Showing papers on "Necking published in 1972"

Plastic flow and fracture of metallic glass

Abstract: The tensile flow and fracture behavior of three Pdo.8Sio2-based alloys in the glassy, “microcrystalline,” and fully crystalline condition has been studied. The glassy alloys flow plastically to a total strain of approximately 0.5 pct e, and exhibit proportional limit stresses of approximatelyE x 10~2 whereE is Young’s modulus. This plastic flow is accompanied by the formation of shear deformation bands on the specimen surfaces. Fully crystalline alloys are extremely brittle and fracture via intergranular cracking. Fracture surfaces of the amorphous and “microcrystalline” alloys are inclined at 45 deg to the tensile axis and exhibit two morphologically distinct zones. One zone is relatively featureless while the other contains a “river” pattern of local necking protrusions. Detailed comparison of opposing surfaces indicates that fracture is preceded by large local plastic shear which produces the smooth zone while the local necking pattern is produced during rupture. These observations form the basis for the hypothesis that plastic flow in the glassy material occurs via localized strain concentrations and that fracture is initiated by catastrophic, “adiabatic” shear.

A numerical study of necking in circular cylindrical bar

Abstract: The boundary-value problem for a circular cylindrical bar in uniaxial tension is posed in two ways. In one the ends of the specimen are assumed to be cemented to rigid grips; while in the other they remain shear free. With the latter end-condition, necking initiates by a bifurcation from a state of uniform tensile stress. The eigenvalue problem governing bifurcation is solved by means of the finite element method, and a comparison is made between the results obtained here and the conventional maximum load criterion. The post-bifurcation behavior is then determined. The entire deformation history of the bar cemented to rigid grips is also calculated and the effect of end conditions on the deformations in the necked-down bar is assessed.

The macroscopic volume changes of selected polymers subjected to uniform tensile deformation

Abstract: High density polyethylene (PE) , polycarbonate (PC) , and polymethylmethacrylate (PMMA) were subjected to uniaxial tensile deformation up to the onset of instability or necking. Simultaneous readings of longitudinal extension and transverse contraction (width and thickness) were obtained continuously during the loading period. From these data, plds of longitudinal versus “average” transverse strain were produced and it was found that the trends were neither constant nor linear over the full strain range employed. Additional plots of per cent volume change versus longitudinal strain indicate that the PMMA and PC show a maximum volume increase of about 0.6% while PE shows a maximum volume decrease on the order of 2.5%. Similar volume decreases have been noted by others and it would appear that structural changes are the most likely cause of this behavior.

The Effect of Strain History on the Crystallization Behavior of Bulk Poly(ethylene Terephthalate)

Abstract: The crystallization of bulk tensile samples of glassy, amorphous poly(ethylene terephthalate) was studied as a function of various deformation and annealing treatments. Per cent crystallinity, orientation function, crystallite size, and long period spacing were measured to characterize the samples. Crystallization, necking, and orientation occurred during straining when, at a given temperature, the strain rate was sufficient to generate a critical stress level within the material. During subsequent annealing crystallization was nucleated by the crystallites formed during deformation. The observed behavior was consistent with a model proposed by Dismore and Statton for drawn nylon 66 yarns and which was previously discussed by Dumbleton for the case of poly(ethylene terephthalate).

Applications of finite elements in space and time

Abstract: The paper initiates a major extension of the matrix displacement method concerning the analysis of dynamic phenomena in the presence of material and geometric non-linearities. In particular, elasto-plastic behaviour as well as large displacements are taken into account. An iterative procedure of solution of the nonlinear matrix equations is discussed. The application of the theory is described in detail in two examples. The first considers the simple static problem of a rectangular flat strip in a tensile test. The iterative calculation may be carried out for deformations as large as required and shows clearly the necking effect. More ambitious is the second example which demonstrates the non-linear dynamic theory on a cyclindrial deformable billet under the impact by a heavy rigid body. The momentum of the weight and the property of the billet are such that the latter will undergo large plastic deformations. If so required, it is straightforward to incorporate damping and also allow for friction forces on the contacts. The direct applicability of the technique to forging problems is evident. The solution of the dynamic phenomenon is accomplished by extending the discretisation of space also into time. In particular, the inertia forces are taken to vary over a finite time element as a third order polynomial. Exceptional accuracy is achieved by this method.

Mechanism of necking as revealed by ultrasonic cavitation of polyethylene single crystals

Abstract: Ultrasonic cavitation of polyethylene single crystals and single-crystal aggregates in the form of cakes results in lamella fragmentation and necking involving the transformation of lamellar crystals into fibrillar crystals between 20 to 400 A in diameter. The smaller fibrils (∼20–30 A) have a very smooth appearance, whereas the larger ones (∼100–400 A) contain a beady structure about 100 A periodically spaced along the fibrils. The smoother microfibrils are suggested to contain extended chains that are formed by unfolding of molecules directly from the chain-folded lamellae as well as from the folded-chain crystals contained within the beady fibrils. The presence of the chain-folded crystals within the larger beady fibrils is shown in numerous instances to be due to incorporation of mosaic crystalline blocks originally present, but weakly connected to one another, in the lamellar single crystals. The necking process is deduced by observation to involve primarily a mechanical shearing of mosaic c...

Contribution to the theory of forced high-elastic deformation of polymer materials

Abstract: The large reversible and irreversible deformations of network polymers are examined. Expressions are derived for the stress tensor and for the conventional tensile stress. The uniaxial homogeneous extension curve has a maximum and a minimum if it is assumed that the coefficient of friction depends exponentially on the applied stress. The "necking" condition is derived for uniaxial tension. The results are compared with the experimental data.

Thermal fatigue resistance of Al-Si casting alloys for pistons

Abstract: Failures due to thermal fatigue often occur on the pistons made of aluminum alloys, especially on those of Diesel-engines. This paper reports the thermal fatigue resistance of Al-Si casting alloys used for pistons. The resistance to thermal fatigue was evaluated by measuring the number of cycles to failure versus total amount of strain, constraint stress or plastic strain. An apparatus for thermal fatigue testing was developed, where a high frequency induction coil was employed as a heating device. The results obtained were summarized as follows:(1) The thermal fatigue resistance of Al-Si alloy castings was in the following order:AC8B>AC8A> 15% hypersilumin> 18% hypersilumin> 23% hypersilumin. This indicates that increase in Si content lowers the resistance to thermal fatigue.(2) The resistance to thermal fatigue was related to elongation of specimens at tensile tests. Namely, materials with higher ductility showed higher resistance.(3) The hypoeutectic and eutectic Al-Si alloy castings failed owing to stress concentration by an necking effect produced by plastic deformation.(4) The hypereutectic Al-Si alloy castings, especially containing more than 18%Si, failed by preferential cracking in primary Si crystals. The cracking was caused by the difference in thhe amount of thermal expansion between Si crystals and the Al matrix.

Metallic positive expulsion diaphragms

Abstract: High-cycle life ring-reinforced hemispherical type positive expulsion diaphragm performance was demonstrated by room temperature fluid expulsion tests of 13" diameter, 8 mil thick stainless steel configurations. A maximum of eleven (11) leak-free, fluid expulsions were achieved by a 25 deg cone angle diaphragm hoop-reinforced with .110-inch cross-sectional diameter wires. This represents a 70% improvement in diaphragm reversal cycle life compared to results previously obtained. The reversal tests confirmed analytic predictions for diaphragm cycle life increases due to increasing values of diaphragm cone angle, radius to thickness ratio and material strain to necking capacity. Practical fabrication techniques were demonstrated for forming close-tolerance, thin corrugated shells and for obtaining closely controlled reinforcing ring stiffness required to maximize diaphragm cycle life. A non-destructive inspection technique for monitoring large local shell bending strains was developed.

Cold-drawn crystalline polyolefins. Effect of extension rate and temperature

Abstract: Load-extension diagrams are used to analyze the process of necking in various specimens of cold-drawn polyolefins and the effect of extension rate, temperature, and crystallinity.

Fine structure of polyester fiber prepared by the necking drawing

Abstract: Fine structure of polyester fibers prepared by the necking drawing of various undrawn fibers was investigated.The results obtained are as follows; 1. The birefringence of the necking region and the necking draw ratio of polyester fibers are strongly influenced by preorientation of undrawn fibers. For an isotropic undrawn polyester fibers, the maximum value of birefringence of the necking region and necking region and necking draw ratio were about 0.200 and 6 times, respectively.2. The birefringence of the necking region (Δnneck) vs necking draw ratio curves at low necking draw ratio for various undrawn fibers can be interpreted by Kuhn and Grun's theory for rubber photo-elasticity, and at high necking draw ratios, it agrees with Kratky's ‘first borderline case’.3. The experimental coefficients of Kuhn and Grun's theory, and K2=NkT increase with increasing pre-orientation of undrawn fibers.4. The crystallization induced by necking is dependent only on the necking draw ratio of the samples.5. The highest density obtained by means of necking drawing method was 1.369g/cm3 and the crystal structure of this sample is smectic at high orientation.

Plastic flow for axisymmetric-tension specimens with necking

Abstract: Plastic deformation of a tension specimen bounded by a curved surface of revolution is considered. Such a configuration may occur, for example, as a result of necking. The specimen material satisfies the Tresca yield condition and the associated flow rule. Approximate solutions for the stress distribution in the neck were examined in [6]. The extension of notched bars was investigated by numerical and graphic methods in [2, 4]. Below, the problem is solved analytically for a sufficiently smooth neck; a small degree of nonuniformity of the plastic properties is taken into account.