Thermo-mechanically coupled modelling of cellular MgO-C refractories under thermal shock

Refractory materials such as magnesium oxide carbon (MgO-C) composites are used in the steel-making industry for furnaces, ladles or oxygen converters. A new class of composites are cellular MgO-C materials, consisting of carbon foams filled with magnesium oxide and inclusions of gas filled pores. Cellular MgO-C composites have the advantage of significantly improving the thermo-mechanical properties [1]. This contribution focuses on the FEM implementation of a fully coupled thermo-mechanical continuum model. It is based on the theory of porous media (TPM) restricted by a kinematic coupling of the displacement field of all constituents. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermo-mechanical modelling of cellular hybrid refractories

Refractory materials are subjected to both quasi-static and dynamic thermal loading (thermal shock) causing damage up to mechanical failure. Typical refractories are magnesia carbon bricks consisting of periclase (MgO) and carbon inclusions. Recently, a significant improvement of the thermo-mechanical behaviour could be achieved by cellular hybrid composites made of periclase-filled carbon foams. The present contribution focuses on MgO-filled carbon foams and the investigation and optimisation of the structure-property relationship with respect to a reduction of thermally induced stresses and damage. It is a transient as well as static, fully coupled thermo-mechanical problem. According to the fact that, in general, refractories are brittle materials a linear elastic model, with a damage criterion was used. To optimise the structural morphology of the cellular refractories, the effect of micro structural changes has been determined. For the investigation of the thermal shock! behaviour, the results correlate very well with the experimentally motivated Hasselman relation. There is a significant size effect depending on the pore size. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity of micro-and nanostructural epoxy composites at low temperatures

The thermal conductivity of epoxy composites containing not only the traditional fillers quartz, talc, carbon black, and aerosil, but also the very promising carbon nanomaterials is investigated. Two kinds of carbon nanomaterials — multi-wall (MWNT) and single-wall (SWNT) carbon nanotubes — were considered. The influence of their content (from 0.05 to 3.0 wt.%) on the thermal conductivity of MWNT-epoxy composites was studied. The thermal conductivity of epoxy composites was examined in the temperature range from −150 to 150°C. It was found that the introduction of 0.1–1.0 wt.% MWNT enhanced the thermal conductivity of pure epoxy resin by about 40%. A further increase in content of the nanotubes decreased the thermal conductivity. This can be explained by the worsening of nanotube dispersion at their high concentrations. The maximum growth in the thermal conductivity of the epoxy composites, on the entire range of temperatures considered, was observed at a 0.1 wt.% content of MWNT. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 1, pp. 117–126, January–February, 2008.     

Multifibre polymer composites: Prediction of deformation and thermophysical properties

Conclusions A theoretical and experimental investigation was carried out to examine the possibilities of a structural approach for prediction of elastic constants, creep functions and thermophysical characteristics of hybrid polymer composites reinforced with anisotropic fibres of several types. The theoretical solutions were obtained by generalizing the self-consistent method for the case of a three phase model. The effects of brittle fibre breakdown under tension in the direction of reinforcement of a unidirectional hybrid composite were studied under conditions of a short-term loading and a long-term creep. It has been shown that a creep of viscoelastic fibres plays a principal role in creep of the hybrid composite. It is just this creep that significantly increases the fibre damage during creep of the composite.A variant of the solution has been proposed for predicting the thermorheologically complex behavior of hybrid composites containing not only elastic but also viscoelastic thermorheologically simple components with different temperature-time shift factors. The peculiarities of thermal expansion of hybrid composites and the possibilities for a purposeful control of thermal expansion coefficients by hybridization were studied. The considered thermal interval included a region of transition of the polymer matrix from a glass state into a viscoelastic one.The control tests were performed for specimens of organic/glass, organic/carbon, glass/carbon and organic/boron polymer composites with different ratios of fibre volume contents. On the whole, the obtained accuracy of predicting the characteristics of the examined hybrid composites may be considered as acceptable for engineering applications.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 3, pp. 299–313, May–June, 1994.     

Nonlocal elasticity effect on column buckling of multiwalled carbon nanotubes under temperature field

Based on nonlocal theory of thermal elasticity mechanics, an elastic multiple column model is developed for column buckling of MWNTs with large aspect ratios under axial compression coupling with temperature change. The present model treats each of the nested tubes as an individual column interacting with adjacent nanotubes through the intertube van der Waals forces. The thermal effect is incorporated in the formulation. In particular, an explicit expression is derived for the critical axial strain of a double-walled carbon nanotube which clearly demonstrates that small scale effects contribute significantly to the thermo-mechanical behavior of multiwalled carbon nanotubes and cannot be ignored.     

An extended tube model for thermo-viscoelasticity of rubberlike materials: Parameter identification and examples

The extended tube-model was presented by KALISKE & HEINRICH (RCT 72, 602-632) in 1999 as a novel approach for isothermal hyperelasticity of rubberlike materials. This contribution is dedicated to its further development to finite non-linear thermo-viscoelasticity. A non-linear evolution law and a thermo-mechanical coupled free energy formulation are the kernel of the phenomenological approach where the elastic material response is inspired by statistical-mechanical theory. The representation of viscoelasticity is based on a multiplicative decomposition of the deformation gradient. The Helmholtz free energy of the material is formulated in terms of isothermal free energy functions multiplicatively coupled with non-linear temperature evolution functions. The non-linear evolution law for the viscous material branch is solved by applying a predictor-corrector algorithm with an exponential mapping scheme. In today's literature, several sophisticated thermo-mechanical material models are available. However, they are built upon a considerable number of material parameters governing the mechanical and thermal material response which need to be identified for practical application. Therefore, particular emphasis is given to an appropriate parameter identification technique for the thermal field. For the latter, a uniaxial extension test is carried out where the recorded data of the temperature field of the rubber specimen under cyclic loading is used for parameter identification. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the thermophysical and antifriction properties of polyethylene-based compositions

The results of a study of the effect of the filler properties (thermal conductivity, density, particle shape and size, percentage content) on the thermal conductivity of the composite are presented; the factors responsible for the increase in the thermal conductivity of the polymer are indicated. Formulas are recommended for calculating the thermal conductivity of composites based on polyethlene and various fillers.Institute of Polymer Mechanics, Academy of Sciences of the LatvianSSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 259–266, March–April, 1973.     

Thermal dependence of electric conductivity in thermoplastic composites

Thermal dependence of the electric conductivity of thermoplastic composites based on both amorphous (hiPS) and crystallized (PP) polymers is investigated in this study. Two types of carbon black fillers with different values of BET surface area were used as charge conductors. Composites based on crystallized polymer matrices indicate the sharp growth of electric resistivity just before the melting range. This maximum is followed by substantial decrease of resistance at T > T_melt. With the decrease of carbon black concentration both relative growth of resistance at the T T_melt and further dropping resistance at T > T_melt increase. Composites filled with particles of higher surface area are characterized by less pronounced matrix influence on thermal dependence of electric conductivity than composites filled with particles of lower surface area; this can be caused by more pronounced matrix/filler interaction in the first case. The range of temperatures at which the resistance increase occurs does not depend on the type of carbon filler and its concentration. Composites with amorphous matrices are characterized by distinct increase of resistance above glass transition. Thermal treatment of the sample significantly affects the initial values and intensity of the temperature dependence of the resistance.To be presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 526–532, July–August, 1995.     

Influence of Modifiers on the Physicomechanical Properties of Sawdust-Polyethylene Composites

Polymer-wood composites based on recycled polyethylene (RPE) are investigated. Dispersed alder sawdust was utilized as a filler. To improve the compatibility between the nonpolar matrix and the polar wood fibers as a reinforcement, two types of modifiers were used, which differed in their chemical nature and mechanical interaction with the constituents of the composites. The modifiers of the first type (paraffin and OP) improved the dispersibility of sawdust (SD), and those of the second type (Exxelor 1015 and OREVAC) contained groups of maleic anhydride, which interacted with the OH-groups of SD. The effect of the modifiers on the moisture sorption by SD, the dispersibility of the filler in the matrix, and the strength characteristics (ultimate strengths and moduli in tension and bending) of dry and moist RPE–SD composites and on their moisture sorption is estimated. The best results were obtained for the composites modified with paraffin, which is due to the more efficient employment of the strength and rigidity of well-dispersed SD fibers. In their strength characteristics, the RPE-based composites investigated are comparable to composites based on low-density polyethylene.     

A Formulation of Additive Finite Anisotropic Thermo-Plasticity in Logarithmic Lagrangean Strain-Entropy Space

A phenomenological model of rate-independent thermo-plasticity at finite strains is discussed. The formulation is based on an additive decomposition of the strain measure into an elastic and plastic part as proposed by Green and Naghdi. A constitutive model in the logarithmic Lagrangean strain-entropy space is developed capable of modelling isotropic elastic and anisotropic plastic material behaviour. The staggered solution scheme for coupled thermo-mechanical problems employs an isentropic phase for the deformation and an iso-geometrical phase for the thermal field. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal deformation of a composite reinforced with hybrid cloth strips

Conclusion We tested (for mechanical and thermal effects) composites reinforced with hybrid cloth COS and VAI strips; five alternate schemes of material, which differred in terms of the content of VAI layers and layers reinforced with COS, were tested. The elasticity characteristics, tension diagrams, and CLTE of the composites were determined. It was established experimentally that variation in the relative content of the above-indicated layers makes it possible to regulate the thermal expansion of the composite in the longitudinal direction of the reinforcing strips Objectively over significant ranges; in this case, the elastic modulus varies negligibly, while the specific elastic modulus remains virtually unchanged,An alternate scheme for determining the elasticity characteristics and CLTE of laminar polymeric materials reinforced with hybrid cloth strips on the basis of component properties is developed. The model according to which the structural organization of the composite is subdivided into several levels is primarily a computational model. The stress-strain state of the repeating structural elements is evaluated by methods of the strength of materials. The proposed algorithm for computing the physicomechanical characteristics of laminar composites is implemented in the form of a computer program. The experimental elasticity characteristics and CLTE obtained for composites with a different content of COS and VAI layers are compared with those calculated in accordance with the method developed (the computed values correspond to the experimental with an accuracy acceptable for engineering applications).Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 392–401, May–June, 1988.     

Effect of a dispersed filler on the modulus of elasticity of composites in the glassy state

An expression is proposed for predicting the reinforcement of composites by a dispersed filler in both the rubbery and glassy states with allowance for the internal thermal stresses in the polymer matrix. It is shown that by varying the plasticizer concentration in composites based on polyvinyl chloride it is possible to regulate the thermal stresses in the polymer matrix. The time-stress and time-concentration superposition principles are shown to be applicable to the dependence of the relative modulus of elasticity on filler concentration for glassy composites.     

Towards a finite element simulation of coating by means of thermal spraying

Metal sheet forming processes like deep drawing are applied in order to produce carriage parts in mass production. Therefore, forming tools are required that are well protected against wear. For such forming tools, wear resistant surfaces are, e.g., produced by thermal spraying of hard material coatings. The thermal spraying process itself is a highly transient thermo-mechanical process. In order to gain a better understanding of the heat input and transfer during thermal spraying, a simulation framework for thermal spraying processes is presented. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermo-mechanical description of material diffusion during an ultrasonic wire bonding process

The volume diffusion during an ultrasonic wire bonding process leads to a material transport between the wire and the material of the substrate and thus creates an intermetallic phase between them. In order to investigate this process, the thermal and mechanical mechanisms occurring during wire bonding should be studied. For this purpose, finite element simulations based on coupled thermo-mechanical equations are performed to study the temperature and stress distribution in and around the interface. The final objective of the model is to develop a growth law for the intermetallic phases by considering the mechanical work applied to the wire in addition to the temperature increase at the interface. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental and Numerical Investigations for Cyclic Thermal Shocks

Thermal shock is an extreme form of thermo-mechanical loading. Detailed investigations of thermal shock and live time analysis close to reality are necessary in industrial engineering in order to get a good prediction of life expectancy of high quality and safety relevant machine components. The first part of this paper concentrates on experimental investigations of macroscopic quantities like temperature, deformation, damage and crack propagation. Additionally first results on parameter studies for finite element thermal shock simulations on the thermal-mechanical problem are summarized. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Estimation of the optimal distribution of the fibers in carbon/carbon composite

Carbon/carbon (C/C) composites are widely represented in the industry and are utilized for extreme thermal and mechanical loading. Optimization of the fibers distribution allows by still high stiffness to provide reducing of the weight of the components that has crucial importance in aircraft and aerospace industry. A microstructure optimization problem for estimation of the microstructure with minimal compliance is formulated. The design variables of the posed problem are the local fibers distribution and porosity. The volume fractions of the fibers and pores in the whole microstructure are fixed. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of surface treatment of carbon fibers on the strength properties of carbon fiber plastics

Conclusions The strength characteristics of composites based on carbon fibers having a coating of silicon carbide are in direct dependence on the coating thickness and on the porosity, which makes it possible to assume the possibility of increasing the degree of realization of the strength characteristics of fibers having a coating in a composite by increasing the degree of impregnation of the carbon cord with the binder. The latter finds confirmation also in the fact that at a small coating thickness on the carbon fiber (of the order of 5 nm) the porosity of the composite obtained is equal to the porosity of the material based on the carbon fiber without coating. Moreover, as is evident from Fig. 3c, the casing of silicon carbide does not form a continuous coatting over the whole perimeter of the cord. The presence of these prerequisites, and also the high resistance of carbon fibers having a silicon carbide coating to oxidation [9], open up wide prospects for creating new composite materials based on them.Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 603–606, July–August, 1979.     

The effective thermal conductivity of carbon nanotube composites

The effective thermal conductivity of Carbon Nanotube (CNT)-polymer composites has been estimated using analytical and computational models. The analytical approach is based on the Cascade Continuum Micromechanics (CCM) model formulated within the framework of mean-field homogenization and the computational approach is based on numerical homogenization of the composite microstructure using image based Voxel-FEM (Finite Element Method). Comparison of the analytical and computational model predictions with experimental data show that the interfacial thermal resistance is overestimated by the analytical model as a consequence of not taking into account the CNT fiber tortuosity (curviness). (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelastic stability analysis of laminated composite plates: An analytical approach

The paper presents Chebyshev series based analytical solutions for the postbuckling response of the moderately thick laminated composite rectangular plates with and without elastic foundations. The plate is assumed to be subjected to in-plane mechanical, thermal and thermomechanical loadings. In-plane mechanical loading consists of uniaxial, biaxial, shear loadings and their combinations. The temperature induced loading is due to either uniform temperature or a linearly varying temperature across the thickness. The mathematical formulation is based on higher order shear deformation theory (HSDT) and von-Karman nonlinear kinematics. The elastic foundation is modeled as shear deformable with cubic nonlinearity. The thermal and mechanical properties of the composites are assumed to be temperature dependent. The quadratic extrapolation technique is used for linearization and fast converging finite double Chebyshev series is used for spatial discretization of the governing nonlinear equations of equilibrium. The effects of plate parameters and foundation parameters on buckling and postbuckling response of the plate are presented.