Showing papers on "Slip (materials science) published in 1971"

Molecular model of drawing polyethylene and polypropylene.

Abstract: Morphological studies of plastic deformation of single crystals, thin layers, and bulk samples together with mechanical, X-ray and infra-red data revealed the existence of three stages in cold drawing of crystalline polymer: the plastic deformation of the original spherulitic structure, the discontinuous transformation of the spherulitic into fibre structure by micronecking, and the plastic deformation of the fibre structure The initial material, which has low strength and high ductility, consist of stacks of parallel lamellae with few interlamella links It deforms plastically by stack rotation, sliding of lamellae, phase change and twinning of crystal lattice, chain slip and tilt until the predeformed lamellae reach the position of maximum compliance for fracture by micronecking The micronecks transform every single lamella into microfibrils of between one and three hundred angstroms in width, consisting of folded chain blocks broken off the lamella primarily by chain slip in the boundary layers between adjacent mosaic blocks The chains bridging the crack are partially unfolded during the micronecking process They connect in axial direction the blocks in the microfibril as intrafibrillar tie molecules The number of microfibrils per cm of crack length increases with molecular weight The draw ratio of the microfibrils and the axial separation in the microfibril of the originally adjacent crystal blocks increase with the average distance between microfibrils and, hence, decrease with increasing molecular weight The concentration of micronecks in every stack of lamellae in a thin destruction zone produces a bundle of microfibrils of rather uniform draw ratio Such a fibril measuring a few thousand angstroms in width includes the interlamella ties of the original sample as interfibrillar tie molecules connecting adjacent microfibrils The concentration of micronecks also provides the conditions for a nearly adiabatic heating of the generated fibril by the transformation work in the destruction zone The local temperature rise imparts so much mobility to the chains in the crystal blocks that during subsequent cooling to ambient temperature, the long period becomes adjusted to this temperature The more or less random distribution of destruction zones in the neck makes the transformation from spherulitic to fibre structure appear to be a gradual process in spite of the discontinuous transformation in the micronecks The plastic deformation of the new fibre structure can proceed only by longitudinal sliding of microfibrils past each other, a process limited by interfibrillar tie molecules Hence, high molecular weight samples with many interlamella links exhibit a smaller draw ratio than lower molecular weight material The three stages are to some extent intermixed in the neck In the initial neck characterised by a low draw ratio and rather gentle constriction, the transformation into the fibre structure is not complete, so that some of the remains of the original microspherulitic structure are still present in the necked portion They are destroyed during subsequent drawing which completes the transformation and also deforms the fibre structure The sharply constricted mature neck, however, yields a high draw ratio which is composed of the draw ratio of microfibrils and of subsequent sliding motion of the microfibrils The technically important natural draw ratio is the maximum draw ratio obtained with the sample under the conditions of the experiment It seems to be higher than the draw ratio of the microfibrils

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Abstract: Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al2O3. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.

Influence of Shear Stress on Screw Dislocations in a Model Sodium Lattice

Abstract: The behavior of the screw dislocation core in the presence of an external shear stress has been examined for the body-centered cubic and hexagonal close-packed phases of a model sodium lattice, usi...

Calculation of the taylor factor and lattice rotations for bcc metals deforming by pencil glide

Abstract: Values of the Taylor factorM and the corresponding lattice rotations for tension or compression have been computed for grains of various orientations postulated to slip on arbitrary planes in 〈111〉 directions. The stresses in each grain were first obtained by numerically maximizing the work expression subject to the constraints of the pencil-glide yield expressions. The derived value of Mave = 2.733 is slightly lower than in the case of mixed {110}, {112}, and {l23} 〈111〉 slip and the computed rotations are in reasonable agreement with experiments on iron. In compression, 26 pct of the grains are predicted to rotate toward 〈100〉, although the 〈111〉 texture component develops faster. The computational method used can easily be applied to the determination of the mechanics of texture generation for arbitrary deformations.

Origin of paired metamorphic belts and crustal dilation in island arc regions

Abstract: We present quantitative models for the thermal structure of the crust and uppermost mantle in island arc areas and consider the implications of this structure for paired metamorphic belts and crustal extension behind arcs. In an active island arc (e.g., Japan), the following processes are assumed to occur: (1) The oceanic lithosphere slides under the island arc structure along the line of the oceanic trench. The uneonsolidated oceanic sediment that overlies the oceanic lithosphere accumulates against the front of the arc, building it out at a rate of about 1 km/m.y. (2) Heat is generated along the slip zone between the descending lithosphere and the rocks that overlie it. At some depth, melting occurs along the slip zone, magmas are formed, and heat is transferred upward by magmatic convection. At the surface, three distinct thermal zones run parallel to the arc; they are, in order away from the trench toward the arc: (A) zone of low heat flow, where accumulation of unconsolidated ocean floor sediment takes place; (B) zone of average heat flow occupied by previously accumulated sediment or old crust; and (C) zone of high heat flow that overlies that part of the slip zone on which melting occurs and upward transfer of heat takes place by penetrative magmatic convection. The width of each zone is variable and depends largely on the relative rate of movement of the descending lithosphere. Each zone is characterized by its own vertical geothermal gradient, and there is a strong horizontal temperature gradient between zones. At high rates of movement on the slip zone, the deeper part of zone A can provide the P/T conditions appropriate for the formation of glaucophane schists and eclogites, while zone C can give the P/T conditions for andalusite-sillimanite and kyanite-sillimanite metamorphic facies series. Upward and possibly horizontal tectonic transport of the glaucophane schists to the surface is necessary to preserve the high-pressure, low-temperature mineralogy and to give the arrangement of paired metamorphic belts observed around the margins of the Pacific. These vertical movements are probably associated with a deceleration or cessation of movement on the slip zone. The rate of magmatic intrusion into the upper crust required to give the high heat flow in zone C would result in crustal extension (spreading) behind the arc at rates of up to several centimeters, per year in a steady-state situation.

Slip and the clinoenstatite transformation as competing rate processes in enstatite

Abstract: Previous experimental deformation of orthorhombic enstatite at temperatures from 200° to 1000°C has produced clinoenstatite; yet, the natural occurrences of clinoenstatite in deformed rocks is extremely rare. The present experiments were conducted on the premise that transformation to clinoenstatite and slip in orthoenstatite are competing rate-controlled deformation processes, with slip having the higher activation energy. The results show that, at strain rates of 10−3 to 10−7 sec−1, slip occurs in orthoenstatite without transformation to clinoenstatite at temperatures above 1300° to about 1000°C, respectively. Parameters of the creep equations for the two different flow processes were determined empirically using stress-relaxation and temperature-differential creep tests. The boundary between slip in orthoenstatite and transformation to clinoenstatite calculated theoretically from the flow laws agrees well with the experimental boundary. Extrapolation to geologically reasonable strain rates suggests that, above a temperature between 450° and 650°C, clinoenstatite will not form in natural deformation of orthoenstatite except where strain rates are abnormally high. This result appears to explain the near absence of clinoenstatite in naturally deformed pyroxenes.

Slip systems in NiAl single crystals at 300°K and 77°K

Abstract: Single crystals of NiAl have been examined by transmission electron microscopy after plastic deformation at 300°K and 77°K. In agreement with earlier work, it is found that single crystals generally slip along 〈100〉. Further, it is shown that crystals oriented so that there is zero applied stress on the dislocations of b = 〈100〉, deform by the movement of dislocations of b = 〈111〉 on {112} and {110}. A lower limit to the Peierls stress for dislocations of b = 〈111〉 is calculated from the observed spacing of opposite sign screw dislocations, and it is shown that these dislocations are far more difficult to move than mobility calculations would suggest.

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.

2. Faulting of the Great Kanto Earthquake of 1923 as Revealed by Seismological Data

Abstract: The fault parameters of the Great Kanto earthquake of September 1, 1923, are determined on the basis of the first-motion data, aftershock area, and the amplitude of surface waves at teleseismic stations. It is found that the faulting of this earthquake is a reverse right-lateral fault on a plane which dips 34° towards N20°E. The auxiliary plane has a dip of 80° towards S55°E. This means that the foot-wall side moves approximately north-west with respect to the hanging wall side. The strike of the fault plane is almost parallel to that of the Sagami trough, and the slip direction is more or less perpendicular to the trend of the Japan trench. This earthquake is therefore considered to represent a slippage between two crustal blocks bounded by the Sagami trough. A seismic moment of 7.6×1027 dyne-cm is obtained. If the fault dimension is taken to be 130×70 km2, the average slip on the fault plane and the stress drop are estimated to be 2.1m and 18 bars respectively. This slip is about 1/3 of that estimated from geodetic data. This discrepancy may indicate an existence of a pre-seismic deformation which did not contribute to the seismic wave radiation, but the evidence from other observations is not very firm.

Dynamic Shear Cracks with Friction as Models for Shallow Focus Earthquakes

Abstract: Summary In this paper a dynamical model of an earthquake source is investigated. This necessarily idealized model consists of a uniform elastic half space under a shearing pre-stress which tends to produce strike slip on a vertical fault plane. The fault plane is a plane of weakness across which the material is not welded but is initially inhibited from moving by a static frictional resistance which increases with depth. At a certain instant in time and depth in the half space a region of relative slip across the fault plane is initiated which spreads upwards and downwards so as to occupy at all times an infinite strip. Thus we shall be concerned only with two-dimensional SH motion (anti-plane strain). Once slipping occurs only reduced tractions act across the region of slip and it is the resulting stress drop which drives the mechanism. This model is almost the same as that considered by Berg and Weertman but goes further in that the dynamical problem is solved. We here extend previous work by Burridge and Burridge & Willis, in that we now find as part of the solution how the zone of slip spreads as well as the relative displacements, how the increasing friction prevents the crack (zone of slip) from penetrating very deeply, and eventually how it brings the whole mechanism to rest. Finally we calculate the pulse shapes in the far-field radiation and the residual displacements and stresses on the fault plane.

Hardness anisotropy, deformation mechanisms and brittle-to-ductile transition in carbide

Abstract: The slip plane for TiC0.8′ VC0.84 and substoichiometric tantalum carbide has been determined as {110} using microhardness indentation at room temperature. Under the same conditions, HfC0.98 also slips on {110} but TaC0.96 slips on {111}. At low temperatures {110} slip is characteristic of the Group IV and substoichiometric Group V transition metal carbides while stoichiometric Group V carbides probably deform preferentially on {111} at all temperatures. This behaviour is explained in terms of two models for the crystal structures of the carbides. The Group IV carbides are described by a close-packed metal lattice whereas the structure of stoichiometric Group V carbides is more open. Various physical and mechanical properties and the effects of changing carbon content have been correlated on the basis of the models. In particular, an explanation of the brittle-to-ductile transition in carbides is proposed.

Approximate Method in the Kinetic Theory

Abstract: An approximate method for solving the slip problems in the kinetic theory of gases is proposed, and it is shown that extremely good results can be obtained by a simple modification of Maxwell's arguments.

Small Plate Tectonics in the Northeastern Pacific

Abstract: Lithospheric plate motions in the northeastern Pacific were complicated at about 2.5 m.y. B.P. by the movement along a major northeast-trending fault cutting Cascadia Basin. An estimate of the slip rate along this fault gives critical information on the relative motions of four geometrically interdependent blocks. The fault is presently inactive. Seventy km of slip along this fault during 2 m.y. or less gives an average slip rate of about 3-5 cm/yr or greater, and resulting plate motions suggest a significantly greater rate of net subduction along the continental margin off Oregon than off Washington and Vancouver Island. Subduction rate off Oregon is less sensitive to slip rate along this fault than is subduction off Washington.

Elektronenmikroskopische untersuchung der versetzungsanordnung verformter kupfereinkristalle im belasteten zustand

Abstract: Copper single crystals orientated for single glide were deformed in tension at 78°K and irradiated before load-removal with fast neutrons at 4°K or 20°K. The dislocation arrangement which was thus stabilized in the stress-applied state was investigated extensively by transmission electron microscopy and compared with the results of corresponding investigations on unloaded crystals. In stage II dislocation braids and dislocation grids (sheets) are observed both in the stress-removed and the stress-applied states. Groups of primary dislocations of the same sign, however, are a representative feature of the dislocation arrangement in the stress-applied state only. It is concluded from the observations that the grids are formed by secondary slip at the head of piled-up groups and are capable of growth. The observed spatial change in the curvature of free primary dislocations in the stress-applied state points directly to the existence of a quasi-periodic long-range internal stress field whose amplitude is of ...

Improved fatigue resistance of Al-Zn-Mg-Cu (7075) alloys through thermomechanical processing

Abstract: To decrease the accumulation of damage during long-life low-stress cyclic loading, microstructures must accommodate inelastic deformation by homogeneous or “dispersed” slip rather than by localized slip concentrations. In age-hardening aluminum alloys this requirement can be met by introducing a dense and uniform dislocation forest through suitable thermo-mechanical treatments. Such a treatment was developed for Al-Zn-Mg-Cu (7075) alloys, involving a process cycle of solution annealing, partial aging, mechanical working and final aging. The fatigue properties (S-N curves) of commercial and high-purity 7075TMT are compared with conventional 7075-T651 properties; with zero mean stress the alternating stress to cause failure in 107 cycles is more than 25 pct higher for commercial-purity 7075TMT and almost 50 pct higher for high-purity 7075TMT. The results emphasize the importance of microstructural control when high fatigue resistance is required.

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.

Thermal Creep Slip with Arbitrary Accommodation at the Surface

Abstract: An accurate theoretical analysis of the thermal creep slip velocity qT,asy for arbitrary α, the fraction of the molecules that are diffusely reflected from the surface, is carried out by applying the method of elementary solutions to the Bhatnagar‐Gross‐Krook model. Specifically, qT,asy is expressed in terms of two power series that, respectively, converge rapidly for α→1 and α→0, and it is found that qT,asy depends only slightly on α. Some alternate forms of boundary conditions are also discussed, and it is shown that the thermal creep slip velocity does not depend on αp, the momentum accommodation coefficient. These results confirm the essential accuracy of a very general variational expression given recently by one of the authors. Further, some consequences of the present results in the analysis of the thermal transpiration experiments are also discussed.

Stiffness in faulting and in friction experiments

Abstract: Analysis shows that the ‘stiffness’ characteristic of large earthquakes is 4 or 5 orders of magnitude less than the ‘stiffness’ characteristic of typical laboratory experiments on rock. Direct application of laboratory experiments to sliding on faults is therefore uncertain. This discrepancy does not necessarily invalidate the suggestion that earthquakes may be large-scale stick-slip sliding, because stick slip is enhanced in machines with low stiffness.

On giant screw dislocations in ZnS polytype crystals

Abstract: Vapour-grown platelet crystals of ZnS containing long-period polytypes have been found by x-ray topography to contain a single screw dislocation with a very large Burgers vector parallel to the c axis. Individual polytype regions can have highly perfect lattices; the few dislocations observed in them in addition to the giant screw have Burgers vectors in the basal plane. A divergent-beam section topograph arrangement has been used to measure giant Burgers vectors to an accuracy of about 30%. Within this uncertainty, the Burgers vector magnitudes match the polytype repeat period in the five crystals upon which measurements have been made, the repeat periods ranging from 50 A to 106 A. The x-ray topographic studies provide good evidence in support of the periodic slip process for polytype formation.