Showing papers on "Deformation (engineering) published in 2012"

Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles

Abstract: ZnO nanoparticles were prepared by coprecipitation method at 450C. X-ray diffraction result indicates that the sample is having a crystalline wurtzite phase. Transmission electron microscopy (TEM) result reveals that the ZnO sample is spherical in shape with an average grain size of about 50nm. X-ray peak broadening analysis was used to evaluate the crystalline sizes and lattice strain by the Williamson-Hall (W-H) analysis. All other relevant physical parameters such as strain, stress, and energy density values were also calculated using W-H analysis with different models, viz, uniform deformation model, uniform deformation stress model and uniform deformation energy density model. The root mean square strain was determined from the interplanar spacing and strain estimated from the three models. The three models yield different strain values; it may be due to the anisotropic nature of the material. The mean particle size of ZnO nanoparticles estimated from TEM analysis, Scherrers formula and W-H analysis is highly intercorrelated.

Stretchable and highly sensitive graphene-on-polymer strain sensors

Abstract: The use of nanomaterials for strain sensors has attracted attention due to their unique electromechanical properties. However, nanomaterials have yet to overcome many technological obstacles and thus are not yet the preferred material for strain sensors. In this work, we investigated graphene woven fabrics (GWFs) for strain sensing. Different than graphene films, GWFs undergo significant changes in their polycrystalline structures along with high-density crack formation and propagation mechanically deformed. The electrical resistance of GWFs increases exponentially with tensile strain with gauge factors of ~103 under 2~6% strains and ~106 under higher strains that are the highest thus far reported, due to its woven mesh configuration and fracture behavior, making it an ideal structure for sensing tensile deformation by changes in strain. The main mechanism is investigated, resulting in a theoretical model that predicts very well the observed behavior.

Probing topological protection using a designer surface plasmon structure.

Abstract: The ideal elastic limit is the upper bound to the stress and elastic strain a material can withstand. This intrinsic property has been widely studied for crystalline metals, both theoretically and experimentally. For metallic glasses, however, the ideal elastic limit remains poorly characterized and understood. Here we show that the elastic strain limit and the corresponding strength of submicron-sized metallic glass specimens are about twice as high as the already impressive elastic limit observed in bulk metallic glass samples, in line with model predictions of the ideal elastic limit of metallic glasses. We achieve this by employing an in situ transmission electron microscope tensile deformation technique. Furthermore, we propose an alternative mechanism for the apparent 'work hardening' behaviour observed in the tensile stress-strain curves.

Twinning and martensite in a 304 austenitic stainless steel

Abstract: The microstructure characteristics and deformation behavior of 304L stainless steel during tensile deformation at two different strain rates have been investigated by means of interrupted tensile tests, electron-backscatter-diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The volume fractions of transformed martensite and deformation twins at different stages of the deformation process were measured using X-ray diffraction method and TEM observations. It is found that the volume fraction of martensite monotonically increases with increasing strain but decreases with increasing strain rate. On the other hand, the volume fraction of twins increases with increasing strain for strain level less than 57%. Beyond that, the volume fraction of twins decreases with increasing strain. Careful TEM observations show that stacking faults (SFs) and twins preferentially occur before the nucleation of martensite. Meanwhile, both ɛ-martensite and α′-martensite are observed in the deformation microstructures, indicating the co-existence of stress-induced-transformation and strain-induced-transformation. We also discussed the effects of twinning and martensite transformation on work-hardening as well as the relationship between stacking faults, twinning and martensite transformation.

A finite deformation stress-dependent chemical potential and its applications to lithium ion batteries

Abstract: This paper reports the development of a new stress-dependent chemical potential for solid state diffusion under multiple driving forces including mechanical stresses. The new stress-dependent chemical potential accounts for nonlinear, inelastic, and finite deformation. By using this stress-dependent chemical potential, insertion and extraction of lithium ions into a silicon particle is investigated. The distribution and evolution of diffusion-induced stress during the insertion/extraction processes are numerically calculated. Critical particle size is obtained as a function of the charging/discharging rates. It is also found that when plastic deformation occurs, the hoop stresses on the particle surface, contrary to intuition, can become positive even during the charging process, which may explain some of the recent experimental observations.

Mechanical Properties of Materials

Abstract: Preface 1 Mechanical Testing of Materials 2 Introduction to Dislocations 3 Plastic Deformation 4 Strengthening Mechanisms 5 Time Dependent Deformation - Creep 6 Cyclic Stress - Fatigue 7 Fracture 8 Mechanical Behavior in the Micron and Submicron/Nano range Index

Anisotropic elastic properties of flexible metal-organic frameworks: how soft are soft porous crystals?

Abstract: We performed ab initio calculations of the elastic constants of five flexible metal-organic frameworks (MOFs): MIL-53(Al), MIL-53(Ga), MIL-47, and the square and lozenge structures of DMOF-1. Tensorial analysis of the elastic constants reveals a highly anisotropic elastic behavior, some deformation directions exhibiting very low Young's modulus and shear modulus. This anisotropy can reach a $400\ensuremath{\mathbin:}1$ ratio between the most rigid and weakest directions, in stark contrast to the case of nonflexible MOFs such as MOF-5 and ZIF-8. In addition, we show that flexible MOFs can display extremely large negative linear compressibility. These results uncover the microscopic roots of stimuli-induced structural transitions in flexible MOFs, by linking the local elastic behavior of the material and its multistability.

Compression deformation behavior of Ti-6Al-4V alloy with cellular structures fabricated by electron beam melting

Abstract: Ti-6Al-4V alloy with two kinds of open cellular structures of stochastic foam and reticulated mesh was fabricated by additive manufacturing (AM) using electron beam melting (EBM), and microstructure and mechanical properties of these samples with high porosity in the range of 62%∼92% were investigated. Optical observations found that the cell struts and ligaments consist of primary α' martensite. These cellular structures have comparable compressive strength (4∼113 MPa) and elastic modulus (0.2∼6.3 GPa) to those of trabecular and cortical bone. The regular mesh structures exhibit higher specific strength than other reported metallic foams under the condition of identical specific stiffness. During the compression, these EBM samples have a brittle response and undergo catastrophic failure after forming crush band at their peak loading. These bands have identical angle of ∼45° with compression axis for the regular reticulated meshes and such failure phenomenon was explained by considering the cell structure. Relative strength and density follow a linear relation as described by the well-known Gibson-Ashby model but its exponential factor is ∼2.2, which is relative higher than the idea value of 1.5 derived from the model.

Thermally responsive rolling of thin gel strips with discrete variations in swelling

Abstract: The process by which spatial variations in growth transform two-dimensional elastic membranes into three-dimensional shapes is both a fundamentally interesting mechanism of shape selection and a powerful tool for the preparation of responsive materials. From the perspective of lithographic patterning of thin gel sheets, it is most straightforward to prepare materials consisting of discrete regions with different degrees of swelling. However, the sharp variations in swelling at the boundaries between such regions make it impossible for the sheet to adopt a configuration that is free of in-plane stresses everywhere. Thus, the deformation of such materials is not well understood. Here, we consider the geometrically simple case of a photo-crosslinkable poly(N-isopropylacrylamide) copolymer patterned into thin rectangular strips divided into one high- and one low-swelling region. When swelled in an aqueous medium at 22 °C, the sheet rolls into a three-dimensional shape consisting of two nearly cylindrical regions connected by a transitional neck. Heating to 50 °C leads to fully reversible de-swelling back to a flat configuration. We propose a scaling argument based on a balance between stretching and bending energies that relates the curvature of the 3D shape to the width and thickness of the strip, find good agreement with experimental data and numerical simulations, and further demonstrate how this simple geometry provides a powerful route for the fabrication of self-folding stimuli-responsive micro-devices.

Mechanisms of multi-shape memory effects and associated energy release in shape memory polymers

Abstract: Shape memory polymers have attracted increasing research interest due to their capability of fixing a temporary shape and associated deformation energy then releasing them later on demand. Recently, it has been reported that polymers with a broad thermomechanical transition temperature range can demonstrate a multi-shape memory effect (m-SME), where shape recovery and energy release occur in a stepped manner during free recovery. This paper investigated the underlying physical mechanisms for these observed shape memory behaviors and the associated energy storage and release by using a theoretical modeling approach. A multibranch model, which is similar to the generalized standard linear solid model of viscoelasticity, was used for a quantitative analysis. In this model, individual nonequilibrium branches represent different relaxation modes of polymer chains with different relaxation times. As the temperature was increased in a staged manner, for a given temperature, different numbers of branches (or relaxation modes) became shape memory active or inactive, leading to the observed m-SME. For energy release during free recovery, under a tensile deformation of the SMP, stored energy in individual nonequilibrium branches was first transferred into a compressive deformation energy, then gradually declined to zero. Energy release during recovery was a complicated process due to the involvement of multiple relaxation modes.

A study of the heterogeneity of plastic deformation in IF steel by EBSD

Abstract: The objective of this experimental study is to recognize the roles of several quantities like grain size and orientation distributions on the development of plastic heterogeneities. The measurements are performed on an interstitial free (IF) steel by Electron Back Scattered Diffraction (EBSD) at different states of deformation (from 0% to 17% tensile deformation). For each level of deformation, EBSD maps are performed before and after the deformation on exactly the same area. Several parameters as the Grain Orientation Spread (GOS), the Grain Orientation Spread over the grain Diameter (GOS/D) and the Geometrically Necessary Dislocation (GND) densities can thus be determined for different subpopulations of grains ranked as a function of individual grains sizes to follow the evolution of the deformed-induced microstructure. It appears that none of these grain scale measures are deciding and that grain neighborhood interactions play an important role.

Signature of viscous flow units in apparent elastic regime of metallic glasses

Abstract: We characterize and identify the flow units in two typical metallic glasses (MGs), which have markedly different β-relaxation behaviors and mechanical properties. The viscoelastic hysteresis loops are found in the cyclic deformation in the nominal elastic regime of the metallic glasses. We show that the hysteresis loops are related to the activation of the flow units in metallic glasses, and a model is proposed to describe the flow units. We demonstrate that the flow units are both the deformation units of the anelastic and plastic deformation behaviors and the structural origin of the β-relaxation in metallic glasses.

Mechanical Properties of Si Nanowires as Revealed by in Situ Transmission Electron Microscopy and Molecular Dynamics Simulations

Abstract: Deformation and fracture mechanisms of ultrathin Si nanowires (NWs), with diameters of down to ~9 nm, under uniaxial tension and bending were investigated by using in situ transmission electron microscopy and molecular dynamics simulations. It was revealed that the mechanical behavior of Si NWs had been closely related to the wire diameter, loading conditions, and stress states. Under tension, Si NWs deformed elastically until abrupt brittle fracture. The tensile strength showed a clear size dependence, and the greatest strength was up to 11.3 GPa. In contrast, under bending, the Si NWs demonstrated considerable plasticity. Under a bending strain of 20%, the cracks nucleated on the tensed side and propagated from the wire surface, whereas on the compressed side a plastic deformation took place because of dislocation activities and an amorphous transition.

Constitutive equations for elevated temperature flow behavior of commercial purity aluminum

Abstract: In order to study the high-temperature flow stress of commercial purity aluminum (AA1070), isothermal hot compression tests were conducted at the deformation temperatures varying from 350 to 500 °C and strain rates ranging from 0.005 to 0.5 s−1. The results showed that the flow stress of AA1070 was evidently affected by both the deformation temperature and strain rate. The influence of strain was also incorporated in the constitutive equation by considering the effects of strain on material constants which are consist of β, α, n, A and activation energy Q. The predicted flow stress curves using the proposed constitutive equations well agree with the experimental results of the flow stress for AA1070.