Showing papers on "Necking published in 1980"

Void Nucleation Effects in Biaxially Stretched Sheets

Abstract: The effects of void nucleation occurring during the deformation history on forming limit curves are considered for both in-plane and punch stretching employing a constitutive model of a porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated by a two parameter void nucleation criterion. For in-plane stretching, plastic strain controlled nucleation can have, in certain circumstances, a significantly destabilizing effect on the forming limit curve. However, within the framework of plane stress theory which neglects the enhance­ ment of the hydrostatic stress due to necking, a stress controlled nucleation process is not found to be significantly destabilizing. In punch stretching a ductile rupture criterion, which limits the maximum volume fraction of voids, as well as the ap­ pearance of a well defined thickness trough, is adopted as a localized necking criterion. Only plastic strain controlled void nucleation is considered here in out-ofplane stretching. The resulting forming limit curves have the same shape as those obtained previously with void nucleation neglected.

Rubber-Modified Epoxies: Morphology and Mechanical Properties.

Abstract: : A wide spectrum in morphology paralleling a range of phase-separated and dissolved rubber, can be developed in rubber-modified epoxies through control of rubber-epoxy compatibility and cure conditions. These morphologies result in different stress response mechanisms. Dissolved rubber promotes plastic deformation and necking at low strain rates that provide large increases in the elongation to break. Dissolved rubber is ineffective in providing improvement at impact rates. Phase-separated rubber domains can also increase the elongation to break since they promote cavitation at the interfacial boundary. The elongation is limited to the extent of cavitation and therefore large increases in the energy to break are not found. The presence of rubber domains was found to be a necessary but not sufficient condition for impact energy improvement. Optimum materials contained a relatively large amount of dissolved rubber and a low volume fraction of phase separated rubber. These combine high elongations at low strain rates with improved impact properties. (Author)

Effects of pre-strain on plane stress ductile fracture in α-brass

Abstract: The effects of pre-strain on plane stress ductile fracture in a 70/30 alpha brass Austral 207 have been studied using the deep-edge-notched tension (DENT) specimens. The amount of pre-strain varies between 5 and 35%. It is found that both the specific essential work of fracture (w e) and the critical crack opening displacement (δc) decrease with increasing pre-strain. A simple theory for estimating the specific essential work of fracture in the presence of pre-strain is suggested and it gives good agreement with experimental results. Elongations to fracture in the DENT specimens are also predictable from a simple deformation analysis which considers the plastic elongations due to crack initiation, crack propagation and final stretch of a ligament that has reached a necking strain equal to that in a simple plain tension test. Micro-hardness measurements show that the strain localization is more intense near the fracture surface as the pre-strain level is increased and this is suggested to be an explanation for the low δc values obtained in pre-strained specimens.

In situ observation of the spherulite deformation in polybutene-1 (Modification I)

Abstract: Stress-strain behaviour under uniaxial tension was determined for polybutene-1 specimens with various spherulite sizes and degrees of crystallinity. Samples with large spherulites and a high crystallinity level exhibited a very specific behaviour, characterized by a higher elastic modulus, a remarkably homogeneous deformation (absence of necking), a stress whitening phenomenon and a brittle intraspherulitic rupture. The deformation mechanisms have been analysed by means of in situ microscopic observations. It was shown that interlamellar separation, which occurred first in the equatorial regions perpendicular to the tensile axis, is mainly responsible for the observed phenomena.

Study of the thermal effects in the necking of polymers with the use of an infrared camera

Abstract: When a polymer specimen yields under tension by the process of cold-drawing, heat is generated in the necking region. This beating effect has been studied in polycarbonate, polyethylene, polypropylene, and PVC with the use of a thermovision system which produces a real time thermal image of the necking specimen. In this way it was possible to observe the temperature distribution over the surface of the neck and to show that at least 85% of the mechanical work done on the specimen during necking was converted into heat. A semiempirical equation relating the maximum temperature to the drawing rate has been derived.

The effect of chain structure on the annealing and deformation behaviour of polymers

Abstract: In a tensile test ductile thermoplastics may give either uniform deformation or necking. Recently it has been found that those giving uniform deformation either, are known to have extended chain configurations in solution, or have chemical formulae with linear structures and relatively few flexible bonds. The observed differences in behaviour can be predicted from a viscosity retarded rubber elasticity model in which a constant related to chain flexibility is introduced, which quantifies the strain hardening of the material. It is argued that the early development of strain hardening generally interferes with the localization of plastic strain in shear bands or crazes (as well as in a neck) and correlates with the stress cracking performance of high density polythenes.

Superplasticity: Mechanical and Structural Aspects, Environmental Effects, Fundamentals and Applications

Abstract: 1. Historical Introduction.- 1.1 First Observations of Structural Superplasticity.- 1.2 Early Observations of Environmental Superplasticity.- 1.3 The Revival of Interest in Structural Superplasticity.- 2. The Mechanics of Superplastic Deformation and the Assessment of Superplastic Behaviour.- 2.1 The Tensile Test.- 2.2 Constitutive Relations for Superplastic Materials.- 2.3 The Deformation Behaviour of Superplastic Materials.- 2.3.1 The tensile stress-strain curve.- 2.3.2 Tensile stress-strain rate variations.- 2.3.3 Compressive behaviour.- 2.3.4 Torsion testing.- 2.4 The Shape of the Deforming Specimen.- 2.5 Plastic Instability.- 2.5.1 The onset of necking.- 2.5.2 The geometry of neck formation.- 2.6 The Elongations of Rate-Sensitive Materials.- 2.7 The Strain-rate Sensitivity Index, m.- 2.7.1 Determination of m from the $${\sigma _t} - {\dot \varepsilon _t}$$ curve.- 2.7.2 Determination of m using change in strainrate method.- 2.7.3 Determination of m from stress-relaxation tests.- 2.7.4 Comparison of m values determined using different techniques.- 2.7.5 The physical significance of m value measurements.- 2.8 Strain-rate Sensitivity in Environmentally Superplastic Materials.- 2.9 Other Test Procedures for Studying Structural Superplasticity.- 2.9.1 Indentation tests.- 2.9.2 Measurement of internal stresses.- 2.9.3 Measurement of grain boundary shear.- 2.9.4 Damping characteristics.- 2.9.5 Miscellaneous tests.- 3 Structural Superplasticity - Experimental.- 3.1 The Mechanical Properties.- 3.1.1 Characteristics of deformation.- 3.1.2 Necessary conditions.- 3.1.3 Variables of deformation.- (a) Strain-rate.- (b) Strain.- (c) Strain-rate sensitivity index.- (d) Temperature.- 3.1.4 The influence of prior treatments and compositional variations.- 3.1.5 Elongation.- 3.1.6 Macroscopic characteristics: an assessment.- 3.2 Microstructural Studies.- 3.2.1 Qualitative metallography.- (a) Surface studies.- (b) Internal structure.- (c) P.Iarker experiments.- (d) Dynamic experiments.- 3.2.2 Quantitative metallography.- (a) Grain boundary sliding.- (b) Measurements of grain size and shape.- (c) Analysis of texture.- 3.2.3 Assessment of metallographic and topological features.- 3.3 Fracture.- 3.3.1 Macroscopic aspects.- 3.3.2 Microscopic aspects.- 3.3.3 Origin of cavitation.- (a) Phenomenological.- (b) Microstructural.- 4. Structural Superplasticity - Theoretical.- 4.1 Early Theories.- 4.2 Modern Theories.- 4.2.1 Rheological theories.- 4.2.2 Activation energy for superplastic flow.- 4.2.3 Atomistic theories.- (a) Diffusional flow mechanisms.- (b) Dislocation creep theories.- (c) Grain boundary deformation models.- (d) Multi-mechanisms.- 5. Environmental Superplasticity.- 5.1 Experimental Results.- 5.1.1 Macroscopic characteristics.- (a) Transformational plasticity.- (b) Temperature cycling.- (c) Neutron irradiation.- 5.1.2 Metallographic features.- 5.2 Theoretical Studies.- 5.2.1 Phenomenological approaches.- 5.2.2 Mechanistic models.- 5.2.3 Cavitation.- 6. Applications of Superplasticity.- 6.1 Forming Operations.- 6.1.1 Hydraulic bulging.- 6.1.2 Sheet thermoforming.- 6.1.3 Blow moulding.- 6.1.4 Deep drawing.- 6.1.5 Punch stretching.- 6.1.6. Forging and stamping.- 6.1.7 Extrusion.- 6.1.8 Dieless drawing.- 6.1.9 Powder metallurgy processes.- 6.2 Theoretical Analyses of Superplastic Forming Processes.- 6.2.1 Analysis of hydraulic bulging.- 6.2.2 Analysis of bulging into a 90 V-groove.- 6.2.3 Analysis of extrusion.- 6.2.4 Regression analyses.- 6.3. Practical Developments.- 6.3.1 Alloys.- 6.3.2 Design considerations.- 6.3.3 Tooling.- References.- Author Index.

Flow and fracture of a Ni-Fe metallic glass

Abstract: When brittle failure modes are bypassed, sheet tensile specimens of Ni49Fe29P14B6Si2 glass exhibit abrupt shear failures, coincident with yielding; at low to intermediate temperatures no reduction of area is evident in the narrow shear zone. At higher temperatures (up to 0.87 Tg, where Tg is the glass transition temperature) plastic flow at ordinary strain rates is different only in that it is less localized, i.e. yielding induces readily evident necking through the thickness of the sheet; failure still generally occurs by shear rupture through the neck. In the low temperature regime, the yield stress (σy) decreases slowly with increasing temperature; it varies from 3.04 to 1.96 GPa between 77 and 568 K (T/Tg≏0.82). At higher temperatures, where necking is observed, σy decreases rapidly, apparently approaching zero near Tg (∼694 K).

Strain-rate sensitivity and effects of strain rate in sheet forming

Abstract: The strain-rate dependencies of flow strength and strain-hardening behaviour are particularly important in sheet-forming operations because they influence strain distribution and necking resistance. Measurements made in uniaxial and biaxial tensile tests on low-carbon steels have shown that strain-rate sensitivity is a function of both strain and strain rate, and that it varies with dissolved interstitial solute content. It is concluded that some simple constitutive equations which are commonly used to describe flow-strength–strain-rate relationships do not provide generally reliable bases for predictions of limit strains in sheet-forming operations. Variations in the plastic behaviour of the workpiece material which resultfrom changes in the speed of pressing can have important influences on performance in practical forming operations, but pressing speed–formability relationships are usually complicated by simultaneous changes in process variables such as lubricant behaviour and the extent of def...

An anomaly in the necking behavior of polyethylene, Part 2

Abstract: Dumb-bell shaped specimens of three polyethylenes were subjected to constant uniaxial tensile loads at test temperatures from 298 to 353 K. For the high density polyethylenes, a marked transition appeared in the neck/fracture behavior. At a certain stress level, the instantaneous fracture of the neck formed at high loads was replaced by the formation of a neck that resisted fracture for a considerable time. This transition was more gradual for the medium density polyethylene. Furthermore, at all test temperatures the transition shifts towards higher nominal stresses with increasing molecular weight. Mainly on the basis of measurements of the local strain rate in the neck forming region, a hypothesis is proposed which explains the appearance of the marked transition. The draw ratios and the densities of the fractured neck fibers were also measured and are in accordance with current molecular deformation theories.

Molecular Interpretation of the Load-Elongation Curve of Crystalline Polymers

Abstract: The load-elongation curve of a semicrystalline polymer with lamellar structure may have four distinct parts: the initial elastic part, the yield area, the necking area where the lamellar morphology is transformed into the fibrous structure, and the drawing area of the fibrous material up to the failure of the sample. The neck formation stops the elastic deformation of the lamellar structure and hence reduces the upper yield load to the load needed for the continuous necking. The drawing of the fibrous structure extends the interfibrillar tie molecules and shears the crystal blocks. The increase of the contribution of taut interfibrillar tie molecule to the axial elastic modulus of the drawn sample comes to an end as soon as the tie molecule gets so far unfolded that at least one end is completely pulled out of the crystal block it was originally anchored in. Swelling of the amorphous regions so much weakens these areas that they are not able to exert enough force on the lamellae for their complet...

An anomaly in the necking behavior of polyethylene, Part 3

Abstract: Differential scanning calorimetry (DSC) measurements, polarized light microscopy and scanning electron microscopy (SEM) studies are reported on neck fibers formed by constant uniaxial tensile loading of polyethylene specimens at temperatures ranging from 298 to 353 K. The DSC measurements indicate that the temperature of the melting peak (Tpeak) of the neck fibers is closely related to the fibrilinity, i.e., the content of complete fibrillar structure, and that Tpeak and thus fibrillinity of the fractured neck fibers is sensitive to the nominal stress in the region of marked transition. A previously proposed hypothesis concerning the appearance of a marked transition in the necking/fracture behavior of high density and high molecular weight polyethylene is thereby supported. The polarized light microscopy showed a correlation between the zone length of the transformation from spherulitic to fibrillar structure and the previously reported distinctness in neck formation. The crystallinity determinations obtained from the DSC measurements and the SEM observations confirmed the suggestion previously made that the density decrease in the fractured neck fibers of a high density polyethylene with Mn = 21.6 × 103 and Mw = 199 × 103 is a result of void formation.

Metallurgical Characterization of Superplastic Forming

Abstract: : A systematic characterization of the superplasticity for regular- grade Ti-6Al-4V, ELI-grade Ti-6Al-4V, simulated coil-rolled Ti-6Al-4V, Ti-3Al-2. 5V, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V, and Ti-15V-3Cr-3Sn-3Al sheets with several different microstructures and texture was performed. The effects of alloy chemistry, grain size, volume fraction of constituent phases, and anomalous microstructures on the superplastic parameters were determined by incremental strain-rate, constant stress, constant strain rate and biaxial constant stress cone-forming tests. The strain-rate time (or equivalently, strain) dependences of flow stress and strain-rate sensitivity were identified as the most important superplasticity parameters, with the continuous changes in alloy microstructures during superplastic deformation requiring proper consideration. In the alpha-beta and near-alpha titanium alloys, the flow stress decreases and necking resistance increase with decreasing grain size at 850-950 F (1562 - 1742 F) with increasing strain and time the flow stress at constant strainrate increases and the strain rate at constant applied stress decreases as a consequence of increasing grain size. Ti-6Al-4V alloys with elongated-alpha have significantly higher flow stress than equiaxed regular grade Ti-6Al-4V. The flow stress at a constant strain rate of different alpha-beta alloys is uniquely related to grain size, beta transus temperature, and volume fractions of constituent phases at the test temperature.

Applications of Superplasticity

Abstract: As described in the previous Chapters the characteristic features of the deformation of alloys which exhibit structural superplasticity is that extremely high strains can be achieved with low applied stresses with a high strain-rate sensitivity during deformation. The high strain-rate sensitivity makes these alloys resistant to localised deformation (necking or thinning). Thus it is not surprising that at an early stage it was concluded that these features could be taken advantage of in forming operations of various types. The resemblance between the behaviour of superplastic materials and thermoplastics above their glass transition temperature is clear both in their stress-strain and stress-strain rate dependence, and this suggested that forming techniques for the latter could readily be adopted for the former. Thus it was envisaged that processes such as vacuum thermoforming, drape forming, bottle-blowing etc., could be utilised for forming superplastics.

The stability of tensile deformation of single ductile fibre-ductile matrix composites with weak interfaces

Abstract: The embedded molybdenum fibre in a copper matrix composite was elongated apparently uniformly although the interface was so weak that necking in the fibre could not be suppressed by the matrix. To explain this result, a possible mechanism was suggested where suppression of necking in the fibre is caused by strain hardening of the composite as a whole and by an increment in strain rate in the cross-section where the fibre starts necking, but the incremental deformation amount and the incremental strain rate of the cross-section are small. On the basis of this mechanism, and with the aid of Hart's criterion, a new instability approach to tensile behaviour of the composite is presented. It was found that the stability of the composite is determined mainly by the strain-hardening exponent of the composite, which is determined by the modified rule of mixtures. The present derived condition of instability of the composite is in good agreement with that proposed by Mileiko.

Necking molding method of metallic can

Abstract: PURPOSE:To mold the necking without causing wrinkles, etc, by reducing the inside of the lower end part of the die-ring two times, and making it a regular taper angle by means of plastic working CONSTITUTION:The can barrel end 3 is intruded along the inside guide face of the guide ring 4 by lowering the metallic mold 2, and the lowering center ring 10 is inserted into the can barrel end 3 The can barrel end 3 is guided to the reducing face of the die-ring 7 by lowering the die-ring 7, and the shoulder part having a taper angle of 20-30 deg is molded Furthermore, by lowering the center ring 10 and the die-ring 7, the can barrel end is pressed into the circumferential space 14 of the die-ring 7 and is reduced, and the half-molded neck part connected to the shoulder part 13 is plastically worked After that, the neck part connected to the shoulder part 13 of the can barrel 1 is reduced continuously by use of the molding die metallic mold 20 having the same structure as the foregoing by carrying out the afore- mentioned method, the molding workability of necking is elevated

Factors Affecting Necking Elongation of Steel Sheets

Abstract: Susumu MIZUNUMA, Shigehiro YAMAGUCHI, Mitsunobu ABE, and Satohiro HAYAMI Synopsis: Necking behaviour in tensile tests has been investigated for typical steel sheets. Local elongation in spontaneously necked region depends mainly upon the strain diffusibility of materials. Using specimens having artificial necking, a fomula describing this strain diffusibility has experimentally been deduced in relation to temperature and strain rate dependences of flow stress, and stress ratio. Furthermore, the relationship between necking elongation and macro-factors affecting it is formulated as follows,

Morphology of Creep Fracture of a Phase‐Separated Borosilicate Glass

Abstract: A phase-separated borosilicate glass with interconnected microstructure was stretched in the viscoelastic temperature range. When elongated, significant necking and subsequent fracture occurred. SEM study of the fractured samples revealed an enormous amount of crazing on the surface and void formation in the bulk of the samples near the fractured edges.