Showing papers on "Necking published in 1970"
TL;DR: In this paper, the effect of dynamic strain-ageing on the initiation and propagation of ductile fracture in mild steel was examined using scanning electron microscopy and it was shown that this crack density increases with strain.
57 citations
TL;DR: In this article, the tensile creep behavior of amorphous, glassy polycarbonate based on bisphenol A was studied and the transition from linear to nonlinear behavior was characterized.
Abstract: The validity of the theory of linear viscoelasticity over the temperature range 20°-13°C has been tested by studying the tensile creep behavior of the amorphous, glassy polycarbonate based on bisphenol A. The stress threshold, beyond which the mechanical properties deviate from the predictions of linear theory by more than 3%, is found to decrease as the temperature approaches the vicinity of the glass transition temperature (149°C). Extension of the experimental time scale from 10 to 1000 sec likewise diminishes the stress level below which the linear theory is applicable. The onset of nonlinear behavior is marked by a clear increase in the isothermal, isochronous compliance with increasing stress. Such an increase in compliance with stress becomes gradually sharper at increasing stress levels until necking or failure of the specimen occurs. The analytical characterization of the transition from linear to nonlinear behavior, as well as certain molecular implications of such a transition, are dis...
25 citations
TL;DR: In this paper, a change-of-slope method was used to obtain the activation energy for tensile creep in ferromagnetic α iron (Ferrovac•E 99.91% pure) in the temperature range 620°-700°C.
Abstract: The temperature and the stress dependence of tensile creep has been studied in ferromagnetic α iron (Ferrovac‐E 99.91% pure) in the temperature range 620°–700°C. By using the change‐of‐slope method, activation energies for creep were obtained. The modulus‐compensated activation energy is shown to be essentially independent of the initial stress and of strain up to the necking point. The change‐of‐slope method gave a modulus‐compensated activation energy of 65 kcal/mole while a plot of the logarithm of the modulus‐compensated strain rate versus reciprocal of the absolute temperature gave a straight line yielding a modulus‐compensated activation energy of 62 kcal/mole. These are in essential agreement with the value for self‐diffusion in this temperature range. The logarithm of the temperature‐compensated creep rate versus logarithm of stress gave a straight line with a slope of 6.6, i.e., a power‐law dependence of the strain rate on the stress. The fact that the activation energy for creep is the same as t...
24 citations
01 Jan 1970
TL;DR: In this paper, the effect of multiple necking of metal fibers, strain rate, and fiber surface condition on the ductility and deformation behavior of uniaxial metal matrix composites was studied.
Abstract: A model composite system, brass-tungsten, was used to study the effect of multiple necking of metal fibers, strain rate, and fiber surface condition on the ductility and deformation behavior of uniaxial metal matrix composites. Low volume percent (<20 vol pct) composites were made using brass (90 Cu-10 Zn) powders and tungsten wires. Tungsten wires contained in plastically deformed composites exhibited numerous necks per length of wire, with an average spacing between necks of 3 to 5 wire diameters. Composite ductility was shown to increase with decreasing multiple neck spacing and increasing percent reduction-in-area at the necks and was generally independent of strain rate. The multiple necking of tungsten wires in a brass matrix was found to result from local strain hardening of the brass matrix in the vicinity of each neck enabling the matrix to control composite deformation locally. Degrading the brass-tungsten interfacial bond by electropolishing the tungsten wires and coating them with graphite had no effect on the multiple necking phenomenon and composite ductility.
20 citations
TL;DR: In this paper, the nonlocal damage concept developed by Bazant was introduced into the modified Gurson model to reduce the mesh-size dependency and the relationship between the localization limiter and the characteristic length.
Abstract: The ductile fracture behavior of materials is considerably influenced by the characteristic microstructural length which may be related to the average inclusion spacing. However, it is not easy to introduce this critical parameter into micromechanical modelling. An additional problem met at the simulation with micromechanical models is the sensibility of numerical results to the mesh size. In this work the nonlocal damage concept developed by Bazant was introduced into the modified Gurson model. The ability of the nonlocal micromechanical concept to reduce the mesh-size dependency and the relationship between the localization limiter and the characteristic length have been studied by simulating damage development in shear bands, necking regions and at crack tips.
17 citations
TL;DR: The conditions at fracture are eminently favorable for crack-branching because of the high stress level (∼20% of the theoretical), the low effective surface energy for fracture, and the highly preferred crystallographic texture which provides (III) planes of low-effective surface energy in suitable orientations for propagation of the branched wires as mentioned in this paper.
Abstract: Heavily drawn wires (80/99·7% RA) were fractured at temperatures between 20°c and − 100°c at various strain rates An unusual morphology of fracture resulted in all cases, even when plastic strain or necking preceded fracture Specimens broke into three pieces, viz two wire ends, with conical fracture surfaces, and a cylindrical chip with re-entrant conical fracture surfaces and with a small hole where the apices of these cones met Despite some resemblance to a ductile shear failure, evidence shows that the fracture is of ‘brittle’, presumably cleavage, type and that the morphology results from crack-branching (Congleton and Petch 1987) The conditions at fracture are eminently favourable for crack-branching because of (i) the high stress level (∼20% of the theoretical), (ii) the low effective surface energy for fracture, and (iii) the highly preferred crystallographic texture which provides (III) planes of low effective surface energy in suitable orientations for propagation of the branched
16 citations
TL;DR: In this paper, a new definition of equivalent strain is introduced in the context of anisotropic plasticity to express the strain-hardening behavior of rolled sheets, which can be derived from that in the rolling direction and vice versa.
11 citations
TL;DR: In this paper, a high-carbon maraging steel has been subjected to four different heat-treatments to vary its tensile properties and the stresses necessary to initiate cracking of TiC particles in the steel were then determined.
Abstract: A high-carbon maraging steel has been subjected to four different heat-treatments to vary its tensile properties. The stresses necessary to initiate cracking of TiC particles in the steel were then determined. In all cases cracking began at about the point when visible necking occurred in the tensile specimens. The results are discussed in terms of the influence of the maraging precipitate particles in homogenizing flow and changing the local stresses at the carbide particles. The results show that suitable precipitate dispersions can both significantly increase the strength and minimize cracking of brittle inclusion particles.
5 citations
TL;DR: In this article, the surface energy for the coalescence of cavities by internal microscopic necking was derived from a rigid-plastic model for ductile fracture, and it was shown that the surface energies for coalescence will be a function of the mean free-path between the cavities or second phase particles.
Abstract: An estimate of the surface energy for the coalescence of cavities by internal microscopic necking is derived from a rigid-plastic model [1] for ductile fracture. The. results suggest that the surface energy for coalescence will be a function of the mean-free-path between the cavities or second phase particles. The theoretical surface energy for coalescence of cavities in steels was found to be about an order of magnitude less than experimental estimates of the total surface energy for ductile crack propagation. Since the surface energy measured in experiments includes a contribution from pre-coalescence straining ahead of the advancing crack, it is concluded that the theoretical surface energy for coalescence is of the right order and that the theoretical model for ductile fracture by internal necking of cavities may be closely related to the true mechanism of ductile fracture in real materials.
4 citations
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01 Mar 1970
TL;DR: In this paper, the necking process of an axisymmetric tension specimen made of elastic-plastic, strain-hardening material is analyzed by a generalized J sub 2-flow theory for large deformations.
Abstract: : The necking process of an axisymmetric tension specimen made of elastic-plastic, strain-hardening material is analyzed by a generalized J sub 2- flow theory for large deformations The governing equations are solved in a Kantorovich type approach based on a variational principle The effects of both geometric nonlinearities resulting from large deformations and the physical nonlinearities arising from plastic material behavior are considered Numerical results have been found for the stress and deformation histories in the specimen up to a 50% reduction of the neck radius The shape of the neck, for the first time, is rationally calculated
TL;DR: The neck angle is very sensitive to the temperature and propagation rate as discussed by the authors and the plots of necking angle against the propagation rate can be superimposed to a master curve by shibbing the plot along the rate axis.
Abstract: This paper describes the necking phenomena of undrawn polyester fibers in stretching under various conditions in the vieinity of glass transition temperature. Neck ungle is used as a parameter whichexpresses the characteristics of the necking geometry. This parameter means the angle between the direction of the necking shoulder and the fiber axis. The neck angle is very sensitive to the temperature and propagation rate. The plots of necking angle against the propagation rate abtained at different temperatures cone be superimposed to a master curve by shibting the plot along the rate axis. The shift factor log aT-temperature relationship agrees quite well with the WLF equation, with exception at temperatures lower than the apparent Tg under external force.The necking phenomena are characterized by the necking draw ratio, the necking tension and the birefringence of the necking region. These factors depend a priori on the neck angle irrespective of the drawing condition.Finally, it must be recognized that the necking is a relaxation phenomena within a large wale of temperature and therefore the necking always occurs at temperatures lower than Tg even in the very low rate of stretching.
TL;DR: In this paper, the microvoid volume fraction of polycrystalline materials is identified as a micro-damage scalar field variable, and inclusion into an elastic-viscoplastic constitutive theory enables description of rate-dependent compressible inelastic deformation that includes material degradation culminating in fracture.
Abstract: Spall type fracture of a circular plate caused by mechanical impact and high strain rate fracture of a tensile bar caused by dynamic load application are described using a viscoplastic-damage type of constitutive model. Identification of the microvoid volume fraction of polycrystalline materials as a microdamage scalar field variable, and inclusion into an elastic-viscoplastic constitutive theory enables description of rate-dependent compressible inelastic deformation that includes material degradation culminating in fracture. Calculations simulating the spall fracture and fragmentation of a circular plate induced by high velocity impact, and the necking localization and fracture at high strain rate of a dynamically loaded smooth tensile bar, are shown.
TL;DR: In this paper, the development of plastic strain around a circular hole and a quasi-elliptical hole was experimentally analyzed by the Moire method at successive stages of fatigue, and the following characteristics were clarified concerning the plastic behavior in the specimens under low cycle fatigue.
Abstract: Low cycle fatigue tests were carried out for specimens with circular hole and quasi-elliptical hole under constant load pulsating tension. The materials used are mild steel (SM 41) and quenched and tempered high tensile strength steel (HT 60). The development of plastic strain around a circular hole and a quasi-elliptical hole was experimentally analyzed by the Moire method at successive stages of fatigue, and the following characteristics were clarified concerning the plastic behavior in the specimens under low cycle fatigue.The following three types of fracture were confirmed in the low cycle fatigue test for two kinds of steels.Type-A : A ductile fracture occurs after the necking of the whole part of the net section.Type-B : A fatigue crack initiates after the necking solely at the notch root and strain at the tip of crack increases as it extends.Type-C : A fatigue crack initiates and propagates without a little strain increase at the tip of the crack.In the case of type-A fracture the maximum equivalent strain at the fracture position becomes from 50 to 60 percent, while in type-C fracture it becomes below 20 percent at the stage of crack initiation. In the case of type-B fracture the maximum equivalent strain at the fracture position lies between type-A and type-C values. These strain values are irrespective of materials, size and shape of specimens, and the number of cycles to cause fracture of type-A or crack initiation of type-B and C.The following relation exists, in the case of type-A fracture, without concern to materials, size and shape of specimens, ΔemNf=e0/2where Δem is the steady state creep rate, Nf is the number of cycles to cause fracture of type-A or crack initiation of type-B and C, e0 is the equivalent plastic strain at fracture in simple tension.In the case of type-C fracture, however, the formula no longer holds good and the critical number of cycles to cause fracture, which indicates that the relation between Δemand Nf deviates from the formula, is larger in SM 41 than in HT 60.
TL;DR: In this paper, a method for investigating the resistance of aluminum alloys to thermal fatigue and the phenomena in thermal fatigue tests of Al-Si base alloy castings was presented, which was found that a high rate thermal cycling could be achieved with a thermal fatigue testing apparatus of high frequency induction heating type (not the Coffin type).
Abstract: Failures due to thermal fatigue often occur in aluminum alloy pistons and cylinder heads of engines, especially in those of Diesel-engines owing to their high operating temperatures.This paper reports a method for investigating the resistance of aluminum alloys to thermal fatigue and the phenomena in thermal fatigue tests of Al-Si base alloy castings.The results obtained were summarized as follows:(1) It was found that a high rate thermal cycling could be achieved with a thermal fatigue testing apparatus of high frequency induction heating type (not the Coffin type.) (2) The number of cycles to failure was decreased with the increase in temperature amplitude, thermal stress amplitude, and plastic strain amplitude.(3) Specimens were tested at an average temperature of 300°C; they were radially expanded in the section heated to the maximum temperature and necking appeared in the both ends of the expanded section. The deformation of specimens was related with the distribution and variation of hardness in the specimens. The hardness was decreased in the section heated to above 300°C; and was minium in the necked parts, wehre the fracture took place.
01 Jan 1970
TL;DR: The phenomenon of cold-drawing (formation and steady propagation of an abrupt contraction in stretching cylindrical samples) is one of the qualitative peculiarities of polymers and plays an important role in the production processes of strong fibres as discussed by the authors.
Abstract: The phenomenon of cold-drawing (formation and steady propagation of an abrupt contraction — “necking” — in stretching cylindrical samples) is one of the qualitative peculiarities of polymers and plays an important role in the production processes of strong fibres [1–4].
TL;DR: In this article, the authors investigated the two-dimensional extension of polyethylene and polypropylene film by optical microscopy and found that the ultimate stress in 2D extension is close to the necking stress in uniaxial tension.
Abstract: The two-dimensional extension of polyethylene and polypropylene film has been investigated by optical microscopy. It has been established that there is no two-dimensional plastic flow (two-dimensional necking) and that failure is quasi-brittle in character. In a small region near the cracks that develop, uniaxial orientation of the polymer associated with stress concentrations near the crack tip is observed. Measurements show that the ultimate stress in two-dimensional extension is close to the necking stress in uniaxial tension.
TL;DR: In this paper, the damage mechanics of steels are studied using experimental and theoretical methods, and the applicability of FEM based GursonTvergaard structural damage models are tested in estimating localised necking and fracture of the tensile tests.
Abstract: The damage mechanics of steels is studied using experimental and theoretical methods. Tensile tests and other tests are performed on a low alloy steel with varied grain sizes. Experimental results are analyzed using computer programs to obtain true stress-true strain curves. These results are simulated using micro structural dislocation dynamics models and theories on grain boundary, solution and dispersion hardening of multiphase and dispersion hardened alloyed materials. Using these models the initial discontinuous yielding and subsequent work hardening is satisfactorily predicted. Temperature and strain rate effects are also included. The applicability of FEM based GursonTvergaard structural damage models are tested in estimating localised necking and fracture of the tensile tests. Mechanics of microstructural damage is visualized with a 2-D multibody dynamics simulation program.