Research Papers

J. Eng. Mater. Technol. 2019;141(3):031001-031001-11. doi:10.1115/1.4042661.

In this paper, the finite strain viscoelastic constitutive model for particulate composite solid propellants is proposed considering strain rate, large deformation/large strain, thermorheological behavior, stress softening due to microstructural damage, compressibility, and chemical age hardening. The compressible Mooney–Rivlin hyperelastic strain energy density function is used along with the standard model of viscoelasticity. To model the compressibility, the dilatational strain energy is taken as the hyperbolic function of the determinant of deformation gradient. The stress-softening phenomenon during cyclic loading (Mullin's effect) due to microstructural damage is described by an exponential function of the current magnitude of intensity of strain and its previous maximum value. The variation of material properties with time are studied using the isothermal accelerated aging technique through simulation and experimental investigation. The comparison of predictions based on the proposed model with the uniaxial experimental data demonstrates that the proposed model successfully captures the observed behavior of the solid propellants.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031002-031002-6. doi:10.1115/1.4042863.

Cold metal transfer (CMT) welding provides many advantages for welding of dissimilar materials and thin sheets with its superior heat input control mechanism. In this study, AA6061 and AA7075 aluminum alloys were joined with CMT welding. The effect of welding parameters on hardness, tensile strength, and corrosion rate was investigated. The Tafel extrapolation method was carried out to determine the corrosion rates of AA6061 and AA7075 base metals and AA6061–AA7075 joints. Increasing heat input was found to be detrimental for both mechanical properties and corrosion resistance. The outcomes showed that CMT welding produces adequate joints of AA6061–AA7075 in terms of mechanical properties and corrosion resistance, favorably with welding parameters that provide low heat input.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031003-031003-10. doi:10.1115/1.4042864.

An attempt is made to investigate the dynamic compressive response of multilayered specimens in bilayered and trilayered configurations, using a split Hopkinson pressure bar (SHPB) and finite element analysis. Two constituent metals comprising the multilayered configurations were Al 6063-T6 and IS 1570. Multiple stack sequences of trilayered and bilayered configurations were evaluated at three different sets of strain rates, namely, 500, 800, and 1000 s−1. The experiments revealed that even with the same constituent volume fraction, a change in the stacking sequence alters the overall dynamic constitutive response. This change becomes more evident, especially in the plastic zone. The finite element analysis was performed using abaqus/explicit. A three-dimensional (3D) model of the SHPB apparatus used in the experiments was generated and meshed using the hexahedral brick elements. Dissimilar material interfaces were assigned different dynamic coefficients of friction. The fundamental elastic one-dimensional (1D) wave theory was then utilized to evaluate the stress–strain response from the nodal strain histories of the bars. Predictions from the finite element simulations along with the experimental results are also presented in this study. For most cases, finite element predictions match well with the experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031004-031004-7. doi:10.1115/1.4042663.

A polymer matrix system of thermoset fiber-reinforced composites helps protect its high modulus and strength fibers from an adverse environment and transfers the load to the reinforced fibers. However, when subjected to a high temperature that exceeds its postcuring-stage temperature, the polymeric matrix will decompose or be charred. To address this issue, various techniques have been developed to improve the flame-retardant property of the polymeric matrix. One of these techniques is to either delay ignition or release moisture to extinguish the flame by combining other chemicals or reactively modifying the epoxy resin. Graphene oxide (GO) nanofilms deposited on top of composite surfaces were compared with the test results of nanocomposite coatings of GO and nanoclay particles on composite surfaces. GO thin film applied to the surface of fiber-reinforced composites acts as a heat shield to quickly dissipate heat and eliminate local heat formation. Thermal tests, such as thermogravimetric analysis (TGA), 45-deg burn tests, vertical burn tests, and surface paint adhesion tests were accomplished. Average burn lengths and the average burn areas were reduced with nanoparticle inclusion to the nanoclay samples and graphene samples. TGA analysis indicated that the nanoclay inclusion samples, as well as the graphene inclusion samples, have a higher percentage weight loss than that of the base sample. GO inclusion samples were less affected than nanoclay inclusion samples during the vertical as well as 45-deg burn tests. In addition, there were no signs of damage to the GO thin film that was secondarily bonded to the surface of composite panels for the burn test.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031005-031005-13. doi:10.1115/1.4042744.

Spring-back of poly(methyl methacrylate) (PMMA) at large strains, various embossing temperatures, and release temperatures below glass transition is quantified through modified unconfined recovery tests. Cooling, as well as large strains, is shown to reduce the amount of spring-back. Despite reducing the amount of spring-back, these experiments show that there is still a substantial amount present that needs to be accounted for in hot embossing processes. Spring-back is predicted using finite element simulations utilizing a constitutive model for the large strain stress relaxation behavior of PMMA. The model's temperature dependence is modified to account for cooling and focuses on the glass transition temperature region. Spring-back is predicted with this model, capturing the temperature and held strain dependence. Temperature assignment of the sample is found to have the largest effect on simulation accuracy. Interestingly, despite large thermal gradients in the PMMA, a uniform temperature approximation still yields reasonably accurate spring-back predictions. These experiments and simulations fill a substantial gap in knowledge of large strain recovery of PMMA under conditions normally found in hot embossing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031006-031006-7. doi:10.1115/1.4042745.

Reinforced polyester composites are widely used in many applications where puncture resistance is important. This research is aimed at understanding whether exposure to different temperatures, high and very low, affects the puncture resistance of a glass cloth reinforced polyester composite and the unreinforced polyester matrix. The temperature range investigated was +80 °C to −80 °C. A hemispherical drop-weight impactor was used for the puncture tests in an environmentally controlled chamber. The reinforced composite specimens are more puncture-resistant compared with the unreinforced polyester at all temperatures. The damage created by the impact reduces with decreasing temperatures, while the energy absorbed remains constant with temperature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031007-031007-5. doi:10.1115/1.4042746.

A new approach of measuring through-thickness Young's moduli of composite materials using nanoindentation was proposed. First, an approximate expression of the reduced modulus of nanoindentation was introduced for orthotropic composites. Second, spherical nanoindentation was conducted for an E-glass fiber/vinyl ester composite system, and measured Young's modulus was quite consistent with the previously reported value for a similar material system.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031008-031008-11. doi:10.1115/1.4042865.

The effect of underloads is mostly quantified by the averaged effect on the fatigue crack growth rate, and the transient behavior is rarely investigated. The objective of this paper is to study the mechanisms behind the effect of underloads, periodic underloads, and underloads combined with overloads. A single underload smashes the material around the crack tip, producing a depression on crack flank and a local reduction of contact forces at the minimum load. The reduction of plastic elongation behind the crack tip has an immediate effect on crack opening level, which rapidly disappears with crack propagation. The smashing associated with the compressive force occurs mainly behind the crack tip position where the underload was applied. The effect of the underload is intimately linked to reversed plastic deformation, which explains its enhanced effect for kinematic hardening. The decrease of load below the minimum baseline load is the main loading parameter. The application of periodic underloads extends the effect of a single underload. The effect of the underload is enhanced by the presence of obstacles in the form of residual plastic deformation, which explains the great effect of underloads applied after overloads.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031009-031009-14. doi:10.1115/1.4042867.

The additive manufacturing (AM) process is unique in that it can facilitate anisotropy because of the layer-by-layer deposition technique intrinsic to this process. In order to develop a component for a desired application, it is necessary to understand the mechanics that facilitate this material behavior. This study investigates how build orientation affects the mechanical performance of as-built direct metal laser sintered (DMLS) stainless steel (SS) GP1 (also referred to as 17-4PH) through strain-controlled monotonic tension and completely reversed low-cycle fatigue (LCF) testing. The anisotropic behavior of DMLS SS GP1 is assessed for samples built along the horizontal plane. Fracture surfaces were found to exhibit ductile responses that were consistent with the σε curves. Constitutive models (i.e., Ramberg–Osgood, Hahn) based upon linear elasticity and nonlinear plasticity are presented and used to simulate the monotonic discontinuous stress–strain yielding response of this material, which are found to be in agreement with the experimental data. A collection of low-cycle fatigue tests reveals initial strain hardening to stabilization, followed by softening to fracture. Tensile and fatigue material constants determined from experimental findings are also presented in this study. Plasticity effects on the life of varying build orientations are explored.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):031010-031010-15. doi:10.1115/1.4042868.

It has been observed that tension twins (TTs) are triggered in rolled polycrystalline magnesium alloys under tensile loading applied along the rolling direction (RD) or the transverse direction. This is surprising because these alloys have a near-basal texture, and TTs would therefore cause extension (instead of contraction) along the normal direction. In this work, the origin of these anomalous TTs is first examined by performing crystal plasticity-based finite element simulations using model textures, wherein the c-axis in one grain is systematically tilted toward the loading direction (RD), with the other grains maintained in ideal basal orientation. It is shown that strong basal slip is triggered in the former, which through its effect on the local stress distribution plays a catalytic role in activating TTs. The above behavior is also observed in a simulation performed with an actual texture pertaining to a rolled AZ31 Mg alloy. Most importantly, when basal slip is suppressed, evolution of TTs is found to be very much retarded. The present results corroborate well with experimental observations.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Mater. Technol. 2019;141(3):034501-034501-5. doi:10.1115/1.4042660.

Additive manufacturing (AM) offers a fabrication process that provides numerous advantages when compared with traditional fabrication methods. Specifically, AM technology allows for the creation of porous media where porosity and permeability can be precisely controlled. When manufacturing metallic artifacts for biomedical use (e.g., bone implants), the investment in a laser sintering machine can be prohibitive for the budget-conscious enterprises limiting the study and use of this technology. Electroforming, electroplating, and electrotyping have been used for decades to replicate the complex shape of unique artifacts and can be viable techniques to create complex metallic shapes starting from a conductive mandrel. We investigated a fabrication technique that combines the stereolithographic additive manufacturing of a polymeric mandrel with electroforming, to obtain porous composites of polymers and metals. The fabrication method to electroform a porous artifact is presented, and an analytical model of the combined properties of the composite material is provided.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(3):034502-034502-9. doi:10.1115/1.4042869.

To understand the formation of direct chill (DC)-casting defects, e.g., butt curl and crack formation, it is essential to take into account the effect of temperature variation, strain rate, and their role in the constitutive behavior of the DC-cast alloys. For the correct prediction of defects due to thermal stresses during DC casting, one needs to rely on the fundamentals of mechanisms that may be relevant to the temperatures at below solidus temperatures. This research work aims to find a suitable physically based model for the as-cast aluminum alloys, namely AA3104, AA5182, and AA6111, which can describe the constitutive behavior at below solidus temperatures during complex loading conditions of temperatures and strain rates. In the present work, an earlier measured and modeled (Alankar and Wells, 2010, “Constitutive Behavior of As-Cast Aluminum Alloys AA3104, AA5182 and AA6111 at Below Solidus Temperatures,” Mater. Sci. Eng. A, 527, pp. 7812–7820) stress–strain data are analyzed using the Voce equation and Kocks–Mecking (KM) model. KM model is capable of predicting the hardening and recovery behavior during complex conditions of strain, strain rate, and temperatures during DC casting. Recovery is dependent on temperature and strain rate, and thus, relevant parameters are determined based on the temperature-sensitive annihilation rate of dislocations. For the KM model, we have estimated k1 parameter as a function of temperature, and k2 has been further modeled based on the temperature and strain rate. KM model is able to fit the constant temperature uniaxial tests within 1.5% of the regenerated data.

Commentary by Dr. Valentin Fuster

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