Showing papers on "Necking published in 1979"
TL;DR: In this paper, a tensile testing method is described in which the samples are tested atconstant local true strain rate, and it is concluded that the positive curvature of the log σ flow curve is responsible for the stabilization of flow localization associated with cold drawing.
Abstract: The methods of conventional tensile testing as applied to solid polymers are compared and reviewed critically. Experiments were performed using these techniques, and it is shown that large variations in local strain rate occur while necking and cold-drawing take place. A new tensile testing method is described in which the samples are tested atconstant local true strain rate. This technique is based on the use of a diameter transducer, an exponential voltage generator and a closed-loop testing machine. Flow curves for poly(vinyl chloride) and high density polyethylene were determined at room temperature over the strain rate range of 10−1 to 10−4 sec−1. It is shown that the flow behaviour of these two polymers can be approximated by the constitutive relation:\(\sigma = K \cdot \exp [(\gamma _ \in /2) \in ^2 ] \cdot \dot \in ^m\), whereK andγ∈ are constants andm, the rate sensitivity, is in the range 0.02 to 0.06. It is concluded that the positive curvature of the log σ flow curve is responsible for the stabilization of flow localization associated with cold drawing, and that the rate sensitivity plays a much smaller role.
401 citations
TL;DR: In this paper, the stress-strain response of polyethylene and amorphous polycarbonate has been determined in tension and in compression at superimposed pressures up to 1104 MPa(160 ksi).
Abstract: The stress-strain response of crystalline high density polyethylene and of amorphous polycarbonate has been determined in tension and in compression at superimposed pressures up to 1104 MPa(160 ksi). Strain softening occurred in the polycarbonate at low pressures but was inhibited by pressure. Tensile necking occurred in both materials, but was promoted by pressure in polyethylene and inhibited in polycarbonate. The initial modulus, E, and the flow stress, σ, at a given offset strain varied linearly with the mean pressure, P, with essentially the same pressure coefficient, α. Thus, E = (1+αP)E0 and σ = (1+αP)σ0, where E0 and σ0 are values at zero mean pressure. In polyethylene, the coefficient, σ0, was the same in tension and compression, indicating that the strength differential between tension and compression was a simple manifestation of pressure-dependent yielding. In polycarbonate the coefficient, σ0, was different in tension and in compression, implying an effect due to the third stress invariant or to anisotropy. The results suggest a constitutive model for polymers in which the flow stress is linearly dependent on mean pressure, but in which inelastic volume change is negligible. The results also suggest that the pressure dependence of flow stress in polymers is the same as that of the initial modulus.
197 citations
TL;DR: In this article, the stable and unstable plastic flow of stainless steel-clad aluminum and aluminum-clad stainless steel sandwich sheet materials deformed in uniaxial tension have been investigated.
Abstract: The stable and unstable plastic flow of stainless steel-clad aluminum and aluminum-clad stainless steel sandwich sheet materials deformed in uniaxial tension have been investigated. For the clad sheet materials studied experimentally, stable deformations were uniform in the component layers, and the assumption of isostrain was used in modeling the deformation behavior. The rule of mixtures, an average of component properties weighted by cross-sectional area fractions, was applied to determine sandwich uniaxial true stress-true strain curves from those of the components. In addition, measurements of residual stress distributions in deformed tensile specimens gave insight into states of stress during loading. A model to determine the magnitude of stresses which are generated by component normal plastic anisotropy differences was developed as well. With this knowledge of the stress state, predictions of uniform elongation of the clad sheet materials were made which compared favorably to experimental measurements. As for ductile monolithic sheet materials, stable flow of sandwich sheet materials in tension was limited by diffuse necking, which leads to local instability at higher strains. This local instability gives rise to a through-thickness localized thinning which terminates macroscopic deformation. Conditions for local instability in uniaxial tension have been developed for sandwich as well as monolithic sheet materials. Predictions from these models are in agreement with measurements.
94 citations
TL;DR: In this paper, the authors measured the stress exponents m of the strain rate for the material at flow at these temperatures and found that the exponents decrease to unity as the test temperature approaches Tg.
84 citations
TL;DR: In this article, the transformation of the homogeneous, natural presentation of the strain and stress states to the standard formulation and vice versa is established for the large strain domain and the results apply immediately to the description of simple finite elements which may serve as subelements for the derivation of higher order finite elements.
84 citations
TL;DR: In this paper, the deformation of the superplastic Zn-22% Al eutectoid is divisible into three distinct regions: region I, region II and region III.
Abstract: The mechanical behaviour of the superplastic Zn-22% Al eutectoid is divisible into three distinct regions. Experiments show the deformation is quasi-uniform at intermediate strain rates in region II, but neck formation is important at low strain rates in region I. Extensive cavitation occurs in regions I and II, but fracture in region I is due to necking. The results provide strong evidence for a decrease in the true value of the strain rate sensitivity in region I.
54 citations
TL;DR: In this paper, the boundary value problem for the plane-strain uniaxial tension of a rectangular bar is posed in two ways, in one case the ends of a specimen of compressible elastic-plastic material are assumed to remain shear free.
46 citations
TL;DR: In this paper, failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum, and failure was produced in the drawing area of the forming limit diagram.
Abstract: Failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum. By using narrow blanks and no lubrication, lateral contraction was possible, and failures could be produced in the drawing area of the forming limit diagram. For this deformation regime, diffuse instability led to localized necking. As in monolithic materials, the development of the localized neck in stainless steel clad aluminum determined the forming limit, and predictions of the strain levels for the onset of local instability correlated well with the observed forming limit strains. By preventing lateral contraction, failures in stretching were produced. The forming limit strains in this case depended on the strains at the onset of diffuse instability in much the same manner as is observed for monolithic materials. The strains at the onset of diffuse instability were predicted using a generalized rule of mixtures, and agreement between measured values and values predicted from component properties was good when the strain-path dependence of the instability strain for the individual components was taken into account. The diffuse necking process in stretching of stainless steel clad aluminum led to local thinning when deformations involved small degrees of biaxiallity. On the other hand, nonuniform through thickness straining of the component layers in specimens strained close to balanced biaxial stretching appeared to control the localization process and gave rise to forming limit strains lower than expected from observations of punch formed monolithic sheet materials. For all deformation modes, localized flow culminated in delamination and fracture.
31 citations
TL;DR: In this paper, the necking of a cylindrical, elastic-plastic, strain-hardening bar in axisymmetric tension was analyzed using the Lagrangian formulation.
27 citations
TL;DR: In this article, a tensile test on polyvinyl chloride has been made to find the conditions under which uniform extension, diffuse necking, localized necking or thermal fracture occur.
TL;DR: In this paper, the surface craze characteristics and the role they play in the fracture processes are reported as a function of strain-rate (10−2−10+2 min−1) from scanning electron microscopy studies of fracture topographies and edges of polycarbonate specimens fractured in tension at room temperature.
TL;DR: In this paper, the effect of strain rate sensitivity on the delay in necking of sheet tensile specimens is analysed for a fairly general material constitutive law on the basis of two approximations, and the results are similar in form, although not in detail, to those of a previous analysis for round tensile bars.
TL;DR: In this article, the authors used the empirical relationship σ = Kϵ n, commonly measured in the tension test, to determine the final strain distribution achieved in a pressed part.
TL;DR: In this paper, the authors show a marked transition at a certain stress level from a neck formation followed by instantaneous fracture to the formation of a neck which resists fracture for a considerable time.
Abstract: Tensile creep measurements at constant load on nonoriented polyethylene have shown a marked transition at a certain stress level from a neck formation followed by instantaneous fracture to the formation of a neck which resists fracture for a considerable time. The transition, which shifts towards shorter time and higher nominal stress with increasing molecular weight, has been studied for 16 polyethylenes of different molecular weights, degrees of branching and crystalline structures. The marked. transition has only been observed for high density polyethylene of high molecular weight. Deformation measurements show a more distinct necking for the high density than for the medium density polyethylenes. This is consistent with current molecular deformation theories. A hypothesis for the transition is proposed based on the distinctness of the neck process in the high density polyethylene and the large difference in strength between the spherulitic structure and the fibrillar structure. The dependence of the transition on molecular weight is expected since the number of tic chains incrcrtses with increasing molecular weight.
TL;DR: In this article, the influence of prior deformation of martenite at room temperature upon the subsequent mechanical response of the alloy at increasing degrees of metastability is described, and the experimental results indicate that the tensile strength, the mean work-hardening rate and the volume fraction of martensite transformed up to necking are primarily controlled by the temperature of deformation.
TL;DR: In this paper, the deformation of circular membranes of a strain-rate dependent, work-hardening metal is analyzed based on the strain increment theory and numerical solutions for aluminum membranes are obtained under various types of loading as well as impulsive pressure pulses caused by underwater wire explosions.
Abstract: The plastic deformation of circular membranes of a strain-rate dependent, work-hardening metal is analyzed based on the strain increment theory. Numerical solutions for aluminum membranes are obtained under various types of loading as well as impulsive pressure pulses caused by underwater wire explosions. When the duration of a loading pulse is less than one-third of the time required to finish the deformation of the membrane, the deflection of a deformed membrane depends almost only on the impulse of the loading pulse and the instantaneous profile of the membrane is found to be trapezoidal during deformation. The relation between impulse of loading and deflection is calculated and illustrated. The forming limit due to peripheral necking is discussed. The neglect of strain-rate dependency of material leads to a notable underestimation of the forming limit.
TL;DR: In this paper, the effects of material parameters on the plastic instability in uniaxial tension of commercial A-75 titanium sheet tested at 300-700 K and strain rates of 10−4−10−2s−I were investigated employing gridded specimens.
TL;DR: In this paper, the effect of oxidation on the ductility of a single fiber composite was investigated and it was shown that a compressive stress component can result when the composite zircaloy/ZrO2 is elastically strained.
TL;DR: In this article, the details of the shear mode of ductile fracture in sheet tensile specimens of a pearlitic 0.9% steel have been investigated using the techniques of optical, scanning electron and transmission electron microscopy.
01 Jan 1979
TL;DR: In this article, the cooling velocity and the stretch-ratio of the melt were varied as characteristic parameters of the PE-monofilaments produced by the melt-spinning process.
Abstract: PE-monofilaments are produced by the melt-spinning process. The cooling velocity and the stretch-ratio of the melt were varied as characteristic parameters of the melt-spinning process. By increasing the cooling velocity and the cold drawing of the obtained threads, the elastic recovery after a 25% — elongation can be improved to complete reversibility. From tensile test runs a new deformation mechanism can be identified which explains this behaviour. The main features of this deformation mechanism are that no necking occurs during the deformation and that the deformation of the noncrystalline plays an important role. Apparently this mechanism is correlated with an imperfect crystalline structure.
TL;DR: In this article, a new approach based on the phenomena of stress wave propagation is presented to analyse necking in the uniaxial tension test, where the initiation of necking has been associated in this analysis with a stage, wherein the stress level at any section of the specimen acquires an instability stress value, and the stress wave velocity becomes zero at the section concerned.
TL;DR: In this paper, the authors studied the environmental stress cracking of polyethylene in nonreacting, nonswelling liquids using uniaxial creep tests and found that the liquid does not play a significant role in this last case.
Abstract: The environmental stress cracking (ESC) of polyethylene in nonreacting, nonswelling liquids has been studied using uniaxial creep tests. For active liquids, three types of behavior have been recognized. At low stresses, “pure” ESC occurs; at intermediate stresses, time to failure is largely controlled by the ability of the liquid to flow into a growing crack; and at high stresses, ESC is in competition with failure by necking, and the latter prevails. The liquid does not therefore play a significant role in this last case. Nonactive liquids produce results similar to those observed in air. It is believed that this is because these liquids are unable to flow into growing cracks sufficiently quickly even at low stresses and thus the liquid does not influence failure behavior. This criterion for activity of the liquid is largely determined by the viscosity of the liquid and by the spreading coefficient of the liquid on the solid—a parameter defining the ability of the liquid to wet the solid.
TL;DR: In this paper, the shape evolution of tensile specimens was studied and it was shown that the flow stress is a function of the strain and strain rate, and that shape evolution is independent of deformation history if S(e,e') = f(e)e/sup m/m/n.
01 Jan 1979
TL;DR: Theoretical and experimental description of the Mechanical Behaviour of metals during metal forming processes is discussed in this article, where the authors present an approach for generalized three-dimensional Extrusion of Sections from Round Billets by conformal transformation.
Abstract: On the Prediction of Necking in Anisotropic Sheets.- On the Theoretical and Experimental Description of the Mechanical Behaviour of Metals during Metal Forming Processes.- Spin-Forging of Sheet Metal Cones Having Various Cone Angles from 70/30 Brass and Commercially Pure Aluminium.- Residual Stresses in Deep Drawn Cups and Sunk Tubes.- On the Complete Numerical Solution of the Axisymmetrical Deep-Drawing Problem.- Application of the Stereophoto- grammetric Methods to the Analysis of the Kinematics and Dynamics of Shells.- Deformations finies en coordonnees locales et application a un probleme d'emboutissage.- Optimum Deformation Rate in Large Inelastic Deformations.- New Trends in Tube and Bar Processing.- Analysis of Large Deformation and Stress in Metal Forming Processes by the Finite Element Method.- Stress and Deformation Analysis of Metal-Forming Processes.- Inhomogeneity of Deformation and Tool Design.- A New Approach for Generalized Three-Dimensional Extrusion of Sections from Round Billets by Conformal Transformation.- Ideal Metal Forming.- The Application of Singular Perturbation Techniques to the Analysis of Forming Processes for Strain-Hardening Materials.- Allowing for a Variable Flow Stress in the Analysis of Metal Working Processes.- Plastic Deformations with Free Boundaries - A Finite Element Approach.- Finite-Element Formulation for the Analysis of Plastic Deformation of Rate-Sensitive Materials in Metal Forming.- Theoretical Solutions of Some Plastic Working Processes in the Light of Experimental Evidence.- Analysis of Metal Forming by the Rigid-Plastic Finite Element Method Based on Plasticity Theory for Porous Metals.- Mechanics of Creep in Metal Forming.- Modeling of the Tool-Workpiece Interface.- Drawing of Fiber Reinforced Tubes.- Stresses and Strains in Deformation of Clad Metals.- Joint Examination Project of Validity of Various Numerical Methods for the Analysis of Metal Forming Processes - Report Given and Comments Made at the Pound Table Discussion of the Symposium.
TL;DR: In this article, an extension of a model of Barenblatt for isothermic necking in polymers is proposed, and it is shown that at sufficiently low temperatures, uniform necking is no longer stable and self-oscillations are observed.
Abstract: The phenomenon of oscillatory necking in the stretching of polyethylene films is described. We propose an extension of a model of Barenblatt for isothermic necking in polymers, and we show that oscillatory necking may arise, for example, in polymers for which the effect of diffusion of stresses exceeds that of diffusion of oriented material. In such polymers, at sufficiently low temperatures, uniform necking is no longer stable and self-oscillations are observed.
01 Jan 1979
TL;DR: In this article, the authors investigated the pressure-dependent shear stress-strain behavior of two diverse polymers, namely, highly crystalline polytetrafluoroethylene (PTFE) and completely amorphous polycarbonate (PC).
Abstract: In a previous study, the dependence of the shear stress-strain behavior of two polymers, polyoxymethylene (POM) and polypropylene (PP) on hydrostatic pressure was well documented [l]. Furthermore, at a critical shear strain of 0.500, both polymers exhibited a region of concentrated shear strain (girdle) in the central portion of the test specimens. Evidently, instabilities similar to those usually observed in tensile specimens (necking) may also develop in highly deformed torsion specimens. Other researchers have reported the pressure dependence of the shear modulus and the shear yield stress of polymers, but none has reported observing a girdle [2–5]. This study was directed toward the investigation of the pressure-dependent shear stress-strain behavior of two diverse polymers; namely, highly crystalline polytetrafluoroethylene (PTFE) and completely amorphous polycarbonate (PC). In addition, physical changes in the specimens were monitored to ascertain the generality of the shear strain girdle as a manifestation of shear yielding in these materials.