Showing papers on "Necking published in 1969"

Strain‐Rate Dependence of the Tensile Failure of a Polycrystalline Material at Elevated Temperatures

Abstract: A general discussion is presented of the failure modes which can occur in uniaxial tensile tests of a polyphase, polycrystalline material at various strain rates at elevated temperatures. Specific results in the temperature range 0.56–0.70Tm are given using Type 316 stainless steel as a representative material. The intrinsic plastic failure is determined solely by plastic deformation, leading first to necking and concluding by separation at a point. However, this intrinsic plastic failure may be interrupted at relatively high strain rates by the formation of cracks at inclusions within the neck. At low strain rates the onset of appreciable necking is postponed and the whole failure process is determined by intergranular crack formation. At intermediate strain rates intergranular cracks form within the neck, and a shear mode of failure is possible. Some conclusions regarding future studies are included.

Apparatus for and method of necking in end portions of tubular members

Abstract: This disclosure relates to apparatus for and method of necking in one or both end portions of tubular members. The disclosure particularly pertains to the necking in of tubular members which have internal surfaces which may be readily scratched by relative movement between the tubular member and the die center. Accordingly, a suitable die assembly is provided wherein it may be utilized in the necking in of a tubular member without there being relative movement between the die center thereof and the tubular member during the necking in of the tubular member.

The Work-Hardening Process in Metals

Abstract: Tangling process of dislocations, especially at the beginning, has been continuously observed with a 500 kV electron microscope mainly on aluminum, and relations between cross slip and tangles are clarified. On the basis of these results, the work-hardening processes in metals are discussed. They are classified into the following two types: (A) The obstacle barrier strongly depends on the local deformation history, (B) the barrier is free from the history so that it is always constant. These types are closely related to the stacking fault energy and the homologous temperature; i.e. , deformation accompanied by the loop-formation belongs to the type (A) and deformation through the interaction products of dislocations belongs to the type (B). Various characteristics of deformation, such as necking, the temperature dependence and the effect of solute atoms, can be analysed in terms of these characters of obstacles.

Drawing of single‐crystal mats of linear polyethylene

Abstract: Drawing of single-crystal mats of linear polyethylene has been investigated. Drawing is possible at temperatures higher than about 90°C. The drawing is accompanied by distinct necking, with a large decrease in the thickness of the mat and a very high maximum draw ratio, sometimes over 30. The maximum draw ratio is approximately proportional to the thickness of the lamellae. This behavior strongly suggests the unfolding of chains during drawing. A change of orientation of crystal axes occurs before necking without change of lamellar orientation. The a axis orients in the drawing direction; the b axis orients perpendicular to the direction of drawing; and the chain axis tilts away from the thickness direction of the mat. The structure of films drawn from mats is characterized by a distinct double orientation of crystals. This biaxial orientation in the drawn films has a high degree of correlation with the orientation of crystal axes observed before necking, and suggests that necking takes place in such a way that the chain tilts gradually about the b axis and ultimately unfolds. The postulate of formation of transitory two-dimensional crystals in necking seems useful in explaining the double orientation in the drawn film. The orientation behavior of crystal axes observed before necking is not always similar to that observed in the deformation of a single crystal. The difference is thought to be due to the effect of forces induced by drawing that act in the direction normal to the lamellae within a mat.

Plane strain necking of V-notched and un-notched tensile bars

Abstract: An analytical nonsteady solution for plane strain necking of V-notched, rigid/perfectly-plastic tensile bars was obtained by E.H. Lee in 1952. In his solution the plastic zone remains geometrically similar, a velocity discontinuity occurs on the rigid-plastic boundary, and when carried to the point of separation, the plastic flow results in a wedge-shaped neck. In this paper a new analytical solution is given for the same problem. However, the plastic zone does not remain geometrically similar, velocity discontinuities are absent, and plastic flow results in a convexshaped neck. Comparisons with measurements of neck profiles obtained under conditions approximating plane strain indicate that the new solution is a good representation of the physical situation n metals. Thus, it appears to be useful for estimating basic stress-strain relations from tensile tests after necking, and for estimating the stresses and strains required for fracture from notched bar tests.

A Theory for the Effects of Strain-Rate-Sensitivity on Ductile Fracture

Abstract: A theory for ductile fracture in a material with a strain-rate-sensitive yield stress is proposed, based on the hypothesis that fracture is the result of internal necking between adjacent microscopic cavities. The theory is developed for a plane-strain element containing uniformly distributed cavities and this model is used to examine the effects of strain-rate-sensitivity on the strain to fracture in ductile materials. The results suggest possible explanations for both “blue-brittleness” behaviour and the “superplasticity” effect.

Orientation behavior of crystalline and noncrystalline phases during orthogonal-biaxially stretching

Abstract: Orientation behavior of crystalline and noncrystalline phases of an extruded isotactic polypropylene film during orthogonal-biaxially stretching was investigated by means of simultaneous measurements of x-ray diffraction and birefringence. The orthogonal-biaxially stretching was performed under the following three fashions; simultaneous, alternative, and strip biaxial.In general, the orientation behavior of the crystalline phase can be classified into the following three types; α, β, and γ type. The α type orientation is characterized by a preferential orientation of crystal b-axis toward the film normal, around which the crystal a* and c-axis orient randomly; i.e., preferential planar orientation of (040) crystal plane along the film surface. This behavior may be interpreted in terms of lamella orientation parallel to the film surface, accompanying the lamella twisting around its axis and the lamella necking along its axis so that the crystal c-axis orients parallel to the film surface due to-the straining of tie-chains between the lamellae.Since the oriented crystal having its b-axis perpendicular to the film surface is stable enough against possible crystal slipages along (010) and (110) planes, the only possible deformation may be (110) twining due to increasing biaxial stress, and the two possible slip planes are inclined by equal angle, 54° with respect to the film normal. This produces the β type orientation.When the biaxial stress is removed, the oriented noncrystalline chains generate restoring force. This force acts the β type oriented crystal so as to make further (110) twinning. Therefore, (110) plane rotates about 36° and orients almost parallel to the film plane. This produces the γ type orientation.The relative proportion of the three type orientations after the highly biaxial stretches, varies with the stretching and releasing conditions. The apperance of the γ type orientation resembles to the (110) twinning of polyethylene crystal under rolling pointed out by Frank et al.

Deformation behavior in metal fiber-metal matrix composites.

Abstract: : A model system, brass-tungsten, was used to study the effect of multiple fiber necking, strain rate, and fiber surface condition on the ductility and deformation behavior of uniaxial metal matrix composites. Low volume percent (<20 v/o) composites were made using brass (90Cu-10Zn) powders and tungsten wires. Multiple necking of tungsten wires in a brass matrix was found to be a result of a load transfer from the wire to the surrounding matrix. Composite ductility was shown to increase with decreasing multiple neck spacing and increasing reduction of area at the neck and was generally independent of strain rate. The ability of metal matrix composites to undergo simple forming operations such as cold rolling and closed-die forging was also shown. (Author)