately the variation of material properties and temperature field in the thickness direction. Furthermore, the assumed strain mixed formulation is used to prevent locking as well as maintaining kinematic stability of the finite element model for thin plates and shells. The thermal buckling behavior under uniform or nonuniform temperature rise across the thickness is analyzed. Numerical results are compared with those of the previous works. In addition, the changes of critical buckling temperature due to the effects of temperature field, volume fraction distributions, and system geometric parameters are studied.

Layered versus multiphase magneto-electro-elastic composites

Abstract: Several researchers have focused on developing material properties for homogeneous magneto-electro-elastic multiphase composite materials. The candidate materials for this study are piezoelectric BaTiO 3 barium titanate as the embedded material with magnetostrictive CoFe 2 O 4 cobalt iron oxide as the matrix material. The materials are evaluated in terms of modeling the physical problem of the free vibration an infinite plate. Multiphase material properties vary depending upon the ratio of fiber material to matrix material. Actual electromagnetic materials are modeled as layered materials with the ratio of constituent materials being controlled by varying the number and thickness of layers of each material. Frequencies of vibration are compared for the layered materials versus the multiphase materials as a measure of the accurateness of the derived material constants. Multiphase material predictions for frequency agree quite well with layered materials for the problem that is studied.

Thermodynamic framework for the constitutive modeling of asphalt concrete: theory and applications

Abstract: The response of an asphalt concrete pavement to external loading depends on its internal structure. Using a recent framework that associates different natural (stress-free) configurations with distinct internal structures of the body, asphalt concrete is modeled. The authors assumed asphalt concrete to be a mixture of aggregate matrix and asphalt mortar matrix with evolving natural configurations. The evolution of the natural configuration is determined using a thermodynamic criterion, namely, the maximization of the rate of dissipation. Appropriate choices for the Helmholtz potential, the rate of dissipation and the other thermodynamic criteria are assumed to describe how energy is stored and the manner of the rate of dissipation. As an example, a specific form for the Helmholtz potential and the rate of dissipation function that leads to a generalized "upper convected Burgers's model" were chosen, its linearized version being the viscoelastic model that is usually used for modeling asphalt concrete. This model is just one example of how a class of thermodynamically consistent models can be generated to describe the nonlinear behavior of materials such as asphalt concrete. The uniaxial compressive and tensile creep of asphalt concrete for 2 different types of specimens and test methods are modeled. Details are provided of the numerical scheme used to solve the initial value problem, and the experimental data of Monismith and Secor (1962) is compared with predictions of the model.

Non‐linear analysis of the thermo‐electro‐mechanical behaviour of shear deformable FGM plates with piezoelectric actuators

Abstract: This paper investigates the non-linear bending behaviour of functionally graded plates that are bonded with piezoelectric actuator layers and subjected to transverse loads and a temperature gradient based on Reddy's higher-order shear deformation plate theory. The von Karman-type geometric non-linearity, piezoelectric and thermal effects are included in mathematical formulations. The temperature change is due to a steady-state heat conduction through the plate thickness. The material properties are assumed to be graded in the thickness direction according to a power-law distribution in terms of the volume fractions of the constituents. The plate is clamped at two opposite edges, while the remaining edges can be free, simply supported or clamped. Differential quadrature approximation in the X-axis is employed to convert the partial differential governing equations and the associated boundary conditions into a set of ordinary differential equations. By choosing the appropriate functions as the displacement and stress functions on each nodal line and then applying the Galerkin procedure, a system of non-linear algebraic equations is obtained, from which the non-linear bending response of the plate is determined through a Picard iteration scheme. Numerical results for zirconia/aluminium rectangular plates are given in dimensionless graphical form. The effects of the applied actuator voltage, the volume fraction exponent, the temperature gradient, as well as the characteristics of the boundary conditions are also studied in detail.

Elasto-plastic hemispherical contact models for various mechanical properties

Abstract: This work uses the finite element technique to model the elastoplastic deformation of a hemisphere contacting a rigid flat for various material properties typical of aluminium, bronze, copper, titanium and malleable cast iron. Additionally, this work conducted parametric finite element method (FEM) tests on a generic material in which the elastic modulus and Poisson's ratio are varied independently while the yield strength is held constant. A larger spectrum of material properties are covered in this work than in most previous studies. The results from this work are compared with two previously formulated elastoplastic models simulating the deformation of a hemisphere in contact with a rigid flat. Both of the previously formulated models use carbon steel mechanical properties to arrive at empirical formulations implied to pertain to various materials. While both models considered several carbon steels with various yield strengths, they did not test materials with various Poisson's ratios or elasti...

Onset of failure in finitely strained layered composites subjected to combined normal and shear loading

Abstract: A limiting factor in the design of fiber-reinforced composites is their failure under axial compression along the fiber direction. These critical axial stresses are significantly reduced in the presence of shear stresses. This investigation is motivated by the desire to study the onset of failure in fiber-reinforced composites under arbitrary multi-axial loading and in the absence of the experimentally inevitable imperfections and finite boundaries. By using a finite strain continuum mechanics formulation for the bifurcation (buckling) problem of a rate-independent, perfectly periodic (layered) solid of infinite extent, we are able to study the influence of load orientation, material properties and fiber volume fraction on the onset of instability in fiber-reinforced composites. Two applications of the general theory are presented in detail, one for a finitely strained elastic rubber composite and another for a graphite–epoxy composite, whose constitutive properties have been determined experimentally. For the latter case, extensive comparisons are made between the predictions of our general theory and the available experimental results as well as to the existing approximate structural theories. It is found that the load orientation, material properties and fiber volume fraction have substantial effects on the onset of failure stresses as well as on the type of the corresponding mode (local or global).

Measurement technique for high frequency characterization of semiconducting materials in extruded cables

Abstract: Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.

An equation of state for silicate melts. i. formulation of a general model

Abstract: A simple, empirical, volume-explicit equation of state (EOS) is developed for use in describing the volumetric properties of materials that exhibit large thermal expansivities and strongly non-linear, pressure-dependent compressibilities. Existing and commonly used EOS expressions, like the Universal and Birch-Murnaghan equations, develop singularities at critical values of temperature and pressure that prevent the calculation of derived thermodynamic properties for this class of substances. The proposed EOS reduces to a simple polynomial form at low-pressure; this form is utilized in the literature to describe the properties of silicate melts at 105 Pa. At high-pressures, the proposed EOS can be parameterized to yield a finite, non-zero volume limit, unlike other formulations. Thermodynamic properties can be calculated using the proposed EOS without the need for iterative solutions and all derivative and integral representations of the equation are analytic. A detailed example is developed demonstrating how the proposed EOS can be utilized in modeling the Gibbs free energy of amorphous silica at elevated temperature and pressure. The thermodynamic model described in the example accounts for vibrational contributions to the energy of the system as well as configurational effects arising from increases in the coordination number of Si with temperature and pressure. Vibrational contributions are modeled at elevated pressure by the proposed EOS. Configurational contributions are described by a simple associated solution model for the entropy of mixing. Parameters are calibrated from molecular dynamical simulations of the configurational and volumetric properties of amorphous silica. The calibration is used to calculate the phase diagram of silica to 3000 K and 15 GPa, which compares quite favorably with experimental data on melting.