Showing papers on "Deformation (engineering) published in 1970"

The deformation of plastically non-homogeneous materials

Abstract: Many two-phase alloys work-harden much faster than do pure single crystals. This is because the two phases are not equally easy to deform. One component (often dispersed as small particles) deforms less than the other, or not at all, so that gradients of deformation form with a wavelength equal to the spacing between the phases or particles. Such alloys are ‘plastically non-homogeneous’, because gradients of plastic deformation are imposed by the microstructure. Dislocations are stored in them to accommodate the deformation gradients, and so allow compatible deformation of the two phases. We call these ‘geometrically-necessary’ dislocations to distinguish them from the ‘statistically-stored’ dislocations which accumulate in pure crystals during straining and are responsible for the normal 3-stage hardening. Polycrystals of pure metals are also plastically non-homogeneous. The density and arrangement of the geometrically-necessary dislocations can be calculated fairly exactly and checked by electr...

The relation between polycrystal deformation and single-crystal deformation

Abstract: A phenomenological description of crystallographic slip and pencil glide in single crystals is outlined, with emphasis on the behavior under prescribed strains Theoretical relations are established between these single-crystal properties and the behavior of quasi-homogeneous, quasi-isotropic polycrystals deforming uniformly on a macroscopic scale, at subdiffusive temperatures Experimental comparisons between single crystals and polycrystals are reviewed, considering flow stress, work hardening, temperature and strain rate effects, and various effects of grain size

On the Structure of Stress-Strain Relations for Time-Dependent Plastic Deformation in Metals

Abstract: The paper is concerned with the structure of multiaxial stress-strain relations in timedependent metal plasticity, as for transient creep and rate sensitive yielding. First. a general kinematical relation is developed between the macroscopic inelastic strain tensor and microstructural slip displacements, as modeled either by continuum shearing on crystallographic planes of individual grains or by the motion of discrete dislocation lines. It is assumed that at any given slipped state, the rate of slipping on a particular system is governed by the resolved shear stress on that system (or by the local \"forces\" on dislocation lines). 'This leads to the primary result of the paper: Components of the macroscopic inelastic strain rate tensor are deriz!able, at each instant in the course of deformation, from a potential function of stress, General features of the flow potential surfaces in stress space are discussed, and some specific functional forms are examined, Linear viscoelasticity and tirre-independent plasticity are developed as limiting cases of the flow potential formulatic!, and the appropriateness of a potential function for stationary creep is discussed,

Physical properties of trabecular bone

Abstract: The purpose of this investigation was to determine the relation between compressive strength and density of human vertebral bone, the relation between trabecular orientation and compressive strength, and the effects of the strain rate on compressive strength. A total of 71 samples obtained at 23 post mortem examinations were investigated. Density determinations were made and two different densities were calculated: (1) the apparent density as wet weight divided by the total sample volume; and (2) real density as wet weight divided by the volume of bone matrix determined by water displacement. Compression test measurements were made on the fresh samples using an Instron materials testing machine at controlled deformation rates. The results indicated a highly significant relation between apparent density and compressive strength. Similar relations were found between dry weight per total sample volume and ash weight per total sample volume and compressive strength. As the apparent density increased, the compressive strength of the sample decreased. A negative regression between real density and compressive strength was found in one of the experimental series of this investigation where, as the density of the bone matrix increased the compressive strength of the sample decreased. This relation was not significant in the other two experimental series. The strength of vertebral bone was significantly higher along the superior-inferior axis or vertical axis than along the two other directions. Samples showed a significantly greater ability to withstand compression when the deformation rate was 1 cm per minute than they did when the rate was 0.01 cm per minute.

Structure and properties of thermal-mechanically treated 304 stainless steel

Abstract: Mechanical and thermal-mechanical treatments of 304 stainless steel enables yield strengths of over 200,000 psi to be obtained with elongations better than 10 pct. Electron microscopy, X-ray, and magnetic techniques show that during deformation, strain induced γ → ∈ → α transformation occurs with further thermal nucleation of α achieved by aging up to 400°C. The yield strength is linearly proportional to the amount of ° irrespective of the treatment used to form α. The yield strength is given by αy = 225f + 48.65 ksi, where ƒ is the volume fraction of martensite. Softening occurs by aging at 500°C and above due to a decrease in percent α which may occur by renucleation of γ. The system is an unusual form of composite strengthening; hard martensite particles are formed within the austenite, and the percent α (and thereby the mechanical properties), can be controlled by the mechanical/thermal-mechanical processing.

The formation of micro shear bands in polystyrene and polymethylmethacrylate

Abstract: The conditions under which amorphous polystyrene and polymethylmethacrylate deform inhomogeneously by the formation of fine shear bands have been investigated. The thermal history of the sample and the temperature and strain rate at which the deformation is carried out have been found to be critical. Stress-strain data have been obtained in order to test the criterion for inhomogeneous deformation presented in the preceeding paper. In addition, stress-birefringence measurements have been made on polystyrene and the possible influence of molecular transition processes on the plastic deformation process is discussed.

The Direct Measurement of the Strength of Metals on a Sub-Micrometre Scale

Abstract: Four types of experiment have been carried out to investigate the strength properties of annealed metals when the stressed volume is small enough to lie between the existing dis­locations in a crystal. These are (i) indentation experiments of a soft metal surface with a hard stylus, (ii) blunting of a soft metal tip against a hard surface, (iii) compression of individual metal crystals, and (iv) bending of thin filaments. The experiments were performed in either a scanning electron microscope or a transmission electron microscope with the use of micro­-loading devices capable of applying loads down to 0.1 mgf (10 -6 N). In the blunting experi­ments carried out in the transmission electron microscope it was possible to observe disloca­tions directly in the tip during loading. The majority of the experiments were carried out on gold. The results showed that strengths similar to the theoretical value can be achieved but in the experiments in which the stress was applied at an external surface (experiments (i), (ii) and (iii) above) the strength was strongly dependent on the condition of the interface. The strength that could be sustained by a region of perfect crystal in contact with a hard metallic surface was about five times less than the theoretical strength. These relatively low strengths are probably due to interfacial tractions producing very high local stresses. The introduction of some polymeric or amorphous layer at the interface raised the strength to the theoretical level. It is suggested that this is due to the fact that the polymeric layer removes most of the stress concentration. Transmission electron microscopy through aluminium tips during blunting showed that plastic deformation could take place at quite low stresses in a dislocation-free crystal with no dislocations remaining in the crystal during the early stages of blunting. Dislocation build up only occurred in the later stages of deformation. The yield stress was found to decrease with plastic strain in all the experiments, and could fall to values which were not substantially greater than those observed in large specimens. Comparison of the compressive strength of two spherical gold crystals, 0.5 μ m and 2 mm in diameter respectively, showed that the small crystal was only twice as strong as the large crystal after they had both undergone equal amounts of compression. The maximum strengths observed for gold ( μ /20) are greater than those calculated by Kelly (1966) ( ca . μ /50) and are nearer the value deduced by Brown & Woolhouse (1970) for the generation of dislocations around precipitate particles in alloys. The low strengths observed on deformed crystals are considered in the context of dislocation generation in small volumes and it is concluded that although a source mechanism of the Frank–Read type may be able to operate on this scale, some other source mechanism may also exist.

The strain rate dependent plastic flow behavior of zirconium and its alloys

Abstract: The effect of strain rate (10−5 { $$ \sim $$ } 10−1 min−1) on the plastic flow behavior was examined by means of tension tests over a range of temperatures (RT { $$ \sim $$ } 500°C). The principal material examined was Zircaloy-2, and the others were iodide zirconium and binary alloys of Zr-0.1 pct O and Zr-1.5 pct Sn by weight. In Zircaloy-2, the behavior was characterized by a sudden increase of flow strength with decreasing strain rate; concurrently the ductility decreased. Other interrupted and hold time experiments showed that the process is essentially that of a strain rate-induced strengthening phenomenon, with several features which could be identified with the strain aging process. It was further shown that the critical range of strain rates and temperatures at which the anomalous behavior took place could be correlated with the minimum in the strain rate sensitivity of flow stress. A similar but less pronounced flow behavior was observed with pure zirconium and its binary alloys, but with no marked change in the tensile ductility. From these results, the role of oxygen and tin on the early stage of plastic flow behavior was discussed in terms of dislocation-impurity interaction mechanisms. It was however concluded that these elements are not directly responsible for the ductility loss at slow strain rate in Zircaloy-2.

Mechanical properties of engineering materials

Abstract: This chapter discusses the mechanical properties of engineering materials. The mechanical properties of those materials that are commonly used in engineering need to be understood to determine the size and shape of each member to carry its respective load. First, it is necessary to define some of these properties. When a direct force is applied to a structural member it tends to lengthen if the force is tensile, or to shorten if the force is compressive. Strain is a measure of the deformation, and direct strain is the deformation determined along the line of action of the direct force. If a length of mild steel bar is subjected to an axial tensile load the extension of the specimen is, at first, very small and would be undetectable by direct measurement, such as with a scale. It is necessary, therefore, to make use of an instrument known as an extensometer. This instrument is clamped over a known length of the specimen, and the elongation of that length is magnified so that it can be determined easily. As the tensile load increases so the extension increases until a point is reached where the extension increases rapidly and becomes measurable by dividers and a scale.

The Influence of Austenite Strength Upon the Austenite-Martensite Transformation in Alloy Steels

Abstract: The resistance of austenite to plastic deformation (austenite flow stress) was measured using a high temperature tensile apparatus. The flow stress was then correlated with the Ms temperature as determined magnetically during subsequent cooling. In one part of the study, the flow stress of the austenite was varied only by work hardening the austenite, allowing the austenite composition, which is known to affect Ms, to be held constant. A decrease in Ms temperature with increasing austenite flow stress was observed. This observation was supported by the observation of a decrease in the amount of austenite transformed at 25°C. In the other part of the study, a series of alloy steels of different chemical compositions was tested. A decrease in Ms temperature with increasing austenite flow stress was again observed. Strengthening of austenite by plastic deformation was shown not to change the chemical driving force for transformation. The effect of deformation on Ms temperature thus results from its influence on either the nucleation or the growth process. While the effect of austenite deformation on martensite nucleation is uncertain, specific nucleation models can account for only approximately one-third of the nonchemical free energy change which accompanies transformation. A proposal, consistent with the observations, was made that the energy expended for the deformation of austenite during martensite plate growth could reasonably account for a substantial part of the nonchemical free energy change.

Plastic deformation of polypropylene. VI. mechanism and properties

Abstract: Small- and wide-angle X-ray investigation of drawing and annealing of polypropylene quenched films in wide ranges of draw ratio (λ = 1-20) and of temperature (20°-150°C) shows the discontinuity of the elementary act of transformation in the microneck of the original lamella of the microspherulitic into the microfibrils of the final fiber structure. The long period, the orientation, and the crystalline disorder change abruptly. The continuity of macroscopical deformation in the neck is a consequence of superposition of the effects of a great many microneck zones which are more or less randomly scattered over the whole neck area. In the first stages of plastic deformation before the appearance of micronecks the lamellae parallel to the draw direction are compressed and those perpendicular to the draw direction become physically separated. This was checked by noting the drastic reduction of scattering after the introduction of dodecane which fills the holes between lamellae created in the initial st...

Plastic Deformation in Fine-Grain Ceramics

Abstract: Plastic deformation in fine-grain (i.e., ≤ 10 µ)ceramics is discussed. It is shown that fine-grain polycrystals can be exceptionally ductile, the fine grain size enhancing diffusional deformation and grain boundary sliding processes. The deformation is sensitive to both grain size and temperature.

Work strengthening by a deformation-induced phase transformation in 'MP Alloys'

Abstract: Work strengthening and microstructure were investigated for a class of alloys, designated “MP Alloys”, containing 20 pct Cr, 10 pct Mo, and the remainder cobalt and nickel in proportions ranging from 60Co∶10Ni to 30Co∶40Ni. These alloys, in the fully annealed, homogenized condition, have a fcc structure with yield strengths ranging from about 45 to 60 ksi. Deformation at room temperature rapidly increases the yield strength of the alloys to about 250 ksi. Structural analyses by X-ray and electron diffraction techniques indicate that this marked increase in strength is associated with a deformation-induced martensitic transformation forming a network of extremely thin hcp platelets within the fcc grains. The nature of this martensitic transformation was studied as a function of alloy composition, deformation temperature, and structural variables, such as the platelet size, thec/a ratio of the hcp phase, and twinning.

Compression Strength of Polyester Resin Reinforced with Steel Wires

Abstract: The behaviour of polyester resin reinforced with hard-drawn steel wires and with fully-softened wires of the same steel has been studied in compression tests on free-standing cylinders in which the wires are parallel with the direction of the applied load. Despite the predictions of various theories of the compressive strength of composites, the two materials behave dissimilarly, and their strengths fall well below values predicted by models based on treatments of the fibres as col umns on an elastic foundation. Fibres appear to buckle into helical shapes rather than into the planar forms usually postulated, and it is likely that the lower elastic energy required for this deformation could be the factor responsible for the disagreement between theory and experiment.

Strength and deformation of pyrite and pyrrhotite

Abstract: Brittle fracture of pyrite, twinning and translation-gliding plastic deformation of pyrrhotite

Quantitative structural characterization of the mechanical properties of isotactic polypropylene

Abstract: This investigation introduces quantitative morphological criteria for defining the structural state of a polymer sample and develops a general structural state deformation model for characterizing mechanical behavior. This general model discards the present fragmented view of the autonomy of observed mechanical processes (yielding, fracture, recovery, etc.) and brings the fabrication process, the mechanical tests, and even the construction of the sample (fiber or film) into focus as simply different aspects of a single process of deformation. The fabrication, fracture, yielding, and recovery behavior of two series of uniaxially drawn isotactic polypropylene films and two series of drawn isotactic polypropylene fibers, totaling thirty different structural states, have been deter-mined. The structural state of each of these samples is known quantitatively. The measurements were made over a range of strain rates from 1 to 1,000,000%/min and temperatures from 23° to -196°C. This study has resulted in...

An experiment proving that dilatancy is a pervasive volumetric property of brittle rock loaded to failure

Abstract: An Experiment Proving that Dilatancy is a Pervasive Volumetric Property of Brittle Rock Loaded to Failure. Dilatancy, that is, volumetric expansion relative to the compression resulting from elastic deformation is often observed in the compression of rock specimens to failure. This phenomenon is of considerable importance to the understanding of the processes of rock failure and in their application to geological and engineering problems. Most of the observations to date have been made on small cylindrical specimens of rock, with a high surface area to volume ratio; frequently by means of resistance strain gauges cemented to the surface of the specimen. The possibility of the measured dilatancy being a superficial phenomenon due to anomalous surface deformations cannot be excluded. Measurements of dilatant deformation on the inside and outside of hollow cylindrical specimens during compression under homogeneous stresses provide a means of resolving this uncertainty. Superficial dilatancy would result in a relative diminution of the cross-section of the hole as that of the outside of the specimen increases with deformation; whereas pervasive volumetric dilatancy would result in similar relative increases in the cross-section of both the hole and the specimen. Experiments on hollow cyclindrical specimens of quartzite show that dilatancy is a pervasive volumetric phenomenon.

Plastic Deformation of Corundum Single Crystals

Abstract: The stress-strain curves and the defect structure of corundum single crystals (sapphire and ruby) were studied. The influence of impurity (Cr) presence, crystallographic orientation, temperature, and deformation rate was investigated. Chromium makes corundum harder and causes a yield point phenomenon. The yield point was also increased by the transition from 60° to 90° orientation of the specimens, by lowering the temperature, and by an increase in the deformation rate. In 60°-specimens the deformation occurs by means of gliding on basal planes in 〈1120〉 and 〈1010〉 directions. In 90°-samples beside this one gliding in (1010), (1011), (2021) and (2243) is found. [Russian Text Ignored.]