Papers by Keyword: Mechanical Anisotropy

Abstract: Stable structures of pentagonal B_xN_y monolayers of different stoichiometric ratios were investigated through density functional theory calculations. Combining the energy and phonon dispersion, two stable pentagonal B_xN_y structures, B₂N₄‒I and B₃N₃‒I, are predicted. Under uniaxial and biaxial tensile strains, B₂N₄‒I and B₃N₃‒I show anisotropy mechanical behaviours in terms of Young’s modulus and intrinsic strength. B₂N₄‒I possesses larger Young’s modulus (up to 206 N/m) and intrinsic strength (up to 40 GPa) compared with those of B₃N₃‒I. Particularly, due to the low symmetry and prominent anisotropy, uniaxial tensile strain can uniquely tailor the band gap and trigger the transition from a direct to an indirect band gap in semiconducting B₃N₃‒I.

Abstract: In this study, using experiment results obtained by electron backscatter diffraction, information on crystal orientation is introduced into a computational model for crystal plasticity simulation considering the effects of grain boundaries and dislocation sources to express the effect of the microstructure of ultrafine-grained metals. Finite-element simulations are performed for a polycrystal of an aluminum plate under biaxial tension. The multiscale crystal plasticity simulations depict the yield surface of the ultrafine-grained aluminum produced by accumulative roll-bonding processes. The anisotropic material coefficients of a higher-ordered yield function for ultrafine-grained aluminum are derived using a genetic algorithm.

Abstract: Chiral sculptured thin films (STFs) Glancing-angle deposition (GLAD) thin films are nanoengineered to meet the requirements of a variety of applications such as micro filters, sensors, and waveguides due to their unique frequency characteristics which cannot be achieved by conventional solid materials. For the design, it is necessary to understand the elastic properties of STFs. To facilitate this, we report on our newly developed advanced micro-scale vibration testing process. In the testing, specially designed micro-specimens with surface areas of tens by tens of microns are excited using a piezoelectric (PZT) actuator and the resonance frequencies are detected by a laser device in the vertical or lateral directions successfully. The anisotropy elastic modulus of STFs composed of helical nanosprings are identified on the basis of vibration testing. The thin film shows strong characteristic anisotropy that the solid one hardly can attain. The micro-scale testing technique can be extended to other materials and microstructures.

Abstract: Severe plastic deformation (SPD), where metals are deformed up to very high strain values, leads to a very small grain size and a high strength of the material. ECAP (Equal Channel Angular Pressing) is one of the SPD methods, and involves the extrusion of a metal billet through two intersecting channels with identical cross-section and forming an angle between them. The material undergoes shearing as it crosses from one channel to the other, but its external dimensions are not altered. Shearing occurs along a single plane, which may lead to anisotropy in the mechanical properties of the material after ECAP. Compression, tension and shearing tests along various directions in the as-processed specimens indicated the presence of mechanical anisotropy in ECAP processed aluminum.

Abstract: Natural porous materials, like wood or bone, are multiscale cellular composite structures which exhibit mechanical (such as elasticity and strength) and functional (such as the thermal or acoustic insulating properties) behaviors dependent on the measuring direction. They show the best performance/weight ratio among all materials because their response is optimized in the needed direction by removing matter where not strictly functional, giving as a result a strong structural as well as morphological anisotropy. A new approach has been developed to mimic this behavior in polymeric foams, in which the mechanical and/or functional response of the cellular structure is tailored in a specific direction through the control of the spatial distribution and configuration of reinforcing particles. In order to demonstrate the concept, polymeric foams were produced with micro- or nano-sized reinforcement distributed along specific directions by means of the magnetic field. The effects of particles content, production parameters, and magnetic field strength were investigated and related to the mechanical (both elastic and magneto-elastic) performances.

Abstract: The plastic anisotropy was studied on aluminium sheets with layers of different purity (A: 5N and B: 2N+) produced by accumulative roll bonding (ARB). Both material layers show a contrasting recrystallization behavior where A and B are discontinuously and continuously recrystallized, respectively. Global textures were measured by neutron diffraction. The mechanical anisotropy was measured by tensile testing after different numbers of ARB cycles. The planar anisotropy decreases with the number of ARB cycles while the normal anisotropy reaches a plateau after 4 cycles. Simulations of the Lankford parameters were carried out with the help of the viscoplastic self-consistent scheme (based on the global texture) and compared with the experimental data. Deviations of the simulated values from those of experiment are discussed with regard to through-thickness texture and material heterogeneities.

Abstract: Several multilevel plasticity models that make use of the crystallographic texture have been developed in the past for the prediction of deformation textures. State-of-the-art models that consider grain interaction, such as Alamel and VPSC, are known to give superior deformation texture predictions compared to the well-known (full constraint) Taylor model. In this paper, these models are assessed on a different basis, namely their ability to predict plastic anisotropy in single-phase steel sheet. A wide range of mechanical tests is considered: uniaxial tension, plane strain tension, simple shear and sheet normal compression. Furthermore, the sensitivity of the anisotropy predictions is analyzed, considering the variability in textures measured by routine XRD. The considered grain interaction models clearly produce improved predictions of plastic anisotropy over the Taylor model.

Abstract: The present study explores a way to improve predictions of the mechanical anisotropy of textured polycrystalline aggregates. The underlying hypothesis is that grain-shape-dependent backstresses developed during the elastic-plastic transition influence the selection of active slip systems inside individual grains. Recently, a model was developed and applied successfully to electro-deposited pure iron with a columnar grain structure \cite{Delannay2011}. In the present study, we first suggest another definition of the boundary separation distance experienced by individual slip systems. Then, the model is adapted from the case of spheroidal grains, considered initially, to the more general situation of ellipsoidal grains. A combined effect of grain size, grain shape and texture on plastic anisotropy at yielding is illustrated in case of a rolled IF steel sheet.

Abstract: Deformation anisotropy of samples from rolled sheet and extruded rod of AZ31 alloy was investigated in the present work. A strong basal plane texture is detected formed during rolling and extrusion, and both rolled and extruded samples exhibit similar mechanical behavior: tensile yield strength is the highest in the specimens parallel to the longitudinal direction, and decrease continuously as the specimen orientation departs from the longitudinal direction. Using texture analysis and optical microscopy it has been found that, the obvious anisotropy can be explained by texture and orientation factor during tension and compression. Basal slip and twinning are restricted when tensile load is applied in the rolling and extrusion direction, which results in high tensile yield strength along the two directions.

Abstract: In order to quantify the plastic anisotropy of the ultrafine grained aluminium alloy AA6016 produced by accumulative roll-bonding (ARB) the Lankford parameter is measured by tensile testing as a function of the number of ARB cycles. The experimental results are compared with those from texture-based Taylor simulations. Increasing differences between experiment and theory at higher number of ARB cycles may be attributed to highly oriented microstructural features.