Showing papers in "International Journal of Applied Mechanics in 2013"

Numerical manifold method (NMM) simulation of stress wave propagation through fractured rock mass

Abstract: The present work is devoted to the simulation of stress wave propagation through fractured elastic media, such as rock mass, by using the numerical manifold method (NMM). A single fracture is used to verify the capability and accuracy of the NMM in modeling fractured rock mass. The frequency-dependence on stress wave transmission across a fracture is analyzed. The influence of the fracture specific stiffness on the wave attenuation and effective wave velocity is discussed. The results from the NMM have a good agreement with those obtained from a theoretical displacement discontinuity method (DDM). Taking the advantage that the NMM is able to simulate highly fractured elastic media with a consistent mathematical cover system, a numerical example of stress wave propagation through a fractured rock mass with numerous inherent fractures is presented. It is showed that the results are reasonable and the NMM has a high efficiency in simulating stress wave propagation through highly fractured rock mass. A safety assessment of a tunnel under blast is conducted by using the NMM subsequently. The potential application of the NMM to a more complex fractured rock mass is demonstrated.

Size-dependent piezoelectricity and elasticity due to the electric field-strain gradient coupling and strain gradient elasticity

Abstract: A size-dependent nonclassical Bernoulli–Euler beam model based on the strain gradient elasticity is proposed for piezoelectric nanowires. The governing equations and the corresponding boundary conditions are naturally derived from the variational principle. Different from the classical piezoelectric beam theory, the electric field–strain gradient coupling and the strain gradient elasticity are both taken into account. Static bending problem of a cantilever piezoelectric nanobeam is solved to illustrate the effect of strain gradient. The present model contains material length scale parameters and can capture the size dependent piezoelectricity and elasticity for nanoscale piezoelectric structures. The numerical results reveal that the deflections predicted by the present model are smaller than that by the classical beam theory and the effective electromechanical coupling coefficient is dramatic enhanced by the electric field–strain gradient coupling effect. However, the differences in both the deflections and effective EMC coefficient between the two models are very significant when the beam thickness is very small; they are diminishing with the increase of the beam thickness. This model is helpful for understanding the electromechanically coupling mechanism and in designing piezoelectric nanowires based devices.

Inhomogeneous large deformation kinetics of polymeric gels

Abstract: This work examines the dynamics of nonlinear large deformation of polymeric gels, and the kinetics of gel deformation is carried out through the coupling of existing hyperelastic theory for gels with kinetic laws for diffusion of small molecules. As finite element (FE) models for the transient swelling process is not available in commercial FE software, we develop a customized FE model/methodology which can be used to simulate the transient swelling process of hydrogels. The method is based on the similarity between diffusion and heat transfer laws by determining the equivalent thermal properties for gel kinetics. Several numerical examples are investigated to explore the capabilities of the present FE model, namely: a cube to study free swelling; one-dimensional constrained swelling; a rectangular block fixed to a rigid substrate to study swelling under external constraints; and a thin annulus fixed at the inner core to study buckling phenomena. The simulation results for the constrained block and one-dimensional constrained swelling are compared with available experimental data, and these comparisons show a good degree of similarity. In addition to this work providing a valuable tool to researchers for the study of gel kinetic deformation in the various applications of soft matter, we also hope to inspire works to adopt this simplified approach, in particular to kinetic studies of diffusion-driven mechanisms.

Axial crushing of triangular tubes

Abstract: In the present paper, the mechanical behavior of large deformation of a regular equilateral triangular tube under quasi-static axial crushing is reported, which is a polygon with an acute angle and odd number of sides. Based on the results from nonlinear finite element analysis (FEA), a new type of inextensional basic plastic collapse folding element is proposed to describe the plastic progressive collapse. The progressive folding around the stationary horizontal hinges and inclined traveling hinges are involved to develop the new basic folding element. Two types of inextensible deformation modes are discovered, i.e., diamond mode and rotational symmetrical mode. The average crushing load for each mode is predicted from the super-folding element theory, which was proposed from the previous investigation on the axial crushing of square columns. A rigid-plastic material model and a kinematically admissible model are involved in this theory. The results are further validated against experiments. The approximate quasi-static theoretical predictions for the mean crushing loads of triangular tubes provide reasonable agreement with the corresponding experimental results.

Bending of fgm plates by a simplified four-unknown shear and normal deformations theory

Abstract: The bending response of FGM plates is presented based upon a simplified shear and normal deformations theory. The present simplified theory is accounted for an adequate distribution of transverse shear strains through the plate thickness and tangential stress-free on the plate surfaces. The effect of transverse normal strain is also included. The number of unknown functions involved here is only four as against six in case of other shear and normal deformations theories. The principle of virtual work is employed to derive the governing equations. A comparison with the corresponding results is made to check the accuracy and efficiency of the present theory. Additional results for all stresses are investigated through-the-thickness of the FGM plate.

A moving kriging interpolation-based meshless local petrov–galerkin method for elastodynamic analysis

Abstract: A meshless local Petrov–Galerkin method (MLPG) based on the moving Kriging interpolation for elastodynamic analysis is presented in this paper. The present method is developed based on the moving Kriging interpolation for constructing shape functions at scattered points, and the Heaviside step function is used as a test function in each subdomain to avoid the need for domain integral in symmetric weak form. Since the shape functions constructed by this moving Kriging interpolation have the delta function property, the essential boundary conditions can be implemented easily, and no special treatment techniques are required. The discrete equations of the governing elastodynamic equations for two-dimensional solids are obtained using the local weak-forms. The Newmark method is adopted for the time integration scheme. Some numerical results are compared to that obtained from the exact solutions of the problem and other (meshless) methods. This comparison illustrates the efficiency and accuracy of the present method for solving the static and dynamic problems.

Isogeometric simulation for buckling, free and forced vibration of orthotropic plates

Abstract: Buckling, free and forced vibration analyses of orthotropic plates are studied numerically using Isogeometric analysis. The present formulation is based on the classical plate theory (CPT) while the NURBS basis function is employed for both the parametrization of the geometry and the approximation of plate deflection. An efficient and easy-to-implement technique is used for imposing the essential boundary conditions. Numerical examples for free and forced vibration and buckling of orthotropic plates with different boundary conditions and configurations are considered. The numerical results are compared with other existing solutions to show the efficiency and accuracy of the proposed approach for such problems.

The Effects of Elastic Stiffening on the Evolution of the Stress Field Within a Spherical Electrode Particle of Lithium-Ion Batteries

Abstract: Poor cyclic performance of lithium-ion batteries is calling for efforts to study its capacity attenuation mechanism. The internal stress field produced in the lithium-ion battery during its charging and discharging process is a major factor for its capacity attenuation, research on it appears especially important. We established an electrochemical –mechanical coupling model with the consideration of the influence of elastic stiffening on diffusion for graphite anode materials. The results show that the inner stress field strongly depends on the lithium-ion concentration field, greater concentration gradients lead to greater stresses. The evolution of the stress field is similar to that of the concentration gradient but lags behind it, which shows hysteresis phenomenon. Elastic stiffening can lower the concentration gradient and increase elastic modulus, which are two major factors influencing the inner stress field. We conclude that the latter is more dominant compared to the former, and elastic stiffening acts to increasing the internal stress.

Stress analysis of thick laminated plates using trigonometric shear deformation theory

Abstract: A trigonometric shear deformation theory (TSDT) taking into account transverse shear deformation effect as well as transverse normal strain effect is presented. The inplane displacement field uses sinusoidal function in terms of thickness coordinate to include the shear deformation effect. The cosine function in thickness coordinates is used in transverse displacement to include the effect of transverse normal strain. Governing equations and boundary conditions of the theory are obtained using the principle of virtual work. The results of displacements and stresses for static flexure of simply supported symmetric and anti-symmetric cross-ply laminated square plates subjected to parabolic load and line load are obtained. The results obtained by present theory are compared with those of classical, first-order and higher-order plate theories.

Periodic and chaotic responses of an axially accelerating viscoelastic beam under two-frequency excitations

Abstract: This study focuses on the steady-state periodic response and the chaotic behavior in the transverse motion of an axially moving viscoelastic tensioned beam with two-frequency excitations. The two-frequency excitations come from the external harmonic excitation and the parametric excitation from harmonic fluctuations of the moving speed. A dynamic model is established to include the finite axial support rigidity, the material derivative in the viscoelastic constitution relation, and the longitudinally varying tension due to the axial acceleration. The derived nonlinear integro-partial-differential equation of motion possesses space-dependent coefficients. Applying the differential quadrature method (DQM) and the integral quadrature method (IQM) to the equation of the transverse motion, a set of nonlinear ordinary differential equations is obtained. Based on the Runge–Kutta time discretization, the time history of the axially moving beam is numerically solved for the case of the primary resonance, the super–harmonic resonance, and the principal parametric resonance. For the first time, the nonlinear dynamics is studied under various relations between the forcing frequency and the parametric frequency, such as equal, multiple, and incommensurable relationships. The stable periodic response and its sensitivity to initial conditions are determined using the bidirectional frequency sweep. Furthermore, chaotic motions are identified using different methods including the Poincare map, the maximum Lyapunov exponent, the fast Fourier transforms, and the initial value sensitivity. Numerical simulations reveal the characteristics of the periodic, quasiperiodic, and chaotic motion of a nonlinear axially moving beam under two-frequency excitations.

Improvement of the peeling strength of thin films by a bioinspired hierarchical interface

Abstract: The peeling behavior of a thin film bonded to a substrate is investigated by using the cohesive interface model. We compare the peeling processes of film/substrate interfaces with three different geometric shapes, including a flat interface, a curved interface of sinusoidal shape, and a wavy interface with two-level sinusoidal hierarchy. The effect of the peeling angle on the maximal peeling strength is also examined. It is demonstrated that the peeling strength can be significantly improved by introducing a hierarchical wavy morphology at the film/substrate interface. This study may be helpful for the design of film/substrate systems with enhanced mechanical properties.

A yield criterion and plastic analysis for physically asymmetric sandwich beam with metal foam core

Abstract: A yield criterion for physically asymmetric sandwich cross-sections is proposed in this paper. Using the yield criterion, analytical solutions for the large deflections of fully clamped asymmetric slender sandwich beams transversely loaded by a flat punch at the midspan are derived considering the core strength effect and interaction of bending and axial stretching. Finite element (FE) method is employed to predict the large deflection behavior of the sandwich beams. Good agreement is achieved between the analytical predictions and FE results. Effects of asymmetric factor, core strength and loading punch size are also discussed. It is demonstrated that core strength and loading punch size have significant influences on the load-carrying and energy absorption capacities of physically asymmetric metal sandwich beams while the asymmetry effect could be neglected when the deflection exceeds sandwich beam depth.

Buckling analysis of functionally graded skew plates: an efficient finite element approach

Abstract: In the present study, an attempt has been made to present the Co finite element formulation based on third order shear deformation theory for buckling analysis of functionally graded material skew plate under thermo-mechanical environment. Here, prime emphasis has been given to study the influence of skew angle on the buckling behavior of functionally graded plate. Two dissimilar homogenization schemes, namely Mori–Tanaka scheme and Voigt rule of mixture are employed to sketch their influence for the interpretation of data. Temperature-dependent material properties of the constituents of the plate are considered to perform thermal analysis. Numerical examples are solved using different mixture of ceramic and metal plates to generate the new results and relative imperative conclusions are highlighted. The roles played by the different factors like loading condition, volume fraction index, skew angle, boundary condition, aspect ratio, thickness ratio and homogenization schemes on buckling behavior of the FGM skew plates are presented in the form of tables and figures.

Continuum Damage Mechanics Analysis of Thin-Walled Tube Under Cyclic Bending and Internal Constant Pressure

Abstract: Ratcheting and fatigue damage of thin-walled tube under cyclic bending and steady internal pressure is studied. Chaboche's nonlinear kinematic hardening model extended by considering the effect of continuum damage mechanics employed to predict ratcheting. Lemaitre damage model [Lemaitre, J. and Desmorat, R. [2005] Engineering Damage Mechanics (Springer-Verlag, Berlin)] which is appropriate for low cyclic loading is used. Also the evolution features of whole-life ratcheting behavior and low cycle fatigue (LCF) damage of the tube are discussed. A simplified method related to the thin-walled tube under bending and internal pressure is used and compared well with experimental results. Bree's interaction diagram with boundaries between shakedown and ratcheting zone is determined. Whole-life ratcheting of thin-walled tube reduces obviously with increase of internal pressure.

Thermally induced vibrations of solar panel and their coupling with satellite

Abstract: A coupled thermal-structural model of a laminated composite plate is proposed by using the absolute nodal coordinate formulation, the transverse shear and normal deformations through element thickness are included. The dynamic equations of structure are established by applying the d'Alembert's principle and then solved numerically to determine dynamic responses and transient heat conduction in the structure due to the nonlinear elastic force and thermal radiation. A cantilevered flexible solar panel subjected suddenly to a solar radiation is examined, it is found that by considering the coupling between the thermal and structural responses, thermal flutter of the composite panel can be well predicted. The coupled behavior of the composite solar panel with a satellite is also analyzed by idealizing it as a rigid-flexible multibody system in the low earth orbit, in which a natural coordinate formulation is established to analyze the attitude of the satellite rigid hub, the thermal snap phenomenon is also well predicted.

Experimental Characterization, Modeling and Parametric Identification of the Hysteretic Friction Behavior of Viscoelastic Joints

Abstract: Viscoelastic joints connecting solids are essential components of mechanical systems. Viscoelastic components have inherent damping in their structure. Moreover, energy losses in structural vibrations are strongly linked to the friction properties of joints. In this work, a new visco-tribological model was developed by coupling the rheological linear generalized Maxwell model and Dahl friction model. A method for parametric identification is proposed. Parameters of the model are identified from dynamic mechanical analysis (DMA) tests for different excitation frequencies. Comparison between measurements and simulations is performed and the validity of the proposed model is discussed.

Asymptotic analysis of generalized thermoelasticity for axisymmetric plane strain problem with temperature-dependent material properties

Abstract: In this paper, a new formulation for the generalized thermoelasticity in an isotropic elastic medium with temperature-dependent material properties is established. The governing equations for the generalized axisymmetric plane strain problem are derived. The asymptotic solutions for an infinite cylinder with the boundary subjected to the thermal shock are obtained under the linear assumption. Numerical results for the propagation of the thermal and elastic waves and the distributions of the displacement, temperature and stresses are given and illustrated graphically. Using these solutions, some phenomenon involved in the generalized thermoelastic problem are obtained, and the jumps at the wavefronts are observed clearly. The comparison is made with results obtained in the temperature-independent case and the influence of the temperature dependency of material properties on the propagation of thermal and elastic waves are also discussed.

Stochastic seismic response of base-isolated buildings

Abstract: The stochastic response of base-isolated building considering the uncertainty in the characteristics of the earthquakes is investigated. For this purpose, a probabilistic ground motion model, for generating artificial earthquakes is developed. The model is based upon a stochastic ground motion model which has separable amplitude and spectral non-stationarities. An extensive database of recorded earthquake ground motions is created. The set of parameters required by the stochastic ground motion model to depict a particular ground motion is evaluated for all the ground motions in the database. Probability distributions are created for all the parameters. Using Monte Carlo (MC) simulations, the set of parameters required by the stochastic ground motion model to simulate ground motions is obtained from the distributions and ground motions. Further, the bilinear model of the isolator described by its characteristic strength, post-yield stiffness and yield displacement is used, and the stochastic response is determined by using an ensemble of generated earthquakes. A parametric study is conducted for the various characteristics of the isolator. This study presents an approach for stochastic seismic response analysis of base-isolated building considering the uncertainty involved in the earthquake ground motion.

Vibroacoustic analysis of stiffened plates with nonuniform boundary conditions

Abstract: This paper deals with vibroacoustic analysis of bidirectional stiffened thin plates with nonuniform discrete elastic edge restraints. The displacement-like governing equation of motion of the stiffened plate is derived by applying energy approach. A series of simple polynomials satisfying the Rayleigh–Ritz convergence criteria as well as the edge boundary conditions are then applied to discretize the governing equation, and the transverse displacements of the plate are solved considering the excitation of an incident acoustic plane wave. The commonly-adopted vibroacoustic indicators of elastic plane surface, i.e., mean square velocity (MSV), radiation efficiency (σ) and sound transmission loss (STL), are calculated for the stiffened plate with uniform and nonuniform discrete boundary conditions (BCs). Numerical studies are performed and results are discussed in detail for the vibroacoustic performance of the plate with different edge elastic restraints.

Dynamic pull-in instability and vibration analysis of a nonlinear microcantilever gyroscope under step voltage considering squeeze film damping

Abstract: In this paper, a nonlinear model is used to analyze the dynamic pull-in instability and vibrational behavior of a microcantilever gyroscope. The gyroscope has a proof mass at its end and is subjected to nonlinear squeeze film damping, step DC voltages as well as base rotation excitation. The electrostatically actuated and detected microgyroscopes are subjected to coupled flexural-flexural vibrations that are related by base rotation. In order to detune the stiffness and natural frequencies of the system, DC voltages are applied to the proof mass electrodes in drive and sense directions. Nonlinear integro differential equations of the system are derived using extended Hamilton principle considering nonlinearities in curvature, inertia, damping and electrostatic forces. Afterward, the Gelerkin decomposition method is implemented to reduce partial differential equations of microgyroscope deflection to a system of nonlinear ordinary equations. By using the 4th order Runge–Kutta method, the nonlinear ordinary equations are solved for various values of damping coefficients, air pressures, base rotation and various initial gaps between the proof mass electrodes and the substrates. Results show that the geometric nonlinearity increases the dynamic pull-in voltage and also consideration of the base rotation gives an improved evaluation of the dynamic instability. It is shown that the squeeze film damping has a considerable influence on the dynamic deflection of the microgyroscopes.

Generalized multi-symplectic method for dynamic responses of continuous beam under moving load

Abstract: Based on the multi-symplectic idea, a generalized multi-symplectic integrator method is presented to analyze the dynamic response of multi-span continuous beams with small damping coefficient. Focusing on the local conservation properties, the generalized multi-symplectic formulations are introduced and a fifteen-point implicit structure-preserving scheme is constructed to solve the first-order partial differential equations derived from the dynamic equation governing the dynamic behavior of continuous beams under moving load. From the results of the numerical experiments, it can be concluded that, for the cases considered in this paper, the structure-preserving scheme is generalized multi-symplectic if the viscous damping c ≤ 0.3751 when the continuous beam is under a constant-speed moving load and the structure-preserving scheme is generalized multi-symplectic if the viscous damping c ≤ 0.3095 when the continuous beam is under a variable-speed moving load with fixed step lengths Δt = 0.05 and Δx = 0.025. Similar to a multi-symplectic scheme, the generalized multi-symplectic scheme also has two remarkable advantages: the excellent long-time numerical behavior and the good conservation property.

Temperature and strain rate sensitivity of ultrafine-grained copper under uniaxial compression

Abstract: Uniaxial compressive experiments of ultrafine-grained (UFG) copper fabricated by equal channel angular pressing method were performed at temperatures ranging from 77 K to 573 K under quasi-static and dynamic loading conditions. Based on the experimental results, the influence of temperature on flow stress, strain hardening rate and strain rate sensitivity (SRS) were investigated carefully. The results show that the flow stress of UFG copper displays much larger sensitivity to testing temperature than that of coarse grained copper. Meanwhile, both the strain hardening rate and its sensitivity to temperature of UFG copper are lower than those of its coarse counterpart. The SRS of UFG copper also shows apparent dependence on temperature. Although the estimated activation volume of UFG-Cu is on the order of ~10 b3, which is on the same order with that of grain boundary diffusion processes, these processes should be ruled out as dominant mechanisms for UFG-Cu at our experimental temperature and strain rate range. Instead, it is suggested that the dislocation-grain boundary interactions process might be the dominant thermally activated mechanism for UFG-Cu.

A direct simulation method for the effective in-plane stiffness of cellular materials

Abstract: A simulation model is presented in order to provide the near-exact geometrical description of the actual cellular materials and to determine the effective in-plane stiffness of them. The model consists of a geometric input generator and a micromechanical model. The input generator is used to describe the cellular material geometry including the cell wall thicknesses, cell connectivity, vertex and center coordinates through the in situ specimen images while the micromechanical model is used to incorporate the geometrical description and the mechanical behavior of the cell walls. For model validation, a case study is performed on Nomex honeycomb specimens, the results of which are expressed in terms of the effective in-plane stiffness properties and compared to the measured values provided in the literature.

Micro-slip induced damping in the contact of nominally flat surfaces

Abstract: It is well known that the friction between interfaces at bolted joints plays a major role in the damping of assembly structures. Friction can be either induced by macro-slipping or micro-slipping. The aim of this paper is to model and quantify the dissipated energy by micro-sliding down to the scale of roughness between two flat surfaces in order to compute the damping ratio. It was assumed that the coefficient of friction between two materials is constant and that friction is the only source of energy dissipation. An experimental study was conducted to measure static normal load and dynamic tangential load without any coupling between these two main directions. A rheological contact model based on the Extended Greenwood Model with micro-contacts and statistical distributions was developed and studied. Experimental results and simulations are compared in order to assess and discuss the model.

Pressure driven steady flow in constricted channels of different cross-section shapes

Abstract: The influence of the cross-section shape on pressure driven viscous flow through a uniform channel is assessed by presenting analytical flow solutions of velocity distribution, volume flow rate and shear stress for different cross-section shapes. Next, a simplified flow model through a non-uniform constricted channel is formulated which accounts for flow inertia, viscosity and cross-section shape. The model outcome is quantified for different fluids, flows and geometrical properties relevant to physiological flows. It is seen that a commonly applied quasi-one-dimensional (1D) model is not accurate indicating the need to account for the cross-section shape.

Three-dimensional pressure-dependent elastoplastic cosserat continuum model and finite element simulation of strain localization

Abstract: A pressure-dependent elastoplastic Cosserat continuum model for three-dimensional problems is presented in this paper. The nonassociated Drucker–Prager yield criterion is particularly considered. Splitting the scalar product of the stress rate and the strain rate into the deviatoric and the spherical parts, the consistent algorithm of the pressure-dependent elastoplastic model is derived in the three-dimensional framework of Cosserat continuum theory, i.e., the return mapping algorithm for the integration of the rate constitutive equation and the closed form of the consistent elastoplastic tangent modulus matrix. The matrix inverse operation usually required in the calculation of elastoplastic tangent constitutive modulus matrix is avoided, that ensures the second order convergence rate and the computational efficiency of the model in numerical solution procedure. A comparison is performed between the classical and Cosserat continuum model through the numerical results of three-dimensional shear structure, tensile specimen, footing on a soil foundation, and soil slope stability. It illustrates that mesh dependency and numerical difficulties exist in classical model, while Cosserat model possesses the capability and performance in keeping the well-posedness of the boundary value problems with strain softening behavior incorporated. The relationship between the internal length scale and the width of shear band and the load-carrying capability of the structure has also been demonstrated.

Deformation of graphene induced by adsorption of peptides: a molecular dynamics study

Abstract: Graphene has potential applications in a variety of fields including electronics, photonics and chemical-biosensing. In this study, we perform molecular dynamics (MD) simulations to study the interactions between graphene sheets and biomolecules. The bending behavior of graphene induced by adsorption of peptides is investigated. The influence of peptide size, number, and alignment on the deflection of graphene sheets is studied in detail. The van der Waals (VDW) interaction plays a dominant role in the interaction between peptides and graphene. Our study provides valuable information for the experimental design of nanodevices incorporating graphene with biomolecules.

Wave reflection in piezoelectric half-plane

Abstract: The assumption of quasi-static electric field in the problem of wave reflection in piezoelectric half-plane results in missing an independent electric wave mode at the piezoelectric boundary, which leads to oversimplified solutions of reflected waves in a strong piezoelectric medium if only elastic bulk wave boundary conditions are considered. The paper presents a novel solution to address the issue by using the inhomogeneous wave theory and introducing a virtual reflection wave mode in addition to the elastic bulk wave modes. The virtual wave is assumed to satisfy the Snell's law as well as the piezoelectric boundary condition and can be treated in the same way as the elastic bulk waves. The analysis results show that this virtual wave always propagates along the boundary for any incident angle and can be treated as a pseudo surface wave. The energy transmission analysis reveals that this surface wave transmits zero energy and does not violate the energy conservation between the incident and the reflected elastic bulk waves. In addition, the analysis also reveals an interesting result that the quasi-transverse, not the quasi-longitudinal, incident wave will be fully reflected and no quasi-longitudinal reflected wave will be generated if the incident angle is beyond a critical angle.

An elastoplastic constitutive model for porous materials

Abstract: A micromechanics-based elastoplastic constitutive model for porous materials is proposed. With an assumption of modified three-dimensional Ramberg–Osgood equation for the compressible matrix material, the variational principle based on a linear comparison composite is applied to study the effective mechanical properties of the porous materials. Analytical expressions of elastoplastic constitutive relations are derived by means of micromechanics principles and homogenization procedures. It is demonstrated that the derived expressions do not involve any additional material constants to be fitted with experimental data. The model can be useful in the prediction of mechanical properties of elastoplastic porous solids.

A THERMO-DAMAGE STRENGTH MODEL FOR THE SiC-DEPLETED LAYER OF ULTRA-HIGH-TEMPERATURE CERAMICS ON HIGH TEMPERATURE OXIDATION

Abstract: At high temperatures above 1650°C, the SiC-depleted layer of ultra-high-temperature ceramics which has high porosity appears during the oxidation process. In this present paper, based on the studies of the oxidative mechanisms and the fracture mechanisms of ultra-high-temperature ceramics under normal and high temperatures, a thermo-damage strength model for the SiC-depleted layer on high temperature oxidation was proposed. Using the model, the phase transformation, microstructure development and fracture performance in the SiC-depleted layer on high temperature oxidation were studied in detail. The study showed that the porosity is mainly related to the oxidation of SiC. And while the SiC is substantially completely oxidized, only a very small part of matrix is oxidized. The fracture strength of the SiC-depleted layer degrades seriously during the high temperature oxidation process. And the bigger the initial volume fraction of SiC, the lower the fracture strength of the SiC-depleted layer is. This layer may become the origin of failure of material, thus the further researches should be undertaken to improve the oxidation behavior for the ultra-high-temperature ceramics in a wider temperature range.