0

IN THIS ISSUE

### Guest Editorial

J. Eng. Mater. Technol. 2012;134(3):030301-030301-1. doi:10.1115/1.4006612.
FREE TO VIEW

This special issue of ASME Journal of Engineering Materials and Technology includes 14 papers on heterogeneous solids and composite materials, which are selected from those in the topical area orally presented at the 2011 ASME Applied Mechanics and Materials Conference (McMat 2011), May 31–June 2, 2011, Chicago and the 2011 International Conference on Heterogeneous Materials Mechanics (ICHMM 2011), May 22–26, 2011, Shanghai, China.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2012;134(3):031001-031001-12. doi:10.1115/1.4006510.

A general solution for the stress and strain fields in a three-layer composite tube subjected to internal and external pressures and temperature changes is first derived using thermo-elasticity. The material in each layer is treated as orthotropic, and the composite tube is regarded to be in a generalized plane strain state. A three-layer ZRY4-SiCf /SiC-SiC composite cladding tube under a combined pressure and thermal loading is then analyzed and optimized by applying the general solution. The effects of temperature changes, applied pressures, and layer thickness on the mechanical behavior of the tube are quantitatively studied. The von Mises’ failure criterion for isotropic materials and the Tsai-Wu’s failure theory for composites are used, respectively, to predict the failure behavior of the monolithic ZRY4 (i.e., Zircaloy-4) inner layer and SiC outer layer and the composite SiCf /SiC core layer of the three-layer tube. The numerical results reveal that the maximum radial and circumferential stresses in each layer always occur on the bonding surfaces. By adjusting the thickness of each layer, the effective stress in the three-layer cladding tube under the prescribed thermal-mechanical loading can be changed, thereby making it possible to optimally design the cladding tube.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031002-031002-5. doi:10.1115/1.4006509.

The mechanics of a small-scaled bilayer film-substrate system subject to temperature variation is studied. The modified couple stress theory is employed to take account of the size effects that are usually observed in small-scaled structures. In addition, the effect of weak bonding between the film and substrate is examined by using a linear slip-type model. Exact solutions are derived and the closed-form expressions for residual thermal stress and curvature of the system are given. Modified Stoney’s formulas are also presented for the bilayer system with perfect interface or imperfect interface between the film and the substrate.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031003-031003-9. doi:10.1115/1.4006508.

This study introduces a simplified micromechanical model for analyzing a combined viscoelastic–viscoplastic response of unidirectional fiber reinforced polymer (FRP) composites. The micromechanical model is derived based on a unit-cell model consisting of fiber and matrix subcells. In this micromechanical model, a limited spatial variation of the local field variables in the fiber and matrix subcells is considered in predicting the overall time-dependent response of composites. The constitutive model for the polymer matrix is based on Schapery’s viscoelastic and Perzyna’s viscoplastic models. An incremental stress–strain relation is considered in solving the time-dependent and inelastic response. A linearized prediction and iterative corrector scheme are formulated to minimize errors from the linearization within the incremental stress–strain relation such that both the micromechanical constraints and the nonlinear constitutive equations are satisfied. The goal is to provide the accurate effective stress–strain relations of the composites and the corresponding viscoelastic and viscoplastic deformation in the polymeric matrix. The micromechanical model is verified by comparing the time-dependent response of the glass FRP composites having several off-axis fiber orientations with experimental data available in the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031004-031004-6. doi:10.1115/1.4006507.

Thermal and mechanical response of a carbon fiber polymer matrix composite plate subjected to a transverse impact and in-plane pulsed electromagnetic loads is studied. Heat transfer analysis in the electrified composite plate for the electric currents of various waveforms (direct current (DC), alternating current (AC), and pulsed) is conducted, and the effects of the waveform parameters (characteristic time, peak, etc.) on the current-induced heating in the composite are investigated. The results show that pulsed electric currents cause significant temperature rises only in the regions immediately adjacent to the electric contact (composite-electrode interface). As for the mechanical response of the composite plate, it is found that the characteristics of the electromagnetic field (waveform, duration of application, and intensity) can significantly reduce deflection and stresses in the plate, and concurrent application of a pulsed electromagnetic load can effectively mitigate the effects of the impact load in the electrically conductive composites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031005-031005-8. doi:10.1115/1.4006506.

We present a semi-analytical approach to study the energy dissipation in carbon nanotube (CNT) beam oscillators under gigahertz excitation. The energy dissipation properties are quantified by the quality factor (Q factor) and associated anelastic properties. Our study reveals that the Q factor is related to the tube radius through an inverse relation for both single walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) beam oscillators. At frequency close to the resonance range, significant energy dissipation is observed due to the activation of phonon modes that serve as a major mechanism for energy dissipation in SWCNTs. For MWCNTs, a registration dependent potential (RDP) is introduced to study the effect of intertube registration. Interlayer friction arising from the $π$ bond overlap is shown to contribute significantly to the additional energy dissipation. Based on the extensive simulation studies, an analytical formula for estimating the Q factors of MWCNTs is proposed. Validation of the analytical prediction with the available experimental data yields a good agreement and quantifies the roles of different factors contributing to the energy dissipation through anelastic relaxation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031006-031006-7. doi:10.1115/1.4006505.

A consistent asymptotic expansion multiscale formulation is presented for analysis of the heterogeneous column structure, which has three dimensional periodic reinforcements along the axial direction. The proposed formulation is based upon a new asymptotic expansion of the displacement field. This new multiscale displacement expansion has a three dimensional form, more specifically, it takes into account the axial periodic property but simultaneously keeps the cross section dimensions in the global scale. Thus, this formulation inherently reflects the characteristics of the column structure, i.e., the traction free condition on the circumferential surfaces. Subsequently, the global equilibrium problem and the local unit cell problem are consistently derived based upon the proposed asymptotic displacement field. It turns out that the global homogenized problem is the standard axial equilibrium equation, while the local unit cell problem is completely three dimensional which is subjected to the periodic boundary condition on axial surfaces as well as the traction free condition on circumferential surfaces of the unit cell. Thereafter, the variational formulation and finite element discretization of the unit cell problem are discussed. The effectiveness of the present formulation is illustrated by several numerical examples.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031007-031007-7. doi:10.1115/1.4006504.

This paper studies the dynamic electromechanical response of multilayered piezoelectric composites under ac electric fields from room to cryogenic temperatures for fuel injector applications. A shift in the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral/monoclinic phases with decreasing temperature was determined using a thermodynamic model, and the temperature dependent piezoelectric coefficients were obtained. Temperature dependent coercive electric field was also predicted based on the domain wall energy. A phenomenological model of domain wall motion was then used in a finite element computation, and the nonlinear electromechanical fields of the multilayered piezoelectric composites from room to cryogenic temperatures, due to the domain wall motion and shift in the MPB, were calculated. In addition, experimental results on the ac electric field induced strain were presented to validate the predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031008-031008-7. doi:10.1115/1.4006503.

Theoretical prediction of percolation thresholds universally applicable for various composites remains a major theoretical challenge. In the work done by Xu (2011, “Ellipsoidal Bounds and Percolation Thresholds of Transport Properties of Composites,” Acta Mech., 223 , pp. 765–774), a variational method is developed to predict optimal percolation thresholds for transport properties of three dimensional composites subjected to full dispersion of fillers. In this paper, simplified formulae are provided for engineering applications of 3D composites. New formulae are derived for optimal percolation thresholds of 2D composites, i.e., laminates and thin films, and for composites containing a combination of fillers with different aspect ratios. The effects of dimensionality and waviness are especially discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031009-031009-8. doi:10.1115/1.4006501.

The flexural response of a three-dimensional (3D) layer-to-layer orthogonal interlocked textile composite has been investigated under quasi-static three-point bending. Fiber tow kinking on the compressive side of the flexed specimens has been found to be a strength limiting mechanism for both warp and weft panels. The digital image correlation (DIC) technique has been utilized to map the deformation and identify the matrix microcracking on the tensile side prior to the peak load in the warp direction loaded panels. It has been shown that the geometrical characteristics of textile reinforcement play a key role in the mechanical response of this class of material. A 3D local–global finite element (FE) model that reflects the textile architectures has been proposed to successfully capture the surface strain localizations in the predamage region. To analyze the kink banding event, the fiber tow is modeled as an inelastic degrading homogenized orthotropic solid in a state of plane stress based on Schapery Theory (ST). The predicted peak stress is in agreement with the tow kinking stress obtained from the 3D FE model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031010-031010-9. doi:10.1115/1.4006500.

Polycrystalline tungsten is considered as an important material in aerospace, automobile, and energy industries due to its excellent thermal and mechanical properties. While grain boundaries (GBs) are perceived to play a major role in polycrystalline tungsten failure resistance, experimental data are scarce on explicit contribution of GBs to tungsten failure resistance. The present work focuses on understanding the effect of GB property variation on fracture resistance of polycrystalline tungsten. The cohesive finite element method is used for the simulation of crack propagation in polycrystalline tungsten microstructures. The results show a significant effect of GB property variation on change of crack propagation patterns during tungsten fracture. A variation of 10% in GB fracture energy resulted in distinctly different crack patterns with different primary crack propagation direction and the microcrack density. Based on the observed microstructural fracture attributes, a relation between cohesive energy dissipation and microcrack density in polycrystalline tungsten microstructures is proposed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031011-031011-6. doi:10.1115/1.4006499.

Various aspects of the mechanical behavior of epoxy-based nanocomposites with graphene platelets (GPL) as additives are discussed in this article. The monotonic loading response indicates that at elevated temperatures, the elastic modulus and the yield stress are significantly improved in the composite as compared to neat epoxy. The activation energy for creep is smaller in neat epoxy, which indicates that the composite creeps less, especially at elevated temperatures and higher stresses. The composites also exhibit larger fracture toughness. When subjected to cyclic loading, fatigue crack growth rate is smaller in the composite relative to neat epoxy. This reduction is important by at least an order of magnitude at all stress intensity factor amplitudes. Optimal property improvements in the monotonic, cyclic, and fracture behaviors are obtained for very low filling fraction of approximately 0.1 wt. %. Similar differences in the mechanical behavior are observed when the composite is probed on the local scale by nanoindentation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031012-031012-10. doi:10.1115/1.4006498.

To investigate the effects of coating layer thickness on stress and the debonding behavior near the interface of coating layer and substrate, multiscale analysis is a must since molecular dynamics (MD) simulations can only be performed on models with thicknesses of about tens of nanometers on common computers, but the real thicknesses of such layers are around 300–1200 nm. In this work, generalized particle dynamics (GP) modeling for Al coated on Fe is first developed by using an atomistic domain near the layer interface and having high-scale particles far from that region to reduce degrees of freedom. Results show that the thicker coatings experience lower local average shearing stresses for a given shear strain. However, it is found that when the layer thickness reaches a large value, further increase of the layer thickness will not greatly benefit the reduction of the stress, thereby not increasing the allowable load. This trend is consistent with the simulation for Al2 O3 coated on Fe by a hierarchical multiscale analysis which is formulated by proposing a nanoscale-based key variable, Gdb , called debonding energy density. This variable, defined by the debonding energy per unit area, is used to characterize material bonding strength in realizing that failure originates from the atomistic and nanoscale. The difference and connection of this low-scale fracture variable, Gdb , with crack energy release rate, GIC , in traditional fracture mechanics is illustrated and how Gdb can be easily determined through atomistic simulation is exemplified. To make the new variable effective in engineering applications, Gdb is used as input to a macroscopic scale finite element model. The obtained layer-thickness effect directly confirms the existence of a critical thickness, predicted by the GP method. This work is an effort in developing material failure theory from lower scales where material fracture originates but with applications in continuum scale via both hierarchical and concurrent multiscale analyses.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031013-031013-5. doi:10.1115/1.4006502.

A parametric study is carried out to shed light on the elastoplastic behavior of a nanowire under lithiation. The Li-ion diffusivity is assumed to be significantly higher at near-saturation than at dilute concentration. It leads to the prediction of an Li-ion diffusion jam and consequently a topologically steep step moving along the wire. The analysis shows that the different plastic flow rates due to the different constraint conditions along the longitudinal and radial directions result in apparent anisotropic volume expansion. Either lower yield strength, smaller strain hardening ratio, or higher charging rate would cause greater swelling anisotropy. The numerical results are compared with the experimental observation of an SnO2 nanowire (Huang , 2011, “In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode,” Science, 330 , pp. 1515–1520) to suggest its elastoplastic properties under lithiation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(3):031014-031014-12. doi:10.1115/1.4006583.

In this work, an atomistic-based finite temperature multiscale interphase finite element method has been developed, and it has been applied to study fracture process of metallic materials at finite temperature. The coupled thermomechanical finite element formulation is derived based on continuum thermodynamics principles. The mesoscale constitutive relations and thermal conduction properties of materials are enriched by atomistic information of the underneath lattice microstructure in both bulk elements and interphase cohesive zone. This is accomplished by employing the Cauchy–Born rule, harmonic approximation, and colloidal crystal approximation. A main advantage of the proposed approach is its ability to capture the thermal conduction inside the material interface. The multiscale finite element procedure is performed to simulate an engineering nickel plate specimen with weak interfaces under uni-axial stretch. The simulation results indicate that the crack propagation is slowed down by thermal expansion, and a cooling region is found in the front of crack tip. These phenomena agree with related experimental results. The effect of different loading rates on fracture is also investigated.

Commentary by Dr. Valentin Fuster