Showing papers on "Necking published in 1993"

Bifurcation and stability of dissipative systems

Abstract: The first theme concerns the plastic buckling of structures in the spirit of Hill's classical approach. Non-bifurcation and stability criteria are introduced and post-bifurcation analysis performed by asymptotic development method in relation with Hutchinson's work. Some recent results on the generalized standard model are given and their connection to Hill's general formulation is presented. Instability phenomena of inelastic flow processes such as strain localization and necking are discussed. The second theme concerns stability and bifurcation problems in internally damaged or cracked colids. In brittle fracture or brittle damage, the evolution law of crack lengths or damage parameters is time-independent like in plasticity and leads to a similar mathematical description of the quasi-static evolution. Stability and non-bifurcation criteria in the sense of Hill can be again obtained from the discussion of the rate response.

Method and apparatus for performing multiple necking operations on a container body

Abstract: Method and apparatus for performing multiple necking operations, such as by utilizing a plurality of die necking stations. In one aspect of the present invention a plurality of venting port(s) are incorporated on a necking assembly which performs a necking operation on a necked container body. In another aspect of the present invention the container body is centered with respect to a necking assembly which produces a double-neck container body configuration.

“Necking” in high-speed spinning revisited

Abstract: The conditions required for “necking” in melt-spinning are discussed. The adopted criterion defines necking as a filament-radius profile R(x) exhibiting two inflexion points (R = 0 ), one characterized by a negative, the other one by a positive, third gradient R'. Analysis of the dynamics of spinning of a non-isothermal corotational Maxwell fluid leads to the conclusion that the conditions of necking require either long relaxation times (Deborah number > 1 2 ), or a large, positive tension gradient F possibly controlled by inertia. In inertialess Newtonian filaments, the necking condition can also be satisfied by local reduction of viscosity. The above three mechanisms of necking, viscoelastic, inertial, and viscous, are analyzed and discussed.

Ductile fracture in sheets under transverse strain gradients

Abstract: The fully plastic fracture of a metal sheet subjected to a small transverse gradient of tensile strain near a reinforcement is modeled as mode I fracture under transverse plane strain (TPS). Necking and fracture were analyzed by assuming that they were set by prior uniform strains and then necking displacements. Equations for the spreading of TPS necking and fracture were thus derived for a sheet with strain gradient. Experiments on tapered specimens confirmed the expected fracture displacements within 12 percent, but Moire studies suggest the agreement may be fortuitous. In any event, in-plane transverse displacements and normal strains in the crack growth direction, as well as shear strains, were negligible. This should simplify any future numerical analysis.

Structural basis of high-strength high-modulus polymers

Abstract: Various ways of producing high-strength and high-modulus flexible-chain polymers are compared. Multistage zone drawing is considered as a promising technique, whose potentialities are still great. It is shown that the mechanical properties of end products drastically depend on many structural parameters of starting material, such as the molecular weight and molecular weight distribution, the morphology, the length and regularity of the folds, the number and type of defects, as well as on the temperature-rate conditions of the drawing. It is emphasized that the transformation of the initial structure into a microfibrillar one on necking occurs through the unfolding of macromolecules and subsequent stress-induced crystallization. With account of the kinetic theory of strength and of the influence of initial morphology on the strengthening and mechanical destruction that compete during the drawing, one can choose an appropriate morphology of the starting material and an optimum drawing regime. The fine structure of ultradrawn polymers is investigated in detail. The structural basis for the discrepancy in experimentally achieved mechanical properties and theoretical estimates are discussed and further improvement of polymer mechanical behavior is considered.

Necking in plane strain under bending with constant tension

Abstract: A s part of a study of failure of plates at built-in supports, modes of plane strain failure under increasing bending with constant tension are calculated for an elastic, power-law plastic material with kinematic hardening. Under increasing bending, the moment reaches a maximum when the bending curvature (normalized by the initial plate thickness) is approximately twice the strain-hardening exponent. At the maximum moment, different boundary conditions result in different failures. Under applied bending moment, rotational instability occurs. Under applied bending angle, stable localized kinking occurs. Kinking leads to high curvatures and then tensile necking. Large further applied bending curvature gives tensile necking. For example, with a strain-hardening exponent of n = 0.1 and a constant tensile force of 90% of that for necking under pure tension, a bending curvature of 0.865 (normalized by the initial plate thickness) is required for tensile instability. For most practical cases (that is, for the radius of curvature greater than the order of thickness), bending decreases the tensile necking force by 7–17% for a strain-hardening exponent of n = 0.2–0.05. Reversal before reaching the maximum moment requires almost the same curvature change to reach a maximum moment as for monotonic loading.

Mechanical properties of blends of nylon with chemically modified ABS

Abstract: Tensile, flexural and impact properties were measured of a heterogeneous polymer blend system, consisting of nylon 6 and a chemically modified ABS (MABS). It was found from the tensile tests that nylon-richer blends show yielding behaviour and nylon-leaner blends show necking behaviour. The addition of MABS increases the modulus, whereas the tensile strength and percentage elongation at break decrease and go through a minimum. The impact strength is increased to a maximum of approximately three times when 20 wt% of MABS is added to nylon 6. In order to interpolate the mechanical properties observed, empirical equations are given which are found to describe the experimental data rather well. Photomicrographs were taken of the tensile fractured surfaces using a scanning electron microscope.

Ductile fracture before localized necking in a strip under tension

Abstract: Unexpected cracking with a 22° opening angle grew out of a rough-sheared edge before appreciable necking in an 0.79 by 37 mm mild steel strip, which normally fractures after diffuse and then localized oblique necking. From the crack tip, two shear bands formed at 55° to the load direction, consistent with isotropic plane stress characteristics (53° was predicted from anisotropy, but necking in thin strips occurred at 67°). Photomicrographs showed that the 22° crack growth occurred by first tunnelling at mid-thickness, and then spreading along through-thickness shear planes. Springback on unloading caused a 0.038 mm crack closure and local buckling. This form of cracking illustrates a size effect in fracture under macroscopically plane stress. It also gives an example of a local mechanism triggering a fracture mode that can require more total work than an alternative.

The molecular orientation and mechanical properties of poly(ethylene terephthalate) under uniaxial extension

Abstract: The molecular orientation, induced crystallization, and mechanical properties of amorphous poly (ethylene terephthalate) (PET), stretched at different temperatures and strain rates, were studied. The results indicate that the orientation parameter [P 2 (cos θ)] of oriented PET varies strongly with both temperature and strain rate, but the strain-induced crystallinity seems dependent on temperature only. It was also found that as draw ratio increased, the Young's modulus, yield stress, and necking ratio were greatly improved in the stretching direction, but slightly declined in the transverse direction. A rubber network model was used to make a quantitative description of the orientation development with draw ratio

Device for flaring out pipes

Abstract: The device for necking pipes forces the pipe wall outward in such a way that a neck is formed around the hole. The device has a body to be inserted into the interior of the pipe and a die which is movable radially to the pipe, the outer diameter of the die corresponding to the inner diameter of the desired neck. The cylindrical outer surface of the body is dimensioned, at least in the vicinity of the necking location, in such a way that it corresponds to the cylindrical interior of the pipe so as to support the pipe from the inside during the necking operation. A bore hole or opening which is directed at least substantially radially is located at the body in the region of the cylindrical surface and is dimensioned in such a way that it receives the die or its lower part with a sliding guiding fit. The die can be moved in the guide through the bore hole or opening in the direction of the latter. An elongated wedge is displaceable in the axial direction in the interior of the body and its surface and/or groove forms a flat wedge angle with the axial direction. The lower end of the die moves in a sliding manner so as to contact said wedge surface and/or groove.

Pressure-sensitive adhesive sheet for surface protection

Abstract: PURPOSE: To obtain the subject sheet which hardly suffers a phenomenon such as necking when stretched and which, even when a metal plate having the sheet stuck to the surface is subjected to, e.g. drawing, can quickly follow the deformation of the metal plate so that it is hardly broken and scarcely mars the surface of the metal plate. CONSTITUTION: This adhesive sheet is made by forming a pressure-sensitive adhesive layer on one side of a film base. The film base contains a polypropylene and at least 40wt.% olefin elastomer consisting mainly of ethylene and propylene and has a modulus in tension of 3,000-7,000kg/cm 2 with no yield point and a recovery from deformation under a tensile load of at most 70%. COPYRIGHT: (C)1994,JPO&Japio

Analysis of mechanical behavior in cold‐drawing deformation of polymers

Abstract: When stretched under uniaxial stress, ductile polymers usually exhibit unstable plastic deformation, which embodies two phases: (a) yielding with the formation of a neck and (b) cold-drawing with the propagation of necking shoulders. The mechanical state associated with this deformation behavior is analyzed. A general treatment of the constitutive function of flow stress in the plastic state is presented, in which a series of relations among various characterizing parameters are formulated. Three mechanical criteria for necking deformation and propagation of necking shoulders are then provided. Lastly, a mathematical model is described which can be used in calculations to fit the contour of the necking shoulder

Quantum calculation of giant magnetoresistance in layered magnetic films.

Abstract: We compute the magnetoresistance of a step-potential model of an FeCr multilayer. We use a Boltzmann-equation method incorporating exact quantum-mechanical wave functions obtained by a transfer-matrix method. The only parameters required are the (spin-dependent) potentials and effective masses of the bulk materials, and the bulk and surface densities of point scatterers. Oscillatory magnetoresistances of the order of several percent are obtained without spin-dependent scattering, due to Fermi-surface necking effects.

Temperature-Induced transition

Abstract: A nitrogen-strengthened austenitic stainless steel was tested in uniaxial tension at room temperature (295 K) and in liquid nitrogen (76 K). A transition in ductile fracture appearance from a cup-cone fracture at room temperature to shear fracture at cryogenic temperature is observed and correlated to deformation behavior and micromechanisms (void nucleation and strain localization) of fracture. The flow stresses, fracture stresses, and strain hardening rates are all higher at liquid nitrogen temperature compared to those at room temperature, and the significant increases in plastic flow stresses are accompanied by planar deformation mechanisms. At both temperatures, primary void nucleation is observed mainly at scattered, large patches of sigma phase, and initial primary void growth is associated with tensile instability (necking) in the specimen. Postuniform elongation at 295 K leads to secondary void nucleation from small, less than 1 μm in diameter, microalloy particles, leading directly to failure; the strain required for secondary void growth and coalescence is highly localized and does not contribute to macroscopic elongation. At 76 K, uniform strain increases, total strain decreases, and strain localization into shear bands between the primary voids and the surface of the neck leads directly to failure. Secondary void nucleation, growth, and coalescence are limited to shear bands and also do not contribute to the macroscopic elongation. The observations of void nucleation are characterized in terms of a continuum analysis for the interfacial stress at voidnucleating particles. The critical interfacial stress for void nucleation at the lower temperature correlates with the increased flow properties of the matrix.

Necking in non-isothermal high-speed spinning with radial viscosity variation

Abstract: This paper is a continuation and extension of previous work by Ziabicki and Tian (J. Non-Newtonian Fluid Mech., 47 (1993) 57-75) on the necking phenomenon in non-isothermal high-speed melt spinning. The concept of flows with dominating extension as well as the possibility of radial viscosity variation, caused by temperature and orientation effects, have been taken into account. Under the assumption of the quasi-elongational approximation, various mechanisms of necking are discussed in greater detail. These are viscous, viscoelastic, inertial and, in particular, the radial viscosity mechanism. It is shown that, in general, any radial viscosity distribution across the filament intensifies the inertial mechanism. For sufficiently high radial viscosity variations a local reduction in axial velocity profiles is not necessary for the onset of necking.

Characteristics of shear banding in dual phase steel

Abstract: The evolution process of shear banding in a ferrite–martensite dual phase steel has been investigated via in situ tensile testing in a scanning electron microscope. Shear band type deformation localisation occurs at the maximum loading point of uniaxial tension. Necking occurs simultaneously and locally. Voids nucleate in ferrite domains and on the interfaces between the two component phases or grain boundaries. The void volume fraction is greater within the shear band than away from the band and is greater in the interior than at the specimen surface. The results also show that void damage promotes the initiation of shear bands and the development of shear banding stimulates further void damage.MST/1819

Fracture of Brittle and Quasi-Brittle Engineering Materials

Abstract: The study of fracture of engineering materials involves a number of science and engineering disciplines. Continuum fracture mechanics is deeply rooted in the problem of fracture because it treats the relationship between a crack or inhomogeneity and the stress state in a material. Physics and chemistry are important because they help to explain the reactions between the environment and the crack tip. Finally, materials science is essential in understanding the relations among bond rupture, structure, processing and performance of a material. Fracture of materials can be divided into two broad categories - ductile and brittle fracture. Ductile fracture is associated with appreciable plastic deformation. "Cup and cone" fracture demonstrated in metals due to tensile overload is a classic example of ductile fracture. The different stages of such fracture are shown in Figure l.l(a). At the maximum load, plastic deformation is concentrated in a small gage length of the specimen and necking begins. Once this necked region has formed, fracture begins at the center of the specimen and extends along the dashed lines, finally producing the familiar cup and cone fracture. In fcc metals, plastic deformation continues on the conjugate slip planes until the specimen has necked down to a sharp point. Polycrystalline metals with second phase particles fail due to initiation, growth and coalescence of micro-voids formed in the necked region. Similarly, semicrystalline polymers exhibit necking which leads to localized strengthening of the specimen. Then the specimen elongates due to the propagation of this neck along the gage length. This ductile fracture in polymers is different from that in metals, in which all subsequent deformation is confined to the neck region. Alternatively, brittle fracture is associated with little or no deformation. A brittle material behaves elastically up to the maximum load at which catastrophic failure occurs (Fig. 1.1(b )). Silicate glasses are the most common example of such fracture. Brittle fracture is controlled by microscopic inclusions, surface and interior flaws and defects and pores present in the material. An intermediate category of fracture, known as quasi-brittle fracture, has recently been defined. A quasi-brittle material, a title which encompasses many polycrystalline ceramics and cementitious materials, shows measurable deformation prior to failure. The deformation, however, is not associated with dislocation motion. At the onset of nonlinearity in the load-displacement relationship existing flaws in the material start growing and new flaws form (Fig. 1.1(c)). Such materials are characterized by a softening curve after the peak load. This softening branch of the load-displacement relationship is associated with stable crack growth in the material before the final fracture. The failed specimen, however, need not look any different from a classically-brittle failed specimen. In a review article such as this, it is useful to first build the necessary background, and various aspects of fracture mechanics are reviewed in the next section. This review is followed by discussions of the microstructural aspects of toughening mechanisms, short crack fracture in comparison to long crack fracture, the statistical nature of fracture and its implications for quasi-brittle materials, and slow crack growth and other environmental effects on various fracture processes are presented. The implications to cementitious materials are discussed throughout.

Tool for neck-in work of metallic can

Abstract: PURPOSE: To provide a tool for neck-in work of a metallic can, by which the development of fold at the end part of the metallic can can be restrained and the neck-in work can be applied to the further thinned metallic can. CONSTITUTION: In the inner side of a necking die 1, a necking part 12 shrinking the diameter in order along the inserting direction of the metallic can, a parallel part 13 connected to this necking part 12 and parallel-arranged to the center axis and a renecking part 14 connected to this parallel part 13 and shrinking the diameter in order along the inserting direction are provided. Further, in a columnar core 2 arranged in the inside of this necking die 1, a parallel part 23 and a renecking part 24 are arranged so as to correspond to the parallel part 13 and the renecking part 14 of the necking die 1. A projecting length L to the parallel plane to the center axis of the renecking part 14 is set to 5-10 times of the thickness at the end part of the metallic can to be worked and a projecting length (h) to the plane crossed at the right angle to the center axis is set to 0.5-1.0 times of the thickness. COPYRIGHT: (C)1995,JPO