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research-article  
Hossein Sepiani, Maria A. Polak and Alexander Penlidis
J. Eng. Mater. Technol   doi: 10.1115/1.4040003
We present a phenomenological three-dimensional nonlinear viscoelastic constitutive model for time dependent analysis. Based on Schapery's single integral constitutive law, a solution procedure has been provided to solve nonlinear viscoelastic behaviour. This procedure is applicable to three dimensional problems and uses time- and stress-dependent material properties to characterize the nonlinear behaviour of material. The equations describing material behaviour are chosen based on the measured material properties in a short test time frame. This estimation process uses the Prony series material parameters and the constitutive relations are based on the non-separable form of equations. Material properties are then modified to include the long term response of material. The presented model is suitable for development of a unified computer code that can handle both linear and nonlinear viscoelastic material behaviour. The proposed viscoelastic model is implemented in a user-defined material algorithm in ABAQUS (UMAT) and the model validity is assessed by comparison with experimental observations on polyethylene for three uniaxial loading cases, namely, short term loading, long term loading, and step loading. A part of the experimental results have been conducted by Liu (Liu, 2007), while the rest are provided by an industrial partner. The research shows that the proposed FEM model can reproduce the experimental strain-time curves accurately, and concludes that with proper material properties to reflect the deformation involved in the mechanical tests, the deformation behaviour observed experimentally can be accurately predicted using the FEM simulation.
TOPICS: Viscoelasticity, Constitutive equations, Finite element analysis, Materials properties, Finite element methods, Deformation, Finite element model, Mechanical testing, Computers, Simulation, Viscoelastic materials, Stress, Algorithms
research-article  
Pham Quang Trung, David Lee Butler and Idapalapati Sridhar
J. Eng. Mater. Technol   doi: 10.1115/1.4040004
Shot peening is a cold working process, which is used to enhance the properties of materials, especially the fatigue life as it induces large compressive residual stresses in the subsurface of materials. In this paper, the effect of the shot peening process on the topography of the shot peened surface and the distribution of the residual stresses in the subsurface of the material was systematically investigated. A technique to estimate the shot peening coverage was employed using a finite element model which was further developed using experimental results for increased accuracy. The comparison between the numerical and experimental studies gives a good agreement of the distribution of the residual stresses in the subsurface of the shot peened material. The shot peening pressure and media size are two main factors affecting on the presence of compressive residual stresses in the subsurface of the material.
TOPICS: Pressure, Residual stresses, Shot peening, Alloy steel, Stress, Materials properties, Fatigue life, Finite element model, Work hardening
research-article  
Tianshun Dong, Xiaodong Zheng, Guolu Li, Haidou Wang, Ming Liu, Xiukai Zhou and Yalong Li
J. Eng. Mater. Technol   doi: 10.1115/1.4040005
Tungsten inert gas arc (TIG) process was employed to remelt Fe-based coating deposited by plasma spraying. Subsequently, the microstructure, interface and the wear resistance of the coatings before and after remelting were studied. The results showed that the lamellar structure, pores and inclusions of Fe-based coating were eliminated and the porosity significantly decreased from 4% to 0.4%. The as-sprayed coating contained unmelted region, single crystal region and twins region, while, microcrystalline region and nanocrystalline region were observed in the remelting coating. There was no element diffusion and dissolution phenomenon at the interface, thus, the bonding form between the as-sprayed coating and substrate mainly was mechanical bonding. On the contrary, the diffusion transfer belt (DTB) emerged at the interface of the remelting coating and substrate, the remelting coating was bonded with the substrate metallurgically. Additionally, the average microhardness and elastic modulus of the remelting coating increased by 33.4% and 53.2% respectively, compared with the as-sprayed coating. During wear process, the as-sprayed coating exhibited obvious brittle fracture characteristics, while the remelting coating appeared typical plastic deformation characteristics and its mass loss reduced by 39.5%. Therefore, TIG remelting process significantly improved the microstructure, mechanical properties and wear resistance of Fe-based coating.
TOPICS: Coatings, Wear resistance, Diffusion (Physics), Bonding, Plasma spraying, Mechanical properties, Brittle fracture, Elastic moduli, Microhardness, Porosity, Tungsten, Crystals, Deformation, Wear, Belts
research-article  
Jeffrey B. Allen
J. Eng. Mater. Technol   doi: 10.1115/1.4039896
In this study, we developed a multi-order, phase field model to compute the stress distributions in anisotropically elastic, inhomogeneous polycrystals and study stress driven grain boundary migration. In particular, we included elastic contributions to the total free energy density, and solved the multicomponent, non-conserved Allen-Cahn equations via the semi-implicit Fourier spectral method. Our analysis included specific cases related to bicrystalline planar and curved systems as well as polycrystalline systems with grain orientation and applied strain conditions. The evolution of the grain boundary confirmed the strong dependencies between grain orientation and applied strain conditions and the localized stresses were found to be a maximum within grain boundary triple junctions.
TOPICS: Density, Grain boundaries, Simulation, Stress, Anisotropy, Engineering simulation, Junctions
research-article  
Ahmad A Mousa, Gert Heinrich, Udo Wagenknecht and Omar Arrabeyat
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

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