Showing papers on "Deformation (engineering) published in 2013"

3D Studies of Indentation by Combined X-Ray Tomography and Digital Volume Correlation

Abstract: Hardness testing obtains material properties from small specimens via measurement of load-displacement response to an imposed indentation; it is a surface characterisation technique so, except in optically transparent materials, there is no direct observation of the assumed damage and deformation processes within the material. Three-dimensional digital image correlation (digital volume correlation) is applied to study deformation beneath indentations, mapping the relative displacements between high-resolution synchrotron X-ray computed tomographs (0.9 μm voxel size). Two classes of material are examined: ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). The measured displacements for Hertzian indentation in Al-SiC are in good agreement with an elastic-plastic finite element simulation. In alumina, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured, in situ under load, for the first time. Potential applications are discussed of this characterization technique, which does not require resolution of microstructural features.

Near-ideal theoretical strength in gold nanowires containing angstrom scale twins

Abstract: Low-dimensional materials containing defects such as twin boundaries are known to fail well below their theoretical strength due to surface imperfections. Here, Wang et al. observe strengths close to the ideal limit in gold nanowires with angstrom scale twins, where homogeneous dislocation nucleation controls deformation.

The Physics of Deformation and Fracture of Polymers

Abstract: 1. Structure of non-polymeric glasses 2. Structure of solid polymers 3. Overview of deformation and fracture mechanisms 4. Small strain elastic response 5. Linear visco-elasticity of polymers 6. Rubber elasticity 7. Inelastic behaviour of non-polymeric glasses 8. Inelastic behaviour of glassy polymers 9. Plasticity of semi-crystalline polymers 10. Deformation instabilities in extensional plastic flow of polymers 11. Crazing in glassy homo and hetero polymers 12. Fracture of polymers 13. Toughening of brittle polymers.

Deformable, Programmable, and Shape‐Memorizing Micro‐Optics

Abstract: The use of shape memory polymers is demonstrated for deformable, programmable, and shape-memorizing micro-optical devices. A semi-crystalline shape memory elastomer, crosslinked poly(ethylene-co-vinyl acetate), is used to prepare various micro-optic components, ranging from microlens and microprism arrays to diffraction gratings and holograms. The precise replication of surface features at the micro- and nanoscale and the formation of crosslinked shape memory polymer networks can be achieved in a single step via compression molding. Further deformation via hot pressing or stretching of micro-optics formed in this manner allows manipulation of the microscopic surface features, and thus the corresponding optical properties. Due to the shape memory effect, the original surface structures and the optical properties can be recovered and the devices be reprogrammed, with excellent reversibility in the optical properties. Furthermore, arrays of transparent resistive microheaters can be integrated with deformed micro-optical devices to selectively trigger the recovery of surface features in a spatially programmable manner, thereby providing additional capabilities in user-definable optics.

Plastic Strain Mapping with Sub-micron Resolution Using Digital Image Correlation

Abstract: Digital image correlation (DIC) of images obtained using scanning electron microscopy has been used to study, quantitatively, the plastic deformation of stainless steel at the microstructural scale. An artificial speckle pattern was generated by the remodelling of a deposited gold layer. A new experimental setup was shown to accelerate the remodelling process and promote the formation of finer nano-scale speckles with sizes ranging 30 nm to 150 nm and of similar spacing. The effects of surface preparation on speckle morphology are discussed. The high density of speckles enabled displacement mapping with resolution of one displacement vector each 0.2 × 0.2 μm2 of surface area. It is shown that sub-micron resolution is necessary to capture the plastic deformation associated with the formation of slip bands in stainless steel, which are an important component of the deformation of these materials at the microscale. Electron backscatter diffraction (EBSD) was used to reconstruct the surface grain boundaries and enabled these deformation features to be linked to the microstructure.

Stiffness of sands through a laboratory test database

Abstract: Deformations of sandy soils around geotechnical structures generally involve strains in the range small (0·01%) to medium (0·5%). In this strain range the soil exhibits non-linear stress–strain behaviour, which should be incorporated in any deformation analysis. In order to capture the possible variability in the non-linear behaviour of various sands, a database was constructed including the secant shear modulus degradation curves of 454 tests from the literature. By obtaining a unique S-shaped curve of shear modulus degradation, a modified hyperbolic relationship was fitted. The three curve-fitting parameters are: an elastic threshold strain γe, up to which the elastic shear modulus is effectively constant at G0; a reference strain γr, defined as the shear strain at which the secant modulus has reduced to 0·5G0; and a curvature parameter a, which controls the rate of modulus reduction. The two characteristic strains γe and γr were found to vary with sand type (i.e. uniformity coefficient), soil state (i....

Mechanical properties and scaling laws of nanoporous gold

Abstract: Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell porous microstructure of the material. Molecular dynamics simulations then reveal that the uniaxial tensile deformation in such porous materials is accompanied by an accumulation of stacking faults in ligaments along the loading direction and their junctions with neighboring ligaments, as well as the formation of Lomer–Cottrell locks at such junctions. The tensile strain leads to progressive necking and rupture of some ligaments, ultimately resulting in failure of the material. The simulation results also suggest scaling laws for the effective Young's modulus, yield stress, and ultimate strength as functions of the relative mass density and average ligament size in the material.

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys

Abstract: The main types of microstructure are (1) lamellar – formed after slow cooling when deforma‐ tion or heat treatment takes place at a temperature in the single-phase β-field above the socalled beta-transus temperature Tβ (at which the α+β→β transformation takes place), consisting of colonies of hexagonal close packed (hcp) α-phase lamellae within large body centered cubic (bcc) β-phase grains of several hundred microns in diameter, and (2) equiaxed – formed after deformation in the two-phase α+β field (ie, below Tβ), consisting of globular α-phase dispersed in β-phase matrix [7-8]

Modeling bending of α-titanium with embedded polycrystal plasticity in implicit finite elements

Abstract: An accurate description of the mechanical response of α-titanium requires consideration of mechanical anisotropy. In this work we adapt a polycrystal self-consistent model embedded in finite elements to simulate deformation of textured α-titanium under quasi-static conditions at room temperature. Monotonic tensile and compressive macroscopic stress–strain curves, electron backscattered diffraction and neutron diffraction data are used to calibrate and validate the model. We show that the model captures with great accuracy the anisotropic strain hardening and texture evolution in the material. Comparisons between predictions and experimental data allow us to elucidate the role that the different plastic deformation mechanisms play in determining microstructure and texture evolution. The polycrystal model, embedded in an implicit finite element code, is then used to simulate geometrical changes in bending experiments of α-titanium bars. These predictions, together with results of a macroscopic orthotropic elasto-plastic model that accounts for evolving anisotropy, are compared with the experiments. Both models accurately capture the experimentally observed upward shift of the neutral axis as well as the rigidity of the material response along hard-to-deform crystallographic direction.

Mechanochromic Photonic Gels

Abstract: Polymer gels are remarkable materials with physical structures that can adapt significantly and quite rapidly with changes in the local environment, such as temperature, light intensity, electrochemistry, and mechanical force An interesting phenomenon observed in certain polymer gel systems is mechanochromism - a change in color due to a mechanical deformation Mechanochromic photonic gels are periodically structured gels engineered with a photonic stopband that can be tuned by mechanical forces to reflect specific colors These materials have potential as mechanochromic sensors because both the mechanical and optical properties are highly tailorable via incorporation of diluents, solvents, nanoparticles, or polymers, or the application of stimuli such as temperature, pH, or electric or strain fields Recent advances in photonic gels that display strain-dependent optical properties are discussed In particular, this discussion focuses primarily on polymer-based photonic gels that are directly or indirectly fabricated via self-assembly, as these materials are promising soft material platforms for scalable mechanochromic sensors

Some Aspects of High Manganese Twinning-Induced Plasticity (TWIP) Steel, A Review

Abstract: High manganese twinning-induced plasticity (TWIP) steel is a new kind of structural material and possesses both high strength and superior plasticity and can meet the weight-lightening requirement for manufacturing vehicle body. The excellent formability of the TWIP steel comes from the extraordinary strain hardening effect during plastic deformation. The reduction of specific weight by aluminum alloying and strain hardening effect can lead to an effective weight reduction of the steel components, and provide a better choice for materials in vehicle body design. The TWIP effect in high Mn steels is generally associated with the successive workhardening generated by twins and influenced by some factors, such as Mn content, Al addition revealed by stacking fault energy (SFE), grain size, deformation temperature and strain rate. The present review introduces some aspects of the TWIP steels relating to their physical metallurgy, influencing factors associated with their deformation mechanisms, and a prospect for the future investigation is also described. Moreover, as a potential candidate for replacing Ni-Cr austenitic stainless steel, researches on the oxidation behavior and corrosion resistance of Fe-Mn-Al-C system steels are also reviewed.

An exact solution for thermal buckling of annular FGM plates on an elastic medium

Abstract: The buckling of heated functionally graded material (FGM) annular plates on an elastic foundation is studied analytically. A conventional Pasternak-type elastic foundation is assumed to be in contact with plate during deformation, which acts in both compression and tension. The equilibrium equations of an annular-shaped plate are obtained based on the classical plate theory. Each thermo-mechanical property of the plate is assumed to be graded across the thickness direction of plate based on the power law form, while Poisson’s ratio is kept constant. Among all combinations of free, simply-supported, and clamped boundary conditions, existence of bifurcation buckling for various edge supports is examined and stability equations are obtained by means of the adjacent equilibrium criterion. An exact analytical solution is presented to calculate the thermal buckling load by obtaining the eigenvalues of the stability equation. Three types of thermal loading, namely; uniform temperature rise, transversely linear temperature distribution and heat conduction across the thickness type are studied. Effects of thickness to outer radii, inner to outer radii, power law index, elastic foundation coefficient, and thermal loading type on critical buckling temperature of FG plates are presented.