Showing papers on "Necking published in 2000"

High quality Ge on Si by epitaxial necking

Abstract: We show that pure Ge grown selectively on SiO2/Si substrates in 100 nm holes is highly perfect at the top surface compared to conventional Ge lattice-mismatched growth on planar Si substrates. This result is achieved through a combination of interferometric lithography SiO2/Si substrate patterning and ultrahigh vacuum chemical vapor deposition Ge selective epitaxial growth. This “epitaxial necking,” in which threading dislocations are blocked at oxide sidewalls, shows promise for dislocation filtering and the fabrication of low-defect density Ge on Si. Defects at the Ge film surface only arise at the merging of epitaxial lateral overgrowth fronts from neighboring holes. These results confirm that epitaxial necking can be used to reduce threading dislocation density in lattice-mismatched systems.

In-Situ Simultaneous Synchrotron Small- and Wide-Angle X-ray Scattering Measurement of Poly(vinylidene fluoride) Fibers under Deformation

Abstract: Poly(vinylidene fluoride) (PVDF) fibers spun at different take-up speeds (10.6−61.0 m/min) were subjected to stretch−hold deformation at room temperature and in-situ simultaneous synchrotron small- and wide-angle X-ray scattering measurements. Crystal transformation from α to β form and morphological changes in lamellar and fibrillar structures were analyzed in detail. All fibers were found to yield at an early stage of deformation, resulting in alternating necked and unnecked regions along the fiber. From the two-dimensional (2-D) wide-angle X-ray diffraction (WAXD) patterns, mixed α and β (minor fraction) forms were found to coexist in the undrawn fibers. Deformation assisted in the conversion from the α-form into the β-form. In necked regions, more α to β transformation took place than in unnecked regions. The overall crystallinity index and unit cell parameters of the α form did not change significantly prior to necking. From the 2-D small-angle X-ray scattering (SAXS) patterns, two kinds of equatoria...

Dynamics of weakly strain-hardening fluids in filament stretching devices

Abstract: We investigate the transient viscoelastic response of weakly strain-hardening fluids to imposed elongational deformation in filament-stretching devices. We combine time-dependent finite-element simulations with quantitative experimental measurements on a rheologically well-characterized test fluid to investigate how well the device reproduces the ideal transient uniaxial extensional viscosity that is predicted theoretically. A concentrated polymer solution containing 5.0 wt% monodisperse polystyrene is used as the test fluid and the experiments are conducted using the filament-stretching rheometer, developed by Spiegelberg et al. The axisymmetric numerical simulations incorporate the effects of viscoelasticity, surface tension, fluid inertia and a deformable free surface. Single and multi-mode versions of the Giesekus constitutive equation are used to model the rheology of the shear-thinning test fluid. Excellent agreement between the measured transient Trouton ratio and the numerical predictions over a range of deformation rates is reported. The numerical simulations also reveal some important aspects of the fluid kinematics exhibited by weakly strain-hardening fluids during stretching—including a rapid necking of the filament diameter near the axial mid-plane of the fluid column, and an associated elastic recoil phenomenon near the rigid end-plates. This necking instability of a viscoelastic filament can be understood through a generalized Considere criterion, as recently documented by Hassager et al. As a consequence of this necking, spatial and temporal homogeneity in the extensional deformation of the filament is never achieved, even at large Hencky strains. This is in sharp contrast to the numerical and experimental studies for strongly strain-hardening dilute polymer solutions that have been reported to-date. Nonetheless, the present computational rheology study shows that filament stretching devices can still be used to accurately extract the transient extensional viscosity function for weakly strain-hardening fluids, provided that the evolution history of the tensile force at the end-plate and the filament radius at the mid-plane are carefully measured and that the experimental data are correctly processed.

Effect of pre-straining and grain size on the limit strains in sheet metal forming

Abstract: Forming processes of metal sheets are generally limited by plastic instability phenomena and flow localization. The occurrence of these phenomena is dependent on the material properties such as strain-hardening exponent, strain rate sensitivity, anisotropy parameters and grain size and is also dependent on the strain path. The formability of the sheet metals can be assessed by the forming limit diagram (FLD). In this study, a theoretical model using the ‘many slices’ approach is introduced to simulate the neck growth. The effects of changing strain path and grain sizes on the limit strains are then investigated both theoretically and experimentally. The low carbon steel ST12 and austenitic stainless steel 321 are used in the experimental approach. The theoretical and experimental FLDs of these sheets are obtained for different grain sizes and after pre-straining in uniaxial and biaxial tension parallel to the prior rolling direction. It is shown that the limit strains are quite sensitive to the gr...

Strain rate dependence of the work of fracture response of an amorphous poly(ethylene-naphthalate) (PEN) film

Abstract: The essential work of fracture (EWF) response of an amorphous film of high molecular weight poly(ethylene-2, 6-naphthalate) (PEN) was determined on tensile-loaded deeply double-edge notched (DDEN-T) specimens at various deformation rates (v = 1, 10 and 100 mm/min) at ambient temperature. The DDEN-T specimens showed full ligament yielding (marked by a load drop in the force-displacement curves) based on which the yielding was separated from the subsequent necking. The yielding related specific essential work of fracture (W e'y ) changed parallel with the yield strength (σ y ), whereas the critical crack tip opening displacement (e y ,O) remained practically constant in the v range studied. As a consequence, W e'y (v) could well be estimated by the product of σ y (v),e y ,O. Necking occurred by cold drawing superimposed by some strain-induced crystallization at the highest deformation rate. The necking-related EWF terms strongly depended on the deformation (strain) rate. Based on previous results on amorphous PENs of various molecular weight (MW), it was argued that increasing deformation rate corresponds to decreasing MW and vice versa. This was reasoned by assuming that only a part of the entangled molecular chain participates in the load distribution under high strain rate conditions.

The influence of notch on the acoustic emission generated during tensile testing of nuclear grade AISI type 304 stainless steel

Abstract: Acoustic emission (AE) generated during tensile deformation of a nuclear grade AISI type 304 stainless steel with the presence of notches has been studied. Comparative analysis of AE activity from unnotched and notched specimens with varying notch lengths, as opposed to similar results reported in the literature for beryllium and aluminium alloys, has been made. It can be inferred from the present results that: (a) prior to and during yielding, higher acoustic activity is observed in notched specimens unlike those reported for beryllium and aluminium alloys and also the total AE counts increase with increase in the initial notch length, and (b) beyond yielding, acoustic activity of both the notched and unnotched specimens reduces. These observations have been explained by considering the increased localised deformation at the notch tip prior to and during yielding and by taking into account the phenomenon of blunting at the notch tip beyond yielding. An analysis of the total AE counts (N) with respect to the stress intensity factor (K) operative at the notch tip during macroyielding shows N to bear a relationship to K as N=AKm where A and m are constants. The value of m is found to lie between 1.11 and 1.93 unlike the reported value of m≥4 for aluminium alloys. The observed acoustic emission activity just before and during tearing of the notched specimens is also noted to be higher than that observed during necking elongation of unnotched specimens.

Damage of short-fiber-reinforced metal matrix composites considering cooling and thermal cycling

Abstract: Mechanical properties and damage evolution of short-fiber-reinforced metal matrix composites (MMC) are studied under a micromechanics model accounting for the history of cooling and thermal cycling. A cohesive interface is formulated in conjunction with the Gurson-Tvergaard matrix damage model. Attention is focused on the residual stresses and damages by the thermal mismatch. Substantial stress drop in the uniaxial tensile response is found for a computational cell that experienced a cooling process. The stress drop is caused by debonding along the fiber ends. Subsequent thermal cycling lowers the debonding stress and the debonding strain. Micromechanics analysis reveals three failure modes. When the thermal histories are ignored, the cell fails by matrix damage outside the fiber ends. With the incorporation of cooing, the cell fails by fiber end debonding and the subsequent transverse matrix damage. When thermal cycling is also included, the cell fails by jagged debonding around the fiber tops followed by necking instability of matrix ligaments.

Welded aluminium alloy connections: test results and BS8118

Abstract: Heat-treatable aluminium alloys are known to suffer from severe loss of strength in the vicinity of welds. This investigation examines the structural effect of a reduced strength zone adjacent to the welds of cruciform connections and assesses its effects on the load carrying capacity, ductility and mode of failure. The results indicate that the load carrying capacity for short connection lengths are lower than values predicted with current methods of design and that the mode of failure is usually with a distinctive dish-like necking around the welded finger within the RSZ of the joint.