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### Guest Editorial

J. Eng. Mater. Technol. 2011;134(1):010301-010301-1. doi:10.1115/1.4005422.
FREE TO VIEW

Advancement on modeling, microstructure characterization, property measurement, and processing in polymer based advanced materials raised interest to integrated computational materials engineering. How to advance computational materials science to guide the processing of advanced materials is an emerging field, reflected in the mission of Materials Genome Initiative recently announced by White House.

Commentary by Dr. Valentin Fuster

### BRIDGING MICROSTRUCTURE, PROPERTIES, AND PROCESSING OF POLYMER-BASED ADVANCED MATERIALS

J. Eng. Mater. Technol. 2011;134(1):010901-010901-5. doi:10.1115/1.4005407.

Characterization of modified humic substances based binders for iron ore agglomeration was examined by chemical analysis, optical density, Fourier transform infrared spectrum (FTIR), and thermogravimetry and differential scanning calorimetry (TG–DSC). Chemical analysis displays the proportion of fulvic acid (FA) to humic acid (HA) in the binder is 1:10. Compared with the HA, the FA possesses more functional groups. Meantime, optical density ratio analysis shows that the molecular weight and aromatization degree of the FA are smaller than those of the HA. FTIR spectra further confirm aromatic and aliphatic fractions are associated with various types of oxygen-rich groups including carboxyl and hydroxyl groups. TG–DSC and chemical analysis indicate structural changes of the binder including thermal decomposition, dehydroxylation and/or decarboxylation are caused during heating. The structural characterization of the binder ensures its good performance in the field of iron ore agglomeration.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010902-010902-5. doi:10.1115/1.4005410.

In this study, a qualitative equivalence between the electrical percolation threshold and the effective thermal conductivity of composites filled with cylindrical nanofillers has been recognized. The two properties are qualitatively compared on a wide range of aspect ratios, from thin nanoplatelets to long nanotubes. Statistical continuum theory of strong-contrast is utilized to estimate the thermal conductivity of this type of heterogeneous medium, while the percolation threshold is simultaneously evaluated using the Monte Carlo simulations. Statistical two-point probability distribution functions are used as microstructure descriptors for implementing the statistical continuum approach. Monte Carlo simulations are carried out for calculating the two-point correlation functions of computer generated microstructures. Finally, the similarities between the effective conductivity properties and percolation threshold are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010903-010903-7. doi:10.1115/1.4005418.

The effect of coated rutile titanium dioxide (TiO2 ) and a disazopyrazolone dye (azo dye) on the mechanism and the kinetic of photo-oxidation of unvulcanized styrene butadiene rubber (SBR) composites under accelerated UV-visible irradiation (λ > 290 nm at 35 °C) has been investigated using several techniques such as infrared spectroscopy, UV-visible spectrophotometry, and gel fraction measurements. Different photo-products resulting from the photo-ageing such as alcohol, ketone, or acids as well as cross-linked network were identified. The incorporation of TiO2 rutile and an azo dye into the matrix did not modify the mechanism of photo-oxidation. However, they have a significant effect on the kinetic of photoproducts accumulation. Both fillers protect the matrix from photo-oxidation. TiO2 rutile, thanks to its inorganic coating at the surface, dissipates part of the UV-visible radiation received by the polymer. The activity mechanism of azo dye consisting of a combination of its function as an antioxidant and light absorber, presents a better stabilizing effect compared with TiO2 rutile.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010904-010904-6. doi:10.1115/1.4005419.

In this study, a hierarchical multiscale homogenization procedure aimed at predicting the effective mechanical properties of silica/epoxy nanocomposites is presented. First, the mechanical properties of the amorphous silica nanoparticles are investigated by means of molecular dynamics (MD) simulations. At this stage, the MD modeling of three-axial tensile loading of amorphous silica is carried out to estimate the elastic properties. Second, the conventional twp phase homogenization techniques such as finite elements (FE), Mori-Tanaka (M-T), Voigt and Reuss methods are implemented to evaluate the overall mechanical properties of the silica/epoxy nanocomposite at different temperatures and at constant weight ratio of 5%. At this point, the mechanical properties of silica obtained in the first stage are used as the inputs of the reinforcing phase. Comparison of the FE and M-T results with the experimental results in a wide range of temperatures reveals fine agreement; however, the FE results are in better agreement with the experiments than those obtained by M-T approach. Additionally, the results predicted by FE and M-T methods are closer to the lower bound (Reuss), which is due to lowest surface to volume ratio of spherical particles.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010905-010905-11. doi:10.1115/1.4005420.

This work aims to investigate the dynamic behavior of polypropylene organoclay nanocomposites. The nanocomposite was obtained by mixing the polypropylene matrix with a masterbatch of polypropylene modified anhydride maleic and montmorillonite organoclay (pp-nanocor). The dynamic behavior was investigated by using split Hopkinson pressure bars, at different strain rates and different temperatures. The obtained nanocomposite exhibits a good dispersion and a partially exfoliated morphology. To study the effect of nanocomposite dispersion and morphology on the dynamic behavior, another nanocomposite was prepared by melt mixing of polypropylene and a modified montmorillonite (dellite) (PP dellite). The dynamic property results for PP-nanocor show an increase of both Young’s modulus and yield stress with the increasing organoclay concentration. However, PP-dellite nanocomposites present poor mechanical properties compared with those of PP-nanocor.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010906-010906-8. doi:10.1115/1.4005421.

Hydroxyapatite (HA) whisker reinforced polyetheretherketone (PEEK) composites have been investigated as bioactive materials for load-bearing orthopedic implants with tailored mechanical properties governed by the volume fraction, morphology, and preferred orientation of the HA whisker reinforcements. Therefore, the objective of this study was to establish key structure-property relationships and predictive capabilities for the design of HA whisker reinforced PEEK composites and, more generally, discontinuous short fiber-reinforced composite materials. HA whisker reinforced PEEK composites exhibited anisotropic elastic constants due to a preferred orientation of the HA whiskers induced during compression molding. Experimental measurements for both the preferred orientation of HA whiskers and composite elastic constants were greatest in the flow direction during molding (3-axis, C33 ), followed by the transverse (2-axis, C22 ) and pressing (1-axis, C11 ) directions. Moreover, experimental measurements for the elastic anisotropy and degree of preferred orientation in the same specimen plane were correlated. A micromechanical model accounted for the preferred orientation of HA whiskers using two-dimensional implementations of the measured orientation distribution function (ODF) and was able to more accurately predict the orthotropic elastic constants compared to common, idealized assumptions of randomly oriented or perfectly aligned reinforcements. Model predictions using the 3-2 plane ODF, and the average of the 3-1 and 3-2 plane ODFs, were in close agreement with the corresponding measured elastic constants, exhibiting less than 5% average absolute error. Model predictions for C11 using the 3-1 plane ODF were less accurate, with greater than 10% error. This study demonstrated the ability to accurately predict differences in orthotropic elastic constants due to changes in the reinforcement orientation distribution, which will aid in the design of HA whisker reinforced PEEK composites and, more generally, discontinuous short fiber-reinforced composites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010907-010907-10. doi:10.1115/1.4005406.

In this study, the effect of moderate magnetic fields on the microstructure of a structural epoxy system was investigated. The changes in the microstructure have been quantitatively investigated using wide angle X-ray diffraction (WAXD) and pole figure analysis. The mechanical properties (modulus, hardness, and strain rate sensitivity parameter) of the epoxy system annealed in the magnetic field were probed with the aid of instrumented nanoindentation, and the results are compared to the reference epoxy sample. To further examine the creep response of the magnetically annealed and reference samples, short 45 min duration creep tests were carried out. An equivalent to the macroscale creep compliance was calculated using the aforementioned nanocreep data. Using the continuous contact compliance (CCC) analysis, the phase lag angle, tan (δ), between the displacement and applied force in an oscillatory nanoindentation test was measured for both neat and magnetically annealed systems through which the effect of low magnetic fields on the viscoelastic properties of the epoxy was invoked. The comparison of the creep strain rate sensitivity parameter, A/d(0), from short term(80 s), creep tests and the creep compliance J(t) from the long term (2700 s) creep tests with the tan (δ) suggests that former parameter is a more useful comparative creep parameter than the creep compliance. The results of this investigation reveal that for the epoxy system cured under low magnetic fields both the quasi-static and viscoelastic mechanical properties have been improved.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010908-010908-7. doi:10.1115/1.4005412.

Electrospun polymer nanofibers are attractive due to their unique volume-to-surface area, chemical, electrical, and optical properties. Department of Homeland security has interest in applications with polymeric scintillation detectors that directly discriminate between neutron and gamma radiations using manufacturing techniques that are inexpensive and which can be effectively implemented to produce large area detectors. Lithium-6 (6 Li) isotope has a significant thermal neutron cross-section and produces high energy charged particles upon thermal neutron absorption. In this research, 6 Li loaded polymer composite was successfully spun onto a stationary stainless steel target creating a thermal neutron scintillator made of randomly oriented fibers. Fiber mats thus obtained were characterized using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) for morphology, and fluorospectroscopy for optical properties. Additionally, the fiber mats were characterized for polymeric properties including microstructure evaluation and response to thermal neutrons, alpha, beta, and gamma radiation using suitable radiation facilities. Fiber matrix was made out of an aryl vinyl polymer and a wavelength shifting fluor with efficient resonant energy transfer characteristics. The mats produced had scintillation fibers having diameters from 200 nm to 3.2 μm.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010909-010909-6. doi:10.1115/1.4005405.

The present paper investigates the microstructure and mechanical properties’ aspects of AISI 4140 steel front axle beams developed by roll and hot-die forging processes. Microstructure of the processed beams exhibited tempered martensite, and nonmartensitic products, such as retained austenite and ferrite at the case and core, respectively. Fatigue testing results indicate that roll forged beams have demonstrated 37% higher fatigue lives (Weibull B50 life) compared to hot-die forged beams, despite similar quasi-static tensile properties. The improved fatigue performance of the roll forged beams over hot-die forged beams is attributed to the fine, close texture and rationalized material flow in the beams processed by the roll forging process. Finite element analysis and experimental strain measurements of subject component indicate that the stress levels due to fatigue loads are well below the static yield strength and endurance limit of AISI 4140 steel; however, the notches present in the form of flash or partition lines of the forged beams have initiated the fatigue failures of the beams.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010910-010910-8. doi:10.1115/1.4005411.

After extensive studies starting in the 1970s in relation to miscibility and piezoelectric properties, the blends of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) have been revisited with the aim of assessing their mechanical behavior. Depending on the amount of PVDF, either amorphous or semicrystalline blends are produced. Typically, the blends remain amorphous when their PVDF content does not exceed 40 wt. %. Blend composition influence on the values of the glass transition temperature, Tg , and on its mechanical expression, Tα , is extensively discussed. Then, emphasis is put on the stress-strain behavior in tension and compression over the low deformation range covering the elastic, anelastic, and viscoplastic response. The reported data depend, as expected, on temperature and strain rate and also, markedly, on blend composition and degree of crystallinity. Molecular arguments, based on the contribution of the glass transition motions are proposed to account for the observed behavior. Thanks to the understanding of phenomena at the molecular level, accurate models can be selected in the view of mechanical modeling.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):010911-010911-9. doi:10.1115/1.4005417.

A microstructure-based finite element analysis model was developed to predict the effective elastic property of cellulose nanowhisker reinforced all-cellulose composite. Analysis was based on the microstructure synthesized with assumption on volume fraction, size, and orientation distribution of cellulose nanowhiskers. Simulation results demonstrated some interesting discovery: With the increase of aspect ratio, the effective elastic modulus increases in isotropic microstructure. The elastic property anisotropy increases with the aspect ratio and anisotropy of nanowhisker orientation. Simulation results from microstructure-based finite element analysis agree well with experimental results, comparing with other homogenization methods: upper bound, lower bound, and self-consistent models. Capturing the anisotropic elastic property, the microstructure-based finite element analysis demonstrated the capability in guiding materials design to improve effective properties.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2011;134(1):011001-011001-9. doi:10.1115/1.4004069.

The effect of texture on grain boundary character distribution (GBCD) in thermomechanically processed oxygen-free high-conductivity copper has been investigated. Copper samples were cold rolled to a reduction in thickness of 50% and then annealed for 60 min in the range of 400–600°C. GBCD and texture were measured using electron backscatter diffraction. The fraction of special boundaries (Σ3, Σ9, and Σ27) varied from 59% to 71%, with the maximum in the sample annealed at 500°C. The results indicate that cold rolling provided a strong texture of brass type. It was found that the sample annealed at 500°C have texture components of cube, Goss, rotated-Goss, and Y orientations. These texture components were in relation with the formation of annealing twins and Σ3 boundaries. It was also shown that twin-induced GBCD evolution occurred by strain-induced boundary migration, multiple twinning, and conventional recrystallization. Annealing at 600°C caused full recrystallization and grain growth, showing a strong cube recrystallization texture. The grain growth was found to hinder the formation of special boundaries.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011002-011002-15. doi:10.1115/1.4004488.

The low in-plane modulus of honeycombs may be used for compliant structures with a high elastic limit while maintaining a required modulus. Numerical and finite element (FE) studies for a functional design of honeycombs having a high shear strength, (τpl *)12 and a high shear yield strain, (γpl *)12 are conducted with two material selections—mild-steel (MS) and polycarbonate (PC) and five honeycomb configurations, when they are designed to be a target shear modulus, G12 * of 6.5 MPa. A numerical study of cellular materials theory is used to explore the elastic limit of honeycombs. FE analysis is also employed to validate the numerical study. Cell wall thicknesses are found for each material to reach the target G12 * for available cell heights with five honeycomb configurations. Both MS and PC honeycombs can be tailored to have the G12 * of 6.5 MPa with 0.1–0.5 mm and 0.3–2.2 mm cell wall thicknesses, respectively, depending on the number of vertical stacks, N. The PC auxetic honeycomb with θ= −20 deg shows high shear flexibility, when honeycombs are designed to be the $G12*$ of 6.5 MPa; a 0.72 MPa (τpl *)12 and a 13% (γpl *)12 . The authors demonstrate a functional design with cellular materials with a large design space through the control of both material and geometry to generate a shear flexible property.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011003-011003-12. doi:10.1115/1.4004829.

A method is presented for adapting the classical Bishop-Hill model to the requirements of elastic/yield-limited design in metals of arbitrary crystallographic texture. The proposed Hybrid Bishop-Hill (HBH) model, which will be applied to ductile FCC metals, retains the “stress corners” of the polyhedral Bishop-Hill yield surface. However, it replaces the ‘maximum work criterion’ with a criterion that maximizes the projection of the applicable local corner stress state onto the macroscopic stress state. This compromise leads to a model that is much more accessible to yield-limited design problems. Demonstration of performance for the HBH model is presented for an extensive database for oxygen free electronic copper. The design problem considered is a hole-in-a-plate configuration of thin sheets loaded in uniaxial tension in arbitrary directions relative to the principal directions of material orthorhombicity. Results obtained demonstrate that HBH-based elastic/yield limited design is capable of predicting complex and highly nonintuitive behaviors, even within standard problems.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011004-011004-10. doi:10.1115/1.4005269.

Ultrasonic additive manufacturing (UAM) has proven useful in the solid-state, low tempe’rature fabrication of layered solid metal structures. It is necessary to optimize the various process variables that affect the quality of bonding between layers through investigation of the mechanical strength of various UAM builds. We investigate the effect of the process parameters tack force, weld force, oscillation amplitude, and weld rate on the ultimate shear strength (USS) and ultimate transverse tensile strength (UTTS) of 3003-H18 aluminum UAM built samples. A multifactorial experiment was designed and an analysis of variance was performed to obtain an optimal set of process parameters for maximizing mechanical strength for the tested factors. The statistical analyses indicate that a relatively high mechanical strength can be achieved with a process window bounded by a 350 N tack force, 1000 N weld force, 26 μm oscillation amplitude, and about 42 mm/s weld rate. Optical analyses of bond characterization did not show a consistent correlation linking linear weld density and bonded area of fractured surfaces to mechanical strength. Therefore, scanning electronmicroscopy (SEM) was conducted on fractured samples showing a good correlation between mechanical strength and area fraction that shows ductile failure.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011005-011005-9. doi:10.1115/1.4005268.

High-strength low alloy steels (HSLA) have been designed to replace high-yield (HY) strength steels in naval applications involving impact loading as the latter, which contain more carbon, require complicated welding processes. The critical role of HSLA-100 steel requires achieving an accurate understanding of its behavior under dynamic loading. Accordingly, in this paper, we experimentally investigate its behavior, establish a model for its constitutive response at high-strain rates, and discuss its dynamic failure mode. The large strain and high-strain-rate mechanical constitutive behavior of high strength low alloy steel HSLA-100 is experimentally characterized over a wide range of strain rates, ranging from 10−3 s−1 to 104 s−1 . The ability of HSLA-100 steel to store energy of cold work in adiabatic conditions is assessed through the direct measurement of the fraction of plastic energy converted into heat. The susceptibility of HSLA-100 steel to failure due to the formation and development of adiabatic shear bands (ASB) is investigated from two perspectives, the well-accepted failure strain criterion and the newly suggested plastic energy criterion [1]. Our experimental results show that HSLA-100 steel has apparent strain rate sensitivity at rates exceeding 3000 s−1 and has minimal ability to store energy of cold work at high deformation rate. In addition, both strain based and energy based failure criteria are effective in describing the propensity of HSLA-100 steel to dynamic failure (adiabatic shear band). Finally, we use the experimental results to determine constants for a Johnson-Cook model describing the constitutive response of HSLA-100. The implementation of this model in a commercial finite element code gives predictions capturing properly the observed experimental behavior. High-strain rate, thermomechanical processes, constitutive behavior, failure, finite elements, Kolsky bar, HSLA-100.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011006-011006-9. doi:10.1115/1.4005267.

This paper investigates the effect of shear stresses on the determination of residual stresses in isotropic and orthotropic materials by the slitting method. A great deal of research effort is focused on the estimation of the residual stress component normal to the slit face using strain data measured by strain gauges installed on the top or the back surface of the stressed specimens. However, the slitting process will also release two in-plane and out-of-plane shear stress components, which may influence the measured strains. For the two specimens of carbon/epoxy and glass/epoxy laminated composites as well as a steel specimen, the distribution of released strains on the top and the back surfaces due to the shear stresses is calculated using finite element method and compared with those due to the residual normal stress. The results show that on the back surface, the shear stresses have a very small effect on the measured strains. However, on the top surface, strains due to the residual shear stresses are significant compared with those due to the residual normal stress and cannot be ignored. A method using two top surface strain gauges in both sides of the slit is presented to separate the effects of normal and shear stresses from each other. Also, strains due to the in-plane and the out-of-plane shear stresses could be isolated from each other. If these separations could be carried out successfully, the residual shear stress can be calculated by the proposed formulation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011007-011007-6. doi:10.1115/1.4005266.

This paper reports an anomaly in the yield strength of dislocation interacting with stacking fault tetrahedra (SFT) in Cu, reveals atomic mechanisms that are responsible for the anomaly, and further shows the thermodynamic driving force for the atomic mechanisms to prevail. Instead of monotonically increasing with the area of intersection cross-section, the yield strength first increases and then decreases with the area. The decrease, or the anomaly, is due to a change of atomic mechanism of the interactions—the SFT goes through a morphological transformation. The thermodynamic driving force for the transformation derives from the competition between the elastic energy of dislocations and the stacking fault energy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011008-011008-8. doi:10.1115/1.4005347.

In the current work, the vibration characteristics of single-walled carbon nanotubes (SWCNTs) under different boundary conditions are investigated. A nonlocal elastic shell model is utilized, which accounts for the small scale effects and encompasses its classical continuum counterpart as a particular case. The variational form of the Flugge type equations is constructed to which the analytical Rayleigh–Ritz method is applied. Comprehensive results are attained for the resonant frequencies of vibrating SWCNTs. The significance of the small size effects on the resonant frequencies of SWCNTs is shown to be dependent on the geometric parameters of nanotubes. The effectiveness of the present analytical solution is assessed by the molecular dynamics simulations as a benchmark of good accuracy. It is found that, in contrast to the chirality, the boundary conditions have a significant effect on the appropriate values of nonlocal parameter.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011009-011009-8. doi:10.1115/1.4005348.

In this article, the design and development of a biaxial tensile test device and its specimen are described. The device, which was designed for evaluating the mechanical characteristics of a thin film specimen under in-plane uniaxial and biaxial tensile stress states, consists of four sets of a piezoelectric actuator, a load cell, a linear variable differential transformer (LVDT), and an actuator case including lever structures with displacement amplification function. The structures fabricated by wire electrical discharge machining are able to amplify the actuator’s displacement by a factor of 3.8 along the tensile direction. The biaxial test specimen prepared using conventional micromachining processes is composed of a cross-shaped film section and chucking parts supported by silicon springs. After square holes in four chuck parts are respectively hooked with four loading poles, the film section is tensioned to the directions where the poles get away from the center of the specimen. Tensile strain rate can be individually controlled for each tensile direction. Raman spectroscopic stress analyses demonstrated that the developed biaxial tensile test device was able to accurately apply not only uniaxial but also biaxial tensile stress to a single-crystal silicon (SCS) film specimen.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2011;134(1):011010-011010-12. doi:10.1115/1.4005349.

The smart sandwich structures have been widely used in the aerospace, automobile, marine, and civil engineering applications. A typical smart sandwich structure is usually comprised of two stiff face skins separated by a thick core with variety of embedded sensors to monitor the performance of the structures. In this study, the smart composite sandwich structure (CSS) samples are fabricated with glass microballoons syntactic foam core and resin infused glass-fiber face skins (with piezoelectric fiber composite sensors (PFCS) embedded inside the resin infused glass-fiber face skins). One of the main concerns associated with embedding sensors inside composite structures is the structural continuity, compatibility, and interface stress concentrations caused by the significant differences in material property between sensor and host structures. PFCS are highly flexible, easily embeddable, highly compatible with composite structures and their manufacturing processes, which makes them ideal for composite health monitoring applications. In this study, in-plane tensile, tension–tension fatigue, short beam shear, and flexural tests are performed to evaluate the effect on strengths/behavior of the CSS samples due to embedded PFCS. Then carefully planned experiments are conducted to investigate the ability of the embedded PFCS to monitor the stress/strain levels and detect damages in CSS using modal analysis technique. The tensile tests show that both the average ultimate strength and the modulus of elasticity of the tested laminate with or without embedded PFCS are within 7% of each other. The stress–life (S-N) curves obtained from fatigue tests indicates that the fatigue lives and strengths with and without the PFCS are close to each other as well. From short beam and flexural test results, it is observed that embedded PFCS leads to a reduction of 5.4% in the short beam strength and 3.6% in flexural strength. Embedded PFCS’s voltage output response under tension–tension fatigue loading conditions has been recorded simultaneously to study their ability to detect the changes in input loading conditions. A linear relationship has been observed between the changes in the output voltage response of the sensor and changes in the input stress amplitude. This means that by constantly monitoring the output response of the embedded PFCS, one could effectively monitor the magnitude of stress/strain acting on the structure. Experiments are also performed to explore the ability of the embedded PFCS to detect the damages in the structures using modal analysis technique. Results from these experiments show that the PFCS are effective in detecting the initiations of damages like delamination inside these composite sandwich structures through changes in natural frequency modes. Hence embedded PFCS could be an effective method to monitor the health of the composite sandwich structures’ in-service conditions.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Mater. Technol. 2011;134(1):014501-014501-7. doi:10.1115/1.4003107.

The effects of postrolling heat treatment on the mechanical property and microstructure of 1050 aluminum alloy and 304 stainless steel (SS) clad metals were investigated. Clad metals were made by cold rolling after surface treatments of both sheets followed by heat treatment at 500 °C for various annealing times. The effects of transformation of microstructure at the interface on bonding strength are discussed. The initial clad roll bonding of Al/stainless steel clad metal was bonded by mechanical locking at the interface. The protruding stainless steel in the interface is the diffusion route and forms the better joint with aluminum in the annealing process, which results in the enhancement of the bonding strength. Intermediate layers were formed for over 2 h. It resulted in the weakening of the bonding strength and the fracture surface transforms into a brittle structure. As Al/stainless steel clad metals were under 13% reduction ratio, it had the optimum bond strength with a heat treatment for 1 h at 500 °C.

Commentary by Dr. Valentin Fuster