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In this work, we numerically investigated the heat transfer effectiveness of different phase change materials (PCMs) when infused in a plate-fin heat sink with a fixed volume fraction of thermal conductivity enhancer. The PCM's ability to absorb and release large amounts of thermal energy at constant temperature is a desired feature in transient electronics cooling applications. In this study, we focused on examining the effect of the number of fins, type of PCM, heat flux, PCM volume fraction, and heat sink bottom wall thickness. The results showed that increasing the number of fins improved the performance of the PCM-infused heat sink. When a heat flux of 4000 W/m2 was applied for 30 minutes on a plate-fin heat sink infused with paraffin wax, the maximum temperature did not exceed 70°C in the four-fin design, while it exceeded 80°C in the two-fin design. A salt hydrate PCM outperformed paraffin wax and RT35. The bottom wall of the heat sink acted as a thermal spreader and a nonlinear relationship existed between the bottom wall thickness and the maximum electronics temperature. Compared to the two- and four-fin heat sink models, the zero-fin model required the longest time to fully melt the entire PCM due to the additional amount of PCM present in the heat sink gaps.

The rapidly increasing use of thin membranes; especially in the fabrication of flexible electronics, has motivated investigating the wrinkling behavior of these membranes. Wrinkling has an undesirable effect since wrinkles degrade the surface accuracy of structures incorporating thin membranes. This work introduces and evaluates an approach based on including optimized geometric features in thin membranes to suppress their wrinkling behavior. The ability of the optimized geometric features to suppress wrinkling in a 25 μm thick polyimide membrane subjected to uniaxial stretching is investigated computationally and experimentally in this work. A finite element model is developed to predict the wrinkling behavior of the membrane. An experimental setup equipped with 3D digital image correlation system is used to determine the wrinkles pattern, amplitude, and wavelength. Symmetric circular holes are introduced in the membrane to redistribute the stress field, eliminate the fluctuation in the minor principal stress in the membrane, and suppress wrinkles. The location and size of the circular holes are determined by coupling the Non-Linear Programming by Quadratic Largrangian technique and finite element simulations. An optimal design is obtained and the experimental results show that the optimally designed holes effectively suppressed the wrinkling behavior.

Cellular solids’ acoustic properties, such as dispersion, direction dependence, and phase velocities, are inherently coupled to the dominant deformation modes in their ligaments. Understanding and ...

Composite laminates often comprise notch-like features such as drilled assembly holes or geometric discontinuities resulting from damage. These features induce stress concentrations in their vicinities and can, through incompletely understood temperature-dependent damage mechanisms, reduce laminates’ load carrying capacity. Accordingly, this work investigates the effect of elevated temperatures on the notch sensitivity of carbon fiber-reinforced polymer composites and on the initiation and progression of notch-triggered damage mechanisms. This work employs an experimental approach in which tensile notched and un-notched specimens are loaded in controlled temperatures. Specimens are tested at multiple temperatures, and their nominal and local behavior in the vicinity of a circular notch are observed using digital image correlation. Results demonstrated a decreasing trend in the elastic moduli and ultimate tensile strength of notched specimens with increasing temperatures. However, the global response of notched samples at 50 $$^{\circ }$$ C surprisingly deviated from the expected trend and exhibited 8% higher tensile strength than that observed at 25 $$^{\circ }$$ C. Moreover, the notch sensitivity was found to decrease with increasing temperatures. Two main temperature-sensitive notch-triggered damage mechanisms were observed, namely transverse cracks and axial splitting. Transverse cracks were evident at all considered temperatures, while axial splitting was absent at room temperature.

Honeycomb structures, owing to their microstructural periodicity, exhibit unique and complex acoustic properties. Tuning their acoustic properties typically involves either changing their topology or porosity. The former route can lead to topologies that may not be readily amenable for large-scale production, while the latter could negatively affect the honeycombs’ weight. An ideal approach for tailoring the acoustic behavior of honeycombs should neither affect their porosity nor should they require customized and expensive fabrication methods. In this work, a novel honeycomb design that alters the microstructural topological features in a relatively simple way, while preserving the porosity of the honeycombs, to tune the acoustic properties of the honeycombs is proposed. The proposed honeycomb can be fabricated using the traditional approach employed to mass produce honeycomb structures; that is by bonding identical corrugated sheets with two periodic thicknesses. The acoustic behavior of the proposed honeycomb in terms of dispersion and phase velocities is analyzed using the finite element method. Simulation results demonstrate the potential of the designed honeycomb to exhibit tailored acoustic behavior at a constant porosity or mass. For example, it is demonstrated that the phase velocities of asymmetric and symmetric waves traversing the proposed honeycomb of aluminum with 90% porosity can be tuned by 30% and 17%, respectively.

Composite sandwich structures are renowned for their ability to provide high flexural stiffness to weight ratios. This ability made them appealing to aerospace, wind turbines, and marine industries. The functionality of sandwich structures primarily depends on the stiffness and strength of the two composite sheets bounding their inner cores. However, when sandwich structures are subjected to transverse impacts or transverse concentrated loads, their cores can fail due to local buckling. This can subsequently trigger the failure of the entire sandwich structure. To prevent such failure, inner cores with increased buckling strengths are sought. However, currently, increasing the buckling strength of sandwich structures' cores mostly comes at the cost of increasing their weights. Conversely, this work aims to increase the buckling strength of one of the most common sandwich structure cores, namely the hexagonal honeycomb core, without increasing its weight. This is accomplished through superposing periodic imperfections to its cell walls. The introduced imperfections follow a sinusoidal form and are applied to each cell wall as a perturbation. Finite element simulations are used to determine the effect of applying the imperfections to the cell walls and to determine the imperfection level that leads to significant improvement in the core's buckling strength. Results demonstrate that the buckling strength of honeycomb cores can be enhanced by more than 30% with the introduction of a minimal amount of sinusoidal periodic imperfections.

Periodic cellular solids, due to their ability to provide high stiffness and strength to weight ratios, have become common in engineering applications requiring weight minimization. Such applications relate to aerospace, marine, renewable energy, and automotive industries. The continuously increasing industrial relevance of periodic cellular solids has motivated developing methodologies to improve their elastic and failure properties. Introducing hierarchical features in periodic cellular solids proved as one of the most effective methods to improve their in-plane elastic and strength properties. This approach was applied to aluminum honeycombs and resulted in more than doubling their in-plane stiffness. However, the effect of introducing hierarchical features on the out-of-plane properties of periodic cellular solids in general and honeycombs in particular has not been fully investigated yet. Accordingly, this work investigates the effect of introducing hierarchical features on the out-of-plane behavior of honeycombs. In particular, this work investigates, using finite element computations, the effect of introducing hierarchy on the buckling strength of hexagonal honeycombs. Results show that hierarchy when carefully introduced, can enhance the buckling strength of honeycombs by more than 170% without increasing their weight.

Micro-architectured lattices are a promising subclass of cellular solids whose inner topologies can be tailored to enhance their stiffness. Generally, enhancing lattices' stiffness is achieved by increasing their connectivity. This strategy gives rise to a stiffer response by forcing lattices' ligaments to deform mainly in an axial manner. Conversely, this work is interested in developing micro-architectured lattices with enhanced stiffness, but whose cell walls deform in a flexural manner. Such structures can be more ductile and exhibit better energy mitigation abilities than their stretching dominated counterparts. Enhancing the stiffness of bending dominated lattices without increasing their connectivity can be realized by transforming them to hierarchical ones. This work explores, using experimentally verified finite element simulations, the effect of fractal-inspired hierarchy and customized nonfractal-based hierarchy on stiffness, anisotropy, and deformation mechanisms of an anisotropic bending dominated diamond lattice. Results show that fractal-inspired hierarchy can significantly enhance the stiffness of bending dominated lattices without affecting their deformation mechanisms or anisotropy level; ill-designed hierarchy can have a detrimental effect on lattice's stiffness; and customized hierarchy are more effective than fractal-inspired hierarchy in enhancing lattices' stiffness as well as can be more compatible with traditional, reliable, mass-producing manufacturing processes.

Additive manufacturing of complex structures with functional and programmable attributes (i.e., 4D printing) enables complex and innovative designs with tunable, adaptable or shape memory properties. To capitalize on the potential of 4D printing, it is instrumental to establish the knowledge base needed to predict the behavior of 4D printed structures. Accordingly, this work is dedicated to study the stability and shape recovery properties of 4D printed honeycomb structures using thermoplastic shape memory polymer (SMP). In particular, the dimensional stability of samples subjected to heat at temperatures around the glass transition temperature of the SMP material was assessed. In addition, the local shape recovery properties were investigated in samples programed into various temporary shapes with highly heterogeneous stored strain fields. The as-printed samples experienced dimensional instability manifested through a time dependent and heat induced strain accumulation when subjected to temperatures of ±10 °C around the SMP glass transition temperature. The rate of strain accumulation was temperature dependent and exhibited a high strain accumulation rate initially, followed by a relatively saturated response after about 10 min of temperature exposure. The total strain accumulation in the honeycomb samples was heterogeneous due to local variation in the deposition direction during printing. In the recovery experiments, despite the dissimilar levels of stored strain during programing, full local recovery was achieved independent of magnitude or nature of the stored strains. However, the local strain recovery rates varied in different regions and was attributed to different levels of stored strain and variation in the deposition direction. Overall, the work sheds important quantitative insight into the heterogeneous material response, dimensional stability and shape recovery of 4D printed structures manufactured using fused deposition and thermoplastic SMP.

A Master of Science thesis in Mechanical Engineering by Vibhor Bhargava entitled, “A Theoretical Approach towards the Modeling of Vibrothermography Using Finite Element Methods”, submitted in April 2019. Thesis advisor is Dr. Bassam Abu-Nabah and thesis co-advisor Dr. Maen Alkhader. Soft and hard copy available.

Apparent eddy current conductivity (AECC) spectroscopy recently established its potential capabilities as a practical approach to assess nonmagnetic metal coating thicknesses over nonmagnetic metals while eliminating the measurement sensitivity to lift-off in a ±25.4 µm range. It reduces the electromagnetic problem to three unknown parameters, namely the clad thickness of interest along with clad and substrate conductivities. This study exploits the use of triple-frequency AECC measurements to retrieve these unknowns following the plane-wave approximation and the Nelder-Mead optimization approach. The three frequencies, their incremental factor and operating range selection criteria is investigated to achieve the desired accuracy in clad thickness estimation. The proposed approach is validated for different Al 1230 clad thicknesses over Al 2024 substrates. It is demonstrated that selecting any three frequencies operating between 0.1 and 15 MHz with a 10 incremental factor delivers a 3% uncertainty in a clad thickness estimation of interest.

Aluminum honeycomb cores are widely used in sandwich structures due to their high stiffness-to-weight ratios and very low densities. However, owing to their porous architecture, honeycomb cores are inherently week and are susceptible to damage due to inadvertent or improper loadings on their encompassing sandwich structure. This damage can potentially lead to the failure of the sandwich structure, and therefore it should be detected and evaluated, preferably using nondestructive methods. Common nondestructive techniques have limited effectiveness in inspecting aluminum honeycombs due to their porous structure and dispersive properties. Since honeycombs are less dispersive at sub-ultrasound frequencies, inspecting them using low and sub-ultrasound frequencies has been introduced lately as a promising alternative to ultrasound inspection. However, this approach requires a priori knowledge of the wave propagation characteristics in the inspected material, which is not readily available for most commercially available aluminum honeycombs, especially the ones manufactured by joining thin corrugated sheets. Thus, this work utilizes finite element computations to assess the low frequency wave propagation characteristics (i.e. phase velocity and dispersive properties) in commercially available aluminum honeycombs made by bonding thin corrugated sheets. Results illustrate that the dispersive behavior and acoustic anisotropy of the studied honeycombs are more significant at higher porosities and high frequencies as well as identify the frequencies below which honeycombs exhibit their least dispersive acoustic behavior.

Carbon fiber-reinforced vinyl-ester polymer composites are increasingly used as structural members in applications (e.g., marine crafts and offshore structures) where they can be frequently exposed to the environmental elements of moisture and cold temperature fluctuations that cause freeze-thaw cycles. These harsh elements can individually and possibly synergistically damage carbon fiber-reinforced vinyl-ester composites. More importantly, their damage can accumulate over time and significantly degrade the structural properties, long-term integrity and durability of carbon fiber-reinforced vinyl-ester composites. This work experimentally investigates the individual and cooperative degrading effects of moisture and freeze-thaw cycles on the structural properties of carbon fiber-reinforced vinyl-ester composites, particularly on their flexural stiffness and strength. Results show that the combined damaging effects of moisture and freeze-thaw cycles are more significant than their individual effects, confirming the synergy between the damage mechanisms of the two elements.

Eddy current technology brings forward many advantages which can offer attractive solutions to assess metallic coating thicknesses over nonmagnetic metals. Available single and multi-frequency based impedance models are sensitive to lift-off deviations from those measured over the calibration blocks as well as different combinations of metallic coatings over nonmagnetic metal substrates. Apparent eddy current conductivity (AECC) spectroscopy has proven to overcome these drawbacks and offer a one order of magnitude improvement in estimating coating thickness in the lift-off range of ±25.4 μm. However, this technique at its present state is highly impractical due the requirement of a large coil diameter to meet the plane-wave approximation. This study simplifies the previous AECC technology to operate at a single frequency, which is validated using COMSOL simulations in agreement with the plane-wave approximation to estimate coating thicknesses, thus making this technique more practical.

The recent developments in apparent eddy current conductivity (AECC) spectroscopy have made it a viable candidate for clad thickness estimation where both clad and substrate are nonmagnetic metals. It negates the previous requirement of establishing a probe characteristic curve with a consistent lift-off distance between the measured sample and the calibration blocks. This eliminates the adverse effects in measurement uncertainty that occurred earlier due to liftoff variations. This study presents a practical approach for clad thickness estimation using AECC spectroscopy targeting clad thicknesses relevant to the industry. The technique once validated both numerically and experimentally has shown to deliver a measurement uncertainty of 3% over the entire coating thickness range of interest at a ±25.4μm lift-off range. Furthermore, it also reduces the coil diameter requirement to as low as 3 mm without the need of any correction factors.

Piezoelectrically active cellular solids are reminiscent of passive structural cellular solids, and therefore, depending on their inner cellular architecture, their cellular ligaments can deform locally by either bending or axial stretching. Three main cellular solid structures (i.e. hexagonal, tetragonal and triangular) that exemplify bending and stretching dominated piezoelectrically active cellular solids are considered. Three-dimensional finite element models were developed to understand the relationships between cellular structure, deformation modes and their effective electromechanical properties. The principal elastic, dielectric and piezoelectric properties of piezoelectric 3-1 cellular solids are insensitive to inner structure or topology in the longitudinally poled systems and highly sensitive to structure in the transversely poled systems. The in-plane electromechanical properties are highly sensitive to cellular architecture and connectivity as well. The effective out-of-plane elastic properties for all the three cellular structures depend linearly on relative density (i.e. stretching dominated), while the dependence of the in-plane effective elastic properties is linear for triangular and tetragonal cellular structures (i.e. stretching dominated) and generally non-linear for hexagonal honeycombs (i.e. bending dominated). Amongst the longitudinally poled systems, the triangular structures exhibit the highest in-plane stiffness properties. Amongst the transversely poled systems, the tetragonal structure exhibits the best overall combination of piezoelectric figures of merit.

Recent improvements in apparent eddy current conductivity (AECC) spectroscopy lends itself as a practical approach to assess nonmagnetic metal coating thicknesses over nonmagnetic metals. It reduces the measurement sensitivity to lift-off while bracketing the conductivity range of interest. It eliminates the need for establishing a probe characteristic curve measured at a fixed lift-off over coated calibration blocks where coated sample conductivity and measurement lift-off deviations can adversely affect the accuracy of coating thickness estimation. In this study, the accuracy of estimating different coating and clad thicknesses relevant to the industry using AECC spectroscopy is validated numerically and experimentally, respectively. It is demonstrated that AECC spectroscopy delivers 3% uncertainty in nonmagnetic metal clad thickness estimation over nonmagnetic metals in a ±25.4 µm lift-off range. It also reduces the measurement sensitivity to coil diameter to as low as 3 mm for high conductivity applications without the need of applying any correction factors.

Additive manufacturing has revolutionized the entire design cycle across a wide range of industries. The majority of materials utilized in 3D printing result in rather rigid structures with a fixed set of properties. Four-dimensional printing (4D printing) addresses this limitation by relaxing the geometric/properties rigidity over time. The careful selection of structural designs and the use of active and smart materials enables a unique set of shapes and properties to be programmed and achieved. This deviation from rigid structures with a fixed set of properties enables novel and unique applications in robotics, deployable structures, biomedical, and aerospace industries. The current work is focused on an interesting class of structures, 2D auxetic cellular solids, which exhibit a negative Poisson’s ratio. 4D printing of such structures allows for tunable structural stiffness, Poisson’s ratio, and rigidity. This work aims to experimentally investigate the tunability of stiffness and Poisson’s ratio in 4D printed auxetic structures fabricated using a thermoplastic shape-memory polymer. The attained structural stiffness was tuned in the range of 0.179−0.242 kN/mm while the in-plane Poisson’s ratio was controlled from -0.33 up to even positive magnitudes of +0.69. This wide range of elastic properties was obtained from a single structure programmed to different deformation levels. Experimental evaluation of 4D printed SMP structures under constant cyclic programming/recovery conditions induced residual strains which consequently affected the mechanical properties by degrading the Poisson’s ratio and structural stiffness. The magnitude of induced residual strains depends on the level of applied deformation during programing and the number of applied programing/recovery cycles. At the local level, full-field measurements revealed the localization of strains within the complex heterogeneous structure of the axially loaded auxetic honeycomb samples.

Characterizing the local deformations and strain fields in ligaments of periodic honeycomb materials is instrumental to applications such as cores of sandwich structure as well as honeycomb shaped tunable phononic crystals, compliant metamaterials, and 4D printed shape memory materials. Such applications employ honeycomb shaped materials whose nonlinear behavior is controlled by localized deformations in their ligaments. These local deformations can exhibit values and complex non-uniform states that far exceed the overall macroscopic deformations and strains sustained by the honeycomb material in value and complexity. Each of the aforementioned applications can be better designed with an improved understanding of the evolution of local strain fields in honeycombs' ligaments during their deformation. This work aims to characterize the local strains and deformations developing in honeycomb ligaments near honeycombs’ macroscopic onset of yielding. Finite element computations and the full field strain measurement technique digital image correlation (DIC) are used to measure and characterize the local deformations in the ligaments of concaved and convex honeycombs. Results show nonlinear strain gradients and significant localization in deformed ligaments. Moreover, complex state of stress comprising biaxial normal stresses and shear stresses exists near plastic zones.

Aluminum honeycomb cores, widely used in lightweight composite sandwich structures, are inherently weak and highly susceptible to buckling, permanent deformation and damage. This damage can detrimentally affect the structural integrity of sandwich structures and should be detected and repaired. Detecting damage to honeycomb cores is difficult as cores are sandwiched between composite sheets and not accessible. Typically, for such scenario a non-destructive testing method such as ultrasound is appealing. However, with the periodic and porous structure of honeycomb cores, at ultrasound frequencies they are dispersive and exhibit high transmission loss; thus they are incompatible with standard ultrasound techniques. Conversely, at sub-ultrasound frequencies honeycomb cores are less dispersive. Therefore low frequency elastic waves can potentially be used to non-destructively inspect honeycomb cores. To enable low frequency non-destructive testing of honeycombs, it is instrumental first to characterize their sub-ultrasound wave propagation and dispersion characteristics. Accordingly, this work, using finite element analysis, provides insights into the propagation characteristics of low frequency acoustic elastic waves in aluminum honeycomb cores in terms of phase velocity; dispersion and directionality characteristics; and frequency band gaps. Also, the effect of relative density and defects in the form of deformation on sub-ultrasound wave characteristics are explored.

Marine industries are increasingly embracing carbon fiber–vinylester resin composites as structural materials for military and civilian naval crafts. This move is driven by the ability of vinylester to better tolerate ultraviolet (UV) radiation and moisture as compared to other resins. Vinylester composites, even with long-term exposure to UV and moisture, exhibit limited surface damage which has a minor effect on their bulk properties. However, this surface-confined damage can impact the composite flexural strength which can be critical for marine crafts; particularly, as marine crafts are often subjected to out-of-plane loads (e.g., hull slamming). This work reports on a series of experiments aimed to investigate the individual and synergistic damaging effects of UV radiation and moisture on the flexural properties of carbon fiber vinylester composites. Results show that marine environment-induced damage can significantly reduce the flexural strength of carbon fiber–vinylester laminas (−10% to −40%).

Cellular solids and piezoelectrically active materials are important to a wide range of engineering applications in their own right. However, the integration of these two classes of materials into a hybrid porous electroactive composite promises to result in materials with a unique combinations of properties. In this work a multifunctional electroactive cellular material engineered by employing piezoelectrically active constituents in an auxetic and topologically tailored cellular solid is proposed, and its unique properties are highlighted using finite element analysis.

A finite element model is developed to characterize the complete electromechanical properties of the most general form of elastically anisotropic and piezoelectrically active foams with honeycomb structures. Four classes of piezoelectric honeycomb structures are identified depending on the relative orientation of the poling direction with the porosity direction (longitudinal and transverse) and the geometry of the honeycombs (isotropic and anisotropic). It is observed that: (i) Most of the elastic, dielectric, and piezoelectric constants of the longitudinally porous honeycomb foams exhibit linear dependence on the volume fraction (or relative density) of the material; (ii) The electromechanical properties of transversely porous foam structures (with the exception of C22 and κ22) exhibit significant dependence on the shape of the porosity; (iii) The piezoelectric figures of merit of the longitudinally porous foams do not exhibit significant dependence on the shape of the porosity; (iv) The piezoelectric figures of merit of the transversely porous foams exhibit a strong dependence on the shape of the porosity with the hexagonal foams exhibiting enhanced hydrostatic strain coefficient and lower acoustic impedance while the square foams exhibiting enhanced piezoelectric coupling constant and hydrostatic figure of merit; (v) In transversely porous anisotropic honeycomb structures, the shear elastic constants such as C12 and C66 and some figures of merit are enhanced significantly when compared to their isotropic counterparts. For example, in the PZT–7A transversely porous anisotropic honeycomb structures with 10% relative density, the hydrostatic figure of merit is expected to be 2485% greater than that predicted for the transversely porous isotropic honeycomb structures.

With the growing world demand on composite products, composite manufacturers are increasingly moving towards adopting automated composite manufacturing. However, current automated machines are not optimized and have throughput limitations. Most of them only target specific composite processes. Because the shortage of understanding on the processing factors and knowledge on precise process control, the products get high probability to come with defect. This paper provides an experimental methodology to explore the relationship between the structural properties of composite parts and processing factors. The paper also provides composite manufacturers a guide for processing condition optimization, productivity improvement and cost reduction. The raw material involved in this paper is an aerospace grade carbon fiber prepreg and was used to fabricate composite samples in a hot press machine system in the experiment. To investigate the effects of processing factors, specimens were fabricated using a series of combinations of different factor values. Three point bending flexural test was performed to evaluate the quality of each sample. Collected data are analyzed by statistical methods and fitted to a multiple linear regression model, which provides insights for composite manufacturing.

The thermo-mechanical response of single crystal and polycrystalline high purity copper is systematically compared at low and high strain rates. The mechanical response of each type of material is very different in terms of strain hardening, although both are distinctly strain rate sensitive. A simplified interpretation of the Taylor–Quinney coefficient, in which the strain dependence is not considered, shows a clear (almost linear) increase of this factor with the strain rate, while the two types show distinct trends. This factor increases with the strain rate but remains markedly lower than the classical value of 0.9. The stored energy of cold work is found to be relatively independent of the strain rate, with the polycrystal storing more energy than the single crystal. A microstructural study (transmission electron microscopy) of representative specimens of each type at low and high strain rates reveals a basically similar microstructure, despite dissimilar values of energy storage. It is proposed that a higher level of storage of the energy of cold work by polycrystalline copper is due to the presence of grain boundaries in this group.

A new shear-compression experiment for investigating the influence of hydrostatic pressure (mean stress) on the large deformation shear response of elastomers is presented. In this new design, a nearly uniform torsional shear strain is superposed on a uniform volumetric compression strain generated by axially deforming specimens confined by a stack of thin steel disks. The new design is effective in applying uniform shear and multiaxial compressive stress on specimens while preventing buckling and barreling during large deformation under high loads. By controlling the applied pressure and shear strain independently of each other, the proposed setup allows for measuring the shear and bulk response of elastomers at arbitrary states within the shear-pressure stress space. Thorough evaluation of the new design is conducted via laboratory measurements and finite element simulations. Practical issues and the need for care in specimen preparation and data reduction are explained and discussed. The main motivation behind developing this setup is to aid in characterizing the influence of pressure or negative dilatation on the constitutive shear response of elastomeric coating materials in general and polyurea in particular. Experimental results obtained with the new design illustrate the significant increase in the shear stiffness of polyurea under moderate to high hydrostatic pressures.

The initial and subsequent yield surfaces for an anisotropic and pressure-dependent 2D stochastic cellular material, which represents solid foams, are investigated under biaxial loading using finite element analysis. Scalar measures of stress and strain, namely characteristic stress and characteristic strain, are used to describe the constitutive response of cellular material along various stress paths. The coupling between loading path and strain hardening is then investigated in characteristic stress–strain domain. The nature of the flow rule that best describes the plastic flow of cellular solid is also investigated. An incremental plasticity framework is proposed to describe the pressure-dependent plastic flow of 2D stochastic cellular solids. The proposed plasticity framework adopts the anisotropic and pressure-dependent yield function recently introduced by Alkhader and Vural [Alkhader M., Vural M., 2009a. An energy-based anisotropic yield criterion for cellular solids and validation by biaxial FE simulations. J. Mech. Phys. Solids 57(5), 871–890]. It has been shown that the proposed yield function can be simply calibrated using elastic constants and flow stresses under uniaixal loading. Comparison of stress fields predicted by continuum plasticity model to the ones obtained from FE analysis shows good agreement for the range of loading paths and strains investigated.

Rapid advances in additive manufacturing techniques promise that, in the near future, the fabrication of functional cellular structures will be achieved with the desired cellular microstructures tailored to specific applications. It is therefore essential to develop a detailed understanding of the relationship between macroscopic mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect of topology and microstructural irregularity (or non-periodicity) on deformation modes of cellular structures under both uniaxial and biaxial stress states. A simple quantitative technique based on the partition of elastic strain energy into bending and stretch components is used to identify the distribution of deformation modes at a microstructural level. The relationship between nodal connectivity, morphological regularity and deformation modes is then explored through their influence on biaxial yield surfaces as obtained from finite element analyses.

The initial yield surface of 2D lattice materials is investigated under biaxial loading using finite element analyses as well as by analytical means. The sensitivity of initial yield surface to the dominant deformation mode is explored by using both low- and high-connectivity topologies whose dominant deformation mode is either local bending or strut stretching, respectively. The effect of microstructural irregularity on the initial yield surface is also examined for both topologies. A pressure-dependent anisotropic yield criterion, which is based on total elastic strain energy density, is proposed for 2D lattice structures, which can be easily extended for application to 3D cellular solids. Proposed criterion uses elastic constants and yield strengths under uniaxial loading, and does not rely on any arbitrary parameter. The analytical framework developed allows the introduction of new scalar measures of characteristic stresses and strains that are capable of representing the elastic response of anisotropic materials with a single elastic master line under multiaxial loading.

Fiber-reinforced polymer (FRP) composites used in the construction of composite-based civil and military marine crafts are often exposed to aggressive elements that include ultraviolet radiation, moisture, and cyclic loadings. With time, these elements can individually and more so cooperatively degrade the mechanical properties and structural integrity of FRP composites. To assist in increasing the long-term reliability of composite marine crafts, this work experimentally investigates the cooperative damaging effects of ultraviolet (UV), moisture, and cyclic loading on the structural integrity of carbon fiber reinforced vinyl-ester marine composite. Results demonstrate that UV and moisture can synergistically interact with fatigue damage mechanisms and accelerate fatigue damage accumulation. For the considered composite, damage and S–N curve models with minimal fitting constants are proposed. The new models are derived by adapting well-known cumulative fatigue damage models to account for the ability of UV and moisture to accelerate fatigue damaging effects.

Engineering research may provide tools to the individual as well as to the public in general, to effectively monitor wellness and health patterns, such as metabolic rate and weight control. Ketone bodies and acetone gas emissions in exhaled breath and skin, in particular, may be used as biomarkers of fatty acid metabolism and may be used in diet control. Two types of technologies, resistive chemosensors and chemomechanical actuators are outlined here as examples of such tools currently under development and of great promise.

Periodic cellular (lattice) materials, by virtue of their periodic structures and associated geometric impedance mismatch, exhibit wave dispersion, frequency dependent transmissibility, and directional characteristics that are inherently dependent on their constituent material and mesoscale microstructural features. These characteristics render lattice materials as potential candidates to perform as low frequency phononic crystals and metamaterials for radar, sonar, wave guiding, wave modulation and isolation applications. Accelerating the wide-spread implementation of lattice materials as phononic crystals hinges on establishing the ability to engineer them to exhibit application-tailored properties and tunable behavior (e.g. to activate/deactivate band gaps). Achieving tunability and application-oriented tailorablity requires, first, establishing an understating of phononic, acoustic, wave dispersion and directional properties of the lattices and how they are affected by lattices’ inherent features. Accordingly, using Bloch’s theorem in conjunction with finite element analysis, this work investigates the relationships between inherent microstructural features (such as lattice symmetry, relative density (i.e. volume fraction) and constituent material) and the acoustic properties (such as wave dispersion, band gaps, and acoustic anisotropy) of architectured lattice materials. The coupling between microstructural features and band gaps is investigated in a hexagonal lattice geometry which is inspired by the two dimensional Bravais family of lattices. Results illustrate that band structure and phononic properties are highly sensitive to relative density and can scale non-uniformly with it as eigenmodes are associated with relative density dependent deformation mechanisms. Moreover, results show that band gaps can potentially be activated and deactivated using macroscopic strain fields. The latter opens horizons for realizing cellular based phononic crystals with tunable properties.Copyright © 2014 by ASME

An experimental approach, based on Split Hopkinson Pressure Bar (SHPB) apparatus, is developed to elucidate failure of viscoelastic elastomers under combined shear and high pressures such as are encountered in explosive and/or armor-impact scenarios. In this experimental arrangement, thin cylindrical polyurea specimens with an aspect ratio (Diameter to thickness) greater or equal to 15 are tested, up to failure, using Split Hopkinson Pressure Bar (SHPB). Specimens with large aspect ratio are used to guarantee the close approximation of a triaxial state of stress in the specimen upon loading; hence the measured normal stress would be approximately equal to the hydrostatic pressure in the specimen. Friction at the loading interfaces forces the stress state to deviate from uniformity, restrict both the circumferential and radial displacements and lead to the development of shear stresses and strains. Hence, induced failure occurs under conditions combining high-strain-rate, high pressure and shear stresses. By using this setup, repeatable failure modes were detected and elucidated using finite element simulations.

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.

A new shear-compression experiment is developed to characterize the influence of hydrostatic pressure on the shear constitutive response of nearly incompressible viscoelastic materials undergoing large deformations. In this design, a uniform torsional shear stress is superposed on a uniform hydrostatic compressive state of stress generated by axially deforming samples confined by a stack of thin steel disks. The new design is effective in applying uniform multiaxial compressive strain while preventing buckling and barreling during inelastic deformation. In addition, it allows for the direct measurement of the stress and strain fields during the deformation history. The new shear-compression setup is developed to aid in characterizing the influence of pressure or negative dilatation on the shear constitutive response of viscoelastic materials in general and Polyurea in particular. Experimental results obtained with this technique illustrate the significant increase in the shear stiffness of polyurea under moderate to high hydrostatic pressures.

Rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular microstructures tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect cellular topology and microstructural irregularity (or non-periodicity) on overall mechanical response of cellular solids. Compressive response of various 2D topologies such as honeycombs, stochastic Voronoi foams as well as tetragonal and triangular lattice structures have been investigated as functions of quantitative irregularity parameters.

Current processing techniques enable the manufacture of cellular cores to prescribed cell sizes and densities. Moreover, the rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular topologies tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and topology in cellular structures. The present work reports the initial results of a computational investigation in this direction. The fundamental issues addressed in the present study are (i) generation of stochastic cellular structures by using Voronoi tessellations, (ii) quantitative measure of cellular topology, (iii) uniqueness of mechanical response, (iv) specimen size effect, (v) boundary effect, and (vi) high-strain-rate effects.