Showing papers on "Necking published in 1998"

Breathable elastic film/nonwoven laminate

Abstract: A breathable elastic laminate is formed by bonding a film including an elastic water vapor-soluble polymer to a neckable nonwoven web such that when the film is relaxed, the web is in a necked state. The breathable laminate is stretchable in a direction parallel to the narrowing or necking of the web. The laminate possesses excellent water vapor permeability but acts as a barrier to the passage of odor-causing chemicals including ammonia.

Material Instabilities in Solids

Abstract: Introduction to Material Instabilities in Solids (E. van der Giessen & R. de Borst). Stability of Layered Geological Structures: An Asymptotic Solution (Y. Leroy & N. Triantafyllidis). Analysis of Shear Failure in Concrete Materials (K. William, et al.). On the Effects of Inertial Coupling on the Wave-Speeds of Elastic-Plastic Fluid-Saturated Porous Media (B. Loret & E. Rizzi). Microstructured Solids: Non-Linear Model and Analysis of Magneto-Elastic Wave Processes (V. Erofeyev & S. Kovalev). Thermodynamics of Crystal Viscoplasticity and Instability Phenomena (P. Perzyna). Instability Issues in Single Crystal Plasticity (P. Steinmann). On the Use of Strain-Softening Models for the Simulation of Strong Discontinuities in Solids (J. Oliver, et al.). Thermomechanics Based Theory and Analysis of Cracking Localization in Concrete Dam under Earthquake Excitation (H. Horii, et al.). Localisation Patterns in Ductile and Brittle Geomaterials (J. Desrues).In-Plane Crushing of a Polymeric Honeycomb (S. Papka & S. Kyriakides). Experimental and Numerical Investigation of Size Scale Effects in Concrete Fracture (M. van Vliet & J. van Mier). Post-Peak Behaviour of Rocks and Natural Building Stones in Uniaxial Compression (I. Vardoulakis, et al.). Inelastic Deformation of F.C.C. Single Crystals by Slip and Twinning (A. Staroselsky & L. Anand). Effects of Strain Paths on Sheet Metal Limit Strains (P. Wu, et al.). Three-Dimensional Analysis of Localized Necking (A. Benallal). Effects of Heterogeneities and Localization on Polymer Deformation and Recovery (M. Boyce & C. Chui). Strain Localization and Void Growth in Polymers (A. Steenbrink & E. van der Giessen). Nonaffine Network Model for Glassy Polymer and Prediction of Instability Propagation (Y. Tomita & T. Adachi). Damage Localisation in Short Fibre Cementitious Composites (B. Karihaloo & J. Wang). Fracture Instabilities in Heterogeneous Materials: Brittleness, Size Effects and Fractality (A. Carpinteri). Compression Fracture-Mechanics of Damage Localization and Size Effect (Z. Bazant). Nonlinear Modelling of Geomaterials and Self-Organization Phenomena (V. Nikolaevskiy). Quasi-Static and Dynamic Characteristics of Strain Gradient Dependent Non-Local Constitutive Models (F. Oka, et al.). Higher-Order Damage Models for the Analysis of Fracture in Quasi-Brittle Materials (M. Geers, et al.). On Gradient Regularization for Numerical Analyses in the Presence of Damage (C. Comi & L. Driemeier). Localisation of Damage in Quasi-Brittle Materials and Influence of Chemically Activated Damage (G. Pijaudier-Cabot, et al.). Nonlocal Damage Effects on Plastic Flow Localization under Dynamic Loading (V. Tvergaard & A. Needleman). Instabilities and Size Effects in Plasticity: Continuum and Dislocation Approaches (H. Zbib). Macroscopic Modelling of Stationary and Propagative Instabilities (L. Sluys & W. Wang). A Discussion of Strain Gradient Plasticity Theories and Application to Shear Bands (N. Fleck & J. Hutchinson). A Beam Theory for Gradient Continua (H.-B. Muhlhaus & P. Hornby). Recent Progress on Gradient Theory and Applications (E. Aifantis). Index.

Extensional deformation, stress relaxation and necking failure of viscoelastic filaments

Abstract: We investigate the transient viscoelastic behavior of weakly strain-hardening fluids in filament stretching devices during uniaxial elongation and following the cessation of stretching. The numerical results are compared with experimental observations on a concentrated shear-thinning polystyrene solution which is well characterized by a multi-mode Giesekus model. The finite element computations incorporate the effects of viscoelasticity, surface tension, and fluid inertia and the time-dependent moving-boundary problem is solved using the code POLYFLOW. A detailed comparison of multi-mode computations with single-mode solution is presented in order to examine the differences in the predicted viscoelastic behavior and the role of the fluid relaxation spectrum. The evolution in the transient Trouton ratio at different deformation rates is compared with experimental measurements and with the theoretical predictions of ideal homogeneous uniaxial elongation. Simulations of the filament stretching device using the multi-mode viscoelastic model demonstrate a significant improvement in the agreement between the predicted and observed extensional viscosity at short times. The computed Trouton ratio is also in good agreement with theoretical expectations for ideal homogeneous uniaxial extension, despite the strongly nonhomogeneous viscoelastic necking of the fluid column observed during elongation in the filament stretching device. Following the cessation of elongation, numerical simulations predict an interesting and complex evolution in the kinematics of the fluid filament. Initially the tensile stresses in the column relax in the non-linear form predicted theoretically, indicating that filament stretching devices can be used to monitor transient extensional stress relaxation, provided that the evolution of the tensile force at the end-plate and the filament radius at the mid-plane are carefully measured. However, at longer times after cessation of stretching, the local extension rate at the axial mid-plane begins to increase rapidly, leading to a 'necking failure' that is greatly accelerated compared to that expected in a corresponding Newtonian filament. The calculations show that this unstable necking is not driven solely by the surface tension but also by the viscoelasticity of the fluid, and is coupled with significant elastic recoil of the material near the end-plates. The rate of necking in the column is a sensitive function of the extensional viscosity predicted by the constitutive model, in particular the magnitude and the rate of strain-hardening that occurs during uniaxial elongation. This phenomenon can also be simply and accurately described by an appropriate set of coupled one-dimensional thin filament equations that use the finite element computations to provide a suitable initial condition for the axial distribution of the polymeric stresses in the filament. (C) 1998 Elsevier Science B.V. All rights reserved.

Rapid conversion of elastic energy into plastic shear heating during incipient necking of the lithosphere

Abstract: An important and novel mechanism for ductile failure of the lithosphere is identified here, which is intrinsic to the thermal-mechanical feedback in a temperature dependent plastic body with coupled elastic fields. Both a temperature-dependent power-law visco-elasto-plastic rheology and a temperature-dependent elasto-plastic rheology are employed to study in a self-consistent fashion the deformation of the lithosphere subject to extension by means of a two-dimensional, finite-element code. A structural perturbation initially localizes elasto-plastic deformation only in its immediate vicinity. However, after 800,000 years have elapsed the localized zone of deformation takes off in a ‘crack-like’ fashion and travels to the bottom of the lithosphere in about 50,000 years time. When the plate is severed, thermal runaway is caused by mechanical heating triggered by the rapid energy transfer of the globally stored elastic energy into localized plastic dissipation in the ductile fault.

Crystal plasticity forming limit diagram analysis of rolled aluminum sheets

Abstract: Numerical simulations of forming limit diagrams (FLDs) are performed based on a rate-sensitive polycrystal plasticity model together with the Marciniak-Kuczynski (M-K) approach Sheet necking is initiated from an initial imperfection in terms of a narrow band The deformations inside and outside the band are assumed to be homogeneous, and conditions of compatibility and equilibrium are enforced across the band interfaces Thus, the polycrystal model needs only to be applied to two polycrystalline aggregates, one inside and one outside the band Each grain is modeled as an fcc crystal with 12 distinct slip systems The response of an aggregate comprised of many grains is based on an elastic-viscoplastic Taylor-type polycrystal model With this formulation, the effects of initial imperfection intensity and orientation, initial distribution of grain orientations, crystal elasticity, strain-rate sensitivity, single slip hardening, and latent hardening on the FLD can be assessed The predicted FLDs are compared with experimental data for the following rolled aluminum alloy sheets: AA5754-0-A, AA5754-0-B, AA6111-T4-A, AA6111-T4-C, and AA6111-T4-D

A view on ductile‐fracture modelling

Abstract: Theoretical models of ductile fracture are reviewed in terms of experimental results from metallurgical studies of ductile fracture in metals and alloys. It is shown that the plastic limit-load model, which is based on a criterion of void coalescence by internal microscopic necking of the intervoid matrix, is fully consistent with scanning electron microscope (SEM) observations of both the ductile-fracture surface and the microstructure immediately adjacent to the fracture surface. On the other hand, the dilational-plastic models of ductile fracture, which are based on the dilational-growth of spherical voids to some arbitrary critical void-volume fraction, are inconsistent with the microstructural observations of ductile fracture. This inconsistency between the dilational-plastic models and experimental results is shown to be the combined effect of neglecting the controlling influence of extensional void-growth and the failure to incorporate a physically realistic criterion of void coalescence. The problems of modelling the ductile crack-growth process by both analytical and numerical (finite element) studies, where problems of uniqueness of the plastic velocity field may occur, are also considered. The limitations of the finite-element method in modelling void-coalescence problems, where the equations of plasticity are of second-order hyperbolic form, are also discussed.

Rift processes at the Valles Marineris, Mars: Constraints from gravity on necking and rate-dependent strength evolution

Abstract: Recent spherical harmonic representations of the gravity field of Mars have sufficient resolution to examine the regional crustal and lithospheric structure of the Valles Marineris and are used as constraints for rift modeling. While gravity signatures of individual troughs are not evident, a broad 190–260 mgal low indicates that the central chasmata as a whole are compensated at a depth of 30–80 km, representing the thickness of crust surrounding the troughs. Furthermore, at the time large-scale relief was established, the effective thickness of the elastic lithosphere was 20 m W m−2. The calculated range of crustal thickness and heat flow can be used to test two semianalytic models of rift formation. In the first model, the distinction between single (“narrow rift”) and multiple (“wide rift”) troughs is determined by the wavelength of necking instabilities. In the second model, narrow-versus-wide morphology is controlled by the evolution of lithospheric strength during rifting. The large-scale, parallel, multiple troughs of the central Valles Marineris are morphologically similar to terrestrial wide rifts, but a lack of distinct faulting in some of the chasmata could imply that the faulted troughs may have formed as isolated narrow rifts. Our results are only marginally consistent with necking leading to a wide rift; allowable combinations of crustal thickness and heat flow lead to decoupling within the lithosphere resulting in a second principal necking wavelength that is smaller than the main trough spacing, for which evidence is equivocal. At high heat flow (>40 m W m−2), however, necking of the strong upper crust alone can yield a shorter wavelength characteristic of a single trough, allowing narrow-rift origins of faulted chasmata. In contrast, the inferred range of crustal thickness and heat flow poorly match the narrow-rift regime of the strength-evolution model but are in good agreement with its predictions for wide rifting. Furthermore, the distinction between wide rift and core complex in this model places an upper bound on heat flow of 70 m W m−2.

Behavior and rupture of hydrided ZIRCALOY-4 tubes and sheets

Abstract: The mechanical behavior and rupture mechanisms of ZIRCALOY-4 guide tubes and sheet containing 150 to 1200 wt ppm hydrogen have been investigated at room temperature. Sheets were notched to study the influence of geometrical defects on rupture. It is shown that hydrides strengthened the material, as maximum stresses sustained by the material are increased with increasing hydrogen contents. On the other hand, ductility is reduced. The material also exhibits a strong anisotropy due to its pronounced texture. Metallographic examinations have shown that damage by hydride cracking is a continuous process that starts after the onset of necking. Notches reduce ductility. A modified Gurson-Tvergaard model was used to represent the material behavior and rupture. Numerical simulation using the finite element method demonstrates the strong influence of plastic anisotropy on the behavior of structures and rupture modes.

The role of extensional instability in creating Ganymede grooved terrain: Insights from Galileo High‐Resolution Stereo Imaging

Abstract: Galileo stereo images covering about 1500 km² of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large-scale ridges and troughs spaced ∼6 km apart, corresponding to the “grooves” seen in Voyager images, and (2) small-scale ridges spaced hundreds of meters apart superimposed on the large-scale ridges We interpret the small-scale ridges to be the result of tilt-block normal faulting of the surface brittle layer, while the large-scale ridges may be due to necking of the brittle layer over a ductile substrate The geometry of the tilt blocks revealed in Galileo images leads to a minimum estimation of 51% to 58% extensional strain in the area The strain estimate, when incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension, leads us to estimate a thermal gradient of ∼20 K/km and a strain rate of ∼10−14 s−1 during groove formation

A model for the necking phenomenon in high-speed fiber spinning based on flow-induced crystallization

Abstract: In this work we investigate the use of an inhomogeneous structural model which explicitly takes into account flow-induced crystallization for representing the necking phenomenon in high-speed fiber spinning. For simplicity, we have considered a one-dimensional (cross-section averaged) approximation for an isothermal system with no surface tension and air drag, with or without inertia. Flory’s approach [J. Chem. Phys. 15, 397–408, (1947)] is used to predict the onset of crystallization in the spinline. After the onset of crystallization, the fiber is modeled as an inhomogeneous medium with two separate (meso) phases—one semi-crystalline and the other amorphous. The amorphous phase, before and after the onset of crystallization, is modeled as a viscoelastic fluid, represented here by the extended White–Metzner model. The semi-crystalline phase is modeled as an anelastic solid. We demonstrate neck formation for a variety of processing conditions and material property values consistent with those encountered in practice. In particular, the addition of inertial effects, which can also be important in high-speed fiber spinning, shifts but does not eliminate the window in parameter space over which the inertialess model predicts neck formation. Based on these results, we propose as a mechanism for the neck formation the structural changes within the material induced by the crystallization and the ability of the semi-crystalline phase to rapidly take up high stresses.

Method and punch for necking cans

Abstract: A method is described for reducing the diameter of the open end of a can, such as a beverage can, in a necking station while substantially preventing the formation of pleats in the can. The necking station includes a deformable support punch that is positioned within the open end of the can. The punch includes an elastomeric sleeve and a means for providing for lateral deformation of the sleeve, such as an actuator making an interference fit with the sleeve. In the necking station, the can is inserted into a necking die having a transition zone separating an outer cylindrical bore and an inner bore having a reduced diameter. When the top edge of the can is forced past the transition zone in to the inner bore to reduce the dimension of the upper portion of the can, the sleeve is controllably deformed in a manner such that the lateral portion of the sleeve is placed into supporting engagement with the interior wall of the can, pressing the can against the transition zone of the die. This supporting action of the elastomeric material against the can wall during the reduction in diameter substantially avoids the formation of localized pleats.