Showing papers on "Deformation (engineering) published in 1971"

The friction and creep of polycrystalline ice

Abstract: The work described in this paper falls into two parts. The first is concerned with a study of the deformation of polycrystalline ice (crystal size ca. 1 mm) in uniaxial compression and when subjected to indentation. The uniaxial compression experiments covered strain ratesfrom 10 -9 to 10 -2 s -1 and temperatures from 0 to —48 °C. It is shown that over the whole range of experimental conditions the secondary creep behaviour can be described by a single relation of the type where o is the applied stress, Q an activation energy an d A, a and n are suitable constants. This reduces to the more familiar power law over more restricted portions of the experimental curve. Over the whole range n has a value close to 3 but Q has two distinct values: 120 J mol -1 above — 8 °C: 78 J mol -1 below —8 °C. The in dentation hardness experiments cover loading-times from 10 -4 to 104 s and a temperature range of 0 to — 25 °C. The hardness behaviour may be linked with the creep properties using the analysis of Atkins, Silverio & Tabor (1966) which assumes that the rate-determining process is the diffusion of the hemispherical elastic-plastic zone surrounding the indenter into the undeformed material ahead. There is very good agreement between the hardness data and the creep parameters determined in the uniaxial compression experiments. In addition the hardness experiments enable experiments to be carried out at much higher compressive stresses.

Identification of structural processes in deformation of oriented polyethylene

Abstract: The investigation is concerned with the relation between changes in the submicroscopic structure, as revealed by low angle X-ray scattering in combination with the usual wide angle X-ray diffraction, and changes in the macroscopic sample dimensions during the deformation of oriented low density polyethylene. The samples examined are mainly drawn and rolled sheets possessing a double crystal texture, with a limited additional study on a drawn sample with fibre symmetry and on recently discovered single texture specimens. The deformations include tension and compression along selected sample directions applied mostly at room temperature, but also at various elevated temperatures. The salient feature of most of these experiments is the identity of the macroscopic strain and the changes in the submicroscopic periodicity along the direction in which the sample has been initially oriented. Even when this identity is not obeyed, as for deformation at the highest temperatures, a proportionality between the quantities concerned is always maintained. It is demonstrated how the changes in the structural periodicity can be subdivided into a rotation of unaltered crystallites, interpreted as interlamellar slip, into a change in chain inclination within the crystallites, interpreted as intralamellar slip, and into a change in the separation of the crystallites which includes the extension or compression of interlamellar amorphous material. It is shown that the relative contributions of each of these three effects is a function of the temperature of the deformation, the sample type and the type of stress applied. The results are evaluated and discussed in terms of existing conceptions of an oriented polymer and are related to earlier findings on this subject. It is pointed out in particular that the samples in question represent a very simple mechanical system: a series coupling of the individual structural processes involved suffices to describe the response of the sample to externally imposed stress. The identity relation between changes in structure and macroscopic sample dimensions is also revealed by swelling experiments. This, in addition to equating changes in lamellar separation with changes in sample dimension, also provides some definitive information on the location of the swelling agent.

Deformation of Particles during Briquetting

Abstract: Hardman and Lilley1 recently reported their microscopical observations of different materials which had been compressed to form a coherent compact. They showed that the densification of sodium chloride is brought about by plastic deformation whereas, for sucrose, extensive fragmentation of the original particles occurs during densification. The behaviour of the coal used in their investigation seemed to be intermediate between these two extremes.

Finite‐Strain Theory of Crystalline Elastic‐Inelastic Materials

Abstract: The thermodynamics of elastic‐viscoplastic materials developed in two previous papers is extended to account for finite strains. The description of the geometry of a body subjected to finite deformation is based on the kinematic of a continuum with dislocations suggested by Fox.

Deformation characteristics of sands under cyclic loading

Abstract: A knowledge of the dynamic stress-strain characteristics of sand subjected to controlled values of shear strain commonly encountered in the field during strong ground shaking is often desirable. Cyclic load simple shear tests have been used to investigate soil performance at shear strain amplitudes greater than those possible from many other types of dynamic laboratory testing equipment. It was found that dynamic modulus values determined near the upper limit of shear strain amplitude that might be expected to be induced by seismic shaking (0.1%) were as much as 40% lower than modulus values obtained at the lowest values of shear strain that were investigated (0.01%). Modulus values were also found to increase slightly with increasing numbers of cycles and with increasing relative density. Values of hysteretic damping, determined from hysteresis loops, showed damping to increase with increasing shear strain amplitude and to decrease slightly with increasing numbers of cycles and increasing values of vertical stress. Curves summarizing the effects of the several parameters on dynamic soil properties have been presented.

Behavior of hydrogen in α-phase Ti-Al alloys

Abstract: The solubility of hydrogen in titanium and various Ti-Al alloys is measured by a resistometric technique and by direct observation in the electron microscope. Aluminum is shown to increase the solubility of hydrogen in titanium and a thermodynamic argument is used to demonstrate that this increased solubility can be due to the self-stresses around the hydride introduced as a result of the increase in volume accompanying the Ti+H→TiH transformation. The hysteresis in resistivity as a function of temperature for the high aluminum alloys is interpreted as being due to the irreversible work done in plastic deformation of the matrix around the hydride. particles and is necessary because maximum elastic strains in the matrix are insufficient to accommodate the deformation induced by the transformation.

The mechanical behaviour of polymers under high pressure

Abstract: The effect of pressure on the tensile deformation of amorphous polycarbonate and poly(ethylene terephthalate) and semi-crystalline polychlorotrifluoroethylene and polytetrafluoroethylene was investigated up to 8 kb. Tensile deformations of polycarbonate at atmospheric pressure at temperatures down to 116°K were also performed. The former three polymers showed increases of yield stress, yield strain and elastic modulus, and decreases of fracture strain. Polytetrafluoroethylene behaved in an analogous manner up to 4 kb, beyond which both the ‘yield’ stress and elastic modulus deviated systematically from the lower-pressure behaviour. This was found to correlate with changes of bulk modulus at a solid–solid phase transition near 5 kb. The pressure dependence of yield stress was fitted by modified von Mises and Mohr–Coulomb yield criteria. A material parameter describing this pressure dependence was obtained for these and other polymers and was found to correlate qualitatively with the strength-limit...

Adhesion of viscoelastic materials to rigid substrates. III. Energy criterion for failure

Abstract: Measurements are described of the strength of a model adhesive joint subjected to (1) tensile rupture, with the interface containing a small unbonded region of varying size, and (2) pure shear deformation, in the form of a partly unbonded sheet. These, and previous measurements of resistance to peeling separation, are all shown to be consistent with an energy criterion for adhesive failure. The characteristic failure energy per unit area of interface has been determined for the model adhesive material as a function of the effective rate of detachment, over a wide range covering almost the entire spectrum of viscoelastic response. The values obtained are found to increase from levels only slightly higher than thermodynamic considerations would predict, i.e., 102−103 ergs/cm2, at low rates of crack propagation, up to a value of about 106 ergs/cm2 at high rates when the material responds in a glasslike manner. These results suggest that the failure energy has two components: the (reversible) work of adsorption and the (irreversible) work of deformation of the adhesive in effecting separation.

Deformation of UO2 at High Temperatures

Abstract: The effects of temperature, strain rate, and grain size on the mechanical properties of UO2 were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, Tc, the material fractured in a brittle manner, with no macroscopic plastic deformation. Between Tc and a second transition at a higher temperature, Tt, a small amount of plastic deformation was measured before fracture. Beyond Tt, the strength of UO2 decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above Tt: and where σp and σ88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.

The prediction of the tensile properties of flexible foams

Abstract: An idealized model for flexible foam in tension is developed. The model includes large deformations and cell structure orientation before and after deformation. It is predicted that two quantities affect the initial modulus of an unoriented foam: the density and a cell structure parameter. Data for latex foam show that the model correctly predicts the initial tensile modulus. The model also predicts that cell structure orientation is a reasonable method for achieving a desired modulus without altering the density.

The effects of applied stress on the martensitic transformation in TiNi

Abstract: The deformation behavior of TiNi at 20°C has been investigated as a function of composition and of the prior heat treatment. Wide mechanical property variation and significant differences between the effects of tensile and of compressive loading were observed. Under some conditions anelastic behavior, characterized by a broad hysteresis loop, was reproducibly obtained. The effects of heating the deformed materials aboveA f on the subsequent stress-strain behavior indicate the anomalies observed to be directly related to the martensitic transformation. The effects of stress application on the martensitic transformation are discussed. It is shown that, under some conditions, the stress-assisted transformation structures may be unstable on the removal of the stress. At test temperatures outside theM f toA f range, this can result in anelastic behavior. More complex behavior expected at temperaturesM f < T < Af, is discussed in some detail. It is shown that both the anelastic behavior and the “shape memory” can be accounted for by the effects of applied stress. It is also shown that the mechanical properties in this temperature range can vary markedly with the prior heat treatment, even at temperatures not normally considered of significance. Though based on observations made on TiNi, the phenomena discussed are inherent in materials undergoing a martensitic transformation over a narrow range of temperatures.

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic

Abstract: Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and α-Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.

Dynamic Compressive Strength and Failure of Steel Reinforced Epoxy Composites

Abstract: Results of an experimental program systematically evaluating the deformation and fracture of steel wire reinforced epoxy composite systems are presented. The program involved mechanical testing in the strain rate range 10-5 to 103/sec and impact testing of composite specimens using massive elastic targets at strain rates approximately 104/sec. Specific results presented include static and dynamic prop erties, strain rate sensitivity, information on the nature and character of dynamic fracture, influence of specimen geometry and reinforce ment spacing, and high speed photographs of dynamic failure modes. Further, a simplified energy criterion is proposed for predicting failure modes and critical velocities.

Plastic deformation of polyethylene II. Change of mechanical properties during drawing

Abstract: The crystallinity, elastic modulus, and tensile strength of samples of various draw ratios together with the true stress—strain curves of high-density polyethylene were determined to establish correlations with morphological changes occurring during deformation. Changes of crystallinity at draw ratios below 5, i.e., constancy during drawing of quenched film and a decrease during drawing of annealed film, are explained by the formation of microfibrils with crystallinity independent of the thermal history of the film. The microfibrils slide past each other at higher draw ratios, generating an increasing number of interfibrillar tie molecules, which is reflected in the increasing number of interfibrillar tie molecules, which is reflected in the increase of crystallinity, elastic modulus, and tensile strength. From the true stress—strain curves, the differential work density for the deformation of the volume element was calculated as a function of the draw ratio. It contains two components which reflect two different mechanisms of deformation. The first component, decreasing with increasing draw ratio, can be associated with the destruction of the original microspherulitic structure; the second one, increasing with increasing draw ratio, can be associated with the deformation of the new fiber structure, i.e., with the sliding motion of the microfibrils formed during the first deformation step.

Analytical studies of contact of nominally flat surfaces - Effect of previous loading

Abstract: Previous plastic loading deformation effect on contact area, print number, size distribution and thermal conductivity for flat surfaces

Low pressure hysteresis in the adsorption of organic vapours by porous carbons

Abstract: This paper summarizes the results of experiments carried out over the past 15 years in which low-pressure hysteresis in the adsorption of organic vapours by porous carbons has been observed. These collected data provide substantial support for the hypothesis that low pressure hysteresis in these systems is associated with the intercalation of molecules of adsorbate in narrow pore spaces leading to irreversible changes in the pore structure. Attention is drawn to the close parallels between this type of hysteresis, in which deformation of the pore structure by adsorption forces exceeds the elastic limit of the porous medium, and stress-strain hysteresis in solids brought about by forces applied externally.

Static deformation of silica and silicates

Abstract: Static ductile deformation in quartz, olivine, pyroxenes and placioglase, noting plastic deformation and recovery

Plastic Deformation of ZrB2 Single Crystals

Abstract: The active slip planes, the Burgers vector, and the critical resolved shear stress to induce macroscopic deformation in ZrB2 single crystals were determined. Room-temperature microhardness indentation and high-temperature uniaxial compression loadings were used to induce deformation. Slip occurred in a close-packed α direction on prismatic planes at room temperature and on the basal plane at high temperatures. The high-temperature yield stress and yield drop are discussed in terms of a Widmanstaetten-type precipitate observed in the crystals.

Reaction‐Rate Model for Fracture in Polymeric Fibers

Abstract: Electron paramagnetic resonance (EPR) techniques were used to determine the number of free radicals produced during deformation leading to fracture of nylon 6 fibers. A reaction‐rate molecular model is proposed to explain some of the deformation and bond‐rupture behavior leading to fracture. High‐strength polymer fibers are assumed to consist of a sandwich structure of crystalline‐block and amorphous‐flaw regions along the fiber axis. In the flaw regions, tie chains connecting the crystalline blocks are assumed to have a statistical distribution in length. These chains are, therefore, subjected to different stresses. The length distribution was determined by EPR. The probability of bond rupture was assumed to be controlled by reaction‐rate theory with a stress‐aided activation energy and behavior of various loadings determined by numerical techniques. The model is successfully correlated with experimental stress, strain, and bond‐rupture results for creep, constant‐rate‐of‐loading, and cyclic‐stress tests.

Yield surfaces in prestrained aluminum and copper

Abstract: The analysis of strain-hardening materials subjected to multiaxial states of stress requires more detailed experimental information about the effects of previous plastic deformation on the yield surfaces of real materials than is presently available. To provide insight into some of these effects thin walled tubular specimens of 1100-0 aluminum and annealed OFHC copper were subjected to biaxial stresses through the application of simultaneous axial tension and internal pressure, and the effects of the magnitude, direction, and sequence of prestraining operations on subsequent yield surfaces were determined. It was found that the yield surface behavior depends greatly upon the definition of yielding employed. Use of small proof strain definitions resulted in very anisotropic yield surface characteristics which reflected the effect of previous deformation. On the other hand, use of large proof strains resulted in isotropic yield surface characteristics which were devoid of previous deformation influence. The small proof strain yield curves were found, in general, to expand and translate in the direction of prestrain and, for biaxial prestrains, to be distorted in the vicinity of the loading point. Multiple prestrain sequences in normal directions induce a large negative cross effect similar to Bauschinger effect observed under reversed loading. Such anisotropic behavior was found to contradict the two most commonly used continuum mechanics predictions, the isotropic and kinematic hardening rules.

Deformation of Polyethylene at High Pressure

Abstract: The influence of pressure, deformation rate, and pressure fluid on the mechanical properties of high‐molecular‐weight polyethylene is examined. A miniature motor‐driven testing machine which operates completely enclosed in a high‐pressure vessel is employed. This device allows for the first time in any material measurement of the sensitivity of the flow stress σ to an instantaneous change of strain rate under superposed high hydrostatic pressure. From these data and the pressure dependence of σ it is possible to calculate the activiation volume V* for plastic flow as a function of pressure; it is suggested that V* is the appropriate parameter to describe the pressure dependence of a kinetic process such as plastic deformation. It is found that samples deformed in n‐pentane generally exhibit a reduced flow stress relative to those tested in water. In addition, it is observed that V* for specimens tested in pentane is significantly less than for those tested in water (∼230 A at 1 atm as opposed to ∼266 A3)....

The effect of hydrostatic pressure on the tensile properties of plastics

Abstract: The effect of hydrostatic pressure, up to 112,000 lb/in.2, on the tensile properties of four polymers is reported. The pressure soaking of polystyrene in castor oil had no significant effect on the material's residual properties when tested under ambient conditions. When tested under pressure polystyrene necked, like a metal, and exhibited a brittle ductile transition at 40,000 lb/in2. Between 40,000 to 112,000 lb/in.2 the tensile strength increased by about 30%. Young's modulus and yield strength were only slightly affected by pressure. Similar results were obtained for specimens sheathed to prevent possible plasticization of the polystyrene. Polymethylmethacrylate tested at 112,000 lb/in.2 failed just short of its instability point and with only a slight increase in Young's modulus. These amorphous polymers thus behaved under pressure in a generally similar manner to metals. Pressure had a marked effect on the stress-strain curves of two crystalline polymers polyethylene and nylon. Young's modulus and tensile strength were considerably increased and elongation decreased. Pressure inhibited ‘drawing’ of the materials. Deformation was restricted to a small necked region.

The stiffness of polymers in relation to their structure

Abstract: In the majority of applications of polymers, we are interested in one or more of three basic mechanical properties-stiffness, strength and toughness. To these can be added creep, which becomes important in many engineering applications. Stiffness represents resistance to deformation, and is a much simpler property than strength and toughness, which relate to failure. Strength is the ultimate stress which a material can withstand, before it fails, whether by fracture or by excessive deformation, whilst toughness represents the work required to fracture a material