Materials Science Forum Vols. 654-656

Abstract: In this work, LS-DYNA program was adopted to simulate the loading process of Vanadium Alloy specimen conducted on the Split Hopkinson Pressure Bar (SHPB) in two dimensions. Based on the Johnson-Cook material constitutive relation and criterion of Johnson-Cook failure, the initiation, propagation process of an adiabatic shear band (ASB) and the corresponding distribution of temperature field in the vanadium alloy V-5Cr-5Ti hat-shaped specimen are analyzed. The field of stress, strain and temperature in the tip of an ASB, and the spread speed, the width as well as the type of the ASB are all studied. It is shown that the formation of the ASB is related to the loading velocity and the size of the hat-shaped specimen. And formation of mircocracks and their interlinkage are primary shearing failure mechanism of hat-shaped specimen.

Abstract: Understanding and prediction of physical phenomena in different scales at the same time that are taking place at the tool/workpiece interface during high shear processing is done in different ways combining the latest finite element (FE) and discrete element (DE) analysis technology. The high shear processing is observed during hot rolling of aluminium, when it produces a highly deformed subsurface layer; and also during friction stir welding (FSW), when it results in significant heat generation and flow. The FE analysis is used for macro-scale simulation while the DE method is applied to simulate meso-scale phenomena taking place in the thin, sometimes a few micron thicknesses, surface layer. Different FE models and numerical techniques combined with DE based transient dynamics approaches are discussed in this work.

Abstract: The objective of this study is to clarify the effect of hub contact shape on contact pressure and fatigue life with regard to the selection of a suitable taper design near the end of the fit. A numerical asymmetric-axisymmetric finite element model was developed in order to determine the contact stress state of press-fitted shaft by using four types of tapered contact surfaces on the hub. The variations of fatigue crack initiation life according to the change of tapered contact surfaces on the hub were evaluated by using the Smith-Watson-Topper (SWT) multiaxial fatigue criterion. As the result, comparing with the contact pressure and the fatigue crack initiation life, maximum decrease of contact pressure and maximum increase of fatigue crack initiation life were obtained for the 1/400 m/m tapered hub subjected to a bending load near the fretting fatigue limit. Furthermore, as the change of bending load, the optimal amout of taper in hub which fatigue life gets into maximum is varied. Therefore, we suggest that the best performance, in terms of pressure distribution and fatigue life of press fit, can be obtained by using a proper taper values for the hub element.

Abstract: In cold spray process, the simulation of coating deposition and the arising residual stress analysis are critical for an optimisation of process conditions. However, there are not many published literatures on the role of residual stresses in a cold sprayed coating. In addition, the multi-particles deposition behavior is also not well known, especially when coating of pure titanium powder is considered. This paper considers the development of an explicit finite element model of a cold spray process with defined initial and boundary environment. The explicit finite element model is then used to determine the optimum operating parameters to deposit titanium particles. It is also used to predict the residual stress developed in the coating by examining a fluid/structure interaction. The measured velocity and temperature from Computational Fluid Dynamics model are then use as initial input parameters to the explicit dynamic simulation. The predicted results reveal that Finite Element Method can be used to study the development of residual stress in a cold-sprayed coating as well as to find the optimum operating conditions to deposit coating of titanium particles before doing a real time fabrication.

Abstract: One of the most important aspects of mould design in injection moulding is the provision of suitable and adequate cooling arrangements. Proper cooling channel design in the mould is an important aspect as it affects cycle time and quality of the injection moulded plastic part. A new cooling channel design with copper tube insert can reduce cycle time by optimal and uniform heat transfer in the mould. In this research work a comprehensive FEA transient thermal-structural analysis has been performed with ANSYS simulation software to understand robustness and longevity of an industrial plastic part mould with these cooling channels and compared with conventional straight cooling channels. Autodesk Moldflow Insight (AMI) also has been used to get essential process parameter values for analysis. Result shows that by inserting copper tube in the cooling channels, a mould can increase cooling efficiency and can last for higher number of cycles before fatigue failure, thus increasing production rate.

Abstract: On a metal surface covered with a moisture layer of variable thickness and shape, the dissolved oxygen may induce a spatial separation of the anodic and cathodic reactions on space-time scales characteristic of the roughness, droplet size and the local kinetics of the system. This leads to a spatio-temporal variations in species concentrations, current and potential over the metal surface and thus atmospheric corrosion. Here a fully three-dimensional transient model is developed that addresses the corrosion of a metal under an aerosol droplet. The effects of various parameters, such as exchange current densities, initial concentrations, shape and size of the droplet, and diffusivity of oxygen on ionic, potential and current distributions are investigated.

Abstract: The properties of interphase in polymer composites are often different from those of bulk polymer matrix, which may include chemical, physical, microstructural, and mechanical properties. The nature of interphase is critical to the overall properties and performance of polymer materials, in particular in nanofiller reinforced composites. Experimental efforts have been made to determine the effective interphase thickness and its properties, for example, by nanoindentation and nanoscratch techniques. Yet, it is very difficult to quantify the interphase and its properties because of its nanoscale nature and the unclear boundary. In this regard, computer simulation, e.g., molecular dynamics, provides an effective tool to characterize such interphase and the properties. In this work, molecular dynamics simulations are applied to quantify the interphase thickness in clay-based polymer nanocomposites. Then, the mechanical properties of the so-called effective nanofiller (i.e., the physical size of nanofiller plus the thickness of interphase) will be determined by a series of simulations.

Abstract: Goethite (α-FeOOH) nanorods could be prepared by a surfactant directed approach in aqueous solution at ambient conditions. In this approach, it is observed that the surfactants (e.g, cetyltrimethylammonium bromide (CTAB) and tetraethylamine chloride (TEAC)) play a key role in the growth of goethite nanorods under the reported conditions. The molecular dynamics (MD) method is used to understand the underlying principle governing particle formation and growth through the analysis of the interaction energies between the crystal surfaces and the surfactant molecules. The findings will be useful for understanding the growth mechanism of anisotropic particles and their surface coatings with heterogeneous materials for desired functional properties.

Abstract: The behavior of water molecules with sulfate on the Fe(001) surface has been investigated using a first-principles method based on density-functional theory (DFT) with numerical atomic orbitals as basis functions for the description of valence electrons and nonlocal pseudopotentials for the atomic core. We present results for the adsorption structure and the bonding nature as caused by the adsorption-induced variations in the electron density and the projected density of states. We have found that the structure of absorbed sulfate depends on the coverage of water molecule on the surface. Analysis of electrostatic potential at an aqueous metal interface provides an appropriate framework to understand complicated potential structures. The mechanism of proton transfer through dissociative adsorption and hydrogen bonding of water molecules has been obtained from calculated results.

Abstract: We performed first-principles calculations of stable atomic structure and partial density of states (PDOS) analysis of Ethylene (C2H4) adsorption on Pd(100) surface stacked on fcc-Au using the projector augmented-wave (PAW) method. In case of Pd overlayers on the Au(100) substrate, from the analysis of PDOS around surface, highest occupied states become sharpen compared with the case of C2H4 adsorption on Pd monometallic slab surface in both di- and  adsorption model.