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# Accepted Manuscripts

BASIC VIEW  |  EXPANDED VIEW
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040553
Mechanical properties of Additive Manufactured metal components can be affected by the orientation of the layer deposition. In this investigation, Ti-6Al-4V cylindrical specimens were fabricated by electron beam melting at four different build angles (0°, 30°, 60° and 90°) and tested as per ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials. With the layer-by-layer fabrication suggesting granting anisotropic properties to the builds, strain fields were recorded by digital image correlation in the search for shear effects under uniaxial loads. For the validation of this measuring method, axial strains were measured with a clip extensometer and a virtual extensometer simultaneously. Evidencing the recording of shear strain fields; failure analysis of the specimens at different orientations was conducted. The failure analysis included fractography, optical micrographs of the microstructure distribution, and failure profiles displaying different failure features associated to the layering orientation. Additionally, an experimental study case of how, by a proper energy management in the fabrication process, the failure mode of components can potentially be designed is presented; in this manner, a specimen was fabricated with practically standard strength but null plastic elongation, and whose failure analysis revealed an interesting fracture surface displaying a mix of brittle and ductile features, artificially induced. At the end, remarks about the shear effects found, and an insight of the possibility of designing components by failure for safer structures are discussed.
TOPICS: Manufacturing, Brittleness, Cathode ray oscilloscopes, Electron beams, Stress, Metalwork, Anisotropy, Melting, Shear (Mechanics), Fracture (Materials), Mechanical properties, Design, Failure mechanisms, Fracture (Process), Mechanical behavior, Elongation, Failure, Failure analysis, Fractography, Tensile testing, ASTM International, Energy management
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040554
This paper outlines a microstructure based model relating gamma prime microstructure of Ni-base alloys to their creep behavior. The ability of the model to explain creep of multiple superalloys with a single equation and parameter set is demonstrated. The only parameters that are changed from alloy to alloy are related to the gamma prime characteristics and grain size. This model also allows prediction of creep performance as a function of heat-treatment and explains some apparently contradictory data from the literature.
TOPICS: Creep, Nickel, Superalloys, Alloys, Heat treating (Metalworking), Grain size
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040555
Cellular architectures are promising in a variety of engineering applications due to attractive material properties. Additive Manufacturing (AM) has reduced the difficulty in the fabrication of three dimensional (3D) cellular materials. In this paper, the numerical homogenization method for 3D cellular materials is provided based on a short, self-contained Matlab code. It is an educational description that shows how the homogenized constitutive matrix is computed by a voxel model with one material to be void and another material to be solid. A voxel generation algorithm is proposed to generate the voxel model easily by the wireframe scripts of unit cell topologies. The format of the wireframe script is defined so that the topology can be customized. The homogenization code is then extended to multi-material cellular structures and thermal conductivity problems. The result of the numerical homogenization shows that different topologies exhibit anisotropic elastic properties to a different extent. It is also found that the anisotropy of cellular materials can be controlled by adjusting the combination of materials.
TOPICS: Matlab, Anisotropy, Materials properties, Thermal conductivity, Algorithms, Engineering systems and industry applications, Architecture, Topology, Additive manufacturing, Elasticity, Manufacturing
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040556
Carbon nanotubes (CNTs) are well known as perfect reinforcement for high strength and lightweight composites due to their high specific strength, thermal, electrical and mechanical characteristics. One of the important challenges is to obtain a homogeneous dispersion of CNTs in metal matrix, so development technologies for producing metal matrix composites (MMCs) is of great interest. Melting followed by solidification, may be successfully utilized for synthesizing CNT-reinforced aluminum-based MMCs. In the present study, Al/CNT composites have been produced by direct injection of CNTs in pure aluminum using high-pressure die casting (HPDC) method. The as-produced billets were subjected to cyclic extrusion (CE) to refine CNT agglomerates and to increase CNT dispersion in aluminum. Current investigation demonstrates that more than 50% efficiency of combined HPDC-CE production method has been achieved. The resulting composites demonstrated improved mechanical properties.
TOPICS: Metal matrix composites, Carbon nanotubes, Aluminum, Composite materials, Mechanical properties, Solidification, Extruding, Die castings (Product), Die casting (Process), High pressure (Physics), Melting, Metals
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040409
Cellular materials are found extensively in nature such as wood, honeycomb, butterfly wings and foam-like structures like trabecular bone and sponge. This class of materials proves to be structurally-efficient by combining low weight with superior mechanical properties. Recent studies have shown there are coupling relations between the mechanical properties of cellular materials and their relative density. Due to its favorable stretching-dominated behavior, continuum models of the octet-truss were developed to describe its effective mechanical properties. However, previous studies were only performed for the cubic symmetry case where the lattice angle ?=45°. In this work, we study the impact of the lattice angle on the effective properties of the octet-truss: namely the relative density, effective stiffness, and effective strength. The relative density formula is extended to account for different lattice angles up to a higher order of approximation. Tensor transformations are utilized to obtain relations of the effective elastic, shear moduli, and Poisson's ratio at different lattice angles. Analytical formulas are developed to obtain the loading direction and value of the maximum and minimum specific elastic moduli at different lattice angles. In addition, tridimensional polar representations of the macroscopic strength of the octet-truss are analyzed for different lattice angles. Finally, collapse surfaces for plastic yielding and elastic buckling are investigated for different loading combinations at different lattice angles. It has been found that lattice angles lower than 45° result in higher maximum values of specific effective elastic moduli, shear moduli, and strength.
TOPICS: Trusses (Building), Mechanical properties, Density, Elastic moduli, Shear modulus, Stiffness, Wings, Weight (Mass), Tensors, Bone, Approximation, Buckling, Collapse, Wood products, Poisson ratio, Honeycomb structures
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040333
Fusion welding of nickel-based alloys is often associated with coarse grains and severe segregation, which finally results in the increase of hot cracking susceptibility and poor mechanical properties. Conventional gas tungsten arc welding (GTAW) can aggravate these phenomena, which is mainly due to its high heat input and low cooling rate. In this paper, the cooling rate was enhanced by spraying liquid nitrogen during the welding process. Compared to conventional GTAW, the rapid cooling produced narrower HAZ width and more equiaxed grains in the fusion zone, thus higher hardness distribution was also achieved in this condition. In addition, ?' phase exhibited a dispersed distribution, and segregation has been improved. The results show that the heat affected zone (HAZ) width is decreased by about 50%, and the fusion zone consisting of the finest equiaxed grains and the lowest segregation was obtained, when the heat sink located on one side 10mm away from the weld centerline. Also, fine equiaxed grains and the dispersed distribution of ?' phase could improve the grain boundary strength and reduce the incidence of liquid films along grain boundaries, contributing to prevent nickel-based alloys welding hot cracking from initiating.
TOPICS: Cooling, Nickel, Alloys, Welding, Gas tungsten arc welding, Cracking (Materials), Heat, Grain boundaries, Fracture (Process), Heat sinks, Liquid films, Nitrogen, Plasma spraying, Spraying (Coating processes), Mechanical properties
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040339
The deformation behavior of as-quenched 2024 Al-Cu-Mg alloy has been experimentally studied. The experiments are designed to cool specimens to the desired temperature with a constant cooling rate, i.e. 5 K/s. Isothermal tensile tests are performed over a range of 573-723 K temperature and (0.01, 0.1 and 1 s-1) strain rates to find out the flow stresses and microstructures after deformation. Due to the non-uniform deformation mechanisms (solid solution vs. solid solution and precipitation), two types of Arrhenius model are established for the temperature range of 573-673 K and 673-723 K, respectively. For temperature between 573 and 673 K, the activation energy is dependent on temperature and strain rate, and the value of activation energy decreases with the increases of temperature and strain rate. Compared with the ideal variation trend with no consideration of precipitation, the largest difference of activation energy is found at the tempera-ture of 623 K which is the nose temperature of 2024 alloy.
TOPICS: Alloys, Quenching (Metalworking), Precipitation, Temperature, Deformation, Solid solutions, Stress, Flow (Dynamics), Cooling
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040352
The Shear Modulus of orthotropic thin sheets from three advanced high-strength steels (AHSS) is measured using the anticlastic-plate-bending (APB) experiment. In APB, a thin square plate is loaded by point forces at its four corners, paired in opposite directions. It thus assumes the shape of a hyperbolic paraboloid, at least initially. The principal stress directions coincide with the plate diagonals, and the principal stresses are equal and opposite. Hence, at 45o degrees to these, a state of pure shear exists. A finite element study of APB is reported first, using both an elastic and an elastoplastic material model. This study confirms the theoretical predictions of the stress field that develops in APB. The numerical model is then treated as a virtual experiment. The input Shear Modulus is recovered through this procedure, thus validating this approach. A major conclusion from this numerical study is that the Shear Modulus for these three AHSS should be determined before the shear strain exceeds 2 x 10-4 (or 200 µe). Subsequently, APB experiments are performed on the three AHSS (DP 980, DP 1180 and MS 1700). The responses recorded in these experiments confirm that over 3x10-4 strain (or 300 µe) the response differs from the theoretically expected one, due to excessive deflections, yielding, changing contact conditions with the loading rollers and, in general, the breaking of symmetry. But under that limit, the responses recorded are linear, and can be used to determine the Shear Modulus.
TOPICS: Shear modulus, Stress, Shear (Mechanics), Corners (Structural elements), Finite element analysis, Deflection, Rollers, Shapes, Steel, Computer simulation
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4040222
Arising from long-term high temperature service, the microstructure of nickel-base (Ni-base) superalloy components undergo thermally- and deformation-induced aging characterized by isotropic coarsening and directional coarsening (rafting) of the $\gamma'$ precipitates. The net result of the morphological evolutions of the gamma primeparticles is a deviation of the mechanical behavior from that of the as-heat treated properties. To capture the influence of a rafted and isotropic aged microstructure states on the long-term constitutive behavior of a Ni-base superalloy undergoing thermomechanical fatigue, a temperature-dependent crystal viscoplasticity (CVP) constitutive model is extended to include the effects of aging. The influence of aging in the CVP framework is captured through the addition of internal state variables that measure the widening of the $\gamma$ channels and in-turn update the material parameters of the CVP model. Through the coupling with analytical derived kinetic equations to the CVP model, the enhanced CVP model is shown to be in good agreement when compared to experimental behavior in describing the long-term aging effects on the cyclic response of a directionally-solidified Ni-base superalloy used in hot section components of industrial gas turbines.
TOPICS: Superalloys, Crystals, Viscoplasticity, Nickel, Industrial gases, Constitutive equations, Mechanical behavior, Turbines, Thermomechanics, High temperature, Deformation, Fatigue, Heat, Temperature
research-article
J. Eng. Mater. Technol   doi: 10.1115/1.4037169
Exfoliated graphite (EXG) was prepared from commercially available natural graphite flakes (NGF), through strong acid treatment followed by thermal shock at 950 oC. The EXG sheets were characterized with respect to their thermal stability via thermo-gravimetric analysis (TGA) and Raman spectra. Their morphology and particle size were evaluated using scanning electron microscope (SEM) and particle size analyzer. The potential of EXG as reinforcement on the mechanical and thermal properties of the dynamically vulcanized polystyrene/styrene butadiene rubber (PS/SBR) composites were evaluated. The influence of EXG on the electrical properties of the composites was measured as well.
TOPICS: Composite materials, Thermal properties, Graphite, Particle size, Raman spectra, Thermal shock, Thermal stability, Electrical properties, Scanning electron microscopes, Styrene-butadiene rubber