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This document is a preface to a special issue of the Journal of Materials Science titled "The Physics of Metal Plasticity." The preface acknowledges the complexity of plastic deformation in solids, particularly in metals, and the challenges in understanding the underlying mechanisms. It pays tribute to Professor Hussein M. Zbib, a respected researcher in the field of solid mechanics, who passed away in 2020. The special issue features 27 contributions that explore various aspects of plastic deformation across different length and time scales, using modeling, simulation, and testing methodologies. The contributions cover topics such as atomistic simulations, dislocation dynamics, continuum modeling, and the mechanical response of polymeric materials. The document also mentions the use of mechanical testing and microscopic characterization in studying the mechanical behavior of materials. [Extracted from the article]

Powder-based laser-directed energy deposition (L-DED) enables rapid prototyping and production of complex geometry Inconel 718 parts for structural applications. However, the heterogeneous microstructure obtained by the L-DED could lead to non-uniform mechanical properties at room temperature. Therefore, evaluating the local heterogeneity in mechanical properties of L-DED manufactured Inconel 718 superalloy is important. The mechanical properties in terms of hardness and local tensile properties of Inconel 718 superalloy have been investigated within the sample volume manufactured by L-DED to determine the local variations in properties. The mechanical properties were compared under the as-deposition and standard solution treatment and aging (STA) conditions. The average hardness and tensile strength properties of L-DED Inconel 718 in as-deposition conditions were significantly lower, while the strain-to-failure is significantly higher than that of its wrought counterparts. STA leads the average mechanical properties of L-DED Inconel 718, which is equivalent to its wrought counterpart. The hardness varied along the deposition direction in as-deposited L-DED Inconel 718. Interestingly, the local variability in the hardness along the deposition direction in L-DED Inconel 718 was eliminated after STA. The tensile strength properties varied along the deposition direction and were found to be lowest near the surface. The local variation trend in tensile strength properties was the same even after STA. The local ductility followed a trend relative to the strength in L-DED Inconel 718. A careful assessment shows that local variation in the mechanical properties was significant along the deposition direction, while it was minimal along the scanning direction of L-DED Inconel 718 under as-deposition and STA conditions. The local heterogeneity in the mechanical properties in L-DED Inconel 718 is explained in terms of the respective microstructure and deformation characteristics. [ABSTRACT FROM AUTHOR]

Simulation models are used to emulate real-world phenomena, and errors are inevitable in the numerical computation process. Owing to that, simulation models need to be verified and validated to ensure the models and their implementations are correct. In this paper, V&V has been done for the micro3d discrete dislocation dynamics (DDD) model by comparing simulation results with corresponding theory, including any analytical solutions, other numerical solutions and experimental data. DDD simulations are a powerful simulation methodology that can help researchers better understand the plastic behavior of crystalline materials. In this study, parametric analyses for DDD simulations parameters have been performed. In addition, simulation results are verified and validated. [ABSTRACT FROM AUTHOR]

Deflection and stress calculated from an experimentally validated, high-fidelity finite element model (FEM) of a photovoltaic module experiencing mechanical load was compared to results from a simplified FEM treating the module laminate as a homogenized composite using a rule of mixtures approach, and further compared to analytical calculations treating the module as a Kirchoff-Love flat plate. The goal of this study was to determine the error incurred by analyzing module mechanics with varying levels of simplification, since resolving the aspect ratios of a module is computationally expensive. Homogenized FEMs were found to underpredict peak deflection under a 1.0 kPa load by between 13 and 19% for lower and upper bound application of the rule of mixtures. However, module shape was captured, implying that a useful replication of a resolved model could be achieved with a reduced, calibrated material stiffness. Homogenized stress results captured glass layer tensile stress components to within 46 to 52% at a sample location of interest, though agreement was poor through the remainder of the laminate due to the lack of material resolution. For plate theory, deflection was overpredicted by 45 to 67% for upper and lower bound homogenizations, and frame-adjacent module shapes were not adequately replicated. Stress results mirrored FEM trends but magnitudes were not well correlated to resolved model values. These results support the use of homogenized laminate models for module shape derivation, though resolved models remain necessary for stress analyses. The accuracy of plate theory was found to be inadequate for most applications. [ABSTRACT FROM AUTHOR]

Discrete Dislocation Dynamics (DDD) simulations are a powerful simulation methodology that can predict a crystalline material's constitutive behavior based on its loading conditions and micro-constituent population/distribution. In this paper, a 3D DDD model with spiral dislocation sources is developed to study size-dependent plasticity in a pure metal material (taken here as Aluminum). It also shows, for the first time, multipole simulations of spirals and how they interact with one another. In addition, this paper also discusses how the free surface of a crystalline material affects the plasticity generation of the spiral dislocation. The surface effect is implemented using the Distributed Dislocation Method. One of the main results from this work, shown here for the first time, is that spiral dislocations can result in traditional Frank–Read sources (edge or screw character) in a crystal. Another important result from this paper is that with more dislocation sources, the plastic flow inside the material is more continuous, which results in a lowering of the flow stress. Lastly, the multipole interaction of the spiral dislocations resulted in a steady-state fan-shaped action for these dislocation sources. [ABSTRACT FROM AUTHOR]

A numerical code has been designed and implemented to find the stress at any material point in a holed wedge material due to any number of crystal or real screw dislocations in the wedge. The code implements the image dislocation method together with the collocation-point method to find stresses at any material point. We demonstrate that the combined analytical–numerical method (analytical with the image dislocations and numerical with the collocation-point method) is valid in that it produces stress fields giving rise to zero traction on free surfaces as well at an arbitrary number of collocation points on void surfaces. In addition, the method makes it possible to find the Peach–Koehler force (PKF) acting on a real screw dislocation at any point in the wedge, as well as to determine points of equilibrium, where the PKF is zero. Numerous and easy to interpret contour plots reveal the utility of this method which was carried out using the mathematical software MATLAB. [ABSTRACT FROM AUTHOR]

Indentation hardness tests are widely used in engineering to test the hardness of a material, which can be performed on a macroscopic scale or microscopic scale. In the present work, copper specimens with different thicknesses were indented with a mechanically-instrumented Rockwell tester. The effects of spacing between indents, indent distance to the edge of the sample, and specimen thickness on the hardness measurement were systematically quantified. Non-linear finite-element modeling was performed, assuming elastic-plastic material response with strain hardening. The two-dimensional numerical modeling of stand-alone and multiple indentations helped to shed more light on the deformation field under indentation, especially the size of the plastic zones and any interaction between them. In addition to the geometric effects, the influence of strain hardening of the test material was also investigated using the finite element analysis. The combined experimental and numerical results provided a range of indent spacings and thicknesses, which yield small to no effect on hardness values. The simulations also quantified the size and shape of the plastic zone under an indent. Lastly, the modeling results showed a competition between the effect of intrinsic material hardening and indent spacing on the value of the measured hardness. [ABSTRACT FROM AUTHOR]

Discrete dislocation dynamics (DDD) codes enable researchers and scientists to explore the mechanical behavior of a material as impacted by its composition and microstructure. Understanding the strengthening mechanisms is very important for the development of new materials with improved and desired mechanical properties. One of the material strengthening/hardening mechanisms is solution hardening, and this method can be fundamentally understood from particle misfit models and the theory/implementation of dislocation dynamics. The overlapping of the eigenstrain fields of the misfit particles or solutes and the dislocations impedes the motion of dislocations and results in material hardening. This article incorporates the misfit particle model in a 3D DDD code in an attempt to capture this phenomenon and find the strength of solid solutions (for a binary Cu–Ni system) from the simulated stress–strain diagram. This study/research finds a good agreement between the simulation results and experimental data. The authors also correlate the strength differentials as a function of solute concentrations and compare them with relations in the literature. [ABSTRACT FROM AUTHOR]

Standard text-book dislocation theory for the elastic fields of a dislocation is provided in an infinite medium. For problems involving free surfaces, or image stresses, the solution for such elastic fields is more complex. The current article extends the state of knowledge for problems with free surfaces to address the stress field of a screw dislocation in a plate (two planar free surfaces) containing a hole/void. Such surfaces interact and thus affect the stress field of a dislocation in deviation of the infinite medium solution. The numerical solutions provided herein, based on reflective image stresses and the collocation-point method, have been verified to the extent available and such solutions allow for further applied problems. One main finding of the paper is that the equilibrium location of a screw dislocation is always closer to the hole/void surface than the planar free surface. [ABSTRACT FROM AUTHOR]

The influence of carbon nanotube dispersion techniques (surfactant-assisted dispersion, ultrasonication and magnetic stirring) and surface functionalization of carbon nanotubes on mechanical properties of nanocomposites fabricated via powder metallurgy techniques is characterized. Functionalized multiwalled carbon nanotubes dispersed using a combination of techniques results in Al/functionalized multiwalled carbon nanotube composites with excellent microstructure and enhanced mechanical properties. Nonfunctionalized carbon nanotubes dispersed using zwitterionic surfactants show good dispersion patterns on Al powder surfaces and results in composites with good strength and relative densities are also obtained. Carbon nanotubes dispersed solely via shear mixing techniques resulted in highly embrittled composites with poor microstructure. Also discussed in this study is the usefulness of scanning electron microscopy in conjunction with image analysis techniques in characterizing porosity in Al/carbon nanotube composites. [ABSTRACT FROM AUTHOR]

Introduction: Morphologic features of computed tomography (CT) scans of the brain can be used to estimate intracranial pressure (ICP) via an image-processing algorithm. Clinically, such estimations can be used to prognosticate outcomes and avoid placement of invasive intracranial monitors in certain patients with severe traumatic brain injury. Features on a CT scan that may correlate with measurements of low ICP are sought. Methods: A measure is proposed that is a function of the distribution of cerebrospinal fluid (CSF) in and around the brain. In our method, we present an algorithm that semiautomatically segments brain parenchyma from CSF, and apply standard image processing calculations. The ratio of CSF volume to the size of the intracranial vault (ICV) or volume inside the skull, csf(v) /icv(v) is calculated and then plotted against the actual recorded ICP, yielding a relationship between the image features and ICP. Results: We analyzed a total of 45 scans from 20 patients with severe traumatic brain injury (TBI). We showed that a ratio csf(v)/icv(v) > .034 correlates with an ICP < 20 mmHg (P = .0046). For csf(v)/icv(v) ≤ .034, a distinction between low and high ICP cannot be effectively estimated by this univariate measure. Conclusion: This method permits a noninvasive means of identifying patients who are low risk for having elevated ICP; by following Brain Trauma Foundation guidelines strictly such a patient may be subjected to an unnecessary, invasive procedure. This work is a promising pilot study that will need to be analyzed for a larger population. [ABSTRACT FROM AUTHOR]

Measurement and modeling of gas flows in microelectromechanical systems (MEMS) scale channels are relevant to the fundamentals of rarefied gas dynamics (RGD) and the practical design of MEMS-based flow systems and micropumps. We describe techniques for building robust, leak-free, rectangular microchannels which are relevant to micro- and nanofluidic devices, while the channels themselves are useful for fundamental RGD studies. For the first time, we report the isothermal steady flow of helium (He) gas through these channels from the continuum to the free-molecular regime in the unprecedented Knudsen range of 0.03–1000. On the high end, our value is 20-fold larger than values previously reported by Ewart et�al (2007 J. Fluid Mech.584 337–56). We accomplished this through a dual-tank accumulation technique which enabled the monitoring of very low flow rates, below 10−14�kg s−1. The devices were prebaked under vacuum for 24�h at 100��C in order to reduce outgassing and attain high Kn. We devised fabrication methods for controlled-depth micro-gap channels using silicon for both channel ceiling and floor, thereby allowing direct comparisons to models which utilize this simplifying assumption. We evaluated the results against a closed-form expression that accurately reproduces the continuum, slip, transition, and free-molecular regimes developed partly by using the direct simulation Monte Carlo method. The observed data were in good agreement with the expression. For Kn > ∼100, we observed minor deviations between modeled and experimental flow values. Our fabrication processes and experimental data are useful to fundamental RGD studies and future MEMS microflow devices with respect to extremely low-flow measurements, model validation, and predicting optimal designs. [ABSTRACT FROM AUTHOR]

We report a study of the electrical and mechanical effects of the inclusion of a thin layer of multiwalled carbon nanotubes (MWCNTs) into the surface of polydimethylsiloxane (PDMS) as a method of creating an electrically actuated, flexible microfluidic valve. Samples of PDMS with various surface loadings of MWCNTs were prepared and tested using a uniaxial tension tester, combined with a four-point probe electrical test. In contrast to other works reporting the inclusion of MWCNTs in the bulk of the material, we have found that inclusion of the MWCNTs on the surface only has no discernible effect on the mechanical properties of the PDMS samples, but causes a significant and repeatable change in the electrical performance. We have found that a loading of 4.16 g m−2 results in an electrical resistivity of 7.31 × 10−4 Ω cm, which is 200% lower than that previously reported for bulk inclusion samples. The microstructure of the MWCNTs was found to consist of both individual fibers and spherical clumps of fibers. We suggest that, due to the microstructure of the MWCNTs used in this study, the mechanical properties can be modeled as a thin layer of particulates, while the electrical properties can be modeled as a thin bed of bulk MWCNTs. © 2012 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Analogous to mechanical systems, modeling the electromagnetic (EM) performance of large and complex metamaterial structures requires the use of effective-medium/constitutive properties. Constitutive properties are critical for efficiently bridging the gap between subwavelength geometry and performance at the mesoscale. In this article, the temperature-dependent effective medium properties for a metamaterial electric-inductive-capacitive (ELC) resonator are described analytically. ELC structures are commonly used in metamaterial designs to provide a tailored electric response to EM waves. An equivalent circuit model, coupled with analytic expressions for the capacitances, inductance, and resistance of the ELC resonator, is utilized to describe how thermally induced mechanical strain and changes in material properties manifest as temperature-dependent permittivity and permeability curves for the metamaterial. The resulting analytic expressions account for the effects of spatial dispersion and losses. This article also details how the process may be expanded to similarly describe the temperature-dependent constitutive properties of metamaterial magnetic resonators. [ABSTRACT FROM PUBLISHER]

Photonic crystals (PCs) are synthetic materials that are used to control light propagation. PCs have a frequency bandgap where light is forbidden to propagate. This bandgap is strongly tied to the microstructure of the photonic crystal. Three-dimensional tungsten photonic crystal in a Lincoln-log microstructure has been suggested as a strong alternative filter in photovoltaic cells with significantly high power efficiency. PCs have also been suggested as sensors for submicron damage. Therefore, mechanical characterization of three-dimensional photonic crystals becomes of interest. Here we report on mechanical characterization of tungsten PC using means of micro-indentation. We also present a three-dimensional finite element simulation of the structural response of a Tungsten photonic crystal under micro-indentation load. Stresses developed in the PC can be used to quantify the level of damage in the crystal. We compare our simulation results with the experimental observations of a Vickers and Knoop micro-indentation experiments of tungsten PC. The FE models were proven able to simulate the mechanical response of the PC with a good accuracy. The calibrated FE models can be further used to realize the mechanical behavior of PC under different thermal and mechanical stresses when used as filters in photovoltaic cells or to simulate the effect of damage in PC sensors. [ABSTRACT FROM AUTHOR]

Finite element (FE) modeling of the human dentate mandible is the method of choice currently used for simulating structural fracture analyses in the mandibular region. A finite element model of a parasymphyseal fracture with an internal rigid fixation plate-screw system has been developed to compare the effects of including frictionless/frictional contact boundary conditions at the fracture site. It is common practice to ignore contact boundary conditions in FE modeling of mandibular fractures due to the non-linearities causing increased computational requirements. The stress distributions and displacements of the mandibular fracture region indicate a significant difference resulting from the introduction of realistic contact boundary conditions. These current findings suggest that even though the modeling of extreme situations, i.e. non-contact modeling of unhealed fractures, may provide insight to non-union problems, future mandibular fracture models should include frictional contact boundary conditions. This is in order to capture more realistic behavior of the system to be analyzed. [ABSTRACT FROM AUTHOR]

Finite element (FE) modeling of the human dentate mandible is the method of choice currently used for simulating structural fracture analyses in the mandibular region. A finite element model of a parasymphyseal fracture with an internal rigid fixation plate-screw system has been developed to compare the effects of including frictionless/frictional contact boundary conditions at the fracture site. It is common practice to ignore contact boundary conditions in FE modeling of mandibular fractures due to the non-linearities causing increased computational requirements. The stress distributions and displacements of the mandibular fracture region indicate a significant difference resulting from the introduction of realistic contact boundary conditions. These current findings suggest that even though the modeling of extreme situations, i.e. non-contact modeling of unhealed fractures, may provide insight to non-union problems, future mandibular fracture models should include frictional contact boundary conditions. This is in order to capture more realistic behavior of the system to be analyzed. [ABSTRACT FROM AUTHOR]

Experimental and numerical methods are used to explore the stresses generated around bone screws used in rigid internal fixation of mandibular fractures. These results are intended to aid in decisions concerning both the design and the use of these bone screws. A finite element (FE) model of a human mandible is created with a fixated fracture in the parasymphyseal region. The mandibular model is anatomically loaded, and the forces exerted by the fixation plate onto the simplified screws are obtained and transferred to another finite element submodel of a screw implant embedded in a trilaminate block with material properties of cortical and cancellous bone. The stress in the bone surrounding the screw implant is obtained and compared for different screw configurations. The submodel analyses are further compared to and validated with simple axial experimental and numerical screw pull-out models. Results of the screw FE analysis (FEA) submodel show that a unicortical screw of 2.6mm major diameter and 1.0mm pitch will cause less bone damage than a bicortical screw of 2.3mm major diameter and 1.0mm pitch. The results of this study suggest that bicortical drilling can be avoided by using screws of a larger major diameter. [ABSTRACT FROM AUTHOR]

A model was developed to calculate the radius of curvature produced by stresses in lattice-mismatched multilayers grown epitaxially on a planar substrate. The analysis was done for any number ( N) of layers using beam bending theory and strain partitioning theory introduced by our group earlier. This model is reduced to a limiting case of a two-layer structure (film-substrate). The variation of the radius of curvature with the relative thicknesses and other material properties of the layers was determined and compared to finite element calculations. The analytical model was further verified by applying it to a symmetric tri-laminate structure. [ABSTRACT FROM AUTHOR]

In this study, using the equations for static equilibrium and Hooke's law for isotropic materials under a plane stress condition, the elastic fields associated with each layer have been calculated. A model has been developed to calculate the elastic fields in multilayers grown epitaxially on a planar substrate. The strain partitioning in this model reduces to the limiting case of a two-layer structure that is available in the literature. We have chosen a particular system (SiGe) and calculated strain energy density for any number (N) of layers of graded Ge (SixGe1-x) on silicon. The model was compared with experiment for graded SiGe grown over a planar silicon substrate. The results revealed qualitative agreement between the calculated strain energy density and the experimentally observed dislocation density in the layers. The effect of the choice of a strain partitioning model on the calculation of a film/epilayer critical thickness is demonstrated. [ABSTRACT FROM AUTHOR]

This work focuses on an anisotropic elasticity treatment of strain partitioning in planar thin layers/films caused by heteroepitaxial crystal growth. In particular, orthotropic materials are considered here. Previous work mostly focused on isotropic elasticity. Here, under the assumption of plane stress and using Hooke’s law for linear elasticity, equilibrium equations, and typical strain definitions, one obtains a solution for the strains, in the two planar directions, in the substrate and the epilayer. The strains in a particular layer are expressed as a function of the elastic constants, thicknesses, and lattice parameters of both layers. These results show that the anisotropic solution differs considerably from the isotropic treatment. The current results can also be used to predict the onset of misfit dislocation formation. [ABSTRACT FROM AUTHOR]

Analytical closed-form solutions for the Mode II crack tip plastic zone shape have been derived for a semi-infinite crack in an isotropic elastic-plastic solid under both plane stress and plane strain conditions. Two yield criteria have been applied: the Von Mises and Tresca yield criteria. The results indicate that the Tresca zone is larger in size than the Von Mises zone. The results also reveal an intricate dependence on Poisson's ratio in plane strain conditions. [ABSTRACT FROM AUTHOR]

Presents a study which uses three-dimensional multiscale simulations of irradiated metals to reveal the mechanisms underlying plastic flow localization in defect-free channels. Methods; Findings of dislocations and dislocation pinning; Conclusion.

Objective: 1) Develop a hyperelastic computational model of skin viscoelastic properties for analysis of wound closures, and 2) Apply the model to variations of the rhombic skin flap to quantify closure force vectors and optimize wound closure tension. Method: A computational model employing the finite element method was created to simulate skin defect closure employing rhombic transposition flaps. Variables of transposition angle, flap width, and tissue undermining were analyzed. Outcome measures of tissue stress, strain, and wound distortion were evaluated and optimized for a standard defect. Results: A second order Yeoh hyperelastic model was fit to previously published experimental skin data with good approximation of observed properties. In the analysis of transposition flap closures of the 60 to 120 degree rhomboid defect, the model suggests that a biomechanically ideal flap design is constructed with distal flap angle of 30 degrees, as is employed in the Webster flap, with the donor site near margin oriented parallel to the short axis of the defect, as in the traditional Limberg flap. This configuration minimizes tissue stress and strain and most evenly distributes wound tension across the closure line. Conclusion: The model quantitatively demonstrates several recognized principles of the rhombic flap. Square defects, as compared with rhomboid defects, close with lower tissue strain, but form a larger standing cutaneous deformity. The Webster flap best distributes wound tension across the incision line, and the Dufourmental modification alters the closure force vector. [ABSTRACT FROM AUTHOR]

This work reports on a recent advancement in three-dimensional dislocation dynamics modelling. A method is presented for the treatment of dislocation image stresses resulting from the presence of nearby traction-free surfaces. The image stress-field of a dislocation segment below a finite-sized free surface is obtained by the distribution of N generally prismatic rectangular or square dislocation loops padding the area (the external bounding surfaces of the simulation box). The unphysical tractions created at surface collocation points by sub-surface crystal dislocations are annulled by proper determination of the loops' Burgers vectors. The image stresses on a dislocation segment are simply those stresses resulting from the surface loops. The accuracy can be improved by increasing the number of collocation points (i.e. surface loop density). [ABSTRACT FROM AUTHOR]