Plastic Metals Research Articles

Rigid-Plastic Model of Chipping During Metal Cutting

A rigid-plastic model of shearing chip formation is proposed. The model is based on the Mohr-Coulomb yield criterion and allows one to determine the size of chipped elements in orthogonal cutting of plastic and brittle metals.

Nanosized mechanocomposites and solid solution in immiscible metal systems

Mechanical alloying is the most perspective method for preparation of nanosized mechanocomposites in non-mixed liquid and solid metals. It is known that plastic metals get fragile in the presence of the liquid metal phase, and this phase spreads over the surface of solid metal due to good wettability. Temperature of milling bodies can rise by several hundred degrees in the high-energy planetary ball mill. Low melting metals, such as gallium, indium, tin, and bismuth can melt on the surface of the balls. So, a pair composed of liquid and solid metal can form in an activator. In such cases, it can be assumed that mechanical activation of metal systems with positive mixing enthalpies, can allow forming a morphologically metastable structure with unusually high concentration of inter-phase boundaries due to easy-melting component spreading along the boundaries of particles formed by comminution of the solid component. Nanosized mechanocomposites were prepared for Cu-Bi and Fe-Bi systems. The content of Bi in these systems must be lower than 3 at. (10 wt.)%. The obtained mechanocomposites represent nanosized particles of copper (or iron) coated with a layer of bismuth in 2-3 atoms. Mechanical activation of the formed composites leads to the formation of supersaturated solid solutions.

A general framework for generating convex yield surfaces for anisotropic metals

The aim of this paper is to develop a general procedure to create yield surfaces (both isotropic and anisotropic) for elastic plastic metals with particular reference to sheet metal forming operations. Due to the fact that the forming limit of sheet metals requires a very accurate prediction of the yield limits and their normals, it is desirable to create yield functions that offer independent control of the yield points and the normal to the yield surface at a particular stress state without affecting its global convexity properties and the yield points at other states of stress. We achieve this by creating a class of yield surfaces that are obtained by the intersection of a number of elementary convex surfaces (such as planes and cylinders); the edges and corners that are a result of the intersection are smoothened by aLp-norm smoothing technique to give rounded corners. The yield surfaces generated by this procedure are centrosymmetric in nature and are thus limited in their applicability.

Stress-strain diagrams of metals under large uniform compressive strains

A new method was worked out for obtaining compressive stress-strain diagrams for large uniform plastic strains. For the first time correct compressive stress-strain diagrams of copper M2, steels St. 3, 45, 12KhN3A, U8A, and of the high-strength cast iron VCh-50 were obtained. In correct experiments at uniform state of stress the previously suggested postulate of perfect plasticity was confirmed for all the materials without exception. It was established that the compressive stress-strain diagram is described by a parabola fully determined by three fundamental constants, viz., yield point, maximal stress, and the strain at which this is attained. It is shown that for plastic metals the stress-strain diagram for large strains can be predicted from two initial points of the curve, and also from the results of standard tensile tests.

Three-dimensional axisymmetric flow in perfectly plastic metals

A method is presented for solving the three-dimensional axisymmet-ric field equations for a perfectly plastic material which obeys the von-Mises yieldcriterion and the Levy-Mises flow law. The method is used for the particular case inwhich a small axisymmetric perturbed flow is superposed on a uniform flow withoutflow reversal taking place. The method then leads to solving a fourth order differen-tial equation for the velocity potential. The special case of a thick cylindrical shellunder compressive flow is examined. The solution so obtained, being derived fromthe three dimensional theory, includes a correct treatment of transverse shear distor-tion. A preferred mode of instability is identified having a wave-length in reasonableagreement with that obtained experimentally by other workers.

Three-dimensional axisymmetric flow in perfectly plastic metals

A method is presented for solving the three-dimensional axisymmetric field equations for a perfectly plastic material which obeys the von-Mises yield criterion and the Levy-Mises flow law. The method is used for the particular case in which a small axisymmetric perturbed flow is superposed on a uniform flow without flow reversal taking place. The method then leads to solving a fourth order differential equation for the velocity potential. The special case of a thick cylindrical shell under compressive flow is examined. The solution so obtained, being derived, from the three dimensional theory, includes a correct treatment of transverse shear distortion. A preferred mode of instability is identified having a wave-length in reasonable agreement with that obtained experimentally by other workers.

Pore expansion in plastic metals under spall

Pore expansion in plastic metals under spall

Finite element analysis of elastic–plastic materials displaying mixed hardening

AbstractThe problem of non‐radial loading (in the stress space) of structures of elastic–plastic metals is discussed. A concept of combined isotropic and kinematic hardening is proposed. For a material displaying such mixed hardening, constitutive equations are derived and incorporated in a finite element program for the analysis of elastic–plastic 2D‐structures. When determining the numerical solution of the non‐linear equations, the program enables the investigator to select an optimal combination of step by step and iteration procedures. The computed results are compared with experimental results and with results from calculations based on the overlay model, Reference 25.

The relationship of axial strain induced by cyclic torsion to metal stability and fatigue failure in Ti-6A1-4V

The alloy Ti-6Al-4V was cycled with reversed torsional strains. The torsional strain amplitudes were all below the yield strain (pseudoelastic fatigue); therefore, plastic deformation is small and the conventional techniques for studying fatigue in plastic metals are ineffective. However, it was observed that the second order axial strain accumulation “Ronay effect” can be used to monitor the metal during fatigue. At a torsional strain amplitude of about ± 0.005, the axial strain accumulation began to increase rapidly. It was observed that the cycles to failure vs strain amplitude curve changes from ( S) type to ( H) type at strains of about 0.005. The axial strain accumulation and the knee of the fatigue life curve appear to be related. The observed results are explained by assuming that the knee, which occurs at strains well below yield, results from inelastic deformation at the grain and phase boundaries. A test with a single specimen showed that the point of increasing axial strain accumulation could also be located by continually increasing the torsional strain amplitude.

The kinetics of the thermofluctuation-Induced micro- and macrocrack growth in plastic metals

The growth rate of main cracks at different temperatures and under the static load has been investigated in aluminum and zinc foils. The observation of the crack growth during loading has been made by means of general microfilming as well as by electron microscope. It has been shown that the growth of main cracks occurs by means of the initiation, development, and coalescence of secondary microcracks which arise at the tip of the main one. The interaction and the growth of cracks of all dimensions, from submicroscopic and up to macroscopic, takes place at constant rate, without any steps. The rate depends exponentially upon the applied stress and the reciprocal test temperature; the activation energy of the crack growth process is near to that of sublimation for the metals.

The effect of load on the cohesive strength of virgin surfaces of plastic metals

Virgin surfaces of copper, carbon steel, brass and lead have been obtained by the method of controllable internal breakdown. These surfaces have been found to feature high values of the cohesion coefficient which is, however, different for different materials. Cohesive strength is found to be a linear function of the applied load value in the load interval up to 0.6 H v. In the case of lead the linearity is shown to fail in the range of load values exceeding 0.6 H v. No failure of cohesion due to elastic stress relief was observed under the experimental conditions.

Cohesion of juvenileous surfaces of plastic metals: Wear, Vol 11, No 5 (May 1968) pp 333–340

Cohesion of juvenileous surfaces of plastic metals: Wear, Vol 11, No 5 (May 1968) pp 333–340

Cohesion of juvenileous surfaces of plastic metals

A method of the controllable internal breakdown is described which permits juvenile surfaces of different solids to be produced and maintained in their initially pure state for a practically unlimited period of time. Cohesive properties of copper, lead, brass and steel have been studied and these materials are shown to have a high cohesive ability, provided their surfaces are virgin. The cohesive strength obtained for copper and lead was equal to the applied compressive force. The value of the coefficient of cohesion has been investigated as a function of the number of repeated test cycles and the degree of alignment of the contacting surfaces.

Relation between hardness and resistance to cold welding [antifriction property] of metals and alloys

1. The resistance to cold welding or seizing which is characterized by the mean punch pressure at which a strong bond is achieved, depends on the alloy content of the solid solution (Fig. 3). The increase in the seizing resistance is proportional to the strengthening owing to alloying (cf. Fig. 2). 2. The ratio of cold welding pressure of any alloy to its hardness as-annealed is constant and about 1.5. For alloys containing higher concentrations of alloying elements and showing a high strainhardening rate, this ratio is slightly over 2 (Fig. 4, broken line). For pure plastic metals, this ratio as determined by the samethod likewise is 1.5 [8]. 3. The extra pressure needed for cold welding per at-% of the alloying element determines the specific cold welding pressure for each alloy which depends on the solubility limit. 4. The fact that the welding pressure/hardness ratio determined in this study for metals and alloys at different deformations required as for cold welding is constant shows that the deformation of the compressed metals along their interface can be very approximately described by the strain magnitude arbitrarily calculated as the ratio of initial to final thickness of the specimens. 5. Transfer of metal and the degree of surface damage during, rubbing are governed by the shear strength of the weld centers formed. The heat developed and the pi tion afforded by oil or other films are here important. For this reason a true picture of the transfer of metal during friction can be obtained only from experiments with the lubricant to be actually used in service in the particular antifriction application.

The plastic deformation of iron and the formation of Neumann lines

The object of the investigation described in the present paper was to ascertain the manner in which wide variations in speed affect the mechanism of deformation in plastic metals. Iron was selected as the first metal for experiment, mainly because it is known that rapid deformation produced by shock is accompanied in this metal by special features, known as Neumann lines or lamellæ. When a piece of nearly pure iron, of suitable size and shape, with one face polished and etched, is subsequently subjected to plastic deformation at a moderate rate, the crystal surfaces, when examined under the microscope after deformation, show the well-known appearance of slip bands (1). The present experiments were undertaken in the first instance to ascertain whether the character, number and appearance of such slip bands would be appreciably affected by varying the rate of deformation over a wide range. For the sake of convenience, deformation by compression has been employed, the metal being used in the form of small rectangular prisms, measuring in some instances 0·44 inch by 0·44 inch in section by 0·7 inch in height, and 0·3 inch by 0·3 inch in section and 0·5 inch in height. These were prepared with considerable care, particularly in regard to their length, and the amount of deformation applied to them was measured in every case. In the majority of experiments these small prisms were placed in a compression testing machine, and were surrounded by a hardened steel ring of such size as to limit the deformation exactly to the desired amount. A similar guard-ring was employed in connection with deformation by a blow from a falling weight. The actual amount of deformation could be varied, while still employing the same ring, by inserting small flat strips of hardened steel as packing-pieces under the specimen of iron.

The plastic deformation of iron and the formation of Neumann lines

The object of the investigation described in the present paper was to ascertain the manner in which wide variations in speed affect the mechanism of deformation in plastic metals. Iron was selected as the first metal for experiment, mainly because it is known that rapid deformation produced by shock is accompanied in this metal by special features, known as Neumann lines or lamellæ. When a piece of nearly pure iron, of suitable size and shape, with one face polished and etched, is subsequently subjected to plastic deformation at a moderate rate, the crystal surfaces, when examined under the microscope after deformation, show the well-known appearance of slip bands (1). The present experiments were undertaken in the first instance to ascertain whether the character, number and appearance of such slip bands would be appreciably affected by varying the rate of deformation over a wide range. For the sake of convenience, deformation by compression has been employed, the metal being used in the form of small rectangular prisms, measuring in some instances 0.44 inch by 0.44 inch in section by 0.7 inch in height.