Showing papers on "Necking published in 1973"

Fracture in cold upset forging -a criterion and model

Abstract: Deformation behavior and fracture of three iron alloys during cold working conditions were studied systematically under a range of stress and strain states by means of the upset test. As described in a previous publication by the authors, an anomalous strain behavior and sub-surface void formation are observed prior to fracture at the bulge surface. This anomaly, in the form of a perturbation in the strain path, has features that are similar to the strain instability in biaxial stretching of sheet material. Based on these experimental observations, a mathematical model of localized necking and fracture is presented that permits evaluation of the effects of material characteristics and process parameters. A fracture criterion involving total strains at failure is also proposed. The criterion accurately fits experimental data obtained in this study as well as that presented by other investigators. Essential features of the fracture criterion can be derived from the mathematical model of instability and fracture.

Thermal Instability Strain in Dynamic Plastic Deformation

Abstract: When a metal is deformed slowly, geometrical and material effects combine to limit the plastic deformation. It can be shown that for a given deformation geometry the limiting, or necking, strain will be a function of the workhardening exponent, n,* in the constituitive relation.

A Theory for the mechanical properties of metal-matrix composites at ultimate loading

Abstract: A theory describing the strain, ultimate strength, and work during uniform strain to ultimate loading of metal-matrix composites deformed in tension parallel to the reinforcement is presented. These quantities may be calculated for composites of arbitrary volume fraction using only the component stress-strain curves. The theory is based on the systematic application of a macroscopic principle commonly used to predict the ultimate strength of ductile monolithic materials—namely, that necking occurs when the load borne by the material is maximized. For brittle reinforcing elements, the results are identical to those of previous workers. For ductile reinforcing elements, necking strains intermediate between those of the components and ultimate strengths increasing smoothly with volume fraction from that of the matrix to that of the reinforcement are predicted. The theory can be used to predict the variation of composite ultimate properties with any parameter of interest. In this paper the variation with volume fraction and yield strength of the matrix are studied, with both exact solutions and useful approximations being derived.

Double stage necking

Abstract: This disclosure relates to the necking-in of can bodies for the purpose of applying smaller diameter or special ends thereon. Previously experienced extreme difficulties in properly neckingin can bodies have been eliminated by first necking-in a larger than normal end portion of a can body to an intermediate diameter and then utilizing tooling for performing the desired necking in operation to further neck-in the extreme end portion only of the previously necked-in longer end portion.

Method and apparatus for necking and flanging can bodies

Abstract: A cylindrical can body is positioned between axially aligned necking and flanging dies which are forced together so that the dies force the can body ends completely into and onto the dies to neck and flange the can body ends. The dies are then pulled apart, with the can body being first clamped to the flanging die to pull the necked end of the can body from the necking die. The can body is gripped and held between its ends to allow the flanging die to be then pulled off the flanged end of the can body.

Thermal fracture of plastics

Abstract: The extension and fracture under tension of poly(vinyl chloride) strips of different lengths have been studied. It was found that, in spite of decreasing strain rate, long pieces are subject to a type of brittle fracture (necking rupture) occurring immediately after neck formation. This result, which could be expected from previous work, is shown to depend on the elastic energy stored in the longer test pieces. This energy is converted to heat under conditions of neck formation, leading to a temperature rise of 24 ± 4°C, determined by a thermal imaging system. With short test pieces the rise in temperature is much smaller, and a transition to stable neck propagation occurs. Optical and electron micrographs provide additional evidence of heating at the fracture surface. A semiquantitative treatment of the adiabatic process gives upper and lower limits for the predicted temperature change neglecting thermal losses. It is also possible to estimate a minimum length for the initiation of adiabatic deformation. Both calculations give results in reasonable agreement with experiments. Observations with polycarbonate show a similar length effect at low temperatures.

Finite deformation of elasto-plastic solids

Abstract: A theoretical basis is established for analysis of finite deformation of metals. The observation that finite deformation of such elastoplastic materials may be viewed as a process rather than an event leads to derivation of a complete initial and boundary value problem distinguished by its quasilinear nature. This feature of the formulation motivates adoption of an incremental approach to numerical problem solving. Numerical solution capability is established for problems of plane stress and plane strain. The validity of the theory and numerical analysis is demonstrated by consideration of a number of problems of homogeneous finite deformation for which analytic solutions are available. Subsequently the analysis is employed for the investigation of necking in flat metal tensile bars. The results of this investigation provide the first full numerical solutions for tensile necking in plane stress and plane strain. In addition a basis is provided for assessment of the validity of stress-strain relations inferred from tensile test data.

ESR study of structural changes in high‐density polyethylene subjected to cyclic tensile stress

Abstract: Bulk-crystallized high-density polyethylene is subjected to cyclic tensile stress and then irradiated by γ-rays to “spin-label” polymer molecules. The change of crystalline texture in the fatigue process before initiation of necking is detected by the ESR method. The line-resolution parameter of the ESR spectrum, which reflects the degree of regularity of molecular arrangement, first decreases, then increases somewhat, and finally becomes almost constant during fatigue cycling. The concentration of radicals trapped in a fatigued sample is larger than that in the unfatigued one. The excess radicals disappear during heating by a process of low activation energy in the temperature region of the mechanical γ dispersion. The degree of crystallinity evaluated from x-ray diffraction and density scarcely changed during fatigue cycling. The size of the mosaic block crystals estimated from the radical trapping capacity showed a rapid decrease at the initial stage of fatigue. The x-ray paracrystalline analysis indicates a similar but not so drastic decrease. These results lead to the conclusion that the slight disordering of crystalline texture accompanying the decomposition of mosaic block crystals in the lamellae occurs at the initial stage of the fatigue process, following which crystals reform somewhat and then remain unchanged until the initiation of necking.

Necking die assembly with internal rollers

Abstract: A die assembly for forming a reduced-diameter neck on the end of a cylindrical can body having a side lap wherein no orientation of the can lap relative to the necking die is required. The can body is pushed into a ring necking die and one or more rollers roll along the inside of the can body to press the can body end outwardly against the inwardly facing reduced-diameter die surface of the necking die, the rollers being yieldable at all times away from the die so that they may roll over the can lap wherever it may be encountered.

Apparatus for controlling the winding of web material on a dye beam

Abstract: This specification discloses apparatus for controlling the winding of web material on a dye beam, wherein the tension on the material is reduced as the material increases in diameter and the opposite outer edges of the material are prevented from curling and/or necking as the material is wound upon the dye beam.

Local and gross deformations in cracked metallic plates

Abstract: The local deformation near a crack tip were investigated when a cracked specimen was stressed beyond general yielding. Crack opening displacements, and near tip strains were measured by the moire technique. The gross elongation in the cracked-section was also measured. These three quantities were found to be linearly related to each other. Surface strain distributions near a crack are influenced by the local necking at the crack tip, which is controlled by the plate thickness.

The Mechanical Behavior in Tension and Fracture Characteristics of Stainless Steel Thin Rolled Foils.

Abstract: Tensile specimens in the thickness range 3.76 μ to 1.02 mm, prepared from thin rolled type 304 stainless steel foils, and recrystallized to a lateral grain size ∼ 15-30 μ, were strained to fracture in an Instron testing machine at an elongation rate of 8.33 x 10-4 mm/sec in order to experimentally determine the effect of foil thickness on mechanical behavior in tension. Fracture surfaces of broken samples were subsequently examined by scanning electron microscopy to observe transverse foil necking, and characterize the fracture mode.

The Necking of Cylindrical Bars Under Lateral Fluid Pressure

Abstract: A cylindrical specimen is observed to neck when subjected to fluid pressure on the lateral surface only. Moreover, the uniform elongation of the specimen at the onset of necking is found to be the same as that in simple tension. A theoretical analysis for the necking problem is presented as an attempt to explain this phenomenon. The experimental investigation is also fully described.

Brittle fracture of precompressed steel as affected by hydrostatic pressure, temperature and strain concentration

Abstract: Highly precompressed 1020 HR steel, 0.65 prestrain at 400°F (204°C), tested in nominally uniform tension at −80°F (−62°C) fractures at about 110,000 psi (760 MN/m2) with less than 0.02 plastic strain. Yet the addition of a hydrostatic pressure of less than 7000 psi (48 MN/m2) converts this visually brittle fracture to a ductile one with appreciable necking. The explanation of this surprising experimental result is shown to follow directly and simply from the combination of a tensile stress criterion of fracture, strain concentration and the low tangent modulus of the stress-strain curve in tension beyond the Bauschinger transition region of a few percent of plastic strain. Temperature dependence and strain-rate dependence of brittle fracture similarly are predictable in an almost trivial manner from the appropriate stress-strain curves for different amounts of precompression. So also is the amazingly high ductility or fracture toughness of the most complex of perforated or notched statically loaded structures of mild steel in an undamaged or fully annealed state.

Studies on the Low Cycle Fatigue of Steels (2 nd Report)

Abstract: A Method for prediction of the life of necked-out type fatigue fracture under low cyclic loading was proposed. Practically, in case of the necked-out type fatigue fracture, it may be assumed that fatigue life has been used up when necking began. In such a case, when accumulated permanent strain, em, amounts to critical value, eus, the fatigue failure occurs, and the critical value is nearly equal to the general elongation of material, eu0. Then a relation between strain and number of cycles is expressed approximately as follows;Δem·Nus=eus-e0=eu0-e0where Δem is a cyclic creep rate at the stationary stage which occupied major part of whole fatigue life, Nus is number of cycles to the fatigue failure and e0 is a permanent strain at the first cycle. The cyclic creep rate Δem is correlated to nominal maximum stress, σmax, and stress ratio, R, and the following relation is assumed.(σB-σmax) /σY= (∑ Ai·Ri) ·ΔemβConstants, Ai, and exponent, β, in the above equation are able to be obtained from statical test results. In order to prove the validity of these equations, mild steel specimens were tested under constant repeated axial loads with several stress ratios, and the test results were compared with the values calculated by the equations, and fairly good agreements between them were obtained.

The Influence of a Dispersion on the Ductility and Fracture Behavior of Iron

Abstract: Iron and iron--thoria strips were produced by a technique involving coprecipitation, hydrogen reduction, compaction, and hot rolling. Flat specimens of iron and two thoria-dispersion strengthened alloys were pulled in tension at temperatures in the range 77 to 373 deg K. The low-temperature ductility of the hot-rolled powder compacts was improved by the dispersion, and the ductilebrittle transition temperature was reduced. The onset of mechanical twinning, discontinuous yielding, and Luders band failures was suppressed to lower temperatures in the dispersion hardened material. The necking and fracture behavior was strongly influenced by the dispersion. Stringers of thoria particles acted as crack initiation sites, particularly if they lay transverse to the loading direction. (auth)

Finite deformation of elasto-plasti c solids ole

Abstract: A theoretical basis is established for analysis of finite deformation of metals. The obser-vation that finite deformation of such elasto^plastic materials may be viewed as a process ratherthan an event leads to derivation of a complete initial-and boundary-value problem distinguish-ed by its quasi-linear nature. This feature of the formulation motivates adoption of an incre-mental approach to numerical problem solving. Numerical solution capability is establishedfor problems of plane stress and plane strain. The validity of the theory and numerical analy-sis is demonstrated by consideration of a number of problems of homogeneous finite deforma-tion for which analytic solutions are available.Subsequently the analysis is employed for the investigation of necking in flat metal tensilebars. The results of this investigation provide the first full numerical solutions for tensilenecking in plane stress and plane strain. In addition a basis is provided for assessment of thevalidity of stress-strain relations inferred from tensile test data.

Application of the dislocation theory of fracture to polymeric materials

Abstract: The macrocharacteristics of the mechanical strength of crystalline polymers are estimated on the basis of the dislocation theory of fracture. The values obtained for the breaking and safe stresses and the necking stress are in good agreement with the experimental results.