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IN THIS ISSUE

### Editorial

J. Eng. Mater. Technol. 2017;139(2):020201-020201-2. doi:10.1115/1.4035706.
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In March 2016, to honor the seminal contributions of George in microstructure plasticity and instability and as a part of the celebration of his 70th birthday, Hanyang University in Seoul, South Korea, hosted an International Symposium on “Multi-Physical Solutions for Harsh Environments: Computations and Experiments.” It was chaired by Taehyo Park and Xi Chen with this dedicated special issue in the ASME Journal of Engineering Materials and Technology (JEMT). The event spanned 2 days and was attended by researchers from the U.S., Europe, and Asia, including University Professors and Researchers from National Labs and Industries. Many of the technical papers associated with the symposium are included in this Special Issue.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2017;139(2):021001-021001-8. doi:10.1115/1.4035253.
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Drag force is usually exerted on bridge piers due to running river water. This force is calculated empirically based on drag coefficients stated in design codes and specifications. Different values of drag coefficients have been reported in literature. For example, AASHTO LRFD Bridge Design Specifications uses a drag coefficient of 1.4 and 0.7 for square-ended and semicircular-nosed pier, respectively, while Coastal Construction Manual (FEMA P-55) recommends a value of two and 1.2 for square and round piles, respectively. In addition, many researchers have obtained other different values of drag coefficient under similar conditions (i.e., similar range of Reynolds number) reaching to 2.6 for square object. The present study investigates the drag coefficient of flow around square, semicircular-nosed, and 90 deg wedged-nosed and circular piers numerically using finite element method. Results showed that AASHTO values for drag force coefficient varied between very conservative to be under-reckoning. The study recommends that AASHTO drag coefficient values should be revised for different circumstances and under more severe conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021002-021002-11. doi:10.1115/1.4035268.
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Engineering polymers generally exhibit asymmetric yield strength in tension and compression due to different arrangements of molecular structures in response to external loadings. For the polymeric materials whose plastic behavior follows the Drucker–Prager yield criterion, the present study proposes a new method to predict both tensile and compressive yield strength utilizing instrumented spherical indentation. Our method is decomposed into two parts based on the depth of indentation, shallow indentation, and deep indentation. The shallow indentation is targeted to study elastic deformation of materials, and is used to estimate Young's modulus and yield strength in compression; the deep indentation is used to achieve full plastic deformation of materials and extract the parameters in Drucker–Prager yield criterion associated with both tensile and compressive yield strength. Extensive numerical computations via finite element method (FEM) are performed to build a dimensionless function that can be employed to describe the quantitative relationship between indentation force-depth curves and material parameters of relevance to yield criterion. A reverse algorithm is developed to determine the material properties and its robustness is verified by performing both numerical and experimental analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021003-021003-7. doi:10.1115/1.4035273.
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Car bomb attack exhibits considerable different effects on structures when compared with the bare explosive blast. In this paper, a postdisaster investigation is presented for an existing bridge under accidental car bomb blast loading. Based on the analysis of the explosive properties, the crack distribution and deformation of the blast loaded girders are studied. Numerical analysis is conducted to verify the findings by simulating the truck isolation effect with steel plate. Both field data and numerical results indicate that the isolation effect of the vehicle can significantly affect the blast loading distribution on structures. Specifically, the shock wave propagation is isolated directly under the explosive source with the delayed arriving time. Peak values of overpressure within the steel plate isolating region are diminished while the pressures are magnified outside the isolating region due to reflection and wave merging. The results can be applicable to determine the essential blast-resistant design criteria to reduce the probability of blast induced failures.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021004-021004-9. doi:10.1115/1.4035280.
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Magnesium (Mg) alloys have been widely used in automotive and aerospace industries due to its merits of exceptional lightweight, super strong specific strength, and high corrosion-resistance, where intermetallic compounds with a small volume are very critical to achieve these excellent performance. This study proposes a reverse analysis that can be employed to extract elastoplasticity-dependent creep property of commercial die-cast Mg alloys and their intermetallic compounds from instrumented indentation with two sharp indenters. First, the creep deformation that obeys the Norton's law ($ε˙$  = A$σn$) is studied, and the parameters of A and n are determined from two indentation experiments conducted with different sharp indenters. Then, a numerical algorithm and dimensional function developed is extended to extract the elastoplasticity of various metallic materials by focusing on the loading stage of indentation experiments. By considering the full loading history with both linear increase and holding stages of loads, we propose a framework of reverse analysis to determine both elastoplasticity and creep properties simultaneously. Parallel indentation experiments on pure magnesium and aluminum and Mg alloys are performed, and the results agree well with the numerical predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021005-021005-15. doi:10.1115/1.4035292.
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The degradation of elastic stiffness is investigated systematically within the framework of continuum damage mechanics. Consistent equations are obtained showing how the degradation of elastic stiffness can be decomposed into a part due to cracks and another part due to voids. For this purpose, the hypothesis of elastic energy equivalence of order n is utilized. In addition, it is shown that the hypothesis of elastic strain equivalence is obtained as a special case of the hypothesis of elastic energy equivalence of order n. In the first part of this work, the formulation is scalar and applies to the one-dimensional case. The tensorial formulation for the decomposition is also presented that is applicable to general states of deformation and damage. In this general case, one cannot obtain a single explicit tensorial decomposition equation for elastic stiffness degradation. Instead, one obtains an implicit system of three tensorial decomposition equations (called the tensorial decomposition system). Finally, solution of the tensorial decomposition system is illustrated in detail for the special case of plane stress.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021006-021006-12. doi:10.1115/1.4035293.
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The thermodynamically consistent framework accounting for the thermomechanical behavior of the microstructure is addressed using the finite-element implementation. In particular, two different classes of the strain gradient plasticity (SGP) theories are proposed: In the first theory, the dissipation potential is dependent on the gradient of the plastic strain, as a result, the nonrecoverable microstresses do not have a value of zero. In the second theory, the dissipation potential is independent of the gradient of the plastic strain, in which the nonrecoverable microstresses do not exist. Recently, Fleck et al. pointed out that the nonrecoverable microstresses always generate the stress jump phenomenon under the nonproportional loading condition. In this work, a one-dimensional finite-element solution for the proposed strain gradient plasticity model is developed for investigating the stress jump phenomenon. The proposed strain gradient plasticity model and the corresponding finite-element code are validated by comparing with the experimental data from the two sets of microscale thin film experiments. In both experimental validations, it is shown that the calculated numerical results of the proposed model are in good agreement with the experimental measurements. The stretch-passivation problems are then numerically solved for investigating the stress jump phenomenon under the nonproportional loading condition.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021007-021007-10. doi:10.1115/1.4035325.
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Hole cold expansion and bolt clamping force are usually applied to improve the fatigue performance of bolted joints. In order to investigate the effects of hole cold expansion and bolt clamping force and reveal the mechanism of these two factors on the fatigue damage of bolted joint, a continuum damage mechanics (CDM) based approach in conjunction with the finite element method is used. The damage-coupled Voyiadjis plasticity constitutive model is used to represent the material behavior, which is implemented by user material subroutine in abaqus. The elasticity and plasticity damage evolutions of the material are described by the stress-based and plastic-strain-based equations, respectively. The fatigue damage of joint is calculated using abaqus cycle by cycle. The fatigue lives of double-lap bolted joints with and without clamping force at different levels of hole cold expansion are all obtained. The characteristics of fatigue damage corresponding to the different conditions are presented to unfold the influencing mechanism of these two factors. The predicted fatigue lives and crack initiation locations are in good agreement with the experimental results available in the literature. The beneficial effects of hole cold expansion and bolt clamping force on the fatigue behavior of bolted joint are presented in this work.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021008-021008-9. doi:10.1115/1.4035617.
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This paper presents the energy absorption of target materials with combinations of polyurethane (PU) foam, PU sheet, SiC inserts, and SiC plate bonded to glass fiber reinforced composite laminate backing during impact loading. SiC inserts and SiC plates are bonded as front layer to enhance energy absorption and to protect composite laminate. The composite laminates are prepared by hand lay-up process and other layers are bonded by using epoxy. Low-velocity impact is conducted by using drop mass setup, and mild steel spherical nosed impactor is used for impact testing of target in fixed boundary conditions. Energy absorption and damage are compared to the target plates when subjected to impact at different energy levels. The energy absorbed in various failure modes is analyzed for various layers of target. Failure in the case of SiC inserts is local, and the insert under the impact point is damaged. However, in the other cases, the SiC plate is damaged along with fiber failure and delamination on the composite backing laminate. It is observed that the energy absorbed by SiC plate layered target is higher than SiC inserts layered target.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021009-021009-6. doi:10.1115/1.4035326.
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We investigate a domain decomposition method (DDM) of finite element method (FEM) using Intel's many integrated core (MIC) architecture in order to determine the most effective MIC usage. For this, recently introduced high-scalable parallel method of DDM is first introduced with a detailed procedure. Then, the Intel's Xeon Phi MIC architecture is presented to understand how to apply the parallel algorithm into a multicore architecture. The parallel simulation using the Xeon Phi MIC has an advantage that traditional parallel libraries such as the message passing interface (MPI) and the open multiprocessing (OpenMP) can be used without any additional libraries. We demonstrate the DDM using popular libraries for solving linear algebra such as the linear algebra package (LAPACK) or the basic linear algebra subprograms (BLAS). Moreover, both MPI and OpenMP are used for parallel resolutions of the DDM. Finally, numerical parallel efficiencies are validated by a two-dimensional numerical example.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021010-021010-7. doi:10.1115/1.4035486.
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Nowadays, the numerical method has become a very important approach for solving complex problems in engineering and science. Some grid-based methods such as the finite difference method (FDM) and finite element method (FEM) have already been widely applied to various areas; however, they still suffer from inherent difficulties which limit their applications to many problems. Therefore, a strong interest is focused on the meshfree methods such as smoothed particle hydrodynamics (SPH) to simulate fluid flow recently due to the advantages in dealing with some complicated problems. In the SPH method, a great number of particles will be used because the whole domain is represented by a set of arbitrarily distributed particles. To improve the numerical efficiency, parallelization using message-passing interface (MPI) is applied to the problems with the large computational domain. In parallel computing, the whole domain is decomposed by the parallel method for continuity of subdomain boundary under the single instruction multiple data (SIMD) and also based on the procedure of the SPH computations. In this work, a new scheme of parallel computing is employed into the SPH method to analyze SPH particle fluid. In this scheme, the whole domain is decomposed into subdomains under the SIMD process and it composes the boundary conditions to the interface particles which will improve the detection of neighbor particles near the boundary. With the method of parallel computing, the SPH method is to be more flexible and perform better.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021011-021011-12. doi:10.1115/1.4035487.
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The steel plate reinforced concrete (SC) walls and roofs are effective protective structures in nuclear power plants against aircraft attacks. The mechanical behavior of the concrete in SC panels is very complicated when SC panels are under the action of impacting loading. This paper presents a dynamic material model for concrete subjected to high-velocity impact, in which pressure hardening, strain rate effect, plastic damage, and tensile failure are taken into account. The loading surface of the concrete undergoing plastic deformation is defined based on the extended Drucker–Prager strength criterion and the Johnson–Cook material model. The associated plastic flow rule is utilized to evaluate plastic strains. Two damage parameters are introduced to characterize, respectively, the plastic damage and tensile failure of concrete. The proposed concrete model is implemented into the transient nonlinear dynamic analysis code ls-dyna. The reliability and accuracy of the present concrete material model are verified by the numerical simulations of standard compression and tension tests with different confining pressures and strain rates. The numerical simulation of the impact test of a 1/7.5-scale model of an aircraft penetrating into a half steel plate reinforced concrete (HSC) panel is carried out by using ls-dyna with the present concrete model. The resulting damage pattern of concrete slab and the predicted deformation of steel plate in the HSC panel are in good agreement with the experimental results. The numerical results illustrate that the proposed concrete model is capable of properly charactering the tensile damage and failure of concrete.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021012-021012-8. doi:10.1115/1.4035488.
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This research aims to describe the behavior of C45 steel and provide better understanding of the thermomechanical ductile failure that occurs due to accumulation of microcracks and voids along with plastic deformation to enable proper structural design, and hence provide better serviceability. A series of quasi-static tensile tests are conducted on C45 steel at a range of temperatures between 298 K and 923 K for strain rates up to 0.15 s−1. Drop hammer dynamic tests are also performed considering different masses and heights to study the material response at higher strain rates. Scanning electron microscopy (SEM) images are taken to quantify the density of microcracks and voids of each fractured specimens, which are needed to define the evolution of internal defects using an energy-based damage model. The coupling effect of damage and plasticity is incorporated to accurately define the constitutive relation that can simulate the different structural responses of this material. Good correlation was observed between the proposed model predictions and experiments particularly at regions where dynamic strain aging (DSA) is not present.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021013-021013-11. doi:10.1115/1.4035616.
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An investigation of the mechanical strain rate, inelastic behavior, and microstructural evolution under deformation for an as-cast pearlitic gray cast iron (GCI) is presented. A complex network of graphite, pearlite, steadite, and particle inclusions was stereologically quantified using standard techniques to identify the potential constituents that define the structure–property relationships, with the primary focus being strain rate sensitivity (SRS) of the stress–strain behavior. Volume fractions for pearlite, graphite, steadite, and particles were determined as 74%, 16%, 9%, and 1%, respectively. Secondary dendrite arm spacing (SDAS) was quantified as 22.50 μm ± 6.07 μm. Graphite flake lengths and widths were averaged as 199 μm ± 175 μm and 4.9 μm ± 2.3 μm, respectively. Particle inclusions comprised of manganese and sulfur with an average size of 13.5 μm ± 9.9 μm. The experimental data showed that as the strain rate increased from 10−3 to 103 s−1, the averaged strength increased 15–20%. As the stress state changed from torsion to tension to compression at a strain of 0.003 mm/mm, the stress asymmetry increased ∼470% and ∼670% for strain rates of 10−3 and 103 s−1, respectively. As the strain increased, the stress asymmetry differences increased further. Coalescence of cracks emanating from the graphite flake tips exacerbated the stress asymmetry differences. An internal state variable (ISV) plasticity-damage model that separately accounts for damage nucleation, growth, and coalescence was calibrated and used to give insight into the damage and work hardening relationship.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021014-021014-5. doi:10.1115/1.4035765.
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A new class of truss structure based on superelastic shape memory alloy (SMA) wire has been developed by weaving superelastic SMA wire through two perforated facesheets. A gap was maintained between the facesheets while weaving and the ends of wire forming the truss legs are anchored in each facesheet. The resulting structure has a modified pyramidal configuration and is capable of undergoing large recoverable deformations typical of superelastic SMA. A four-unit cell truss specimen has been tested under static load cycles to investigate the compressive response. The truss specimen underwent a hysteretic loop and demonstrated minimal permanent deformation closely resembling the behavior of bulk SMA. A finite element model of the truss was generated and the analysis results were compared with the experimental response. The present work is an attempt to demonstrate an SMA-based truss structure having energy absorption capabilities with minimum permanent deformation. These truss structures may be applied for damage mitigation in composites subjected to impact and blast loads.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021015-021015-9. doi:10.1115/1.4035766.
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Recently, there was a collision accident involving vehicle–concrete median barrier in South Korea, and unfortunately, passengers on the opposite direction road were killed by the flying broken pieces of concrete generated by the collision. Primarily after this accident, we felt the need for developing an improved concrete median barrier up to level of SB6 impact severity in order to minimize the amount of broken pieces of concrete and any possibility of traffic accident casualty under the impact loading of truck. Accordingly, in this study, several designs of concrete median barriers have been examined, and a preliminary study has been conducted for developing and verifying appropriate collision model. First, type of vehicle was selected based on impact analysis on rigid wall. Then, the effects of element size and other key parameters on the capacity of the concrete median barrier under impact were studied. It was found that the key parameters for controlling behaviors of the median barrier under impact loading were contact option, threshold value, and mesh and boundary conditions. Furthermore, as a parametric study, effect of geometry and amount of wire-mesh or steel rebar in concrete median barrier on impact resistances of median barrier for reducing the collision debris were investigated. The amount of volume loss after the collision of truck was compared for various reinforcement ratios.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021016-021016-8. doi:10.1115/1.4035618.
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This paper presents a two-way linked computational multiscale model and its application to predict the mechanical behavior of bone subjected to viscoelastic deformation and fracture damage. The model is based on continuum thermos-mechanics and is implemented through the finite element method (FEM). Two physical length scales (the global scale of bone and local scale of compact bone) were two-way coupled in the framework by linking a homogenized global object to heterogeneous local-scale representative volume elements (RVEs). Multiscaling accounts for microstructure heterogeneity, viscoelastic deformation, and rate-dependent fracture damage at the local scale in order to predict the overall behavior of bone by using a viscoelastic cohesive zone model incorporated with a rate-dependent damage evolution law. In particular, age-related changes in material properties and geometries in bone were considered to investigate the effect of aging, loading rate, and damage evolution characteristics on the mechanical behavior of bone. The model successfully demonstrated its capability to predict the viscoelastic response and fracture damage due to different levels of aging, loading conditions (such as rates), and microscale damage evolution characteristics with only material properties of each constituent in the RVEs.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021017-021017-10. doi:10.1115/1.4035489.
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Motivated by the already developed micromechanical approach (Abdul-Latif et al., 2002, “Elasto-Inelastic Self-Consistent Model for Polycrystals,” ASME J. Appl. Mech., 69(3), pp. 309–316.), a new extension is proposed for describing the mechanical strength of ultrafine-grained (ufg) materials whose grain sizes, d, lie in the approximate range of 100 nm < d < 1000 nm as well as for the nanocrystalline (nc) materials characterized by $d≤100 nm$. In fact, the dislocation kinematics approach is considered for characterizing these materials where grain boundary is taken into account by a thermal diffusion concept. The used model deals with a soft nonincremental inclusion/matrix interaction law. The overall kinematic hardening effect is described naturally by the interaction law. Within the framework of small deformations hypothesis, the elastic part, assumed to be uniform and isotropic, is evaluated at the granular level. The heterogeneous inelastic part of deformation is locally determined. In addition, the intragranular isotropic hardening is modeled based on the interaction between the activated slip systems within the same grain. Affected by the grain size, the mechanical behavior of the ufg as well as the nc materials is fairly well described. This development is validated through several uniaxial stress–strain experimental results of copper and nickel.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021018-021018-8. doi:10.1115/1.4035490.
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Dielectric elastomers (DEs) have been attracting great attention in the field of electro-mechanical actuation and sensing. In this paper, we develop a new type of silicone-based DEs by incorporating multiwalled carbon nanotubes (MWNTs) to the DEs as fillers. The dispersion of MWNTs during the material processing plays a significant role in deciding the final properties of the nanocomposites. In this work, acetone and ultrasonication along with characterization tools such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to examine the MWNT dispersion quality within DE nanocomposites. Furthermore, microstructural MWNT dispersion and filler–matrix interfacial bonding as well as the overall dynamic mechanical responses are investigated to reveal the correlation between them. It is concluded that the processing of DE nanocomposites strongly affects the dynamic mechanical properties, which can inversely provide with microstructural information for the nanocomposites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021019-021019-7. doi:10.1115/1.4035705.
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Application of steel fiber reinforced cementitious composites (SFRCC) in the construction of protective structures against extreme loading conditions, such as high-velocity impact and blasts, is an active area of research. It is a challenging task to capture the material behavior under such harsh conditions where strain rate of loading exceeds beyond 104 s−1. In this paper, an effort is made to simulate numerically the multihits of short projectiles on SFRCC panels. A total of 90 numbers of SFRCC panels consist of various core layer materials, thicknesses, fiber volumes, and angle of obliquity, are tested under high-velocity impacts of short projectiles. In numerical simulations, the boundary conditions and impact loading sequence are maintained, similar to that used during impact tests. In order to carry out a realistic numerical simulation, in-service munitions and ammunitions are used. The numerical response is found to corroborate with experimental results. It is observed that, if two consecutive hits are made within a distance of ten times the diameter of the projectile, then it is considered a case of multihit, else, it is considered as single hit case. The damage contours based on effective plastic strain are found to correlate with impact-tested SFRCC panels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021020-021020-11. doi:10.1115/1.4035767.
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The dynamic response of both thick-walled and thin-walled cylindrical composite structures subjected to underwater impulsive loads is analyzed. In the case of thick-walled structures, a novel experimental setup, the underwater shock loading simulator (USLS), is used to generate the impulsive loads. Deflection and core compression are characterized using high-speed digital imaging. The experiments are supported by fully dynamic numerical calculations which account for fluid–structure interactions (FSIs) and damage and failure mechanisms in the materials. The analysis focuses on the effect of varying structural attributes and material properties on load-carrying capacity, deformation mechanisms, and damage. Results show that cylindrical sandwich structures have superior blast-resistance than cylindrical monolithic structures of equal mass with only relatively minor increases in wall thickness. In the case of thin-walled structures, a unique computational framework based on a coupled Eulerian–Lagrangian (CEL) approach is developed to study the structural collapse and damage evolution under large impulsive loads which induces an implosion event. Simulations are carried out for a range of hydrostatic pressure and impulsive load intensity, with different loading configurations. Ply level stress analysis provides an insight on the stress–structural deformation–damage evolution relationship during the severe explosion-induced implosion event. The experiments, computations, and structure–performance relations developed in the current study offer approaches for improving the blast-mitigation capabilities of cylindrical composite sections in critical parts of marine structures, such as the keel, hull, and pipes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021021-021021-4. doi:10.1115/1.4035497.
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With the rapid emerging of two-dimensional (2D) micro/nanomaterials and their applications in flexible electronics and microfabrication, adhesion between thin film and varying substrates is of great significance for fabrication and performance of micro devices and for the understanding of the buckle delamination mechanics. However, the adhesion energy remains to be difficult to be measured, especially for compliant substrates. We propose a simple methodology to deduce the adhesion energy between a thin film and soft substrate based on the successive or simultaneous emergence of wrinkles and delamination. The new metrology does not explicitly require the knowledge of the Young's modulus, Poisson's ratio, and thickness of the 2D material, the accurate measurement of which could be a challenge in many cases. Therefore, the uncertainty of the results of the current method is notably reduced. Besides, for cases where the delamination width is close to the critical wrinkle wavelength of the thin film/substrate system, the procedure can be further simplified. The simple and experimentally easy methodology developed here is promising for determining/estimating the interface adhesion energy of a variety of thin film/soft substrate systems.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021022-021022-4. doi:10.1115/1.4035498.
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As a clean energy technology, the development of electric vehicles (EVs) is challenged by lightweight design, battery safety, and range. In this study, our simulations indicate that using a flexible structure of battery module has the potential to overcome the limitations in battery-powered EVs, contributing to a new design. Specifically, we focus on optimizing the structure of vehicle battery packs, aiming to improve the crashworthiness of EVs through frontal crash simulations. In addition, by considering battery packs as energy-absorption components, it is found that occupant compartment acceleration (OCA) is greatly reduced at an optimal working pressure of 4 MPa for battery module.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021023-021023-8. doi:10.1115/1.4035494.
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Evolving dislocation-density pile-ups at grain-boundaries (GBs) spanning a wide range of coincident site lattice (CSL) and random GB misorientations in face-centered cubic (fcc) bicrystals and polycrystalline aggregates has been investigated. A dislocation-density GB interaction scheme coupled to a dislocation-density-based crystalline plasticity formulation was used in a nonlinear finite element (FE) framework to understand how different GB orientations and GB-dislocation-density interactions affect local and overall behavior. An effective Burger's vector of residual dislocations was obtained for fcc bicrystals and compared with molecular dynamics (MDs) predictions of static GB energy, as well as dislocation-density transmission at GB interfaces. Dislocation-density pile-ups and accumulations of residual dislocations at GBs and triple junctions (TJs) were analyzed for a polycrystalline copper aggregate with Σ1, Σ3, Σ7, Σ13, and Σ21 CSLs and random high-angle GBs to understand and predict the effects of GB misorientation on pile-up formation and evolution. The predictions indicate that dislocation-density pile-ups occur at GBs with significantly misoriented slip systems and large residual Burger's vectors, such as Σ7, Σ13, and Σ21 CSLs and random high-angle GBs, and this resulted in heterogeneous inelastic deformations across the GB and local stress accumulations. GBs with low misorientations of slip systems had high transmission, no dislocation-density pile-ups, and lower stresses than the high-angle GBs. This investigation provides a fundamental understanding of how different representative GB orientations affect GB behavior, slip transmission, and dislocation-density pile-ups at a relevant microstructural scale.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(2):021024-021024-9. doi:10.1115/1.4036020.
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Distributed temperature sensing (DTS)-based fiber optic sensors are widely used for monitoring spatially continuous temperature distribution in structures. In this research, hydro-thermal (H-T) coupled analysis is used to monitor seepage conditions in an embankment dam. Variably saturated two-dimensional heat transport (VS2DHI), a computer code developed by the U.S. Geological Survey, was used for this coupled analysis. From the coupled analysis, the temperature profile for a dam with an artificially generated crack clearly showed the location of the crack. In addition, it turned out that the temperature change in the dam took much longer than the seepage time due to the additional time required for heat transfer. The study shows that temperature variation in the dam is comparable to the seepage condition with time delay for heat transfer. This study also shows the possibility that temperature data may serve as a tool to diagnose prior seepage conditions and past incidents of a dam.

Commentary by Dr. Valentin Fuster